Sensors and Transducers Other Macmillan titles of related interest B. Allen, Analogue Electronics for Higher Studies W.A. Atherton, From Compass to Computer J.C. Cluley, Transistors for Microprocessor Systems Donard de Cogan, Solid State Devices - A Quantum Physics Approach C.W. Davidson, Transmission Lines for Communications, second edition M.E. Goodge, Analog Electronics B.A. Gregory, An Introduction to Electrical Instrumentation and Measurement Systems, second edition Robin Holland, Microcomputer Fault-finding and Design, second edition Paul A. Lynn, An Introduction to the Analysis and Processing of Signals, third edition R.I. Mitchell, Microprocessor Systems-An Introduction Noel M. Morris, Electrical Circuit Analysis and Design M.S. Nixon, Introductory Digital Design P. Silvester, Electric Circuits L.A.A. Warnes, Electronic Materials B.W. Williams, Power Electronics-Devices, Drivers, Applications and Passive Components, second edition New Electronics Series Series Editor: Paul A. 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Keating Department of Cybernetics University of Reading Second Edition -- MACMillAN © M.J. Usher and D.A. Keating 1996 All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission. No paragraph of this publication may be reproduced, copied or transmitted save with written permission or in accordance with the provisions of the Copyright, Designs and Patents Act 1988, or under the terms of any licence permitting limited copying issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London W1P 9HE Any person who does any unauthorised act in relation to this publication may be liable to criminal prosecution and civil claims for damages. First edition 1985 Reprinted 1990 Second edition 1996 Published by MACMILLAN PRESS LTD Houndmills, Basingstoke, Hampshire RG21 6XS and London Companies and representatives throughout the world ISBN 978-0-333-60487-8 ISBN 978-1-349-13345-1 (eBook) DOl 10.1007/978-1-349-13345-1 A catalogue record for this book is available from the British Library Contents Preface vii Acknowledgements ix 1 Introduction 1 1.1 Analogue and digital quantities 1 1.2 Classification of sensing devices 2 1.3 Sensors, transducers and actuators 3 1.4 Types of transducer 4 1.5 Transducer parameters 6 1.6 Measurement systems 9 1.7 Exercises 11 2 Analogies between Systems 12 2.1 Analogies 12 2.2 Mechanical and electrical systems 12 2.3 Fluid systems 16 2.4 Thermal systems 17 2.5 Other systems: radiant, magnetic, chemical 18 2.6 Exercises 20 3 Physical Effects available for Use in Transducers 21 3.1 Representation of transducers 21 3.2 Self-generators 23 3.3 Modulators 29 3.4 Modifiers 37 3.5 Exercises 39 4 Transducer Bridges and Amplifiers 40 4.1 Transducer bridges 40 4.2 Transducer amplifiers 43 4.3 Practical operational amplifier characteristics 46 4.4 Exercises 50 5 Transducers for Length 52 5.1 Classification of length transducers 52 5.2 Displacement transducers 52 5.3 Velocity transducers 71 5.4 Strain transducers 73 5.5 Exercises 78 v vi CONTENTS 6 Transducers for Temperature 83 6.1 Scale of temperature 83 6.2 Temperature transducers 83 6.3 Exercises 90 7 Transducers for Light 93 7.1 Light and its properties 93 7.2 Classification of photodetectors 94 7.3 Thermal photodetectors 95 7.4 Photon detectors 99 7.5 Exercises 107 8 Other Transducers 109 8.1 Acceleration transducers 109 8.2 Force transducers 114 8.3 Pressure transducers 115 8.4 Flow transducers 118 8.5 Microphones 126 8.6 Exercises 128 9 Actuators 131 9.1 Electromagnetic actuators 131 9.2 Electrostatic actuators 140 9.3 Electro-optic devices 142 9.4 Piezoelectric actuators 145 9.5 Exercises 145 10 Measurement Systems 147 10.1 Solid-state transducers 147 10.2 Resonator sensors 150 10.3 Optical fibre transducers 153 10.4 Pyrometry 162 10.5 Ultrasonic measurement systems 167 10.6 Exercises 172 11 Digital Transducers and Interfacing 175 11.1 Digital measurements 175 11.2 Digital transducers 176 11.3 Interfacing 180 11.4 Smart sensors 188 11.5 Exercises 189 Solutions to Exercises 192 References and Bibliography 216 Index 219 Preface Most quantities that we need to measure are inherently analogue. There is nothing very digital about a length or a temperature and although light may be considered to consist of photons most measurements involve such large numbers that the process is effectively analogue. Our own senses are analogue so it is hardly surprising to find that the vast majority of physical sensors are also analogue. It is only since the developments in microprocessor technology that digital transducers have become important, and have sometimes captured an undue proportion of attention; however, they still have to measure analogue quantities and most digital transducers therefore employ exactly the same physical principles as their analogue counterparts. This book discusses most of the transducers in current use, whether digital or analogue. The coverage is primarily from the measurand standpoint; for example, the different types of length transducer are discussed and compared together in one chapter, although we have also included chapters summarising the various transducer technologies, such as solid-state or fibre-optic devices. Digital transducers are dealt with in the same way, under length or temperature as appropriate, but a chapter is also devoted to their summary and classification. The words 'sensor' and 'transducer' are widely used in referring to sensing devices, the former having gained in popularity in recent years. This is a pity because 'transducer' stresses the change in form of energy basic to the sensing process and leads to an elegant and powerful classification of devices. The word 'transducer' is used here when considering a complete sensing device, in which there is bound to be a change in form of energy; the word 'sensor' is reserved for devices which 'respond to a stimulus' but are not energy converting, such as a thermistor, which simply changes its resistance in response to temperature. The most important transducer parameters are 'responsivity' and 'detectivity'; the former refers to the response of a transducer to the applied measurand and the latter to the least input measurand that can be detected. The use of these two separate words removes the ambiguity of the word 'sensitivity', unfortunately sometimes used in describing transducers, which may refer to the response either to the desired input or to an undesired input, or both together. These two responses must be distinguished and this is correctly accomplished by 'responsivity' and 'detectivity', which are used throughout the book. The aim of the book is to provide an integrated account of the principles and vii viii PREFACE properties of the most important types of physical transducer. The first chapter discusses the types of physical energy and the corresponding signals, and identifies the three basic types of transducers: self-generators, modulators and modifiers. A synthesis of the subject is attempted in chapter 2, describing the analogies that exist between different types of physical system and showing how our understanding can sometimes be improved by considering an analogy of a particular device or circuit. Chapter 3 starts with a three-dimensional representation of all possible transducers and goes on to consider the basic physical mechanisms available for transduction. Chapter 4 develops the relevant expressions for amplifiers and transducer bridges that are required before the detailed descriptions of the basic transducers for length, temperature and light are given in chapters 5, 6 and 7. For each of these quantities the physical background and measurement standards are first explained, followed by both a theoretical treatment of the basic transducers and a description of their practical design and application. Chapter 8 includes the application of the basic transducers to several important fields of measurement, such as acceleration, force, pressure, flow and sound. Although transducers are usually thought of as input devices, output transducers are important in measurement systems, being usually referred to as 'actuators', and chapter 9 is devoted to the various types available. Chapter 10 is concerned with measurement systems, showing how transducers and actuators are used in complete systems, and including solid-state sensors, resonator sensors, optical fibre sensors, pyrometry and ultrasonics. The final chapter gives a summary and classification of digital transducers and an introduction to interfacing to computer systems. Many worked examples are given, together with a set of exercises at the end of each chapter, full solutions being provided at the end of the book. The first edition of this book was devoted specifically to transducers and their characteristics, but in the second edition the authors have extended the coverage to include both instrumentation, in chapter 10, and digital transducers and interfacing, in chapter 11. The book is therefore subtitled 'Characteristics, Applications, Instrumentation, Interfacing'. It is intended as a basic under graduate text for students in engineering, physics and information technology. Acknowledgements The authors wish to thank the former and present heads of the Cybernetics Department, Peter Fellgett and Kevin Warwick, for their encouragement and suggestions regarding the lecture courses on which the book is based, and Christine Usher who did much of the typing and corrections. They also acknowledge the feedback from the many Cybernetics and Engineering students who acted as willing guinea-pigs* during the development of the courses. *Guinea-pigs are nocturnal animals that mostly sleep during the day. ix