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Characterisation of pneumatic servovalve for

controlling a continuum robot for Minimally

invasive surgery application

G. Chen a,

aUnilever R&D, Port Sunlight, United Kingdom

M.T. Pham, T. Maalej, H. Fourati, R. Moreau, S. Sesmat b

bLaboratoire Ampere, UMR CNRS 5005, INSA-Lyon, Universite de Lyon,

F-69621, France

Abstract

This paper presents a global characterization and an analytical model of the staticflow stage of an electro-pneumatic servo valve Atchley 200PN. This study will beused to design control scheme in our application for a continuum robot for minimallyinvasive surgery. Firstly, the experimental measurements are carried out using 3Dgraphs where a set of curves gives the output mass flow rate as a function of theelectrical input of the electronic stage for different values of the output pressure.

The exhaust and supply pressures, during these tests, are assumed to be constant.Moreover, 2D classical curves given by some constructors can be reconstructed,such as mass flow gain, pressure gain and mass flow characterization. Secondly, anapproximation of the mass flow stage characteristics of this five-way proportionalvalve by a polynomial function is described. The elaborated model enables a goodreproduction of the pressure gain and the global mass flow characterization curvesto be obtained.

1 Introduction

Biologically-inspired continuum robots [1] have attracted much interest fromrobotics researchers during the last decades to improve the capability of ma-nipulation in constrained space. These kinds of systems are characterized bythe fact that their mechanical components do not have rigid links and discrete

Corresponding author.Email address: [email protected] (G. Chen).

Preprint submitted to IN-TECH 28 March 2011


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joints in contrast with traditional industry robots. The design of these robotsare inspired by movements of animals parts such as tongues, elephant trunksand tentacles etc. The unusual compliance and redundant degrees of freedomof these robots provide strong potential to achieve delicate tasks successfullyeven in cluttered and/or unstructured environments such as undersea opera-

tions [2], urban search and rescue, wasted materials handling [3], MinimallyInvasive Surgery [47].

In view of its special structure, it is posed special challenges in the actua-tion. There are two means of actuation for continuum robots, cable-driven,hydraulic and pneumatic driven ones. In our laboratory, a continuum robotbased on pneumatic power was designed for medical applications. Fig. 2(a)shows the design of the Colobot. The robotic tip has 3 DOF (Degree of Free-dom), which is a unique unit with 3 active pneumatic chambers regularly

disposed at 120 degrees apart. These three chambers are used for actuation;The outer diameter of the tip is 17 mm and the diameter of the inner hole is8mm. The weight of the prototype is 20 grams. The internal pressure of eachchamber is independently controlled by using pneumatic jet-pipe servovalves.The robot can bend in space with 3dofs. This robot was designed for safeguidance of the process of minimally invasive surgery of endoscopy. The robotcan bend to any direction by adjusting the pressure of three chambers.

The Colobot [8] designed for our work, is a small-scaled continuum robot. Dueto the size requirement of the robot, there are challenges on how to miniaturizesensor system integrated into the small-scale robot to implement automaticguidance of progression inside the human colon. This section will present thedetailed design of the Colobot and its fibre-optic proximity sensor system.

In order to the automatic piloting of the electro-pneumatic system, it isnecessary to know the mathematical model of the power modulator. Servo-distributor manufacturers do not provide sufficient characteristics to obtaina model of the flow rate stage of the pneumatic components in their docu-mentation [7]. No precise characteristics are provided by FESTO. This is whythe global static characteristics (port P) of the proportional valve, were estab-lished. Moreover, experimental measurements carried out can give a precise

knowledge of the mass flow rate delivered by the flow stage of this proportionalvalve and then a simulation model can be deduced.

1.1 Colobot

The difference between our robotic tip and other existing continuum robotsis the size. Our design is inspired by pioneer work [9] on a flexible micro-

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actuator (FMA) based on silicone rubber. Fig. 1(a) shows our design of theColobot. The robotic tip has 3 DOF (Degree of Freedom), which is a unique

(a) Colobot

Active chamber

Passive chamber

d8

17mm

(b) Cross section of Colobot

Fig. 1. Colobot and its cross section

unit with 3 active pneumatic chambers regularly disposed at 120 degrees apart.These three chambers are used for actuation; three other chambers shownin fig. 1(b) are designed to optimize the mechanical structure in order toreduce the radial expansion of active chambers under pressure. The outerdiameter of the tip is 17 mm that is lesser than the average diameter ofthe colon. The diameter of the inner hole is 8mm, which is used in order toplace the camera or other lighting tools. The weight of the prototype is 20grams. The internal pressure of each chamber is independently controlled byusing pneumatic jet-pipe servovalves. The promising result obtained from thepreliminary experiment showed that this tip could bend up to 120 and theresonance frequency is 20 Hz.

2 Modeling and experimental characterization of pneumatic ser-

vovalves

During an electro-pneumatic control, the follow up of the power transfer

from the source to the actuator is achieved through one or several open-ings with varying cross-section called restrictions: this monitoring organ is theservovalve [10]. The COLOBOT device is provided by three jet pipe micro-servovalves Atchley 200PN [11], which allow the desired modulation of airinside the different active chambers in Fig. 1(b). In this component, a motoris connected to an oscillating nozzle, which deflects the gas stream to one ofthe two cylinder chambers (Fig. 2(a)). A voltage/current amplifier allows tocontrol the servovalves by the voltage [12]. A first pneumatic output of thiscomponent is directly connected to one of the robot chambers, and a sec-

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ond output is left unconnected. A sensor pressure (UCC model PDT010131)(Fig. 2(b)) is used to measure the pressure in each of the three COLOBOTrobot chambers. The measured pressure, comprised between 0 and 10 bars,was used to determine the servovalve control voltage.

(a) Atchley servovalve 200PN (b) Pressure sensor

Fig. 2. Atchley servovalve and pressure sensor

As the three servo valves used for the COLOBOT actuator are identical, arandom servovalve was chosen for the mass flow and pressure characteriza-tion. The pressure gain curve is the relationship between the pressure and thecurrent control when the mass flow rate is null. It is performed by means ofthe pneumatic test bench shown in Fig 3. A manometer was placed down-stream of the servovalve close by the utilization orifice in order to measure thepressures. Fig. 4 shows the pressure measurements Pn and Pp carried out foran increasing and a decreasing input current. It appears that the behavior ofthe servovalve is quite symmetric but with a hysteresis cycle. Arrival in stopframe couple creates pressure saturation at -18 mA, respectively +18 mA, forthe negative current, respectively for the positive current. In the Fig. 3, wesubstitute the manometer on the test bench for a static mass flow-meter toplot the mass flow rate gain curve (mass flow rate with respect to the inputcurrent). This curve presented in Fig. 5 shows a non linear hysteresis.

Fig. 3. Pressure gain pneumatic characterization bench

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20 15 10 5 0 5 10 15 20 25Control current (mA)

P

pP

n

Measurements

Fig. 4. Pressure gain characterization

20 15 10 5 0 5 10 15 20 25Control current (mA)

Fig. 5. Mass flow gain characterization

Because of the specific size of Colobots chambers, the experimental massflow rate inside the chamber is very small, the current input and the pres-sure variations are small enough to neglect the hysteresis and consider linearcharacteristics for Fig. 4 and Fig. 5.

3 Mass flow rate modelling

3.1 Experimental flow modeling

Experimental flow modelling allows determining the experimental servovalvemass flow. Because of the unidirectional characterization used in the flow-meter two following montages must be used:

Positive flow montage

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Montage described in figure 6(a) was used for positives flows. Power pressure( ps = 4.2bars ) was chosen and a tap placed downstream of the flow-meterallowing to adjust the operating point. (adjust the pressure and used flowcoming from the servovalve orifice (Pp))

(a) For positive mass flow rate

(b) For negative mass flowrate

Fig. 6. Pneumatic characterization bench of the curve

Negative flow montage

For negative flows, flow-meter direction must be changed to measure the flow.The new montage consist of connecting the opening path to the servovalvepower through a device using a flow-meter, a tap, and a pressure regulator(Fig. 6(b)). An electrical control signal is applied to measure the servovalveoutlet flow by varying the pressure with a tap connected to the flow-meterexit. From this control signal, we go up to the maximum value and then

back to the same signal to go through a complete cycle and one measureagain the flow rate for the same operating pressure values. This procedure,performed for a chosen step voltage of 1V, allows forming two global flow ratematrices for the ascent and descent. To reduce the complexity of the staticflow model, one performs an average of the measured rates between the ascentand descent (Fig. 7) which allows the work on one measurement set and toobtain a single operating mass flow model (medium) for the servovalve. The3D-map showing the evolution of the mean mass flow rate ( Fig. 7) allowsthe extraction of the network of global characteristic curves that describesonly the operating flow rate evolution according to the operating pressure,for a given constant electrical control signal, as referenced in Figure 11(a).

After a detailed analysis of this curves network, some remarks arise about theservo valve behaviour. It is noted that for positive mass flow, the deflection ofthe flow could be regarded as a hysteresis phenomenon, it corresponds to anabrupt drop in the pressure adjustment: it is a prohibited zone of pressure.This is due to an abrupt switch of flow regime within the servo valve. Also, thecurves network shows significant regular areas with relatively constant areasflow for an electrical control signal above 7v. Finally, the curves as referencedin Figure 11(a) are quite different and show the influence of the control signalon the flow parameters such as on the sonic conductance corresponding to the

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Fig. 7. 3D-map of the mean flow rate global characteristicsequivalent effective area and on the critical pressure ratio. It is noted thatthe flow decreases when the pressure increases. In the same way, the networkof control-pressure global characteristic curves is extracted from Figure 7. Itdescribes only the operating flow rate evolution according to the operatingelectrical control signal, for a given constant pressure, as shown schematicallyin Figure 11(b). The same remarks made for the network of pressure-flowglobal characteristic curves are valid for the one of control-pressure globalcharacteristic curves.

(a) Constant control voltage

(b) Constant pressure

Fig. 8. Global flow gain characteristics curves

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4 Approximations of the servo-distributor flow stage characteris-

tics

In this section, the approximations of the Atchley servo-distributor flow stage

characteristics by generalised polynomial functions are presented.

4.1 The polynomial approximations

The mass flow rate is function of the pressure and the input control voltage sothe polynomial approximations will be of multivariable type. The curve shapesof the global characteristics justify the choice of functions of a polynomial typeand of their polynomial degrees. The mathematical model that describes themass flow rate is a polynomial function of the following form:

qmest(p,u) =n

i=0

m

j=1

aijuipj (1)

with the coefficients aij are linear according to the two polynomials of controlvoltage ui and pressure pj. qmest represents the mass flow estimated accordingto this approximation and based on linear regression technique. The abovemethod gives only one global model and may be used for the simulation mod-els. T

4.2 The analytical simulation model

We calculate the infinite norm (max (abs ( ))) when = qmqmest is the errorbetween the measured mass flow and the one estimated by 1. This procedure isexecuted in a loop taking in each iteration the maximum value of this infinitenorm to form a matrix which depends on the degree of polynomials ui andpi . Figure ?? shows the variation of the error generated by the polynomialapproximation in function of control voltage and pressure polynomials degrees.Thus it allows to compare the influence of increasing polynomials degrees onthe maximum error by observing the relative gain in the error percentagefor each degree increase. In our case, several approximations have been madewith some polynomial degrees and we stop the loop to third iteration sincethe increase in the degree of polynomials does not gain significant error. Thedegree ofu = 3 and the degree ofu = 4 and the approximation error = 0.032.Finally, the average of the polynomial coefficients is:

aij = [0.17530.0] (2)

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Fig. 9. The error variation according to the degrees of the polynomials u and p(servo valve no1)(Mean between the phase of descent and ascent phase

We plot also the relative error

= (qmqmist)/qm generated by the polynomialapproximation between the estimated and the measured polynomial flows.Figure 9 show that this error is negligible except in a few points where thereare peaks. This confirms the choice efficience of the polynomial approximation.

1.4.3 Comparison of measures and polynomial approximations

To validate the relevance of the flow mathematical model previously identified,we represent on the same figure the points determined by the developed modeland the gain control voltage-flow experimental curve for a constant pressure.The comparison between the model curve and measured points was performedfor two chosen constant values of pressure p = 0 and p = 4. We notice in Figure11 that the difference between the curve of the chosen mathematical modeland the measures flow is negligible. This difference is also small between thesame mathematical model and the measured pressure-flow curve for constantcontrol voltage values (Fig. 11). Thus the choice of the degree of polynomialsin pressure and voltage has helped to identify the coefficients of model Equ.1 to reproduce the same flowOs gain behaviour of the servovalve.

Figure 10. Control voltage-flow characteristics curves

Figure 11. Pressure-flow characteristics curves - Comparison between the math-ematical model and experimental measurements Measurements made on theservo valves served both as a reference for the simulation model and for the

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Fig. 10. Mass flow rate error

(a)

(b)

Fig. 11. Control voltage-flow characteristics curves- Comparison between the math-ematical model and experimental measurements

identification of coefficients of theoretical non-linear model of the servo valve.This model will be used in the non-linear servoing of COLOBOT.

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5 CONCLUSIONS AND FUTURE WORKS

This paper presented a complete robotic system for semi-autonomous colonoscopy.It is composed of a microtip, a proximity multi-sensor system and high level

real-time control system for guidance control of this robot. This system wasfocused on its guidance ability of endoscope inside the human colon with thefiber optic proximity sensors. Colobot is a continuum robot made of siliconerubber. It has three DoF with its outer diameter of 17mm and the weight of 20gram. The pneumatic actuators of ColoBot are independently driven throughthree servovalves. The kinematic model of this soft robot was developed basedon the geometric deformation and validated its correction. A method using acircumscribed circle is utilized to calculate the safe reference position and ori-entation of the Colobot. While kinematic-based orientation control used thesereference paths to adjust the position of Colobot inside the colon to achieve

guidance. Experimental results of guidance control with a transparent tubeverified the effectivity of kinematic control and guidance control strategy. Inthe near future, the proposed method will be tested in a vitro environment.

References

[1] G. Robinson, J. Davies, Continuum robots - a state of the art, in: IEEEInternational Conference on Robotics and Automation, Detroit Michigan, USA,1999, pp. 28492853.

[2] D. Lane, J. David, G. Robinson, D. OBrien, J. Sneddon, E. Seaton, E. A., Theamadeus dextrous subsea hand: Design, modeling, and sensor processing, IEEEJournal of Oceanic engineering 24 (1) (1999) 96111.

[3] G. Immega, K. Antonelli, The KSI tentacle manipulator, in: IEEE InternationalConference on Robotics and Automation, Nagoya, Japan, 1995, pp. 3149 3154.

[4] N. Simaan, R. Taylor, P. Flint, A dexterous system for laryngeal surgery- multi-backbone bending snake-like slaves for teleoperated dexterous surgical toolmanipulation, in: IEEE International Conference on Robotics and Automation,New Orleans, USA, 2004, pp. 351357.

[5] P. Dario, C. Paggetti, N. Troisfontaine, E. Papa, T. Ciucci, M. Carrozza,M. Marcacci, A miniature steerable end-effector for application in an integratedsystem for computer-assisted arthroscopy, in: IEEE International Conference onRobotics and Automation, Albuquerque, USA, 1997, pp. 15731579.

[6] J. Piers, D. Reynaerts, H. Van Brussel, G. De Gersem, H. T. Tang, Designof an advanced tool guiding system for robotic surgery, in: Proceedings of theInternational Conference on Robotics and Automation, Taipei, Taiwan, 2003,pp. 26512656.

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[7] Y. Bailly, Y. Amirat, Modeling and control of a hybrid continuum activecatheter for aortic aneurysm treatment, in: IEEE International Conference onRobotics and Automation, Barcelona, Spain, 2005, pp. 924929.

[8] G. Chen, M. T. Pham, T. Redarce, Development and kinematic analysisof a silicone-rubber bending tip for colonoscopy, in: IEEE/RSJ IntemationalConference on Intelligent Robots and Systems, Beijing, China, 2006, pp. 168173.

[9] K. Suzumori, S. Iikura, H. Tannaka, Applying a flexible-micro-actuator roboticmechanisms, IEEE control systems 12 (1) (1992) 2127.

[10] S. Sesmat, Modelisation, simulation et commande dune servovalveelectropneumatique (in french), Ph.D. thesis, INSA de Lyon (1996).

[11] Atchley Controls, Jet Pipe catalogue.

[12] R. Atchley, A more reliable electrohydraulic servovalve, in: Robot VI

Conference, Detroit, USA, 1982.

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