Sensors and Transducers

Sensors and Transducers Sensors and Transducers Third edition Ian R. Sinclair OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI Newnes An imprint ofButterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801-2041 A division ofReed Educational a nd Professional Publishing Ltd A member ofthe 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 ofthis 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 ofthis publication) without the written permission ofthe copyright holder except in accordance with the provisions ofthe Copyright, Designs and Patents Act 1988 or under the terms ofa 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 ofthis publication should be addressed to the publishers British Library Cataloguing in Publication 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 ofthe ever-increasing role ofthese components and ofim- provements in design. New tables ofpr operties and illustrations have also been added. The topic ofswitches an d switching actions has also been added because so many types ofsensor are intended ultimately to provide a switching action. Ian Sinclair Preface to First Edition The purpose ofthis book is to explain and illustrate the use ofsensors and transducers associated with electronic circuits. The steady spread ofelec- tronic circuits into all aspects oflife,but particularly into all aspects of control technology, has greatly increased the importance ofsensors which can detect, as electrical signals, changes in various physical quantities. In addition, the conversion by transducers ofphysical quantities into electronic signals and vice versa has become an important part ofelectronics. Because ofthis, the range ofpossible 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 ofsensors forspeci¢c purposes. In this book, you will ¢nd described a very large range ofdevices, some use d 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 ofdevices, much more than it would be possible to present in a more specialized text. In addition, I have assumed that the physical principles ofeach 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 electrical 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 ofmathematics has been avoided unless absolutely essential to the understanding ofa 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 x PREFACE TO FIRST EDITION 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 ofspeci¢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 Introduction A sensor is a device that detects or measures a physical quantity, and in this book the types ofsensors that we are concerned with are the types whose output is electrical. The opposite device is an actuator, which converts a signal (usually electrical) to some action, usually mechanical. A transducer is a device that converts energy from one form into another, and here we are concerned only with the transducers in which one form of energy is electrical. Actuators and sensors are therefore forms of transducers, and in this book we shall deal with actuators under the heading oftransducers. 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 ofenergy conversion. The purpose ofa sensor is to detect and measure, and whether its e¤ciency is 5% or 0.1% is almost immaterial, provided the ¢gure is known. A transducer, by contrast, is intended to convert energy, and its e¤- ciency is important, though in some cases it may not be high. Linearity of response, de¢ned by plotting the output against the input, is likely to be important for a sensor, but of much less signi¢cance for a transducer. By contrast, e¤ciency ofconversion is important for a transducer but not for a sensor. The basic principles that apply to one, however, must apply to the other, so that the 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 mechanicalêlectrical transducer, and also because switch action is such an important part ofthe action of many types ofsensors and transducers. Classi¢cation ofsensors is conventio nally by the conversion principle, the quantity being measured, the technology used, or the application. The xii INTRODUCTION organization ofthis book is, in general, by the physical quantity that is sensed or converted. This is not a perfect form of organization, but no form is, because there are many òne-o¡' devices that sense or convert for some unique purpose, and these have to be gathered together in an àssortment' chapter. Nevertheless, by grouping devices according to the sensed quantity, it is much easier for the reader to ¢nd the information that is needed, and that is the guiding principle for this book. In addition, some of the devices that are dealt with early in the book are those which form part ofother sensing or transducing systems that appear later. This avoids having to repeat a description, or refer forward for a description. Among the types ofenergy that can be sensed are those classed as radiant, mechanical, gravitational, electrical, thermal, and magnetic. Ifwe consider the large number ofprinci plesthatcanbeusedinthedesignof sensors and transducers, some 350 to date, it is obvious that not all are of equal importance. By limiting the scope ofthis book to sensors and transducers with electrical/electronic inputs or outputs ofthe six forms listed above, we can reduce this number to a more manageable level. Several points should be noted at this stage, to avoid much tedious repeti- tion in the main body ofthe book. One is that a fairnumber ofphysical e¡ects are sensed or measured, but have no requirement for transducers ^ we do not, for example, generate electricity from earthquake shocks though we certainly want to sense them. A second point is that the output from a sensor, including the output from electronic circuits connected to the sensor, needs to be proportional in some way to the e¡ect that is being sensed, or at least to bear some simple mathematical relationship to the quantity. This means that ifthe output is to be used formeasurements, then some form of calibration can be carried out. It also implies that the equation that connects the electrical output with the input that is being sensed contains various constants such as mass, length, resistance and so on. Ifany ofthese quantities is varied at any time, then recalibration ofthe equipment will be necessary. Sensors can be classed as active or passive. An active or self-generating sensor is one that can generate a signal without the need for any external power supply. Examples include photovoltaic cells, thermocouples and piezoelectric devices. The more common passive sensors need an external source of energy, which for the devices featured in this book will be electrical. These operate by modulating the voltage or current ofa supply. Another class ofpassive sensors, sometimes called modi¢ers, use the same type ofenergy at the output as at the input. Typical ofthese types is a diaphragm used to convert the pressure or velocity oscillations ofsound waves into movements ofa solid sheet. Another point that we need to be clear about is the meaning of resolution as applied to a sensor. The resolution of a sensor measures its ability to detect a change in the sensed quantity, and is usually quoted in terms ofthe smallest change that can be detected.

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