Sensors, actuators, and their interface

The book in front of you introduces the subjects of sensing and actuation. At first it would seem that nothing could be easier; we may think we know what a sensor is, and certainly we know what an actuator is. But do we know it so well that it actually escapes us? There are literally thousands of devices all around us that qualify in either category. In Chapter 1 there is an example that lists many of the sensors and actuators just in a car. The count is approximately 200, and this is merely a partial list The approach adopted here is to view all devices as belonging to three categories: sensors, actuators, and processors (interfaces). Sensors are the devices that provide input to systems and actuators are those devices that serve as outputs. In between, linking, interfacing, processing, and driving are the processors. In other words, the view advocated in this text is one of general sensing and actuation. In that sense, a switch on the wall is a sensor (a force sensor), and the light bulb that turns on as a result is an actuator (it does something). In between there is a ‘‘processor’’ – the wiring harness or, in case a dimmer is used, an actual electronic circuit – that interprets the input data and does something with it. In this case it may be no more than a wiring harness, but in other cases it may be a microprocessor or an entire system of computers.

Sensors, Actuators, and their Interfaces

As sensors and actuators are normally not (and have not been) treated in academic curricula as a subject

in its own right; many students and current professionals often find themselves limited in their knowledge and dealing with topics and issues based on material they may have never encountered Until now.

This book brings sensors, actuators and interfaces out of obscurity and integrates them for multiple disciplines including electrical, mechanical, chemical, and biomedical engineering Real world cases, worked examples, and problem sets with selected answers provide both fundamental understanding and how industry develops sensor systems Students and professionals from any of these disciplines will easily learn the foundational concepts and then be able to apply them to cross-discipline requirements.

The idea is simple A sensor system in general is made of three components:

1 Inputs (sensors)

2 Outputs (actuators)

3 Processor (the unit to which the inputs and outputs are connected and performs all, or the most, tasks needed to interface them)

Sensors, Actuators, and their Interfaces focuses on the broad area of detection, outlining and simplifying the

understanding of theory behind sensing and actuation It is an invaluable textbook for undergraduate and graduate level courses, as well as a reference for professionals who were never afforded the opportunity

to take an introductory course.

ABOUT THE AUTHOR

Nathan Ida is currently a Distinguished Professor of Electrical and Computer Engineering at the University of Akron

He received his Bachelor and Master degrees from Ben-Gurion University of the Negev (Israel) and his Ph.D from Colorado State University in 1983 Dr Ida has written five successful books in electromagnetics including the

undergraduate textbook Engineering Electromagnetics (now in its second edition) He teaches several courses at

the University of Akron including a very popular course on sensors and actuators, which is the foundation of this book He has researched and developed systems for industrial use that help control the thickness of rubber during tire production, oil leak sensing systems, wireless sensors and networks, and many other systems for several clients

He is a Fellow of the IEEE, ASNT (American Society for Nondestructive Testing), and ACES (Applied Computational Electromagnetics Society)

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Sensors, Actuators, and

their Interfaces

Sensors, Actuators, and

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1.4 Classification of Sensors and Actuators 12

1.5 General Requirements for Interfacing 16

2.2.2 Impedance and Impedance Matching 35

2.2.3 Range, Span, Input and Output Full Scale, Resolution, and Dynamic

Range 39 2.2.4 Accuracy, Errors, and Repeatability 42

2.2.5 Sensitivity and Sensitivity Analysis 45

2.2.6 Hysteresis, Nonlinearity, and Saturation 52

2.2.7 Frequency Response, Response Time, and Bandwidth 56

3 Temperature Sensors and Thermal Actuators 67

3.1 Introduction 67

3.1.1 Units of Temperature, Thermal Conductivity,

Heat, and Heat Capacity 69

3.2 Thermoresistive Sensors: Thermistors, Resistance Temperature Sensors,

and Silicon Resistive Sensors 70

3.2.1 Resistance Temperature Detectors 70

3.2.1.1 Self-Heat of RTDs 783.2.1.2 Response Time 80

3.2.2 Silicon Resistive Sensors 81 3.2.3 Thermistors 84

3.3 Thermoelectric Sensors 88

3.3.1 Practical Considerations 94 3.3.2 Semiconductor Thermocouples 101 3.3.3 Thermopiles and Thermoelectric Generators 102

3.4 p-n Junction Temperature Sensors 104

3.5 Other Temperature Sensors 109

3.5.1 Optical and Acoustical Sensors 109 3.5.2 Thermomechanical Sensors and Actuators 110

4.4.2.1 The Photoelectric Effect 1334.4.2.2 Quantum Effects: The Photoconducting Effect 1354.4.2.3 Spectral Sensitivity 137

4.4.2.4 Tunneling Effect 137

4.5 Quantum-Based Optical Sensors 138

4.5.1 Photoconducting Sensors 138 4.5.2 Photodiodes 142

4.5.3 Photovoltaic Diodes 147 4.5.4 Phototransistors 150

4.6 Photoelectric Sensors 153

4.6.1 The Photoelectric Sensor 153 4.6.2 Photomultipliers 154

4.7 Coupled Charge (CCD) Sensors and Detectors 156

4.8 Thermal-Based Optical Sensors 159

4.8.1 Passive IR Sensors 160

4.8.1.1 Thermopile PIR 1604.8.1.2 Pyroelectric Sensors 1624.8.1.3 Bolometers 165

4.9 Active Far Infrared (AFIR) Sensors 166

5.3 The Electric Field: Capacitive Sensors and Actuators 180

5.3.1 Capacitive Position, Proximity, and Displacement Sensors 183

5.3.2 Capacitive Fluid Level Sensors 187

Sensors 208

5.4.2 Hall Effect Sensors 211

5.5 Magnetohydrodynamic (MHD) Sensors and Actuators 218

5.5.1 MHD Generator or Sensor 219

5.5.2 MHD Pump or Actuator 219

5.6 Magnetoresistance and Magnetoresistive Sensors 222

5.7 Magnetostrictive Sensors and Actuators 224

DC Motors (BLDC Motors) 2455.9.2.3 AC Motors 247

5.9.2.4 Stepper Motors 2485.9.2.5 Linear Motors 254

5.9.3 Magnetic Solenoid Actuators and Magnetic Valves 256

5.10 Voltage and Current Sensors 259

6.3 Force Sensors 283

6.3.1 Strain Gauges 283 6.3.2 Semiconductor Strain Gauges 285

6.3.2.1 Application 2886.3.2.2 Errors 288

6.3.3 Other Strain Gauges 292 6.3.4 Force and Tactile Sensors 292

6.4 Accelerometers 297

6.4.1 Capacitive Accelerometers 298 6.4.2 Strain Gauge Accelerometers 300 6.4.3 Magnetic Accelerometers 301 6.4.4 Other Accelerometers 302

6.5 Pressure Sensors 305

6.5.1 Mechanical Pressure Sensors 305 6.5.2 Piezoresistive Pressure Sensors 310 6.5.3 Capacitive Pressure Sensors 314 6.5.4 Magnetic Pressure Sensors 314

6.6 Velocity Sensing 315

6.7 Inertial Sensors: Gyroscopes 319

6.7.1 Mechanical or Rotor Gyroscopes 320 6.7.2 Optical Gyroscopes 321

6.8 Problems 324

7.1 Introduction 335

7.2 Units and Definitions 337

7.3 Elastic Waves and Their Properties 340

7.3.1 Longitudinal Waves 341 7.3.2 Shear Waves 349 7.3.3 Surface Waves 349 7.3.4 Lamb Waves 350

7.4 Microphones 350

7.4.1 The Carbon Microphone 350 7.4.2 The Magnetic Microphone 352 7.4.3 The Ribbon Microphone 354 7.4.4 Capacitive Microphones 354

7.5 The Piezoelectric Effect 357

7.5.1 Electrostriction 361 7.5.2 Piezoelectric Sensors 361

7.6 Acoustic Actuators 363

7.6.1 Loudspeakers 363 7.6.2 Headphones and Buzzers 369

7.6.2.1 The Magnetic Buzzer 3697.6.2.2 The Piezoelectric Headphone and Piezoelectric Buzzer 371

7.7 Ultrasonic Sensors and Actuators: Transducers 373

7.7.1 Pulse-Echo Operation 377 7.7.2 Magnetostrictive Transducers 380

8.3.1 Metal Oxide Sensors 406

8.3.2 Solid Electrolyte Sensors 409

8.3.3 The Metal Oxide Semiconductor (MOS) Chemical Sensor 413

8.4 Potentiometric Sensors 413

8.4.1 Glass Membrane Sensors 414

8.4.2 Soluble Inorganic Salt Membrane Sensors 417

8.4.3 Polymer-Immobilized Ionophore Membranes 418

8.4.4 Gel-Immobilized Enzyme Membranes 419

8.4.5 The Ion-Sensitive Field-Effect Transistor (ISFET) 420

8.5 Thermochemical Sensors 421

8.5.1 Thermistor-Based Chemical Sensors 421

8.5.2 Catalytic Sensors 422

8.5.3 Thermal Conductivity Sensor 425

8.6 Optical Chemical Sensors 425

8.7 Mass Sensors 429

8.7.1 Mass Humidity and Gas Sensors 431

8.7.2 SAW Mass Sensors 431

8.8 Humidity and Moisture Sensors 432

8.8.1 Capacitive Moisture Sensors 433

8.8.2 Resistive Humidity Sensor 435

8.8.3 Thermal Conduction Moisture Sensors 436

8.8.4 Optical Humidity Sensor 437

9.3.2 Scintillation Sensors 465 9.3.3 Semiconductor Radiation Detectors 466

9.3.3.1 Bulk Semiconductor Radiation Sensor 4679.3.3.2 Semiconducting Junction Radiation Sensors 470

9.4 Microwave Radiation 474

9.4.1 Microwave Sensors 476

9.4.1.1 Radar 4769.4.1.2 Reflection and Transmission Sensors 4799.4.1.3 Resonant Microwave Sensors 4829.4.1.4 Propagation Effects and Sensing 487

9.5 Antennas as Sensors and Actuators 487

9.5.1 General Relations 487 9.5.2 Antennas as Sensing Elements 489 9.5.3 Antennas as Actuators 494

10.3.3.4 Valves 53110.3.3.5 Others 533

10.4 Smart Sensors and Actuators 533

10.4.1 Wireless Sensors and Actuators and Issues Associated

with Their Use 538

10.4.1.1 The ISM and SRD Bands 53810.4.1.2 The Wireless Link and Data Handling 54010.4.1.3 Transmitters, Receivers, and Transceivers 542

10.4.2 Modulation and Demodulation 542

10.4.2.1 Amplitude Modulation 54310.4.2.2 Frequency Modulation 54410.4.2.3 Phase Modulation 54510.4.2.4 Amplitude Shift Keying 54710.4.2.5 Frequency Shift Keying 54810.4.2.6 Phase Shift Keying 548

10.4.3 Demodulation 549

10.4.3.1 Amplitude Demodulation 54910.4.3.2 Frequency and Phase Demodulation 549

10.4.4 Encoding and Decoding 550

10.4.4.1 Unipolar and Bipolar Encoding 55010.4.4.2 Biphase Encoding 550

11.2.1 The Operational Amplifier 570

11.2.1.1 Differential Voltage Gain 57111.2.1.2 Common-Mode Voltage Gain 57111.2.1.3 Bandwidth 571

11.2.1.4 Slew Rate 57211.2.1.5 Input Impedance 57311.2.1.6 Output Impedance 57311.2.1.7 Temperature Drift and Noise 57311.2.1.8 Power Requirements 573

11.2.2 Inverting and Noninverting Amplifiers 573

11.2.2.1 The Inverting Amplifier 57411.2.2.2 The Noninverting Amplifier 575

11.2.3 The Voltage Follower 577

11.2.4 The Instrumentation Amplifier 577

11.2.5 The Charge Amplifier 578

11.2.6 The Integrator and the Differentiator 580

11.2.7 The Current Amplifier 581

11.5.1.4 Dual-Slope A/D Converter 59811.5.1.5 Successive Approximation A/D 600

11.6 Bridge Circuits 606

11.6.1 Sensitivity 607 11.6.2 Bridge Output 611

11.7 Data Transmission 614

11.7.1 Four-Wire Transmission 614 11.7.2 Two-Wire Transmission for Passive Sensors 615 11.7.3 Two-Wire Transmission for Active Sensors 615 11.7.4 Digital Data Transmission Protocols and Buses 618

11.8 Excitation Methods and Circuits 618

11.8.1 Linear Power Supplies 619 11.8.2 Switching Power Supplies 621 11.8.3 Current Sources 624

11.8.4 Voltage References 625 11.8.5 Oscillators 626

11.8.5.1 Crystal Oscillators 62711.8.5.2 LC and RC Oscillators 629

11.9 Noise and Interference 635

11.9.1 Inherent Noise 635 11.9.2 Interference 636

12.2.8 Memory 664 12.2.9 Power 666 12.2.10 Other Peripherals and Functionalities 669 12.2.11 Programs and Programmability 670

12.3 General Requirements for Interfacing Sensors and Actuators 670

12.3.1 Signal Level 671 12.3.2 Impedance 672 12.3.3 Response and Frequency 676 12.3.4 Input Signal Conditioning 677

12.3.4.1 Offset 67712.3.4.2 Scaling 68112.3.4.3 Isolation 68312.3.4.4 Loading 684

12.3.5 Output Signals 684

Preface with Publisher’s

Acknowledgements

The book in front of you introduces the subjects of sensing and actuation At first it

would seem that nothing could be easier; we may think we know what a sensor is, and

certainly we know what an actuator is But do we know it so well that it actually escapes

us? There are literally thousands of devices all around us that qualify in either category.

In Chapter 1 there is an example that lists many of the sensors and actuators just in a car

The count is approximately 200, and this is merely a partial list! The approach adopted

here is to view all devices as belonging to three categories: sensors, actuators, and

processors (interfaces) Sensors are the devices that provide input to systems and

actuators are those devices that serve as outputs In between, linking, interfacing,

processing, and driving are the processors In other words, the view advocated in this

text is one of general sensing and actuation In that sense, a switch on the wall is a sensor

(a force sensor), and the light bulb that turns on as a result is an actuator (it does

something) In between there is a ‘‘processor’’ – the wiring harness or, in case a dimmer

is used, an actual electronic circuit – that interprets the input data and does something

with it In this case it may be no more than a wiring harness, but in other cases it may be

a microprocessor or an entire system of computers

CHALLENGES

The process of sensing and actuation permeates the whole spectrum of science and

engineering The principles involved in sensing and actuation derive from all corners of

our knowledge, sometimes from corners obscure to all but the most specialized experts

And the principles are mixed It is not unusual for a sensor to span two or more

dis-ciplines Take, for example, an infrared sensor It may be built in different ways, but one

method is to measure the temperature rise produced by the infrared radiation Thus, a

number of semiconductor thermocouples are built, and their temperature relative to a

reference temperature is measured Not a particularly complex sensor but, to fully

understand it, one would have to resort to theories of heat transfer, optics, and

semi-conductors, at the very least In addition, one must at least consider the electronics

needed to make it work and the interfacing with a controller, such as a microprocessor It

would therefore be difficult, nay, impossible, to cover all principles and all theories in

any detail Necessarily then, we must limit ourselves to a pragmatic approach with

‘‘high-level’’ detail and, at times, to limited explanations It is not reasonable to assume

that one can be proficient in all areas of science and, fortunately, for successful

appli-cation of sensors and actuators, this ‘‘high-level’’ approach is sufficient That is, we will

often view a device as a ‘‘black box’’ with its inputs and outputs and operate on these

rather than concern ourselves with the physics and the detailed operation of the ‘‘black

box.’’ Nevertheless, ignoring the box entirely is detrimental: the user must understand in

xv

sufficient detail the principles involved, the materials used, and the construction ofsensors and actuators The textbook is sufficiently detailed to allow reasonable under-standing of principles.

To bridge the divide between the theory of sensors and actuators and their cation and to gain insight into design of sensing and actuation, we note that most sensorshave electrical output, whereas most actuators have electrical input Virtually all inter-facing issues in sensors and actuators are electrical in nature This means that tounderstand and use these devices, and certainly to interface them and integrate them into

appli-a system, requires elements of electricappli-al engineering Conversely, the sensed quappli-antitiespertain to all aspects of engineering, and electrical engineers will find that mechanical,biological, and chemical engineering issues must be considered alongside electricalengineering issues This multidisciplinary textbook is intended for all engineers and allthose interested in sensing and actuation Each discipline will find components in it thatare familiar and others that need to be learned This is, in fact, the lot of the present-dayengineer, an engineer who must either assimilate various disciplines or work in teams toaccomplish tasks across disciplines However, not all sensors or actuators are ‘‘elec-trical.’’ Some have nothing to do with electricity A meat thermometer will sense thetemperature (sensor) and display it (actuator) without any electrical signal beinginvolved The expansion of a bi-metal piece activates a dial against a spring, so thewhole process is mechanical Similarly, a vacuum motor in a car can open an air con-ditioning vent by entirely mechanical means

MULTIDISCIPLINARY APPROACH

In each chapter the student will find a number of examples taken from varied areas andadapted to emphasize the issues discussed Many examples are based on actual experi-ments, some are based on simulations, and some deal with theoretical issues At the end

of each chapter there is a set of problems, further expanding on the content of the chapterand exploring details and applications related to the subject matter An effort has beenmade to make the examples and problems realistic, applicable, and relevant wheneverpossible, while still keeping each problem focused and self-contained Because of theuniqueness of the subject, that is, the multidisciplinary focus, the student is faced withusing units that he or she may not be familiar with To mitigate this, a section on units isincluded in Chapter 1 Some chapters containing unfamiliar units also contain a sectionthat defines these units and the conversions between them SI units are used throughout

as a rule but, on occasion, common units (such as the PSI or the electron-volt) are alsodefined and used because of their widespread use

ORGANIZATION

The textbook is divided into three main parts The first two chapters serve as an duction and expose the general properties and issues involved in sensing and actuation.The second part includes seven chapters, each dealing with a class of sensors These aregrouped by broad area of detection For example, those sensors and actuators based onacoustic waves – from audio microphones to surface acoustic waves and including

ultrasonic devices – are grouped together Similarly, sensors and actuators based on

temperature and heat are grouped together This grouping scheme does not indicate any

exclusivity An optical sensor may well use thermocouples to sense, but it is classified as

an optical sensor and discussed in conjunction with optical sensors Similarly, a

radia-tion sensor may use a semiconducting juncradia-tion for sensing, but its funcradia-tion is to

sense radiation, and therefore it is classified and discussed as such The third part of

the textbook includes the last two chapters that deal exclusively with interfacing and

circuits that are needed for interfacing Emphasis is given to the microprocessor as a

general-purpose controller In addition, a chapter on micro-electro-mechanical devices

(MEMS) and smart sensors is included and placed between the second and third parts of

the text

The text includes 12 chapters Chapter 1 is an introduction Following a short historical

perspective, we define the various terms including sensors, transducers, and actuators

Then, the issue of classification of sensors is introduced followed by a short discussion of

sensing and actuation strategies and the general requirements for interfacing

Chapter 2 discusses the performance characteristics of sensors and actuators We

discuss the transfer function, span, sensitivity and sensitivity analysis, errors,

non-linearities, as well as frequency response, accuracy, and other properties including issues

of reliability, response, dynamic range, and hysteresis At this point the discussion is

general, although the examples given rely on actual sensors and actuators

Chapters 3 through 9 look at classes of devices, starting with temperature sensors

and thermal actuators in Chapter 3 The starting point is thermoresistive sensors,

including metal resistance temperature detectors (RTDs), silicon resistive sensors, and

thermistors These are followed by thermoelectric sensors and actuators We discuss

metal junction and semiconductor thermocouples, as well as Peltier cells, both as

sen-sors and as actuators PN junction temperature sensor and thermo-mechanical devices

are introduced, as are thermal actuators One interesting aspect of temperature sensors is

that in many common applications the sensor and actuator are one and the same,

although this duality is not limited to thermal devices The whole class of bimetal

sen-sors is of this type with applications in thermostats, thermometers, and in MEMS, a topic

that will be expanded upon in Chapter 10

The important issue of optical sensing is the subject of Chapter 4 Thermal and

quantum-based sensors are discussed, first through the photoconducting effect and then

through silicon-based sensors that include photodiodes, transistors, and photovoltaic

sensors Photoelectric cells, photomultipliers, and CCD sensors are a second important

class, followed by thermal-based optical sensors that include thermopiles, infrared

sensors, pyroelectric sensors, and bolometers Although one seldom thinks of optical

actuators, these do exist, and they conclude the chapter

In Chapter 5 we address electric and magnetic sensors and actuators Naturally, a

large number of devices fall into this class, and, as a consequence, the chapter is rather

extensive It starts with electric and capacitive devices, followed by magnetic devices

We discuss here a variety of sensors and actuators including position, proximity, and

displacement sensors, as well as magnetometers, velocity, and flow sensors The

prin-ciples involved, including the Hall effect and magnetostrictive effect, are discussed side

by side with more common effects A rather extensive discussion of motors and

sole-noids covers many of the principles of magnetic actuation, but capacitive actuators are

discussed as well

Chapter 6 is dedicated to mechanical sensors and actuators The classical straingauge is featured as a generic device for sensing of forces and the related quantities ofstrain and stress But it is also used in accelerometers, load cells, and pressure sensors.Accelerometers, force sensors, pressure sensors, and inertial sensors take the bulk of thechapter Mechanical actuators are exemplified by the bourdon tube, bellows, andvacuum motors.

Chapter 7 discusses acoustic sensors and actuators By acoustic, we mean sensorsand actuators based on elastic, sound-like waves These include microphones, hydro-phones based on magnetic, capacitive, and piezoelectric principles, the classical loud-speaker, ultrasonic sensors and actuators, piezoelectric actuators, and surface acousticwave (SAW) devices Thus, although acoustics may imply sound waves, the frequencyrange here is from near zero to many GHz

Chemical sensors and actuators are among the most common, ubiquitous, and,unfortunately, least understood devices by most engineers For this reason, Chapter 8discusses these in some detail A fairly large section of the existing chemical sensors,including electrochemical and potentiometric sensors, thermochemical, optical, andmass sensors, is given Chemical actuation is not neglected; it is much more prevalentthan normally thought Actuators include catalytic conversion, electroplating, cathodicprotection, and others

Chapter 9 introduces radiation sensors Aside from classical ionization sensors, wetake a wider view that includes nonionizing, microwave radiation as well Here we look

at reflection, transmission, and resonant sensors Since any antenna can radiate power, itcan serve as an actuator to affect specific tasks such as cauterization during surgery,low-level treatment for cancer or hypothermia, and microwave cooking and heating.The subject of Chapter 10 is micro-electro-mechanical sensors and actuators, orMEMS, as well as smart sensors It is somewhat different than the previous chapters inthat it discusses methods of sensor production in addition to classes of sensors How-ever, the importance of these sensors and actuators justifies their introduction and thedeviation in the usual method of presentation Some of the methods of production arefirst given, followed by a number of common classes of sensors and actuators includinginertial and electrostatic sensors and actuators, optical switches, valves, and others Inthe context of smart sensors, we emphasize issues associated with wireless transmission,modulation, encoding, and sensors networks

Chapters 11 and 12 form a unit on interfacing Many of the common circuitsapplicable to interfacing are described in Chapter 11 These start with the operationalamplifier and its many applications, followed by power amplifiers and pulse widthmodulation circuits for use with actuators The A/D and D/A in their various forms,including voltage to frequency and frequency to voltage converters, follow these before

we get into discussion of bridge circuits and data transmission methods A section onexcitation circuits deals with linear and switching power supplies, current and voltagereferences, and oscillators The chapter ends with a discussion on noise and interference.Chapter 12 introduces the microprocessor and its role in interfacing sensors and actua-tors Although the emphasis is on 8-bit microprocessors, the issues addressed are generaland pertain to all microprocessors In this last chapter we deal with the architecture,memory, and peripherals of the microprocessor, the general requirements for interfa-cing, and properties of signals, resolution, and errors

xviii Preface with Publisher’s Acknowledgements

The discussion in this book focuses on sensors and actuators as independent

compo-nents There will be little discussion about systems; rather, we will talk on the sensor/

actuator level, the lowest level at which these devices are useful as building blocks for

the engineer and a bit lower than that into the operation and physical principles For

example, magnetic resonance imaging (MRI) is an extremely useful system for medical

diagnostics and for chemical analysis relying on sensing the precession of molecules

(normally hydrogen) in the body or in a solution But the system is so very complicated

and its operation so intrinsically linked with this complexity that the principle, that of

precession, cannot really be utilized on a low level Discussion of a system of this type

requires discussion of ancillary issues including superconductivity, generation of

uni-form, high magnetic fields, interaction between DC and pulsed, high-frequency

mag-netic fields, and the atomic level issues of excitation and precession All of these are

interesting and important, but they are beyond the scope of this text A different example

is radar A ubiquitous system but one that again requires many additional components to

operate and to be useful although on the low level is not different than a flashlight and

our eye, the flashlight sending a beam (an actuator), and the eye receiving a reflection

(the sensor) We will discuss sensors based on the principle of reflection of

electro-magnetic waves that is identical to radar, without the need to discuss the ancillary issues

of how radar is made to work

RESOURCES

Instructors who adopt this book should check the publisher’s website page for this

book to see the latest instructor resources: solutions to problems, PowerPoint slides,

student projects, etc They and more will be developed as the book finds acceptance

for a growing number of courses To inquire about availability, send an e-mail to

marketing@scitechpub.com

CONCLUSION

This book has been in the works over a number of years with considerable feedback

from students in electrical, mechanical, civil, chemical, and biomedical engineering,

both undergraduate and graduate The subject was taught each summer, either as a

traditional course or online Much of the text, very fittingly, was written on the TGV

(Train a Grande Vitese) during a twice daily, 230 km commute between Paris and Lille,

France, traveling at over 300 km/h during fall 2009, summer 2010 and summer 2011

I have made use of a variety of sources, but much of the material, including all

exam-ples, problems, circuits, and photographs arose from my own and my students’ work on

sensors and actuators When experimental data is indicated, it means that the experiment

has actually been carried out and the data collected specifically for, either the purpose of

the given example or something very close Simulation is an important issue in all

aspects of engineering, and the present subject is no exception For this reason, some

examples and problems, especially in Chapter 11, rely on or assume simulated urations In examples and problems I tried to be as practical as possible without unne-cessarily complicating the issues In some cases, simplifications had to be resorted

config-to Nevertheless, many of the examples and problems can serve as starting pointsfor more complex developments and, indeed, for implementation in the laboratory orextended projects

Nathan IdaAugust 2013

PUBLISHER’S ACKNOWLEDGEMENTS

Authors work very hard and long to complete textbooks, and the best textbooks resultfrom rewriting in order to improve them In the process of iterations, an author’s bestfriend can be the ‘‘other pair of eyes’’ brought to the process by peer reviewers who wantlittle more than to see a good, clear textbook result for them and their students Thepublisher gratefully acknowledges the valuable comments of this book’s manuscript bythe following selfless reviewers:

Prof Fred Lacy – Southern University, Los Angeles

Dr Randy J Jost – Ball Aerospace and Technology Systems and Adjunct Faculty,Colorado School of Mines

Prof Todd J Kaiser – Montana State UniversityProf Yinchao Chen – University of South CarolinaProf Ronald A Coutu, Jr – Air Force Institute of Technology, OhioProf Shawn Addington – Virginia Military Institute

Mr Craig G Rieger – ICIS Distinctive Signature LeadProf Kostas S Tsakalis – Arizona State UniversityProf Jianjian Song – Rose-Hulman Institute of Technology, Indiana

C H A P T E R 1

Introduction

1.1 INTRODUCTION

It would be a cliche´ to say that sensors are important or that they are in widespread use

And this is not because it is so often stated, but rather because it is an understatement In

The senses

The five senses—vision, hearing, smell, taste, and touch—are universally recognized as the

means by which humans and most animals perceive their universe They do so through

optical sensing (vision), acoustic sensing (hearing), chemical sensing (smell and taste), and

mechanical (or tactile) sensing (touch) But humans, animals, and even lower level organisms

rely on many other sensors as well as on actuators Most organisms can sense heat and

estimate temperature, can sense pain, and can locate a sensation on and in the body Any

stimulus on the body can be precisely located Touching of a single hair on the body of an

animal is immediately located exactly through the kinesthetic sense If an organ is affected,

the brain knows exactly where it occurred Organisms can sense pressure and have a

mechanism for balance (the inner ear in humans) Some animals such as bats can echolocate

using ultrasound, while others, including humans, make use of binaural hearing to locate

sounds Still others, such as sharks and fish (as well rays and the platypus), sense variations

in electric fields for location and hunting Birds and some other animals can detect magnetic

fields and use these for orientation and navigation Pressure is one of the main mechanisms

fish use to detect motion and prey in the water, and vibration sensing is critical to a spider’s

ability to hunt Bees use polarized light to orient themselves, as do some species of fish And

these represent only a small selection of the sensing mechanisms used by organisms.

Organisms also have a variety of actuators to interact with their environment In humans,

the hand is an exquisite mechanical actuator capable of a surprising range of motion, but it is

also a tactile sensor The feet, as well as many muscles, allow interaction with the

environ-ment But here as well there are other mechanisms that can be used to affect actuation A

human can use its mouth to blow away dust and can close and open its eyelids, a cat can

unsheathe its claws, and a chameleon can move each eye independently and shoot its

tongue to catch a fly Other actuators allow for voice communication (vocal chords in

humans) or the stunning of prey (ultrasound in dolphins, electrical shock in eels), direct

mechanical impact used by some species of shrimp, and many other specialized functions.

With respect to the sensory and actuation diversity in organisms we are still very far

behind and our mimicking of natural sensors and actuators is still in its infancy It has taken

the better part of 40 years to develop a working artificial heart, whereas seemingly simple

organs, such as the esophagus, have not yet been developed Where are we in comparison

with the nose of a dog?

1

fact, they are in such widespread use that we take them for granted, just as we take forgranted computers or cars.

Yet, whereas most people will acknowledge their existence, sensors, and to asmaller extent actuators, are not as visible as other devices The main reason is that theyare usually integrated in larger systems and the operation of the sensor or actuator isnormally not directly observable nor is it usually self-contained That is, a sensor oractuator can rarely operate on its own, but rather it is part of a larger system that mayinclude many sensors, actuators, and processing elements, as well as auxiliary compo-nents such as power supplies and drive mechanisms, among others For this reason, mostpeople only come in contact with sensors and actuators indirectly A few examples may

be useful here to demonstrate these statements

A car may contain dozens of sensors and actuators derived from various disciplinesbut we almost never come into direct contact with any of them The engine temperaturesensor (often more than one) exists, but how many drivers know where it is, how it isconnected, and exactly what it measures? As a matter of fact, it does not actuallymeasure engine temperature, but rather coolant temperature in the engine Air bags incars save lives and reduce injury by deploying before the driver’s head hits the steeringwheel This is done by one or more accelerometers (acceleration sensors) that detect thedeceleration of a vehicle involved in an accident, activating an explosive charge(actuator) that fills the bag with a gas In between the sensors and the actuator there is aprocessor that decides whether an accident has occurred based on set sensing para-meters Ask the driver and he or she would probably not know that accelerometers areinvolved and most likely will not know where these sensors are physically located Itmay come as a surprise to many to find out that actuation of the airbag is through anexplosion using substances that an explosives expert would find familiar Anotherexample is the catalytic converter in the car This is a unique chemical actuator whosepurpose is to convert otherwise noxious gases into more benign substances through theuse of a number of sensors, and by doing so reducing pollution Most people do notknow where this device is or what it does and are ignorant as to its operation

Similarly, when we change the temperature setting at home, an actuator is activated

to operate the furnace (or heater) or air conditioner, and somewhere there is a sensor (or

a number of sensors) to turn off the furnace or air conditioner at the set temperature.Even if we have an idea of where these sensors/actuators are, it is usually vague.Moreover, we know very little of what type of sensors/actuators are involved and evenless about how they operate or how they are connected, the types of signals they use, etc

In most homes in the United States there is at least one thermostat that regulates heatingand/or cooling in the house The homeowner would be hard pressed to know where thetemperature sensor is (if indeed a separate temperature sensor is used) or if a ‘‘classical’’thermostat is used instead It would probably come as a surprise to many to find out that

a fairly primitive mercury switch combined with a bimetal sensor/actuator is used inmany low-cost thermostats

And how many of us have ever given much thought to the pop-up lid on almost anyjar or can of food we consume? Yet it is there to detect if the jar or can is properly sealedand hence to detect possible spoiled food It is, in effect, a pressure sensor, and perhapsone of the most common sensor we come in contact with

All in all, it is estimated that in daily life a person comes into contact with a fewhundred sensors and actuators in the home, transport, work, and entertainment, although

as a rule most people are scarcely aware of them

1.2 A SHORT HISTORICAL NOTE

We tend to think of sensors and actuators as a product of the information age and of the

rapid development of electronics associated with it Indeed, as far as the sheer number

and variety of sensors available and their sophistication, this perception is justified

However, sensors existed before electronics, before the transistor, before the vacuum

tube, and even before electricity We shall see in Chapter 3 that some of the most

common temperature sensors we use today, thermocouples, have been in use since 1826

when Antoine Cesar Becquerel used them for the first time to measure temperature The

Peltier effect, which allowed heating and refrigeration in space starting in the early

1960s and then became a fixture in portable coolers and heaters, was discovered in 1834

by Charles Athanase Peltier The device has been used as a thermoelectric generator

since at least the 1890s and later was further developed for cooling and heating

pur-poses Resistive temperature sensing based on the change in conductivity of metals has

been in use since 1871 when William Siemens proposed its use with a platinum wire

The modern equivalent is the thermoresistive sensor (or resistive temperature detector

[RTD]) and most of these use platinum wires And there are more The photoelectric

sensor has been in widespread use since the early 1930s, as were others, including

photoemissive sensors Actuators based on thermal expansion have existed since the

mid-1880s, and actuators based on electric motors have been used since the invention of

the electric motor in 1824 by Michael Faraday Air speed on modern aircraft and in

wind tunnels is measured using a Pitot tube, invented in 1732 by Henri Pitot to measure

water flow in rivers And then there is the compass, a sensor critical to the development

of the modern world that existed in Europe since at least 1100 CE and in China since

2400 BCE

It should be noted that electronics had its beginning with the invention of the

electronic vacuum tube Invented in 1904 by John A Fleming, it remained in its simple

form (a diode) until 1906 when Lee De Forest developed the audion—the first electronic

amplifier—a step that started the electronic age, and with it the development and use of

newer sensors that were electronic in nature or that required electronics for their

operation

But before there were sensors in the way we know them today there were others that

could be called ‘‘primitive’’ or ‘‘natural’’ sensors, and of course, actuators Our five

senses and the senses of animals are yet to be challenged by modern sensors in

sensi-tivity and sophistication The acuity of a dog’s sense of smell searching for a lost person,

explosives in an airport, or in some cases cancer in the body of a person, or that of a pig

searching for truffles in the French or Italian countryside are yet to find their match

Similarly, a shark can detect traces of blood at distances of 30 km or more No tactile

sensor can even come close to our own skin, in which we can locate the touching of

a single hair The dexterity of the human hand is the subject of much imitation in

robotics and mechatronics, but no existing tactile sensors comes close The binaural

positioning capabilities of animals is legendary A fox can locate a mouse under thick

snow by hearing alone, pouncing directly on it through the snow Other senses are even

more amazing An elephant can both generate infrasound for long-distance

commu-nication and ‘‘hear’’ infrasound through its legs, which then propagate the vibration to

its inner ear through its bone structure The use of ultrasound by bats to locate insects in

flight and to avoid obstacles constitutes a refined system of ultrasound generation

(actuation) and sensing of an incredible resolution Dolphins are not far behind in these

capabilities, using ultrasound not only to detect (sensor) but also to communicate and tostun prey (actuator) There are also indications that animals can detect impeding earth-quakes or storms It is likely that they use their highly acute senses to detect precursorssuch as electric fields, variations in magnetic fields, and minute tremors to which we,and sometimes our instruments, are oblivious Some of these exceptional capabilities aredue in part to highly developed processors—the brains of these animals.

But primitive sensing is not limited to the five senses Traveling through EasternEurope one may still encounter the use of fish as sensors for water quality In many awell, one can find a small fish or two, typically trout They serve two purposes First, thefish eat insects that have fallen into the well, helping to keep it clean But moreimportantly, being sensitive to water quality, the fish will die, or at least show signs ofdistress, when the water is no longer safe to drink A dead fish in a well is a clearindication not to drink the water The same, seemingly primitive method is in use in theUnited States in some municipal water treatment facilities After all the chemical testsare done, and the water is properly treated, the final ‘‘test’’ is a minnow left in the waterovernight If it is alive the next morning, the water is ‘‘safe.’’ One can buy commercialwater quality testing systems in which small fish are placed in the water to be tested andtheir breathing pattern is monitored electronically for signs of stress (primarily changes

in breathing rate) This is then correlated to water quality In earlier times, going back to

at least tenth-century France, salamanders were kept in water sources for exactly thesame purpose and for the same reason—their sensitivity to any change in water quality(the concern was mainly poisoning of water sources, apparently a common occurrence atthe time) The canary in the coal mine is another example It turns out that the canary isquite sensitive to methane and other noxious gases Methane or carbon monoxideaccumulation is indicated when the bird stops singing Higher concentrations will kill it.These are clear indications of the need to evacuate the mine before an explosion occurs.And it is rather curious that canaries were used for the purpose as late as 1986 Otheranimals, most notably cats, were used in a similar manner Miners also noticed thatmethane and carbon monoxide changed the color and intensity of their gas-burninglanterns and have used these as ‘‘sensors’’ for the presence of methane and carbonmonoxide We will see that the modern equivalent sensors are used in a similar but morecontrolled manner

Plants have also been harnessed in our quest to improve our environment For ages,wine producers have been relying on the simple rose bush to detect fungi that attack andcan devastate their grape vines The rose, it turns out, is much more sensitive to fungi andhence shows signs of its presence much earlier than the grape vine To this day one can seebeautiful roses growing at the edges of vineyards, serving to detect the fungus and acting

as a warning to the vintner Of course, the roses also add a welcome splash of color

1.3 DEFINITIONS

Sensors and actuators are unique devices First, they come in a wide range of types thatsometimes defy classification Also, the operating principles of sensors and actuatorsspan the whole spectrum of physical laws They are used in all engineering disciplinesand for almost all conceivable applications It is therefore not surprising that variousdefinitions of sensors and actuators may be found and, more importantly, that all of

these definitions are more or less correct and more or less useful For example, sensor,

transducer, probe, gauge, detector, pickup, receptor, perceptron, transmitter, and

trans-ponder are often used interchangeably and sometimes incorrectly In particular, there

seems to be confusion between the terms transducer and sensor, in spite of the fact that

these are very distinct terms Similarly, the terms actuator, driver, and operating element

are often used interchangeably And often also, actuators will be called by their function or

by their primary use (motor, valve, solenoid, etc.) rather than using the term actuator Then

there is the ‘‘babel’’ of units that again, because of the various disciplines involved,

includes almost all possible combinations of units, at times with little regard to standards

To add another dimension of uncertainty as to what a sensor or actuator is, it should

be noted that sometimes the boundary between the two is blurred There are sensors that

double as actuators and devices that perform both functions For example, a bimetallic

switch is a temperature sensor that activates a switch or creates a direct contact (cooking

thermometer, thermostat) Since it performs both the functions of a sensor and an

actuator, it is difficult to decide what it is and the only proper definition would be to call

it a sensor-actuator In some cases even the quantity sensed is not obvious An example

is the common fuse One might say that it senses current and disconnects the circuit But

in fact it is not the current that is the direct cause of fusing, but heat generated by the

current Therefore one might also say that the fuse senses temperature In either case it is

clearly a sensor-actuator whose stated function is current sensing

We will try to properly define these and other useful terms before continuing, and

then stick to these definitions to avoid confusion A proper, useful definition that

encompasses the array of devices we need to deal with is not easily found Nevertheless,

we will start with the dictionary, both to see what has been defined and to demonstrate

the inadequacy of these definitions

SENSOR

1 A device that responds to a physical stimulus and transmits a resulting impulse

(Webster’s New Collegiate Dictionary, 1998)

Problem: What is an impulse? Does every sensor ‘‘transmit’’ an impulse?

2 A device, such as a photoelectric cell, that receives and responds to a signal or

stimulus (American Heritage Dictionary, 3rd ed., 1996)

Problem: The definition uses an example (photoelectric cell) that may not be

representative of all sensors What does ‘‘receives’’ mean?

3 A device that responds to a physical stimulus (as heat, light, sound, pressure,

magnetism, or a particular motion) and transmits a resulting impulse (as for

mea-surement or operating a control) (Webster’s New World Dictionary, 3rd ed., 1999)

Problem: What is ‘‘impulse’’ and why ‘‘as for measurement or operating a

control’’?

TRANSDUCER

1 A device that is actuated by power from one system and supplies power usually in

another form to a second system (Webster’s New Collegiate Dictionary, 1998)

Problem: Why ‘‘power’’ and is a transducer an actual physical device?

2 A substance or device, such as a piezoelectric crystal, that converts input energy ofone form into output energy of another (from: trans-ducere—to transfer, to lead)

(American Heritage Dictionary, 3rd ed., 1996)

Problem: What is meant by ‘‘substance’’ and ‘‘input energy’’? Is the example

of the piezoelectric crystal appropriate and representative?

3 A device that is actuated by power from one system and supplies power usually inanother form to a second system (a loudspeaker is a transducer that transforms

electrical signals to sound energy) (Webster’s New World Dictionary, 3rd ed.,

1990)

Problem: Is the loudspeaker a transducer or does transduction occur in the

loudspeaker as part of its function?

Note: Some use transducer as a term that covers both sensors and actuators.

ACTUATOR

1 A mechanism for moving or controlling something indirectly instead of by hand

(Webster’s New Collegiate Dictionary, 1998)

Problem: Does it require specifically a motion? Does that mean that a direct

control such as in a thermostat does not qualify as actuation?

2 One that activates, especially a device responsible for actuating a mechanical device

such as one connected to a computer by a sensor link (American Heritage

Dic-tionary, 3rd ed., 1996)

Problem: Does an actuator have to be a mechanical device (see first

defini-tion)? An example is given, but is it appropriate as a definition?

3 One that actuates; a mechanical device for moving or controlling something

(Webster’s New World Dictionary, 3rd ed., 1990)

Problem: Does it have to be a mechanical device? Does it have to move or

control something?

These definitions (and there are others) show what the problem is: one can easily takethe definition of ‘‘transducer’’ to mean both a sensor or an actuator and the definitions arenot broad enough to represent the wide variety of sensors and actuators in existence Forexample, a loudspeaker is clearly an actuator—it converts electrical energy into acousticenergy But one can connect the same loudspeaker as an input device and use it as amicrophone Now the same device is a sensor—it senses pressure (stimulus) but it is also

a transducer (the conversion of energy is from acoustic to electrical) And this duality isnot limited to loudspeakers—many actuators can operate as sensors or actuators (otherthan, perhaps, the power levels involved—an actuator usually needs to supply morepower than a sensor can generate or needs to operate and therefore a microphone isphysically much smaller than a loudspeaker) So we are back to the original question:what is a sensor, what is an actuator, and what is a transducer? To add to this, somewriters have taken the position that a transducer is more than a sensor; it includes a

‘‘sensing element’’ and an ‘‘energy conversion element’’ as well as auxiliary elementssuch as filters, signal conditioning, perhaps a power source, etc Others have taken theexact opposite view, whereby a transducer is part of a sensor Others simply assume thatthey are one and the same—a transducer is just another name for a sensor And these

views are only a small selection What is it then? Who is right? The answer is, of course,

that everybody is right simply because all of the above are true under given conditions and

stemmed from the complexity and variety of sensors, actuators, and transducers and the

physical laws involved as well as the construction of the devices

To better understand these issues, consider again the loudspeaker and

micro-phone, but now let’s specify what we are talking about First, let’s look at a magnetic

loudspeaker (there are other types) If we use it as a microphone, the motion of the

loudspeaker cone moves a coil in a magnetic field and this generates a voltage across

the coil When connected in a circuit, a measurable current appears in the circuit

This is a passive sensor—it generates power and does not require an external power

source to sense Thus our statement that energy is converted (transducer) is correct In

fact, in principle, we could connect two loudspeakers as in Figure 1.1 Speaking into

loudspeaker 1 results in sound being generated by loudspeaker 2 (a direct connection

between a sensor and an actuator) Transduction from sound pressure to electrical

voltage occurs in one loudspeaker, while transduction from electrical current to

pressure waves occurs in the other loudspeaker, and the process is reversible This is

the same idea we used as children to communicate using two tin cans and a string

(Figure 1.2) Here transduction is from sound waves to vibrations in the string and

vice versa

Usually a direct connection between sensors and actuators is not possible and we

need to use a processing element—in this case an amplifier—as in Figure 1.3 This is

typically the way sensors and actuators operate and interact

Power and Transduction

Consider now a (simplified) telephone link that includes a carbon microphone and a

loudspeaker The carbon microphone (to be discussed in Chapter 7) operates on the

principle of changes in resistance: acoustic power moves a membrane, which in turn

Loudspeaker Loudspeaker

FIGURE 1.1 ¢ Two loudspeakers used

to demonstrate the ideas of sensing, actuation, and transduction.

Metal can Metal can

Tight string

FIGURE 1.2 ¢ Another sensor and actuator with transduction at each end.

Sensor Amplifier Actuator

(processor)

FIGURE 1.3 ¢ The three elements of a sensor-actuator system The amplifier is the

‘‘processor’’ or ‘‘controller’’ in the system.

presses on carbon particles This changes the resistance between the two electrodes of themicrophone Suppose we again connect the microphone directly to a loudspeaker, as in

Figure 1.4 No communication can occur since the microphone does not convert power—

acoustic power is converted into changes in resistance, but not into any form of usablepower, and to communicate we need power Interchanging between the loudspeaker andmicrophone is of no use either The loudspeaker does generate power, but the microphonecannot convert this power into acoustical power Now the microphone is not a transducer,

but it clearly is a sensor (active sensor) To make the system in Figure 1.4 work, we need

to add a power source as in Figure 1.5 Now the changes in resistance result in changes

in current in the circuit that then result in changes in the position of the loudspeaker’scone and these changes result in variations in air pressure (sound waves) In this system

we can view the microphone and the battery as a transducer (lending credence to the ideathat the sensor is part of the transducer) or we can view them as a sensor (lendingcredence to the idea that the transducer is part of the sensor) We could, and perhapsshould, keep them separate and view the microphone as a sensor, the sensor plus thebattery as the transducer, and the loudspeaker as the actuator By doing so we can avoidsome difficulties Specifically, in this case, since the microphone cannot serve both as asensor and an actuator, by viewing them as separate functional elements, one is nottempted to automatically assume functional duality between them while, at the sametime, not excluding it either On the other hand, we will also have to be flexible, as inthe case of the transducer—sometimes the transducer will be clearly identifiable as aseparate element from the sensor, sometimes it will include the sensor

Following this rather long introduction, the definitions we will use are as follows:

A device or mechanism capable of performing a physical action or effect

Carbon microphone Earphone

Telephone line

FIGURE 1.4 ¢ A

telephone link that

cannot work The

microphone now is

an active sensor and

requires power for

transduction.

Telephone line

Carbon microphone Earphone

Battery +

These are very general definitions and encompass all (or nearly all) available

devices Even the term ‘‘device’’ should be understood in the broadest sense For

example, a paper strip imbued with a glucose-sensitive substance used to test for sugar

in the blood is a ‘‘device.’’ At times we may have to narrow these definitions For

example, it is common to assume that most sensors have an electrical output or that

(most) actuators perform some type of motion or involve the exertion of force We

shall make no such assumptions here, but shall do so often in subsequent chapters In

some cases the output of a sensor will indeed be electrical, but in others it can be

mechanical Similarly the physical action of an actuator may not involve force at all,

such as in the case of a lightbulb used as the output in a system or a display used to

monitor status In fact, an actuator may perform a chemical action, such as the

con-version of carbon monoxide (CO) into carbon dioxide (CO2) in the catalytic converter

of a car

A more general definition is for a sensor to be the input to a system, whereas an

actuator is an output In this view, one that we will adopt to a large extent in this text,

sensors of various types and complexity serve as inputs to systems, whereas actuators

serve as outputs In between there is a processor that accepts the inputs, processes the

data, and acts through the actuators connected to the output of the system In general,

one may say that the processor interfaces between the sensors and actuators This is

shown in Figure 1.6 The sensors and actuators can be of a very general nature A switch

on the front of a washing machine is a sensor and a light-emitting diode (LED) showing

that the washing machine is in operation is an actuator It is not absolutely necessary that

an actuator physically produces motion or force, but rather that it acts as an output to the

system producing an effect Many actuators are in fact mechanical and based on the use

of motors However, even a motor needs to be understood in its broadest sense, as these

may be electrical (direct current [DC], alternating current [AC], continuous, stepping,

linear, etc.), pneumatic, or even a micromachined electric motor And some motors can

easily serve as sensors Indeed, a small DC motor used to sense wind speed operates as a

generator and is viewed as a sensor, whereas the same motor used to run a fan is

considered an actuator

The processor or controller itself may be trivially simple or terribly complicated

depending on needs It may be as simple as a direct connection or it may be an amplifier,

a set of resistors, a filter, a microprocessor, or a distributed system of computers In

extreme cases the processor is not necessary at all In these cases the sensor acts as well

as an actuator Bimetal thermometers and thermostats are typical examples, in that the

expansion of metals is a measure of temperature and that same expansion can be viewed

directly on a scale or it can operate a switch

Processor (controller)

Actuator Actuator Actuator

Actuator

Output 1 Output 2 Output 3

Output m

.

Input 1 Input 2 Input 3

Input n

.

EXAMPLE 1.1 Sensors and actuators in the car

A modern car contains dozens of sensors and actuators All are connected to a processor (often

called an electronic control unit [ECU]) as inputs and outputs as shown in Figure 1.6 (sometimes

multiple control units may be used, each dedicated to a set of related functions) Some of the

‘‘sensors’’ are switches or relays used to detect conditions (e.g., that the air conditioning is on oroff, that the transmission is in gear, that the doors are closed, and many others), whereas othersare true sensors Most of the actuators are solenoids, valves, or motors, but some are indicatorssuch as the low oil pressure lamp or an ‘‘open door’’ buzzer Not all cars have the same sensorsand actuators depending on the make and model Most of the sensors and actuators in a car aremonitored by an onboard diagnostics (OBD) system that gives the driver, the mechanic, andregulators an indication of the condition of the systems in the car A partial list of sensors andactuators monitored by the OBD system is given below In addition to those listed, many sensorsare ‘‘hidden’’ within other components For example, the cruise control system uses pressuresensors to maintain speed and the voltage regulator uses current and voltage sensors to keep thevoltage constant, but these are not monitored directly Similarly there are many other actuatorsnot monitored by the OBD system, including motors and valves within other systems such asthose used to open and close windows, doors, sunroofs, etc It should also be noted that many ofthese sensors are ‘‘smart sensors,’’ often containing their own microprocessors

Sensors

Crankshaft position (CKP) sensor A/C low-side temperature sensor

Camshaft position (CMP) sensor (two) A/C evaporator temperature sensor

Heated oxygen sensor (HO 2 S) (two or four) A/C high-side temperature sensor

Mass air flow (MAF) sensor A/C refrigerant overpressure

Manifold absolute pressure (MAP) sensor Left A/C discharge sensor

Intake air temperature (IAT) sensor Right A/C discharge sensor

Engine coolant temperature (ECT) sensor Power steering pressure (PSP) switch

Throttle position (TP) sensor (one to four) Input/turbine speed sensor

Fuel composition sensor (for alternative fuels) Output speed sensor

Fuel temperature sensor (one or two) Secondary vacuum sensor

Fuel rail pressure sensor Alternative fuel gas mass sensor

Engine oil temperature sensor Accelerator pedal position sensor (two)

Turbocharger boost sensor (one or two) Barometric pressure sensor

Knock sensor (KS) (one or two) Brake boost vacuum (BBV) sensor

Exhaust gas recirculation sensor (one or two) Wheel speed sensor (one on each wheel)

Evaporative emission control pressure sensor Left heater discharge sensor

Fuel level sensor (one or two) Right heater discharge sensor

Transmission fluid temperature (TFT) sensor Driver lumbar vertical sensor

A/C refrigerant pressure sensor Driver belt tower vertical sensor

Tire pressure monitor (TPM) system sensor (four) Front vertical sensor

Vehicle stability enhancement system (VSES) sensor Lumbar forward/aft sensor

Yaw rate sensor Lumbar up/down sensor

Lateral accelerometer sensor Left front mirror vertical position sensor

Steering sensor Right front mirror vertical position sensor

Brake fluid pressure sensor Driver front vertical sensor

Left front/driver side impact sensor (SIS) Driver rear vertical sensor

Electronic front end sensor (one or two) Driver seat assembly horizontal sensor

Outside air temperature sensor Twilight photocell

Ambient air temperature sensor Seat back heater sensor

Passenger compartment temperature sensor (one or two) Telescope position sensor

Output air temperature sensor (one or two) Tilt position sensor

Solar load sensor (one or two) Security system sensor

Right-hand panel discharge temperature sensor Automatic headlamp leveling device (AHLD)

Evaporative emission (EVAP) system leak detector Left-hand sun load sensor

Left front position sensor GPS antennas, satellite antennas, radio antennas,

ultrasound and accelerometers for theft prevention, etc.

Right front position sensor

Left rear position sensor

Right rear position sensor

Level control position sensor

Actuators

Turbocharger wastegate solenoid (two) Reverse inhibit solenoid

Exhaust gas recirculation (EGR) solenoid Pressure control (PC) solenoid

Secondary air injection (AIR) solenoid A/T solenoid

Secondary air injection switching valve (two) Torque converter clutch (TCC)/shift solenoid

Secondary air injection (AIR) pump Brake band apply solenoid

EVAP purge solenoid valve Intake manifold runner control (IMRC) solenoid

Evaporative emission (EVAP) vent solenoid Left front ABS solenoid (two)

Intake manifold tuning (IMT) valve solenoid Right front ABS solenoid (two)

Shift/timing solenoid Secondary air injection switching valve (two)

1–4 upshift (skip shift) solenoid Evaporative emission system purge control valve

Line pressure control (PC) solenoid Exhaust pressure control valve

Shift pressure control (PC) solenoid Intake plenum switchover valve

Intake resonance switchover solenoid Electronic brake control module (EBCM) control valve

1.4 CLASSIFICATION OF SENSORS AND ACTUATORS

Sensors and actuators may be classified in any number of ways Classification can

be based on the physical laws governing their operation, on their application, or onsome other convenient distinction between them There is no single method of classi-fication general enough to include all types and therefore various classifications areused for various purposes However, certain distinctions between classes of sensors and

actuators can be useful Starting with sensors, we can distinguish between active and passive sensors An active sensor is a sensor that requires an external power source.

Active sensors are also called parametric sensors because of the dependence of theiroutput on changes in sensor properties (parameters) Simple examples are sensors likestrain gauges (resistance changes as a function of strain), thermistors (resistancechanges as a function of temperature), capacitive or inductive proximity sensors(capacitance or inductance is a function of position), and others In all of these, thesensing function is a change in the device properties, but they can only be used after asource is connected so that an electric signal can be modulated by the respectiveproperty change In contrast, passive sensors operate by changing one or more of theirown properties to generate an electric signal Passive sensors are sensors that do

not require external power sources These are also called self-generating sensors.

Examples are thermoelectric sensors, solar cells, magnetic microphones, piezoelectricsensors, and many others

Note: Some sources define active and passive sensors in exactly the opposite way.

Another distinction that can be made is between contact and noncontact sensors, a

distinction that may be important in certain applications For example, strain gauges arecontact sensors, but a proximity sensor is not However, it should be understood that the

Right front inlet valve solenoid Front washer motor

Right front outlet valve solenoid Rear washer motor

Right rear outlet valve solenoid Coolant thermostat

Left front TCS master cylinder isolation valve Injectors (air, fuel) (one per cylinder)

Right front TCS master cylinder isolation valve Electric door motors

Exhaust solenoid valve short to ground (GND) Cooling/heating fans in compartment

Throttle actuator control (TAC) motor Starter motor

Mirror motor (one on each side) Catalytic converter

Tilt/telescope motor

same sensor may sometimes be used in either mode (e.g., a thermistor measuring

the temperature of an engine is a contact sensor, but when measuring ambient

tem-perature in the car it is not) Sometimes we even have a choice as to how a sensor may

be mounted Other sensors can only be used in one mode For example, a Geiger tube

cannot be a contact sensor since radiation must penetrate into the tube from the outside

Sensors are sometimes classified as absolute or relative An absolute sensor reacts to

a stimulus in reference to an absolute scale An example is the thermistor Its output is

absolute That is, its resistance relates to the absolute temperature Similarly the

capaci-tance proximity sensor is an absolute sensor—its capacicapaci-tance variations are due to the

physical distance to the sensing position A relative sensor’s output depends on a

relative scale For example, the output of a thermocouple depends on the temperature

difference between two junctions The sensed (measured) quantity is the temperature

dif-ference rather than absolute temperature Another example is the pressure sensor All

pressure sensors are relative sensors When the reference pressure is vacuum, the sensor is

said to be absolute, although the very idea of vacuum is relative A relative pressure sensor

senses the pressure difference between two pressures such as, say, that in the intake

manifold of an internal combustion engine and atmospheric pressure

Most classification schemes use one or more of the ‘‘descriptors’’ associated with

sensing Sensors may be classified by the application, by the physical phenomena used,

by the detection method, by sensor specifications, and many others Some of the

pos-sible classifications are shown in Table 1.1, but it should be borne in mind that ad hoc

classifications are common For example, one may classify sensors as low or high

temperature, low or high frequency, low or high accuracy, etc., when specific

appli-cations are considered It is also common to classify sensors by materials used Thus

one can talk of semiconductor (silicon) sensors, biological sensors, and the like

Sometimes even the physical size is used as a method of classification (miniature

sensors, microsensors, nanosensors, etc.) Many of these qualifications are relative and

depend on the application area A ‘‘miniature’’ sensor in an automobile may not be on

the same scale as a miniature sensor in a laptop computer or cell phone For example,

the airbag deploying system uses accelerometers, as do cell phones to orient the display

when the phone is flipped Likely the two sensors will be on a vastly different scale in

size

The classification of actuators is somewhat different in that actuators are understood

(in most cases) to generate motion, apply force (i.e., a motor in the general sense), or

generate an effect Thus some of the classification schemes rely on motion descriptors

while others rely on physical laws used for activation Therefore, in addition to the

classifications in Table 1.1, which apply to actuators as well as to sensors, there are

others, as can be seen in Table 1.2.

One of the main difficulties in discussing sensors is that there are so many different

sensors, sensing a myriad of quantities, using various principles and physical laws that it

is rather difficult to discuss them in a logical way Often these various issues are so

intertwined that some sensors even defy classification

The approach to sensing taken in this text is to look at sensors and actuators based

on the broad area of detection or actuation This has the advantage that in a particular

class of sensors, only one or a few related physical principles are used, simplifying

understanding of the theory behind sensing and actuation Thus we will discuss

tem-perature sensors, optical sensors, magnetic sensors, chemical sensors, and so on Each

of these classes of sensors is based on a few principles at most, sometimes on a

single principle However, in each class the same principles are used for a variety of

physical sensing and actuating quantities For example, an optical sensor may be used to

measure light intensity, but it may also be used to measure temperature Conversely, a

temperature sensor may be used to measure light intensity, pressure, temperature, or air

speed Similarly, when we talk about magnetic sensors, the principles may be applied to

sense position, distance, temperature, or pressure

EXAMPLE 1.2 Classification of the pop-up lid used on food jars

The pop-up lid on food jars detects pressure loss in the jar When the lid pops-up, it creates avisual indication of pressure loss inside the jar It is therefore a sensor-actuator

Classification as a sensor

Area of detection: mechanical sensor

Measurand: pressure

Application: consumer products

Specifications: low cost

Type: passive (it requires no power to operate)

Classification as an actuator

Area of detection: mechanical actuator

Application: consumer products

Specifications: low cost

Type: linear

Power: low

Other classifying terms may be used For example, we might say it is a visual actuator, or that

it is an embedded sensor-actuator (i.e., embedded in the production or integral with the lid asopposed to a separate, attached sensor) One can say as well that this is not a pressure sensor, butrather a ‘‘spoilage’’ or even biological sensor and it simply uses pressure as an indication ofspoilage

TABLE 1.2 ¢ Additional classification methods for actuators

MEMs actuators Nanoactuators

1.5 GENERAL REQUIREMENTS FOR INTERFACING

Sensors and actuators almost never operate by themselves They are more often parts ofmore complex systems and function within these larger systems It is indeed a rareoccurrence when the specifications of sensors or actuators match the needs of the sys-tem Therefore most sensors and actuators need to be interfaced with the system in

which they operate A simple, yet very general configuration is shown in Figure 1.7.

Here a sensor is connected to a ‘‘processor’’ to sense a physical property, say, perature An actuator, also connected to the processor, reacts to the sensed temperature

tem-in some way, such as by displaytem-ing the temperature, clostem-ing a valve, turntem-ing on a fan at apredetermined temperature, or any of a number of other possible functions The pro-cessor is viewed here as some sort of a controller that may be a microprocessor or asimpler circuit that implements the needs of the system

To make this example more concrete, suppose the sensor is a thermocouple and theactuator is a motor whose speed is proportional to temperature (operating a fan to cool acomputer processor) As we shall see later, a thermocouple is a passive sensor, so it doesnot require a power source to operate However, its output is very low—on the order of10–50mV/
C The motor operates at 12 V DC, whereas the controller, which we will take

to be a small microprocessor, operates at 5 V DC Apart from the fact that we mustprovide power to operate the processor and the controller, as well as to program theprocessor, we must also provide interfacing circuits between the sensor and the micro-processor and between the microprocessor and the actuator A possible implementation is

shown in Figure 1.8 Here, the thermocouple is placed in contact with the computer

Thermocouple

50 µ V/ºC

Output driver Microprocessor Fan

EXAMPLE 1.3 Classification of an oxygen sensor

Oxygen sensors are common in vehicles All vehicles that use catalytic converters must porate these sensors

incor-Broad area of detection: chemical (or electrochemical)

Measured output: voltage

Physical law: electrochemical

Specifications: high temperature

Area of application: automotive

Power: none (the sensor is a passive sensor—it does not require external power to operate)Thus the oxygen sensor is a high-temperature, passive, electrochemical sensor used inautomotive applications whose output is a voltage and senses oxygen concentration in the exhaustflow of vehicles

processor or its heat sink to sense its temperature Because the thermocouple measures

temperature difference, a reference temperature T0must be available (say, the ambient

temperature) The signal from the thermocouple is amplified to a more convenient signal

so that the range is between zero and 5 V (5 V represents the highest input voltage of the

processor and therefore should correspond to the highest temperature the system is

expected to sense) This signal is an analog signal It must be converted to a digital signal

before the microprocessor can act on it This is the function of the analog-to-digital

converter (A/D or ADC), which may be internal or external to the microprocessor

The amplifier and the ADC may be viewed as forming a transducer The microprocessor

responds to this input by supplying an output signal that is proportional to the

tempera-ture Since it is a digital device, it supplies a digital signal that must be converted back

into an analog signal The digital-to-analog converter (D/A or DAC) does that A power

amplifier is provided to indicate that a small power signal must somehow operate a higher

power motor In practice, this function may be achieved by different means, but the

configuration here demonstrates the principle The DAC together with the power

ampli-fier form a transducer

In addition, there may be other requirements that influence the design For example,

we may need to isolate the actuator from the microprocessor for safety or functional

reasons This is particularly true if the actuator operates at grid voltages (usually 120–

480 V AC)

The need for interfacing and the method of interfacing a sensor or actuator should be

taken into account in the design since it may influence the choice of sensors, actuators, and

the processor For example, temperature sensors with digital outputs are available, and if

used instead of the thermocouple, the system is simplified considerably On the other hand,

a digital sensor is not, by itself, necessarily the best approach in all cases Similarly we may

choose to use a 5 V motor instead of a 12 V motor to simplify power management, if such a

choice is practical The choice of processor is in itself influenced by the sensor and actuator

Some microprocessors include ADCs internally and some have proportional outputs ideally

suited to drive power devices (pulse width modulation [PWM] modules), allowing the

elimination of the DAC and power amplifier and replacing these with a single transistor

Figure 1.9 shows a different implementation using some of these alternative options and a

semiconductor temperature sensor This is much simpler because many of the requirements

of interfacing have been integrated into the sensor and the microprocessor Of course, there

are consequences to any alternative design The configuration in Figure 1.9 can operate at

up to about 100
C (the upper temperature limit of semiconducting temperature sensors is

less than 125
C), whereas thermocouples can operate at well over 2000
C

Interfacing of any device depends on the specifications of the device and the

requirements of the system to which the device is interfaced, but in almost all cases this

Thermocouple

Amp. Signal

conditioning

Output driver

Logic (program) A/D

D/A (PWM)

Motor/Fan

12 V Processor

5 V Power supply

50 µ V/ºC

FIGURE 1.8 ¢ A complete system for sensing of temperature and activation of a fan

to cool a device.

involves conversion of one sort or another Conversion of voltages, currents, and dances are very common, but sometimes interfacing may involve conversions in otherparameters, such as frequency These conversions take place in the ‘‘transduction’’ section

impe-of the system and may involve multiple steps While it may look simple enough in ciple, the actual implementation in an interfacing circuit may be very complex Forexample, a piezoelectric sensor may generate a few hundred volts that will instantly destroy

prin-a microprocessor On the other hprin-and, the impedprin-ance of the sensor is prprin-acticprin-ally infinite,whereas the input impedance of the microprocessor may be much lower, severely loadingthe sensor and, at best, influencing its properties (sensitivity, output, linearity, etc.) or, atworst, rendering it useless Thus we need to both reduce the voltage from a few hundredvolts to about 5 V and match the impedance of the sensor to that of the microprocessor.Other sensors have totally different properties and requirements A magnetic sensor usuallyincludes a coil that has very low impedance, so now we have exactly the opposite problem.For all of these reasons, interfacing circuits vary from one application to another andcover the whole spectrum of electronic circuits Many of these will be discussed in

is the pascal (Pa) Similarly we will occasionally mention units such as bar, rem, curie,electron-volt, and the like, all non-SI units, but we will keep these occurrences to theminimum necessary A short discussion of relevant units and conversion between them

is supplied at the beginning of the chapters in which they are relevant

1.6.1 Base SI Units

The SI units are defined by the International Committee for Weights and Measures and

includes seven base units as shown in Table 1.3 The base units are defined as follows: Length The meter (m) is the distance traveled by light in a vacuum during a time

interval equal to 1/299,792,458 s

Logic (program)

D/A (PWM)

Processor

5 V

Motor/Fan

outputdriver

12 V

Power supply

5 V Silicon temp.

Mass The kilogram (kg) is the prototype kilogram, a body made of a platinum-iridium

compound and preserved in a vault in Sevres, France

Time The second (s) is the duration of 9,192,631,770 periods of the radiation

corre-sponding to the transition between the two hyperfine levels of the ground state of the

cesium-133 atom

Electric current The ampere (A) is the constant current that, if maintained in two straight

conductors of infinite length and of negligible circular cross section, placed 1 m apart in a

vacuum, produces between the conductors a force of 2107newtons per meter (N/m).

Temperature The kelvin (K) unit of thermodynamic temperature is 1/273.16 of the

thermodynamic temperature of the triple point of water (the temperature and pressure at

which ice, water, and water vapor are in thermodynamic equilibrium) The triple point of

water is 273.16 K at 611.73 Pa

Luminous intensity The candela (cd) is the luminous intensity in a given direction of a

source that emits monochromatic radiation of frequency 5401012

Hz and has a radiation

intensity in that direction of 1/683 watts per steradian (W/sr) (see Section 1.6.3 below).

Amount of substance The mole (mol) is the amount of substance of a system that

contains as many elementary entities as there are atoms in 0.012 kg of carbon-12 (The

entities may be atoms, molecules, ions, electrons, or any other particles.) The accepted

number of entities (i.e., molecules) is known as Avogadro’s number and equals

approximately 6.022141023

1.6.2 Derived Units

Most other metric units in common use are derived from the base units We will discuss

some of these in the following chapters as it becomes necessary, but it is useful to note

here that these units have been defined for convenience based on some physical law,

even though they can be expressed directly in the base units For example, the unit of

force is the newton (N) This is derived from Newton’s law of force as F ¼ ma The unit

of mass is the kilogram and the unit of acceleration is meters per second squared (m/s2)

Thus the newton is in fact kilogram meters per second squared (kgm/s2

)

N¼ mass  accelerationð Þ ¼ kg m

s2

Similarly the unit of electric potential is the volt (V) The derived unit starts from the

definition of electric field intensity in terms of force F and charge q (Coulomb’s law):

TABLE 1.3 ¢ The base SI units