Position Sensors

A resource on position sensor technology, including background, operational theory, design and applications

This book explains the theory and applications of the technologies used in the measurement of linear and angular/rotary position sensors. The first three chapters provide readers with the necessary background information on sensors. These chapters review: the working definitions and conventions used in sensing technology; the specifications of linear position transducers and sensors and how they affect performance; and sensor output types and communication protocols. The remaining chapters discuss each separate sensor technology in detail. These include resistive sensors, cable extension transducers, capacitive sensors, inductive sensors, LVDT and RVDT sensors, distributed impedance sensors, Hall Effect sensors, magnetoresistive sensors, magnetostrictive sensors, linear and rotary encoders, and optical triangulation position sensors.

• Discusses sensor specification, theory of operation, sensor design, and application criteria
• Reviews the background history of the linear and angular/rotary position sensors as well as the underlying engineering techniques
• Includes end-of-chapter exercises

Position Sensors is written for electrical, mechanical, and material engineers as well as engineering students who are interested in understanding sensor technologies.

About the Author xv

Preface xvii

Trademarks xix

About the Companion Website xxi

1 Sensor Definitions and Conventions 1

1.1 Is it a Sensor or a Transducer? 1

1.2 Position versus Displacement 5

1.3 Absolute or Incremental Reading 6

1.4 Contact or Contactless Sensing and Actuation 7

1.5 Linear/Angular Configuration 10

1.6 Position Velocity and Acceleration 11

1.7 Application versus Sensor Technology 13

1.8 Operational Lifetime 13

1.9 Questions for Review 14

References 16

2 Specifications 17

2.1 About Position Sensor Specifications 17

2.2 Measuring Range 18

2.3 Zero Span and Full Scale 18

2.4 Repeatability 21

2.5 Nonlinearity 22

2.6 Hysteresis 28

2.7 Calibrated Accuracy 31

2.8 Drift 33

2.9 What does all this Accuracy Stuff Mean to Me? 34

2.10 Temperature Effects 36

2.11 Response Time 40

2.12 Damping 42

2.13 Cross Sensitivity 44

2.14 Shock and Vibration 45

2.15 Electromagnetic Compatibility 48

2.16 High Voltage Pulse Protection 50

2.17 Power Requirements 52

2.18 Intrinsic Safety Explosion Proofing and Purging 54

2.19 Reliability 62

2.20 Questions for Review 66

3 Output Types and Communication Protocols 69

3.1 Analog Output Types 69

3.2 Digital Output Types 73

3.3 SSI 73

3.4 CANbus 80

3.5 PROFIBUS 104

3.6 HART 106

3.7 Questions for Review 117

4 Resistive/Potentiometric Sensing 119

4.1 Resistive Position Sensors 119

4.2 Resistance 120

4.3 History of Resistors and Resistive Position Sensors 122

4.4 Position Sensor Design 124

4.5 The Resistive Element 127

4.6 The Wiper 129

4.7 Linear and Rotary Mechanics 130

4.8 Signal Conditioning 131

4.9 Advantages/Disadvantages 132

4.10 Typical Performance Parameters 132

4.11 Specifications and Application 135

4.12 Manufacturers 136

4.13 Questions for Review 136

5 Cable Extension Transducers 139

5.1 Cable Extension Transducer History 139

5.2 Cable Extension Transducer Construction 140

5.3 Signal Conditioning 144

5.4 Application 144

5.5 Advantages/Disadvantages 148

5.6 Typical Performance Specifications 148

5.7 Manufacturers 149

5.8 Questions for Review 150

6 Capacitive Sensing 153

6.1 Capacitive Position Sensors 153

6.2 Capacitance 154

6.3 Dielectric Constant 157

6.4 History of Capacitive Position Sensors 158

6.5 Capacitive Position Sensor Design 159

6.6 Electronic Circuits for Capacitive Sensors 163

6.7 Guard Electrodes 169

6.8 EMI/RFI 170

6.9 Typical Performance Specifications and Application 172

6.10 Manufacturers 173

6.11 Questions for Review 173

7 Inductive Sensing 175

7.1 Inductive Position Sensors 175

7.2 Inductance 177

7.3 Permeability 182

7.4 History of Inductive Position Sensors 183

7.5 Inductive Position Sensor Design 184

7.6 The Coil and Bobbin 185

7.7 Core 188

7.8 Signal Conditioning 189

7.9 Advantages 192

7.10 Typical Application and Performance Specifications 192

7.11 Manufacturers 194

7.12 Questions for Review 194

8 The LVDT and RVDT 197

8.1 LVDT and RVDT Position Sensors 197

8.2 History of the LVDT and RVDT 198

8.3 LVDT and RVDT Position Sensor Design 199

8.4 Coils 200

8.5 Core 202

8.6 Carrier Frequency 206

8.7 Demodulation 207

8.8 Signal Conditioning 211

8.9 Synchronization 217

8.10 Calibration 219

8.11 Advantages 221

8.12 Typical Performance Specifications and Application 222

8.13 Manufacturers 224

8.14 Questions for Review 225

References 226

9 Distributed Impedance 227

9.1 Distributed Impedance Position Sensors 227

9.2 History 229

9.3 Operational Theory 229

9.4 The Distributed Impedance Sensing Element as a Transmission Line 234

9.5 Periodic Structures 235

9.6 Hybrid Waves 235

9.7 Distributed Impedance Sensor Design 236

9.8 Electronics 237

9.9 Advantages 239

9.10 Typical Performance Specifications and Applications 239

9.11 Infinite Resolution? 241

9.12 Calibration 241

9.13 Manufacturers 243

9.14 Questions for Review 243

10 The Hall Effect 247

10.1 Hall Effect Sensors 247

10.2 The Hall Effect 248

10.3 History of the Hall Effect 250

10.4 Hall Effect Position Sensor Design 251

10.5 The Hall Effect Element 254

10.6 Electronics 254

10.7 Linear Arrays 257

10.8 Advantages 258

10.9 Typical Performance Specifications and Applications 259

10.10 Manufacturers 260

10.11 Questions for Review 261

11 Magnetoresistive Sensing 263

11.1 Magnetoresistive Sensors 263

11.2 Magnetoresistance 264

11.3 History of Magnetoresistive Sensors 271

11.4 Magnetoresistive Position Sensor Design 273

11.5 The Magnetoresistive Element 275

11.6 Linear Arrays 276

11.7 Electronics 276

11.8 Advantages of Magnetoresistive Sensors 278

11.9 Typical Performance Specifications and Applications 278

11.10 Manufacturers 281

11.11 Questions for Review 281

12 Magnetostrictive Sensing 285

12.1 Magnetostrictive Sensors 285

12.2 Magnetostriction 286

12.3 History of Magnetostriction 288

12.4 Magnetostrictive Position Sensor Design 289

12.5 Waveguide 291

12.6 Position Magnet 295

12.7 Pickup Devices 299

12.8 Damp 301

12.9 Waveguide Suspension 302

12.10 Electronics 303

12.11 Angular/Rotary Magnetostrictive Sensors 307

12.12 Advantages 308

12.13 Typical Performance Specifications 309

12.14 Application 311

12.15 Manufacturers 313

12.16 Questions for Review 313

13 Encoders 315

13.1 Linear and Rotary 315

13.2 History of Encoders 315

13.3 Construction 317

13.4 Absolute versus Incremental Encoders 319

13.5 Optical Encoders 320

13.6 Magnetic Encoders 321

13.7 Quadrature 323

13.8 Binary versus Gray Code 325

13.9 Electronics 326

13.10 Advantages 328

13.11 Typical Performance Specifications and Applications 329

13.12 Manufacturers 331

13.13 Questions for Review 331

14 Optical Triangulation 335

14.1 Linear Sensing 335

14.2 History 336

14.3 Construction 337

14.4 Light Sensor 340

14.5 Electronics 343

14.6 Laser 347

14.7 Advantages 349

14.8 Typical Performance Specifications and Applications 350

14.9 Manufacturers 350

14.10 Questions for Review 351

Bibliography 353

Glossary of Sensor Terminology 355

Index 363

David S. Nyce is founder and owner of Revolution Sensor Company in Apex, North Carolina, US. He was formerly a Divisional General Manager and Director of Technology for the Sensors Group of MTS Systems Corporation, and was Chief Engineer or VP of Engineering at several other sensor manufacturing companies.  Mr. Nyce has more than 30 years of experience developing sensors of many types for industrial, automotive, military, medical, and commercial use.