MECHATRONICS - airwalkbooks.comairwalkbooks.com/images/pdf/pdf_95_1.pdf · ME407 – MECHATRONICS MODULE I: Introduction to Mechatronics: Structure of Mechatronics system. Sensors

MECHATRONICS

(For B.E./B.Tech. Mechanical Engineering Students)

Dr. S. RAMACHANDRAN, M.E., Ph.D.,

Professor – Mech.

Dr. V.J.K. KISHOR SONTI

Professor – ECE

Sathyabama Institute of Science & Technology

Chennai – 119

AIRWALK PUBLICATIONS

(Near All India Radio)

80, Karneeshwarar Koil Street,

Mylapore, Chennai – 600 004.

Ph.: 2466 1909, 94440 81904

Email: [email protected], [email protected]

www.airwalkbooks.com, www.srbooks.org

© First Edition: 10th

July – 2018

This book or part thereof should not be reproduced in any form without the

written permission of the publisher.

Price: Rs. 200/-

ISBN: 978-93-88084-10-9

Typesetting by: Akshayaa DTP, 48E, Sri Gangaiya Avenue, 2nd

Cross Street, Ramapuram Chennai – 89, Mobile: 9551908934.

Printed at:

ME407 – MECHATRONICS

MODULE I: Introduction to Mechatronics:

Structure of Mechatronics system. Sensors − Characteristics − Temperature,

flow, pressure sensors. Displacement, position and proximity sensing by

magnetic, optical, ultrasonic, inductive, capacitive and eddy current methods.

Encoders: incremental and absolute, gray coded encoder. Resolvers and

synchros. Piezoelectric sensors. Acoustic Emission sensors. Principle and types

of vibration sensors.

MODULE II: Actuators:

Hydraulic and Pneumatic actuators − Directional control valves, pressure

control valves, process control valves. Rotary actuators. Development of simple

hydraulic and pneumatic circuits using standard Symbols.

MODULE III: Micro Electro Mechanical Systems (MEMS):

Fabrication: Deposition, Lithography, Micromachining methods for MEMS,

Deep Reactive Ion Etching (DRIE) and LIGA processes. Principle, fabrication

and working of MEMS based pressure sensor, accelerometer and gyroscope.

MODULE IV: Mechatronics in Computer Numerical Control (CNC) machines:

Design of modern CNC machines − Mechatronics elements − Machine

structure: guide ways, drives. Bearings: anti-friction bearings, hydrostatic

bearing and hydrodynamic bearing. Re-circulating ball screws, pre-loading

methods. Re-circulating roller screws. Typical elements of open and closed

loop control systems. Adaptive controllers for machine tools. Programmable

Logic Controllers (PLC) − Basic structure, input / output processing.

Programming: Timers, Internal Relays, Counters and Shift registers.

Development of simple ladder programs for specific purposes.

MODULE V: Mechatronics in Robotics:

System modeling − Mathematical models and basic building blocks of general

mechanical, electrical, fluid and thermal systems. Mechatronics in

Robotics-Electrical drives: DC, AC, brushless, servo and stepper motors.

Harmonic drive. Force and tactile sensors. Range finders: ultrasonic and light

based range finders

MODULE VI: Robotic vision system:

Image acquisition: Vidicon, charge coupled device (CCD) and charge injection

device (CID) cameras. Image processing techniques: histogram processing:

sliding, stretching, equalization and thresholding. Case studies of Mechatronics

systems: Automatic camera, bar code reader, pick and place robot, automatic

car park barrier system, automobile engine management system.

*********

CONTENTS

MODULE – IINTRODUCTION TO MECHATRONICS

1.1 – 1.95

1.1. Introduction to mechatronics 1.1

1.2. Need for mechatronics 1.3

1.3. Concepts of mechatronics approach 1.4

1.4. Classification of mechatronics 1.6

1.5. Emerging areas of mechatronics 1.9

1.6. System 1.11

1.6.1. Structure of mechatronic system 1.12

1.7. Measurement system 1.14

1.8. Control system 1.15

1.8.1. Basic terminology used in control system 1.16

1.8.2. Types of control system 1.16

1.8.3. Basic terms used in closed loop control system 1.19

1.8.4. Comparison between open loop and closed loop

control system

1.20

1.8.5. Application which use automatic control system 1.20

1.8.6. Analogue and digital control systems 1.25

1.8.7. Sequential controllers 1.25

1.8.8. Microprocessor based controllers 1.28

1.9. Sensors 1.33

1.9.1. Classification of sensors 1.34

1.9.1.1. Contact sensors 1.36

1.9.1.2. Non-contact sensors 1.36

1.9.2. Static and dynamic characteristics of sensor 1.36

1.10. Potentiometers 1.42

1.11. Linear variable differential transformer (LVDT) 1.46

1.12. Capacitance sensors 1.48

1.13. Strain gauges 1.53

1.14. Eddy current sensors 1.56

1.15. Hall effect sensor 1.58

1.16. Velocity sensor 1.62

1.16.1. Tachometer 1.62

1.16.2. Hall-effect sensor 1.63

1.16.3. Laser doppler velocimetry 1.64

1.16.3.1. Principle of the instrument 1.64

1.16.3.2. Instrumental set-up 1.64

1.17. Temperature sensors 1.66

1.18. Light sensors 1.75

1.19. Encoder 1.80

1.19.1. Incremental linear encoder 1.81

1.19.2. Absolute linear encoder 1.82

1.20. Rotary encoders 1.83

1.20.1. Absolute encoder 1.83

1.20.2. Incremental encoder 1.84

1.21. Resolver 1.86

1.21.1. Synchros 1.87

1.22. Piezoelectric sensor 1.88

1.23. Acoustic emission sensors 1.91

1.24. Vibration sensors 1.92

1.24.1. Vibrometer (or) seismometer 1.92

1.24.2. Accelerometer 1.93

1.24.3. Laser Doppler Vibrometer (LDV) 1.94

ii Contents

Module II Hydraulic and Pneumatic Actuators and Circuits

2.1 Hydraulic And Pneumatic Actuators . . . . . . . . . . . . . . . . . . 2.1

2.2 Linear Actuators: (Hydraulic and Pneumatic Cylinders). 2.1

2.2.1 Single acting hydraulic and Pneumatic cylinder(actuator) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2

2.2.2 Double acting Hydraulic and Pneumatic cylinder(actuator) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3

2.2.3 Cylinder Cushioning . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4

2.2.4 Side loads and the stop tube . . . . . . . . . . . . . . . . . . . 2.6

2.2.5 Telescopic cylinder . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6

2.2.6 Through Rod cylinder . . . . . . . . . . . . . . . . . . . . . . . . . 2.7

2.2.7 Tandem cylinder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8

2.2.8 Turn Cylinder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8

2.3 Rotary Actuators: (Hydraulic Motors) . . . . . . . . . . . . . . . . . 2.9

2.3.1 Gear Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9

2.3.2 Vane Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10

2.3.3 Piston Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11

2.4 Hydraulic Valves – Direction Controls Pressure – ProcessControls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13

2.5 Direction Control Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.14

2.5.1 Pilot operated check valve . . . . . . . . . . . . . . . . . . . . 2.16

2.5.2 Two way valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.17

2.5.3 Four way valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.18

2.5.4 Various flow configurations of Four way three positionValves (4/3 valves) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.25

2.5.5 Rotary Four way valves . . . . . . . . . . . . . . . . . . . . . . 2.26

2.5.6 Shuttle Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.27

Contents iii

2.6 Pressure Control Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.28

2.6.1 Pressure Relief Valve . . . . . . . . . . . . . . . . . . . . . . . . . 2.28

2.6.2 Compound Relief valve . . . . . . . . . . . . . . . . . . . . . . 2.30

2.6.3 Pressure Reducing Valve . . . . . . . . . . . . . . . . . . . . . . 2.31

2.6.4 Unloading Valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.32

2.6.5 Sequence Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.33

2.6.6 Counter Balance Valve . . . . . . . . . . . . . . . . . . . . . . . 2.35

2.7 Hydraulic Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.36

2.7.1 Pressure Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.37

2.7.2 Temperature Switch . . . . . . . . . . . . . . . . . . . . . . . . . . 2.38

2.8 Process (Flow) Control Valves . . . . . . . . . . . . . . . . . . . . . . . 2.39

2.8.1 Needle Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.39

2.8.2 Easy Read and Adjust Flow Control Valves . . . . . 2.40

2.8.3 Globe valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.40

2.8.4 Gate valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.41

2.8.5 Non-pressure compensated flow control valve . . . . 2.42

2.8.6 Pressure compensated flow control valve . . . . . . . . 2.42

2.8.7 Servo Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.44

2.8.8 Non-adjustable compensated flow control valve. . . 2.45

2.8.9 Flow Control Valve Symbols . . . . . . . . . . . . . . . . . . 2.46

2.9 Hydraulic Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.47

2.10 Hydraulic Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.48

2.10.1 Hydraulic circuits and Pneumatic circuits - symbols 2.49

2.11 Single Acting Hydraulic Cylinder Circuit (ReciprocationCircuit) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.52

2.12 Double Acting Hydraulic Cylinder Circuit (ReciprocationCircuit) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.53

iv Mechatronics - www.airwalkbooks.com

2.13 Continuous Reciprocation Circuit . . . . . . . . . . . . . . . . . . . 2.54

2.14 Quick Return Motion Circuits . . . . . . . . . . . . . . . . . . . . . . 2.55

2.15 Sequencing Circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.57

2.16 Synchronising Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.59

2.17 Accumulator Circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.61

2.17.1 Weight loaded (or) Gravity type accumulator . . . 2.61

2.17.2 Spring loaded type accumulator . . . . . . . . . . . . . . 2.62

2.17.3 Gas loaded accumulator . . . . . . . . . . . . . . . . . . . . . 2.62

2.17.3.1 Non-separator type Gas loaded accumulator . . . 2.63

2.17.3.2 Separator type gas loaded accumulator . . . . . . . 2.63

2.17.4 Applications of Accumulator circuits. . . . . . . . . . . 2.63

2.17.4.1 Accumulator as an auxiliary power source . . . . 2.64

2.17.4.2 Accumulator as a compensator for internal (or)

external leakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.65

2.17.4.3 Accumulator as an emergency power source: (Safety

circuits) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.66

2.17.4.4 Accumulator as a hydraulic shock absorber

(Industrial circuit) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.67

2.18 Safety Circuits (Fail-safe Circuits) . . . . . . . . . . . . . . . . . . 2.68

2.19 Pressure Intensifier Circuit (or) Punching Press Circuit 2.70

2.20 Hydraulic Circuit for Table Movement of Milling Machine(or) Surface Grinder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.71

2.21 Meter Out Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.72

2.22 Hydraulic Fork Lift Circuits (Meter-in Circuit) . . . . . . . 2.73

2.23 Bleed Off Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.75

2.24 Regenerative Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.76

2.25 Counter Balance Valve Circuits . . . . . . . . . . . . . . . . . . . . 2.79

Contents v

2.26 Pneumatic System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.80

2.26.1 Pneumatic Fundamental . . . . . . . . . . . . . . . . . . . . . 2.80

2.26.2 Advantages of Pneumatic Systems over HydraulicSystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.80

2.26.3 Disadvantages of pneumatic systems. . . . . . . . . . . 2.80

2.26.4 Applications of Pneumatic systems . . . . . . . . . . . . 2.80

2.26.5 Comparison of Pneumatic System to a HydraulicSystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.81

2.26.6 Basic Principle of Pneumatic Systems . . . . . . . . . 2.82

2.26.7 Important properties of Air . . . . . . . . . . . . . . . . . . 2.82

2.26.8 Ideal Gas Laws (Perfect Gas Laws) . . . . . . . . . . . 2.83

2.26.9 Compressed Air System. . . . . . . . . . . . . . . . . . . . . . 2.84

2.27 Control Elements in Pneumatic System . . . . . . . . . . . . . 2.86

2.27.1 Control of Single-Acting Air Cylinder: . . . . . . . . . 2.94

2.27.2 Double-Acting Cylinder . . . . . . . . . . . . . . . . . . . . . . 2.95

2.28 Pneumatic Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.98

2.28.1 Graphic Symbols for Trio Units and Assemblies 2.98

2.29 PLC and Microprocessor Uses. . . . . . . . . . . . . . . . . . . . . . 2.99

2.30 Logic Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.103

2.30.1 Pressure Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . 2.106

2.31 Switching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.107

2.32 Electro Pneumatic Circuits . . . . . . . . . . . . . . . . . . . . . . . 2.112

2.33 Electro Hydraulic Circuits . . . . . . . . . . . . . . . . . . . . . . . . 2.113

2.34 Robotic Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.114

vi Mechatronics - www.airwalkbooks.com

MODULE – III: MICRO ELECTROMECHANICAL SYSTEMS (MEMS)

3.1 – 3.24

3.1. Micro Electro Mechanical Systems (MEMS) 3.1

3.2. Sensor 3.3

3.2.1. Type of sensor 3.3

3.2.2. Fabrication of mems materials 3.3

3.3. Few MEMS materials 3.4

3.3.1. Silicon 3.4

3.3.2. Polymers 3.4

3.3.3. Metals 3.5

3.4. Deposition process 3.6

3.4.1. MEMS deposition technology can be classified

into two major groups

3.6

3.4.2. Chemical Vapour Deposition (CVD) 3.6

3.4.3. Physical Vapour Deposition (PVD) 3.7

3.5. Lithography 3.7

3.5.1. Etching 3.8

3.6. Wet etching 3.9

3.7. Dry etching 3.10

3.8. LIGA process 3.12

3.8.1. Steps to follow 3.13

3.8.2. Process flow 3.13

3.8.3. Principle, fabrication and working of MEMS

based pressure sensor

3.14

3.8.4. Principle 3.15

3.8.5. Fabrication of MEMS based pressure sensor 3.15

3.9. Working of MEMS pressure sensor 3.16

3.10. Fabrication and working of MEMS based accelerometer 3.17


3.10.2. Fabrication of MEMS accelerometer 3.18

3.10.3. Working of accelerometer 3.20

Contents vii

3.11. Principle, fabrication and working of mems based gyroscope 3.22


3.11.2. Fabrication 3.22

3.11.3. Working 3.24

3.11.4. Application of gyroscope 3.24

3.12. Electro chemical etching (ECE) 3.24

MODULE – IV : MECHATRONICS INCNC MACHINES AND PLC

4.1 – 4.84

4.1. Design of modern CNC machines 4.1

4.2. Mechatronics elements 4.2

4.3. CNC machine structure 4.2

4.3.1. Static load 4.3

4.3.2. Dynamic load 4.3

4.3.3. Thermal load 4.4

4.4. Slides and slideways (or) guide ways 4.5

4.4.1. Factors considered for design of guideways 4.6

4.4.2. Important types of slideways (or) guideways (or)

bearings

4.6

4.5. Friction slideways 4.7

4.5.1. V-slideways 4.9

4.5.2. Flat and dovetail slideways 4.10

4.5.3. Cylindrical guideways 4.10

4.6. Anti friction linear motion (lm) guideways (or) bearings 4.11

4.6.1. Recirculating linear ball bearings 4.11

4.6.2. Recirculating Ball Bush 4.12

4.6.3. Recirculating Ball Screw and nut 4.13

4.6.3.1. Preloading 4.14

4.6.3.2. Pre-loading Methods 4.14

4.7. Hydrostatic type slideways 4.15

4.8. Hydrodynamic bearings 4.16

viii Contents

4.9. Aerostatic slideways 4.18

4.10. Drives – spindle drives and feed drives 4.19

4.10.1. Spindle brive 4.19

4.10.2. Electric motors 4.20

4.10.3. Spindle drive motors 4.20

4.10.4. Requirements of CNC spindle drives 4.21

4.11. Feed drives 4.22

4.11.1. Requirements of CNC feed drive 4.22

4.11.2. Feed drive motors 4.22

4.12. Feed back devices 4.24

4.12.1. Closed loop control system 4.24

4.12.2. Types of feed back devices 4.25

4.12.3. Positioning feed back devices 4.25

4.12.3.1. Linear Transducers 4.26

4.13. Adaptive Controllers (AC) for machine tools 4.30

4.14. Programmable logic controller 4.32

4.14.1. Functions of a PLC 4.35

4.15. Basic structure 4.36

4.15.1. The CPU 4.38

4.15.2. The memory 4.38

4.15.3. Buses 4.38

4.15.4. Input/output section 4.39

4.16. Input and output processing 4.40

4.17. Programming 4.44

4.18. PLC ladder programming 4.46

4.18.1. Symbols used in ladder programming 4.48

4.18.2. Functional blocks 4.50

4.18.3. Programming examples 4.52

4.19. Logic gates 4.52

4.20. Real time ladder programming example 4.60

4.20.1. Latching 4.62

Contents ix

4.21. Mnemonics 4.63

4.22. Ladder programs and instruction lists 4.64

4.22.1. Branch Codes 4.67

4.23. Timers, counters and internal relays 4.68

4.24. Data handling 4.78

4.24.1. Data movement 4.79

4.24.2. Data comparison 4.80

4.24.3. Arithmetic operations 4.81

4.24.4. Control with a PLC 4.82

4.25. Selection of PLC 4.83

MODULE – V MECHATRONICS IN ROBOTICS

4.1 – 4.94

5.1. Mathematical model 5.1

5.2. Electrical system building block 5.10

5.3. Actuators (or) Drives 5.30

5.3.1. Factors considered for selecting drive system 5.31

5.3.2. Types of drives 5.31

5.4. Pneumatic power drives 5.32

5.5. Hydraulic drives 5.32

5.6. Electrical drives 5.32

5.6.1. Types of electrical drives 5.33

5.7. DC Servomotor 5.33

5.8. Servo motor 5.35

5.8.1. Servomechanism 5.36

5.8.2. DC servomotor construction and working principle 5.36

5.8.3. Types of D.C. motors 5.38

5.8.4. DC Servomotor control theory 5.41

5.8.5.1. DC servo motor advantages 5.44

5.8.5.2. DC servo disadvantages 5.44

5.9. AC Servomotor system 5.45

5.9.1. AC servomotor construction 5.46

x Contents

5.9.2. AC servomotor working principle 5.47

5.9.3. Torque-speed characteristics 5.48

5.9.4. Advantages of AC servomotor 5.49

5.9.5. Comparison between AC and DC servo motors 5.50

5.9.6. Comparison between stepper motor and servo

motors

5.50

5.10. Stepper motor 5.51

5.10.1. Construction and working principle 5.52

5.10.2. Drive modes 5.53

5.10.3. Specifications of stepper motor 5.56

5.10.4. Type of stepper motor 5.57

5.10.5. Advantages of stepper motor 5.61

5.10.6. Disadvantages of stepper motor 5.62

5.11. Selection of motors 5.63

5.12. Comparison of pneumatic 5.64

5.13. End-effectors 5.65

5.14. Grippers 5.66

5.15. Drive system for grippers 5.66

5.16. Harmonic drive 5.67

5.17. Force sensors 5.69

5.17.1. Strain gauge 5.69

5.17.2. Piezoelectric sensor 5.70

5.17.3. Microswitches 5.71

5.18. External sensors 5.71

5.19. Contact type 5.72

5.19.1. Limit switches 5.72

5.19.2. Slip sensors 5.73

5.19.3. Torque sensor 5.74

5.19.4. Tactile sensor 5.76

5.20. Non contact type 5.80

5.20.1. Range sensor 5.81

5.20.2. Proximity sensors 5.87

Contents xi

MODULE – VI : ROBOTIC VISION SYSTEM 6.1 – 6.48

6.1. Robotic vision system – machine vision 6.1

6.2. Image acquisition (sensing and digitizing) function in

robotic vision system

6.3

6.2.1. Imaging devices 6.4

6.3. Vidicon camera (analog camera) 6.4

6.4. Digital camera 6.5

6.4.1. CCD 6.6

6.4.2. Charge Injection Devices (CID) 6.7

6.5. Lighting techniques 6.8

6.5.1. Front light source 6.9

6.5.2. Back light source 6.10

6.5.3. Other miscellaneous devices 6.11

6.6. Analog-to-digital conversion 6.12

6.7. Image storage / frame grabber 6.15

6.8. Image processing and analysis techniques 6.15

6.8.1. Image data reduction 6.16

6.8.2. Segmentation 6.17

6.8.3. Feature extraction 6.21

6.8.4. Object recognition 6.22

6.9. Other algorithms 6.23

6.10. Robotic applications 6.24

6.10.1. Inspection 6.25

6.10.2. Identification 6.25

6.10.3. Visual serving and navigation 6.26

6.10.4. Bin picking 6.26

6.11. Traditional and mechatronics designs 6.27

6.11.1. Possible design solutions 6.28

6.12. Case studies of mechatronic systems 6.38

Short questions and answers SQA.1 – SQA.46

Index I.1 – I.4

xii Contents

MODULE – I

INTRODUCTION TO

MECHATRONICS

* Introduction to Mechatronics: Structure of Mechatronics system.

Sensors – Characteristics – Temperature, flow, pressure sensors.

Displacement, position and proximity sensing by magnetic, optical,

ultrasonic, inductive, capacitive and eddy current methods.

Encoders: incremental and absolute, gray coded encoder. Resolvers

and synchros. Piezoelectric sensors. Acoustic Emission sensors.

Principle and types of vibration sensors.

1.1. INTRODUCTION TO MECHATRONICS

H The word “Mechatronics” originated from Japanese-English. It was

created by Tetsuro Mori a Japanese engineer of the “Yaskawa Electric

Corporation”. The word “Mechatronics” was even registered as a

trademark by the company in 1971. In course of time the company

released the right of using the word in public. The word Mechatronics

was coined by integrating Electronic controls in Mechanisms.

H Mechanism is a machine or part of a machine which by virtue of its

geometry and relative motion controls or transmits or constrains the

movement of other parts. For example a cam mechanism can be used

as timer as shown in Fig. 1.1.

H By rotating the lever the toy moves up and down. In general the size

of the cam or in other words any mechanical mechanism is large and

heavy. This increases the cost and time of the end product. It also

requires specialized tooling which cannot be used for any other

purpose except for manufacturing only those components. Moreover,

if there were space or weight constraints in the design, then creating

a conventional control mechanism becomes very challenging.

H As the field of Electronic Engineering advanced the electronic

components shrunk in size. These components have control application

like counter, timer, etc. Another example, as shown in Fig. 1.2 is

winding watch and smart watch. The winding watch requires precision

parts of small size to make the mechanism which controls the

movements of the watch hands indicating the time.

H Although the winding watch does not require any power source the

engineering involved to make it is huge. Even to adding function like

a date or day or year or stop watch in it involves a lot of changes

in the basic design and subsequent tooling. All this changed with the

advancement of electronic components. Smart watch is one such

1.2 Mechatronics – www.airwalkbooks.com

example. It has no moving parts. It redefined the functionality of

watch from just indicating time to limitless possibilities.

H When engineers blended the advantages of electronic components with

mechanisms the new field of Mechatronics emerged. The word is

formed by taking “Mecha” from mechanisms and “Tronics” from

Electronics. From the French standard NF E 01-010 Mechatronics is

defined as an “approach aiming at the synergistic integration of

mechanics, electronics, control theory, and computer science

within product design and manufacturing, in order to improve

and/or optimize its functionality”. Thus Mechatronics is (MCT) a

multidisciplinary field of engineering. It is a system which brings

together combination of systems engineering, mechanical engineering,

electrical engineering, telecommunications engineering, control

engineering and computer engineering.

1.2. NEED FOR MECHATRONICS

H Any engineering product is the end result of many branches of

technologies brought together. Although an organisation may

departmentalize the contribution of the different technologies, the

boundaries between the technologies is a blur. For instance, let’s

consider an Automobile Engine.

Introduction to Mechatronics 1.3

H In the early period to start an engine people have to mechanically

crank it. To make thinks simpler people integrated the engine with a

small motor powered by a battery power source, as a starter to replace

the function of cranking. This led to the usage of an ignition key.

Now with the advancement in the information technology we can start

an engine by remote control devices.

H Hence, by integrating technologies, there is a huge advancement in

the engineering product or system. Mechatronics is the resultant

technology of integration. To incorporate the advantages of each

technology in a product or in an engineering system it is necessary

to lean on Mechatronics.

1.3. CONCEPTS OF MECHATRONICS APPROACH

H Mechatronics by nature is a unified approach to solve engineering

problems or create engineering products or make engineering systems.

Hence Mechatronic approach explores new possible way to

incorporate the advantage of a new technology in applications which

are already available. This upgrades the application to such a level

that it was neither thought possible nor considered feasible earlier.

H Every technology has its parameters. Parameters are nothing but a

measurable quantity. For instance, in mechanical engineering the

parameters are like temperature, pressure, velocity, displacement, etc.

In the same way electrical parameters are like voltage, current,

resistance etc.

H Technologies are integrated concurrently by integrating these

parameters. The relative information is used in a creative way to make

a product or solve an engineering problem. Although the advancement


in electronics and information systems is recent this methodology

already exists.

H For instance, let’s consider a Rheostat. A rheostat is a variable resistor

which is used to control the current flowing in a circuit by moving

the sliding contact called the wiper over the coil wound as shown

Fig. 1.4. With a mechanical sliding motion the flow of current is

controlled. The mechanical movement is now replaced by electronics

circuits and digital signals. Hence digital potentiometer is made

available in the market.

1. Controlling system

2. Controlled system

H Controlling System is the intelligence system which is programmable

to suit the application needs. Here the signals are perceived at the

input, interpreted by knowledge and a proper response is directed

through the planning and control.

H The controlled system in general is the active system which can be

mechanical or chemical or others where the system’s state is sensed

by sensors and as per the requirement the actuators are activated to

produce the desired results.

H “World” is the end user requirement or can be considered as the

desired output from the system. Hence the key conceptual approach

in mechatronics is the way in which the parameters of different

technologies interact with each other, thereby fusing different


technologies concurrently into one core system. This system thus

emerged draws the advantages and flexibilities of all the technologies

which are integrated.

1.4. CLASSIFICATION OF MECHATRONICS


H Mechatronics as discussed is a branch of engineering which is the

result of fusing two or more engineering technologies. Hence

Mechatronics can be classified based on the technologies which are

fused together. Though such technological ideology may already exists

will now be considered as a part of multidisciplinary field of

Mechatronics. Fig. 1.6 illustrates the how technologies are merged.

1. Based on the level of fusion let’s try to classify mechatronics

Level 1:

Here two major engineering technologies are fused hence level would

comprise of:

H Electromechanical Engineering (Electrical and Mechanical)

H Digital Systems (Electrical and Computer)

H Digital controls (Computer and Control)

H Sensor and Actuators (Mechanical and Control)

Level 2:

Here three major engineering technologies are fused and they are:

H Micro-control

H Analog Systems

H Simulation

H Modeling

Level 3:

H The Fig. 1.6 illustrates only a symbolic representation of fusion of

technologies. When the boundary of engineering classification is no

longer applicable then the products / systems / solutions is of

Mechatronics level 3 classification.


2. Based on Mechatronics product it was classified by Japan society for

Promotion of Machine Industry (JSPMI) into the following categories

Class I:

H Mechanical products which are fused with electronics to enhance or

increase functionality come under Class I. Example for this is a

numerically controlled machine or a variable speed drives, etc.

Class II:

H When traditional mechanical systems are upgraded with internal

electronics then it comes under Class II. A modern sewing machines

or a Digital Odometer is an apt example for that.

Class III:

H Class III is systems that retain the functionality of the traditional

mechanical system, but the internal mechanisms are replaced by

electronics. A classic example is the digital watch.

Class IV:

H Class IV products are designed with integrated mechanical and

electronic technologies in a synergistic way. Examples include right

from photocopiers, to smart washing machines.

3. Based on the Behavioral characteristic of the system

(a) Automated Mechatronic Systems

H An Automated mechatronic system is capable of handling materials

and energy, communicating with its environment and is characterized

by self-regulation, which enables it to respond to predictable changes

in its environment in a pre-programmed fashion. An overwhelming

majority of current mechatronic systems belong to this category.

(b) Intelligent Mechatronic Systems

H An Intelligent mechatronic system is capable of achieving given goals

under conditions of uncertainty. In contrast to automated systems,

which are, by definition, pre-programmed to deliver given behavior

and are therefore predictable, intelligent systems may arrive at

specified goals in an unpredictable manner.


(c) Intelligent Mechatronic Networks

H Intelligent mechatronic networks are capable of deciding on their own

behavior by means of negotiation between constituent autonomous

units (the network nodes). Each of constituent units is itself an

intelligent mechatronic system.

1.5. EMERGING AREAS OF MECHATRONICS

H Machine vision.

H Automation and robotics.

H Servo-mechanics.

H Sensing and control systems.

H Computer-machine controls.

H Expert systems.

H Industrial goods.

H Consumer products.

H Mechatronics systems.

H Medical mechatronics, medical imaging systems.

H Structural dynamic systems.

H Transportation and vehicular systems.

H Mechatronics as the new language of the automobile.

H Computer aided and integrated manufacturing systems.

H Computer-aided design.

H Engineering and manufacturing systems.

H Bio-mechatronics.

H Packaging.


H Microcontrollers / PLCs.

H Mobile apps.

H M & E Engineering.

H Consumer products: Security camera, microwave oven, etc.

H Implant-devices: Artificial cardiac Pacemaker, etc.

H Defense: Unmanned air, ground and underwater vehicles, jet engines,

etc.

H Robotics: Welding robots, Material handling robots etc.

H Automotive industry: Anti-lock braking system (ABS), Multi-point

fuel injection etc.

H Non-conventional vehicles: electro-bicycles, electro scooters, invalid

carriages, etc.

H Office equipment: copy and fax machines etc.

H Computer peripherals: printers, plotters, disk drives etc.

H Photo and video equipment: Thermal Camera, Camcorders etc.

H Simulators: Car simulator, Plane simulator, etc.

H Entertainment Industry: sound and illumination systems.

H Network-centric, distributed systems.

H Aviation, space and military applications.

Advantages of Mechatronics

H Comparatively low cost without compromising quality.

H Perform complicated and precise movements of high quality.

H High reliability, durability and noise immunity.

H Constructive compactness of modules.


H Systems can be controlled and monitored remotely (Unmanned

systems).

H Redesign functional modules of sophisticated and complex systems as

per specific purposes of the customer.

H Flexibility in the system design.

H Increasing the optimal production limits by increasing the machine

utility to the highest extent.

Disadvantages of Mechatronics

H Different expertise required.

H System design relies more on innovation rather to the conventional

method.

H More complex safety issues.

H Increase in component failures.

H Increased power requirements.

H Lifetimes change/vary as components of different technologies are

used.

1.6. SYSTEM

A system is defined as a set of

interdependent or interacting

components connected to form a

complex/intricate whole which is

designed for a specific purpose. A

system consists of an object

which is under study, enclosed by

a boundary to the surrounding

environment. By varying the input

conditions of surrounding the

output from the object under

study is analyzed. This i s

illustrated in the Fig. 1.7.


H System defined thus is a generalized one. Our whole universe is

comprised of systems performing specific functions. In engineering

context a system can be from simple home appliance like flat iron to

a complex production line.

H In Mechatronics a simple example of a system is a Car. In a car

there is the engine, transmission of motion from the engine to wheels

and many other mechanical parts. There are electrical components like

the batteries, lights etc.

H There are electronic control components in the stereo, brake system,

fuel injection systems etc. Now a day’s modern cars are equipped

with navigators, automated safety devices, anti-theft devices etc. On

whole a Car is product of Mechatronics.

1.6.1. Structure of Mechatronic system

H Fig. 1.8 illustrates the structure of mechatronic system. The elements

are summarized below for clarity.

1. Actuators and Sensors.

2. Signals and Conditioning.

3. Digital Logic System.


4. Data Acquisition system and software.

5. Computer and display devices.

H Process can be mechanical or chemical or any other in the

mechatronic system. For understanding the elements of a mechatronic

system let us consider a mechanical system.

1. Sensors

H The parameters of the mechanical system like pressure, temperature,

displacement etc, are sensed by the respective sensor and are

converted into a signal. This signal is input to a signal conditioning

unit.

2. Signal Conditioning

H The signal obtained from the sensor is converted according to the

requirement. Signals are of two types one is an analog signal and the

other is a digital signal. Hence there are two types of convertor DAC

(Digital to Analog Convertor) and ADC (Analog to Digital Convertor).

3. Digital Logic system

H This is actually the control unit where the signal is analyzed and

proper response or feedback is given to the system. In this unit only

PLC or Micro–controller or any other control circuit are there. With

advancements in the system this unit is interfaced with a computer.

This enables much easier control for the end user.

4. Computer Systems

H In Computers data of the system is acquired by data acquisition units

and stored as data logs. From this data logs one can monitor and

analyze the overall functioning of the system. There are special data

logger or other related devices which are now available with an

interface to connect with a computer.

H Computers are also equipped with display unit. This display unit is

now programed through software to control the entire system. An apt

example for this is BMS (Building Management System).


H BMS software shows the facility’s Air-condition system, CCTV,

Electrical system, etc. With the control terminal of the BMS control

room, one can control all the integrated system of the facility.

1.7. MEASUREMENT SYSTEM

H It is essential to know the state of a system. State of the system is

determined by the Properties/Parameters of the system. In a

mechanical system the properties/parameters are temperature, pressure,

displacement, etc. In electrical system the parameters are current,

voltage, resistance, etc. Mechatronics systems are an integration of

technologies, hence it is a must that parameters of one technology is

read by another technology. To enable that capability a Measurement

system is required. A measurement system is composed of three

components as illustrated in Fig. 1.9.

H The parameters of the systems are read by an appropriate sensor or

transducer. This is in the form of a signal which can either be digital

or analog. As per the system requirement the signal is processed by

a DAC or ADC units by the signal processor.

H This signal processed is shown on the display screen. The display

panel has the control unit which sets the limits of the parameters or

is pre-programed as per the reading. It also stores the data in log files

in the form of readings, tables or graphs etc., as per the design.

H Digital weighing machine can be considered as a simple example to

illustrate the above system as shown in Fig. 1.10 (a).


H When a load is placed on the machine it is actually placed on a strain

gauge. This strain gauge is strained. The strain is converted into

millivolts. This voltage signal is amplified. The amplified voltage is

programed by the logic units to give the analogous reading in

kilograms or pounds etc., at the display unit.

1.8. CONTROL SYSTEM

H In many systems, it is not enough just to measure a parameter. It is

also required to control the parameter. A parameter is either

maintained as constant or varied in a pre-programmed way. To control

a parameter, say pressure, the following is required to be considered:

1. To control any parameter, the first requirement is the real-time

reading of the parameter. Hence the first requirement is to

know the pressure level in the system under observation to

control that.

2. Once that parameter is measured, it must be compared to a

standard. After measuring the pressure of system in bars or

pascals, it must be compared to a standard to know if the

pressure is high or low in the system.

3. After comparison, if the parameter is within the desired range,

then it is maintained otherwise control action is taken. There

are numerous ways to control the variable parameters.

A general control system is illustrated in Fig. 1.10(b).


1.8.1. Basic Terminology used in Control System

(a) Reference Variable or Input: Reference variable is that benchmarked

variable which is used to compare with the system output to know if the

output is in the specified desired level. It is like when petrol is bought from

the bunk, the operator types the amount say Rs.100/- in the counter. The

counter runs a dispensing petrol until it reaches Rs.100/-. This Rs.100/- amount

is ‘Reference Variable’ of the System.

(b) Output: It refers to the actual response of the system as per the input fed

to the system.

(c) Feedback: The output of a system is measured. This measure is in the

form of a signal which is fed to the control circuit. This path from the output

to the control unit is considered as feedback. Refer Fig 1.12.

(d) Error: The difference between the reference variable and the system output

is called error.

(e) Disturbance: Those signals which disturb the system by affecting the

reference variable or other control features are considered as ‘Disturbance’.

(f) Actuating Signal: The response signal due to the error which actuates the

system to change the output is called “Actuating Signal”.

(g) Control or feed forward Elements: The components which are connected

between control unit and the output unit are considered as the feed forward

elements.

(h) Controlled Output: The parameter (Pressure, Temperature, etc.) which is

regulated/Guided/controlled for the system is called “Controlled Output”.

(i) Feedback element: The elements which are used to generate feedback in

the system are the feedback elements.

1.8.2. Types of Control System

H Fig. 1.10 illustrates a general control system. It has not mentioned

how the controlling is done. There are two basic ways in which a

system is controlled and they are


(a) Open Loop Control System

H In this system, the control parameter is simply regulated. Just like a

fan regulator which merely regulates the speed of fan with various

settings. Here the output is only regulated as per the pre-programed

set up. An open loop control system can be illustrated as shown in

Fig. 1.11.

Advantages and Disadvantages of Open Loop

Control System

Advantages Disadvantages

(a) Manufacturing cost is low as it is

very simple.

(a) Control is limited as per the

pre-programing.

(b) Ease of control and Maintenance. (b) Control is manually operated and

hence it is slow and subjected to

human error.

(c ) Pre-programed as per the

requirement.

(c ) Output opt imiza tion is not

possible as there is no feedback.

(d) Very useful in application where

the output is difficult to measure or

economically not feasible.

(d) This system cannot be automated.

(e) It is very economical to use in

applications where the control output

requirement levels are clear.

(e) Cannot be used in complex

applications where the control output

has to be monitored and maintained

even with all variations.


(b) Closed Loop Control System

H In this system, as illustrated in Fig. 1.12, the control parameter is

instantaneously controlled. This is achieved by the means of a

feedback. From the output a feedback is generated. This generated

signal is compared with the set conditions in the control system. If

there is a difference, an error is generated. To compensate the error,

control is activated and output is varied to match the set condition.

This process continues till the error is nil or zero.

H A closed loop control system can be explained from the working

principle of a compressor in an Air conditioning unit. On turning on

an AC unit, an user sets the temperature as 21°C. Now the unit must

maintain the room temperature to 21°C. The thermostat measures the

temperature of the room and converts it to a signal. This signal is

compared analogously with the set temperature 21°C.

H If the temperature of the room is more said 26°C then the error is

positive. This results in activating or switching on the compressor

which is a key component in the AC unit for regulating the

temperature. The comparison of room temperature with the set

temperature is continuous. As soon as the temperature of the room

drops to 21°C the error becomes zero. Depending on the programing

of the AC unit, the compressor will be switched off. Hence the

compressor will cut-in or cut-off as per the fluctuations of the room

temperature.


1.8.3. Basic terms used in Closed Loop Control System

H Process element: It is the element of the system which is to be

controlled. It can be a room where the temperature is controlled or

a tank where water level is controlled etc.

H Measurement Element: The element which is used to measure the

state of the process element is called Measurement Element.

H Reference point or Set point: It is the standard signal which is set

in the system to control the output.

H Comparison Element: This element compares the reference value to

the measured value. The difference between them is considered as

error.

(Error = Reference value − Measured value)

H Control Element: This element reads the error signal and produces

a signal to correct the error.

H Correction element: It is that element which receives a signal from

the control element and makes changes in the output accordingly.

H Controlled Variable: It is that parameter which is controlled by the

control system. It is the temperature of the room which is controlled.

H Manipulated Variable: To control the output or the controlled

variable there is a variable which is changed and it is called

manipulated variable.

For Air conditioning Compressor system Example

Process Element Room

Measurement Element Thermostat

Reference Point Set Cooling Temperature

Comparison Element Electronic control circuit (It compares the

signals)

Control Element Electronic control (as per the program

generates the signal to correct)

Correction Element Compressor on/off switch

Manipulated Variable Temperature of the AC unit

Controlled Variable Temperature of the room


1.8.4. Comparison between Open loop and Closed loop Control System

Feature Open loop System Closed Loop System

1. Cost Low High

2. Feedback No feedback is there Feedback is there

3. Accuracy Limited to

pre-programing

As per the efficiency

of feedback

4. Construction Simple Complex

5. Non-lineraity System can malfunction Well within the

specified range of

non-linearity.

6. Stability Stable as per the

pre-program condition

Continuously active

with feedback.

7. Response time Slow as it is manually

operated

Instantaneous as it is

automated

8. Output Optimization Not possible Possible within the

limits of the control

system.

9. Maintenance Easy Difficult

10. Disturbance

Handling

Chances of having a

disturbance are limited.

Depends on the

systems failsafe’s and

signal filters efficiency.

1.8.5. Application which use Automatic Control System

H A closed loop control system is an Automatic control system. In such

systems the control parameter is either pre-defined as per design

specification or set by the user as per within the range of the designed

specification. Some of applications which use Automatic control

system are as follows.


(a) Automatic Tank level indicator control system

H Fig 1.13(a) illustrates the schematic diagram of the Tank level

indicator system. The water is stored in a tank. Inside the tank there

is a float. This float raises or dips as per the level in the tank. This

float is connected to a level transmitter which uses the property of

the float to measure the water level in the tank.

H This measured level is transmitted by the Level Transmitter as a signal

to the Level controller. In the level controller, there is a comparison

element which compares signal from the level transmitter with the

standard pre-set signal. If there is a difference in two signals then

error is generated. (Error = Reference value − Measured value) Based

on the error that is if level is low, then control valve is activated to

open and water flows into the tank. If the error is null or negative

(In case of pre-design failsafe or malfunction) then the control valve

is closed. Fig 1.13(b) illustrates the control system block diagram of

the Tank level indicator system.


Tank Level Indicator Control System

Process Element Water level in the tank

Measurement Element Float indicator

Reference Point Set Level point

Tank Level Indicator Control System

Comparison Element Level Controller

Control Element Level Controller

Correction Element Control valve open/close

Manipulated Variable Water

Controlled Variable Tank level

(b) Lubrication Oil cooling system

H Lubrication is very important for engines. It not only reduces the wear

and tear, but also regulates the temperature of the engine. But

lubricant’s mechanical properties like viscosity and density change

completely if its temperature crosses a certain limit. Hence it is

important to maintain the temperature of the lubricants within that

limit in which its mechanical property remains unaffected. Fig. 1.14

illustrates the Lubrication oil cooling system.


H From the engine hot lubricant comes out and enters the heat

exchanger. In the heat exchanger the hot lubricant’s heat is exchanged

with cold water. The cool oil goes to the oil sump. The temperature

of the oil at the sump is measured by a thermostat. This temperature

is fed to the control valve. The control valve compares the temperature

with set temperature. If there is difference then it adjusts the flow of

water by adjusting the valve opening.

H Hence the flow rate of water changes as per the temperature of the

oil in the sump. This oil is then pumped into the engine. If the

lubricant’s temperature is beyond the limit, it means the lubricant is

old and has lost its mechanical property and it is time to replace with

fresh stock.

H In order to safeguard the engine from entry of hot lubricant, there is

a failsafe system in place. A thermostat measures the temperature of

the lubricant entering into the engine. This temperature is compared

with the set temperature range at the failsafe. If the temperature is

excess, then the failsafe with trip/stop the engine.

Lubrication oil cooling system Engine failsafe system

Process Element Lubricant Lubricant

Measurement Element Temperature of

lubricant

Temperature of

lubricant

Reference Point Set temperature at valve Set Temperature at

Failsafe

Comparison Element Control valve Failsafe

Control Element Control Valve Failsafe

Correction Element Control Valve

open/close

Failsafe Tripper on/off

Manipulated Variable Water flow rate Engine tripper

Controlled Variable Temperature of

Lubricant

Engine On/Off

(Emergency)


(c) Automatic shaft speed control system

H Fig. 1.15 illustrates the schematic and block diagram of the control

system used to control the speed of the shaft. The speed of the shaft

is measured by the Tachogenerator. The measured speed is sent to

the Differential Amplifier. Differential Amplifier boosts this signal and

compare with set speed signal by the resistance potentiometer. If there

is an error then accordingly a signal is given to the motor to increase

or decrease the speed. Thus the speed of shaft which is coupled with

motor is increased or decreased to get in level with the set speed.

Shaft Speed Control System

Process Element Shaft

Measurement Element Tachogenerator

Reference Point Set speed by resistance potentiometer

Comparison Element Differential Amplifier

Control Element Differential Amplifier

Correction Element DC Motor

Manipulated Variable DC Motor Speed

Controlled Variable Shaft Speed


1.8.6. Analogue and Digital Control systems

H There are two kinds of signal

which can be used in the

control process. They are

digital and analogue signal.

Analogue signal are continuous

signal which varies with time.

Digital signals are signals that

represent a sequence of discrete

values. They are illustrates in

Fig. 1.16.

H Based on the measuring device and control element these signals have

to be converted into the other. Hence we have Digital to analogue

Convertor (DAC) and Analogue to Digital Convertor (ADC). Hence

the control system with these convertors can be illustrated with

previous example of Shaft speed controller as shown in Fig. 1.17.

H The measured shaft speed by tachogenerator is in analog which is

converted into digital by ADC before sending it to the differential

amplifier which is in microprocessor unit. The response signal from

microprocessor unit is converted from digital to analog signal before

feeding to motor through the signal amplifier. Thus both ADC and

DAC are used in control system.

1.8.7. Sequential Controllers

H In process or a plant, there are operations which occur in a sequence.

In some cases like a production line, the output of first operation

becomes the input of second operation and in other cases the same


object is subjected to different operations in a sequence like a product

undergoing a series of quality checks. Each operation in the sequence

must be controlled to get the desired end product/output. To facilitate

that, these plants or process is equipped with sequential controllers.

Let us understand this with the following example.

A. Domestic Washing Machine

In an automatic domestic washing machine, once it is loaded with

laundry, there are number operations machine has to perform to wash them.

They can be listed out as

(a) Pre-wash: In this operation, the closed washing machine drum is filled

with cold water by which laundry gets soaked. Then the machine spins the

drum gently. As per the timer set by Program or by the user, this process

continues.

(b) Main wash: In this operation first the cold water is drained. Then hot

water (Temperature set by the user) along with detergent or any other washing

agent fills the drum. Then the machine spins in normal wash speed (also set

by the user) to wash the laundry. As per the timer set by program or user,

the process goes on.


(c) Rinsing: In this operation, the washed water in the drum is drained and

it is refilled with cold or hot water as per the settings. Then the drum again

spins removing soapy detergent and dirt from the clothes. The time is again

preset by the program or by the user. Depending on the kind of fabric, this

cycle is repeated.

(d) Drier: The rinsed water is drained from the drum. Then the drum spins

expelling water from the laundry. Once the drum comes to rest the drum door

can be opened. The laundry washed will be wet but not soaking wet. Then

the laundry removed from the machine and hung on a wire in the sun to get

them dried manually.

H The operations Pre-wash, Main-wash, Rinsing and Drier are carried

out in sequence. Apart from that, there is also emergency stop and

reset options in the washing machines. Each operation has its own

control parameters and set points to conduct them.

H The domestic washing machine controls the open/close of water inlet

valve. It measures and senses the water level in the drum. It

opens/closes the drain valve by sensing the level of the water in the

drum. It also controls the temperature of the water and the speed of

the drum. In all the operations all these parameters come into play.

Every operation is also timed by timer switches.


H In earlier days, the mechanical control was used. The function of the

timer was performed by cam switches. The timer switch was made

by synching a small motor with sliding or point contact which follows

the profile of the cam as illustrated in the Fig. 1.19(a).

H There are many limitations in cam switches. Today they are replaced

by microprocessor control which is also referred as a microcontroller.

A simple microprocessor with memory is integrated on one chip

known as embedded microcontroller.

H Microcontrollers can be pre-programed to perform the same logical

operations that are required for a washing machine or any other

applications. The advanced adoptable form of the microcontroller is

the “Programmable Logic Controller”. It is used for complicated

system where the process condition varies and a great deal of

flexibility is required. A PLC can be programmed and reprogrammed

as the situation demands for producing the desired output.

1.8.8. Microprocessor based Controllers

H A Microprocessor is also known as the Central Processing Unit

(CPU). It is the brain of computer, household appliances and

electronic devices. A Microprocessor is not a stand alone device, it

must be integrated with input/output device along with memory to

perform functions.


H When a microprocessor is integrated with memory unit, input, output

units and programmed for a particular control application of a system

or a plant, it becomes a Microcontroller. Different kinds of configured

microcontrollers are used in applications as the control element.

H When the demands of the application became complicated with higher

flexibility, a pre-programmed micro-controller was not a viable option.

To meet this demand, PLC (Programmable Logic Controller) came

into the market. The main feature of a PLC is its programmable ability

as per the requirement of the output. Fig 1.20 gives the general

architecture of a PLC.

H Let us now study some of the commonly used applications which

uses microprocessor based system for control.

(a) Automatic camera

H The automatic modern camera has automatic focusing and exposure.

The microprocessor based system is used for controlling the focusing

and exposure.

H Switch is on to activate the automatic camera system. Then the camera

is pointed at the object to be photographed. The microprocessor takes

the input using range sensor and sends an output signal to the lens

position drive in order to position the lens to achieve correct focusing.


Then the lens position is fed back to the microprocessor so that this

feed back signal will be used to modify the lens position according

to the input from the range sensor.

H As soon as the photograph is taken, the microprocessor sends signal

to the motor drive to advance the free space for the next photograph.

(b) Copying Machine

H The copying machine is a best example for mechatronic system. This

machine has analog and digital circuit, sensors, actuators, and

microprocessors. The operating procedure is given here.

H The operator places the original in a loading place and presses a

button to start. The original is taken to the platen glass. A high

intensity light source scans the original and transfers the original’s

image to a drum. A blank piece of paper is taken from the loading

cartridge and the image is transferred on to the paper. During this,

ink toner powder is heated to bond to the paper according to the

image. This is known as electrostatic deposition. Then the xerox copy

is delivered to the appropriate bin which is known as sorting

mechanism.


H Analog circuits control the lamp, heater and other power circuits in

the machine. Digital circuits control the digital displays, indicator

lights, buttons, and switches forming the user interface. Other digital

circuits include logic circuits and microprocessors which co-ordinate

all of the functions in the machine.

H Optical sensors and microswitches detect the presence or absence of

paper, its proper positioning and whether (or) not doors and latches

are in their correct positions. Other sensors include encoders used to

track motor rotation. Actuators include servo and stepper motors that

load and transport the paper, turn the drum and index the sorter.

(c) Engine Management System


H This system is useful to manage the ignition and fuelling requirements

of the engine. Let us consider the four stroke internal combustion

engine in which each cylinder has piston and connected to the

common crankshaft. We have already studied the operation of 4 stroke

engine which are given here briefly.

H When the piston moves down, the inlet valve opens and air fuel

mixture is sucked into the cylinder (suction stroke). When the piston

moves up, the inlet valve closes and the airfuel mixture is compressed,

(compression stroke). At the end of compression stroke, the spark is

thrown on the airfuel mixture and gets ignited. Sudden explosion takes

place and so the piston is pushed down to execute the expansion

stroke. This is known as working stroke (or) power stroke. As soon

as the piston reaches the bottom dead centre, it moves up again to

execute exhaust stroke to send out the burnt gases. During this time,

the exhaust valve is open.

H The different pistons have their power strokes at different times to

enable the common crank shaft rotate continuously.

H The power and speed of the engine can be controlled by changing

the ignition timing and airfuel mixture ratio. Now-a-days, the

microprocessors are used for controlling ignition timing and air-fuel

ratio.

H The important elements of

microprocessor are shown in

Fig. 1.23.

H To control the ignition timing, the

crankshaft rotates and drives the

distributor. The distributor makes

electrical contacts for each spark plug

in turn and a timing wheel. The

timing wheel produces pulses as the

indication of the crankshaft position.

The microprocessor then adjusts the

timing so that at ‘right’ moments of

time, the high voltage pulses are sent

to the distributor.


H To control the airfuel mixture, the microprocessor activates the

solenoid valve to open the inlet valve according to the throttle position

and engine temperature as inputs. The amount of fuel injected into

the air stream are determined by an input from a sensor of mass rate

of air flow and the microprocessor then gives an output to control a

fuel injection valve.

1.9. SENSORS

H Sensors and transducers are used widely in describing measurement

instruments. The usage of the word sensor is rooted from USA,

whereas transducer is rooted from Europe. The meaning of the word

Sensor is “to perceive” and Transducer is “to lead across”.

H A Sensor is defined as a device that detects a change in a physical

stimulus and turns it into a signal which can be measured or recorded.

H A Transducer is defined as a device that transfers power from one

system to another in the same or in the different form (Strain to

voltage).

H Hence a sensor by itself will sense the state of a system and a

transducer with logic circuitry can be used to ascertain the same.All

transducers would thus contain a sensor and most (though not all)

sensors would also be transducers. Fig. 1.24 shows the sensing process

in terms of energy conversion.

H Most of the time the output signal generated would be a voltage

analogous to the input signal. It sometimes may be in a wave form

whose frequency is proportional to the input or a pulse train

containing the information in some other form.


H In Mechatronics sensing the parameters of system is key function to

know the state of the system. Only by knowing/measuring the state

of the system appropriate control measures requirements can be

ascertained.

1.9.1. Classification of Sensors

H There are many ways sensors can be classified. Some of the ways

are discussed as given here.

(a) Based on the stimulus that is the response signal, sensors can be classified

as per table given here.

Table 1.1

S.No. Stimulus

1. Acoustic Wave (Amplitude, Phase, Polarization), Spectrum, Wave

Velocity.

2. Electric Charge, Current, Potential, Voltage, Electric field

(amplitude, phase polarization & spectrum).

3. Magnetic Magnetic field (amplitude, phase, polarization,

spectrum), Magnetic flux, Permeability

4. Optical Wave (amplitude, phase, polarization, spectrum), Wave

velocity, Refractive Index, Emissivity, Reflectivity,

Absorption.

5. Thermal Temperature, Flux, Specific heat, Thermal conductivity.

6. Mechanical Position (Linear, Angular), Acceleration, Force, Stress,

Pressure, Strain, Mass, Density, Moment, Torque,

Shape, Roughness, Orientation, Stiffness, Compliance,

Crystallinity, Structural.

(b) Based on the Power requirement

H Active Sensor: In these sensors, the output signal is generated by a

physical phenomenon of transduction (Like a generation of voltage

due to temperature difference at thermocouple or in a thermometer

where the level of mercury raises with the change in temperature). It


does not require any power source to measure the parameter of the

system. Hence it is also considered as Self-generating Transducer.

H Passive Sensor: These sensors require external power source to

function. Most of them work on the principles which are dependent

on electrical energy like resistance, inductance and capacitance. A

simple example is digital weighing machine which uses a battery

operated Whetstone bridge to measure weight. The machine stops

functioning when the battery is drained.

(c) Based on the type of output Signal

H Analog Sensors: When the signal output of a sensor is in analog

form, then those sensors are called analog sensors. Analog devices

produces a continuous signal which is proportional to the measured

parameter. A simple example is the Bourdon tube where pressure is

measured by elastic property of the tube.

H Digital Sensor: When the signal output of a sensor is in digital form,

then those sensors are called digital sensors. A simple example is

piezoelectric transducer used measure stress by natural phenomenon

piezoelectric effect.

(d) Based on conversion of a parameter to an electrical parameter

H Primary Sensor: When a sensor’s output signal is in the form of

electrical quantity like current, voltage etc., then they are considered

as the primary sensor. Thermocouple sensor or Hall Effect sensor are

typical examples of such kind of sensor.

H Secondary Sensor: When a sensor’s output signal is in any other

form than electrical quantity like voltage and current, then such

quantity (like displacement or strain) are transformed into voltage or

current. The function of secondary sensor is to transform other

quantities into electrical quantities like voltage or current.

(e) Based on the parameter measured

Some of the common parameters which are measured to know the state

of a system are

H Displacement

H Pressure


H Velocity

H Temperature

H Light

H Level

H Flow

H Proximity

1.9.1.1. Contact sensors

Contact sensors respond to physical contact such as touch, slip and torque

sensors

1.9.1.2. Non-contact sensors

1.9.2. Static and Dynamic Characteristics of Sensor

H A system basically exists in two states. One is called the Transient

state and the other is called Steady state. Transient state is a state

where the system is subjected to a sudden change. Steady state is a

state when the system reaches equilibrium. A sensor should work well

in both these states. In the steady state, the system is in a static

condition and hence the characteristics of a sensor in that state are

called as Static characteristics. In transient condition, the system is in

a dynamic condition and hence the characteristics of a sensor in that

state are called Dynamic characteristics.


A. Static Characteristics of a Sensor

(i) Accuracy: It is the extent to which a sensor is capable of measuring

the parameter’s value to the actual/true value in the system. The

accuracy of a sensor can be expressed in following ways:

(a) Point accuracy: When accuracy is specified at only one

particular point of scale it does not give any information about

the accuracy at any other Point on the scale it is called point

accuracy.

(b) Accuracy as percentage of scale span: When a sensor has a

uniform scale, its accuracy can be expressed in terms of scale

range.

(c) Accuracy as percentage of true value: Accuracy can also be

specified in terms of a percentage true value of the quantity

being measured.

(ii) Precision: The ability of the sensor to give the same measurement

at all time if the parameter measured remains constant in the system

is considered as precision. Precision is related to the variance of a

set of measurements taken by the sensor. For instance, a level

indicator sensor will indicate the same level in real time unless there

is an actual change in the level. But if the reading is changing when

it senses the same level then the precision of the sensor is bad. Hence

it is very much related to the repeatability, reproducibility and

reliability of the sensor. Precision are composed of two characteristics

(a) Conformity: Consider a weight of 3.451 Kg. When a weighing

machine with a sensor reads that measure it reads as 3.5 Kg

as it is scaled in that way. Though there is no deviation from

the value this error is caused due to the limitation of the sensor.

(b) Number of Significant figures: When a set of reading of the

same quantity is taken by a sensor then each reading is a

significant figure. These significant figures convey the actual

information about the magnitude & the measurement precision

of the quantity. The precision can be mathematically expressed

as


P = 1 − Xn − X

__n

Xn

P = Precision

Xn = Value of nth measurement

X

__n = Average of the set of measured values

For example let’s consider the following set of readings

S.No Reading Measured

1 32

2 31

3 30

4 29

5 32

The average value Xn = (32 + 31 + 30 + 29 + 32) ⁄ 5 = 30.8

The precision of the 4th reading is

P = 1 − [(30.8 − 29) ⁄ 30.8] = 0.941 = 94.1%

(iii) Error: The algebraic difference between the true value (At) and

indicated value (Am) of the parameter measured by the sensor is

called error (e).

e = At − Am

(iv) Repeatability/Reproducibility: The repeatability and reproducibility

of a sensor are its ability to give the same output for repeated

applications of the same input value. Repeatability is also defined as

the measure of deviation of test results mean value.

(v) Reliability: The reliability of a sensor is defined as the possibility

that it will perform its assigned functions for a specific period of

time under given conditions. The reliability of the sensor is affected

not only by the choice of individual part in it but also by the

manufacturing methods, quality of maintenance and the type of user.


(vi) Sensitivity: The sensitivity denotes the smallest change in the

measured variable to which the instrument responds. It is defined as

the ratio of the changes in the output of an instrument to a change

in the value of the quantity to be measured. Mathematically it is

expressed as

Sensitivity = Infinitesimal change in output

Infinitesimal change in input =

∆ q0

∆ q1

(vii) Linearity: Linearity is defined as the ability of the sensor to

reproduce the input characteristics of the measured parameter of the

system in a linear and symmetrical manner.

H Graphically an ideal relationship between the input and output would

be like the straight line as shown in Fig. 1.24(b). Practically the

relationship between the input and output would be like the actual

curve which is called as the calibration curve of the sensor.

H Linearity as mathematical quantity is defined as the percentage ratio

of maximum deviation between the ideal line and actual calibration

curve to full scale deflection.

% Linearity =

Maximum deviation of outputfrom idealized straight line

Full scale deflection × 100

It is expected to have a sensor as linear as possible as it is very much

related to its accuracy.


(viii) Resolution: Resolution is defined as the smallest increment in the

assured value that can be detected by the sensor. The smallest value

of input change which will produce an observable change in the

output of the sensor is called resolution. It is also referred as the

discrimination of the sensor. It is also known as the degree of

fineness with which measurements can be made. For example, if a

micrometer with a minimum graduation of 1 mm is used to the

nearest 0.5 mm, then by interpolation, the resolution is estimated as

0.5 mm.

(ix) Threshold: If the input quantity is varied from zero onwards, the

output does not change until some minimum value of input is

exceeded. This minimum value of input is called Threshold. Hence

resolution is the smallest measurable input change and threshold is

the smallest measurable input.

(x) Drift: The drift is the gradual shift in the sensor’s output over an

extended period of time during which the value of input variable

does not change. Drift may be classified into three categories:

(a) Zero Drift: It is defined as the deviation in the sensor’s output

with time, from its zero value when the variable measured is

a constant.

(b) Span drift or sensitivity drift: If there is proportional change

in the indication all along the upward scale, the drifts is called

span drift or sensitivity drift.

(c) Zonal drift: In case the drift occurs only at a portion of span

of a sensor, it is called zonal drift.

(xi) Stability: The ability of a sensor to retain its performance throughout

its specified operating life and the storage life is defined as stability.

(xii) Tolerance: The maximum allowable error in the measurement is

specified in terms of some value is called tolerance. It is closely

related to accuracy as many sensors manufactured specify accuracy

in terms of tolerance values. Hence tolerance indicates the maximum

allowable deviation of the manufactured sensor from a specified

value.


(xiii) Range or span: The range of a sensor is defined as 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. The load cell can only measure

from 20 to 100 N. Below 20 N or above 100 N it will not show

any reading. Hence the range of the load cell is between 20 to 100

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

(xiv) Bias: The constant error that exists in the sensor throughout its range

is called bias. It can be eliminated by calibrating the sensor.

(xv) Hysteresis: If the input variable is increased the output of the

measuring sensor also increases. This is given by curve 1. When the

input variable is decreased the output of the sensor also decreases.

This is given by curve 2. The difference between the two curves is

called hysteresis. Hysteresis is generally expressed as a percentage

of full scale reading.

(xvi) Dead Space: There will be no output response for certain range of

input values in a sensor which is known as Dead space/Dead band.

Backlash in gears is a good example of dead space. There will be

no output until the input has reached a particular value. The time

taken by a sensor to respond with an output after providing an input

is called Dead time.


B. Dynamic Characteristics of Sensor

(i) Response time: When a sensor is suddenly given an input (by turning it

on) or subjected to sudden change in the input, then there will considerable

delay before it indicates a specified percentage (98%) of actual measure of

the input as output. This delay/lag is called the Response time of the sensor.

(ii) Time Constant: It is 63.2% of the response time when subjected to step

input.

(iii) Rise Time: The time taken for the sensor’s output to rise to a specified

percentage of the steady state output. It is mostly considered as the time taken

to rise from 10% of steady state value to 90-95% of the steady state value.

(iv) Settling time: The time taken for the output of a sensor to settle within

a specified percentage of the steady state value is called settling time.

1.10. POTENTIOMETERS

H The Potentiometer is a displacement transducer. It uses a variable

resistance transduction principle to indicate the position or track the

displacement.

Construction

H As illustrated in Fig. 1.25, potentiometer consists of three terminals.

The one in the middle is known as the wiper/slider, and the other

two are known as ends. The wiper is a movable contact where

resistance is measured with respect to it and either one of the end

terminals.


Working Principle

H “The potential difference across any length of a wire of uniform

cross-section and uniform composition is proportional to its length

when a constant current flows through it.”

H A battery is connected to a potentiometer wire “AB” with a switch

control. Current ‘I’ from the battery flows through potentiometer wire

AB forming the primary circuit.

H A primary cell of potential

difference ‘E’ is connected

in series with a positive

terminal of the battery

along with a galvanometer,

High resistor and a jockey

forming the secondary

circuit.

H If the potential difference

between AJ and the

potential difference of the

primary cell (E) is same

then there will be no

deflection in the galvanometer. AJ is called balancing length.

The potential difference between AJ (VAJ) is expressed as

VAJ = I x r x l


Where I – Current in the primary circuit

r – Resistance of unit length of potentiometer wire

l – Varying length on the balancing length of the potentiometer

Hence

E = I x r x l

H Since ‘I’ and ‘r’ are constant E is proportional to l. This voltage ‘E’

is what is measured in real time application which is directly

calibrated to indicate the length/displacement.

Some applications of Potentiometer

H It is used to adjust the level of analog signals.

H It is used as a control inputs for electronic circuits.

H It is used as light dimmer in lamps.

H Preset potentiometers are widely used throughout electronics wherever

adjustments must be made during manufacturing or servicing.

H User-actuated potentiometers are widely used as user controls, and

may control a very wide variety of equipment functions.

H In consumer electronics, it is used as such as volume controls and

position sensors.

Audio control linear potentiometers (“faders”)

H Low-power potentiometers, both linear and rotary, are used to control

audio equipment, changing loudness, frequency attenuation and other

characteristics of audio signals.

H The ‘log pot’ is used as the volume control in audio power amplifiers,

where it is also called an “audio taper pot”, because the amplitude

response of the human ear i s approximately logarithmic .

Potentiometers used in combination with filter networks act as tone

controls or equalizers.


H Television Potentiometers were formerly used to control picture

brightness, contrast, and color response.

H A potentiometer was often used to adjust “vertical hold”, which

affected the synchronization between the receiver’s internal sweep

circuit (sometimes as a multi-vibrator) and the received picture signal,

along with other things such as audio-video carrier offset, tuning

frequency.

H Motion Control Potentiometers can be used as position feedback

devices in order to create “closed loop” control, such as in a servo

mechanism Transducers

H Potentiometers are also very widely used as a part of displacement

transducers because of the simplicity of construction and because they

can give a large output signal.

H Computation in analog computers, high precision potentiometers are

used to scale intermediate results by desired constant factors, or to

set initial conditions for a calculation.

H A motor-driven potentiometer may be used as a function generator,

using a non-linear resistance card to supply approximations to

trigonometric functions.

H Linear Track Systems, Gate Positioning,Injection Molding Machines,

Spray Painting Robots, Liquid Level Measurement, Railroad Track

Laying Equipment.

H Medical − Cat Scan Tables, Bone Densitometers, MRI Tables.

H In Automobiles − Brake Position, Clutch Position, Steering Position,

Throttle Position, Suspension Flexure, Body Movement, Crash

Testing.


1.11. LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

H LVDT is a lso cal led as

Differential transformer or

Linear Variable Displacement

transformer. It is an electrical

transformer used for

measuring linear

displacement. There is also a

transformer used to measure

angular displacement referred

to as Rotary Variable

Displacement Transformer

(RVDT).

H These transducers are non-contact type hence they have long life and

they produce very accurate results. They work using AC supply, hence

there are no electronic components in it. Even in extreme temperatures

they can be used. LVDT uses an electromagnet coupling to convert

linear displacement into an equivalent electrical signal.

Construction:

H It consists of primary coil wound on the middle of a hollow

cylindrical rod which is connected to AC supply. On the same hollow

cylindrical rod, two secondary coils of equal turns are wound at an

equal distance from the primary coil on either side. The two secondary

coils are connected in series in opposition to each other to get net

induced EMF as the difference between them.

E0 = ES1 − ES2

Where

E0 − Net Emf

ES1 − EMF induced at secondary coil S1

ES2 − EMF induced at secondary coil S2

A movable soft iron core is placed inside this hollow cylinder.


Working Principle

H When supply is given to the primary coil, current flow through the

coil creating a varying magnetic field in it. This magnetic field

interacts with the secondary coil producing EMFs in them. When the

iron core is in the center the EMF produced by both the coils are

equal. Hence the net EMF of the coil is zero. The position of the

iron core is considered as Null position.

H If the core moves forward towards the secondary coil S2, then the

EMF induced in that coil will become greater hence the net EMF

will be negative.

ES1 < ES2 , E0 = ES1 − ES2 = − ve

H If the core moves backward towards the secondary coil S1, then the

EMF induced in that coil will become greater hence the net EMF

will be positive.

ES1 > ES2 , E0 = ES1 − ES2 = + ve

H Thus by using the magnitude and polarity of the net EMF induced,

LVDT measures the displacement of the core from the Null

position.


Some applications of LVDT

H Displacement − extensometers, temperature transducers, butterfly valve

control, servo valve displacement sensing, Precision gap between torch

and surface of Welding work.

H Deflection of Beams, Strings, or Rings.

H Load cells, force transducers, pressure transducers.

H Thickness Variation of Work Pieces-dimension gauges, thickness,

surface irregularities and profile measurements, product sorting by

size.

H Fluid Level and fluid flow measurement, position sensing in hydraulic

cylinders.

H Velocity & Acceleration-automotive suspension control.

1.12. CAPACITANCE SENSORS

H A capacitance sensor is a variable capacitor. It is used for measuring

displacement, pressure, etc.

Construction

H The capacitance sensor shown in Fig. 1.29 consists of two parallel

metal plates separated by substance like air called dielectric. In normal

capacitor the distance between the plates are fixed and this makes the


capacitance of the capacitor constant. In a capacitance sensor the

distance between the plates is a variable and hence there will be

change in the capacitance which can be measured easily.

Working Principle

H In capacitive transducers, the value of the capacitance changes when

there is a change in the input parameter’s value which is to be

measured. This change in capacitance is measured and calibrated

against the input parameter’s value. Thus the value of the input

quantity is measured.

The capacitance C between the two plates of a capacitance transducer is

C = ε0 × εr × (A ⁄ d)

Where

εo – Absolute permittivity

εr − Relative permittivity

A – Area of the plate

d – Distance between the plates


H The product of εo × εr is called Dielectric Constant of the capacitor.

Hence as per the formula of calculating capacitance, the value of

capacitance varies due to

(a) Dielectric Constant

(b) Area of the plate

(c) Distance between the plates

H Hence depending on the parameter which is used to change the

capacitance of the capacitive transducer, there are three types of

capacitive transducers.

1. Changing Dielectric Constant type

H The dielectric material between the two plates is changed (Fig 1.30),

due to which the capacitance of the transducer also changes. When

the input value changes, corresponding to that the value of the

dielectric constant also changes. Hence the capacitance of the

transducer also changes.

H This capacitance change is calibrated to measure the input parameter

value directly. This principle is used for measurement of level in the

hydrogen container, where the change in level of hydrogen between

the two plates results in change of the dielectric constant of the

capacitive transducer. This principle can also be used for measurement

of humidity and moisture content of the air.


2. Changing Area of the Plates type

H When the area of the plates changes, the capacitance of the variable

capacitive transducer also changes. This principle is used in the

torque-meter, used for measurement of the torque on the shaft. This

comprises of the sleeve that has teeth cut axially and the matching

shaft that has similar teeth at its periphery.

3. Changing Distance (Linear/Angular) between the Plates type

H In these capacitive transducer, distance between the plate is variable,

while the area of the plates and the dielectric constant remain constant.

This is commonly used variable capacitive transducer. For

measurement of the displacement of the object, one plate of the

capacitive transducer is kept fixed, while the other is connected to

the object.

H When the object moves, the plate of the capacitive transducer also

moves, this results in change in distance between the two plates and

the change in the capacitance. The changed capacitance is measured

easily and it is calibrated against the input quantity, which is

displacement. This principle can also be used to measure pressure,

velocity, acceleration etc.


Some applications of Capacitance Sensors

H Level Control of Liquids, Solids.

H Pile-up Control.

H Monitoring at Hazardous Area Environments, High Temperature

Environments, High Pressure Wash-down Environments.

H Feed Hopper Level Monitoring.

H Small Vessel Pump Control.


H Suitable for use in Environments with Inconsistent Power Supplies.

H Grease Level Monitoring.

H Pharmaceuticals Manufacturing.

H Suitable for Use with Chemicals.

H Pipeline Leak Detection.

H Vessel Leak Detection.

1.13. STRAIN GAUGES

H Strain gauges are used for measuring displacement, stress, strain

and force. These are classified as mechanical, optical or electrical

gauges depending on the principle of operation. Among them, the

most commonly used and modernized gauge is the electrical resistance

type gauge. It is popular as the process of measurement is simple and

advantageous over all other gauges.

Construction: Electrical resistance type strain gauge


H The strain gauge shown in Fig. 1.33 has sensing element in the form

of a thin metallic resistive foil grid made of about 3 to 6 µm thick.

This grid is put on the base of a plastic film of 15 to 16 µm thick

and is laminated with another thin film on the top. Gauge sensing

length is marked along with center markings on the length and width

of the grid. Leads to the gauge are soldered to wire made of silver

clad copper of 120-160 µm diameter and 2500 µm of length.

H The strain gauge is bonded to the measuring object with a dedicated

special adhesive. Strain occurring on the measuring site is transferred

to the strain sensing element via the gauge base through the leads

which are normally connected to Whetstone’s bridge or any other

resistance measuring device. For accurate measurement, the strain

gauge and adhesive should match the measuring material and

operating conditions including temperature.

Working Principle

H Strain gauge is basically a wire which goes back and forth as in

Fig. 1.34. Depending on the direction of deformation the active axis

is aligned with the object on which strain is to be measured.The input

can be anything like force, torque or pressure which can deform the

element on which measurement is to be made. This deformation is

transferred to the strain gauge which changes the resistance of the


gauge which is measured in the Whetstone’s bridge or any other

resistance measuring device. This is directly calibrated to indicate the

parameter which is to be measured by the sensor.

The strain is measured by the following equation

∆ R

R = Gf ε

Where R – Base Resistance

∆ R – Change in Resistance

Gf – Gauge factor which is the constant of proportionality

ε – Strain

H Based on the type of element there are different forms of strain gauge

as shown in Fig. 1.35.

H The strain is affected by temperature. Due to thermal stress, the

expansion and contraction of the wire changes the resistance of the

wire. Hence proper material is used to withstand this and an

appropriate correction factor is used to improve accuracy of the gauge.

Strain gauges are used for measuring

H Stress analysis

H Forces

H Moments


H Pressures

H Accelerations

H Displacements

H Vibrations

1.14. EDDY CURRENT SENSORS

H Eddy current is circular current which is induced by a conductor when

it is kept in a changing magnetic field. Eddy current sensors are

used as proximity sensors to detect the presence or absence of

non-magnetic conductive material. It is a non-contact sensor.

Construction

H The eddy current sensor consists of a coil wound on a ferrite core

connected to a signal amplifier and a signal processor as shown in

Fig. 1.36. The coil has an AC supply. The coil and the core

arrangement are made inside a probe. This probe is connected to the


signal processing unit with the signal amplifier and signal analyzer.

As per the application, the signal processing unit can be calibrated to

indicate the parameter which is to be measured.

Working Principle

H When supply is given to the sensing coil, it produces a magnetic field.

When this magnetic field cuts the work piece, it generates Eddy

current in it. The Eddy current in the work piece generates a magnetic

field opposite to the sensing coil’s magnetic field.

H Eddy current is very weak. Hence the magnetic field it produces is

also very weak. When two magnetic field clashes, it produces a small

distortion which is detected by the signal processing unit. These small

distortion signals are recognized when it is amplified and then are

easily analyzed. Based on the applications, this sensor is used with

the signal processing unit programmed.

H This sensor is very accurate and cost of construction is low. It can

be used in extreme conditions as it is not temperature sensitive. Only

problem is the distance from the target must be less.

Some Applications of Eddy Current Sensor

H Automation requiring precise location.

H Machine tool monitoring.

H Final assembly of precision equipment such as disk drives.

H Precision stage positioning.

H Drive shaft monitoring.

H Vibration measurements.


1.15. HALL EFFECT SENSOR

H ‘Hall Effect’ is used to measure magnetic field. The sensor applying

‘Hall effect’ principle is used to measure position, displacement,

level and flow. It is also a non-contact sensor.

Hall Effect Principle

H When a thin conductor is powered by battery, then current will pass

through the conductor in a straight line. But when that conductor is

subjected to a magnetic field, then current flow will be disturbed by

a force called Lorentz force. Hence electrons will move to one side

of the plate and the positive poles to the other side of the plate,

creating a potential difference in between the two sides of the plate.

This is measured with a multimeter. This process of obtaining a

measurable voltage is called Hall Effect.

Basic types of Hall Effect Sensors

There are basically two kinds of Hall Effect sensors based on the output

signals which can be either analog or digital. Any Hall Effect sensor has a

Hall element which produces the voltage due to Hall Effect. This voltage is

very low, hence it is amplified by a High Gain Amplifier. The amplified


voltage is an analog signal. When the application requires analog output, this

circuitry is enough. However, if the application demands a digital output, then

this analog signal is fed to a Schmitt Trigger. It converts analog signal into

a digital signal.

Hall Effect Switch

H The Hall Effect switch is nothing but a digital Hall Effect sensor with

pre-set output condition. Fig. 1.39 illustrates a typical Hall Effect

Switch. The voltage output of the sensor is conditioned. If the voltage

is less than the set standard, then it is in the ‘off’ state and if it is


more than the set standard, then it is in the ‘on’ state. Thus, it is

used as a switch.

Hall Effect Sensor for determining RPM of Wheel

H Here the Hall Element with a permanent magnet is placed near the

rotating disc. The gap between the sensor and the teeth of the disc


is fixed and very less. Hence, when the disc is in motion the teeth

cuts the magnetic field at regular intervals forming a pattern. This

pattern gives an output in the form of a square wave signal which

can be easily processed to determine the RPM of the shaft.

Hall Effect Sensor for determining Liquid Level

H Hall effect transducers are

used for sensing position,

displacement and

proximity when the object

to be sensed is fit with a

small permanent magnet.

Normally, the fuel level in

an automobile fuel tank is

determined by this Hall

effect transducer as shown

in Fig. 1.41.

H The magnet will be attached to a float. As the fuel level raises (or)

lowers, the float with magnet distance from the Hall effect sensor

changes. Due to this, Hall voltage will be induced which measures

the distance of the float from the sensor. Hence the fuel level can be

determined.

Some applications of Hall effect Sensor

H Wheel Speed sensors − RPM.

H Crankshaft/Camshaft position sensors.

H Hall Effect Switches.

H MEMS Compasses.

H Proximity Sensors.


1.16. VELOCITY SENSOR

Velocity sensors are used to measure the velocity or speed by taking

consecutive position measurements at known intervals and computing the time

rate of change of the position values or directly finding it based on different

principles.

1.16.1. Tachometer:

H Tachometer is directly used to find the velocity at any instant of time,

and without much of computational load.

H This measures the speed of rotation of an element.

H There are various types of tachometers in use but a simple design is

based on the Fleming’s rule, which states “the voltage produced is

proportional to the rate flux linkage”.

H A conductor (basically a coil) is attached to the rotating element which

rotates in a magnetic field (stator).

H As the speed of the shaft increases, the voltage produced at the coil

terminals also increases.

H Figure 1.42 shows the schematic of tachometer.


H Magnet is placed on the rotating shaft and a coil on the stator.

H The voltage produced is proportional to the speed of rotation of the

shaft.

H This information is digitised using an analog to digital converter and

passed on to the computer.

1.16.2. Hall-Effect Sensor

Hall-Effect sensor is also called as a velocity measuring sensor.

H If a flat piece of conductor material called the Hall chip, is attached

to a potential difference on its two opposite faces as shown in

Fig. 1.43, the voltage across the perpendicular face is zero. If a

magnetic field is imposed at right angles to the conductor, the voltage

is generated on the two other perpendicular faces.


H Higher the field value, higher is the voltage level.

H If one provides a ring magnet, the voltage produced is proportional

to the speed of rotation of the magnet.

1.16.3. Laser Doppler Velocimetry

H The Laser Doppler velocimeter is a unique technique of measuring

the instantaneous point velocities in a fluid flow without using any

probe. It was developed as a result of research work carried out by

the National Aeronautics and Space Administration, U.S.A around the

year 1968.

1.16.3.1. Principle of the Instrument

H By detecting the Doppler shift in the frequency of the scattered light,

the instrument measures the velocity at a point in the fluid, flowing

in a glass walled conduit or channel. These scattered lights are

originating from minute suspended particles in the flow that happen

to cross the point of measurement defined by two intersecting laser

beams. In this, instrument laser source is preferred as it gives a

narrow, intense and truly parallel light beam of high spectral purity.

1.16.3.2. Instrumental set-up

The instrument can be set up in either of two modes of measurements.

They are namely, the Reference beam mode and the interference fringe mode

Fig. 1.44 shows the set-up in the reference beam mode.


The instrument has four components and they are as follows

(i) Laser source Helium-neon.

(ii) Beam splitter.

(iii) Light pick-up unit and

(iv) Signal processor.

H The light from a 2 mill watt Helium-Neon laser is split up by means

of a Beam splitter into a strong (95% power) scattering beam and a

weak (5% power) reference beam. By using adjustment scale, the

beam inclination θ could be raised (provided in beam splitter) and

also by means of a lens in the beam splitter, the intensity of reference

beam could be attenuated.

H The two equally inclined beams to the flow direction are made to

intersect at the point of velocity measurement in the channel. When

illuminated by the strong scattered beam, it scatters light in all

directions. The optimum range of size of scattering particles is in

between 0.5 to 5 microns. Too fine particles will lead to Brownian

motion and too large particles will increase the system noise by

excessive masking of the light picked up. Ordinary tap water contains

adequate fine suspended particles to provide enough scattering. By

using the intermittent glittering of the laser beam, the presence of

large particles in water can be indicated. The light scattered by

suspended particles is Doppler-shifted. The magnitude of the shift

depends on the direction in which it is picked up.

H The light pick-up unit is so positioned on the other side of the

channel, and its telescope is so focused, that it picks up the reference

beam as well as scattered light in the same direction originating from

the point of intersection of the two beams. The light is focused on

to a tiny electronic device which is called as P.I.N. photodiode, where

optical mixing takes place and a weak electrical signal emerges,

having a frequency equal to the Doppler shift. For boosting the

Doppler signal to a more appropriate level for transmission to the

Signal processor unit, the Light pick-up unit uses a built-in pre

amplifier. The Doppler signal is first passed through a set of three

sharp band-pass filters to reduce the noise content of the Doppler

signal. Then it is amplified and later the frequency of the Doppler


signal is converted to a proportional voltage by an f − V converter.

The converter has fast response due to which it can track closely the

Doppler frequency. The instantaneous output voltage can be recorded

on a strip chart recorder or can be digitized and stored on a Compact

disc. The mean voltage, averaged over a few seconds, is indicated by

a panel meter.

1.17. TEMPERATURE SENSORS

Temperature can be measured by using any one of the following

principle.

1. Materials change in length, volume (or) pressure of the system as a

result of change in temperature.

2. By measuring change in electrical resistance as a result of change in

temperature.

3. By measuring voltage between two dissimilar metals as a result of

difference in temperature.

4. By measuring change in radiated energy as a result of change in

temperature.

(a) Liquid in glass Thermometer


H Liquid in Glass Thermometers make use of thermal expansion of a

liquid enclosed in a bulb when it is exposed to the system for which

temperature has to be measured. The temperature can be determined

by measuring the level of the liquid (mercury (or) alcohol) in the

capillary attached to the bulb.

H The standard clinical thermometer is the example for the liquid in

Glass thermometer.

H Liquid filled mechanical thermometer is shown in Fig. 1.45.

It consists of:

1. Temperature sensor in the form of an immersible bulb.

2. Spiral spring (measuring element) coupled to the bulb through

a capillary tube and

3. An indicating (or) recording attachment (pointer).

H When the temperature sensor bulb is exposed to the thermal medium,

the mercury enclosed in the bulb expands and this change in volume

drives the spiral spring through a capillary link. The pointer coupled

to the spring deflects as a function of temperature and it indicates the

exact temperature.

(b) Resistance Temperature Detector

H Most of the metal’s resistance will increase with the increase of

temperature [But carbon’s resistance will decrease with the increase

of temperature]. The resistance of a highly conducting material

increases with an increase in temperature. But, the resistance of

semiconductor generally decreases with an increase in temperature.

H The temperature sensing device is placed in contact with the system

for which the temperature is to be measured.

H The measure of its resistance indicates the temperature of the system.

H The Resistance Temperature Detector (RTD) is used to measure

temperature with the variation of metal resistance, where as, the

thermistor is used to measure temperature with the variation of semi

conductor resistance.


H RTD is used to measure temperature ranging from cryogenic

( − 200°C) to 600°C. This is sensitive and highly stable. These RTDs

are made up of materials such as platinum, Cu, Ni and tungsten. The

RTD consists of a wire which is wound in the shape of coil to achieve

small size and improve thermal conductivity.

H Several forms of RTDs have been developed for temperature

measurements depending upon their requirement, such as speed of

response, environmental conditions and ability to withstand vibration

(or) corrosion.

H The Fig. 1.46(a) shows open wire RTD in which platinum wire is

wound in the form of a free spiral (or) held in place by an insulated

carrier such as silica (or) ceramic in the form of a perforated coil

former. The lead wires are in direct contact with the gas (or) liquid

for which the temperature has to be measured. Such RTD has an

excellent response time, small conduction errors and small heating

errors.

H Resistance Vs temperature for most of the metals are given by

quadratic equations.

R = R0 (1 + α (T − T0) + β (T − T0)2 + … )

where R0 is the resistance at absolute temperature T0 and α and β are material

constants depending on the purity of the material used.


H The resistance Vs temperature curve is shown in Fig. 1.46(b) which

shows RTD’s characteristics for different metals.

(c) Thermistor

H Thermistor is used to measure temperature based on the principle of

change in semiconductor resistance with change in temperature. The

characteristics of thermistor depend on the particular behaviour of

semiconductor resistance versus temperature.

H When the temperature of the material is increased, the molecules starts

vibrating. Further more increase of temperature causes more vibrations

and as a result, the volume occupied by the atoms in the metal lattice

will increase. Electron flow through the lattice becomes very difficult,

which causes electrons in the semiconductor to detach, resulting in

increased conductance.

H Final conclusion is that an increase in temperature improves

conductance.

H The semiconductor becomes good conductor of current when its

temperature is increased. And also the change in semiconductor’s

resistance with respect to temperature is highly nonlinear. But for

metal, its resistance is increased (ie conductance is decreased) when

its temperature is increased.

The thermistor curves are plotted by the following non-linear equation.

1

T = A + B ln R + C (ln R)3

where T = Temperature in K

R = Resistance of thermistor

A, B, C = Curve fitting constants

H The temperature range of thermistor is in between − 250°C and

650°C. The advantage of the thermistor is its high sensitivity. Because

of the non-linear behaviour of the thermistor, it makes difficult to use

the thermistor as a primary measurement device.

H The thermistor can be fabricated in many forms, including discs, beads

and rods as shown in Fig. 1.47.


H The thermistors vary in size from a bead of 1 mm diameter to a disc

of several centimeters in diameter and several centimeters thick. By

varying manufacturing process and using different semiconducting

materials, a manufacturer can provide a wide range of resistance

values at any particular temperature. Because of thermistor’s small

size, they respond very rapidly to changes in temperature. But the

main disadvantage of the thermistor is its non-linearity. Generally, the

resistance-temperature relationship for a thermistor are given by the

equation

R = K eβ ⁄ t

where R = Resistance at temp t

K and β are constants

(d) Thermocouples

H Thermocouples are used to measure temperature. They are based on

the principle that a current flows in a closed circuit made up of two

dissimilar metals, if the junctions of the two metals are kept at

different temperatures.


H When two conductors of dissimilar material are joined to form a

circuit, the following effect is noted.

H When the two junctions are at different temperatures T1 and T2, small

emf’s are produced at the junctions, and the algebraic sum of these

causes a current. This effect is known as Seebeck effect.

H If both junctions are at the same temperature, there is no net emf.

But, if there is a difference in temperature between the two junctions,

there is an emf. This thermocouple voltage is proportional to the

junction temperature difference.

V = α (T1 − T2)

where α is called Seebeck coefficient.

The standard thermocouple configuration is shown in Fig. 1.49.


H Thermocouple consists of two different metallic wires, A and B. These

two wires are attached to a voltage measuring device. There are two

junction 1 and 2. Junction 2 is normally maintained at 0°C by being

immersed in ice water. This junction is known as reference junction

temperature (0°). The other junction 1 has to be installed at a point

where temperature has to be measured. This junction is known as

measuring junction.

H By measuring the voltage difference, we can determine the

temperature at the measuring junction. For a given pair of

thermocouple metals and a reference temperature, a standard reference

table can be compiled for converting voltage measurements to

temperatures.

H The following examples show how to find temperature by measuring

the voltage. A standard two junction thermocouple configuration is

used to measure the temperature.

Example 1: The reference junction is held at constant temperature of 10°C.

But we have thermocouple table with respect to the reference temperature 0°C

as shown here.

Junction temperature (°C) Output voltage (mV)

0 0

10 0.507

20 1.019

30 1.536

: :

: :

80 4.186

90 4.725

100 5.268

What is the output voltage when the measuring junction is exposed to

100°C?


H By applying law of intermediate temperature, we can write

V100 ⁄ 0 = V100 ⁄ 10 + V10 ⁄ 0. The voltage measured for the temperature

of 100°C relative to the reference junction at 10°C can be calculated

as follows

V100 ⁄ 10 = V100 ⁄ 0 − V10 ⁄ 0

= (5.268 − 0.507) mV

= 4.761 mV

Example 2: The reference junction is held at constant temperature of 20°C.

Use thermocouple table referenced to 0°C. What is the output voltage when

measuring junction is exposed to 80°C.

H By applying law of intermediate temperature, we can write.

V80 ⁄ 0 = V80 ⁄ 20 + V20 ⁄ 0

H The voltage measured for the temperature of 80°C relative to the

reference junction at 20°C can be calculated as follows.

V80 ⁄ 20 = V80 ⁄ 0 − V20 ⁄ 0

= 4.186 − 1.019

= 3.167 mV.

(e) Radiative Temperature Sensing

H A body at a temperature greater than 0K radiates electromagnetic

energy in an amount that depends on its temperature and physical

properties. Radiative temperature sensor need not be in contact with

the surface to be measured. Because the radiation emitted by a body

is proportional to the fourth power of its temperature.

ie Qradiation = σ T4

where σ = Stefan Boltzman Constant = 5.67 × 10− 8 W

m2 K4


H An optical pyrometer identifies the temperature of a surface by the

colour of the radiation emitted by the surface. When a body is heated,

it initially becomes dark red, turns to orange and finally attains a

white colour. The actual temperature measurement is based upon the

determination of the variations in colour of the object, and comparing

it with known values generated with a heated element.

Optical pyrometer is shown in Fig. 1.50.

H The radiation from the source is viewed through a lens and filter

arrangement, along with a standard lamp placed in the optical path

of the incoming radiation. A red filter is used to eliminate source of

the uncertainities resulting from variation of radiation properties with

wavelengths. By suitable adjustment of the lamp current, the colour

of the lamp filament is made to match with the colour of the incoming

radiation. When balance conditions are attained, the filament will

disappear in the total incoming radiation field. At this moment, by

measuring the lamp heating current, we can measure the temperature

of the radiating body.


1.18. LIGHT SENSORS

H The Light Sensors are passive devices which convert “light energy”

(visible or invisible like the infra-red) to electrical signals. Light

sensors are also known as “Photoelectric Devices” or “Photo Sensors”

as they convert light energy (photons) into electricity (electrons).

Photoelectric devices can be grouped into two main categories, one

which generate electricity when illuminated, such as Photo-voltaic or

Photo-emissive, and the other which change their electrical properties

in some way such as Photo-resistors or Photo-conductors. Hence they

are classified as:

A. Photo-emissive Cells

Principle

H These photo-devices are light sensitive materials like caesium that

releases free electrons when struck by a photon of sufficient energy.

The energy of the photons depends on the frequency of the light.

Higher the frequency, greater is the energy of the photons which

converts light energy into electrical energy.


Construction and Working

H The photo emissive cell consists of a glass envelope with a vacuum

inside. The envelope also contains a light sensitive cathode and an

anode. When light strikes the cathode, negative electrons are emitted

and are attracted by the positive anode. The value of this current is

proportional to the intensity of light falling on the cathode. The PEC

(Photo Emissive cells) can be used as part of a potential divider circuit.

H This basic design is called a photo emissive cell or phototube. In a

slightly different design it is called a photo multiplier where there is

a series of plates (each plate consisting of the above components in

it) are arranged so that one incoming photon releases multiple

electrons-effectively amplifying an incoming light signal so it

produces a bigger electrical response. Photoemissive cells are the

oldest and most elaborate way of turning light into electricity.

B. Photo-conductive Cells

Principle

H These photo-devices when exposed to light, their electrical resistance

varies. When a semiconductor material is exposed to light, current


f lows through i t and this phenomenon is referred to as

Photoconductivity. Hence exposure to light increases the current for

a given applied voltage in the circuits connected with these devices.

This change can be measured to sense the intensity of light. The most

common photoconductive material is Cadmium Sulphide used in LDR

photocells.


H Photoconductive Cell is also called the Photoresistor or Light

Dependent Resistor or LDR.The snake like track shown in

Fig. 1.52(a) is the Cadmium Sulphide (CdS) film which also passes

through the sides. On the top and bottom are metal films which are

connected to the terminal leads. It is designed in such a way as to

provide maximum possible contact area with the two metal films. The

structure is housed in a clear plastic or resin case, to provide free

access to external light.

H The main component for the construction of LDR is Cadmium

Sulphide (CdS), which is a photoconductor and contains no or very

few electrons in the absence of light. In the absence of light, it is

designed to have a high resistance in the range of mega ohms. When

the sensor is exposed to light, the electrons becomes free due to

photoconductivity. Hence the conductivity of the material increases.

H When the light intensity exceeds a certain frequency, the photons

absorbed by the semiconductor give band electrons the energy

required to jump into the conduction band. This causes the free

electrons or holes to conduct electricity and thus dropping the

resistance dramatically (1 Kiloohm). The equation to show the relation

between resistance and illumination can be written as

R = A × Ea

Where

E – Illumination (lux),

R – Resistance (Ohms), A,

a – constants


H The value of ‘a’ depends on the CdS used and on the manufacturing

process. Values usually range between 0.7 and 0.9.

C. Photo-voltaic Cells

Principle

H These photo-devices generate an emf which is proportional to the

radiant light received. They are similar in effect to photoconductivity.

Here two semiconductor materials are sandwiched and exposed to

light which creates a voltage of approximately 0.5 V. The most

common photovoltaic material is Selenium used in solar cells.


H A typical silicon solar cell is composed of a thin wafer consisting of

an ultra-thin layer of phosphorus-doped (N-type) silicon on top of a

thicker layer of boron-doped (P-type) silicon. Hence form a PN


junction. When the cell surface is exposed to sunlight a potential

difference is created in PN junction. Hence there is a flow of

electrons.

H Individual solar cells can be connected together in series to form solar

panels which increases the output voltage or connected together in

parallel to increase the available current. Commercially available solar

panels are rated in Watts, which is the product of the output voltage

and current (Volts times Amps) when fully lit.

D. Photo-junction Devices

Principle

H These photo-devices are mainly true semiconductor devices such as

the photodiode or phototransistor which use light to control the flow

of electrons and holes across their PN-junction. Photo-junction devices

are specifically designed for detector application and light penetration

with their spectral response tuned to the wavelength of incident light.

Construction and working


H The construction of the Photodiode light sensor is similar to that of

a conventional PN-junction diode, except that the diodes outer casing

is either transparent or has a clear lens to focus the light onto the

PN junction for increased sensitivity. The junction will respond to

light, particularly longer wavelengths such as red and infra-red rather

than visible light.

H The current-voltage characteristic (I/V Curves) of a photodiode with

no light on its junction (dark mode) is very similar to a normal signal

or rectifying diode. When the photodiode is forward biased, there is

an exponential increase in the current, the same as for a normal diode.

When a reverse bias is applied, a small reverse saturation current

appears which causes an increase of the depletion region, which is

the sensitive part of the junction.

H When used as a light sensor it operates in the reverse biased mode.

When the photodiode is exposed to light the PN junction in diode

produces more hole/electron pairs and this increases the leakage

current. As per the materials used for making the diode light

sensitivity levels are set along with the corresponding leakage current

at various levels of exposure to light.Thus, the photodiode’s current

is directly proportional to the light intensity it is exposed to.

1.19. ENCODER

H Encoder is a digital optical device that converts motion into a

sequence of digital pulses.

H By counting a single bit or by decoding a set of bits, the pulses can

be converted to relative or absolute measurement.

The types of encoder are given below.


1.19.1. Incremental Linear Encoder

Working

H Figure 1.55 shows the incremental linear encoder where it has

transparent scale with an opaque grating.

H The thickness of the grating lines and the gap between them is made

equal of range in microns.

H On one side, the scale is provided with a light source and a condenser

lens.

H On the other side, there are light sensitive cells.

H The resistance of the cells decreases whenever a beam of light falls

on them.

H A pulse is generated each time a beam of light is intersected by the

opaque line. This pulse is fed to the controller, which updates a

counter.


1.19.2. Absolute Linear Encoder

H Opaque black is 1 and transparent glass represents 0. Hence top most

track (4) value is 16 [ i.e., 24 = 16 ]. Next track (3) value is 0 because

transparent [ i.e. 03 = 0 ]. Next track (2) value is 0 because transparent

[i.e. 02 = 0]. Next track (1) value is 2 because 21 = 2. Next track (0)

value is 0 because transparent.

H The principle of absolute linear encoder is same as that of the

incremental linear encoder. The difference is that it gives an absolute

value of the distance covered at any time.

H Thus the missing pulses at high speed are less in chance. In this type,

the output is digital.

H Figure 1.56 shows the scale which is marked with sequence of opaque

and transparent strip.

H This scale shows that opaque black represents 1 and the transparent

glass represents zero.

H The left most column will show a binary number as 00000 − The

next column will show a binary number 00001.


H The absolute encoder provides exact rotational position of the shaft

whereas the incremental encoder gives relative position of the shaft

in terms of digital pulses.

H This encoder consists of a glass disc with a number of accurately

etched equidistant slots along the periphery as shown in

Fig. 1.57(a) & (b).

H The outermost track has an equivalent value of 1. (i.e., 20 = 1).

Similarly, other consecutive tracks have the values as given below.

21 = 2, 22 = 4 ; 23 = 8, 24 = 16 … so on.

H The sum of the shaded area sensed by scanner gives the displacement

value.

H The encoder is directly mounted on the motor shaft or with some

gearing to enhance the accuracy of measurement.

H To avoid noise in the encoder, a gray scale is some times used.

H A gray code allows only one of the binary bits in a code sequence

to change between radial lines.

H It avoids the changes in binary output of the absolute encoder when

the encoder oscillates between points.

1.20.2. Incremental encoder

H The encoder disk of an incremental encoder consists of one track and

two sensors as shown in Fig. 1.58(a).

H The output of these encoders are called channel A and channel B as

shown in Fig. 1.58(b). When the shaft rotates, pulse trains occur on

these channels with a frequency proportional to the rotational speed.

The waves are formed with a 90° phase difference. These signals are

fed to a comparator which determines the magnitude and sign of error.

H The incremental encoder provides more resolution (more accurate) at

lower cost compared to the absolute encoder.


H The encoder is directly mounted on the servomotor shaft (or) at the

end of lead screw as shown in Fig. 1.59.

H The actual distance moved by the machine slide table is calculated

from the rotary motion by knowing lead (lead = pitch × no. of start)

of the lead screw.


1.21. RESOLVER

H Resolver is also used to measure the angular position of lead screw

thereby to measure the position of machine slide.

H The resolver consists of stator and rotor windings which are mounted

at right angles to each other as shown in Fig. 1.60. It has similar

construction features as a small A/C motor.


H In a resolver, the output signal as a function of rotation is obtained

by inductive coupling between the stator and rotor. If an A/C voltage

is applied to one of the stator coils, a maximum voltage will appear

at the rotor coil, when these two coils are in line and the voltage

will disappear for ± 90° shift.

H When the shaft is rotated, the induced voltage in one rotor coil follows

a sine curve and the voltage induced in the other follows a cosine

curve. So the phase angle φ depends on the angular position of the

rotor shaft.

H Hence for example, if the rotor is rotated 90 mechanical degrees, the

output voltage of the rotor winding is shifted by 90 electrical degrees.

1.21.1. Synchros

H Synchros is a rotary transformer, whose primary-to-secondary coupling

may be varied by physically changing the relative orientation of the

two windings. It is also known as selsyn synchros widely used for

measuring the angle of a rotating machine. Example: Antenna

platform.


H The physical construction of a synchro is similar to an electric motor.

The primary winding of the transformer, is fixed to the rotor which

is excited by an alternating current. This exited rotor AC current by

electromagnetic induction, flows in three Y-connected secondary

windings fixed at 120 degrees to each other on the stator.

H A synchro will fall into one of these eight functional categories

(i) Torque Transmitter. (ii) Control Transmitter.

(iii) Torque Differential Transmitter. (iv) Control Differential Transmitter.

(v) Torque receiver. (vi) Torque Differential Receiver.

(vii) Control Transformer. (viii) Control Receiver Transmitter.

1.22. PIEZOELECTRIC SENSOR

H It is a measuring device that uses the piezoelectric effect, to measure

changes in pressure, temperature, acceleration, strain or force by

converting them to an electrical charge. The following circuit shows

the schematic symbol of the piezoelectric sensor.


Principle and Operation

There are three main operations in the piezoelectric sensor and they are:

(i) Transverse Effect

(ii) Transducer Effect and

(iii) Shear Effect

(i) Transverse Effect

Cx = dxy Fy b

a

where, Cx → The amount of charge.

Fy → Force applied along neutral axis (y)

a → The dimension in line with the neutral axis.

b → The dimension in line with the charge generating axis.

dxy → Piezoelectric coefficient.

(ii) Longitudinal Effect

Cx = dxx Fx n

where, dxx → Piezo electric coefficient for a charge in x-direction

released by forces applied along x-direction.

n → Number of stacked elements.

H The amount of charge displaced (Cx) is directly proportional to the

applied force and independent of the piezoelectric element size and

shape. The charge output can be increased by putting several elements

mechanically in series and electrically in parallel.

(iii) Shear Effect

Cx = 2 dxx Fx n

Applications:

(i) Medical instruments.

(ii) Aerospace parts.


(iii) Nuclear instruments.

(iv) Automotive industry.

It is used to measure force.

H A piezoelectric material uses a phenomenon known as piezoelectric

effect.

H This effect explains that when asymmetrical, elastic crystals are

deformed by a force, an electrical potential will be developed within

the distorted crystal lattice as shown in Figure 1.61(a).

This effect is reversible.

H If a potential is applied between the surface of the crystal, it will

change the physical dimension.

H The magnitude and polarity of the induced charges are proportional

to the magnitude and direction of the applied forces.

H The piezoelectric materials are quartz, tourmaline, rochelle salt, and

others.

H The range of forces that are measured using Piezoelectric sensors are

from 1 to 20 kN.

H These sensors can be used to measure instantaneous change in force

(dynamic force).


1.23. ACOUSTIC EMISSION SENSORS

H An Acoustic Emission Sensor is a type of sensor, which converts the

surface movement caused by an elastic wave into an electrical signal,

which can be processed by the measurement equipment. The

piezoelectric element of Acoustic Emission (AE) sensor should have

high sensitivity and it should convert the surface movement most

efficiently to an electrical voltage.

H AE sensors are designed highly sensitive at a certain frequency or

with a broad frequency response. It is very important to select an

appropriate sensor for a specific AE applications.

The AE method offers the following advantages

H It can observe the progresses of plastic deformation and microscopic

collapse in real time.

H It can diagnose facilities while they are in operation.

H It can locate a flaw by using several AE sensors.

AE sensors are broadly classified into two types.

(i) Resonance Model and (ii) Wide bandwidth model.

H Resonance models are highly sensitive at a specific frequency and

wide bandwidth models possess a constant sensitivity across a wide

band of frequencies.

Applications of AE sensors:

(i) Product testing.

(ii) Tool monitoring.

(iii) Facility diagnosis.

(iv) Safety monitoring in civil engineering projects.

(v) Diagnosis of the integrity of large structures.


1.24. VIBRATION SENSORS

H Vibrations are measured by measuring the displacement, velocity (or)

acceleration of the vibrating body with the help of vibration measuring

instruments.

H The vibration measuring instrument having mass, spring and dash pot

etc. is known as seismic instrument (or) seismic transducer.

H The quantities displacements, velocity (or) accelerations are measured

and displayed in the form of electrical signals with amplifications.

The output of electrical signal is proportional to the quantity to be

measured.

There are 3 types of vibration sensors.

1. Vibrometer (or) seismometer − measures displacement of vibrating

body.

2. Accelerometer − measures acceleration of vibrating body.

3. Laser Doppler Vibrometer (LDV).

1.24.1. Vibrometer (or) Seismometer

H Vibrometer (or) seismometer is designed with low natural frequency

and hence it is known as low frequency transducer.

H The seismometer is shown in Fig. 1.63.


H The relative motion between the mass and vibrating body is converted

into proportional voltage and it can be recorded.

1.24.2. Accelerometer

H As name implies, accelerometer is used to measure acceleration of

the vibrating body

H Accelerometer is designed with high natural frequency and hence it

is known as high frequency transducer.

H After measuring acceleration, with the help of electronics integration

devices, the velocity and displacement − both can be displayed in the

screen.

H Since it is very small and also it measures all quantities displacement,

velocity and acceleration simultaneously, it is widely been used.

H The figure for accelerometer is same as seismometer. But

accelerometer is a high frequency transducer.

H The voltage signals from accelerometer are amplified to display in

bigger screen.

H The double integration device is used to get velocity in the first integration

and displacement in the second integration on the bigger screen.


1.24.3. Laser Doppler Vibrometer (LDV)

H It is a non contact type vibration measuring instrument. A laser beam

from LDV is directed on the surface of interest. The vibration

amplitude and frequency can be extracted from the Doppler shift of

the reflected laser beam frequency due to motion of the surface.

Principles of operation

H It is generally a two beam laser interferometer, which measures the

frequency difference between an interval reference beam and test

beam. The most widely used LDV is Helium-Neon laser. From the

target, test beam is directed and the light scattered from the target is

col lected which interfered wi th the reference beam on a

photodetection.

H From 1.64, it can be observed that the beam from the laser of

frequency f0 is divided into a reference beam and a test beam by a

beam splitter. While passing through the Bragg cell, a frequency shift

of fb is added which is further directed to the target. The motion of

the target adds a Doppler shift to the beam which is given by

fb = 2 × v (t) × cos ( α ) ⁄ λ


where, v (t) is the velocity of target as a function of time.

α is the angle between the laser beam and velocity

vector.

λ is the wavelength of the light.

H From the target, light scatters in all direction, but only some portion

of the light is collected by the LDV and reflected by the beam splitter

to the photodetector. This light has the frequency of f0 + fb + fd. This

scattered light is then combined with the interference beam at the

photodetector. This output of the photodetector is a standard frequency

modulated signal, with the Bragg cell frequency as the carrier

frequency, and the Doppler shift as the modulation frequency.

Applications

Some of the applications of laser Doppler vibrometer are as follows,

(i) Acoustic

(ii) Architecture

(iii) Automotive

(iv) Calibration..

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MECHATRONICS - airwalkbooks.comairwalkbooks.com/images/pdf/pdf_95_1.pdf · ME407 – MECHATRONICS MODULE I: Introduction to Mechatronics: Structure of Mechatronics system. Sensors