Sensors and Transducers Sensors and Transducers Third edition Ian R. Sinclair OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI Newnes An imprint of Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801-2041 A division of Reed Educational and Professional Publishing Ltd A member of the Reed Elsevier plc group First published by BSP Professional Books 1988 Reprinted by Butterworth-Heinemann 1991 Second edition published by Butterworth-Heinemann 1992 Third edition 2001 # I. R. Sinclair 1988, 1992, 2001 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers British Library Catalogu ing in Publi cation Data A catalogue record for this book is available from the British Library ISBN0750649321 Typeset by David Gregson Associates, Beccles, Su¡olk Printed and bound in Great Britain Contents Preface to Third Edition vii Preface to First Edition ix Introduction xi 1 Strain and pressure 1 2 Position, direction, distance and motion 21 3 Light and associated radiation 53 4 Temperature sensors and thermal transducers 87 5 Sound, infrasound and ultrasound 116 6 Solids, liquids and gases 142 7 Environmental sensors 170 8 Other sensing methods 197 9 Instrumentation techniques 206 10 Switch principles 233 11 Switch mechanisms 248 12 Signal-carrying switches 270 Appendix A: Suppliers of sensors and transducers 290 Appendix B: Glossary of terms 293 Index 296 Preface to Third Edition This third edition of Sensors and Transducers has been thoroughly revised to take account of the ever-increasing role of these components and of im- provements in design. New tables of properties and illustrations have also been added. The topic of switches and switching actions has also been added because so many types of sensor are intended ultimately to provide a switching action. Ian Sinclair Preface to First Edition The purpose of this book is to explain and illustrate the use of sensors and transducers associated with electronic circuits. The steady spread of elec- tronic circuits into all aspects of life, but particularly into all aspects of control technology, has greatly increased the importance of sensors which can detect, as electrical signals, changes in various physical quantities. In addition, the conversion by transducers of physical quantities into electronic signals and vice versa has become an important part of electronics. Because of this, the range of possible sensors and transducers is by now very large, and most textbooks that are concerned with the interfaces between electronic circuits and other devices tend to deal only with a few types of sensors for speci¢c purposes. In this book, you will ¢nd described a very large range of devices, some used industrially, some domestically, some employed in teaching to illustrate e¡ects, some used only in research laboratories. The important point is that the reader will ¢nd reference to a very wide range of devices, much more than it would be possible to present in a more specialized text. In addition, I have assumed that the physical principles of each sensor or transducer will not necessarily be familiar. To be useful, a book of this kind should be accessible to a wide range of users, and since the correct use of sensors and transducers often depends critically on an understanding of the physical principles involved, these principles have been explained in as much depth as is needed. I have made the reasonable assumption that elec- trical principles will not be required to be explained in such depth as the principles of, for example, relative humidity. In order for the book to be as serviceable as possible to as many readers as possible, the use of mathematics has been avoided unless absolutely essential to the understanding of a device. I have taken here as my guide the remark by Lord Kelvin that if he needed to use mathematics to explain something it was probably because he didn't really understand it. The text should prove useful to anyone who encounters sensors and transducers, whether from the point of view of speci¢cation, design, servicing, or education. I am most grateful to RS Components for much useful and well-organized information, and to Bernard Watson, of BSP Professional Books, for advice and encouragement. Ian Sinclair April 1988 x PREFACE TO FIRST EDITION [...]... descriptions that appear in this book will apply equally to sensors and to transducers Switches appear in this book both as transducers /sensors in their own right, since any electrical switch is a mechanical^electrical transducer, and also because switch action is such an important part of the action of many types of sensors and transducers Classi¢cation of sensors is conventionally by the conversion principle,... Actuators and sensors are therefore forms of transducers, and in this book we shall deal with actuators under the heading of transducers The di¡erences between sensors and transducers are often very slight A sensor performs a transducing action, and the transducer must necessarily sense some physical quantity The di¡erence lies in the e¤ciency of energy conversion The purpose of a sensor is to detect and measure,... sensed are those classed as radiant, mechanical, gravitational, electrical, thermal, and magnetic If we consider the large number of principles that can be used in the design of sensors and transducers, some 350 to date, it is obvious that not all are of equal importance By limiting the scope of this book to sensors and transducers with electrical/electronic inputs or outputs of the six forms listed above,... moduli are the linear Young's modulus, the shear (twisting) modulus, and the (pressure) bulk modulus For small amounts of strain, the strain is proportional to stress, and an elastic modulus is a quantity that expresses the ratio stress/strain in the 2 Figure 1.1 SENSORS AND TRANSDUCERS The classic method of measuring tensile stress and strain for a wire elastic region, i.e the portion of the stress^strain... material, or which can be measured for a sample of material The stress is stated in units of 12 Table 1.1 modulus SENSORS AND TRANSDUCERS Stress, strain and the elastic constants of Young's modulus and the bulk Stress ˆ strain  Young's modulus (for tensile stress) Example: If measured strain is 0.001 and the Young's modulus for the material is 20  10 10 N/m 2 then stress is: 20  10 10  0.001 ˆ 20  10... the capacitance between the diaphragm and a ¢xed plate, and this change of capacitance can be sensed electronically The formula relating capacitance to spacing is shown in (b) 14 SENSORS AND TRANSDUCERS Figure 1.14 Using a variable reluctance type of sensing system The movement of the diaphragm causes considerable changes in the reluctance of the magnetic path, and so in the inductance of the coil... only movement in a system and requires rather di¡erent methods In addition, 22 SENSORS AND TRANSDUCERS Figure 2.1 The Cartesian co-ordinate system This uses measurements in two directions at right angles to each other as reference axes, and the position of a point is plotted by ¢nding its distance from each axis For a three-dimensional location, three axes, labelled x, y and z, can be used The ¢gure... re£ected many times The extra 10 SENSORS AND TRANSDUCERS Figure 1.9 Optical ¢bre construction The optical ¢bre is not a single material but a coaxial arrangement of transparent glass or (less usefully) plastics The materials are di¡erent and refract light to di¡erent extents (refractivity) so that any light ray striking the junction between the materials is re£ected back and so trapped inside the ¢bre... with a voltmeter whose input resistance is not high enough All forms of sensors are liable to dynamic errors if they are used only for sensing, and to both dynamic and static errors if they are used for measurement Since the development of microprocessors, a new breed of sensors has been developed, termed intelligent or smart sensors This type of system uses a miniature sensor that is integrated on... 1.13 The diaphragm is insulated from the ¢xed backplate, and the capacitance between the diaphragm and the backplate forms part of the resonant circuit of an oscillator Reducing the spacing between the diaphragm and the backplate will increase the capacitance, in accordance with the formula shown in Figure 1.13(b), and so reduce the resonant STRAIN AND PRESSURE 13 Figure 1.12 The aneroid barometer principle . Sensors and Transducers Sensors and Transducers Third edition Ian R. Sinclair OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE. direction, distance and motion 21 3 Light and associated radiation 53 4 Temperature sensors and thermal transducers 87 5 Sound, infrasound and ultrasound 116 6