Mechatronics 15ME54T 2020 - 2021 Mechatronics unit 1

Mechatronics 15ME54T 2020 - 2021

Department of Collegiate & Technical Education, Bengaluru, Karnataka-560001 Page 1

Mechatronics unit 1

15ME54T

E content

Prepared by Mohammed Imran



CHAPTER 1

MECHATRONICS, SENSORS AND TRANSDUCERS

Definition and Introduction:

Mechatronics can be defined as an interdisciplinary approach to engineering

design through an integration of mechanical engineering, electrical engineering,

computer technology, electronics and control engineering.

It is a multi-disciplinary approach to product and manufacturing system

design.

It involves application of electrical, mechanical, control and computer

engineering to develop products, processes and systems with greater

flexibility, ease in redesign and ability of reprogramming.

It concurrently includes all these disciplines.

EXAMPLE: Fully automated washing machine.

Importance of Mechatronics:

In the design of cars, robots, machine tools, washing machines etc, such an

integrated and interdisciplinary approach to engineering design (Mechatronics) is

increasingly being adopted in order to have following advantages:

1. It makes system more reliable.

2. It makes system more flexible.

3. It makes system cheaper.

4. Good quality with less cost.

5. More versatile.

6. Concurrent approach.

7. Highly controllable.

System:



The mechatronics generally involves the systems. A system may be a box

which has an input and an output and we are not concerned with what goes on

inside the box but we concerned about only the relationship between the output

and the input.

Example: an electric motor.

Measurement Systems:

Generally, the measurement system consists of following three elements:

1. A sensor

2. A signal conditioner

3. A display system

1. Sensor: A sensor is a device which senses and detects the quantity being

measured and responds by giving output signal related to quantity being

measured. Ex: thermocouple a temperature sensor. Gives – EMF output

related to temperature value.

2. A signal conditioner: It is a device which takes the signal from the sensor and

manipulates it into a condition suitable for display or control system. Ex:

amplifier which amplifies small EMF into bigger signal.

3. A display system: It is a device in which the output from the signal

conditioner is displayed for. EX: digital display or pointer moving across

scale.



Control Systems:

The control system is the main base for the mechatronic system. The control

system consists of a feedback unit, which gets the signals from the output of a system

and compares with the actual output require and the sends the control signal by

adjusting its output according to the input in order to obtain the required output.

Types of control systems:

Open loop:

In this system, the control action is carried out with reference to the given

input known as set point, without which any feedback from the output.

Advantages:

1. It is relatively simple.

2. Low cost.

3. Good reliability.

4. Easy maintenance.

5. Less chance of breakdown.

Disadvantages:

1. No feedback unit.

2. They are often inaccurate.

3. No correction for error.

Closed loop:



A control system with a feedback unit is known as closed loop system. In this

case, the control action is carried out by the feedback control i.e., the control system

gets the feedback from the output and compares the actual value with the required

value and makes the corrections to modify the input value on the basis of difference

between actual value and required value.

Advantages:

1. It always compares actual value with required value.

2. High system accuracy.

3. Good system response.

Difference between Open loop and Closed loop system:

Sl.

No

.

Open loop Closed loop

1 Not using feedback Using feedback

2 Less accurate More accurate

3 Simple in construction Complicated in construction

4 Optimization in control is not

possible

Optimization in control is possible

5 Easy maintenance & cost is less Difficult to maintain & cost is more

6 Ex. CD deck, Digital

thermometer

Ex. Automatic water level, washing

machine

Basic elements of closed loop system

Comparison elements.

Control elements.

Correction elements.

Process elements.

Measurement element or feedback device.

Elements of closed loop system:

Comparison elements: This element compares output measured value with

required or ref value and produces an error signal.

Error signal= Ref value ± measured value

Control elements: This element decides what action is being taken when it

receives error signal from comparison elements.

Correction elements: The correction element produces change in the process

to correct or change controlled condition through actuator which provides

power to carry out control action ex electric switch on a heater to increase the

temp to produce required temperature.



Process elements: It is what being controlled for example, room whose temp

being controlled.

Feedback device: This measures the processed variable to give feedback

about actual value to the controller.

Sensors and Transducers:

Sensors:

A sensor is an element which produces a signal corresponding to the quantity

being measured. Example: An eclectic resistance temperature element which senses

the temperature in to an equivalent change in resistance as output signal.

Transducers:

Transducers are defined as elements which when subjected to some physical

change experiences a related change. Example: Strain gauge, thermocouple.

Classification of sensors:

Temperature Sensor

Proximity Sensor

Accelerometer

IR Sensor (Infrared Sensor)

Pressure Sensor

Light Sensor

Ultrasonic Sensor



Smoke, Gas and Alcohol Sensor

Touch Sensor

Color Sensor

Humidity Sensor

Tilt Sensor

Flow and Level Sensor

Classification of transducers:

1. Analog transducer:

i) ELECTRO MECHANICAL

B.) Inductive type Vibrating string transducer

ii) Non-self generating type (LVDT)

C.) Capacitive type

D.) Piezo- electric type

E.) Resistance strain gauges

F.) Ionization transducer

G.) Mechano- electric transducers.

H) Opto-electrical transducers.

A) Photo emission transducer

B) Photo conductive transducer

C) Photo voltaic transducer

2. Digital transducers:

i) Frequency domain transducer

A) Electrochemical frequency domain

B ) Opto-electrical frequency

B) Vibrating string transducer

ii) Digital encoders.

Performance Terminology:

Performance features of sensors and transducers:



The following terms are used to define the performance of transducers and

measurement systems as a whole:

1. Range and span

2. Error

3. Accuracy

4. Sensitivity

5. Hysteresis error

6. Non-linearity error

7. Repeatability / reproducibility

8. Stability

9. . Dead band / time

10. Resolution

11. Output impedance.

1. Range and Snap:

Range:

Range is defined the limits between which the input can vary.

Snap:

Snap is the maximum value of the input minus the minimum value. Example,

a load cell for the measurement of forces might have a range of 0 to 50 kN.

2. Error:

Error is the difference between the results of the measurement and the true

value of the quantity being measured.

Error = measured value – true value

Example, if the measured temperature is 25°C when the actual is 24°C, then the error

is +1°C.

Instead if the measured value was 26°C, then the error should be -1°C.

3. Accuracy:

Accuracy is the extent to which the value indicated by a measurement system

might be wrong. In other words, it is the summation of all the possible errors that are

likely to occur.



Example, the temperature – measuring instrument may be specified as having

an accuracy of ±2 °C of full range output. Then if the rang of the sensor is 0 to 200°

the reading given can be expected to be within + or - 10°C of the true reading.

4. Sensitivity:

Sensitivity is defined as the ratio of output to the input i.e. output/input. It is

the relationship indicating how much output you get per unit input. Example, a

temperature measuring instrument sensitivity may be given as ±1°C of the reading per

°C change in temperature.

5. Hysteresis Error:

Transducers can give different outputs for the same input value, whether that value

has been reached by continuously increasing change or decreasing change, this effect

is called hysteresis error as shown in fig.

6. Non-linearity Error:

It may be generally defined as maximum difference from straight line.

Generally a graph of output plotted against input is assumed to give straight

line, if it deviates from assumed straight line we may said error occur as a result of

assumption linearity .it is expressed in percentage of full range output. Example, we

may quote as pressure measuring transducer having a non-linearity error of ± 5% of

full range.



7. Repeatability / Reproducibility:

It can be defined as an ability to give the same output for repeated applications

of the same input values. Mathematically, it can be expressed as percentage of the full

range output.

8. Stability:

It can be defined as an ability of a transducer to five the same output when

used to measure a constant input over a period of time. The change that occurs over a

period of time for the same input is called drift. The drift may be expressed as a

percentage of the full range output.

9. Dead band / Time:

The dead band or dead space of a transducer is the range of input values for

which there is no output. The dead time is the length of time from the application of

an input until the output begins to responds and change.

10. Resolution:

It is a smallest change in the input value that will produce by the considerable

change in the output value.

11. Output Impedance:

When a sensor gives an electrical output is interfaced with electronic circuits it

is necessary to know the output impedance since this impedance is being connected in

either series or parallel with that circuit.



Factors to be considered while selecting a particular type of sensor:

1. Size of displacement

2. Weather displacement is linear or angular

3. Resolution required

4. accuracy required

5. Material of measured object

6. The cost

Displacement, Position and Proximity Sensors

Displacement sensors are concerned with the measurement of the amount by

which some object has been moved. The position sensors are concerned with the

determination of the position of some object with reference point. The proximity

sensors are a form of position sensors and determine when an object has moved

within some particular critical distance of the sensor.

Displacement sensors

1. Potentiometer sensor

The potentiometer consists of a resistance element with a sliding contact

which can be moved over the length of resistance element. This can be used for linear

or angular displacement, the displacement being converted into a potential difference.

A rotary potentiometer as shown in fig, it consists of a circular wire wound

track or film of a conductive plastic over which a rotatable sliding contact can be

rotated. With a constant source of voltage Vs , between 1 and 3, the output voltage Vo

between 2 and 3 is a fraction of input voltage, and fraction depends on R23 between

terminal 2 and 3 to the resistanceR13 between terminal 1 and 3. That is,



So, for rotary potentiometer the output voltage is proportional to angle through

which the slider has rotated, hence an angular displacement can be converted into a

potential difference.

2. Strain gauge element

Strain gauge is a metal wire, metal foil or a strip of semiconductor material;

these elements can be struck onto surfaces like a postage stamp. When subjected to

strain, its resistance R changes, the fractional change in resistance being proportional

to the strain.

i.e.

G is the gauge factor typically values are 2 for metal foil or wire +100 for P-

type, -100 for N-type semiconductor.

For R=100, G=2, the change in resistance due to 0.001 strain is R=RGɛ=0.2 ohm

Strain is the ratio of change in length / original length.

The linear displacement sensor consists of a strain gauge attached to the

cantilever, ring or U-shape flexible elements. When the flexible element is bent or

deformed as a result of forces being displaced, then the electrical resistance strain

gauges mounted on the element is strained and so gives a resistance change, which

can be monitored. The change in resistance is thus a measure of the displacement or

deformation of the flexible element.



3. Capacitive linear Displacement Sensors

The capacitive sensor for the monitoring of linear displacement are as shown

in fig, a, b and c. In fig (a) one of the plates is moved by displacement so that the plate

separation changes. In (b), the displacement causes the area of overlap to change, in

(c) the displacement causes the dielectric between the plates to change.

Since capacitance C of a parallel plate is by:

Capacitive sensors for monitoring of linear displacements might take forms shown.



In (a) if d is changed by a displacement x then capacitance is

The change in the capacitance value is no linear relationship

The nonlinear capacitance can be overcome by using a push-pull displacement sensor.

Push-pull Displacement Sensor: This is an improved type of capacitive sensor.

This consists of three plates with the plates with the upper pair forming one capacitor

and lower pair becomes another capacitor. The displacement moves the central plates

between two outer plates so if initially the displacement between plate 1 and 2 equal

to the distance between plate 2 and 3 then C1=C2 and small displacement x.

Such sensors are used to measure displacement from few mm to hundreds of mm.

Non-linearity and hysteresis are about ±0+0.1% of full range.

4. Linear Variable Differential Transformers (LVDT):

LVDT consists of three coils symmetrically spaced along an insulated tube.

The central coil is a primary coil and the other two are identical secondary coils are

connected in series in such a way that their outputs oppose each other. A magnetic

core is moved through the central tube in response to a displacement.

If ac voltage is applied to the primary winding, equal and opposite voltage will

be generated in the secondary winding and the net output voltage is zero. However,

when the core is displaced from the centre there is resultant output voltage.



The e.m.f. induced in the coil is given by

Where M is mutual inductance (depends on the number of turns and the core). For

sinusoidal input current, the induced voltages can be written as

And the net output voltage with v1>v2 is

Or

With v2>v1

5. Eddy Current Proximity Sensors

It consists of a sensor coil and reference coil, when the coil is supplying an

alternating magnetic field is in close proximity to a metal object, then an eddy current

is produced in the object, which in turn produces a magnetic field. As a result, the

impedance of the coil changes, thereby changing the amplitude of the alternating

current. At some preset value of amplitude the switch will be triggered. These types of

sensors are used for detection of non-magnetic but conductive material.

Advantages:

1. It is relatively inexpensive.



2. Small in size.

3. High reliability.

4. High sensitivity to small displacement.

6. Optical Position Encoders

An encoder sensor is a device which provides a digital output as a result of

linear or angular measurement. These are also called as position encoders. Position

encoders are of two types:

I. Increment encoders

II. Absolute encoders

I. Increment encoders

This is used for measurement of angular displacement. It consists of a disc

having slots around its periphery, LED and light sensor. A beam of light passes

through slot in a disc and is detected by a suitable light sensor. When the disc is

rotated, a pulsed output is produced by the sensor with the number of pulses being

proportional to the angle through which the disc rotates, which is the measure of

angular position of the disc with some datum position. Normally three tracks are used

to monitor the position. The inner track has only one hole to locate the “home”

position of the disc. The other two tracks have a series of equally spaced holes around

the disc.



II. Absolute Encoder

This is used for measurement of angular displacement. It consists of rotating

disc fixed over a shaft whose angular position is to be determined. The rotating disc

has three concentric circles of slots and three sensors to detect the light pulses. The

slots are arranged such a way that, the sequential output from the sensor is a number

in the code each such number has a several digits and each such numbers represent a

particular angular position.

Generally, the encoders have 10 to12 tracks. The binary number bits will be

equal to the number of tracks. When disc rotates by means of shaft, light sensor

detects pulses the sequential output from the sensors in the form of 3-binary bits.

8. Pneumatic Sensors

It involves the use of compressed air, displacement or the proximity of an

object being transformed into a change in air pressure. It consists of inlet and outlet



ports with sensor. Low pressure is allowed to escape through a port in front of the

sensor. So if there is a close object, the air cannot so readily escape ant the result is

that, the pressure increases in the sensor output port. Such sensors are used for

measurement of displacement of fractions of millimeters (3-12mm).

9. Proximity Switches

These are switches which can be activated by the presence of an object. These are

micros witches, which can be activated by the presence of an object, require physical

contact and small operating force to operate.

Reed Switch: It consists of two magnetic switch contacts sealed in a glass

tube. When a magnet is brought close to the switch, the magnet reeds are attracted to

each other and close the switch contacts. It is a non contact proximity switch.

9. Photosensitive Sensors

These are used to detect the presence of an object, opaque object. It consists of

light emitting diode (LED) and a photo detector. When the object to be detected



moves in between LED and photo detector, it breaks the beam of light or by detecting

the light reflected back by the object, where LED and photo detector are in the same

side.

10. Half Effect Sensors

When a beam of charged particles passes through a magnetic field, forces acts

on the particles and the beam is deflected from its straight line path i.e. current

flowing in a conductor can be deflected by a magnetic field, this effect is known as

„Half effect‟ and it is discovered by E.R. Hall in 1879.

Consider the electrons moving in a conductive plate with magnetic field applied at

right angle to the plane of a plate. During this process the electrons deflected on one

side becomes negatively charged while the opposite side becomes positively charged.

This charge separation produces an electrical field in the material. The charge balance

the forces produced by the magnetic field. These results in production of potential

difference called Hall voltage. Hall voltage is the measure of the magnetic density.

The value of this voltage is given by

Where t is the plate thickness and KH is hall constant. At constant current source, the

voltage difference is a measure of magnetic flux density.



Hall Effect Sensor Application

Can be used as position, displacement and proximity sensors.

Can be used to measure the level of fluid

Other applications are commutation of BLDC motors, current and voltage

measurement, wheel speed sensors, angle sensor, rotor position measurement,

distance measurement.

Velocity Sensors:

Tachogenerator:

The variable reluctance tachogenerator used for measurement of angular

velocity. It consists of a toothed wheel of ferromagnetic material which is attached to

the rotating shaft. A pick-up coil is wounded on a permanent magnet. As the wheel

rotates the teeth of the ferromagnetic material, thus the flux linked by a pick-up coil

changes, this will continue for each teeth resulting in cyclic change in the flux linked

produces an alternating e.m.f. in the coil.

If the wheel contain „n‟ teeth and rotates with an angular velocity Ѡ , then the

flux changes with time for the coil can be given as φ = φ0 + φa cos nѠt.

Where φ0 is the mean value of flux and φa is the amplitude of the flux

variation. The induced e.m.f is the N turns of the pick-up coil given as

Limitations:

At low speed the frequency of the output voltage becomes low; hence have to

be designed with large number of poles.

At high speed, impedance of the coil increases which may cause impedance

mismatch.



Force sensors: Strain Gauge Load Cells:

A strain gauge load cell is a force measuring transducer by converting the

strain produced due to be measured into electrical signal. It consists of a cylindrical

tube to which strain gauges are attached. When force applied the cylinder to compress

it, then the strain gauge give a resistance change, which is the measure of the strain

and hence the applied force.

Applications of load cells:

The strain gauge load cells can be used for the following applications:

1. Measurement of force

2. Electronic weighbridge

3. Portable electronic weighing machine

4. Measurement of impact force

5. For feedback control

6. For calculation of C.G.

Fluid Pressure Sensors:

Fluid pressure sensors are devices used to measure the fluid pressure in

industrial processes. The type of pressure to be measured area absolute pressure,

vacuum pressure, differential pressure and gauge pressure. The pressure measurement

involves the monitoring of the elastic deformation of diaphragm, capsule, bellows and

tubes.

a. In diaphragm when pressure is applied on one side, then the centre of the

diaphragm displaces, results in corrugation in diaphragm. This can be

measured by strain gauge attached.

b. Capsules can also be used for pressure measurement, it consists of two

corrugated diaphragms combined together to give greater sensitivity.

c. A bellow type which is more sensitive than capsules for small pressure

measurement.

d. A bellow can be combined with a LVDT to give a pressure sensor with an

electrical output.



Piezoelectric Sensors:

These are devices which when stretched or compressed generates electric

charge with one face positively connected and other face negatively charged results in

voltage production; it is known as piezoelectric effect used to measure pressure,

acceleration and force.

Piezoelectric capacitor, when force F is applied over an area A of the

capacitance C of piezoelectric material between plates changes and produces voltage

which is proportional to applied force F.

Turbine Flow Meter (Liquid Flow Sensor):

The turbine flow meter is used to measure the flow orate of liquid based on the

measurement of the pressure drop occurring when the fluid flows through a turbine

blade in a horizontal tube. It consists of a multibladed rotor that is supported centrally

in the pipe along which the flow occurs. The fluid flow results in rotation of the rotor,

the angular velocity being approximately proportional to the flow rate. The rate of



rotation of the rotor can be calculated by using magnetic pick up the number of pulse

is the number of revolution of rotor.

Liquid Level Sensor:

These are sensors used to monitor the level of liquid in a vessel. The level of

liquid can be measured directly by monitoring the position of the liquid surface or

indirectly by measuring some variable related to the height.

Direct method: A float system uses displacement of float caused by increasing or

decreasing water level in vessel this displacement rotate lever arm in potentiometer.

Results in output voltage related to the height of liquid

Indirect method: This method includes the monitoring of the weight of a vessel

by using load cells. The weight of the liquid is Ahρg. Where, A = cross-section

area of the vessel, h = height of liquid, ρ = density of liquid, g = acceleration due to

gravity.

Temperature Measuring Sensors:

These are the devices which are used to monitor the change in a material by

means of expansion or contraction of solids, liquids or gases, the change in electric

resistance of conductors and semiconductors and thermoelectric e.m.f.s.

Temperature sensor can be grouped as:

1. Bimetallic strip or Thermostat

2. Resistance temperature detectors (RTD)



3. Thermistors

4. Thermo couples

5. Thermo-diodes and transistor

1. Bimetallic strip or Thermostat:

These device consists of two different metal strips having different

coefficients of expansion are bonded together. When the temperature changes the

composite strip bends into a curved strip with the higher coefficient metal on the

outside of the curve. This deformation may be used as a temperature controlled switch

in a thermostat with the domestic heating system. As the bimetallic strip bends due to

change in temperature, the small magnet attracts and enables the sensor to exhibit

hysteresis i.e. switch contact closes at a different temperature from that at which they

open.

2. Resistance Temperature Detector (RTDs):

These are the simple resistive elements like platinum, nickel, etc. , in the form

of coils of wire, whose resistance increases with increase in temperature linearly. This

variation of resistance with tempetature for metals can be used to detect the value of

temperature by using the relation:

Rt = R0 (1 + αt)

Where, Rt is the resistance at a temperature t°C

R0 is the resistance at 0°C



3. Thermistors:

Thermistor are small pieces of material made from the mixture of metal oxides

called as semiconductors in the form of discs, and rods. It is used to measure

temperature changes, relying on the change in its resistance with changing

temperature. In the range of (-40 ‒ 150)°C with accuracy = ±0.35 .

Thermistors are of two types: NTC, negative thermal coefficient, and PTC,

positive thermal coefficient. The resistance of NTC thermistors decrease with increase

in temperature and the resistance of PTC thermistors increase with increases in

temperature.

4. Thermocouple:

A thermocouple is a junction of two different metals joined together. If both

the junctions are at the same temperature then, there is no e.m.f. across the junction. If

there is a difference in temperature between the two junctions, then there is an e.m.f.

across the junction. The value of e.m.f. E depends on the temperature difference

between two metals. Usually one junction is held at 0°C.

5. Thermodiodes and Transistors:

A junction semiconductor diode is widely used as a temperature sensor. When

the temperature of doped semiconductor changes, the mobility of their charge carriers

changes which in turn affects the rate at which the electrons and holes can diffuse

across a p-n junction and sets potential difference V across it which produces the

current I through the junction.

I = I0[ e (eV/Kt-1)]

Where I is the current produced corresponds to the change in temperature T.



Light Sensors:

These are the sensors which detect the presence of light, when light falls on

these sensors by converting light signals into electrical signals. These sensors are also

called as photo-electric transducers.

Photodiodes:

These are the semiconductor junction diodes which are connected into circuit

in reverse bias, giving high resistance. Therefore when light falls on the junction the

diode resistance drops and the current in the circuit increases considerably. These are

variable resistance device controlled by light incident.

Phototransistors:

These are the light sensitive collector-base p-n junction. When there is no

incident light there is a very small collector-to-emitter current. When light is incident,

a base current is produced which is proportional to the light intensity and produces a

collector current which is the measure of light intensity.

A photoresistor:

A photoresistor has a resistor depends on the light intensity falling on it. The

resistance decreases linearly with increase in light intensity. For particular value of

light intensity corresponding decrease in resistance is the measure of light intensity.



Selection of sensors: Following factors to be considered while selecting the

sensors:

1. The nature of the measurement required.

2. Type of output required.

3. Environmental conditions under which the measurement is to be carried out.

4. Level of resolution required.

5. Accuracy, linearity, speed of response, reliability, and maintainability

required.

6. Life and cost of sensor.

7. Power supply required.

8. Ruggedness required etc.

Documents

Mechatronics 15ME54T 2020 - 2021 Mechatronics unit 1