Research ArticleAnalysis of Step Responses in Nonlinear DynamicSystems Consisting of Antagonistic Involvement ofPneumatic Artificial Muscles

Miroslav Rimaacuter1 Peter Šmeringai1 Marcel Fedak1 Michal Hatala2 and Andrii Kulikov1

1Department of Process Engineering Faculty of Manufacturing Technologies Technical University of Kosice with a Seat in PresovSturova 31 08001 Presov Slovakia2Department of Manufacturing Technologies Faculty of Manufacturing TechnologiesTechnical University of Kosice with a Seat in Presov Sturova 31 08001 Presov Slovakia

Correspondence should be addressed to Peter Smeringai petersmeringaitukesk

Received 22 October 2016 Revised 2 January 2017 Accepted 10 January 2017 Published 22 February 2017

Academic Editor Giovanni Berselli

Copyright copy 2017 Miroslav Rimar et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The paper describes a set of experimental measurements carried out on the experimental equipment with a drive based onpneumatic artificial muscles Based on the analysis of the PAMS control systems issue in relation to the issue of a position control acontrol algorithm has been designed and verified The requirements of the control systems do not arise only from the condition ofthe desired positioning point rapid achievement but also from the subsequent dynamics and accuracy repeatabilityThis algorithmenables an efficient way of stabilization of the actuator position in various dynamic conditions during the operation It allowseliminating undesirable vibrations oscillating around the point of the required position and dampening them appropriately Thearticle describes a set of performed verification experimental measurements confirming the applicability in relation to the systemthat controls the position of the actuator utilizing the described algorithmThe algorithm application enables a positive influencingand optimization of the actuator positioning accuracy and a full-valued automation of its operation

1 Introduction

Research and development in manufacturing technologyhave been constantly motivated by new challenges in man-ufacturing companies and firms focused on production ofmanufacturing machines and manipulators The effort tokeep a strong competitive environment leads manufacturersof manufacturing technology to searching for new solutionsto structural nodes inmanufacturing technology It also leadsto introduction of modern technology to manufacturing Forinstance the requirements of modernmanufacturing deviceswhich are able to perform their functions in aggressiveenvironment are currently very demanded It also refers to therequirements for an adapted technical device for examplea device with a modified drive which allows its functioningin the environment where standard types of drives cannot beused due to their negative effect on the work environment

In terms of the need of technical equipment adjustmentsthere may occur multiple factors affecting the choice of anequipment suitable replacement for example the size of adevice and its output with a given size (weight) are importantIn this regard it is possible to consider a more efficientenergy utilization in a manufacturing system Each of thenonconventional drives (electric pneumatic or hydraulic) ischaracterized by its own deficiencies Considering the drivesoptions the power-to-weight ratio is the main characteristicsthanks to which manipulation devices with power unitscomprising pneumatic artificial muscles start to be used inproduction facilities [1ndash4]

Artificial pneumatic muscles have also started to beapplied tomedicine [5]The fundamental principle of a pneu-matic artificial muscle (PAM) functioning allows evocatinga tensile force in one direction Its use for the movement inboth directions requires its use with the involvement of the

HindawiAdvances in Materials Science and EngineeringVolume 2017 Article ID 7168462 14 pageshttpsdoiorg10115520177168462

2 Advances in Materials Science and Engineering

element for a force bond acting in the opposite direction toa PAM force acting This may concern the PAM involvementin a binding interaction with a pulley system and an onerouselement elastic element (eg a spring) or another artificialmuscle

The paper deals with the utilization of the involvementof two pneumatic artificial muscles in an antagonistic con-nection [6] Their wider use is limited due to their significantnonlinearity in cases of higher positioning requirements [7]

In present conditions when a considerable part of man-ufacturing facilities is oriented towards automated operationthe method of control of manufacturing devices with drivesbased on pneumatic artificial muscles has to be considered[8ndash12]

Nowadays the term ldquopneumatic artificial musclerdquo (PAM)starts to be used more often in connection with propulsionsystems Due to its characteristics it is used not only insimple applicationsThanks to a sufficiently advanced controlcomputing it becomes a part of even more complex mecha-nisms or actuators Due to its characteristics it allows suchapplications where under specified conditions electric andhydraulic actuators fail The PAM is characterized by a dis-position of a functionality similar to a human muscle Theseproperties of the PAM are a prerequisite for its use in theareas such asmanufacturing and transportation industries ormedicine [8 9]

Regarding the range of utilization the McKibben type ofthe artificial muscle is currently most widely used and stud-ied The McKibben artificial muscle has a number of mod-ifications such as braided pneumatic actuator pneumaticmuscle-fluidic muscle (FESTO) and rubbertuator These aremodifications of the McKibben artificial muscle that havea significantly high power-to-weight ratio with a sufficientstability of flexibility Their disadvantage is their nonlinearityand therefore insufficiently solved problems of a positioningaccuracy that is needed for a wider use of the PAM inmanufacturing industry [8 9 13ndash15]

2 Characteristics of PneumaticArtificial Muscles

The functional principle of artificial muscles is generallydefined as a conversion of pneumatic energy into mechanicalenergy [9 15] As mentioned in the introduction the artificialmuscle is a traction drive whose characteristics approachto those of a biological muscle It consists of a contractileelement a mounting equipment (to mount the PAM in thestable position) and connecting devices (to connect thepressurized working medium)

With fluid artificial muscles the maximum tractive forceis at the beginning of a contraction During the lift (agradual change in length) it quasi-linearly decreases Theterm ldquomuscle contractionrdquo refers to shortening of its lengthIt is expressed in the form of a percentage ratio to a nominallength of an unloaded artificial muscle membrane From theforegoing it follows that the durability of the artificial muscleis largely dependent upon a contraction and operating pres-sure as well as temperature [15 16]

Functionally the operation of the artificial muscle isbased on a performance of the operating medium which actson the inner wall of the artificial muscle (membrane) thusincreasing its volume Increasing volume causes a change inthe inclination of the artificial muscle braid which results inthe braid shortening The artificial muscle develops a tensileforce by its shortening Based on the above described factors itfollows that artificial muscle reduces its length and increasesits volume with the increasing pressure of a working medium[1 9 13 17]

Work carried out by the artificial muscle can be dividedinto the so-called input work 119882in and output work 119882out[1 9 18] The input work 119882in is carried out by a workingmedium pushing on the inner surface of the artificial musclemembrane [1] The input work acting on the wall of themembrane is defined as [1 9]

119889119882in = int119878119894

(119875 minus 1198750) 119889119897119894sdot 119889119904119894= 1198751015840 sdot 119889119881 (1)

where1198751198750is the absolute pressure of theworking fluid inside

the muscleambient gas 1198751015840 is the relative pressure (deviationof 119875 and 119875

0) 119904119894is the inner surface of the PAM 119889119904

119894is the

difference in surface 119889119897119894is the displacement of the inner

surface and 119889119881 is the total volume change [1 9]The result of the muscle shortening is the output work119882out [1 9]

119889119882out = minus119865 sdot 119889119897 (2)

where 119865 is the tensile force of the PAM acting in the axialdirection of the muscle and 119889119897 is the axial displacement of themuscle [1 9]

Assuming a simplified system if its losses are neglectedthe output work is equal to the input work [1 9]

119889119882out = 119889119882in (3)

Substituting the right sides of (1) and (2) into (3) we obtain[1 9 11]

minus119865 sdot 119889119897 = 1198751015840 sdot 119889119881 (4)

After the adjustment

119865 = minus1198751015840 sdot 119889119881119889119897 (5)

Provided that the shape of the active part of the artificialmuscle is simplification into the ideal shape of a cylinder (Fig-ure 1) where the length of the muscle 119897 is a representation ofthe ideal height of the cylinder 120579 is the angle of the inclinationof themuscle braid fibers from the ideal axis of the cylinder 119899is the number of encircles of network structure fibers aroundthe ideal cylinder of the muscle 119863 is a diameter of thecylinder and 119887 is a total length of the fiber we can estimatethe volume change according to the change in length of thecylinder (119889119881119889119897)

The length 119897 and the diameter 119863 of the cylinder can beexpressed as the function of the angle of the cylinder

Advances in Materials Science and Engineering 3

n encirclesof a fibre

120579120579

L

D n120587D

b

Figure 1 Functional principle of the artificial muscle braid [1]

inclination 120579 with constant quantities 119899 and 119887 indicating thecharacteristics of a particular cylinder [1 9]

119897 = 119887 sdot cos 120579 (6)

119863 = 119887 sdot sin 120579119899 sdot 120587 (7)

Subsequently it is possible to determine the volume of theideal cylinder as [1 9]

119881 = 14 sdot 120587 sdot 1198632 sdot 119897 = 119887

3

4120587 sdot 1198992 sdot sin2 120579 sdot cos 120579 (8)

Equation (5) after these modifications it is possible to derivethe resulting dependence of the muscle tensile force 119865 onthe relative pressure 1198751015840 and the inclination of fibers from thecylinder axis 120579 [1 9]

119865 = minus1198751015840 sdot 119889119881119889119897 = minus1198751015840 119889119881119889120579119889119897119889120579 =

1198751015840 sdot 1198872 (3 cos2 120579 minus 1)4120587 sdot 1198992 (9)

Substituting from the right side of (6) it is alternativelypossible to express the force as a function of 1198751015840 and 119897 [1 9]

119865 = 1198751015840 sdot (31198972 minus 1198872)4 sdot 120587 sdot 1198992 (10)

Of the previously described relations it follows that thedependence of a tensile force and pressure of the workingfluid inside the ideal cylinder (representing a simplified arti-ficial muscle) is liable to a direct proportion and a monotonefunction of fibers inclination from the cylinder axis [1 9]

From the foregoing and in accordance with [15] it ensuesthat the basic possible uses of artificial muscles are

(i) single-acting drive(ii) pneumatic spring

The PAM cannot be used as an air spring without a sufficientoperating pressure [15]

3 Materials and Methods

31 Preparation of Experimental Measurements The aim ofthe experiment realization was to verify the algorithm mod-ularization for controlling the position of the PAMs enabling

an effective way of the actuator position stabilization in var-ious dynamic conditions during the operation in relation tothe positioning accuracy In experiments during the changesin position there was monitored the influence of a staticload placed on the arm of the experimental assembly on thedynamics of the actuator arm stabilization process utilizingthe power based on the pneumatic artificial muscles in antag-onistic involvement There was also observed the effect ofthe proposed position control algorithm on the inaccuracieselimination The effort was to achieve a stable position

In order to measure the position of the arm the repeatedsequences were performed in which the characteristics toachieve the desired point and the actual dynamics of theprocess were followed that is the rate of acceleration in stabi-lizing and its impact on the process of stabilization Owing tothe nature of the system and its behaviour in a direct relationto the stiffness each of the defined characteristic points wasmonitored Increment of the change in a positionwas set froma zero starting position up to the maximum value Based onthe research experiments according to [9] it is not appropri-ate to use the PWM in the range below 20 considering theextreme decrease in durability of the electropneumatic valvesTherefore the range from 0 to 15 was indicated as unusedin the implementation of the experiment As the first value15∘ deflection change was determined while all followingvalues had increment of 15∘

The methodology of the procedure to achieve the objec-tive of the experimental measurements was guided by thefollowing scheme (Figure 2)

Within the elaboration of themethodology formeasuringthe transient characteristics of the PAM the initial require-ments of the functionality of the PAMs control system havebeen set The control system has been designed to allow amodification to its functionalities on the modular arrange-ment of algorithms The method respectively procedure foranalyzing the state of the system during each iteration wasdesigned The designed methodology is not based only onthe premise of the PAMs state online analysis It is also basedon the need of the analysis of the system after its operationusing the data taken during its operation The measuring ofthe system state consists of the PAMs measurements of vari-ables commonly measured on drives working based on thepneumatic artificial muscles such as the pressure in each ofthe PAM and the position of the support armThese variablesprovide information about the state of the system but do notfollow the processes taking place inside each PAM and insidethe transport routes of the working fluid Similarly they donot allow studying the effect of nonlinearity of the PAMssystem Therefore the question arose about how to followthe dependence of processes taking place inside the PAMssystem At the same time the requirement arose of the small-est possible interaction of sensing devices with the PAMssystem in terms of a minimal impact on the functionality andthe system load Therefore a measurement of a supportingarm acceleration was added Monitoring of this physicalquantity envisaged the possibility of a future description oftransitional processes inside the PAMsThere were taken intoaccount the conditions of the influence of pressure of theworking fluid and the time of the working fluid acting on the

4 Advances in Materials Science and Engineering

Measurement of individual quantities

Analysis and synthesis of results Evaluation

Determination of the desired position and the desired deviation

Activation of the algorithm

System setup options

Measuring the position of the actuator arm

Verification of function of the proposed algorithm for stabilization of the actuator arm

Achieving the desired position

Optimizing the position deviation

Measurement of individual quantities

Monitoring the accuracy of the achieved position

Figure 2 Schematic representation of the methodology of the measurement procedures

acceleration of the loaded system supporting element withthe known pressures in each PAM

For themeasurements therewere performed three exper-iments In the experiment number 1 the measurements werecarried out under conditions based on the known knowledgeThen on the basis of the data obtained in the experimentnumber 1 corrections were performed and verified in exper-iments numbers 2 and 3

32 Characteristics of the Experimental Device At theDepartment of Process Technique the Faculty of Manufac-turing Technologies with a Seat in Presov an experimentalassembly of an actuator powered by artificial muscles wasconstructed The experimental actuator assembly comprisesan antagonistic involvement of artificial muscles with agearing to a rotational movement of a rotary arm a controland regulation passage and a pneumatic system supplying aworking fluid into the actuator drive The assembly is com-pleted with the computer control system The control is pro-grammed in LabVIEW and its computing hardware base wasassembled from the components of the series CompactRIO-National Instruments Figure 3 gives a schematic representa-tion of the experimental device

The experimental actuator is divided into smaller parts(1) Antagonistic involvement of two pneumatic artificial

muscles (PAMs)(2) The system for a drive load(3) Pneumatic system(4) The system for measurement of dynamic character-

istics monitoring and control of the experimentaldevice

(5) Elements of a signal system experimental involve-ment

(6) Manual control interfaceThe antagonistic involvement of pneumatic artificial musclesis the involvement in which the exerted force is used for

a rotary movement of the shaft which is loaded by a sup-porting rotary arm In the involvement two fluidic artificialmuscles FESTO MAS-40-1000N-AA-MC-K are used ThePAM inner diameter is 40mm a nominal length 1000mmoperating pressure 0ndash6 bar maximum extension 3 of itsnominal length and maximum hysteresis 25 of the nom-inal length View of the real experimental device is shown inFigure 4

Compressed air is used as a working medium A driveload system consists of the pivoting arm which is connectedto the drive shaft The drive shaft is driven via a chaintransmissionwhose chain is connected to each PAMThe sizeof the load can be controlled via a prismatic connection of thepivot arm and a weight (5 kg)The pneumatic system ensuresthe supply of a pressurized fluid by the compressor GudeAL210824Ewith the air tankThepressure control inside thepneumatic artificial muscles is realized through proportionalelectropneumatic valves PNMX821104224which ensure theinflation of a pressurized medium to each PAM as well asits draining Electropneumatic valves enable the control bya pulse-width modulation (PWM)

The pressure measuring of the operating medium insidethe PAM is provided by pressure sensors working on thebasis of a ceramic measuring cell An electric current loopin which they are connected works with the electric currentin the interval of 4ndash20mA The system for a dynamic char-acteristics measurement monitoring control and regulationof the experimental device involves computational units thatperform tasks of measurement monitoring control andregulation using PC userrsquos connections manual control unitand conducting of electrical variables The essence of controlactivities across the experimental involvement is based onthe control of valves opening and closing using the pulse-width modulation (PWM) The idea of using the PWMwas taken from [7 8 20] In order to ensure measuringmonitoring and controlling operations online a real-timecomputer unit of the CompactRIO NI cRIO 9024 class wasdesigned NI cRIO 9024 is connected to the bus NI cRIO

Advances in Materials Science and Engineering 5

PVZD

PK

V1L V2L

V1R V2R

Interface 24VTTL

AB

C

SP120572

120572

PAML PAMR

PL PR

M

PP II

R

NI 9401NI 9234 NI 9219 RS232 NI cRIO9024

Ethernet PC user interface

LM

Figure 3 Schematic representation of the experimental involvement PAML left pneumatic artificialmuscle PAMR right pneumatic artificialmuscle LM distance of a gravity center of an onerous element M from the pivoting arm shaft 119875R right pressure sensor 119875 pressure 119878Ppotentiometric position sensor 120572 rotation angle of the arm from the neutral position 119875L left pressure sensor 119875R right pressure sensor 119868electric current 119877 electrical resistance 119875K pressure at the compressor outlet V

1LV1R leftright inflation electropneumatic valve V2LV2R

leftright draining electropneumatic valve 119875VZD pressure in the air tank pressure and ABC acceleration sensors Orange colour signalsfrommeasuring devices red colour signals from vibration sensors green colour control signals to control electropneumatic valves and bluecolour transport routes for pressure medium

Figure 4 Experimental device with a drive based on the PAMs

9114The bus provides the functionality of its operations withthe accuracy of 100 ppm and enables a connection of eightmeasuring modules The measured values (pressure at theinput into each PAM arm rotation) are processed as signalsby a measuring card NI 9219 with four analog inputs Forelectropneumatic valves control a digital inputoutput cardNI 9041 was used A direct connection of NI 9041 to thevalves was not possible therefore an independent interfaceworking on the basis of TTL logic (HL 5V) was designed Incase that the 5V voltage is applied to the circuit it switchesthe power supply circuit with 24V for the electropneumaticvalves In order to run the service workplace programs apersonal computer is used The data collection related tothe frequency characteristics during the system operationand which referred to the movement of the rotating arm isprovided by the measuring card NI 9234 that processes theanalog output from the installed vibrometers

Table 1 Acceleration sensors parameters [19]

Characteristics Value UnitsReference sensitivity 20 msdotsminus2 RMSFrequency range 1ndash10000 HzResonant frequency 32 kHzTransversal sensitivity lt5 Range of measurement plusmn4000 msdotsminus2 peak

The elements of the signal system may be divided intoelements implemented directly into the experimental deviceand elements or measuring devices the use of which is notrequired to carry out measurement directly in real timethus they are used during the device operation sporadicallywith a low intensity for creating the indicative overview ofthe system or as a single application In the experimentalinvolvement sensors to measure the acceleration at the endof the supporting rotational arm are installed (Table 1)

For the sensing of the supporting arm rotation size apotentiometric resistive divider with linear characteristicswas used Its working range is 0ndash10 kΩ with a maximumrotation of 270 plusmn 5∘ To measure the ambient temperature athermal hygrometer TESTO 605-H1 was usedThe electricitysupply for the equipment of the experimental device whichhas special requirements of electrical current and voltageis provided by regulated laboratory power supplies DCPower SupplyHY3005The control system allows performingcontrol tasks only by means of signals for electropneumaticvalves opening and closing For the valves manual controla separate device (interface for manual control) was built

6 Advances in Materials Science and Engineering

Figure 5 Taskbar of the designed user interface

PWM0

INICPREM

DBLDBLDBL

DBLDBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBLDBLDBL

DBLDBLDBLDBL

TF

TF

TF

TF

TF

TF

TF

TF

TF

TF

AUTOM

MEASUR

PRESSURECONTROL

CONTROLPROG

SAVEDATA

PWM1

PWM2

PWM3

AI2

AI0

AI1

i

U8

U8

U8

U8

I32

I32

abc

I32

123

I32

123

123

DBL

ldquoPage 2ldquo

ldquoFalserdquo

0

0

10

0

00

Error

Error

uRIO07txt

Open or create

EXT

Errordt 100321 100

Measurement series

Required pressure in R

Required pressure in L

Required positionCluster 2

Start and end of the AM cycle

Cluster 4

Cluster 3

Exec series 2

Pressure requirement to start PWM

Exec series

Required position to start PWM

Tab Control

Measured pressure in L

Measured pressure in R

Pressure-Graph

Pressure tolerance (bar)

Max working pressure

Program mode

PWM L

PWM R

Max R

Max L

PWM U

Alarm

Basic alarm R

Basic alarm L

Saved data for current iteration

Optical monitoring of variables for error detection

stop

PWM R

PWM L

R-inflation

L-inflation

R-deflation

L-deflation

Elapsed time

1

3

2

4

56

7

minus1minus1

minus2

=

=

lt

1kHz(R)ms

Force 15mVV

∘)

Temperature (∘C)

Angle (∘)

Angle (∘)

X or Y 2

Max ∘ LR

Position tolerance (

Position control

Figure 6 Block diagram of the main algorithm

which is independent of the control system In case that thecontrol system is shutdown it is possible to a limited extentto use this device to control and regulate the operation of theexperimental device Another possible use is the positioningof the supporting arm during the maintenance of the device

33 Algorithms of the System Control To work with theexperimental assembly it was necessary to create a user-friendly user interface which allows a manual control of theactuator for testing and the definition of the operation duringthe automatic control of the test equipment The controlsystem is designed so that the operator can input instructionsfor the control system using the user interface of the programin an operational workplace (PC) The control system isrunning on a computing unit NI cRIO 9024 The taskbar ofthe control system userrsquos interface is shown in Figure 5

In Figure 6 the main structure of the ldquocontrol systemrdquo isshown highlighted are the individual parts that constitute the

main subprograms of the main algorithm designed for spe-cialized operations (1) downloading data (2) initializationof variables (3) automatic control (4) position regulation (5)pressure regulation (6) control program and (7) saving thedata

Pressure control is ensured in this subprogram by analgorithm that ensures opening and closing of the electrop-neumatic valves based on the comparison ofmomentary anddesired pressures in each PAM (Figure 7) that is in each exe-cuted iteration of the main program The blocks f1ndashf4 repre-sent the output control signal for PWMcontrol for each PAMinflation and deflation using the electropneumatic valves

Position control algorithm (Figure 8) is designed toperform regulatory operations leading to the opening andclosing of the electropneumatic valves which are basedon data entering the subroutine Its role in each iteration(repetition) of the control program is to evaluate the state of

Advances in Materials Science and Engineering 7

DBL

DBL

123

DBL

DBL

DBL

DBL

TF

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

TF

TF

T

F

T

F

TF

TF

T F

T F

TF

TF

T F T F

T F

T F

T F

T F T F

T F

T F

T F

T F

T F

T F

T F

T F

TF

+

+

+

+

divide

divide

divide

divide

0

0

0

0

0

0 0

0

0

0

0

100

100

100

100

0

100 125 10089

100

divide

Numeric 3

Numeric 4

Max working pressure

Pressure toleration (bar)

Required pressure in R

Required pressure in L

Pressure requirement to start PWM

Right PAM

Left PAM

Max working pressure out

Boolean

f4

f3

f2

f

Pressure alarm

PWM L

PWM R

times

times

gt lt

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

gt

gt

le

le

ge

ge

ge

ge

minus

minus

minus

minus

minus

minus

minus

minus

==

DBL3

2

4 5

DBL3

2

4 5

Figure 7 Block diagram of the subroutine for the pressure regulation

TF

TF

TF

TF T

F

T F

T F

T F

T F

TF

TF T F

TF

DBL

DBL

TF

TF

TF

TF

DBL

DBL

123

DBL3

2

4 5

TF

200

2

0

0

0

0

0

0

0

100

100

100

125 10089

92

Actual position Requirements to start the PWM

The required position

PWM U

The required position

AlarmAlarm L basic

Alarm R basic

Max L position

Max R position

Position control

T

T

F

F

minus1

minus1

+

+

+

+

minus

minus

minus

minus

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

le

ge

ge

=

divide

divide

divide

divide

times

times

times

Tolerance to the required position (∘)

DBL

123

fDBL

123

f2

I32Max∘ LR

Figure 8 Block diagram of the subroutine for the position control

the controlled system and to decide on its further operationA subroutine works as a proportional controller

4 Experimental Measurements

41 Experimental Measurement Number 1 The experimentalactuator was used to carry out experimental measurementsfor the purpose of obtaining information about the static and

dynamic characteristics of the device The principal aim ofthe measuring was to describe the limiting conditions of thedevice operationThe experimentalmeasurementwas carriedout in a room with a relatively stable temperature The initialmeasuring conditions and constant values are presented inTable 2

Based on the information about the possibilities of thePWM regulation presented in [9] the minimal regulated

8 Advances in Materials Science and Engineering

Table 2 Initial conditions and constant parameters for experimental measurements

Initial conditions Value Units AnnotationAmbient temperature 242 [∘C] mdashAir tank pressure Max 75 [bar] The pressure at the electropneumatic valves inletPressure sensors power supply 23 [V] Laboratory power suppliesControl valves power supply 24 [V] Laboratory power suppliesPressure in PAM 2 [bar] Initial operating pressurePressure tolerance 005 [bar] Setting in GUI of the control systemPosition angle of the support arm 0 [∘] mdashPosition tolerance plusmn1 [∘] Setting in GUI of the control system

PWM 20ndash100 [] Determined by the control program in each of its cycles(119891 = 50Hz)

Weight 4002590 Nmm∘ Arm without weights (force determined owing to theshaft axis with the supporting arm rotation to 90∘)

Table 3 The values of the operational parameters for the experimental measurement number 1

Operational characteristics Value UnitsPositioning (1 series of measurement) 0 15 30 45 60 75 80 0 minus15 minus30 minus45 minus60 minus75 minus80 [∘]Maximum pressure 51 [bar]The number of repetitions of a series 100 mdashPositioning step duration 6 [s]Positioning step duration in starting and ending position 16 [s]Mass of weight 65 [kg]

value of the PWM regulation is 20 The use of lowervalues would lead to excessive decreases in durability of theelectropneumatic valves

During the experimental operation the pressure values ineachPAM values of the PWMsignals and rotation and accel-eration of the supporting rotational armweremeasured Dur-ing the measuring the values listed in Table 3 were applied

The measuring was carried out under changing ambientconditions with a room temperature in the range of 242ndash264∘C within 100 series of positioning According to theorder given in Table 3 in each series of experimental mea-surements the automatic control systemwas applied to set upthe size of the rotation angle of the actuator supporting armto the desired position The measurements were performedwithout setting the limits of the device operation to the rec-ommended operatingminima ormaxima so that it was possi-ble to observe the behaviour of the antagonistic involvementof the pneumatic artificial muscles mechanism withoutsoftware restrictions of its operation

After completing the first experimental measurements areview measurement of the limit positions of the supportingarm was conducted which were resulting from the restric-tions of the operational maximum and minimum pressurein each PAM Based on the analysis of the measured valuesexperimental measurement number 3 was performed whoseprinciple consisted in the ambition to prove the differences inthe experimental actuator operation at the start of its opera-tion from the initial state (minimal rotation of the supporting

arm) and from the state of the supporting arm maximumrotation

The data expressing the current position pressure in eachPAM and the acceleration of the supporting arm obtainedbased on each series of measurements were presented graph-ically Between the last iteration of a series of measurementsand the first iteration of the following series of measure-ments when the system started to respond to the change inposition there was a delay in the range of 01ndash04 secondsFigure 9 shows graphs used for the preliminary analysis of thetransient characteristics of the experimental assembly duringthe operation To obtain a description of the mechanismbehaviour without limiting its operating conditions its posi-tioning was not restricted through setting up the minimumand maximum operating pressure during this measurementFor the reason of a better readability of the data the first graphshows the measured data obtained only from the ten consec-utive series of measurements

The processed measured data showed that the measuredcharacteristics recorded in each series of the experiment werenot significantly different In six cases of 100 repeats of themeasurement series there were recorded deviations from thestandard course of the measured dynamic characteristics Apresumed cause is a combination of two factors the operationin the limit positions of the end member of the actuator(75ndash84∘) and the lack of a pressure medium in the air tankIn order to eliminate these phenomena it was necessary toincrease the minimum pressure in the air tank Another

Advances in Materials Science and Engineering 9Po

sitio

n (∘

)

6030 402010 500Time (s)

5010 604020 300Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Posit

ion

(∘)

minus10000

000

10000

10 20 30 40 50 600Time (s)

10 20 40 5030 600Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

minus10000

000

10000

(mmiddotsminus

2)

(mmiddotsminus

2)

Figure 9The graphical representation of a current position and thesupporting arm acceleration

reason why these cases occurred was the mechanism opera-tion without limitation of the PAMminimum andmaximumoperating pressure

The second graph in Figure 9 shows the support armacceleration The acceleration indicates a drop in the sizeof hydraulic shocks with the increased rigidity of the PAMsmechanisms For a more detailed analysis the next graph(the angle of the arm rotation) shows only the data measuredduring the positioning of the supporting arm in the range0ndash84∘ Negative values of the acceleration represent themovement of the supporting arm in the positive directionIn case that the supporting arm exceeds the positioningaccuracy and the actual position is larger than the requiredposition the system responds with a feedback in the form ofa backward movement This fact corresponds with positivevalues of the acceleration

The rigidity of the system was low at low values of thesupporting arm angle In some cases the device failed to reacha stable state of the desired position during the time intervalof one step of the positioningThe lowest value at which it was

1200130014001500160017001800

Posit

ion

(∘)

0000200040006

757 767 777 787 797 807Iteration

minus0004minus0002

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

01020304050

PWM

regu

latio

n (

)

minus10 767 777 797 807757 787Iteration

767 777 787 797 807757Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 10 Analysis of graphs of the experimental measurementnumber 1

possible to stabilize the system during the measurement wasthe position with the angle of 15∘

Figure 10 shows graphs of the device dynamic char-acteristics measured while maintaining the position of thesupporting arm at 15∘ The data are shown in ten series of themeasuringThe first graph shows the course of the supportingarm positioning The principle aim was to maintain thesupporting arm in the position with the desired accuracyduring its positioningThe definition of the position accuracycaused the arm motion to stop immediately after reachingthe accuracy interval It resulted in reaching a real middleposition around 1465∘Therefore it was necessary to considerthe adjustments of the subprogram for the position controlSignificant fluctuations in the position are highlighted in thegraphThese fluctuations occur predominantly in the positivedirection (in the direction of the position increase)

The second graph shows the behaviour of the arm acceler-ationThe largest fluctuations in the arm position correspondto the largest fluctuations in the arm acceleration In theseries number 3 and number 6 the position and accelerationwaveforms are marked When compared with the last graphin Figure 10 there can be seen a direct relation betweenthe occurrence of hydraulic shocks and opening (closing)of electropneumatic valves or between the formation ofhydraulic shocks the size of the PWM regulation and timeof the PWM operation

Figure 11 shows the waveforms of the measured variablesin the fifth series of the experiment Based on a comparison of

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

Advances in Materials Science and Engineering 3

n encirclesof a fibre

120579120579

L

D n120587D

b

Figure 1 Functional principle of the artificial muscle braid [1]

inclination 120579 with constant quantities 119899 and 119887 indicating thecharacteristics of a particular cylinder [1 9]

119897 = 119887 sdot cos 120579 (6)

119863 = 119887 sdot sin 120579119899 sdot 120587 (7)

Subsequently it is possible to determine the volume of theideal cylinder as [1 9]

119881 = 14 sdot 120587 sdot 1198632 sdot 119897 = 119887

3

4120587 sdot 1198992 sdot sin2 120579 sdot cos 120579 (8)

Equation (5) after these modifications it is possible to derivethe resulting dependence of the muscle tensile force 119865 onthe relative pressure 1198751015840 and the inclination of fibers from thecylinder axis 120579 [1 9]

119865 = minus1198751015840 sdot 119889119881119889119897 = minus1198751015840 119889119881119889120579119889119897119889120579 =

1198751015840 sdot 1198872 (3 cos2 120579 minus 1)4120587 sdot 1198992 (9)

Substituting from the right side of (6) it is alternativelypossible to express the force as a function of 1198751015840 and 119897 [1 9]

119865 = 1198751015840 sdot (31198972 minus 1198872)4 sdot 120587 sdot 1198992 (10)

Of the previously described relations it follows that thedependence of a tensile force and pressure of the workingfluid inside the ideal cylinder (representing a simplified arti-ficial muscle) is liable to a direct proportion and a monotonefunction of fibers inclination from the cylinder axis [1 9]

From the foregoing and in accordance with [15] it ensuesthat the basic possible uses of artificial muscles are

(i) single-acting drive(ii) pneumatic spring

The PAM cannot be used as an air spring without a sufficientoperating pressure [15]

3 Materials and Methods

31 Preparation of Experimental Measurements The aim ofthe experiment realization was to verify the algorithm mod-ularization for controlling the position of the PAMs enabling

an effective way of the actuator position stabilization in var-ious dynamic conditions during the operation in relation tothe positioning accuracy In experiments during the changesin position there was monitored the influence of a staticload placed on the arm of the experimental assembly on thedynamics of the actuator arm stabilization process utilizingthe power based on the pneumatic artificial muscles in antag-onistic involvement There was also observed the effect ofthe proposed position control algorithm on the inaccuracieselimination The effort was to achieve a stable position

In order to measure the position of the arm the repeatedsequences were performed in which the characteristics toachieve the desired point and the actual dynamics of theprocess were followed that is the rate of acceleration in stabi-lizing and its impact on the process of stabilization Owing tothe nature of the system and its behaviour in a direct relationto the stiffness each of the defined characteristic points wasmonitored Increment of the change in a positionwas set froma zero starting position up to the maximum value Based onthe research experiments according to [9] it is not appropri-ate to use the PWM in the range below 20 considering theextreme decrease in durability of the electropneumatic valvesTherefore the range from 0 to 15 was indicated as unusedin the implementation of the experiment As the first value15∘ deflection change was determined while all followingvalues had increment of 15∘

The methodology of the procedure to achieve the objec-tive of the experimental measurements was guided by thefollowing scheme (Figure 2)

Within the elaboration of themethodology formeasuringthe transient characteristics of the PAM the initial require-ments of the functionality of the PAMs control system havebeen set The control system has been designed to allow amodification to its functionalities on the modular arrange-ment of algorithms The method respectively procedure foranalyzing the state of the system during each iteration wasdesigned The designed methodology is not based only onthe premise of the PAMs state online analysis It is also basedon the need of the analysis of the system after its operationusing the data taken during its operation The measuring ofthe system state consists of the PAMs measurements of vari-ables commonly measured on drives working based on thepneumatic artificial muscles such as the pressure in each ofthe PAM and the position of the support armThese variablesprovide information about the state of the system but do notfollow the processes taking place inside each PAM and insidethe transport routes of the working fluid Similarly they donot allow studying the effect of nonlinearity of the PAMssystem Therefore the question arose about how to followthe dependence of processes taking place inside the PAMssystem At the same time the requirement arose of the small-est possible interaction of sensing devices with the PAMssystem in terms of a minimal impact on the functionality andthe system load Therefore a measurement of a supportingarm acceleration was added Monitoring of this physicalquantity envisaged the possibility of a future description oftransitional processes inside the PAMsThere were taken intoaccount the conditions of the influence of pressure of theworking fluid and the time of the working fluid acting on the

4 Advances in Materials Science and Engineering

Measurement of individual quantities

Analysis and synthesis of results Evaluation

Determination of the desired position and the desired deviation

Activation of the algorithm

System setup options

Measuring the position of the actuator arm

Verification of function of the proposed algorithm for stabilization of the actuator arm

Achieving the desired position

Optimizing the position deviation

Measurement of individual quantities

Monitoring the accuracy of the achieved position

Figure 2 Schematic representation of the methodology of the measurement procedures

acceleration of the loaded system supporting element withthe known pressures in each PAM

For themeasurements therewere performed three exper-iments In the experiment number 1 the measurements werecarried out under conditions based on the known knowledgeThen on the basis of the data obtained in the experimentnumber 1 corrections were performed and verified in exper-iments numbers 2 and 3

32 Characteristics of the Experimental Device At theDepartment of Process Technique the Faculty of Manufac-turing Technologies with a Seat in Presov an experimentalassembly of an actuator powered by artificial muscles wasconstructed The experimental actuator assembly comprisesan antagonistic involvement of artificial muscles with agearing to a rotational movement of a rotary arm a controland regulation passage and a pneumatic system supplying aworking fluid into the actuator drive The assembly is com-pleted with the computer control system The control is pro-grammed in LabVIEW and its computing hardware base wasassembled from the components of the series CompactRIO-National Instruments Figure 3 gives a schematic representa-tion of the experimental device

The experimental actuator is divided into smaller parts(1) Antagonistic involvement of two pneumatic artificial

muscles (PAMs)(2) The system for a drive load(3) Pneumatic system(4) The system for measurement of dynamic character-

istics monitoring and control of the experimentaldevice

(5) Elements of a signal system experimental involve-ment

(6) Manual control interfaceThe antagonistic involvement of pneumatic artificial musclesis the involvement in which the exerted force is used for

a rotary movement of the shaft which is loaded by a sup-porting rotary arm In the involvement two fluidic artificialmuscles FESTO MAS-40-1000N-AA-MC-K are used ThePAM inner diameter is 40mm a nominal length 1000mmoperating pressure 0ndash6 bar maximum extension 3 of itsnominal length and maximum hysteresis 25 of the nom-inal length View of the real experimental device is shown inFigure 4

Compressed air is used as a working medium A driveload system consists of the pivoting arm which is connectedto the drive shaft The drive shaft is driven via a chaintransmissionwhose chain is connected to each PAMThe sizeof the load can be controlled via a prismatic connection of thepivot arm and a weight (5 kg)The pneumatic system ensuresthe supply of a pressurized fluid by the compressor GudeAL210824Ewith the air tankThepressure control inside thepneumatic artificial muscles is realized through proportionalelectropneumatic valves PNMX821104224which ensure theinflation of a pressurized medium to each PAM as well asits draining Electropneumatic valves enable the control bya pulse-width modulation (PWM)

The pressure measuring of the operating medium insidethe PAM is provided by pressure sensors working on thebasis of a ceramic measuring cell An electric current loopin which they are connected works with the electric currentin the interval of 4ndash20mA The system for a dynamic char-acteristics measurement monitoring control and regulationof the experimental device involves computational units thatperform tasks of measurement monitoring control andregulation using PC userrsquos connections manual control unitand conducting of electrical variables The essence of controlactivities across the experimental involvement is based onthe control of valves opening and closing using the pulse-width modulation (PWM) The idea of using the PWMwas taken from [7 8 20] In order to ensure measuringmonitoring and controlling operations online a real-timecomputer unit of the CompactRIO NI cRIO 9024 class wasdesigned NI cRIO 9024 is connected to the bus NI cRIO

Advances in Materials Science and Engineering 5

PVZD

PK

V1L V2L

V1R V2R

Interface 24VTTL

AB

C

SP120572

120572

PAML PAMR

PL PR

M

PP II

R

NI 9401NI 9234 NI 9219 RS232 NI cRIO9024

Ethernet PC user interface

LM

Figure 3 Schematic representation of the experimental involvement PAML left pneumatic artificialmuscle PAMR right pneumatic artificialmuscle LM distance of a gravity center of an onerous element M from the pivoting arm shaft 119875R right pressure sensor 119875 pressure 119878Ppotentiometric position sensor 120572 rotation angle of the arm from the neutral position 119875L left pressure sensor 119875R right pressure sensor 119868electric current 119877 electrical resistance 119875K pressure at the compressor outlet V

1LV1R leftright inflation electropneumatic valve V2LV2R

leftright draining electropneumatic valve 119875VZD pressure in the air tank pressure and ABC acceleration sensors Orange colour signalsfrommeasuring devices red colour signals from vibration sensors green colour control signals to control electropneumatic valves and bluecolour transport routes for pressure medium

Figure 4 Experimental device with a drive based on the PAMs

9114The bus provides the functionality of its operations withthe accuracy of 100 ppm and enables a connection of eightmeasuring modules The measured values (pressure at theinput into each PAM arm rotation) are processed as signalsby a measuring card NI 9219 with four analog inputs Forelectropneumatic valves control a digital inputoutput cardNI 9041 was used A direct connection of NI 9041 to thevalves was not possible therefore an independent interfaceworking on the basis of TTL logic (HL 5V) was designed Incase that the 5V voltage is applied to the circuit it switchesthe power supply circuit with 24V for the electropneumaticvalves In order to run the service workplace programs apersonal computer is used The data collection related tothe frequency characteristics during the system operationand which referred to the movement of the rotating arm isprovided by the measuring card NI 9234 that processes theanalog output from the installed vibrometers

Table 1 Acceleration sensors parameters [19]

Characteristics Value UnitsReference sensitivity 20 msdotsminus2 RMSFrequency range 1ndash10000 HzResonant frequency 32 kHzTransversal sensitivity lt5 Range of measurement plusmn4000 msdotsminus2 peak

The elements of the signal system may be divided intoelements implemented directly into the experimental deviceand elements or measuring devices the use of which is notrequired to carry out measurement directly in real timethus they are used during the device operation sporadicallywith a low intensity for creating the indicative overview ofthe system or as a single application In the experimentalinvolvement sensors to measure the acceleration at the endof the supporting rotational arm are installed (Table 1)

For the sensing of the supporting arm rotation size apotentiometric resistive divider with linear characteristicswas used Its working range is 0ndash10 kΩ with a maximumrotation of 270 plusmn 5∘ To measure the ambient temperature athermal hygrometer TESTO 605-H1 was usedThe electricitysupply for the equipment of the experimental device whichhas special requirements of electrical current and voltageis provided by regulated laboratory power supplies DCPower SupplyHY3005The control system allows performingcontrol tasks only by means of signals for electropneumaticvalves opening and closing For the valves manual controla separate device (interface for manual control) was built

6 Advances in Materials Science and Engineering

Figure 5 Taskbar of the designed user interface

PWM0

INICPREM

DBLDBLDBL

DBLDBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBLDBLDBL

DBLDBLDBLDBL

TF

TF

TF

TF

TF

TF

TF

TF

TF

TF

AUTOM

MEASUR

PRESSURECONTROL

CONTROLPROG

SAVEDATA

PWM1

PWM2

PWM3

AI2

AI0

AI1

i

U8

U8

U8

U8

I32

I32

abc

I32

123

I32

123

123

DBL

ldquoPage 2ldquo

ldquoFalserdquo

0

0

10

0

00

Error

Error

uRIO07txt

Open or create

EXT

Errordt 100321 100

Measurement series

Required pressure in R

Required pressure in L

Required positionCluster 2

Start and end of the AM cycle

Cluster 4

Cluster 3

Exec series 2

Pressure requirement to start PWM

Exec series

Required position to start PWM

Tab Control

Measured pressure in L

Measured pressure in R

Pressure-Graph

Pressure tolerance (bar)

Max working pressure

Program mode

PWM L

PWM R

Max R

Max L

PWM U

Alarm

Basic alarm R

Basic alarm L

Saved data for current iteration

Optical monitoring of variables for error detection

stop

PWM R

PWM L

R-inflation

L-inflation

R-deflation

L-deflation

Elapsed time

1

3

2

4

56

7

minus1minus1

minus2

=

=

lt

1kHz(R)ms

Force 15mVV

∘)

Temperature (∘C)

Angle (∘)

Angle (∘)

X or Y 2

Max ∘ LR

Position tolerance (

Position control

Figure 6 Block diagram of the main algorithm

which is independent of the control system In case that thecontrol system is shutdown it is possible to a limited extentto use this device to control and regulate the operation of theexperimental device Another possible use is the positioningof the supporting arm during the maintenance of the device

33 Algorithms of the System Control To work with theexperimental assembly it was necessary to create a user-friendly user interface which allows a manual control of theactuator for testing and the definition of the operation duringthe automatic control of the test equipment The controlsystem is designed so that the operator can input instructionsfor the control system using the user interface of the programin an operational workplace (PC) The control system isrunning on a computing unit NI cRIO 9024 The taskbar ofthe control system userrsquos interface is shown in Figure 5

In Figure 6 the main structure of the ldquocontrol systemrdquo isshown highlighted are the individual parts that constitute the

main subprograms of the main algorithm designed for spe-cialized operations (1) downloading data (2) initializationof variables (3) automatic control (4) position regulation (5)pressure regulation (6) control program and (7) saving thedata

Pressure control is ensured in this subprogram by analgorithm that ensures opening and closing of the electrop-neumatic valves based on the comparison ofmomentary anddesired pressures in each PAM (Figure 7) that is in each exe-cuted iteration of the main program The blocks f1ndashf4 repre-sent the output control signal for PWMcontrol for each PAMinflation and deflation using the electropneumatic valves

Position control algorithm (Figure 8) is designed toperform regulatory operations leading to the opening andclosing of the electropneumatic valves which are basedon data entering the subroutine Its role in each iteration(repetition) of the control program is to evaluate the state of

Advances in Materials Science and Engineering 7

DBL

DBL

123

DBL

DBL

DBL

DBL

TF

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

TF

TF

T

F

T

F

TF

TF

T F

T F

TF

TF

T F T F

T F

T F

T F

T F T F

T F

T F

T F

T F

T F

T F

T F

T F

TF

+

+

+

+

divide

divide

divide

divide

0

0

0

0

0

0 0

0

0

0

0

100

100

100

100

0

100 125 10089

100

divide

Numeric 3

Numeric 4

Max working pressure

Pressure toleration (bar)

Required pressure in R

Required pressure in L

Pressure requirement to start PWM

Right PAM

Left PAM

Max working pressure out

Boolean

f4

f3

f2

f

Pressure alarm

PWM L

PWM R

times

times

gt lt

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

gt

gt

le

le

ge

ge

ge

ge

minus

minus

minus

minus

minus

minus

minus

minus

==

DBL3

2

4 5

DBL3

2

4 5

Figure 7 Block diagram of the subroutine for the pressure regulation

TF

TF

TF

TF T

F

T F

T F

T F

T F

TF

TF T F

TF

DBL

DBL

TF

TF

TF

TF

DBL

DBL

123

DBL3

2

4 5

TF

200

2

0

0

0

0

0

0

0

100

100

100

125 10089

92

Actual position Requirements to start the PWM

The required position

PWM U

The required position

AlarmAlarm L basic

Alarm R basic

Max L position

Max R position

Position control

T

T

F

F

minus1

minus1

+

+

+

+

minus

minus

minus

minus

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

le

ge

ge

=

divide

divide

divide

divide

times

times

times

Tolerance to the required position (∘)

DBL

123

fDBL

123

f2

I32Max∘ LR

Figure 8 Block diagram of the subroutine for the position control

the controlled system and to decide on its further operationA subroutine works as a proportional controller

4 Experimental Measurements

41 Experimental Measurement Number 1 The experimentalactuator was used to carry out experimental measurementsfor the purpose of obtaining information about the static and

dynamic characteristics of the device The principal aim ofthe measuring was to describe the limiting conditions of thedevice operationThe experimentalmeasurementwas carriedout in a room with a relatively stable temperature The initialmeasuring conditions and constant values are presented inTable 2

Based on the information about the possibilities of thePWM regulation presented in [9] the minimal regulated

8 Advances in Materials Science and Engineering

Table 2 Initial conditions and constant parameters for experimental measurements

Initial conditions Value Units AnnotationAmbient temperature 242 [∘C] mdashAir tank pressure Max 75 [bar] The pressure at the electropneumatic valves inletPressure sensors power supply 23 [V] Laboratory power suppliesControl valves power supply 24 [V] Laboratory power suppliesPressure in PAM 2 [bar] Initial operating pressurePressure tolerance 005 [bar] Setting in GUI of the control systemPosition angle of the support arm 0 [∘] mdashPosition tolerance plusmn1 [∘] Setting in GUI of the control system

PWM 20ndash100 [] Determined by the control program in each of its cycles(119891 = 50Hz)

Weight 4002590 Nmm∘ Arm without weights (force determined owing to theshaft axis with the supporting arm rotation to 90∘)

Table 3 The values of the operational parameters for the experimental measurement number 1

Operational characteristics Value UnitsPositioning (1 series of measurement) 0 15 30 45 60 75 80 0 minus15 minus30 minus45 minus60 minus75 minus80 [∘]Maximum pressure 51 [bar]The number of repetitions of a series 100 mdashPositioning step duration 6 [s]Positioning step duration in starting and ending position 16 [s]Mass of weight 65 [kg]

value of the PWM regulation is 20 The use of lowervalues would lead to excessive decreases in durability of theelectropneumatic valves

During the experimental operation the pressure values ineachPAM values of the PWMsignals and rotation and accel-eration of the supporting rotational armweremeasured Dur-ing the measuring the values listed in Table 3 were applied

The measuring was carried out under changing ambientconditions with a room temperature in the range of 242ndash264∘C within 100 series of positioning According to theorder given in Table 3 in each series of experimental mea-surements the automatic control systemwas applied to set upthe size of the rotation angle of the actuator supporting armto the desired position The measurements were performedwithout setting the limits of the device operation to the rec-ommended operatingminima ormaxima so that it was possi-ble to observe the behaviour of the antagonistic involvementof the pneumatic artificial muscles mechanism withoutsoftware restrictions of its operation

After completing the first experimental measurements areview measurement of the limit positions of the supportingarm was conducted which were resulting from the restric-tions of the operational maximum and minimum pressurein each PAM Based on the analysis of the measured valuesexperimental measurement number 3 was performed whoseprinciple consisted in the ambition to prove the differences inthe experimental actuator operation at the start of its opera-tion from the initial state (minimal rotation of the supporting

arm) and from the state of the supporting arm maximumrotation

The data expressing the current position pressure in eachPAM and the acceleration of the supporting arm obtainedbased on each series of measurements were presented graph-ically Between the last iteration of a series of measurementsand the first iteration of the following series of measure-ments when the system started to respond to the change inposition there was a delay in the range of 01ndash04 secondsFigure 9 shows graphs used for the preliminary analysis of thetransient characteristics of the experimental assembly duringthe operation To obtain a description of the mechanismbehaviour without limiting its operating conditions its posi-tioning was not restricted through setting up the minimumand maximum operating pressure during this measurementFor the reason of a better readability of the data the first graphshows the measured data obtained only from the ten consec-utive series of measurements

The processed measured data showed that the measuredcharacteristics recorded in each series of the experiment werenot significantly different In six cases of 100 repeats of themeasurement series there were recorded deviations from thestandard course of the measured dynamic characteristics Apresumed cause is a combination of two factors the operationin the limit positions of the end member of the actuator(75ndash84∘) and the lack of a pressure medium in the air tankIn order to eliminate these phenomena it was necessary toincrease the minimum pressure in the air tank Another

Advances in Materials Science and Engineering 9Po

sitio

n (∘

)

6030 402010 500Time (s)

5010 604020 300Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Posit

ion

(∘)

minus10000

000

10000

10 20 30 40 50 600Time (s)

10 20 40 5030 600Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

minus10000

000

10000

(mmiddotsminus

2)

(mmiddotsminus

2)

Figure 9The graphical representation of a current position and thesupporting arm acceleration

reason why these cases occurred was the mechanism opera-tion without limitation of the PAMminimum andmaximumoperating pressure

The second graph in Figure 9 shows the support armacceleration The acceleration indicates a drop in the sizeof hydraulic shocks with the increased rigidity of the PAMsmechanisms For a more detailed analysis the next graph(the angle of the arm rotation) shows only the data measuredduring the positioning of the supporting arm in the range0ndash84∘ Negative values of the acceleration represent themovement of the supporting arm in the positive directionIn case that the supporting arm exceeds the positioningaccuracy and the actual position is larger than the requiredposition the system responds with a feedback in the form ofa backward movement This fact corresponds with positivevalues of the acceleration

The rigidity of the system was low at low values of thesupporting arm angle In some cases the device failed to reacha stable state of the desired position during the time intervalof one step of the positioningThe lowest value at which it was

1200130014001500160017001800

Posit

ion

(∘)

0000200040006

757 767 777 787 797 807Iteration

minus0004minus0002

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

01020304050

PWM

regu

latio

n (

)

minus10 767 777 797 807757 787Iteration

767 777 787 797 807757Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 10 Analysis of graphs of the experimental measurementnumber 1

possible to stabilize the system during the measurement wasthe position with the angle of 15∘

Figure 10 shows graphs of the device dynamic char-acteristics measured while maintaining the position of thesupporting arm at 15∘ The data are shown in ten series of themeasuringThe first graph shows the course of the supportingarm positioning The principle aim was to maintain thesupporting arm in the position with the desired accuracyduring its positioningThe definition of the position accuracycaused the arm motion to stop immediately after reachingthe accuracy interval It resulted in reaching a real middleposition around 1465∘Therefore it was necessary to considerthe adjustments of the subprogram for the position controlSignificant fluctuations in the position are highlighted in thegraphThese fluctuations occur predominantly in the positivedirection (in the direction of the position increase)

The second graph shows the behaviour of the arm acceler-ationThe largest fluctuations in the arm position correspondto the largest fluctuations in the arm acceleration In theseries number 3 and number 6 the position and accelerationwaveforms are marked When compared with the last graphin Figure 10 there can be seen a direct relation betweenthe occurrence of hydraulic shocks and opening (closing)of electropneumatic valves or between the formation ofhydraulic shocks the size of the PWM regulation and timeof the PWM operation

Figure 11 shows the waveforms of the measured variablesin the fifth series of the experiment Based on a comparison of

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

4 Advances in Materials Science and Engineering

Measurement of individual quantities

Analysis and synthesis of results Evaluation

Determination of the desired position and the desired deviation

Activation of the algorithm

System setup options

Measuring the position of the actuator arm

Verification of function of the proposed algorithm for stabilization of the actuator arm

Achieving the desired position

Optimizing the position deviation

Measurement of individual quantities

Monitoring the accuracy of the achieved position

Figure 2 Schematic representation of the methodology of the measurement procedures

acceleration of the loaded system supporting element withthe known pressures in each PAM

For themeasurements therewere performed three exper-iments In the experiment number 1 the measurements werecarried out under conditions based on the known knowledgeThen on the basis of the data obtained in the experimentnumber 1 corrections were performed and verified in exper-iments numbers 2 and 3

32 Characteristics of the Experimental Device At theDepartment of Process Technique the Faculty of Manufac-turing Technologies with a Seat in Presov an experimentalassembly of an actuator powered by artificial muscles wasconstructed The experimental actuator assembly comprisesan antagonistic involvement of artificial muscles with agearing to a rotational movement of a rotary arm a controland regulation passage and a pneumatic system supplying aworking fluid into the actuator drive The assembly is com-pleted with the computer control system The control is pro-grammed in LabVIEW and its computing hardware base wasassembled from the components of the series CompactRIO-National Instruments Figure 3 gives a schematic representa-tion of the experimental device

The experimental actuator is divided into smaller parts(1) Antagonistic involvement of two pneumatic artificial

muscles (PAMs)(2) The system for a drive load(3) Pneumatic system(4) The system for measurement of dynamic character-

istics monitoring and control of the experimentaldevice

(5) Elements of a signal system experimental involve-ment

(6) Manual control interfaceThe antagonistic involvement of pneumatic artificial musclesis the involvement in which the exerted force is used for

a rotary movement of the shaft which is loaded by a sup-porting rotary arm In the involvement two fluidic artificialmuscles FESTO MAS-40-1000N-AA-MC-K are used ThePAM inner diameter is 40mm a nominal length 1000mmoperating pressure 0ndash6 bar maximum extension 3 of itsnominal length and maximum hysteresis 25 of the nom-inal length View of the real experimental device is shown inFigure 4

Compressed air is used as a working medium A driveload system consists of the pivoting arm which is connectedto the drive shaft The drive shaft is driven via a chaintransmissionwhose chain is connected to each PAMThe sizeof the load can be controlled via a prismatic connection of thepivot arm and a weight (5 kg)The pneumatic system ensuresthe supply of a pressurized fluid by the compressor GudeAL210824Ewith the air tankThepressure control inside thepneumatic artificial muscles is realized through proportionalelectropneumatic valves PNMX821104224which ensure theinflation of a pressurized medium to each PAM as well asits draining Electropneumatic valves enable the control bya pulse-width modulation (PWM)

The pressure measuring of the operating medium insidethe PAM is provided by pressure sensors working on thebasis of a ceramic measuring cell An electric current loopin which they are connected works with the electric currentin the interval of 4ndash20mA The system for a dynamic char-acteristics measurement monitoring control and regulationof the experimental device involves computational units thatperform tasks of measurement monitoring control andregulation using PC userrsquos connections manual control unitand conducting of electrical variables The essence of controlactivities across the experimental involvement is based onthe control of valves opening and closing using the pulse-width modulation (PWM) The idea of using the PWMwas taken from [7 8 20] In order to ensure measuringmonitoring and controlling operations online a real-timecomputer unit of the CompactRIO NI cRIO 9024 class wasdesigned NI cRIO 9024 is connected to the bus NI cRIO

Advances in Materials Science and Engineering 5

PVZD

PK

V1L V2L

V1R V2R

Interface 24VTTL

AB

C

SP120572

120572

PAML PAMR

PL PR

M

PP II

R

NI 9401NI 9234 NI 9219 RS232 NI cRIO9024

Ethernet PC user interface

LM

Figure 3 Schematic representation of the experimental involvement PAML left pneumatic artificialmuscle PAMR right pneumatic artificialmuscle LM distance of a gravity center of an onerous element M from the pivoting arm shaft 119875R right pressure sensor 119875 pressure 119878Ppotentiometric position sensor 120572 rotation angle of the arm from the neutral position 119875L left pressure sensor 119875R right pressure sensor 119868electric current 119877 electrical resistance 119875K pressure at the compressor outlet V

1LV1R leftright inflation electropneumatic valve V2LV2R

leftright draining electropneumatic valve 119875VZD pressure in the air tank pressure and ABC acceleration sensors Orange colour signalsfrommeasuring devices red colour signals from vibration sensors green colour control signals to control electropneumatic valves and bluecolour transport routes for pressure medium

Figure 4 Experimental device with a drive based on the PAMs

9114The bus provides the functionality of its operations withthe accuracy of 100 ppm and enables a connection of eightmeasuring modules The measured values (pressure at theinput into each PAM arm rotation) are processed as signalsby a measuring card NI 9219 with four analog inputs Forelectropneumatic valves control a digital inputoutput cardNI 9041 was used A direct connection of NI 9041 to thevalves was not possible therefore an independent interfaceworking on the basis of TTL logic (HL 5V) was designed Incase that the 5V voltage is applied to the circuit it switchesthe power supply circuit with 24V for the electropneumaticvalves In order to run the service workplace programs apersonal computer is used The data collection related tothe frequency characteristics during the system operationand which referred to the movement of the rotating arm isprovided by the measuring card NI 9234 that processes theanalog output from the installed vibrometers

Table 1 Acceleration sensors parameters [19]

Characteristics Value UnitsReference sensitivity 20 msdotsminus2 RMSFrequency range 1ndash10000 HzResonant frequency 32 kHzTransversal sensitivity lt5 Range of measurement plusmn4000 msdotsminus2 peak

The elements of the signal system may be divided intoelements implemented directly into the experimental deviceand elements or measuring devices the use of which is notrequired to carry out measurement directly in real timethus they are used during the device operation sporadicallywith a low intensity for creating the indicative overview ofthe system or as a single application In the experimentalinvolvement sensors to measure the acceleration at the endof the supporting rotational arm are installed (Table 1)

For the sensing of the supporting arm rotation size apotentiometric resistive divider with linear characteristicswas used Its working range is 0ndash10 kΩ with a maximumrotation of 270 plusmn 5∘ To measure the ambient temperature athermal hygrometer TESTO 605-H1 was usedThe electricitysupply for the equipment of the experimental device whichhas special requirements of electrical current and voltageis provided by regulated laboratory power supplies DCPower SupplyHY3005The control system allows performingcontrol tasks only by means of signals for electropneumaticvalves opening and closing For the valves manual controla separate device (interface for manual control) was built

6 Advances in Materials Science and Engineering

Figure 5 Taskbar of the designed user interface

PWM0

INICPREM

DBLDBLDBL

DBLDBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBLDBLDBL

DBLDBLDBLDBL

TF

TF

TF

TF

TF

TF

TF

TF

TF

TF

AUTOM

MEASUR

PRESSURECONTROL

CONTROLPROG

SAVEDATA

PWM1

PWM2

PWM3

AI2

AI0

AI1

i

U8

U8

U8

U8

I32

I32

abc

I32

123

I32

123

123

DBL

ldquoPage 2ldquo

ldquoFalserdquo

0

0

10

0

00

Error

Error

uRIO07txt

Open or create

EXT

Errordt 100321 100

Measurement series

Required pressure in R

Required pressure in L

Required positionCluster 2

Start and end of the AM cycle

Cluster 4

Cluster 3

Exec series 2

Pressure requirement to start PWM

Exec series

Required position to start PWM

Tab Control

Measured pressure in L

Measured pressure in R

Pressure-Graph

Pressure tolerance (bar)

Max working pressure

Program mode

PWM L

PWM R

Max R

Max L

PWM U

Alarm

Basic alarm R

Basic alarm L

Saved data for current iteration

Optical monitoring of variables for error detection

stop

PWM R

PWM L

R-inflation

L-inflation

R-deflation

L-deflation

Elapsed time

1

3

2

4

56

7

minus1minus1

minus2

=

=

lt

1kHz(R)ms

Force 15mVV

∘)

Temperature (∘C)

Angle (∘)

Angle (∘)

X or Y 2

Max ∘ LR

Position tolerance (

Position control

Figure 6 Block diagram of the main algorithm

which is independent of the control system In case that thecontrol system is shutdown it is possible to a limited extentto use this device to control and regulate the operation of theexperimental device Another possible use is the positioningof the supporting arm during the maintenance of the device

33 Algorithms of the System Control To work with theexperimental assembly it was necessary to create a user-friendly user interface which allows a manual control of theactuator for testing and the definition of the operation duringthe automatic control of the test equipment The controlsystem is designed so that the operator can input instructionsfor the control system using the user interface of the programin an operational workplace (PC) The control system isrunning on a computing unit NI cRIO 9024 The taskbar ofthe control system userrsquos interface is shown in Figure 5

In Figure 6 the main structure of the ldquocontrol systemrdquo isshown highlighted are the individual parts that constitute the

main subprograms of the main algorithm designed for spe-cialized operations (1) downloading data (2) initializationof variables (3) automatic control (4) position regulation (5)pressure regulation (6) control program and (7) saving thedata

Pressure control is ensured in this subprogram by analgorithm that ensures opening and closing of the electrop-neumatic valves based on the comparison ofmomentary anddesired pressures in each PAM (Figure 7) that is in each exe-cuted iteration of the main program The blocks f1ndashf4 repre-sent the output control signal for PWMcontrol for each PAMinflation and deflation using the electropneumatic valves

Position control algorithm (Figure 8) is designed toperform regulatory operations leading to the opening andclosing of the electropneumatic valves which are basedon data entering the subroutine Its role in each iteration(repetition) of the control program is to evaluate the state of

Advances in Materials Science and Engineering 7

DBL

DBL

123

DBL

DBL

DBL

DBL

TF

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

TF

TF

T

F

T

F

TF

TF

T F

T F

TF

TF

T F T F

T F

T F

T F

T F T F

T F

T F

T F

T F

T F

T F

T F

T F

TF

+

+

+

+

divide

divide

divide

divide

0

0

0

0

0

0 0

0

0

0

0

100

100

100

100

0

100 125 10089

100

divide

Numeric 3

Numeric 4

Max working pressure

Pressure toleration (bar)

Required pressure in R

Required pressure in L

Pressure requirement to start PWM

Right PAM

Left PAM

Max working pressure out

Boolean

f4

f3

f2

f

Pressure alarm

PWM L

PWM R

times

times

gt lt

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

gt

gt

le

le

ge

ge

ge

ge

minus

minus

minus

minus

minus

minus

minus

minus

==

DBL3

2

4 5

DBL3

2

4 5

Figure 7 Block diagram of the subroutine for the pressure regulation

TF

TF

TF

TF T

F

T F

T F

T F

T F

TF

TF T F

TF

DBL

DBL

TF

TF

TF

TF

DBL

DBL

123

DBL3

2

4 5

TF

200

2

0

0

0

0

0

0

0

100

100

100

125 10089

92

Actual position Requirements to start the PWM

The required position

PWM U

The required position

AlarmAlarm L basic

Alarm R basic

Max L position

Max R position

Position control

T

T

F

F

minus1

minus1

+

+

+

+

minus

minus

minus

minus

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

le

ge

ge

=

divide

divide

divide

divide

times

times

times

Tolerance to the required position (∘)

DBL

123

fDBL

123

f2

I32Max∘ LR

Figure 8 Block diagram of the subroutine for the position control

the controlled system and to decide on its further operationA subroutine works as a proportional controller

4 Experimental Measurements

41 Experimental Measurement Number 1 The experimentalactuator was used to carry out experimental measurementsfor the purpose of obtaining information about the static and

dynamic characteristics of the device The principal aim ofthe measuring was to describe the limiting conditions of thedevice operationThe experimentalmeasurementwas carriedout in a room with a relatively stable temperature The initialmeasuring conditions and constant values are presented inTable 2

Based on the information about the possibilities of thePWM regulation presented in [9] the minimal regulated

8 Advances in Materials Science and Engineering

Table 2 Initial conditions and constant parameters for experimental measurements

Initial conditions Value Units AnnotationAmbient temperature 242 [∘C] mdashAir tank pressure Max 75 [bar] The pressure at the electropneumatic valves inletPressure sensors power supply 23 [V] Laboratory power suppliesControl valves power supply 24 [V] Laboratory power suppliesPressure in PAM 2 [bar] Initial operating pressurePressure tolerance 005 [bar] Setting in GUI of the control systemPosition angle of the support arm 0 [∘] mdashPosition tolerance plusmn1 [∘] Setting in GUI of the control system

PWM 20ndash100 [] Determined by the control program in each of its cycles(119891 = 50Hz)

Weight 4002590 Nmm∘ Arm without weights (force determined owing to theshaft axis with the supporting arm rotation to 90∘)

Table 3 The values of the operational parameters for the experimental measurement number 1

Operational characteristics Value UnitsPositioning (1 series of measurement) 0 15 30 45 60 75 80 0 minus15 minus30 minus45 minus60 minus75 minus80 [∘]Maximum pressure 51 [bar]The number of repetitions of a series 100 mdashPositioning step duration 6 [s]Positioning step duration in starting and ending position 16 [s]Mass of weight 65 [kg]

value of the PWM regulation is 20 The use of lowervalues would lead to excessive decreases in durability of theelectropneumatic valves

During the experimental operation the pressure values ineachPAM values of the PWMsignals and rotation and accel-eration of the supporting rotational armweremeasured Dur-ing the measuring the values listed in Table 3 were applied

The measuring was carried out under changing ambientconditions with a room temperature in the range of 242ndash264∘C within 100 series of positioning According to theorder given in Table 3 in each series of experimental mea-surements the automatic control systemwas applied to set upthe size of the rotation angle of the actuator supporting armto the desired position The measurements were performedwithout setting the limits of the device operation to the rec-ommended operatingminima ormaxima so that it was possi-ble to observe the behaviour of the antagonistic involvementof the pneumatic artificial muscles mechanism withoutsoftware restrictions of its operation

After completing the first experimental measurements areview measurement of the limit positions of the supportingarm was conducted which were resulting from the restric-tions of the operational maximum and minimum pressurein each PAM Based on the analysis of the measured valuesexperimental measurement number 3 was performed whoseprinciple consisted in the ambition to prove the differences inthe experimental actuator operation at the start of its opera-tion from the initial state (minimal rotation of the supporting

arm) and from the state of the supporting arm maximumrotation

The data expressing the current position pressure in eachPAM and the acceleration of the supporting arm obtainedbased on each series of measurements were presented graph-ically Between the last iteration of a series of measurementsand the first iteration of the following series of measure-ments when the system started to respond to the change inposition there was a delay in the range of 01ndash04 secondsFigure 9 shows graphs used for the preliminary analysis of thetransient characteristics of the experimental assembly duringthe operation To obtain a description of the mechanismbehaviour without limiting its operating conditions its posi-tioning was not restricted through setting up the minimumand maximum operating pressure during this measurementFor the reason of a better readability of the data the first graphshows the measured data obtained only from the ten consec-utive series of measurements

The processed measured data showed that the measuredcharacteristics recorded in each series of the experiment werenot significantly different In six cases of 100 repeats of themeasurement series there were recorded deviations from thestandard course of the measured dynamic characteristics Apresumed cause is a combination of two factors the operationin the limit positions of the end member of the actuator(75ndash84∘) and the lack of a pressure medium in the air tankIn order to eliminate these phenomena it was necessary toincrease the minimum pressure in the air tank Another

Advances in Materials Science and Engineering 9Po

sitio

n (∘

)

6030 402010 500Time (s)

5010 604020 300Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Posit

ion

(∘)

minus10000

000

10000

10 20 30 40 50 600Time (s)

10 20 40 5030 600Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

minus10000

000

10000

(mmiddotsminus

2)

(mmiddotsminus

2)

Figure 9The graphical representation of a current position and thesupporting arm acceleration

reason why these cases occurred was the mechanism opera-tion without limitation of the PAMminimum andmaximumoperating pressure

The second graph in Figure 9 shows the support armacceleration The acceleration indicates a drop in the sizeof hydraulic shocks with the increased rigidity of the PAMsmechanisms For a more detailed analysis the next graph(the angle of the arm rotation) shows only the data measuredduring the positioning of the supporting arm in the range0ndash84∘ Negative values of the acceleration represent themovement of the supporting arm in the positive directionIn case that the supporting arm exceeds the positioningaccuracy and the actual position is larger than the requiredposition the system responds with a feedback in the form ofa backward movement This fact corresponds with positivevalues of the acceleration

The rigidity of the system was low at low values of thesupporting arm angle In some cases the device failed to reacha stable state of the desired position during the time intervalof one step of the positioningThe lowest value at which it was

1200130014001500160017001800

Posit

ion

(∘)

0000200040006

757 767 777 787 797 807Iteration

minus0004minus0002

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

01020304050

PWM

regu

latio

n (

)

minus10 767 777 797 807757 787Iteration

767 777 787 797 807757Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 10 Analysis of graphs of the experimental measurementnumber 1

possible to stabilize the system during the measurement wasthe position with the angle of 15∘

Figure 10 shows graphs of the device dynamic char-acteristics measured while maintaining the position of thesupporting arm at 15∘ The data are shown in ten series of themeasuringThe first graph shows the course of the supportingarm positioning The principle aim was to maintain thesupporting arm in the position with the desired accuracyduring its positioningThe definition of the position accuracycaused the arm motion to stop immediately after reachingthe accuracy interval It resulted in reaching a real middleposition around 1465∘Therefore it was necessary to considerthe adjustments of the subprogram for the position controlSignificant fluctuations in the position are highlighted in thegraphThese fluctuations occur predominantly in the positivedirection (in the direction of the position increase)

The second graph shows the behaviour of the arm acceler-ationThe largest fluctuations in the arm position correspondto the largest fluctuations in the arm acceleration In theseries number 3 and number 6 the position and accelerationwaveforms are marked When compared with the last graphin Figure 10 there can be seen a direct relation betweenthe occurrence of hydraulic shocks and opening (closing)of electropneumatic valves or between the formation ofhydraulic shocks the size of the PWM regulation and timeof the PWM operation

Figure 11 shows the waveforms of the measured variablesin the fifth series of the experiment Based on a comparison of

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

Advances in Materials Science and Engineering 5

PVZD

PK

V1L V2L

V1R V2R

Interface 24VTTL

AB

C

SP120572

120572

PAML PAMR

PL PR

M

PP II

R

NI 9401NI 9234 NI 9219 RS232 NI cRIO9024

Ethernet PC user interface

LM

Figure 3 Schematic representation of the experimental involvement PAML left pneumatic artificialmuscle PAMR right pneumatic artificialmuscle LM distance of a gravity center of an onerous element M from the pivoting arm shaft 119875R right pressure sensor 119875 pressure 119878Ppotentiometric position sensor 120572 rotation angle of the arm from the neutral position 119875L left pressure sensor 119875R right pressure sensor 119868electric current 119877 electrical resistance 119875K pressure at the compressor outlet V

1LV1R leftright inflation electropneumatic valve V2LV2R

leftright draining electropneumatic valve 119875VZD pressure in the air tank pressure and ABC acceleration sensors Orange colour signalsfrommeasuring devices red colour signals from vibration sensors green colour control signals to control electropneumatic valves and bluecolour transport routes for pressure medium

Figure 4 Experimental device with a drive based on the PAMs

9114The bus provides the functionality of its operations withthe accuracy of 100 ppm and enables a connection of eightmeasuring modules The measured values (pressure at theinput into each PAM arm rotation) are processed as signalsby a measuring card NI 9219 with four analog inputs Forelectropneumatic valves control a digital inputoutput cardNI 9041 was used A direct connection of NI 9041 to thevalves was not possible therefore an independent interfaceworking on the basis of TTL logic (HL 5V) was designed Incase that the 5V voltage is applied to the circuit it switchesthe power supply circuit with 24V for the electropneumaticvalves In order to run the service workplace programs apersonal computer is used The data collection related tothe frequency characteristics during the system operationand which referred to the movement of the rotating arm isprovided by the measuring card NI 9234 that processes theanalog output from the installed vibrometers

Table 1 Acceleration sensors parameters [19]

Characteristics Value UnitsReference sensitivity 20 msdotsminus2 RMSFrequency range 1ndash10000 HzResonant frequency 32 kHzTransversal sensitivity lt5 Range of measurement plusmn4000 msdotsminus2 peak

The elements of the signal system may be divided intoelements implemented directly into the experimental deviceand elements or measuring devices the use of which is notrequired to carry out measurement directly in real timethus they are used during the device operation sporadicallywith a low intensity for creating the indicative overview ofthe system or as a single application In the experimentalinvolvement sensors to measure the acceleration at the endof the supporting rotational arm are installed (Table 1)

For the sensing of the supporting arm rotation size apotentiometric resistive divider with linear characteristicswas used Its working range is 0ndash10 kΩ with a maximumrotation of 270 plusmn 5∘ To measure the ambient temperature athermal hygrometer TESTO 605-H1 was usedThe electricitysupply for the equipment of the experimental device whichhas special requirements of electrical current and voltageis provided by regulated laboratory power supplies DCPower SupplyHY3005The control system allows performingcontrol tasks only by means of signals for electropneumaticvalves opening and closing For the valves manual controla separate device (interface for manual control) was built

6 Advances in Materials Science and Engineering

Figure 5 Taskbar of the designed user interface

PWM0

INICPREM

DBLDBLDBL

DBLDBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBLDBLDBL

DBLDBLDBLDBL

TF

TF

TF

TF

TF

TF

TF

TF

TF

TF

AUTOM

MEASUR

PRESSURECONTROL

CONTROLPROG

SAVEDATA

PWM1

PWM2

PWM3

AI2

AI0

AI1

i

U8

U8

U8

U8

I32

I32

abc

I32

123

I32

123

123

DBL

ldquoPage 2ldquo

ldquoFalserdquo

0

0

10

0

00

Error

Error

uRIO07txt

Open or create

EXT

Errordt 100321 100

Measurement series

Required pressure in R

Required pressure in L

Required positionCluster 2

Start and end of the AM cycle

Cluster 4

Cluster 3

Exec series 2

Pressure requirement to start PWM

Exec series

Required position to start PWM

Tab Control

Measured pressure in L

Measured pressure in R

Pressure-Graph

Pressure tolerance (bar)

Max working pressure

Program mode

PWM L

PWM R

Max R

Max L

PWM U

Alarm

Basic alarm R

Basic alarm L

Saved data for current iteration

Optical monitoring of variables for error detection

stop

PWM R

PWM L

R-inflation

L-inflation

R-deflation

L-deflation

Elapsed time

1

3

2

4

56

7

minus1minus1

minus2

=

=

lt

1kHz(R)ms

Force 15mVV

∘)

Temperature (∘C)

Angle (∘)

Angle (∘)

X or Y 2

Max ∘ LR

Position tolerance (

Position control

Figure 6 Block diagram of the main algorithm

which is independent of the control system In case that thecontrol system is shutdown it is possible to a limited extentto use this device to control and regulate the operation of theexperimental device Another possible use is the positioningof the supporting arm during the maintenance of the device

33 Algorithms of the System Control To work with theexperimental assembly it was necessary to create a user-friendly user interface which allows a manual control of theactuator for testing and the definition of the operation duringthe automatic control of the test equipment The controlsystem is designed so that the operator can input instructionsfor the control system using the user interface of the programin an operational workplace (PC) The control system isrunning on a computing unit NI cRIO 9024 The taskbar ofthe control system userrsquos interface is shown in Figure 5

In Figure 6 the main structure of the ldquocontrol systemrdquo isshown highlighted are the individual parts that constitute the

main subprograms of the main algorithm designed for spe-cialized operations (1) downloading data (2) initializationof variables (3) automatic control (4) position regulation (5)pressure regulation (6) control program and (7) saving thedata

Pressure control is ensured in this subprogram by analgorithm that ensures opening and closing of the electrop-neumatic valves based on the comparison ofmomentary anddesired pressures in each PAM (Figure 7) that is in each exe-cuted iteration of the main program The blocks f1ndashf4 repre-sent the output control signal for PWMcontrol for each PAMinflation and deflation using the electropneumatic valves

Position control algorithm (Figure 8) is designed toperform regulatory operations leading to the opening andclosing of the electropneumatic valves which are basedon data entering the subroutine Its role in each iteration(repetition) of the control program is to evaluate the state of

Advances in Materials Science and Engineering 7

DBL

DBL

123

DBL

DBL

DBL

DBL

TF

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

TF

TF

T

F

T

F

TF

TF

T F

T F

TF

TF

T F T F

T F

T F

T F

T F T F

T F

T F

T F

T F

T F

T F

T F

T F

TF

+

+

+

+

divide

divide

divide

divide

0

0

0

0

0

0 0

0

0

0

0

100

100

100

100

0

100 125 10089

100

divide

Numeric 3

Numeric 4

Max working pressure

Pressure toleration (bar)

Required pressure in R

Required pressure in L

Pressure requirement to start PWM

Right PAM

Left PAM

Max working pressure out

Boolean

f4

f3

f2

f

Pressure alarm

PWM L

PWM R

times

times

gt lt

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

gt

gt

le

le

ge

ge

ge

ge

minus

minus

minus

minus

minus

minus

minus

minus

==

DBL3

2

4 5

DBL3

2

4 5

Figure 7 Block diagram of the subroutine for the pressure regulation

TF

TF

TF

TF T

F

T F

T F

T F

T F

TF

TF T F

TF

DBL

DBL

TF

TF

TF

TF

DBL

DBL

123

DBL3

2

4 5

TF

200

2

0

0

0

0

0

0

0

100

100

100

125 10089

92

Actual position Requirements to start the PWM

The required position

PWM U

The required position

AlarmAlarm L basic

Alarm R basic

Max L position

Max R position

Position control

T

T

F

F

minus1

minus1

+

+

+

+

minus

minus

minus

minus

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

le

ge

ge

=

divide

divide

divide

divide

times

times

times

Tolerance to the required position (∘)

DBL

123

fDBL

123

f2

I32Max∘ LR

Figure 8 Block diagram of the subroutine for the position control

the controlled system and to decide on its further operationA subroutine works as a proportional controller

4 Experimental Measurements

41 Experimental Measurement Number 1 The experimentalactuator was used to carry out experimental measurementsfor the purpose of obtaining information about the static and

dynamic characteristics of the device The principal aim ofthe measuring was to describe the limiting conditions of thedevice operationThe experimentalmeasurementwas carriedout in a room with a relatively stable temperature The initialmeasuring conditions and constant values are presented inTable 2

Based on the information about the possibilities of thePWM regulation presented in [9] the minimal regulated

8 Advances in Materials Science and Engineering

Table 2 Initial conditions and constant parameters for experimental measurements

Initial conditions Value Units AnnotationAmbient temperature 242 [∘C] mdashAir tank pressure Max 75 [bar] The pressure at the electropneumatic valves inletPressure sensors power supply 23 [V] Laboratory power suppliesControl valves power supply 24 [V] Laboratory power suppliesPressure in PAM 2 [bar] Initial operating pressurePressure tolerance 005 [bar] Setting in GUI of the control systemPosition angle of the support arm 0 [∘] mdashPosition tolerance plusmn1 [∘] Setting in GUI of the control system

PWM 20ndash100 [] Determined by the control program in each of its cycles(119891 = 50Hz)

Weight 4002590 Nmm∘ Arm without weights (force determined owing to theshaft axis with the supporting arm rotation to 90∘)

Table 3 The values of the operational parameters for the experimental measurement number 1

Operational characteristics Value UnitsPositioning (1 series of measurement) 0 15 30 45 60 75 80 0 minus15 minus30 minus45 minus60 minus75 minus80 [∘]Maximum pressure 51 [bar]The number of repetitions of a series 100 mdashPositioning step duration 6 [s]Positioning step duration in starting and ending position 16 [s]Mass of weight 65 [kg]

value of the PWM regulation is 20 The use of lowervalues would lead to excessive decreases in durability of theelectropneumatic valves

During the experimental operation the pressure values ineachPAM values of the PWMsignals and rotation and accel-eration of the supporting rotational armweremeasured Dur-ing the measuring the values listed in Table 3 were applied

The measuring was carried out under changing ambientconditions with a room temperature in the range of 242ndash264∘C within 100 series of positioning According to theorder given in Table 3 in each series of experimental mea-surements the automatic control systemwas applied to set upthe size of the rotation angle of the actuator supporting armto the desired position The measurements were performedwithout setting the limits of the device operation to the rec-ommended operatingminima ormaxima so that it was possi-ble to observe the behaviour of the antagonistic involvementof the pneumatic artificial muscles mechanism withoutsoftware restrictions of its operation

After completing the first experimental measurements areview measurement of the limit positions of the supportingarm was conducted which were resulting from the restric-tions of the operational maximum and minimum pressurein each PAM Based on the analysis of the measured valuesexperimental measurement number 3 was performed whoseprinciple consisted in the ambition to prove the differences inthe experimental actuator operation at the start of its opera-tion from the initial state (minimal rotation of the supporting

arm) and from the state of the supporting arm maximumrotation

The data expressing the current position pressure in eachPAM and the acceleration of the supporting arm obtainedbased on each series of measurements were presented graph-ically Between the last iteration of a series of measurementsand the first iteration of the following series of measure-ments when the system started to respond to the change inposition there was a delay in the range of 01ndash04 secondsFigure 9 shows graphs used for the preliminary analysis of thetransient characteristics of the experimental assembly duringthe operation To obtain a description of the mechanismbehaviour without limiting its operating conditions its posi-tioning was not restricted through setting up the minimumand maximum operating pressure during this measurementFor the reason of a better readability of the data the first graphshows the measured data obtained only from the ten consec-utive series of measurements

The processed measured data showed that the measuredcharacteristics recorded in each series of the experiment werenot significantly different In six cases of 100 repeats of themeasurement series there were recorded deviations from thestandard course of the measured dynamic characteristics Apresumed cause is a combination of two factors the operationin the limit positions of the end member of the actuator(75ndash84∘) and the lack of a pressure medium in the air tankIn order to eliminate these phenomena it was necessary toincrease the minimum pressure in the air tank Another

Advances in Materials Science and Engineering 9Po

sitio

n (∘

)

6030 402010 500Time (s)

5010 604020 300Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Posit

ion

(∘)

minus10000

000

10000

10 20 30 40 50 600Time (s)

10 20 40 5030 600Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

minus10000

000

10000

(mmiddotsminus

2)

(mmiddotsminus

2)

Figure 9The graphical representation of a current position and thesupporting arm acceleration

reason why these cases occurred was the mechanism opera-tion without limitation of the PAMminimum andmaximumoperating pressure

The second graph in Figure 9 shows the support armacceleration The acceleration indicates a drop in the sizeof hydraulic shocks with the increased rigidity of the PAMsmechanisms For a more detailed analysis the next graph(the angle of the arm rotation) shows only the data measuredduring the positioning of the supporting arm in the range0ndash84∘ Negative values of the acceleration represent themovement of the supporting arm in the positive directionIn case that the supporting arm exceeds the positioningaccuracy and the actual position is larger than the requiredposition the system responds with a feedback in the form ofa backward movement This fact corresponds with positivevalues of the acceleration

The rigidity of the system was low at low values of thesupporting arm angle In some cases the device failed to reacha stable state of the desired position during the time intervalof one step of the positioningThe lowest value at which it was

1200130014001500160017001800

Posit

ion

(∘)

0000200040006

757 767 777 787 797 807Iteration

minus0004minus0002

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

01020304050

PWM

regu

latio

n (

)

minus10 767 777 797 807757 787Iteration

767 777 787 797 807757Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 10 Analysis of graphs of the experimental measurementnumber 1

possible to stabilize the system during the measurement wasthe position with the angle of 15∘

Figure 10 shows graphs of the device dynamic char-acteristics measured while maintaining the position of thesupporting arm at 15∘ The data are shown in ten series of themeasuringThe first graph shows the course of the supportingarm positioning The principle aim was to maintain thesupporting arm in the position with the desired accuracyduring its positioningThe definition of the position accuracycaused the arm motion to stop immediately after reachingthe accuracy interval It resulted in reaching a real middleposition around 1465∘Therefore it was necessary to considerthe adjustments of the subprogram for the position controlSignificant fluctuations in the position are highlighted in thegraphThese fluctuations occur predominantly in the positivedirection (in the direction of the position increase)

The second graph shows the behaviour of the arm acceler-ationThe largest fluctuations in the arm position correspondto the largest fluctuations in the arm acceleration In theseries number 3 and number 6 the position and accelerationwaveforms are marked When compared with the last graphin Figure 10 there can be seen a direct relation betweenthe occurrence of hydraulic shocks and opening (closing)of electropneumatic valves or between the formation ofhydraulic shocks the size of the PWM regulation and timeof the PWM operation

Figure 11 shows the waveforms of the measured variablesin the fifth series of the experiment Based on a comparison of

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

6 Advances in Materials Science and Engineering

Figure 5 Taskbar of the designed user interface

PWM0

INICPREM

DBLDBLDBL

DBLDBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBL

DBLDBLDBL

DBLDBLDBLDBL

TF

TF

TF

TF

TF

TF

TF

TF

TF

TF

AUTOM

MEASUR

PRESSURECONTROL

CONTROLPROG

SAVEDATA

PWM1

PWM2

PWM3

AI2

AI0

AI1

i

U8

U8

U8

U8

I32

I32

abc

I32

123

I32

123

123

DBL

ldquoPage 2ldquo

ldquoFalserdquo

0

0

10

0

00

Error

Error

uRIO07txt

Open or create

EXT

Errordt 100321 100

Measurement series

Required pressure in R

Required pressure in L

Required positionCluster 2

Start and end of the AM cycle

Cluster 4

Cluster 3

Exec series 2

Pressure requirement to start PWM

Exec series

Required position to start PWM

Tab Control

Measured pressure in L

Measured pressure in R

Pressure-Graph

Pressure tolerance (bar)

Max working pressure

Program mode

PWM L

PWM R

Max R

Max L

PWM U

Alarm

Basic alarm R

Basic alarm L

Saved data for current iteration

Optical monitoring of variables for error detection

stop

PWM R

PWM L

R-inflation

L-inflation

R-deflation

L-deflation

Elapsed time

1

3

2

4

56

7

minus1minus1

minus2

=

=

lt

1kHz(R)ms

Force 15mVV

∘)

Temperature (∘C)

Angle (∘)

Angle (∘)

X or Y 2

Max ∘ LR

Position tolerance (

Position control

Figure 6 Block diagram of the main algorithm

which is independent of the control system In case that thecontrol system is shutdown it is possible to a limited extentto use this device to control and regulate the operation of theexperimental device Another possible use is the positioningof the supporting arm during the maintenance of the device

33 Algorithms of the System Control To work with theexperimental assembly it was necessary to create a user-friendly user interface which allows a manual control of theactuator for testing and the definition of the operation duringthe automatic control of the test equipment The controlsystem is designed so that the operator can input instructionsfor the control system using the user interface of the programin an operational workplace (PC) The control system isrunning on a computing unit NI cRIO 9024 The taskbar ofthe control system userrsquos interface is shown in Figure 5

In Figure 6 the main structure of the ldquocontrol systemrdquo isshown highlighted are the individual parts that constitute the

main subprograms of the main algorithm designed for spe-cialized operations (1) downloading data (2) initializationof variables (3) automatic control (4) position regulation (5)pressure regulation (6) control program and (7) saving thedata

Pressure control is ensured in this subprogram by analgorithm that ensures opening and closing of the electrop-neumatic valves based on the comparison ofmomentary anddesired pressures in each PAM (Figure 7) that is in each exe-cuted iteration of the main program The blocks f1ndashf4 repre-sent the output control signal for PWMcontrol for each PAMinflation and deflation using the electropneumatic valves

Position control algorithm (Figure 8) is designed toperform regulatory operations leading to the opening andclosing of the electropneumatic valves which are basedon data entering the subroutine Its role in each iteration(repetition) of the control program is to evaluate the state of

Advances in Materials Science and Engineering 7

DBL

DBL

123

DBL

DBL

DBL

DBL

TF

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

TF

TF

T

F

T

F

TF

TF

T F

T F

TF

TF

T F T F

T F

T F

T F

T F T F

T F

T F

T F

T F

T F

T F

T F

T F

TF

+

+

+

+

divide

divide

divide

divide

0

0

0

0

0

0 0

0

0

0

0

100

100

100

100

0

100 125 10089

100

divide

Numeric 3

Numeric 4

Max working pressure

Pressure toleration (bar)

Required pressure in R

Required pressure in L

Pressure requirement to start PWM

Right PAM

Left PAM

Max working pressure out

Boolean

f4

f3

f2

f

Pressure alarm

PWM L

PWM R

times

times

gt lt

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

gt

gt

le

le

ge

ge

ge

ge

minus

minus

minus

minus

minus

minus

minus

minus

==

DBL3

2

4 5

DBL3

2

4 5

Figure 7 Block diagram of the subroutine for the pressure regulation

TF

TF

TF

TF T

F

T F

T F

T F

T F

TF

TF T F

TF

DBL

DBL

TF

TF

TF

TF

DBL

DBL

123

DBL3

2

4 5

TF

200

2

0

0

0

0

0

0

0

100

100

100

125 10089

92

Actual position Requirements to start the PWM

The required position

PWM U

The required position

AlarmAlarm L basic

Alarm R basic

Max L position

Max R position

Position control

T

T

F

F

minus1

minus1

+

+

+

+

minus

minus

minus

minus

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

le

ge

ge

=

divide

divide

divide

divide

times

times

times

Tolerance to the required position (∘)

DBL

123

fDBL

123

f2

I32Max∘ LR

Figure 8 Block diagram of the subroutine for the position control

the controlled system and to decide on its further operationA subroutine works as a proportional controller

4 Experimental Measurements

41 Experimental Measurement Number 1 The experimentalactuator was used to carry out experimental measurementsfor the purpose of obtaining information about the static and

dynamic characteristics of the device The principal aim ofthe measuring was to describe the limiting conditions of thedevice operationThe experimentalmeasurementwas carriedout in a room with a relatively stable temperature The initialmeasuring conditions and constant values are presented inTable 2

Based on the information about the possibilities of thePWM regulation presented in [9] the minimal regulated

8 Advances in Materials Science and Engineering

Table 2 Initial conditions and constant parameters for experimental measurements

Initial conditions Value Units AnnotationAmbient temperature 242 [∘C] mdashAir tank pressure Max 75 [bar] The pressure at the electropneumatic valves inletPressure sensors power supply 23 [V] Laboratory power suppliesControl valves power supply 24 [V] Laboratory power suppliesPressure in PAM 2 [bar] Initial operating pressurePressure tolerance 005 [bar] Setting in GUI of the control systemPosition angle of the support arm 0 [∘] mdashPosition tolerance plusmn1 [∘] Setting in GUI of the control system

PWM 20ndash100 [] Determined by the control program in each of its cycles(119891 = 50Hz)

Weight 4002590 Nmm∘ Arm without weights (force determined owing to theshaft axis with the supporting arm rotation to 90∘)

Table 3 The values of the operational parameters for the experimental measurement number 1

Operational characteristics Value UnitsPositioning (1 series of measurement) 0 15 30 45 60 75 80 0 minus15 minus30 minus45 minus60 minus75 minus80 [∘]Maximum pressure 51 [bar]The number of repetitions of a series 100 mdashPositioning step duration 6 [s]Positioning step duration in starting and ending position 16 [s]Mass of weight 65 [kg]

value of the PWM regulation is 20 The use of lowervalues would lead to excessive decreases in durability of theelectropneumatic valves

During the experimental operation the pressure values ineachPAM values of the PWMsignals and rotation and accel-eration of the supporting rotational armweremeasured Dur-ing the measuring the values listed in Table 3 were applied

The measuring was carried out under changing ambientconditions with a room temperature in the range of 242ndash264∘C within 100 series of positioning According to theorder given in Table 3 in each series of experimental mea-surements the automatic control systemwas applied to set upthe size of the rotation angle of the actuator supporting armto the desired position The measurements were performedwithout setting the limits of the device operation to the rec-ommended operatingminima ormaxima so that it was possi-ble to observe the behaviour of the antagonistic involvementof the pneumatic artificial muscles mechanism withoutsoftware restrictions of its operation

After completing the first experimental measurements areview measurement of the limit positions of the supportingarm was conducted which were resulting from the restric-tions of the operational maximum and minimum pressurein each PAM Based on the analysis of the measured valuesexperimental measurement number 3 was performed whoseprinciple consisted in the ambition to prove the differences inthe experimental actuator operation at the start of its opera-tion from the initial state (minimal rotation of the supporting

arm) and from the state of the supporting arm maximumrotation

The data expressing the current position pressure in eachPAM and the acceleration of the supporting arm obtainedbased on each series of measurements were presented graph-ically Between the last iteration of a series of measurementsand the first iteration of the following series of measure-ments when the system started to respond to the change inposition there was a delay in the range of 01ndash04 secondsFigure 9 shows graphs used for the preliminary analysis of thetransient characteristics of the experimental assembly duringthe operation To obtain a description of the mechanismbehaviour without limiting its operating conditions its posi-tioning was not restricted through setting up the minimumand maximum operating pressure during this measurementFor the reason of a better readability of the data the first graphshows the measured data obtained only from the ten consec-utive series of measurements

The processed measured data showed that the measuredcharacteristics recorded in each series of the experiment werenot significantly different In six cases of 100 repeats of themeasurement series there were recorded deviations from thestandard course of the measured dynamic characteristics Apresumed cause is a combination of two factors the operationin the limit positions of the end member of the actuator(75ndash84∘) and the lack of a pressure medium in the air tankIn order to eliminate these phenomena it was necessary toincrease the minimum pressure in the air tank Another

Advances in Materials Science and Engineering 9Po

sitio

n (∘

)

6030 402010 500Time (s)

5010 604020 300Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Posit

ion

(∘)

minus10000

000

10000

10 20 30 40 50 600Time (s)

10 20 40 5030 600Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

minus10000

000

10000

(mmiddotsminus

2)

(mmiddotsminus

2)

Figure 9The graphical representation of a current position and thesupporting arm acceleration

reason why these cases occurred was the mechanism opera-tion without limitation of the PAMminimum andmaximumoperating pressure

The second graph in Figure 9 shows the support armacceleration The acceleration indicates a drop in the sizeof hydraulic shocks with the increased rigidity of the PAMsmechanisms For a more detailed analysis the next graph(the angle of the arm rotation) shows only the data measuredduring the positioning of the supporting arm in the range0ndash84∘ Negative values of the acceleration represent themovement of the supporting arm in the positive directionIn case that the supporting arm exceeds the positioningaccuracy and the actual position is larger than the requiredposition the system responds with a feedback in the form ofa backward movement This fact corresponds with positivevalues of the acceleration

The rigidity of the system was low at low values of thesupporting arm angle In some cases the device failed to reacha stable state of the desired position during the time intervalof one step of the positioningThe lowest value at which it was

1200130014001500160017001800

Posit

ion

(∘)

0000200040006

757 767 777 787 797 807Iteration

minus0004minus0002

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

01020304050

PWM

regu

latio

n (

)

minus10 767 777 797 807757 787Iteration

767 777 787 797 807757Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 10 Analysis of graphs of the experimental measurementnumber 1

possible to stabilize the system during the measurement wasthe position with the angle of 15∘

Figure 10 shows graphs of the device dynamic char-acteristics measured while maintaining the position of thesupporting arm at 15∘ The data are shown in ten series of themeasuringThe first graph shows the course of the supportingarm positioning The principle aim was to maintain thesupporting arm in the position with the desired accuracyduring its positioningThe definition of the position accuracycaused the arm motion to stop immediately after reachingthe accuracy interval It resulted in reaching a real middleposition around 1465∘Therefore it was necessary to considerthe adjustments of the subprogram for the position controlSignificant fluctuations in the position are highlighted in thegraphThese fluctuations occur predominantly in the positivedirection (in the direction of the position increase)

The second graph shows the behaviour of the arm acceler-ationThe largest fluctuations in the arm position correspondto the largest fluctuations in the arm acceleration In theseries number 3 and number 6 the position and accelerationwaveforms are marked When compared with the last graphin Figure 10 there can be seen a direct relation betweenthe occurrence of hydraulic shocks and opening (closing)of electropneumatic valves or between the formation ofhydraulic shocks the size of the PWM regulation and timeof the PWM operation

Figure 11 shows the waveforms of the measured variablesin the fifth series of the experiment Based on a comparison of

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

Advances in Materials Science and Engineering 7

DBL

DBL

123

DBL

DBL

DBL

DBL

TF

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

DBL

123

TF

TF

T

F

T

F

TF

TF

T F

T F

TF

TF

T F T F

T F

T F

T F

T F T F

T F

T F

T F

T F

T F

T F

T F

T F

TF

+

+

+

+

divide

divide

divide

divide

0

0

0

0

0

0 0

0

0

0

0

100

100

100

100

0

100 125 10089

100

divide

Numeric 3

Numeric 4

Max working pressure

Pressure toleration (bar)

Required pressure in R

Required pressure in L

Pressure requirement to start PWM

Right PAM

Left PAM

Max working pressure out

Boolean

f4

f3

f2

f

Pressure alarm

PWM L

PWM R

times

times

gt lt

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

gt

gt

le

le

ge

ge

ge

ge

minus

minus

minus

minus

minus

minus

minus

minus

==

DBL3

2

4 5

DBL3

2

4 5

Figure 7 Block diagram of the subroutine for the pressure regulation

TF

TF

TF

TF T

F

T F

T F

T F

T F

TF

TF T F

TF

DBL

DBL

TF

TF

TF

TF

DBL

DBL

123

DBL3

2

4 5

TF

200

2

0

0

0

0

0

0

0

100

100

100

125 10089

92

Actual position Requirements to start the PWM

The required position

PWM U

The required position

AlarmAlarm L basic

Alarm R basic

Max L position

Max R position

Position control

T

T

F

F

minus1

minus1

+

+

+

+

minus

minus

minus

minus

lt

lt

lt

lt

lt

gt

gt

gt

gt

gt

le

ge

ge

=

divide

divide

divide

divide

times

times

times

Tolerance to the required position (∘)

DBL

123

fDBL

123

f2

I32Max∘ LR

Figure 8 Block diagram of the subroutine for the position control

the controlled system and to decide on its further operationA subroutine works as a proportional controller

4 Experimental Measurements

41 Experimental Measurement Number 1 The experimentalactuator was used to carry out experimental measurementsfor the purpose of obtaining information about the static and

dynamic characteristics of the device The principal aim ofthe measuring was to describe the limiting conditions of thedevice operationThe experimentalmeasurementwas carriedout in a room with a relatively stable temperature The initialmeasuring conditions and constant values are presented inTable 2

Based on the information about the possibilities of thePWM regulation presented in [9] the minimal regulated

8 Advances in Materials Science and Engineering

Table 2 Initial conditions and constant parameters for experimental measurements

Initial conditions Value Units AnnotationAmbient temperature 242 [∘C] mdashAir tank pressure Max 75 [bar] The pressure at the electropneumatic valves inletPressure sensors power supply 23 [V] Laboratory power suppliesControl valves power supply 24 [V] Laboratory power suppliesPressure in PAM 2 [bar] Initial operating pressurePressure tolerance 005 [bar] Setting in GUI of the control systemPosition angle of the support arm 0 [∘] mdashPosition tolerance plusmn1 [∘] Setting in GUI of the control system

PWM 20ndash100 [] Determined by the control program in each of its cycles(119891 = 50Hz)

Weight 4002590 Nmm∘ Arm without weights (force determined owing to theshaft axis with the supporting arm rotation to 90∘)

Table 3 The values of the operational parameters for the experimental measurement number 1

Operational characteristics Value UnitsPositioning (1 series of measurement) 0 15 30 45 60 75 80 0 minus15 minus30 minus45 minus60 minus75 minus80 [∘]Maximum pressure 51 [bar]The number of repetitions of a series 100 mdashPositioning step duration 6 [s]Positioning step duration in starting and ending position 16 [s]Mass of weight 65 [kg]

value of the PWM regulation is 20 The use of lowervalues would lead to excessive decreases in durability of theelectropneumatic valves

During the experimental operation the pressure values ineachPAM values of the PWMsignals and rotation and accel-eration of the supporting rotational armweremeasured Dur-ing the measuring the values listed in Table 3 were applied

The measuring was carried out under changing ambientconditions with a room temperature in the range of 242ndash264∘C within 100 series of positioning According to theorder given in Table 3 in each series of experimental mea-surements the automatic control systemwas applied to set upthe size of the rotation angle of the actuator supporting armto the desired position The measurements were performedwithout setting the limits of the device operation to the rec-ommended operatingminima ormaxima so that it was possi-ble to observe the behaviour of the antagonistic involvementof the pneumatic artificial muscles mechanism withoutsoftware restrictions of its operation

After completing the first experimental measurements areview measurement of the limit positions of the supportingarm was conducted which were resulting from the restric-tions of the operational maximum and minimum pressurein each PAM Based on the analysis of the measured valuesexperimental measurement number 3 was performed whoseprinciple consisted in the ambition to prove the differences inthe experimental actuator operation at the start of its opera-tion from the initial state (minimal rotation of the supporting

arm) and from the state of the supporting arm maximumrotation

The data expressing the current position pressure in eachPAM and the acceleration of the supporting arm obtainedbased on each series of measurements were presented graph-ically Between the last iteration of a series of measurementsand the first iteration of the following series of measure-ments when the system started to respond to the change inposition there was a delay in the range of 01ndash04 secondsFigure 9 shows graphs used for the preliminary analysis of thetransient characteristics of the experimental assembly duringthe operation To obtain a description of the mechanismbehaviour without limiting its operating conditions its posi-tioning was not restricted through setting up the minimumand maximum operating pressure during this measurementFor the reason of a better readability of the data the first graphshows the measured data obtained only from the ten consec-utive series of measurements

The processed measured data showed that the measuredcharacteristics recorded in each series of the experiment werenot significantly different In six cases of 100 repeats of themeasurement series there were recorded deviations from thestandard course of the measured dynamic characteristics Apresumed cause is a combination of two factors the operationin the limit positions of the end member of the actuator(75ndash84∘) and the lack of a pressure medium in the air tankIn order to eliminate these phenomena it was necessary toincrease the minimum pressure in the air tank Another

Advances in Materials Science and Engineering 9Po

sitio

n (∘

)

6030 402010 500Time (s)

5010 604020 300Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Posit

ion

(∘)

minus10000

000

10000

10 20 30 40 50 600Time (s)

10 20 40 5030 600Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

minus10000

000

10000

(mmiddotsminus

2)

(mmiddotsminus

2)

Figure 9The graphical representation of a current position and thesupporting arm acceleration

reason why these cases occurred was the mechanism opera-tion without limitation of the PAMminimum andmaximumoperating pressure

The second graph in Figure 9 shows the support armacceleration The acceleration indicates a drop in the sizeof hydraulic shocks with the increased rigidity of the PAMsmechanisms For a more detailed analysis the next graph(the angle of the arm rotation) shows only the data measuredduring the positioning of the supporting arm in the range0ndash84∘ Negative values of the acceleration represent themovement of the supporting arm in the positive directionIn case that the supporting arm exceeds the positioningaccuracy and the actual position is larger than the requiredposition the system responds with a feedback in the form ofa backward movement This fact corresponds with positivevalues of the acceleration

The rigidity of the system was low at low values of thesupporting arm angle In some cases the device failed to reacha stable state of the desired position during the time intervalof one step of the positioningThe lowest value at which it was

1200130014001500160017001800

Posit

ion

(∘)

0000200040006

757 767 777 787 797 807Iteration

minus0004minus0002

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

01020304050

PWM

regu

latio

n (

)

minus10 767 777 797 807757 787Iteration

767 777 787 797 807757Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 10 Analysis of graphs of the experimental measurementnumber 1

possible to stabilize the system during the measurement wasthe position with the angle of 15∘

Figure 10 shows graphs of the device dynamic char-acteristics measured while maintaining the position of thesupporting arm at 15∘ The data are shown in ten series of themeasuringThe first graph shows the course of the supportingarm positioning The principle aim was to maintain thesupporting arm in the position with the desired accuracyduring its positioningThe definition of the position accuracycaused the arm motion to stop immediately after reachingthe accuracy interval It resulted in reaching a real middleposition around 1465∘Therefore it was necessary to considerthe adjustments of the subprogram for the position controlSignificant fluctuations in the position are highlighted in thegraphThese fluctuations occur predominantly in the positivedirection (in the direction of the position increase)

The second graph shows the behaviour of the arm acceler-ationThe largest fluctuations in the arm position correspondto the largest fluctuations in the arm acceleration In theseries number 3 and number 6 the position and accelerationwaveforms are marked When compared with the last graphin Figure 10 there can be seen a direct relation betweenthe occurrence of hydraulic shocks and opening (closing)of electropneumatic valves or between the formation ofhydraulic shocks the size of the PWM regulation and timeof the PWM operation

Figure 11 shows the waveforms of the measured variablesin the fifth series of the experiment Based on a comparison of

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

8 Advances in Materials Science and Engineering

Table 2 Initial conditions and constant parameters for experimental measurements

Initial conditions Value Units AnnotationAmbient temperature 242 [∘C] mdashAir tank pressure Max 75 [bar] The pressure at the electropneumatic valves inletPressure sensors power supply 23 [V] Laboratory power suppliesControl valves power supply 24 [V] Laboratory power suppliesPressure in PAM 2 [bar] Initial operating pressurePressure tolerance 005 [bar] Setting in GUI of the control systemPosition angle of the support arm 0 [∘] mdashPosition tolerance plusmn1 [∘] Setting in GUI of the control system

PWM 20ndash100 [] Determined by the control program in each of its cycles(119891 = 50Hz)

Weight 4002590 Nmm∘ Arm without weights (force determined owing to theshaft axis with the supporting arm rotation to 90∘)

Table 3 The values of the operational parameters for the experimental measurement number 1

Operational characteristics Value UnitsPositioning (1 series of measurement) 0 15 30 45 60 75 80 0 minus15 minus30 minus45 minus60 minus75 minus80 [∘]Maximum pressure 51 [bar]The number of repetitions of a series 100 mdashPositioning step duration 6 [s]Positioning step duration in starting and ending position 16 [s]Mass of weight 65 [kg]

value of the PWM regulation is 20 The use of lowervalues would lead to excessive decreases in durability of theelectropneumatic valves

During the experimental operation the pressure values ineachPAM values of the PWMsignals and rotation and accel-eration of the supporting rotational armweremeasured Dur-ing the measuring the values listed in Table 3 were applied

The measuring was carried out under changing ambientconditions with a room temperature in the range of 242ndash264∘C within 100 series of positioning According to theorder given in Table 3 in each series of experimental mea-surements the automatic control systemwas applied to set upthe size of the rotation angle of the actuator supporting armto the desired position The measurements were performedwithout setting the limits of the device operation to the rec-ommended operatingminima ormaxima so that it was possi-ble to observe the behaviour of the antagonistic involvementof the pneumatic artificial muscles mechanism withoutsoftware restrictions of its operation

After completing the first experimental measurements areview measurement of the limit positions of the supportingarm was conducted which were resulting from the restric-tions of the operational maximum and minimum pressurein each PAM Based on the analysis of the measured valuesexperimental measurement number 3 was performed whoseprinciple consisted in the ambition to prove the differences inthe experimental actuator operation at the start of its opera-tion from the initial state (minimal rotation of the supporting

arm) and from the state of the supporting arm maximumrotation

The data expressing the current position pressure in eachPAM and the acceleration of the supporting arm obtainedbased on each series of measurements were presented graph-ically Between the last iteration of a series of measurementsand the first iteration of the following series of measure-ments when the system started to respond to the change inposition there was a delay in the range of 01ndash04 secondsFigure 9 shows graphs used for the preliminary analysis of thetransient characteristics of the experimental assembly duringthe operation To obtain a description of the mechanismbehaviour without limiting its operating conditions its posi-tioning was not restricted through setting up the minimumand maximum operating pressure during this measurementFor the reason of a better readability of the data the first graphshows the measured data obtained only from the ten consec-utive series of measurements

The processed measured data showed that the measuredcharacteristics recorded in each series of the experiment werenot significantly different In six cases of 100 repeats of themeasurement series there were recorded deviations from thestandard course of the measured dynamic characteristics Apresumed cause is a combination of two factors the operationin the limit positions of the end member of the actuator(75ndash84∘) and the lack of a pressure medium in the air tankIn order to eliminate these phenomena it was necessary toincrease the minimum pressure in the air tank Another

Advances in Materials Science and Engineering 9Po

sitio

n (∘

)

6030 402010 500Time (s)

5010 604020 300Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Posit

ion

(∘)

minus10000

000

10000

10 20 30 40 50 600Time (s)

10 20 40 5030 600Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

minus10000

000

10000

(mmiddotsminus

2)

(mmiddotsminus

2)

Figure 9The graphical representation of a current position and thesupporting arm acceleration

reason why these cases occurred was the mechanism opera-tion without limitation of the PAMminimum andmaximumoperating pressure

The second graph in Figure 9 shows the support armacceleration The acceleration indicates a drop in the sizeof hydraulic shocks with the increased rigidity of the PAMsmechanisms For a more detailed analysis the next graph(the angle of the arm rotation) shows only the data measuredduring the positioning of the supporting arm in the range0ndash84∘ Negative values of the acceleration represent themovement of the supporting arm in the positive directionIn case that the supporting arm exceeds the positioningaccuracy and the actual position is larger than the requiredposition the system responds with a feedback in the form ofa backward movement This fact corresponds with positivevalues of the acceleration

The rigidity of the system was low at low values of thesupporting arm angle In some cases the device failed to reacha stable state of the desired position during the time intervalof one step of the positioningThe lowest value at which it was

1200130014001500160017001800

Posit

ion

(∘)

0000200040006

757 767 777 787 797 807Iteration

minus0004minus0002

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

01020304050

PWM

regu

latio

n (

)

minus10 767 777 797 807757 787Iteration

767 777 787 797 807757Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 10 Analysis of graphs of the experimental measurementnumber 1

possible to stabilize the system during the measurement wasthe position with the angle of 15∘

Figure 10 shows graphs of the device dynamic char-acteristics measured while maintaining the position of thesupporting arm at 15∘ The data are shown in ten series of themeasuringThe first graph shows the course of the supportingarm positioning The principle aim was to maintain thesupporting arm in the position with the desired accuracyduring its positioningThe definition of the position accuracycaused the arm motion to stop immediately after reachingthe accuracy interval It resulted in reaching a real middleposition around 1465∘Therefore it was necessary to considerthe adjustments of the subprogram for the position controlSignificant fluctuations in the position are highlighted in thegraphThese fluctuations occur predominantly in the positivedirection (in the direction of the position increase)

The second graph shows the behaviour of the arm acceler-ationThe largest fluctuations in the arm position correspondto the largest fluctuations in the arm acceleration In theseries number 3 and number 6 the position and accelerationwaveforms are marked When compared with the last graphin Figure 10 there can be seen a direct relation betweenthe occurrence of hydraulic shocks and opening (closing)of electropneumatic valves or between the formation ofhydraulic shocks the size of the PWM regulation and timeof the PWM operation

Figure 11 shows the waveforms of the measured variablesin the fifth series of the experiment Based on a comparison of

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

Advances in Materials Science and Engineering 9Po

sitio

n (∘

)

6030 402010 500Time (s)

5010 604020 300Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Posit

ion

(∘)

minus10000

000

10000

10 20 30 40 50 600Time (s)

10 20 40 5030 600Time (s)

minus0006

minus0001

0004

Acce

lera

tion

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

minus10000

000

10000

(mmiddotsminus

2)

(mmiddotsminus

2)

Figure 9The graphical representation of a current position and thesupporting arm acceleration

reason why these cases occurred was the mechanism opera-tion without limitation of the PAMminimum andmaximumoperating pressure

The second graph in Figure 9 shows the support armacceleration The acceleration indicates a drop in the sizeof hydraulic shocks with the increased rigidity of the PAMsmechanisms For a more detailed analysis the next graph(the angle of the arm rotation) shows only the data measuredduring the positioning of the supporting arm in the range0ndash84∘ Negative values of the acceleration represent themovement of the supporting arm in the positive directionIn case that the supporting arm exceeds the positioningaccuracy and the actual position is larger than the requiredposition the system responds with a feedback in the form ofa backward movement This fact corresponds with positivevalues of the acceleration

The rigidity of the system was low at low values of thesupporting arm angle In some cases the device failed to reacha stable state of the desired position during the time intervalof one step of the positioningThe lowest value at which it was

1200130014001500160017001800

Posit

ion

(∘)

0000200040006

757 767 777 787 797 807Iteration

minus0004minus0002

Series 1Series 2Series 3Series 4Series 5

Series 6Series 7Series 8Series 9Series 10

01020304050

PWM

regu

latio

n (

)

minus10 767 777 797 807757 787Iteration

767 777 787 797 807757Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 10 Analysis of graphs of the experimental measurementnumber 1

possible to stabilize the system during the measurement wasthe position with the angle of 15∘

Figure 10 shows graphs of the device dynamic char-acteristics measured while maintaining the position of thesupporting arm at 15∘ The data are shown in ten series of themeasuringThe first graph shows the course of the supportingarm positioning The principle aim was to maintain thesupporting arm in the position with the desired accuracyduring its positioningThe definition of the position accuracycaused the arm motion to stop immediately after reachingthe accuracy interval It resulted in reaching a real middleposition around 1465∘Therefore it was necessary to considerthe adjustments of the subprogram for the position controlSignificant fluctuations in the position are highlighted in thegraphThese fluctuations occur predominantly in the positivedirection (in the direction of the position increase)

The second graph shows the behaviour of the arm acceler-ationThe largest fluctuations in the arm position correspondto the largest fluctuations in the arm acceleration In theseries number 3 and number 6 the position and accelerationwaveforms are marked When compared with the last graphin Figure 10 there can be seen a direct relation betweenthe occurrence of hydraulic shocks and opening (closing)of electropneumatic valves or between the formation ofhydraulic shocks the size of the PWM regulation and timeof the PWM operation

Figure 11 shows the waveforms of the measured variablesin the fifth series of the experiment Based on a comparison of

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

10 Advances in Materials Science and Engineering

1200130014001500160017001800

757 767 777 787 797 807Iteration

Posit

ion

(∘)

00005

00025

00045

757 767 777 787 797 807Iterationminus00015

minus00035

Series 5

0102030

757 767 777 787 797 807IterationPW

M re

gulat

ion

()

Acce

lera

tion

(mmiddotsminus

2)

Figure 11 Analysis of the graphs of the series number 5 of theexperimental measurement number 1

the graph of the acceleration and size of the PWM regulationa dependence of these phenomena is noticeableThe assemblyof the experimental manufacturing device responds to theelectropneumatic valve opening by the armmovement whichcauses a dynamic shock (change in speed or acceleration)in the direction of the desired movement Closing of theelectropneumatic valve gives rise to the effect of inertia thatacts upon the supporting arm causing a pose overshoot in theopposite direction There is evident time difference betweenthe opening and closing of the valve and the system responsein the range of one control program iteration One iterationis considered the minimum time unit (experimental mea-surement number 1) While the experimental actuator wasmoving into different positions there were recorded the sameactions as mentioned above A supporting arm angle of 15∘was used to represent these actions because there is low rigid-ity of the whole system and these actions are better viewable

Based on the data processing obtained in the experimen-tal measurement number 1 the following conclusions havebeen taken

(i) For the implementation of relevant measurements itis sufficient to perform only ten repeats of a properlyperformed experiment

(ii) Identified dynamic shocks generated in experimentalinvolvement are directly linked to the compressedmedium supply system of PAMs

(iii) The algorithm of a position control should be recon-sidered and adapted to new requirements with regardto the positioning accuracy subsequently changeshave been made in the control algorithm

Posit

ion

(∘)

minus55

15253545556575

1750 2250 2750 3250 3750 42501250Iteration

1250 2250 37503250 42501750 2750Iteration

minus0006

minus0004

minus0002

0000200040006

012345

Pres

sure

(bar

)

1750 2250 2750 3250 3750 42501250Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 12 A graphical representation of the experimental measure-ment number 2 positioning accuracy = 1∘

(iv) Taking into account the impact of rigidity of thePAMs on the formation of dynamic shocks in the pneu-matic system as well as the impact on the positioningrepeatability in a full operational interval it is possibleto determine the interval of the operating positionsin which according to preestablished requirementsit will be possible to achieve an adequate positioningaccuracy and sufficient rigidity of the system to ensuresteady required static and transient characteristics ofthe mechanism

42 Experimental Measurement Number 2 Based on theresults of the first experimental measurement changes tothe control system were performed The experimental mea-surement number 2 was performed to verify the assump-tions related to the system behaviour when minimum andmaximum operating pressures were set The experiment wasperformed by a gradual enlargement of the supporting armdeviation of small values The aim was to achieve the mostaccurate description of the mechanism rigidity impact on theactuators ability to stabilize itself in time The measurementstarted after the stabilization of the arm in the position 0∘Deviations in the position started to be slowly stabilized inthe required accuracy (the positioning accuracy selected toplusmn1∘) in the position range of 34∘ and higher (Figure 12) Inthe range of the positions 0ndash34∘ a supporting arm oscillatedaround the requested position but it was not possible to reachits stability or its stabilization took a long time It was due tothe use of the PAMs control system which worked too fastIn case of an excessive oscillations of the arm the size of thearm rotation was recorded by the control systemThe controlsystem has an algorithm applied in such situations where an

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

Advances in Materials Science and Engineering 11

Posit

ion

(∘)

minus55

15253545556575

5350 5850 6350 6850 7350 78504850Iteration

5350 5850 6350 6850 7350 78504850Iteration

minus0006

minus0004

minus0002

000020004

012345

Pres

sure

(bar

)

5350 5850 6350 6850 7350 78504850Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 13 Graphical representation of the experimental measure-ment number 2 positioning accuracy = 15∘

action signal is launched to regulate the position of the armin the opposite direction

When using limits of minimum andmaximum operatingpressures in each PAM the position of max 7000∘ canbe reached This measurement confirms the validity of thesupplemented functions of the algorithm that do not allowexceeding the set operating pressures in the PAMs

Figure 13 shows waveforms of the position accelerationand pressure when positioning with the accuracy of 15∘ Inthe graphs a considerably more stable operation with lowerrates of vibration can be seen after exceeding the positionof 25∘ Pressure characteristics in each muscle differ frompreviousmeasurementsThis is due to the position control bythe method when one of the muscles is not used as a pneu-matic spring with a constant pressure In order to eliminatethe pressure differences before startingmeasurement in eachseries of the experiment it is useful to perform the so-calledinitialization series to set up the device

43 ExperimentalMeasurement Number 3Measurement afterthe Initialization Series Experimental measurement number3was carried out after the initialization series (a device settingup) had been performedThe armwas positioned to themax-imal achievable position when the pressure was set to 55 bar(in the control program a maximum operating pressure pre-determined to 6 bar with a safety fuse of 05 bar) Positioningwas done with the accuracy of plusmn1∘ Graphical representationof the experimental measurements number 3 is shown in Fig-ure 14 Stability of themechanismwas achieved at the positionangle of 30∘ This allows improving the operation stability

910 1010 1110 1210 1310 1410 1510 1610 1710810Iteration

0123456

Pres

sure

(bar

)

minus0004

minus0003

minus0002

minus0001

00001000200030004

0Posit

ion

(∘)

minus20

20406080

100

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

810 1010 1110 1210 1310 1410 1510 1610 1710910Iteration

Acce

lera

tion

(mmiddotsminus

2)

Figure 14 Graphical representation of the experimental measure-ments number 3 positioning accuracy = 1∘

Due to the improvement of the mechanism stabilization ina specific position it is also possible to extend the range ofpositions reaching a stable operation by the value of 4∘

5 Evaluation of the Results ofExperiments and Proposed Modifications ofthe Experimental Device

The aim of the experiments was to describe transientphenomena occurring inside the PAMs The results haveconfirmed the hypothesis about the PAMs and the air tankand time of its effect At the same time there was confirmedthe possibility of automation of the system using appropriatecontrol algorithms for achieving theoretically possible posi-tioning accuracy without the need for the implementation ofcomplicated hardware nodes

From the point of view of the positioning accuracy withmajor differences between positions hydraulic impacts werenot so important compared to minor changes in the positionA pressure difference has a significant impact on the speed ofthe supporting arm Each PAM is controlled by an electrop-neumatic valve with the PWM A high pressure differencesignificantly affects the arm movement acceleration or theamount of kinetic energy that is transferred to the supportingarm in a time period during which the valve is openThe armpositioning accuracy also depends on the way of stopping thesupporting arm or it depends on the method of a brakedmovement of the supporting arm [21ndash23]

Based on the nature of potential functionalities of thementioned pneumatic actuator it follows that the systembraking is performed only by a sudden stop or by the PWM

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

12 Advances in Materials Science and Engineering

With a sudden stop of the supporting arm under theeffect of inertial force the misalignments of the supportingarm occur Taking into account the nonlinearity of the entiremechanical system and the subsequent oscillation of the armaround the equilibrium position (as seen from the experi-mental measurements the equilibrium position need not beequal to the position required) the size of this vibration isdirectly dependent on the supporting arm acceleration

In case of a high pressure difference there is an adequateresponse of the pneumatic system in the form of a hydraulicshock This results in the deviation of the supporting armposition from its equilibrium position The size of thedeviation corresponds to the size of the differential pressureExperimentalmeasurements have shown that at a low rigidityof the mechanical system the deviations of the position areabove the range of the required positioning accuracy specifiedto 1∘ max 15∘ After exceeding the positioning accuracy thecontrol system automatically ensures the arm to move in theopposite direction To adjust the position of the arm a certainperiod of time is required due to which an undesirable delayoccurs Such a delay may require a lower speed of handlingand manufacturing operations in flexible long-term opera-tions In some cases this can be recognized as a bottleneck inthe production system In such cases the use of such a systemwould not be possibleThe use of such a system is not suitablein fluid material handling

Operating range of positions in which such a systemallows its stable operation is greatly influenced by a pressuredifference between the source of theworkingmediumand thepressure in the pneumatic artificial muscles rigidity of eachPAM and a load on the supporting arm (torque acting on theshaft)

Rotating of the actuator arm is provided by using fourelectropneumatic valves enabling the proportional regulation(PWM) The applied PWM regulation allows reducing theamount of the inflated anddeflated fluid from the fluid systemin time The PWM regulation is not possible in its entiretyIn practice it is verified that with this system it is necessaryto limit the use of the PWM control to the range of 20ndash100 This restriction is required to achieve an adequatedurability of electropneumatic valves In the control systemit is limited to the interval of 20ndash89 In case that the valueof the needed PWM in a given iteration is greater than 89the system automatically uses the value of 100 The PWMregulation causes a gradual slowdown of the supporting armmovement However if the formation of hydraulic shocks istaken into account that accurately correspond to the openingand closing of the electropneumatic valves it is obvious thatthe use of a time smoother regulation would be preferableA better solution might be a regulation of a gradual closingof the valve with a continuous flow of the working fluidor a regulation that would prevent from hydraulic shocksformation in a pneumatic system of the PAMs mechanism

A control utilizing the PWM regulation has improvedproperties compared to a control using a maximum pressuredifference dependent on the current pressure in the air tankOn the one hand a limitation of the PWM operating rangeto only 20 and higher causes a slight slowdown of thearm movement On the other hand there is the impact of

hydraulic shocks arising even with the lowest PWM applica-tion With a lower rigidity of a mechanism under the influ-ence of hydraulic shocks it is impossible to stop the support-ing arm in the narrowest range around the desired position

Despite many disadvantages described above the reg-ulation of the supporting arm positioning to the desiredposition using the PWM allows obtaining a relatively stableand repeatable positioning in a significantly greater range ofa rotational angle of the supporting arm

For the experimental actuator designs of alternativeinvolvements of technical equipment of the actuator havebeen created The proposed alternatives of changes of theexperimental actuator differ from the original involvementdiagram The main differences are in elements allowing acontinuous pressure change [24ndash26]

6 Conclusion

The main objective of the described measurements was toelaborate the methodology of online monitoring and controlof propulsion systems in manufacturing technology specif-ically for manufacturing devices with artificial muscles inorder to achieve optimal results of the positioning accuracy

The paper consists of two parts The first part of thepaper contains a description of the experimental devicewith adrive based onpneumatic artificialmuscles (using the PWM)identification of its physical possibilities and a prediction ofa complete mechanism behaviour The second part describesthe realization of experimental measurements analysis andassessment of the measurements results and a design of thecontrol system modification or a modification of the entireactuatorThe article deals with the evaluation of the state andthe possibilities of the experimental involvement In order toachieve the ability to automatically control the position of thesupporting arm a proposal for the actuator optimization hasalso been described

The device was used to carry out experimental measure-ments Their purpose was to analyze the behaviour of thepropulsion system based on a pair of the PAM assembledin antagonistic involvement under different operation con-ditions The experiments were focused on the examinationof the positioning accuracy The data about the currentpressure inside each PAM a value of the PWM and a currentposition and acceleration of the supporting arm have beenrecorded Based on the processed and evaluated measureddata the characteristics of the actuator and processes tak-ing place inside its components during the operation havebeen described Based on the graphic representation of themeasured variables it was possible to identify the source ofexcessive fluctuations in the position of the supporting armas well as the nature of the hydraulic shock occurrence in theexperimental pneumatic actuator systemThemeasured datahave been statistically analyzed Results of the data processingshowed the effect of the PAMs rigidity and the size of the armrotation angle on the stability of the device operation and theaccuracy of the arm positioning

As follows from the carried out experimental measure-ments it is possible to automate the systemswith the PAMsbyappropriate algorithms and using the appropriate hardware

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

Advances in Materials Science and Engineering 13

equipment to achieve the required positioning accuracy inthe technically useful range The measurements showed thatthe PAMs systems (described in this article) are able tooperate with the absolute positioning accuracy 1∘ and less incompliance with the range of oscillation and the value of thegravitational overload It is also possible to interpret the givenfindings that by appropriate algorithmisation and determina-tion of time increment of the measurements in combinationwith the change in the rigidity of the system and its inversionit is possible to achieve a high accuracy even in a narrow rangeof the change in the position in relation to a baseline

After the evaluation of the course and results of the firstexperimental measurement there have beenmodifications ofthe software base of the control system performed by addingthe limits for a maximum and minimum operating pressurein each PAM

The actuator has been provided with acceleration sensorsto identify the shocks inside the pneumatic artificial musclesor the sensors of the changes in the torque acting on theactuator arm

Based on the data obtained by the experimental mea-surements several alternatives have been proposed to supplythe elements to the experimental device that would allow acontinuous deceleration or the acceleration of the supportingarm movement eventually the reduction of the impact ofthe pressure difference value between the air tank and thePAMs (to stop the formation of hydraulic shocks with theundesirable values)

Within the solution of these issues attention has beenpreferably paid to practical options to control a particularexperimental device Searching for other alternatives todescribe the processes running inside the mechanism afterthe device enlargement by adding the proposedmodificationsin order to reduce the impact of the hydraulic shocks on thedevice operation could also be interesting

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This paper is supported by the project VEGA 1033815ldquoResearch of effective combinations of energy sources onthe basis of renewable energiesrdquo This paper is supportedby the Slovak Research and Development Agency under theContract no APVV-15-0602

References

[1] Kopecny and F Solc ldquoMcKibben pneumatic muscle actuator inroboticsrdquo ATampP Journal vol 2 p 62 2003

[2] L Karnık R Knoflıcek and J N Marcincin Mobilnı RobotyMarfy Slezsko Opava Czech Republic 2000

[3] W Huang Shape memory alloys and their application toactuators for deployable structures [PhD thesis] University ofCambridge 1998

[4] K C Wickramatunge and T Leephakpreeda ldquoStudy onmechanical behaviors of pneumatic artificial musclerdquo Interna-tional Journal of Engineering Science vol 48 no 2 pp 188ndash1982010

[5] I Dıaz J J Gil and E Sanchez ldquoLower-limb robotic rehabilita-tion literature review and challengesrdquo Journal of Robotics vol2011 Article ID 759764 11 pages 2011

[6] Z Situm ldquoPneumatski misic kao aktuatorrdquo Znanstveno-popularni casopis Sustavi vol 3 no 5 pp 54ndash60 2009

[7] M Rimar P Smeringai M Fedak and S Kuna ldquoAlternativesfor real time positioning control system in mechatronic systemwith pneumatic artificial musclesrdquo International Journal ofEngineering Research in Africa vol 18 pp 3ndash11 2015

[8] J Pitelrsquo M Balara A Hosovsky and M Tothova PneumatickeUmele Svaly Modelovanie Simulacia Riadenie Technicka Uni-verzita v Kosiciach Kosice Slovakia 2015

[9] S Kuna Vyskum metod real time monitoringu a diagnostikyvyrobnych strojov [dissertation work] Technicka Univerzita vKosiciach Kosice Slovakia 2014

[10] F Zezulka P Fiedler and Z Bradac Prostredky PrumysloveAutomatizace Vysoke Ucenı Technicke v Brne Fakultaelektrotechniky a Komunikacnıch Technologiı Brno CzechRepublic 2002 httpwwwspseiczattsouboryprostredkyprumyslove automatizacepdf

[11] R Ramasamy M Juhari M Mamat S Yaacob N Mohd Nasirand M Sugisaka ldquoAn application of finite element modellingto pneumatic artificial musclerdquo American Journal of AppliedSciences vol 2 no 11 pp 1504ndash1508 2005

[12] H P Anh and N T Nam ldquoModeling and adaptive self-tuningMVCcontrol of PAMmanipulator using online observeroptimized withmodified genetic algorithmrdquo Engineering vol 3no 2 pp 130ndash143 2011

[13] G K Klute J M Czerniecki and B Hannaford ldquoMcKibbenartificial muscles pneumatic actuators with biomechanicalintelligencerdquo in Proceedings of the IEEEASME InternationalConference on Advanced Intelligent Mechatronics (AIM rsquo99) pp221ndash226 Atlanta Ga USA September 1999

[14] D Plettenburg ldquoPneumatic actuators a comparison of energy-to-mass ratiorsquosrdquo in Proceedings of the 9th International Confer-ence on Rehabilitation Robotics (ICORR rsquo05) pp 545ndash549 IEEEChicago Ill USA June 2005

[15] ldquoFESTO Fluidnı svaly DMSPMASrdquo 2004 httpswwwfestocomcaten-gb gbdatadoc SKPDFSKMAS SKPDF

[16] T Szepe ldquoAccurate force function approximation for pneumaticartificial musclesrdquo in Proceedings of the 3rd IEEE InternationalSymposium on Logistics and Industrial Informatics (LINDI rsquo11)Budapest Hungary August 2011

[17] J L Serres Dynamic characterization of a pneumatic muscleactuator and its application to a resistive training device [disser-tation work]Wright State University Dayton Ohio USA 2008

[18] J Pitelrsquo and M Balara ldquoPneumaticky umely sval-perspektıvnyprvok mechatronikyrdquo ATampP Journal vol 4 no 1 pp 59ndash622009

[19] P Smeringai Vyskum metod online monitorovania vyrobnychzariadenı s umelymi svalmi [PhD dissertation] TechnicalUniversity in Kosice 2016

[20] V T Jouppila S AGadsdenGM BoneAU Ellman and S RHabibi ldquoSliding mode control of a pneumatic muscle actuatorsystem with a PWM strategyrdquo International Journal of FluidPower vol 15 no 1 pp 19ndash31 2014

14 Advances in Materials Science and Engineering

[21] D W Repperger C A Phillips A Neidhard-Doll D BReynolds and J Berlin ldquoActuator design using biomimicrymethods and a pneumatic muscle systemrdquo Control EngineeringPractice vol 14 no 9 pp 999ndash1009 2006

[22] A Smeringaiova I Vojtko and K Monkova ldquoExperimentelleAnalyse der Dynamik von ZahnradgetriebenmdashTeil 1rdquo TM-Technisches Messen vol 82 no 2 pp 57ndash64 2015

[23] A Smeringaiova I Vojtko and K Monkova ldquoExperimentelleAnalyse der Dynamik von ZahnradgetriebenmdashTeil 2rdquo TMmdashTechnisches Messen vol 82 no 4 pp 224ndash232 2015

[24] A Panda M Prislupcak J Jurko T Krenicky and M JancıkldquoEvaluation of vibration parameters under machiningrdquo KeyEngineering Materials vol 669 pp 228ndash234 2016

[25] S Salokyova andTKrenicky ldquoAnalysis of the effects of factors inrelation to vibration of technological head during the splittingof construction steels through hydro-abrasive splittingrdquo KeyEngineering Materials vol 669 pp 212ndash219 2016

[26] J Zajac D Mital P Michalik and D Mitalrsquo ldquoVerification ofprocess fluids in mass productionrdquo in Proceedings of the 7thInternational Congress of Precision Machining Key EngineeringMaterials (ICPM rsquo13) vol 581 pp 554ndash559 Miskolc HungaryOctober 2013

Submit your manuscripts athttpswwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

Submit your manuscripts athttpswwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials