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Sensorsand 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 sensorsand 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 sensorsandtransducers 290
Appendix B: Glossary of terms 293
Index 296
Preface to Third Edition
This third edition of SensorsandTransducers 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 sensorsandtransducers 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 andtransducers 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 sensorsand 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 sensorsand 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 sensorsandtransducers Classi¢cation of sensors is conventionally by the conversion principle,... Actuators andsensors are therefore forms of transducers, and in this book we shall deal with actuators under the heading of transducers The di¡erences between sensorsandtransducers 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 sensorsand transducers, some 350 to date, it is obvious that not all are of equal importance By limiting the scope of this book to sensorsandtransducers 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 SENSORSANDTRANSDUCERS 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 SENSORSANDTRANSDUCERS Stress, strain and the elastic constants of Young's modulus and the bulk Stress 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 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 SENSORSANDTRANSDUCERS 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 SENSORSANDTRANSDUCERS 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 SENSORSANDTRANSDUCERS 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