SENSORS FOR INDUSTRIAL INSPECTION

Sensors for Industrial Inspection by

C.LOUGHLIN llkley, West Yorkshire, U.K.

Springer Science+Business Media, B.V.

Sensors for Industrial Inspection by

C.LOUGHLIN llkley, West Yorkshire, U.K.

Springer Science+Business Media, B.V.

Library of Congress Cataloging-in-Publication Data

Loughlin, C, Sensors for industrial inspection I by C. Loughlin.

p. cm. Includes bibliographical references and index. ISBN 978-94-010-5211-5 ISBN 978-94-011-2730-1 (eBook) DOI 10.1007/978-94-011-2730-1

1. Engineering inspection. 2. Detectors--Industrial applications. 1. Title. TS156.2.L68 1992 670.42'5--dc20 92-37362

ISBN 978-94-010-5211-5

Printed on acid-free paper

All Rights Reserved © 1993 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1993 Softcover reprint of the hardcover 1 st edition 1993 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

Library of Congress Cataloging-in-Publication Data

Loughlin, C, Sensors for industrial inspection I by C. Loughlin.

p. cm. Includes bibliographical references and index.

1. Engineering inspection. 2. Detectors--Industrial applications. 1. Title. TS156.2.L68 1992 670.42'5--dc20 92-37362

ISBN 978-94-010-5211-5

Printed on acid-free paper

All Rights Reserved © 1993 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1993 Softcover reprint of the hardcover 1 st edition 1993 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

To Vicki and my parents; for their considerable help and continuous support.

Contents

Introduction ................................................................................................................... xix

Acknowledgements ........................................................................................................ xxi

Chapter 1. Fundamentals of Sensor Materials ....................................................... 1 1.1 Introduction ..................................................................................................... 1 1.2 Atomic Structure .............................................................................................. 1

Ionisation Potential ..................................................................................... 2 1.3 Crystalline Structure ........................................................................................ 2 1.4 Electron Energies ............................................................................................. 3 1.5 Semiconductors ................................................................................................ 4 1.6 Resistance ........................................................................................................ 5

Strain Gauges .............................................................................................. 6 Thermistors ............... ................................................................................... 6 Thermocouples ............................................................................................ 7

1.7 Thermal Radiation ........................................................................................... 8 1.8 Photoelectric Effects ........................................................................................ 9· 1.9 Photon Emission ............................................................................................ 10

Luminescence ............................................................................................ 10 Photodiodes ............................................................................................... 11

1.10 Piezoelectric Effects. ...................................................................................... 11 Ferroelectric Effect ..... .............................................................................. 11

Chapter 2. Distance ................................................................................................. 13 2.1 Introduction .................................................................................................... 13 2.2 Ultrasonic Ranging ........................................................................................ 15

Ultrasonic Transducers ... .......................................................................... 16 Resolution .... .............................................................................................. 18 Interference ............................................................................................... 19 Pulse Rate .................................................. ................................................ 20 Beam Angle ............................................................................................... 21 Range ......................................................................................................... 21 Velocity of Sound ................................................. ...................................... 22 Doppler Effect with Moving Targets ......................................................... 23 Environmental Considerations ........ .......................................................... 24 Installation Techniques ....... ...................................................................... 27 Sensor Interfacing ..................................................................................... 28

viii

2.3

2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11

2.12

Contents

Optical Techniques ........................................................................................ 28 Time of Flight ............................................................................................ 28 Pulse/Echo ................................................................................................. 30 Phase Differences ...................................................................................... 30 Triangulation Techniques ......................................................................... 30 Ranging using Focus ................................................. ................................ 36 Optical Displacement Sensor ................................................ .................... 38 RangefromBrightness .............................................................................. 39 Zone Distance Sensors .............................................................................. 40 Visual Scaling ................................................ ............................................ 41 Optical Interferometry and Diffraction Systems ....................................... 42 Photographic Camera Ranging Techniques ............................................. 42

Capacitive Distance Sensors .......................................................................... 45 Inductive Distance Sensors ............................................................................ 45 Air Gauge ....................................................................................................... 45 X-ray Thickness Gauging .............................................................................. 46 LVDT ............................................................................................................. 46 Magneto-Acoustic Sensor .............................................................................. 47 Time Domain Reflectometer .......................................................................... 48 Rotary / Linear Encoders ............................................................................... 48

Optical Encoders ....................................................................................... 48 Rotary/Linear potentiometers ................................................................... 50

Thickness Measurement ................................................................................ 51 Ultrasonic Thickness Gauging .................................................................. 51

Chapter 3. Movement ............................................................................................. 53 3.1 Introduction .................................................................................................... 53 3.2 Velocity .......................................................................................................... 53

Doppler Effect ........................................................................................... 53 3.3 Accelerometers .............................................................................................. 56

Introduction ......................................................................... ...................... 56 Piezoelectric Accelerometers .................................................................... 57 Piezoresistive Accelerometers ................................................................... 58 Variable Capacitance Accelerometers ...................................................... 59 Force Balancing Accelerometers ...... ........................................................ 59 Environmental Considerations ...... ............................................................ 60 Mounting Considerations .......................................................................... 61 Electrical Interfacing ................................................................................ 61

3.4 Flow Measurement ........................................................................................ 62 Pressure Difference Flow Meters ............................................................. 62 Frequency Shift Flow Meter ...................................................................... 63 Transit Time Flowmeter ............................................................................ 64 Hot Wire Anemometer ............................................................................... 64 Rotating Vanes .......................................................................................... 64 Electromagnetic Flow Sensing .................................................................. 64

3.5 Tachorneters ................................................................................................... 65 AC / DC Generators .................................................................................. 65 Optical Tachometers ................................................................................. 65

3.6 Vibration ........................................................................................................ 65 Machine Vibration Measurement .............................................................. 65

3.7 Tilt Switches .................................................................................................. 67

Contents IX

Chapter 4. Proximity Sensors ................................................................................ 69 4.1 Introduction .................................................................................................... 69

Application Example ....... .......................................................................... 70 4.2 Inductive Proximity Switches ........................................................................ 70

Inductive Sensing ............................................... ........................................ 71 Skin Effect .. ................................................................................................ 71 Magnetic Inductive Proximity Sensors ...................................................... 73 Mounting Considerations .......................................................................... 74 Hysteresis .................................................................................................. 74 Target Approach .............................. > ••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 75 Body Styles and Sizes ................................................................................. 75 Welding Proof ................................................. .......................................... 75 Applications ............................................................................................... 75

4.3 Capacitive Proximity Sensors ........................................................................ 76 Capacitive Sensing .................................................................................... 76 Target Materials ........................................................................................ 77

4.4 Photoelectric Proximity Switches .................................................................. 78 Photoelectric Sensing ................................................................................ 78 Sensor Configurations ............................................................................... 79

4.5 Interfacing Proximity Sensors ........................................................................ 81

Chapter 5. Tactile Sensors ...................................................................................... 83 5.1 Introduction .................................................................................................... 83 5.2 Resistive techniques ....................................................................................... 84

Conductive rubber ..................................................................................... 85 Conductive Plastics ................................................................................... 86 Carbon Fibre ................... .......................................................................... 86 Tactile Arrays ................................................ ............................................ 87

5.3 Piezoelectric Effect. ....................................................................................... 90 Piezoelectric Film ..................................................................................... 90

5.4 Magnetic effects ............................................................................................. 91 Magnetoresistance .................................................................................... 91 Magnetoelastic & Magnetostrictive Materials ......................................... 92

5.5 Optical Techniques ........................................................................................ 94 Touch Vision ................................................ .............................................. 96 Touch Finger ............................................................................................. 97

5.6 3-D Tactile Sensors ...................................................................................... 100

Chapter 6. Surface Finish ..................................................................................... 101 6.1 Introduction .................................................................................................. 101 6.2 Stylus Instruments ........................................................................................ 102

Stylus Protection ..................................................................................... 103 Stylus Selection ........................................................................................ 104 Mounting Arrangements .......................................... ................................ 104 Laser Stylus ............................................................................................. 106

6.3 Profile Analysis ............................................................................................ 106 Sample and Assessment Lengths ............................................................. 106

6.4 Surface Reflection ........................................................................................ 109 Roughness and Waviness from Surface Scattering ................................. 109

6.5 Laser Scanning ............................................................................................. 112 Polarised Laser Scanning ....................................................................... 112

x

6.6

6.7 6.8 6.9

Contents

Visual Analysis of Texture .......................................................................... 113 Statistical Classijication .......................................................................... 113 Structural Approaches ............................................................................. 115 Spectral Approaches (Fourier Analysis) ................................................. 116

Optical Sectioning (Structured Light) .......................................................... 117 Roughness Gauging ..................................................................................... 117 Crack Detection ........................................................................................... 118

Chapter 7. Structural Analysis ............................................................................ 119 7.1 Introduction .................................................................................................. 119 7.2 Photoelastic Stress Analysis ........................................................................ 119

7.3 7.4 7.5

7.6 7.7 7.8 7.9 7.10 7.11

Stress Direction ....................................................................................... 121 Stress Magnitude ..................................................................................... 121 Stress Polarity . ........................................................................................ 125 3-D Model Analysis ................................................................................. 125 Stress Freezing ........................................................................................ 125

Thermoelastic Stress Analysis ..................................................................... 126 Transient Therrnography .............................................................................. 128 Acoustic Emission - Stress Wave Sensing .................................................. 133

Individual Transients ............................................................................... 133 Multiple Transients ................................................................................. 134 Continuous Emission ......................................................... ...................... 134 Applications ....................................................... ...................................... 135 Locating the Source ................................................................................. 135 Sensor Design .......................................................................................... 135

X-ray Inspection. .......................................................................................... 139 Gamma-Radiation ........................................................................................ 141 Beta-Particles ............................................................................................... 142 Viscosity ...................................................................................................... 143 Eddy current sensing .................................................................................... 144 Ultrasonic Inspection ................................................................................... 148

Compression and Shear Waves ............................................................... 148 Velocity .................................................................................................... 149 Wavelength ................................................... ........................................... 149 Probe Design ........................................................................................... 150 Data Acquisition and Display ................................................................. 151

7.12 Acoustic Flaw Detection .............................................................................. 152 7.13 Hardness Testing .......................................................................................... 153

Chapter 8. Temperature Sensing ......................................................................... 157 8.1 Introduction .................................................................................................. 157

Units o/Temperature .............................................................................. 157 8.2 Metallic Resistance Thermometers .............................................................. 157

Platinum Resistance Thermometers ........................................................ 158 Linearity .................................................................................................. 159 Installation .............................................................................................. 160 Low Temperature Resistance Sensors ..................................................... 160

8.3 Thermistors .................................................................................................. 161 Introduction ............................................................... .............................. 161 Thermal Characteristics ................................................... ....................... 161 Current Limiting Applications ................................................................ 162

Contents xi

Positive Temperature Coefficient Thermistors (PTC) ............................ 163 Installation Considerations ..................................................................... 164

8.4 Thennocouples ............................................................................................. 164 Seebeck Effect ......................................................................................... 164 Thermocouple Types ............................................................................... 165 Thermocouple Materials ......................................................................... 167 Thermocouple Construction .......................................... .......................... 167 Law of Intermediate Metals ..................................................................... 167 Cold junction compensation ............ ........................................................ 168 Connecting Cables .................................................................................. 168 Temperature Transmitters ....................................................................... 169

8.5 Semiconductor Temperature Measurement.. ............................................... 169 Silicon Junction Diode ............................................................................ 169 Semiconductor Temperature Sensors ...................................................... 170 Temperature Sensing Integrated Circuits ............................................... 171

8.6 Optical Pyrometers. ...................................................................................... 171 Infrared theory ........................................................................................ 171 Planck's Law ........................................................................................... 171 Black bodies ............................................................................................ 173 Radiation Detectors ................................................................................. 175 Application Considerations ..................................................................... 178

8.7 Thennal Imaging .......................................................................................... 183 8.8 Disappearing Filament ................................................................................. 184 8.9 Thennographic Materials ............................................................................. 184

Temperature sensitive pigments .............................................................. 185 Liquid crystals ... ...................................................................................... 185

8.10 Thennographic Phosphors ........................................................................... 186 8.11 Distributed Fibre Optic Temperature Sensors ............................................. 186

Raman Scattering .................................................................................... 186 Applications ........................................... .................................................. 187

8.12 Thennostats .................................................................................................. 188 Bimetallic Elements ................................................................................. 188

8.13 Ceramic Temperature Sensors ..................................................................... 188 Cooling System Control Application ....................................................... 188

Chapter 9. Pressure Sensing. ................................................................................ 191 9.1 Introduction .................................................................................................. 191 9.2 Capacitance Manometers ............................................................................. 191 9.3 Quartz Pressure Sensors ............................................................................... 192

Electrostatic Pressure Sensors ................................................................ 192 Quartz Resonant Pressure Sensors ......................................................... 192

9.4 Piezoresistive Pressure Sensors ................................................................... 193 9.5 Strain Gauge Pressure Sensors. .................................................................... 193 9.6 LVDT Pressure Transducer ......................................................................... 193 9.7 Silicon Pressure Sensors .............................................................................. 193 9.8 Vacuum Gauges ........................................................................................... 194

Thermal Conductivity Gauges ................................................................. 194 Thermocouple Gauge .............................................................................. 194 Pirani Gauge ........................................................................................... 194 Thermistor Gauge ................................................................................... 196 Ionisation Gauges ................................................................................... 196

xii Contents

Chapter 10. Weight ................................................................................................. 197 10.1 Introduction ................................................................................................ ,. 197 10.2 Check Weighers ........................................................................................... 198 10.3 Dynamic Weighing Systems ........................................................................ 199 lOA Hydraulic and Pneumatic Weighing Systems .............................................. 200

Chapter 11. Moisture/Humidity ............................................................................ 201 11.1 Introduction .................................................................................................. 201 11.2 Dew Point Instruments ................................................................................. 202 11.3 Vapour Pressure Methods / Relative Humidity ........................................... 203

Absolute Humidity of a Gas Mixture ....................................................... 203 Relative Humidity of a Gas Mixture ........................................................ 204 Equilibrium Relative Humidity ................................................................ 204

1104 Infrared Moisture Measurement ................................................................. 204 Infrared Moisture Sensor ........................................................................ 205 Infrared Absorption ..... ............................................................................ 205

11.5 Microwave Techniques ................................................................................ 206 11.6 Quartz Crystal Oscillator ............................................................................. 206 11.7 Conducti vi ty ................................................................................................. 206 11.8 Capacitance Sensors. .................................................................................... 208 11.9 Diffusion Cell Electrolyte ............................................................................ 208 11.10 Calibration ................................................................................................... 209

Chapter 12. Gas Detection ...................................................................................... 211 12.1 Introduction .................................................................................................. 211

Combustible Gas Detectors ..................................................................... 211 Toxic Gas Detectors ................................................................................ 212 Sensor Location ....................................................................................... 212

12.2 Electrochemical Sensors .............................................................................. 213 12.3 Semiconductor Detectors ............................................................................. 213 1204 Catalytic Detector (pellistor) ........................................................................ 214 12.5 Conducting Polymer Gas Detectors ............................................................. 214 12.6 Infrared Spectroscopy .................................................................................. 214

Molecular Vibrations .............................................................................. 214 Absorption of Infrared Light .................................................................. 215 Photoacoustic Spectroscopy .................................................................... 215

12.7 Thermal Conductivity Detector (TCD) ........................................................ 217 Katharometer .......................................................................................... 218

12.8 Photo-Ionisation detector (PID) ................................................................... 219 12.9 Ultrasonic Detectors ..................................................................................... 219 12.10 Paramagnetic Oxygen Analysers ................................................................. 219

Magnetic Wind Instruments .................................................................... 220 12.11 Flame Ionisation Detectors (FID) ................................................................ 220 12.12 Units of Gas Concentration .......................................................................... 221

Chapter 13. Light and Colour Measurement ....................................................... 223 13.1 Introduction .................................................................................................. 223 13.2 Units of Light Measurement.. ...................................................................... 224

Luminous Flux ......................................................................................... 224 Luminous Intensity .................................................................................. 226 Illuminance ........... ................................................................................... 227

Contents xiii

13.3 The Nature of Colour ................................................................................... 228 Hue ................................................... ....................................................... 228 Saturation ................................................................................................ 229 Intensity ................................................................................................... 229

13.4 Human Perception of Colour ....................................................................... 229 Colour Discrimination ............................................................................ 231

13.5 CIEStandards .............................................................................................. 231 The Standard Observer ........................................................................... 231 Colour Sensitivity of the Human Eye ...................................................... 232 CIE Chromaticity Diagram ............................................... ...................... 233 XYZ System .............................................................................................. 233 Yxy Colour System ................................................................................... 235 The Lab system ........................................................................................ 235

13.6 Munsell Colour Whee1. ................................................................................ 237 13.7 RGB Systems ............................................................................................... 237

RGB Colour Space .................................................................................. 238 Colour Triangle ....................................................................................... 239

13.8 Colour Mixing .............................................................................................. 240 Additive and Subtractive Colour Mixing ................................................. 240 Complimentary Colours .......................................................................... 240

13.9 Illumination .................................................................................................. 241 Colour Temperature ................................................................................ 241 Light Sources ............................................... ............................................ 241 Man Made Light Sources ........................................................................ 242 Natural Light Sources ............................................................................. 242 Filters ...................................................................................................... 243 Atmospheric Absorbtion ......................................................................... 244

13.10 Colour Pigments. .......................................................................................... 244 13.11 Colour Matching .......................................................................................... 244

Metamerism ............................................................................................. 245 13.12 Colour Measuring Equipment.. .................................................................... 245

Colour Sensing ........................................................................................ 245 Densitometers .......................................................................................... 245 Effects of Polarisation ............................................................................. 247 Chromameters ......................................................................................... 247 Illuminance Meters ................................................ .................................. 248 Luminance Meters ................................................................................... 248 Spectrophotometers ................................................................................. 248 Spectral Filter Arrays .... .......................................................................... 248

Chapter 14. Optics .................................................................................................. 249 14.1 Introduction .................................................................................................. 249 14.2 Light - Fundamentals .................................................................................. 249 14.3 Laws of optics .............................................................................................. 250

Reflection .. ............................................................................................... 250 Absorption and Transmission ..... ............................................................ 251 Refraction ............................................ .................................................... 252 Diffraction ............................................................................................... 255

14.4 Lenses .......................................................................................................... 258 Real and Imaginary Images .................................................................... 260 Bi-convex Lenses ................................................ ..................................... 260

xiv Contents

Bi-concave Lenses ................................................................................... 261 Ray Diagrams .......................................................................................... 261 Prisms ...................................................................................................... 262 Fresnel Lenses ....................................................... .................................. 263 Depth of Field - Depth of Focus ............................................................. 264 Aperture ................................................................................................... 265 Measuring the Focal Length of an Unknown Lens ................................. 266

14.5 Mirrors ......................................................................................................... 266 Prism Reflectors ..................................................................................... 267

14.6 Lens Systems ............................................................................................... 267 Magnifying glass ..................................................................................... 267 Photocopier ............................................................................................. 268 Projector ... ............................................................................................... 268

14.7 Lens Defects. ................................................................................................ 268 Spherical Abberation. .............................................................................. 268 Chromatic Aberration ............................................................................. 270

14.8 Polarised Light ............................................................................................. 272 Polarisation by Reflection ....................................................................... 273 Double Refraction ................................................................................... 273 Optical Rotation ...................................................................................... 274

14.9 Interference Filters ....................................................................................... 274 Band Pass Filters .................................................................................... 275 Filter Factors .......................................................................................... 275 Correction Filters .................................................................................... 275 Contrast Filters ....................................................................................... 275 Dichroic Filters ....................................................................................... 276

14.10 Holography .................................................................................................. 276

Chapter 15. Lighting ............................................................................................... 279 15.1 Introduction .................................................................................................. 279 15.2 Methods of Illumination .............................................................................. 279

Front lighting .......................................................................................... 279 Specular Illumination (dark field) ................... ........................................ 280 Specular Illumination (light field) ........................................................... 280 Beam Splitter ........................................................... ................................ 280 Split Mirror ............................................................................................. 281 Shadow Illumination ............................................................................... 281 Backlit Illumination ................................................................................. 281 Retroreflector .. ........................................................................................ 282 Back Illumination (condenser) .. .............................................................. 283 Rear Illumination (collimator) ................................................................ 283 Rear Offset Illumination .......................................................................... 283 Structured Light ....................................................................................... 284 Ring Lights .............................................................................................. 284 Diffuse Scattering .................................................................................... 284 Fibre Optic Light Sources ....................................................................... 285 Surface Characteristics ........................................................................... 286 Distribution and Spread of Illumination ................................................. 286

15.3 Colour .......................................................................................................... 288 Spectral Characteristics .......................................................................... 288 Daylight .................................................................................................. 289

Contents xv

15.4 Lighting Power Supplies .............................................................................. 289 Frequency - Ripple and Drift ................................................................. 289 Colour Temperature ................................................................................ 290 Run Up ..................................................................................................... 290 High Frequency Operation ..................................................................... 291

15.5 Handling Heat Dissipation ........................................................................... 291 Dichroic Reflectors .................................................................................. 291

15.6 Protection Against Lamp Shattering ............................................................ 292 15.7 Lamp Life ..................................................................................................... 292

Pre-Ageing of Lamps ............................................................................... 293 Efficiency ................................................................................................. 293

15.8 Incandescent lamps ...................................................................................... 293 15.9 Tungsten Halogen Lamps ............................................................................ 293

Installation Considerations ..................................................................... 294 15.10 Fluorescent Lamps ....................................................................................... 294

Luminescence .......................................................................................... 295 Fluorescent Lamp Output ........................................................................ 296 High Frequency Fluorescents ................................................................. 296

15.11 Sodium Vapour Discharge Lamps ............................................................... 296 15.12 Neon Lamps ................................................................................................. 296 15.13 Electronic Flash Tubes ................................................................................. 297

Xenon Flash Lamps ................................................................................. 297 15.14 UV Illumination (black light lamps) ........................................................... 298 15.15 Light Emitting Diodes (LED's) .................................................................. 298 15.16 Electrol uminescence .................................................................................... 299

Electroluminescent Panels ...................................................................... 299 15.17 Lasers ........................................................................................................... 300

Gas lasers ................................................................................................ 300 Semiconductor Lasers ............................................................................. 300 Laser Safety ............................................................................................. 301

Chapter 16. Cameras .............................................................................................. 303 16.1 Introduction .................................................................................................. 303

Solid State Camera Arrays ...................................................................... 303 Advantage of Solid State Devices ............................................................ 304

16.2 Vidicon Cameras .......................................................................................... 304 Exposure and Scanning ........................................................................... 305 Dark Current ........................................................... ................................ 306 Colour Vidicons ....................................................................................... 306 Choice of Target for Vidicons ................................................................. 308

16.3 Photodiode Arrays ....................................................................................... 309 16.4 Charge Coupled Devices (CCD'S) ............................................................. 310

Transfer of Charge .................................................................................. 310 Infrared Sensitivity .................................................................................. 311 Photosite Shapes ..................................................................................... 312 Area Arrays ..... ........................................................................................ 312 Colour CCD Arrays ................................................................................ 313 2 D Images on Linear Arrays .................................................................. 314 Very High Resolution 2 D Cameras ........................................................ 314 Hexagonal Arrays ................................................................................... 314 Circular Arrays ....................................................................................... 315

xvi Contents

16.5 Charge Injection Devices (CID's) ............................................................... 315 16.6 Ram Cameras ............................................................................................... 315 16.7 Camera Specification & Performance ......................................................... 316

Resolution .. .............................................................................................. 316 Sensitivity ................................................................................................ 319 Crosstalk. ...... ........................................................................................... 320 Linearity and Gamma. ..... ........................................................................ 320 Geometrical Faults .................................................................................. 320 Blooming ................................................................................................. 321 Lag ........................................................................................................... 321 Noise ................................................. ....................................................... 322 Dark Signal ............................................................................................. 322

16.8 Image Intensifiers. ........................................................................................ 322 Channel Plate Intensifier ........................................................................ 324

16.9 Scanning Rates ............................................................................................. 324 Short Exposures ............................................. .......................................... 324 Video Images ........................................................................................... 325

16.10 Flying Spot Laser Scanners ......................................................................... 326 Direction of Reflection ............................................................................ 329 Dimension M easurement ......................................................................... 331

Chapter 17. Image Processing ................................................................................ 3 33 17.1 Introduction .................................................................................................. 333

Nomenclature and Standards .................................................................. 333 17.2 Thresholding ................................................................................................ 334

Fixed Level Thresholding ........................................................................ 334 Dynamic Thresholding ............................................................................ 339 Adaptive Thresholding ............................................................................ 342 Multi-Level Thresholding ........................................................................ 342 Spatially Varying Thresholds ............................................ ...................... 343 Colour Thresholding ............................................................................... 344 Threshold Implementation. ........................................ .............................. 348

17.3 Grey Scale Transformations ........................................................................ 350 Histogram Equalisation .......................................................................... 351 Contrast Expansion ........... ...................................................................... 352 Square intensity ....................................................................................... 352

17.4 Multiple Image Averaging ........................................................................... 353 17.5 Filtering / Convolutions ............................................................................... 355

Mean (Average) ............................................. ........................................... 356 Median Filter ........................................................................................... 358 High Pass ................................................................................................ 358 Low Pass ................................................................................................. 359 Lightest .............................................. ...................................................... 360 Darkest ..... ............................................................................................... 360 Intermediate Results ................................................................................ 361

17.6 Edge Detection ............................................................................................. 362 Horizontal gradient ................................................................................. 363 Vertical Gradient ..................................................................................... 364 Sobel ................................................. ....................................................... 365 Dispersion ............................................................................................... 366 Goodman-Smyth ...................................................................................... 366

Contents xvii

Roberts .................................................................................................... 369 Laplace ............................................... ..................................................... 369 Complex Edge Detectors ................................................. ........................ 370

17.7 Image Transfoffilation .................................................................................. 373 Spatial Transformations .......................................................................... 373

Chapter 18. Application of Machine Vision ........................................................ 379 18.1 Introduction .................................................................................................. 379 18.2 A Low Cost Commercial Vision System. .................................................... 320

System Specification .......................................... ...................................... 380 Inspection Procedure .............................................................................. 381 Simple Thresholds ................................................................................... 381 XY Float ................................................................................................... 383 Window Measurements ........................................................................... 384 Line Gauge Measurements .......................................... ............................ 384

18.3 Area Array Camera Applications. ................................................................ 385 Inspection of a Plastic Moulded Plug Top .............................................. 385 Clothes Peg ............................................................................................. 387 Edge Enhancements ................................................................................ 387

18.4 Line Scan Camera Applications. .................................................................. 388 Line Scan Cameras for 2-D Image Capture ............................................ 389 Line Scan Camera Measurement Systems ............................................... 392 Lighting Considerations ........................................ .................................. 394 Alignment and Focusing .......................................................................... 395 'Ruler' Applications .. .............................................................................. 396 Biscuit Width & Colour Measurement .................................................... 399

18.5 Colour Inspection ......................................................................................... 401 Benefits of using Colour ................................................ .......................... 401 Colour System Selection .......................................................................... 402 Chocolate Biscuit Inspection ................................................................... 402 Colour Sorting ............................................... .......................................... 403 Orange Picking ... .................................................................................... 403

Commercial Organisations .......................................................................................... 405

Bibliography .................................................................................................................. 411

References. ..................................................................................................................... 413

Index .............................................................................................................................. 421

Introduction

Numerous areas of expertise are often required for the inspection of an individual product, with many different sensors being used within a single inspection machine. For this reason it is necessary for the production engineer to have at least a working knowledge of all the different technologies that may be employed.

This book aims to cover the majority of sensors that can be applied on the shop floor and has been designed to assist engineers with little or no previous experience in the various fields. The information that the book contains is of a highly practical nature and is based on the authors considerable first hand experience of varied industrial applications.

Many potentially good applications have come to grief because the companies concerned have tackled them without sufficient understanding of the practical problems that they are likely to encounter. Failure, or at best the excessive difficulties that they experienced, have often coloured the company's perception of automatic inspection and caused them to withdraw, at least for some time, from additional applications. This is very unfortunate as there can be few companies that cannot benefit from a further application of automatic inspection technology.

Acknowledgements

This book covers a very broad range of industrial sensors and this has only been made possible by the numerous sources who have given so freely of their knowledge and time. These I acknowledge by reference within the text as this seems more appropriate than a long list placed here. To any that may have accidentally been omitted I extend my sincere apologies.

I would also like to thank:-

Stephen McClelland for his encouragement that got the book started.

IPS Publications Ltd. for permission to reproduce illustrations from their numerous publications that are such a rich source of information for anyone working in this field.

MCB University Press Ltd. for permission to include extracts and illustrations from their journals' Sensor Review', 'Assembly Automation' and 'Industrial Robot'.

Image Inspection Ltd. for the free loan of their Intelligent Framestore that I used to illustrate the chapter on image processing.