Unit I MECHATRONICS, SENSORS AND TRANSDUCERS 1.1

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ME 2401 - MECHATRONICS

Unit – I MECHATRONICS, SENSORS AND TRANSDUCERS

1.1 INTRODUCTION TO MECHATRONICS:

Consider the modern auto-focus, auto-exposure camera. To use the camera all you

need to do is point it at the subject and press the button to take the picture. The camera

automatically adjusts the focus so that the subject is in focus and automatically

adjusts the aperture and shutter speed so that the correct exposure is given.

Consider a truck smart suspension. Such suspension adjusts to uneven loading to

maintain a level platform, adjusts to cornering, moving across rough ground. etc. to

maintain smooth ride.

Consider an automated production line. Such a line may involve a number of

production processes which are all automatically carried out in the correct sequence

and in the correct way.

The automatic camera, the truck suspension and the automatic production line are

examples of a marriage between electronic control systems and mechanical

engineering. Such control systems generally use microprocessors as controllers arid

have electrical sensors extracting information from the mechanical inputs and

outputs via electrical actuators to mechanical systems.

The term mechatronics is used for this integration of microprocessor control systems,

electrical systems and mechanical system. A mechatronics system is not just a marriage

of electrical and mechanical systems and is more than just a control system; it is a

complete integration of all of them.

In the design now of cars, robots, machine tools, washing machines, cameras, and very

many other machines, such an integrated and interdisciplinary approach to engineering

design is increasingly being adopted Mechatronics has to involve a concurrent approach

to these disciplines rather than a sequential approach of developing, say, a mechanical

system then designing the electrical part and the microprocessor part.

1.1.1 Definition:

“Mechatronics is the synergetic integration of mechanical engineering with

electronics and intelligent computer control in the design and manufacturing of industrial

products and processes”.

www.Vidyarthiplus.com


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1.2 SYSTEMS:

Mechatronics involves what are termed as systems. A system can be thought of as a

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

inside the box but only the relationship between the output and the input. Thus for

example, a motor may be thought of as a system which has as input electric power and as

output the rotation of a shaft.

a) Example: A Motor.

A motor has input as electric power as input and rotation as output. The following

figure shows the representation.

Fig: System

Basically in mechatronics we divide the systems in to 2 types

1. Measurement System.

2. Control System.

Now we will discuss in detail about these 2 systems.

1.2.1 MEASUREMENT SYSTEM:

A Measurement system can be defined as a black box which is used for making

measurements. It has an input the quantity being measured and its output the value of that

quantity.

Example: A temperature measurement system. i.e. Thermometer

Fig: Measurement system



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1.2.1.1 Elements of Measurement system

Measurement System can be considered to be made up of three elements as shown in figure.

1. A sensor which responds to the quantity being measured by giving as its output a

signal which is related to the quantity. Ex. a thermocouple is a temperature sensor.

2. A signal conditioner takes the signal from the sensor and manipulates it into a

condition which is suitable for either display or in the case of a control system, for use to

exercise control. Thus for example the output from a thermocouple is a rather small

e.m.f and might be fed through an amplifier to obtain a bigger signal. The amplifier is

the signal conditioner.

3. A display system where the output from the signal conditioner is displayed. This

might, for example be a pointer moving across a scale or a digital readout.

As an example, consider a digital thermometer. This has an input of temperature to

a sensor probably a semiconductor diode. The potential difference across the sensor is a

constant current.

1.2.2 CONTROL SYSTEM:

A control system can be defined as a block box which can be used to control its

output to some particular value.

Example: A domestic central heating control system.

We can set the required temperature on the thermostat or controller and the pump

can be adjusted to supply water through radiators. So the required temperature can be

maintained in the house.



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In a system when the output quantity is controlled by varying the input quantity then

the system is called as Control system.

The output quantity is called as controlled variable or response and the input quantity

is called as command signal or excitation.

In Control system, we have two types

1. Open loop control system. 2. Closed loop control system.

1.2.1.1 OPEN AND CLOSED-LOOP SYSTEMS:

There are two basic forms of control system one being called and Open loop and

other closed-loop systems. The difference between these can be illustrated by a simple

example.

Consider an electric fire which has a selection switch which allows a 1 KW or a 2

kW heating element to be selected. If a person used the heating element to heat a room, he

or she might just switch on the 1 kW element if the room is not required to be at too high a

temperature.

The room will heat up and reach a temperature which is only determined by the fact

the 1 kW element was switched on, and not the 2 kW elements. If there are changes in

the conditions perhaps someone opening a window, there is no way the heat output is

adjusted to compensate.

This is an example of open loop control in that there is no information fed back to the

element to adjust it and maintain a constant temperature.



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The heating system with the heating element could be made a closed loop system if

the person has a thermometer and switches the 1 kW and 2 kW elements on or off,

according to the difference between the actual temperature and the required temperature, to

maintain the temperature of the room constant.

In this situation there is feedback, the input to the system being adjusted

according to whether its output is the required temperature. This means that the input to the

switch depends on the deviation of the actual temperature from the required temperature.

The difference between them determined by a comparison element. The person in this case.

Illustration of a motor:

To illustrate further the differences between open and closed-loop systems,

consider a motor.

With an open-loop system the speed of rotation of the shaft might be determined

solely by the initial setting of a knob which affects the voltage applied to the motor.

Any changes in the supply voltage, the characteristics of the motor as a result of

temperature changes, or the shaft load will change the shaft speed but not be compensated

for.

There is no feedback loop. With a closed-loop system, however, the initial setting of

the control knob will be for a particular shaft speed and this will be maintained by feedback,

regardless of any changes in supply voltage, motor characteristics or load.

In an open-loop control system the output from the system has no effect on the

input signal. In a closed-loop control system the output does have an effect on the input

signal, modifying it to maintain an output signal at the required value.



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OPEN-LOOP SYSTEMS have the advantage of being

Relatively simple and

Consequently low cost with generally good reliability.

However, there are disadvantages like,

Inaccurate since there is no correction for error.

CLOSED-LOOP SYSTEMS have the advantage of being

Relatively accurate in matching the actual to the required values.

However, there are disadvantages like,

More complex

So more costly and

A greater chance of breakdown as a consequence of the greater number of

components.

BASIC ELEMENTS OF A CLOSED-LOOP SYSTEM:

Generally the closed loop system consists of the following elements

1. Comparison element.

2. Control element.

3. Correction element.

4. Process dement

5. Measurement element.

1. Comparison element

This compares the required or reference value of the variable condition being controlled

with the measured value of what is being achieved and produces an error signal.

It can be regarded as adding the reference signal, which is positive, to the

measured value signal, which is negative in this case:

Error signal = reference value signal - measured value signal.

The symbol used, in, general, for an element at which signals are summed is a

segmented circle, inputs going into segments.



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The inputs are all added; hence the feedback input is marked as negative and the

reference signal positive so that the sum gives the difference between the signals.

2. Control element

This decides what action to take when it receives an error signal.

It may be for example, a signal to operate a switch or open a valve.

The control plan being used by the element may be just to supply a signal which

switches on or off when here is an error, as in a room thermostat or perhaps a signal

which proportionally opens or closes a valve according to the size of the error.

3. Correction element

The correction element produces a change in the process to correct or change the

controlled condition.

Thus it might be a switch which switches on a heater and so increases the temperature of

the process or a valve which opens and allows more liquid to enter the process.

The term actuator is used for the element of a correction unit that provides the power to

carry out the control action.

4. Process element

The process is what is being controlled. It could be a room in a house with its

temperature being controlled or a tank of water with its level being controlled.

5. Measurement element

The measurement element produces a signal related to the variable condition of the

process that is being controlled.

For example, a switch which is switched on when a particular position is reached or a

thermocouple which gives an e.m.f related to the temperature.

FOR TEMPERATURE CONTROLLED CLOSED LOOP SYSTEM

With the closed-loop system illustrated in Fig. above, for a person controlling

the temperature of a room, the various elements are:

Controlled variable - the room temperature

Reference value - the required room temperature

Comparison element - the person comparing the measured value with the required

value of temperature

Error signal - the difference between the measured and required

temperatures.

Control unit - the person

Correction unit - the switch on the fire

Process - the heating by the fire



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Measuring device - a thermometer

AUTOMATIC WATER LEVEL CONTROLLER:

Fig. The automatic control of water level

An automatic control system for the control of the room temperature could involve a

temperature sensor, after Suitable signal conditioning, feeding an electrical signal to the

input of a computer where it is compared with the set value and an error signal generated.

This is then acted on by the computer to give at its output a signal, which, after suitable

signal conditioning, might be used to control a heater and hence the room temperature. Such

a system can readily be programmed to give different temperatures al different times of

the day.

The above figure shows an example of a simple control system used to maintain a

constant water level in a tank. The reference value is the initial setting of the lever arm

arrangement so that it just cuts off the water supply at the required level. When water is

drawn from the tank the float moves downwards with the water level. This causes the

lever arrangement to rotate and so allows water to enter the tank. This flow continues

until the ball has risen to such a height that it has moved the lever arrangement to cut off the

water supply. It is closed loop control system with the elements being:

Controlled variable - the water level in the tank

Reference value - initial setting of the float and lever position

Comparison clement - the lever

Error signal - the difference between the actual and initial

settings of the lever positions

Control unit - the pivoted lever

Correction unit - the flap opening or closing the water supply

Process - the water level in the tank

Measuring device - the floating ball and lever



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1.3 SEQUENTIAL CONTROLLERS:

When a controller operates in a sequence way i.e. Step by step, then that type of

controllers is called as sequence controllers.

In sequential controllers, step 2 is started only after completing step 1 and after

completing step 2 step 3 will be started. In sequential controllers, the control .actions are

ordered in time, which is obtained by an electrical circuit with sets, of relays or cam operated

switches which are wired up in such a way as to give the required sequence.

Now-a-days hardwired circuits and relays are replaced by a microprocessor controlled

system, the sequencing are controlled by software program

Example: Washing machine.

Consider a washing machine; the numbers of sequential operations carried out are,

1. Pre wash cycle -the clothes in the drum are washed with cold water.

2. Main wash cycle - the clothes are washed with hot water.

3. Rinse cycle - the washed clothes are rinsed with cold water number of times.

4. Spinning - the rinsed clothes are spinned to remove water.

The above figure shows the basic washing machine system and gives a rough idea of

its constituent elements.



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The system that is used for the washing machine controller was a mechanical system

which involved a set of cam-operated switches, i.e mechanical switches. Figure below show

the basic principle of one such switch.

When the machine is switched on, a signal electric motor slowly rotates its shaft,

giving an amount of rotation proportional no tune. The rotation turns the controller

cams so that each in turn operates electrical switches and so switches on circuits in the

correct sequence. The contour of a cam determines the time at which it operates a switch.

The contours of the cams and the means by which the program is specified and

stored in the machine. The sequence of instructions and the instructions used in a particular

washing program are determined by the set of cams chosen.

With modern washing machines the controller is a microprocessor and the

program is not supplied by the mechanical arrangement of cams but by a software program.

For the pre-wash cycle an electrically operated valve is opened when a current is supplied

and switched off when it ceases. This valve allows cold water into the drum for a period of

time determined by the profile of the cam or the output from the microprocessor used to

operate its switch.

However, since the requirement is a specific level of water in the washing

machine drum, there needs to be another mechanism which will stop the water going into

the tank, during the permitted time, when it reaches the required level.

A sensor is used to give a signal when the water level has reached the preset level

and give art output front the microprocessor which is used to switch off the current to

the valve. In the case of a cam-controlled valve, the sensor actuates a switch which closes

the valve admitting water to the washing machine drum.

When this event is completed die microprocessor, or the rotation of the cams,



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initiates a pump to empty the drum.

For the main wash cycle, the microprocessor gives an output which starts when lie

pre-wash part of the program is completed: in the case of the cam-operated system

the cam has a profile such that it starts in operation when the pre-wash cycle is completed.

It switches a current into a circuit to open a valve to allow cold water into the drum. This

level is sensed and the water shut off when tine required level is reached.

The microprocessor or cam then supplies a current to activate a switch which

applies a larger current to an electric heater to heat the water. A temperature sensor is

used to switch off the current when the water temperature reaches the preset value.

The microprocessor or cams then switch on the drum motor to rotate the drum. This

will continue for the time determined by the microprocessor or cam profile before switching

off. Then the microprocessor or a cam switches on the current to a discharge pump to empty

the water from the drum.

The rinse part of the operation is now switched as a sequence of signals to open

valves which allow cold water into the machine. Switch it off, operate the motor to rotate

the drum, operate a pump to empty the water from the drum, and repeat this sequence a

number of times.

1.4 MICROPROCESSOR BASED CONTROLLERS:

Microprocessors are rapidly replacing the mechanical cam operated controllers. These

microprocessors are used to control the function. In many simple systems, an embedded

micro controller is used to control or perform the particular task.

A more adaptable form of controller is the programmable logic controller. The

programmable logic controller is defined as a sequential logic device that generates output

signals according to logic operations performed on the input signals. The PLC is a

microprocessor based controller which uses programmable memory to store instructions and

to implement functions such as logic sequence, timing counting and arithmetic to control

events. This PLC can be easily reprogrammed for different tasks. The PLC is shown below.



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The example for input devices are switches relays and limit switches. The examples

for output devices are motor to be controlled, Lamp, relay and solenoid. The controller

monitors the inputs and outputs according to the program stored in the PLC by the operator.

PLC are similar to computers but have certain features which are specific to their use of

controllers. These are,

1. They are rugged and designed to withstand vibrations, temperature, humidity and noise.

2. The interfacing for inputs and outputs is inside the controllers.

3. They are easily programmed and have an easily understood programming language. The

Programming is primari1y concerned with logic and switching operations.

Note:

Micro controller: - Microprocessor with integrated peripherals is called as micro controller

Some of the microprocessor based control system is discussed below.

1.4.1 AUTOMATIC CAMERA:

The modern camera is likely to have automatic focusing and exposure. Figure 1.10

illustrates the basic aspects of a microprocessor-based system that can’ t be used to

control the focusing and exposure.

When the switch is operated to activate the system and the camera pointed at the object

being photographed, the microprocessor takes the input from the range sensor and sends

an output to the lens position drive to move the lens to achieve focusing. The lens

position is fed back to the microprocessor so that the feedback signal can’ t be used to

modify the lens position according to the inputs from the range sensor.

The light sensor gives an input to the microprocessor which then gives an output to



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determine, if the photographer has selected the shutter controlled rather than aperture

controlled mode, the time for which the shutter will be opened. When the photograph

has been taken, the microprocessor gives an output to the motor drive to advance the

film ready for the next photograph.

The program for the microprocessor is a number of steps where the

microprocessor is making simple decisions of the form: is there an input signal of a

particular input line or not and if there is output a signal on a particular output line. The

decisions are logic decisions with the input and output signals either being low or

high to give on-off states.

A few steps of the program for the automatic camera might be of the form:

begin

if battery check input OK

then continue

otherwise stop

loop

read input from range sensor calculate lens movement

output signal to lens position drive

input data from lens position encoder

compare calculated output with actual output stop output when lens in correct

position

send in-focus signal to viewfinder display

etc.



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1.4.2 THE ENGINE MANAGEMENT SYSTEM:

The engine management system of a car is responsible for managing the ignition and

fuelling requirements of the engine.

With a four-stroke internal combustion engine there are several cylinders, each of which

has a piston connected to a common crankshaft and each of which carries out a four-

stroke sequence of operations.

When the piston moves down a valve opens and the air-fuel mixture is drawn into the

cylinder.

When the piston moves up again the valve closes and the air-fuel mixture is

compressed.

When the piston is near the top of the cylinder the spark plug ignites the mixture with a

resulting expansion of the hot gases. This expansion causes the piston to move back

down again and so the cycle is repeated.

The pistons of cacti cylinder are connected to a common crankshaft and their power

strokes occur at different times so that here is continuous power for rotating the

crankshaft.

Fig. Four Stroke Sequence

The power and speed of the engine are controlled by varying the ignition timing and the

air - fuel mixture.

With modem car engines this is done by a microprocessor. Figure shows the basic

elements of a microprocessor control system.



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For ignition timing, the crankshaft drives a distributor which makes electrical contact

for each spark plug in turn and a timing wheel. This timing wheel generates

pulses to indicate he crankshaft position.

The microprocessor then adjusts the timing at which high voltage pulses are sent to the

distributor so they occur at the right moments of time.

To control the amount of air fuel mixture entering a cylinder during the intake strokes,

the microprocessor varies the time for which a solenoid is activated to open the intake

on the basis of inputs received of the engine temperature and the throttle position.

The amount of fuel to be injected into the air stream can be determined by an

input from a sensor of the mass rate of air flow, or computed from other measurements, and

the microprocessor then gives an output to control a fuel injection valve.



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1.5 MECHATRONICS APPROACH:

The domestic washing machine that used cam operated switches in order to control

the washing cycle is out of date. Such mechanical switches are being replaced by

microprocessor. A microprocessor may be considered as being essentially a collection of

logic gates and memory elements that are not wired up is individual components but whose

logical functions are implemented by means of software.

The microprocessor- controlled washing machine can be considered an example of a

mechatronics approach in that a mechanical system has become integrated with electronic

controls. As a consequence, a bulky mechanical system is replaced by a much more compact

microprocessor system which is readily adjustable to give a greater variety of programs.

Mechatronics brings together a number of technologies like, mechanical engineering,

Electronic Engineering, electrical engineering, information technology, computer technology

and control engineering. This can be considered as the application of Computer based digital

control techniques, through electronic and electric interfaces to mechanical engineering

problems.

There are many applications of mechatronics in the mass produced products used in

home. Microprocessor based controllers are to be found in domestic washing machines, dish

washers, microwave ovens, cameras, camcorders, watches, hi-fi and video recorder systems,

central heating controls, sewing machines, etc.. They are to be found in cars in the active

suspension, antiskid brakes, engine control, speedometer display, transmission etc. A large

scale application of mechatronics is a flexible manufacturing engineering system (FMS)

involving computer controlled machines, robots, automatic material conveying and overall

supervisory control.



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PART – A –TWO MARK QUESTIONS 1. Write about Mechatronics?

2. What are the components in a Mechatronics system?

3. What is the use of actuators and sensors?

4. What is the use of digital devices?

5. What is the function of conditioning and interfacing Circuits and graphical displays?

6. Give some examples of Mechatronics systems?

7. What are the important sub-systems involved in Mechatronic system?

8. What is the use of control system?

9. What are the important elements of measurement system?

10. What is the function of sensor?

11. What is the function of signal conditioner?

12. What is the use of Display system?

13. How the control system is classified?

14. What is meant by open loop control system?

15. What is meant by closed loop control system in CNC machine?

16. What are the import elements of a closed loop control system?

17. What is the use of comparison element?

18. What is meant by error signal?

19. What is the use of control element?

20. What is the function of the correction element?

21. What is meant by process element?

22. What is meant by sequence control?

23. Why mechatronic systems are also known as smart devices?

PART – B QUESTIONS

1. Explain the closed loop system with example.

2. What are the basic components of closed loop system? Explain.

3. Describe the sequential controllers.

4. Explain the microprocessor controlled automatic camera.

5. Explain the microprocessor controlled engine management system.

6. Explain the mechatronics approach with its advantages.



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1.0 INTRODUCTION:

SENSORS AND TRANSDUCERS

+ Sensor is used to produce a varying signal according to the quantity

being measured.

+ Sensor is an element in a mechatronic system which acquires a physical parameter and changes it into signal that can be processed by the system.

+ The active element of a sensor is known as transducer.

+ Transducer converts the measured quantity, property (or) condition

into a usable electrical output.

+ The mechatronic system requires sensors to measure physical

quantities such as position, distance, force, strain, temperature, vibration and acceleration. Simply sensors are also called transducers.

2.0 PERFORMANCE TERMINOLOGY:

+ The function of the sensor (or) transducer is to sense (or) detect a parameter such as pressure, temperature flow, motion, resistance,

voltage, current and power.

+ The sensor should be capable of faithfully and accurately detecting

any changes that occur in the measured parameter.

+ The performance of transducers can be defined by using the following terms:

1. Range and span

2. Error

3. Accuracy

4. Sensitivity

5. Hysteresis error

6. Non linearity error

7. Repeatability/Reproducibility

8. Reliability

9. Stability

10. Dead band/time

11. Resolution

12. Backlash

13. Output impedance



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1. Range and Span:

+ The range of a transducer defines the limits between which the input can vary.

+ The difference between the limits (maximum value - minimum value) is known as span.

+ For example a load cell is used to measure force. An input force can vary from 20 to 100 N. Then the range of load cell is 20 to 100 N. And the span of load

cell is 80 N (i.e., 100-20)

2. Error:

+ If the transducer is ideally designed and made from appropriate materials with ideal workmanship, then output will indicate the true value. But in

actual practice the output of the transducer will deviate from the true value.

+ The algebraic difference between the indicated value and the true value of the

measured parameter is termed as the error of the device.

+ Error = Indicated value —true value

+ For example, if the transducer gives a temperature reading of 30° C when the actual temperature is 29° C, then the error is + 1° C. If the actual temperature is

3 1° C, then the error is —1° C.

3. Accuracy:

+ Accuracy is the extent to which the value indicated by the measurement system would be wrong.

+ Accuracy is the summation of all possible errors that are likely to occur.

+ For example, a thermocouple has an accuracy of ± 1° C. This means that reading given by the thermocouple can be expected to lie within + 1° C (o r) —

1° C of the true value.

+ Accuracy is also expressed as a percentage of the full range output (or) full- scale deflection.

+ For example, a thermocouple can be specified as having an accuracy of ±4

% of full range output. Hence if the range of the thermocouple is 0 to 200°

C, then the reading given can be expected to be within + 8° C (or) —8° C of the true reading.

4. Sensitivity:

+ The sensitivity is the relationship showing how much output we can get per

unit input.

+ ie sensitivity = Output / Input

5. Hysteresis error:

+ When a device is used to measure any parameter plot the graph of output Vs value of measured quantity.

+ First for increasing values of the measured quantity and then for decreasing

values of the measured quantity.

+ The two output readings obtained usually differ from each other.



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Fig.1.1 Hysteresis error

+ This is because of a certain amount of internal (or) external friction in the

response of the sensing element.

+ The maximum difference in between any part of output readings so obtained is known as hysteresis error.

+ The hysteresis error can be reduced by proper design and selection of

the mechanical components, introducing greater flexibility and providing

suitable heat treatment to the materials.

6. Non-linearIty error:

+ A linear relationship is assumed between the input and output and hence,

a straight line is drawn in the graph as shown here.

Fig.1.2

+ Some transducers, do not have linear relationship and errors occur as a

result of the assumption of linearity.

+ The error is defined as the maximum difference from the straight line.

+ There are three methods to find the the numerical error. They are

namely, (i) End range value

(ii) Best straight line for all values

(iii) Best straight line, through zero point.

+ In the first method, (fig 1.2), the straight line is drawn by joining the output values at the end points of the range.



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+ In the next method, the straight line is drawn by using the method of least

squares to determine the best fit line by considering all data values are in error. Refer fig (1.3).

+ In the last method, the straight line is drawn by using the method of

least squares to determine the best fit line which passes through the zero

point.

Fig.1.3 Fig.1.4

7. Repeatability/Reproducibility:

+ The repeatability and reproducibility of a transducer are its ability to give the same output for repeated applications of the same input value.

+ Repeatability is also defined as the measure of the deviation of test results mean value.

8. Reliability:

+ The reliability of a system is defined as the possibility that it will perform its

assigned functions for a specific period of time under given conditions.

+ The reliability of a device (or) system is affected not only by the choice of individual parts in system but also by manufacturing methods, quality of

maintenance and the type of user.

9. Stability:

+ The stability of a transducer is its ability to give the same output when used to measure a constant input over a period of time.

+ The term drift is the change in output that occurs over time.

+ The drift can be expressed as a percentage of the full range.

+ Zero drift means if there is change in output when there is zero input.

10. Dead band / time:

+ There will be no output for certain range of input values. This is known as

dead band. There will be no output until the input has reached a particular value.

+ The length of time from the application of an input until the output begins to respond and change is known as Dead time.



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11. Resolution:

+ Resolution is defined as the smallest increment in the measured value

that can be detected.

+ The resolution is the smallest change in the input value which will produce an observable change in the input.

+ Resolution is also known as the degree of fineness with which

measurements can be made.

+ For example, if a micrometer with a minimum graduation of 1mm is.

used to measure to the nearest 0.5 mm, then by interpolation, the resolution is estimated as 0.5 mm.

12. Backlash:

+ Backlash is defined as the maximum distance (or) angle through

which any part of a mechanical system can be moved in one

direction without causing any motion of the attached part.

+ Backlash is an undesirable phenomenon and is important in the precision design of gear trains.

13. Output Impedance:

+ Before defining impedance, we should know about Ohm’ s law.

+ Ohm’ s law is used to define the relationship between voltage V, Current I and

Resistance R.

(i.e.,) V=IR

+ Ohm’ s law can be extended to the AC circuit analysis of resistor, capacitor and inductor elements as

v=ZI

where Z is called impedance of the elements. So impedance is similar to resistance.

+ The sensors produce electrical output.

+ When these sensors are interfaced with an electronic circuit, it is necessary to know the output impedance.

+ This impedance is connected in either series (or) parallel with that

circuit and the inclusion of the sensor will modi1 the behaviour of the

system to which it is connected.

3.0 DISPLACEMENT, POSITION AND PROXIMITY

Displacement Sensors:

The measurement of the amount by which some object has been moved. 1. Potentiometer,

2. Resistance strain gauge,

3. LVDT, 4. Push pull displacement sensor.



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Position Sensors:

+ The determination of the position of some object with reference to

some reference point.

1. Photo electric sensors, 2. H

sensors.

Proximity Sensors:

+ Used to determine when an object has moved to within some particular

critical distance.

1. Pneumatic proximity sensor, 2. Eddy current proximity sensor,

3. Inductive proximity switch, 4. Micro switch,

5. Reed switch.

Factors to be considered while selecting displacement, Position and Proximity sensors:

1. The accuracy required 2. The resolution required

3. The size of the displacement

4. Displacement type (linear or angular)

5. The cost and material made

1. Contact Sensors:

+ The measured object is mechanical contact with the sensor. + In the contact sensors there is a sensing shaft which is direct contact

with the object being monitored.

+ The movement of the shaft may be used to make changes in electrical

voltage, capacitance, resistance.

2. Non-contact sensors:

+ The measured object is no physical contact between the measured

object and the sensor. + In the non-contact sensors the measured object causing a change in the air

pressure in the sensor, or a change in inductance or capacitance.

3.1 DISPLACEMENT SENSORS:

+ A potentiometer can be used to convert rotary or linear

displacement to a voltage.

+ The potentiometers can be classified into three types.



32

1. Potentiometer Sensor

+ Potentiometers consists of a resistance element with a sliding contact

and the sliding contact can be moved over the length of the

element. This sliding contact is called Wiper.

+ The motion of the sliding contact may be linear or rotational. + The Fig.1.5 shows the linear potentiometer and the Fig.1.6 shows the

rotary potentiometer.

+ The rotary potentiometer consists of a circular wire-wound track over which a rotatable sliding contact can be rotated.

+ The wire-wound track may be single turn or helical turn.

Displacement and Position Sensor Types:

The displacement and position sensors are grouped into: 1. Contact sensors

2. Non-contact sensors 1. Rotary

2. Linear 3. Helical potentiometers

Fig.1.5

Fig.1.6



33

Fig.1.7

Advantages of Resistance Potentiometers:

1. They are simple and in expensive, 2. Electrical efficiency is high,

3. Simple in operation.

4. Useful for measurement of large amplitudes of displacement

2. Strain Gauged Element:

+ The change in length divided by original length is called strain. + The strain gauge consists of metal wire, metal foil strip. When

subject to strain, the resistance ‘ R’ changes, and the change in

resistance L is proportional to strain E.

where G is a constant (gauge factor).

+ In the Fig.1.9 the strain gauge is attached to flexible elements in the form of cantilevers, rings, U shapes.

+ When the flexible element is bent, as a result of this the electrical

resistance will change due to force applied by a contact point.

+ The change in resistance is the measure of displacement.

+ The Fig.1.8 and 1.9 shows the strain gauges and strain gauged elements. + The major types of strain gauges are

I. Metal wire strain gauges,

2. Metal foil strain gauges,



34

3. Semiconductors strain gauges.

Fig.1.8

Fig.1.9



35

+ In Metal Wire Strain Gauges a wire stretched between two points

in an insulating medium such as air.

+ The wires may be made of various copper nickel, chrome nickel or nickel

iron alloys. They are about 0.003 mm in diameter and gauge factor of 2. The length of wire is 25 mm or less.

+ In Metal foil strain gauge the foil is usually made up of constantan, and it is etched in a grid pattern onto a thin plastic backing material, usually

polyimide. The foil is terminated at both ends with large metallic pads.

+ The size of the entire gauge is very small and has a length of 5 mm to

15 mm length.

+ In Semiconductor strain gauges the p type and n type silicon semiconductors are used.

+ The semiconductor strain gauges have the gauge factors of about +100 or —

100. In p-type gauges resistance increases with tensile strain. While in n-type,

resistance decreases. Typical thickness is about 0.25 mm and effective length range from 1.25 to 12 mm.

4. Linear Variable Differential Transformer (LVDT)

Fig.1.10

Fig.1.11



36

+ It consists of three coils symmetrically spaced along an insulated tube.

+ The central coil is primary and other two are secondary. + A magnetic core is moved through the central tube, so that the

displacement being monitored.

+ When voltage is supplied to the primary coil, alternating e.m.f.s are induced in the secondary coils.

+ Suppose the magnetic core is in central, the e.rn.f. induced in each coil

is same because of magnetic material in each coil is same and oppose

to each other. So there is no output. + If the core is displaced from the central position there is a greater

amount of magnetic core in one coil than the other. This will create a

higher e.m.f. in one coil and lesser e.m.f. in the other coil. This will

make a net difference in two e.m.f.s and the displacement being

monitored. + The formulas which are used in LVDT are:

1. The e.m.f.s induced in the two secondary coils 1 and 2 are:

where K1, K2 are degree of coupling between the primary and secondary coils.

Advantages of L VDT:

1. High range

2. Friction and electrical isolation

3. Low hysteresis

4. Power consumption is less.

5.Push Pull Displacement Sensor:

+ It has three plates with the upper pair forming one capacitor and the

lower pair forming another capacitor.

+ There is a non-linear relationship form between the change in capacitance AC

and the displacement X.

+ The displacement moves the central plate between the two other plates. + The result of this, the central plate moving downwards and to

increase the plate separation of the upper capacitor and decrease the separation of the lower capacitor.

+ Therefore, the capacitance of a parallel plate capacitor is given by

where C1 is in one arm of an a.c. bridge,



37

C2 is in an other arm of an a.c. bridge.

= Relative permittivity of the dielectric between the

= Permittivity of free space

constant, x = Displacement,

A = The area of overlap between the two plates, d = The plate separation.

3.2 POSITION SENSORS

+ position sensors report the position of an object with respect to a reference part.

+ The information can be an angle as in many degree a dish antenna has turned. + The following are the position sensors.

1. Photoelectric Sensors

+ It is used to detect the object by breaking a beam of light (Refer Fig.1.12(a)) or radiation falling on a device or by detecting the light

reflected back by the object (Refer Fig.1.12(b)).

Fig.1.12

2. Hall effect Sensors

+ Hall effect: Hall effect is defined as when a beam of charged particles

passes through magnetic field, the beam is deflected from its straight line path due to the forces acting on the particles.

+ A current flowing in a conductor like a beam is deflected by a magnetic field.

Fig.1.13

+ The working principle of a Hall effect sensor is that if a strip of

conducting material carries a current in the presence of a transverse ngne1i shown in Fig.1.13.



38

+ The difference of potential is produced between the opposite edges

of the conductor. The magnitude of the voltage depends upon the

current and magnetic field. + In the Fig. the current is passed through leads 1 and 2 of the strip. The

output leads connected with Hall strip.

+ When a transverse magnetic field passes through the strip the

voltage difference occur in the output leads.

+ The hail effect sensor have the advantages of being able to operate as switches and it operate upto 100 KHz.

Fig.1.14

Applications of Hall Effect Sensors:

1. It is used as a Magnetic to electric transducer.

2. It is used for the measurement of the position or displacement of a

structural element. 3. It is used for measurement of current.

4. It is used for measurement of power.

Digital Optical Encoder:

+ A digital optical encoder is a device that converts motion into a

sequence of digital pulses. + By counting or decoding these bits and the pulses can be converted into

relative or absolute position measurements.

+ Encoders are in Rotary, linear configurations.

+ The Rotary encoders are in two forms. 1. Absolute encoder 2. Incremental encoder.

1. Absolute Encoder:

+ The absolute encoder is designed to produce a digital word that distinguishes

‘ N’ distinct positions of the shaft.



39

Fig. 1.15. Components of an optical

encoder

+ The Fig.1.15 shows the basic form of an absolute encoder. + The rotating disc has four concentric circles of slots and four sensors to

detect the light pulses.

+ The slots are arranged in such a way that the output is made in the binary

code.

+ The number of bits in the binary number will be equal to the number of tracks.

+ The most common types of numerical encoding used in the absolute encoder are gray and natural binary codes.

+ To illustrate the action of an absolute encoder, the gray code and

natural binary code disk track patterns for a simple 4 track (4-bit) encoder is shown in Fig.1.16

Fig. 1.16 4-bit gray code absolute encoder disk track patterns

2. Incremental Encoder:

+ Working: A beam of light passes through the slots in a disc and it is detected by a suitable light sensor.

+ When the disc is rotated, the output is shown in terms of pulses and these pulses being proportional to the angle of disc rotation.



40

Fig. 1.17. Incremental

encoder

+ So the angular position of the disc is determined by the number of

pulses produced. In the above Fig. three tracks and three sensors are

used. + The inner track has just one hole and other two tracks have a series of equally

spaced holes.

+ The angle is determined by the number slots on the disc.

3.3 PROXIMITY SENSOR

+ A proximity simply tells the contra! system whether a moving part is

at a certain place. + Proximity sensors come under the non contact type sensors.

+ The following are the some of the proximity sensors.

1. Pneumatic proximity sensor:

+ Working: Low-pressure air is allowed and to escape through a port which is placed in the front position of the sensor. This escaping air reduces the pressure in the nearby sensor output port, when there is no close by object.

Fig. 1.18. Pneumatic proximity

sensor



41

+ If there is a close by object means the air will not escape readily,

so the pressure increases in the sensor output port. This output from the sensor depends on the proximity of objects.

2. Eddy current proximity sensors:

Fig. 1.19. Eddy current

sensor

+ Working: When alternating current is supplied to the coil

means the alternating magnetic field is produced. If there is a metal

object in close proximity to this alternating magnetic field the eddy

current is induced in it. This eddy current will produce a magnetic field

themselves and the impedance of the coil changes the amplitude of the

alternating current.

+ The above Fig. shows the basic form of such sensor and it is used

for the detection of non-magnetic conductive materials.

3. Inductive proximity switch:

+ It is used for the detection of metal objects and it consists of a coil wound around a core.

+ The metal object is close to the coil means it will produce a inductance

change in the coil. This inductive change is being monitored.

4. Microswitch:

+ It is used for determining the presence of an item on a conveyor belt

and this might be actuated by the weight of the item on the belt depressing the belt by a spring loaded platform nearer to the sensor

the presence of item in the conveyor is determined. + The closeness of switch is done by movement of this spring loaded

platform.

Fig. 1.20.

Microswitch

5. Reed switch:

+ It is a non-contact proximity switch. It is used for checking the closure of doors.



42

+ It consists of two magnetic switch contacts sealed in a glass tube.

Fig. 1.21. Reed

switch

+ When a magnet is brought close to the switch, the magnetic reeds are

attracted each other and close the switch contacts.

4.0 VELOCITY AND MOTION:

To detect and monitor the velocity and motion the following sensors are used.

4.1 VELOCITY MEASUREMENT

+ Velocity sensors or tachogenerators are devices that give an output proportional to angular velocity.

+ These sensors find wide application in motor speed control systems.

+ The following are the various velocity sensors.

1. Electro Magnetic Transducer,

+ The most commonly used transducer for measurement of linear velocities is electromagnetic transducer.

+ The electromagnetic transducers are classified into two categories.

1. Moving Magnet Type: 2. Moving coil

type.

+ In moving magnet type the sensing element is a rod that is rigidly

coupled to the device whose velocity is being measured.

+ This rod is a permanent magnet. This permanent magnet is

surrounded by a coil.

+ The motion of the magnet induces a voltage in the coil and the amplitude of the voltage is directly proportional to the velocity.



43

Fig. 2.22. Moving magnet type

transducer

2. Moving coil type velocity transducer:

+ It is operated through the action of a coil moving in a magnetic field.

+ A voltage generated in the coil is proportional to the velocity of the coil. + This is a more satisfactory arrangement due to it forms a closed

magnetic circuit with a constant air gap and the device is contained an

antimagnetic case which reduces the effects of stray magnetic field.

Fig. 2.23. Moving coil type velocity transducer

3. Tachogenerators:

+ A sensor that converts speed of rotation directly into electrical signal is

called a tachogenerator.

+ It is used to convert angular speed into a directly dependent voltage signal.

(a) Toothed Rotor Variable Reluctance Tachogenerator:

+ It is used to measure angular velocity.

+ This tachogenerator consists of a metallic toothed rotor mounted on the shaft whose speed is to be measured.

+ A magnetic pick up is placed near the toothed rotor and this magnetic pick up

consists of a housing, and the housing containing a small permanent



44

magnet with a coil wound around it.

+ When the rotor rotates, the reluctance of the air gap between pickup and

the toothed rotor changes and the rise in e.m.f. is induced in the pickup coil. Finally the output is in the form of pulses and wave shapes.

+ The pulses induced depend upon the number of teeth in the rotor and the rotational speed. When the speed is known, the rotational

speed is calculated by measuring the frequency pulses.

+

Fig. 2.24. Toothed rotor tachometer generator

+ Suppose the rotor has ‘ n’ teeth and the speed of rotation is ‘ N’

r.p.s. and number of pulses per second is ‘ p’ .

+ The number of pulses per revolution = ‘ n’ = n

The advantage of toothed rotor variable reluctance tachogenerator is the

information from this device can be easily transmitted and easy to calibrate.

4. A. C. Generator Form of Tachogenerator:

+ It consists of rotor, which rotates with the rotating shaft and a coil. + When the coil rotates in the magnetic field the e.m.f. is induced.

+ The magnet may be in the form of stationary permanent magnet or electromagnet.

+ The frequency of this alternating e.m.f. is used to measure the

angular velocity. + The output voltage is rectified and it is measured with a permanent magnet

moving coil (PMCC) voltmeter.



45

Fig. 2.25. A.C Tachometer

generator



46

4.2 MOTION SENSORS 1. Stroboscope:

+ Stroboscope is a simple portable manually operated device for periodic or rotary motions measurement.

+ It is a variable frequency flashing light instrument and the flashing is set by the operator.

+ If a strong light is caused to flash on a moving object at the time

each flash occurs. The stroboscope occupies a given position, and the

object will appear to be stationary. + The flashing light whose frequency can be varied and controlled,

and this source is called strobotron.

2 Pyroelectric Sensors:

+ It consists of a polarised pyroelectric crystal with thin metal film

electrodes on opposite faces. (Pyro electric materials, e.g., lithium tantalate are crystalline materials which generate charge in response to

heat flow. When such materials heated to about 610° C in an electric

field, the electric dipoles within the material line up and it becomes polarised as shown in Fig.).

+ Due to the crystal is polarised with charged surfaces, the ions are drawn from the surrounding air and electrons from any measurement

circuit is connected to the sensor to balance the surface charge as shown in Fig.

+ For measurement of a human or heat source motion, the sensing element has to differentiate between general background heat radiation

and a moving heat source. For that a single pyroelectric sensor is not capable to use and dual pyroelectric sensors are used as shown in

Fig.

+ In this dual pyroelectric sensors the sensing element has the one front electrode and two back electrodes. When two sensors being connected means both sensors are receive the same heat signal and

their outputs are cancelled.



47

Fig. 2.26. Pyroelectric sensors



48

+ Suppose a heat source moves from its position means the heat radiation

moves from one of the sensing elements to the other, then the current

is alternates in one direction first and then reversed to the other direction second.

+ A moving human gives an alternating current of 1O A. When the infrared radiation is incident on the dual pyroelectric sensor material and

changes its temperature, the polarisation in the crystal is reduced. A

focusing device is needed to direct the infrared radiation onto the sensor.

5.0 FLUID PRESSURE SENSORS

+ The devices which are used to monitor fluid pressure in industrial

processes is diaphragms, bellows, capsules and tubes. + The types of pressure measurements required are

(1) Absolute pressure measurement, (2) Differential pressure measurements.

+ In absolute pressure measurements the measurement is related to vacuum pressure (zero pressure) and in differential pressure measurement the difference in pressure is measured. The types of pressure measurement devices are discussed below.

1. Diaphragms

+ In this the pressure to be measured is applied to the diaphragm, causing it to deflect, and the deflection being proportional to the applied

pressure. This movement can be monitored by some form of displacement sensor. (Example for displacement sensor is strain

gauge) and it is shown in Fig.2.27.

Fig. 2.27. Diaphragm pressure gauge



49

Fig. 2.28. Diaphragm type strain gauge pressure

transducer

+ A specially designed strain gauge is also used for measuring pressure

and it consisting of four strain gauges with, two measuring the strain in a

circumferential direction while remaining two measure strain in a

radial direction. The four strain gauges are connected to form the arms of a wheatstone bridge a shown in Fig.2.28.

+ The deflection at any point is shown in terms of +ve and —ye sign. The stress distribution on the diaphragm surface is almost ideal for

practical purposes, since both compressive and tensile stresses exit. So this will allow the use of a four arm wheatstone bridge where all the

gauges are active and consequently there is a large output.

+ The strain gauges I and 4 are placed at close to the centre and oriented to read tangential strain and its value is +ve maximum at this point.

+ The gauges 2 and 3 are oriented to read radial strain and it is placed close to the edge as possible.

2. Bellows + A metallic bellows is a series of circular parts as shown in Fig.2.29

and the parts are formed or joined in such a manner that they are

expanded or contracted axially by change in pressure.

Fig. 2.29.



50

Bellows



51

+ The Fig.2.30 shows the bellows can be combined with a LVDT to

give a pressure sensor with an electrical output.

+ The bellows are made up of materials like stainless steel, phosphor bronze, nickel, rubber and nylon.

+ The output pressure is calibrated through the LVDT.

Fig. 2.30. L VDT with bellows

3. Capsule

Fig. 2.31. Capsule

+ Capsules are one of the pressure measuring device and it can be

considered to be just two corrugated diaphragms combined and give

even greater sensitivity. + The capsules are more sensitive in measuring pressure.

4. Tube Pressure Sensors

Fig. 2.32. Tube pressure

sensors

+ In tube pressure measurement the increase in pressure in a tube is

cause the tube in circular cross-section. It is shown in the above Fig.

The tubes having greater sensitivity while the pressure increases. + The tubes are made up of stainless steel and phosphor bronze.

5. Tactile Sensor



52

+ It is one form of pressure sensor and it is used to determine the pressure in

Robotics in such a form fingertips of robotics contact with the object.



53

+ These type of sensors also used in ‘ touch display screens’ where

physical contacts to be sensed.

+ The above Fig.2.33 shows the one form of tactile sensor. + It uses piezo electric polyvinylidene fluoride (PVDF) film.

+ There are two layers of such film is used and it is separated by a soft film which transmits vibrations.

Fig. 2.33. PVDF tactile

sensor

+ The alternating voltage is supplied in the lower PVDF film and this

results in mechanical oscillations of the film.

+ The intermediate film transmits these vibrations to the upper PVDF film. + Due to the piezoelectric effect the vibrations formed are cause an

alternating voltage to be produced across the upper film.

+ So the pressure is applied to the upper PVDF film and its vibrations are

affected the output voltage.

6. Piezoelectric sensor

Fig. 2.34. Sensor equivalent circuit

+ The electrical circuit for a piezo electric sensor is a charge

generator in parallel with capacitance Cs and in parallel with Resistance Rs.

+ The effective circuit is as shown by the Fig. when the sensor is

connected via a cable of capacitance C and resistance RA.

+ The sensor is charged subject to pressure change and the

capacitor will discharge with time. The discharge time depends on the time constant of the circuit.

LIQUID FLOW SENSORS

+ There are many devices used to measure the liquid flow. + The basic principle in measuring flow is the fluid flowing through the pipe

per second is proportional to square root of pressure difference.

+ The following flow measuring devices are used to measure the liquid flow.



54

1. Turbine Flowmeter

+ The Fig.2.35 shows the turbine flowmeter and it consists of a multi-

bladed rotor which is supported in the pipe along with the flow occurs.

+ The rotor rotation depends upon the fluid flow and the angular velocity is proportional to the flow rate.

+ The rotor rotation is determined y the magnetic pick-up, which is

connected to the coil.

+ The revolution of the rotor is determined by counting the number of pulses produced in the magnetic pick up. The accuracy of this instrument is ± 3%.

Fig. 2.35.

2. Orifice Plate

+ It is a simple disc with a central hole and it is placed in the tube through

which the fluid flow.

Fig. 2.36. Orifice plate

+ From the above Fig.2.36 the pressure difference measured between

a point equal to the diameter of the tube upstream and half the diameter of down stream.

+ The accuracy of this instrument is ±1.5%.

LIQUID LEVEL MEASUREMENT The liquid level measurement is done by using

1. Differential pressure sensor and

2. Float system.



55

1. Differential Pressure Sensor

+ In this the differential pressure cell determines the pressure difference

between base of the liquid and atmospheric pressure.

+ The differential pressure sensor can be used in either form of open or closed vessel system.

Fig. 2.37.

2. Float System

+ In this method the level of liquid is measured by movement of a float. + The movement of float rotates the arm and slider will move across

a potentiometer.

+ The output result is related to the height of the liquid.

Fig. 2.38.

6.0 TEMPERATURE SENSORS

+ Temperature measurements are amongst the most common and the most important measurements made in controlling industrial processes.

+ Changes that are commonly used to monitor temperature are, the expansion or contraction of solids, liquids or gases, the change in electrical resistance of conductors, semiconductors and thermoelectric e.m.f.s. The control system which are used to measure the temperature is as follows

1 Thermocouples

+ The most common electrical method of temperature measurement uses the thermocouples.

+ The basic principle of this is, if two different metals are joined together, a potentiometer difference occurs across the junction.

+ The potential difference depends on the metals used and the

temperature of the junction.

+ When both junctions are at the same temperature, there is no net e.ni.f.

But if there is a difference in temperature between the junction the e.m.f. will be produced.

+ This e.m.f. will depend upon the two metals and the temperature



56

between the junctions. One junction is held at 0° C and the equation

which is used to find out the e.m.f. is



57

+

Fig. 2.39. Thermocouple

+ There are three e.m.f.s present in a thermoelectric circuit. In this the

Seebeck e.m.f. is caused by the junction of dissimilar metals and the Pettier e.m.f. is caused by a current flow in the circuit, and the Thomson

e.m.f. which results from a temperature gradient in the materials.

+ It is observed that all thermocouple circuits must involve at least two junctions. In that one of the junctions senses the desired

or unknown temperature.

+ This junction is called the hot or measuring junction. The other junction is usually maintained at a known fixed temperature and this

junction is called the cold or reference junction.

+ If the temperature of the reference or cold junction is known, the temperature of the hot or the measuring junction can be

calculated by using the thermoelectric properties of the materials. + If thermocouple circuit can have other metals in the circuit and they will have

no effect on the thermoelectric e.m.f. + A thermocouple can be used with the reference junction at a

temperature other than

+ 0° C.

+ For that we assume a 0° C junction and the correction has to be

applied using the law of intermediate temperatures.

The equation used in this is

Fig. 2.41. Las’ of intermediate temperature



58

+ Here to maintain the 0° C at one junction a compensation circuit is

Used to provide an e.m.f. which varies with the temperature of the cold junction.

+ When it is added to the thermocouple e.m.f. it will generate a combined e.m.f.

This is shown in Fig.2.42.

Fig. 2.42. Compensation thermocouple

+ In the above Fig.2.42, the wires from the measuring junction are

screwed directly to an isothermal block terminal strip. + The temperature of the block is ambient temperature.

+ This reference temperature is measured by semiconductor

sensor and compensation circuitry develops a voltage Ecomp which is combined with measuring junction and the net voltage across the

voltmeter = T (Temperature being measured).

+ The isothermal block can accept many thermocouple pairs in multichannel instruments with microprocessor computing power

since the T (reference junction sensor now sends its temperature

data to the computer which computes the needed voltage correction for each thermocouple.

+ The thermocouples like E, J, K and T are relatively cheap and it has accuracies

of about ± ito 3%.

+ The noble metal thermocouples are very high cost compared with this and it has accuracies of about ±1% better than the base metal thermocouples.

+ Thermocouples are used in applications ranging from measurement of room air temperature to that of a liquid metal bath. The problems

which may be encountered are

1. Faulty reference junction, 2. Installation faults,

3. Junctions formed by users may involve excessive temperatures or faulty soldering techniques,

4. Gross errors can result due to wrong installation of thermocouple. 2. Resistance Temperature Detectors (RTDs)

+ Resistance temperature detectors (RTDs) or resistance thermometers

are basic instruments for measurement of resistance. + The materials used for RTDs are Nickel, Iron, Platinum, Copper,



59

Lead,

Tungsten, Mercury, Manganin, Silver, etc. + The resistance of most metals increases over a limited temperature

range and the relationship between Resistance and Temperature is

shown below.



60

+ Fig. 2.43. Resistance temperature

detector

+ The Resistance temperature detectors are simple, and resistive

elements in the form of coils of wire and it is shown in the above

Fig.2.44.

+ The equation which is used to find the linear relationship in RTD is

Fig. 2.44. RTD element

Constructional Details ofRTDs:

+ The platinum, nickel and copper in the form wire are the most

commonly used materials in the RTDs. + Thin film platinum elements are often made by depositing the metal on a

suitable substrate wire- wound elements involving a platinum wire held by a high temperature glass adhesive inside a ceramic tube.

+ This is shown in Fig.2.45.

Fig. 2.45.



61

Salient Features ofRTDs:

1. High degree of accuracy.

2. Resistance thermometer is interchangeable in a process without

compensation or recalibration. 3. It is normally designed for fast response as well as accuracy to

provide close control of processes.

3. Thermistors

+ Thermistor is a semiconductor device that has a negative

temperature coefficient of resistance in contrast to positive coefficient

displayed by most metals. + Thermistors are small pieces of material made from mixtures of metal oxides,

such as Iron, cobalt, chromium, Nickel, and Manganese. + The shape of the materials is in terms of discs, beads and rods.

+ The thermistor is an extremely sensitive device because its resistance changes rapidly with temperature.

+ The resistance of conventional metal-oxide thermistors decreases in

a very non-linear manner with an increase in temperature is shown in the Fig.2.46 below.

+ The change in resistance per degree change in temperature is considerably

larger than that which occurs with metals.

Fig. 2.46. Thermistors

+ The simple series circuit for measurement of temperature using a

thermistor and the variation of resistance with temperature for a typical thermistor is shown in the below Fig.2.47.

Fig. 2.47. Thermistor

+ The thermistor is an extremely sensitive device because its resistance

changes rapidly with temperature.

+ Thermistors have many advantages when compared with other temperature sensors.

+ The main disadvantage is highly non-linear behaviour.



62

4. Thermodiodes and Transistors

(a) Thermodiodes:

+ Thermodiode is widely used method for measuring temperature.

When the temperature of doped semiconductors changes, the mobility of their charge carriers changes and this affects the rate at

which electrons and holes can diffuse across ap-n junction.

1. Measurement of temperature, 2. Control of temperature,

3. Temperature compensation,

4. Measurement of thermal conductivity, 5. Measurement of power at high frequencies,

6. Measurement of composition of gases, 7. Providing time delay,

8. Vacuum measurements.

+ The difference in voltage and current through the junction is a function of

the temperature. The equation which is used to find the I is

+ From the above equation the voltage ‘ V’ is proportional to the temperature on Kelvin scale and the potential difference

measurement across a diode at constant current is used to measure

the temperature.

(b) Transistor: + In Thermo transistor the voltage across the junction between the base

and the emitter depends on the temperature. + A common method is use of two transistors with different collector current

and finding the difference in the base-emitter voltages between them, and this difference is the measure of temperature.

Fig. 2.48.

Transistor

+ The thermotransistors can be combined with circuit components on a

single chip to give a temperature sensor. + This is shown in the above Fig.2.48.



63

5. Bimetallic strips

Fig. 2.49. Bimetallic thermostat

+ A Bimetallic thermostat consists of two different metal strips bounded together and they cannot move relative to each other.

+ These metals have different coefficients of expansion and when the temperature changes the composite strips bends into a curved strip, with the higher coefficient metal on the outside of the curve.

+ The basic principle in this is all metals try to change their physical

dimensions at different rates when subjected to same change in temperature.

+ This deformation may be used as a temperature- controlled switch, as in

the simple thermostat.

+ The Fig.2.49 shows the Bimetallic thermostat which was commonly used with domestic heating systems.

7.0 LIGHT SENSORS

1. Photodiodes

+ Diodes like photodiodes and semiconductor diodes are connected into a circuit in reverse bias giving a very high resistance.

+ When light falls on the junction the resistance of the diode will drop and the current in the circuit will rise.

Fig. 2.50.



64

+ The Fig.2.51 shows the diode characteristics.



65

+ If the diode is sufficiently reverse biased, it will breakdown.

+ The current passing through the diode when forward biased only. + If an A.C. voltage is applied across a diode, it can be regarded

as only switching on when forward bias it and being off in the reverse direction.

+ The photodiodes have a very fast response to light and it can be used as a variable resistance device controlled by the light incident on

it.

2. Photo Transistors

The transistors are come in two forms 1. npn, 2. pnp.

Fig. 2.51.

+ The main current flows in at the collector and out at the emitter

in npn transistor. + The main current flowing in at the emitter and out at the collector in pnp

transistor.

+ The phototransistors have a light sensitive collector-base p-n junction.

+ There is a very small collector to emitter current when there is no incident light. Suppose the light is incident a base current is produced

and it is proportional to the light intensity. + So this will produce a collector current and it is used for measure of the light

intensity. + The example for photo transistors is photo Darlington arrangement.

Fig. 2.52. Photo Darlington

arrangement

3. The Photo Resistor

+ Its resistance depends on the intensity of light falling on it, and the resistance will decrease linearly as the intensity increases.

+ The photoresistor like cadmium sulphide has most responsive to



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the light having wavelengths of about 520 mm to 700 mm. 4. Array of Light Sensors

+ This will be used in small space like rooms to determine the variations of light intensity across that space.

e.g., Automatic camera



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8.0 SELECTION OF SENSORS

The factors to be considered while selecting sensors are

1. The nature of output required from the sensor. 2. The nature of measurement required.

3. The accuracy of the sensor.

4. The cost of the sensor.

5. The power requirement of the sensor.

6. The speed response of the sensor.

7. The linearity of the sensor. 8. The Reliability and Maintainability of the sensor.

9. Environmental conditions under which the measurement is to be made. 10. Signal conditioning requirements.

11. The nominal and range of values of the sensor. 12. Suitable output signals from the measurement.

PART- A

1. What is the use of sensors and transducers?

2. Differentiate between Range and Span. 3. Give the formula for finding the repeatability of a transducer.

4. What is hysteresis error?

5. What is the difference between ‘ Accuracy’ and ‘ Precision’ ?

6. What is threshold?

7. What is Dead time and Dead zone?

8. What is resolution?

9. What is Rise time and Settling time?

10. What is meant by Hall effect?

11. What are the velocity and motion sensors?

12. What is non-linearity error?

13. Give the example for measuring force.

14. What are the fluid pressure sensors?

15. What are the liquid flow measuring devices?

16. What are the two types diaphragms?

17. What are the Temperature measuring devices?

18. Give the example for light sensors.

19. What is the basic principle in thermocouples?

20. Give some materials used in thermocouples

21. What is offset voltage of an operational amplifier

22. What is the equation for V of an integrator?

23. What is a precision diode?



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24. What is a comparator?

25. Name an application of a Schmitt trigger.

26. Why integrators are preferred over differentiators in analog computers?

27. What is a voltage follower?

28. What is the advantage of CMOS Schmitt trigger?

PART-B

1. Explain the terminologies used in transducers.

2. What are all the displacement sensors? Explain each one briefly.

3. Explain the position sensors with neat figure.

4. Define proximity and explain the proximity sensors.

5. What are all the velocity and Motion sensors?

6. How the pressure is measured? Explain the pressure sensors neatly.

7. Explain the temperature measurement sensors.

8. Explain the light sensors with neat figure.

9. What are all the points to be considered while selecting the sensors?

10. Explain the signal processing.

11. Explain some applications of operational amplifier.

12. Explain the operation of successive approximation ADC.

13. How do a dual slope ADC and single slope ADC differ?

14. What is Flash ADC ? Discuss.

15. Explain the construction of R-DR ladder DAC.

16. Discuss the various terms associated with ADC.



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Unit I MECHATRONICS, SENSORS AND TRANSDUCERS 1.1