Tài liệu Handbook.of.modern.sensors.physics.designs.and.applications

HANDBOOK OF MODERN SENSORS P H Y S I C S, D E S I G N S, a n d A P P L I C A T I O N S Third Edition Springer New York Berlin Heidelberg Hong Kong London Milan Paris Tokyo HANDBOOK OF MODERN SENSORS P HY S I C S, D E S I G N S, a n d A P P L I C AT I O N S Third Edition JACOB FRADEN Advanced Monitors Corporation San Diego, California With 403 Iillustrations Jacob Fraden Advanced Monitors Corporation 6255 Ferris Square, Suite M San Diego, CA 92121 USA [email protected] Library of Congress Cataloging-in-Publication Data Fraden, Jacob Handbook of modern sensors : physics, designs, and applications / Jacob Fraden.–3rd ed. p. cm. Includes bibliographical references and index. ISBN 0-387-00750-4 (alk. paper) 1. Detectors–Handbooks, manuals, etc. 2. Interface circuits–Handbooks, manuals, etc. I. Title. TA165.F723 2003 681
.2—dc21 ISBN 0-387-00750-4 2003044597 Printed on acid-free paper. AIP Press is an imprint of Springer-Verlag, Inc. © 2004, 1996 Springer-Verlag New York, Inc. All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer-Verlag New York, Inc., 175 Fifth Avenue, New York, NY 10010, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed in the United States of America. 9 8 7 6 5 4 3 2 1 SPIN 10919477 www.springer-ny.com Springer-Verlag New York Berlin Heidelberg A member of BertelsmannSpringer Science+Business Media GmbH To the memory of my father This page intentionally left blank Preface Seven years have passed since the publication of the previous edition of this book. During that time, sensor technologies have made a remarkable leap forward. The sensitivity of the sensors became higher, the dimensions became smaller, the selectivity became better, and the prices became lower. What have not changed are the fundamental principles of the sensor design. They are still governed by the laws of Nature. Arguably one of the greatest geniuses who ever lived, Leonardo Da Vinci, had his own peculiar way of praying. He was saying, “Oh Lord, thanks for Thou do not violate your own laws.” It is comforting indeed that the laws of Nature do not change as time goes by; it is just our appreciation of them that is being refined. Thus, this new edition examines the same good old laws of Nature that are employed in the designs of various sensors. This has not changed much since the previous edition. Yet, the sections that describe the practical designs are revised substantially. Recent ideas and developments have been added, and less important and nonessential designs were dropped. Probably the most dramatic recent progress in the sensor technologies relates to wide use of MEMS and MEOMS (micro-electro-mechanical systems and micro-electro-opto-mechanical systems). These are examined in this new edition with greater detail. This book is about devices commonly called sensors. The invention of a microprocessor has brought highly sophisticated instruments into our everyday lives. Numerous computerized appliances, of which microprocessors are integral parts, wash clothes and prepare coffee, play music, guard homes, and control room temperature. Microprocessors are digital devices that manipulate binary codes generally represented by electric signals. Yet, we live in an analog world where these devices function among objects that are mostly not digital. Moreover, this world is generally not electrical (apart from the atomic level). Digital systems, however complex and intelligent they might be, must receive information from the outside world. Sensors are interface devices between various physical values and electronic circuits who “understand” only a language of moving electrical charges. In other words, sensors are the eyes, ears, and noses of silicon chips. Sensors have become part of everyone’s life. In the United States alone, they comprise a $12 billion industry. VIII Preface In the course of my engineering work, I often felt a strong need for a book that would combine practical information on diversified subjects related to the most important physical principles, design, and use of various sensors. Surely, I could find almost all I had to know in texts on physics, electronics, technical magazines, and manufacturers’ catalogs. However, the information is scattered over many publications, and almost every question I was pondering required substantial research work and numerous trips to the library. Little by little, I have been gathering practical information on everything that in any way was related to various sensors and their applications to scientific and engineering measurements. Soon, I realized that the information I collected might be quite useful to more than one person. This idea prompted me to write this book. In setting my criteria for selecting various sensors for this edition, I attempted to keep the scope of this book as broad as possible, opting for brief descriptions of many different designs (without being trivial, I hope) rather than fewer treated in greater depth. This volume attempts (immodestly perhaps) to cover a very broad range of sensors and detectors. Many of them are well known, but describing them is still useful for students and those who look for a convenient reference. It is the author’s intention to present a comprehensive and up-to-date account of the theory (physical principles), design, and practical implementations of various (especially the newest) sensors for scientific, industrial, and consumer applications. The topics included in the book reflect the author’s own preferences and interpretations. Some may find a description of a particular sensor either too detailed or too broad or, contrary, too brief. In most cases, the author tried to make an attempt to strike a balance between a detailed description and a simplicity of coverage. This volume covers many modern sensors and detectors. It is clear that one book cannot embrace the whole variety of sensors and their applications, even if it is called something like The Encyclopedia of Sensors. This is a different book, and the author’s task was much less ambitious. Here, an attempt has been made to generate a reference text that could be used by students, researchers interested in modern instrumentation (applied physicists and engineers), sensor designers, application engineers, and technicians whose job is to understand, select, and/or design sensors for practical systems. The previous editions of this book have been used quite extensively as desktop references and textbooks for the related college courses. Comments and suggestions from the sensor designers, professors, and students prompted me to implement several changes and correct errors. Jacob Fraden San Diego, California November 2003 Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII 1 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Sensors, Signals, and Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Sensor Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3 Units of Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2 Sensor Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Transfer Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Span (Full-Scale Input) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Full-Scale Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Calibration Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 Nonlinearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 Repeatability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11 Dead Band . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12 Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13 Special Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.14 Output Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.15 Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.16 Dynamic Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.17 Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.18 Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.19 Application Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.20 Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 13 15 16 17 18 19 20 20 22 23 23 23 24 24 25 25 29 31 33 33 35 X Contents 3 Physical Principles of Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Electric Charges, Fields, and Potentials . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Dielectric Constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Faraday’s Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Solenoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Toroid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Permanent Magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Induction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Specific Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Temperature Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.3 Strain Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.4 Moisture Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Piezoelectric Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Piezoelectric Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Pyroelectric Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Hall Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Seebeck and Peltier Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Sound Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11 Temperature and Thermal Properties of Materials . . . . . . . . . . . . . . . . 3.11.1 Temperature Scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11.2 Thermal Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11.3 Heat Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12 Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12.1 Thermal Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12.2 Thermal Convection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12.3 Thermal Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12.3.1 Emissivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12.3.2 Cavity Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13 Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14 Dynamic Models of Sensor Elements . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14.1 Mechanical Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14.2 Thermal Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14.3 Electrical Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14.4 Analogies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 38 44 45 46 50 52 54 55 55 56 59 60 62 64 65 66 72 76 82 86 92 94 95 96 98 99 99 102 103 106 109 111 113 115 117 118 119 119 4 Optical Components of Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Radiometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Photometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 125 129 132 134 Contents 5 XI 4.5 Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Fresnel Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 Fiber Optics and Waveguides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8 Concentrators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9 Coatings for Thermal Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 Electro-optic and Acousto-optic Modulators . . . . . . . . . . . . . . . . . . . . 4.11 Interferometric Fiber-optic Modulation . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 137 140 144 145 146 148 149 Interface Electronic Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Input Characteristics of Interface Circuits . . . . . . . . . . . . . . . . . . . . . . 5.2 Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Operational Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Voltage Follower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Instrumentation Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Charge Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Excitation Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Current Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Voltage References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.4 Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Analog-to-Digital Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 V/F Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.3 Dual-Slope Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.4 Successive-Approximation Converter . . . . . . . . . . . . . . . . . . . 5.4.5 Resolution Extension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Direct Digitization and Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Ratiometric Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Bridge Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.1 Disbalanced Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.2 Null-Balanced Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7.3 Temperature Compensation of Resistive Bridge . . . . . . . . . . . 5.7.4 Bridge Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Data Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.1 Two-Wire Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.2 Four-Wire Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.3 Six-Wire Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Noise in Sensors and Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.1 Inherent Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.2 Transmitted Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.3 Electric Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.4 Bypass Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.5 Magnetic Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.6 Mechanical Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 151 156 156 158 159 161 164 165 169 171 174 175 175 176 181 183 185 186 190 192 193 194 195 200 201 202 203 204 204 205 207 212 214 215 217 XII Contents 5.9.7 Ground Planes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9.8 Ground Loops and Ground Isolation . . . . . . . . . . . . . . . . . . . . 5.9.9 Seebeck Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Batteries for Low Power Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10.1 Primary Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10.2 Secondary Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 219 221 222 223 224 225 6 Occupancy and Motion Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Ultrasonic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Microwave Motion Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Capacitive Occupancy Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Triboelectric Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Optoelectronic Motion Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Sensor Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1.1 Multiple Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1.2 Complex Sensor Shape . . . . . . . . . . . . . . . . . . . . . . . 6.5.1.3 Image Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1.4 Facet Focusing Element . . . . . . . . . . . . . . . . . . . . . . . 6.5.2 Visible and Near-Infrared Light Motion Detectors . . . . . . . . . 6.5.3 Far-Infrared Motion Detectors . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.3.1 PIR Motion Detectors . . . . . . . . . . . . . . . . . . . . . . . . 6.5.3.2 PIR Sensor Efficiency Analysis . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 228 228 233 237 238 240 241 241 241 242 243 244 245 247 251 7 Position, Displacement, and Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Potentiometric Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Gravitational Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Capacitive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Inductive and Magnetic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 LVDT and RVDT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.2 Eddy Current Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.3 Transverse Inductive Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.4 Hall Effect Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.5 Magnetoresistive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.6 Magnetostrictive Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Optical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.1 Optical Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.2 Proximity Detector with Polarized Light . . . . . . . . . . . . . . . . . 7.5.3 Fiber-Optic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.4 Fabry–Perot Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.5 Grating Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.6 Linear Optical Sensors (PSD) . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Ultrasonic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 Radar Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 254 256 258 262 262 264 266 267 271 274 275 275 276 278 278 281 283 286 289 Contents XIII 7.7.1 Micropower Impulse Radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7.2 Ground-Penetrating Radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8 Thickness and Level Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.1 Ablation Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.2 Thin-Film Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.8.3 Liquid-Level Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 291 293 293 296 296 298 8 Velocity and Acceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Accelerometer Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Capacitive Accelerometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Piezoresistive Accelerometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Piezoelectric Accelerometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Thermal Accelerometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Heated-Plate Accelerometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.2 Heated-Gas Accelerometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.1 Rotor Gyroscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.2 Monolithic Silicon Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.3 Optical Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 Piezoelectric Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 303 305 307 309 309 309 310 313 313 314 317 319 321 9 Force, Strain, and Tactile Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Strain Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Tactile Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Piezoelectric Force Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 325 327 334 336 10 Pressure Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 Concepts of Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Units of Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Mercury Pressure Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Bellows, Membranes, and Thin Plates . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Piezoresistive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6 Capacitive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7 VRP Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.8 Optoelectronic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.9 Vacuum Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.9.1 Pirani Gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.9.2 Ionization Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.9.3 Gas Drag Gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 339 340 341 342 344 349 350 352 354 354 356 356 357 XIV 11 Contents Flow Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Basics of Flow Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Pressure Gradient Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Thermal Transport Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Ultrasonic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5 Electromagnetic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6 Microflow Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7 Breeze Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.8 Coriolis Mass Flow Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.9 Drag Force Flow Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 359 361 363 367 370 372 374 376 377 378 12 Acoustic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Resistive Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Condenser Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Fiber-Optic Microphone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4 Piezoelectric Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5 Electret Microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6 Solid-State Acoustic Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381 382 382 383 385 386 388 391 13 Humidity and Moisture Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1 Concept of Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Capacitive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3 Electrical Conductivity Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Thermal Conductivity Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.5 Optical Hygrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.6 Oscillating Hygrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 393 396 399 401 402 403 404 14 Light Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Photodiodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3 Phototransistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.4 Photoresistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.5 Cooled Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.6 Thermal Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.6.1 Golay Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.6.2 Thermopile Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.6.3 Pyroelectric Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.6.4 Bolometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.6.5 Active Far-Infrared Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.7 Gas Flame Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 407 411 418 420 423 425 426 427 430 434 437 439 441 Contents 15 XV Radiation Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1 Scintillating Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 Ionization Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.1 Ionization Chambers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.2 Proportional Chambers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.3 Geiger–Müller Counters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.4 Semiconductor Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 444 447 447 449 450 451 455 16 Temperature Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.1 Thermoresistive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.1.1 Resistance Temperature Detectors . . . . . . . . . . . . . . . . . . . . . . 16.1.2 Silicon Resistive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.1.3 Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.1.3.1 NTC Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.1.3.2 Self-Heating Effect in NTC Thermistors . . . . . . . . . 16.1.3.3 PTC Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2 Thermoelectric Contact Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.1 Thermoelectric Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.2 Thermocouple Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.3 Thermocouple Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3 Semiconductor P-N Junction Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4 Optical Temperature Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4.1 Fluoroptic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4.2 Interferometric Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4.3 Thermochromic Solution Sensor . . . . . . . . . . . . . . . . . . . . . . . 16.5 Acoustic Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.6 Piezoelectric Temperature Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457 461 461 464 465 465 474 477 481 482 484 486 488 491 492 494 494 495 496 497 17 499 500 500 501 503 503 504 505 506 507 508 510 512 512 513 Chemical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.1 Chemical Sensor Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.2 Specific Difficulties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.3 Classification of Chemical-Sensing Mechanisms . . . . . . . . . . . . . . . . 17.4 Direct Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.4.1 Metal-Oxide Chemical Sensors . . . . . . . . . . . . . . . . . . . . . . . . 17.4.2 ChemFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.4.3 Electrochemical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.4.4 Potentiometric Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.4.5 Conductometric Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.4.6 Amperometric Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.4.7 Enhanced Catalytic Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . 17.4.8 Elastomer Chemiresistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.5 Complex Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.5.1 Thermal Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XVI Contents 17.5.2 Pellister Catalytic Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.5.3 Optical Chemical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.5.4 Mass Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.5.5 Biochemical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.5.6 Enzyme Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.6 Chemical Sensors Versus Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . 17.6.1 Chemometrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.6.2 Multisensor Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.6.3 Electronic Noses (Olfactory Sensors) . . . . . . . . . . . . . . . . . . . 17.6.4 Neural Network Signal (Signature) Processing for Electronic Noses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.6.5 “Smart” Chemical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527 530 530 Sensor Materials and Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.1.1 Silicon as a Sensing Material . . . . . . . . . . . . . . . . . . . . . . . . . . 18.1.2 Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.1.3 Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.1.4 Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.1.5 Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.2 Surface Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.2.1 Deposition of Thin and Thick Films . . . . . . . . . . . . . . . . . . . . . 18.2.2 Spin-Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.2.3 Vacuum Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.2.4 Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.2.5 Chemical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.3 Nano-Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.3.1 Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.3.2 Silicon Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.3.2.1 Basic Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.3.2.2 Wafer bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 533 533 536 540 542 543 543 543 544 544 545 546 547 548 549 549 554 555 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.1 Chemical Symbols for the Elements . . . . . . . . . . . . . . . . . . . . Table A.2 SI Multiples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.3 Derivative SI Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.4 SI Conversion Multiples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.5 Dielectric Constants of Some Materials at Room Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.6 Properties of Magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . Table A.7 Resistivities and Temperature Coefficients of Resistivity of Some Materials at Room Temperature . . . . Table A.8 Properties of Piezoelectric Materials at 20◦ C . . . . . . . . . . . . . 557 557 558 558 559 18 514 514 516 519 520 520 523 524 524 564 564 565 565 Contents Table A.9 Physical Properties of Pyroelectric Materials . . . . . . . . . . . . . Table A.10 Characteristics of Thermocouple Types . . . . . . . . . . . . . . . . . . Table A.11 Thermoelectric Coefficients and Volume Resistivities of Selected Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.11a Thermocouples for Very Low and Very High Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.12 Densities of Some Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.13 Mechanical Properties of Some Solid Materials . . . . . . . . . . . Table A.14 Mechanical Properties of Some Crystalline Materials . . . . . . Table A.15 Speed of Sound Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.16 Coefficient of Linear Thermal Expansion of Some Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.17 Specific Heat and Thermal Conductivity of Some Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.18 Typical Emissivities of Different Materials . . . . . . . . . . . . . . . Table A.19 Refractive Indices of Some Materials . . . . . . . . . . . . . . . . . . . Table A.20 Characteristics of C–Zn and Alkaline Cells . . . . . . . . . . . . . . . Table A.21 Lithium–Manganese Dioxide Primary Cells . . . . . . . . . . . . . . Table A.22 Typical Characteristics of “AA”-Size Secondary Cells . . . . . Table A.23 Miniature Secondary Cells and Batteries . . . . . . . . . . . . . . . . . Table A.24 Electronic Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table A.25 Properties of Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XVII 566 566 567 567 568 568 569 569 569 570 571 572 573 573 573 574 576 577 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579 This page intentionally left blank 1 Data Acquisition “It’s as large as life, and twice as natural” —Lewis Carroll, “Through the Looking Glass” 1.1 Sensors, Signals, and Systems A sensor is often defined as a device that receives and responds to a signal or stimulus. This definition is broad. In fact, it is so broad that it covers almost everything from a human eye to a trigger in a pistol. Consider the level-control system shown in Fig. 1.1 [1]. The operator adjusts the level of fluid in the tank by manipulating its valve. Variations in the inlet flow rate, temperature changes (these would alter the fluid’s viscosity and, consequently, the flow rate through the valve), and similar disturbances must be compensated for by the operator. Without control, the tank is likely to flood, or run dry. To act appropriately, the operator must obtain information about the level of fluid in the tank on a timely basis. In this example, the information is perceived by the sensor, which consists of two main parts: the sight tube on the tank and the operator’s eye, which generates an electric response in the optic nerve. The sight tube by itself is not a sensor, and in this particular control system, the eye is not a sensor either. Only the combination of these two components makes a narrow-purpose sensor (detector), which is selectively sensitive to the fluid level. If a sight tube is designed properly, it will very quickly reflect variations in the level, and it is said that the sensor has a fast speed response. If the internal diameter of the tube is too small for a given fluid viscosity, the level in the tube may lag behind the level in the tank. Then, we have to consider a phase characteristic of such a sensor. In some cases, the lag may be quite acceptable, whereas in other cases, a better sight tube design would be required. Hence, the sensor’s performance must be assessed only as a part of a data acquisition system. This world is divided into natural and man-made objects. The natural sensors, like those found in living organisms, usually respond with signals, having an electrochemical character; that is, their physical nature is based on ion transport, like in the nerve fibers (such as an optic nerve in the fluid tank operator). In man-made devices,