Position Sensors - David S. Nyce - ebook

Position Sensors ebook

David S. Nyce

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A resource on position sensor technology, including background, operational theory, design and applications This book explains the theory and applications of the technologies used in the measurement of linear and angular/rotary Position Sensors. The first three chapters provide readers with the necessary background information on sensors. These chapters review: the working definitions and conventions used in sensing technology; the specifications of linear position transducers and sensors and how they affect performance; and sensor output types and communication protocols. The remaining chapters discuss each separate sensor technology in detail. These include resistive sensors, cable extension transducers, capacitive sensors, inductive sensors, LVDT and RVDT sensors, distributed impedance sensors, Hall Effect sensors, magnetoresistive sensors, magnetostrictive sensors, linear and rotary encoders, and optical triangulation Position Sensors. * Discusses sensor specification, theory of operation, sensor design, and application criteria * Reviews the background history of the linear and angular/rotary Position Sensors as well as the underlying engineering techniques * Includes end-of-chapter exercises Position Sensors is written for electrical, mechanical, and material engineers as well as engineering students who are interested in understanding sensor technologies.

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Table of Contents

COVER

TITLE PAGE

ABOUT THE AUTHOR

PREFACE

TRADEMARKS

ABOUT THE COMPANION WEBSITE

1 SENSOR DEFINITIONS AND CONVENTIONS

1.1 IS IT A SENSOR OR A TRANSDUCER?

1.2 POSITION VERSUS DISPLACEMENT

1.3 ABSOLUTE OR INCREMENTAL READING

1.4 CONTACT OR CONTACTLESS SENSING AND ACTUATION

1.5 LINEAR/ANGULAR CONFIGURATION

1.6 POSITION, VELOCITY, AND ACCELERATION

1.7 APPLICATION VERSUS SENSOR TECHNOLOGY

1.8 OPERATIONAL LIFETIME

1.9 QUESTIONS FOR REVIEW

REFERENCES

2 SPECIFICATIONS

2.1 ABOUT POSITION SENSOR SPECIFICATIONS

2.2 MEASURING RANGE

2.3 ZERO, SPAN, AND FULL SCALE

2.4 REPEATABILITY

2.5 NONLINEARITY

2.6 HYSTERESIS

2.7 CALIBRATED ACCURACY

2.8 DRIFT

2.9 WHAT DOES ALL THIS ACCURACY STUFF MEAN TO ME?

2.10 TEMPERATURE EFFECTS

2.11 RESPONSE TIME

2.12 DAMPING

2.13 CROSS SENSITIVITY

2.14 SHOCK AND VIBRATION

2.15 ELECTROMAGNETIC COMPATIBILITY

2.16 HIGH VOLTAGE PULSE PROTECTION

2.17 POWER REQUIREMENTS

2.18 INTRINSIC SAFETY, EXPLOSION PROOFING, AND PURGING

2.19 RELIABILITY

2.20 QUESTIONS FOR REVIEW

REFERENCES

3 OUTPUT TYPES AND COMMUNICATION PROTOCOLS

3.1 ANALOG OUTPUT TYPES

3.2 DIGITAL OUTPUT TYPES

3.3 SSI

3.4 CAN

BUS

3.5 PROFI

BUS

3.6 HART

3.7 QUESTIONS FOR REVIEW

4 RESISTIVE/POTENTIOMETRIC SENSING

4.1 RESISTIVE POSITION SENSORS

4.2 RESISTANCE

4.3 HISTORY OF RESISTORS AND RESISTIVE POSITION SENSORS

4.4 POSITION SENSOR DESIGN

4.5 THE RESISTIVE ELEMENT

4.6 THE WIPER

4.7 LINEAR AND ROTARY MECHANICS

4.8 SIGNAL CONDITIONING

4.9 ADVANTAGES/DISADVANTAGES

4.10 TYPICAL PERFORMANCE PARAMETERS

4.11 SPECIFICATIONS AND APPLICATION

4.12 MANUFACTURERS

4.13 QUESTIONS FOR REVIEW

5 CABLE EXTENSION TRANSDUCERS

5.1 CABLE EXTENSION TRANSDUCER HISTORY

5.2 CABLE EXTENSION TRANSDUCER CONSTRUCTION

5.3 SIGNAL CONDITIONING

5.4 APPLICATION

5.5 ADVANTAGES/DISADVANTAGES

5.6 TYPICAL PERFORMANCE SPECIFICATIONS

5.7 MANUFACTURERS

5.8 QUESTIONS FOR REVIEW

6 CAPACITIVE SENSING

6.1 CAPACITIVE POSITION SENSORS

6.2 CAPACITANCE

6.3 DIELECTRIC CONSTANT

6.4 HISTORY OF CAPACITIVE POSITION SENSORS

6.5 CAPACITIVE POSITION SENSOR DESIGN

6.6 ELECTRONIC CIRCUITS FOR CAPACITIVE SENSORS

6.7 GUARD ELECTRODES

6.8 EMI/RFI

6.9 TYPICAL PERFORMANCE SPECIFICATIONS AND APPLICATION

6.10 MANUFACTURERS

6.11 QUESTIONS FOR REVIEW

REFERENCES

7 INDUCTIVE SENSING

7.1 INDUCTIVE POSITION SENSORS

7.2 INDUCTANCE

7.3 PERMEABILITY

7.4 HISTORY OF INDUCTIVE POSITION SENSORS

7.5 INDUCTIVE POSITION SENSOR DESIGN

7.6 THE COIL AND BOBBIN

7.7 CORE

7.8 SIGNAL CONDITIONING

7.9 ADVANTAGES

7.10 TYPICAL APPLICATION AND PERFORMANCE SPECIFICATIONS

7.11 MANUFACTURERS

7.12 QUESTIONS FOR REVIEW

8 THE LVDT AND RVDT

8.1 LVDT AND RVDT POSITION SENSORS

8.2 HISTORY OF THE LVDT AND RVDT

8.3 LVDT AND RVDT POSITION SENSOR DESIGN

8.4 COILS

8.5 CORE

8.6 CARRIER FREQUENCY

8.7 DEMODULATION

8.8 SIGNAL CONDITIONING

8.9 SYNCHRONIZATION

8.10 CALIBRATION

8.11 ADVANTAGES

8.12 TYPICAL PERFORMANCE SPECIFICATIONS AND APPLICATION

8.13 MANUFACTURERS

8.14 QUESTIONS FOR REVIEW

REFERENCES

9 DISTRIBUTED IMPEDANCE

9.1 DISTRIBUTED IMPEDANCE POSITION SENSORS

9.2 HISTORY

9.3 OPERATIONAL THEORY

9.4 THE DISTRIBUTED IMPEDANCE SENSING ELEMENT AS A TRANSMISSION LINE

9.5 PERIODIC STRUCTURES

9.6 HYBRID WAVES

9.7 DISTRIBUTED IMPEDANCE SENSOR DESIGN

9.8 ELECTRONICS

9.9 ADVANTAGES

9.10 TYPICAL PERFORMANCE SPECIFICATIONS AND APPLICATIONS

9.11 INFINITE RESOLUTION?

9.12 CALIBRATION

9.13 MANUFACTURERS

9.14 QUESTIONS FOR REVIEW

REFERENCE

10 THE HALL EFFECT

10.1 HALL EFFECT SENSORS

10.2 THE HALL EFFECT

10.3 HISTORY OF THE HALL EFFECT

10.4 HALL EFFECT POSITION SENSOR DESIGN

10.5 THE HALL EFFECT ELEMENT

10.6 ELECTRONICS

10.7 LINEAR ARRAYS

10.8 ADVANTAGES

10.9 TYPICAL PERFORMANCE SPECIFICATIONS AND APPLICATIONS

10.10 MANUFACTURERS

10.11 QUESTIONS FOR REVIEW

REFERENCES

11 MAGNETORESISTIVE SENSING

11.1 MAGNETORESISTIVE SENSORS

11.2 MAGNETORESISTANCE

11.3 HISTORY OF MAGNETORESISTIVE SENSORS

11.4 MAGNETORESISTIVE POSITION SENSOR DESIGN

11.5 THE MAGNETORESISTIVE ELEMENT

11.6 LINEAR ARRAYS

11.7 ELECTRONICS

11.8 ADVANTAGES OF MAGNETORESISTIVE SENSORS

11.9 TYPICAL PERFORMANCE SPECIFICATIONS AND APPLICATIONS

11.10 MANUFACTURERS

11.11 QUESTIONS FOR REVIEW

REFERENCES

12 MAGNETOSTRICTIVE SENSING

12.1 MAGNETOSTRICTIVE SENSORS

12.2 MAGNETOSTRICTION

12.3 HISTORY OF MAGNETOSTRICTION

12.4 MAGNETOSTRICTIVE POSITION SENSOR DESIGN

12.5 WAVEGUIDE

12.6 POSITION MAGNET

12.7 PICKUP DEVICES

12.8 DAMP

12.9 WAVEGUIDE SUSPENSION

12.10 ELECTRONICS

12.11 ANGULAR/ROTARY MAGNETOSTRICTIVE SENSORS

12.12 ADVANTAGES

12.13 TYPICAL PERFORMANCE SPECIFICATIONS

12.14 APPLICATION

12.15 MANUFACTURERS

12.16 QUESTIONS FOR REVIEW

REFERENCES

13 ENCODERS

13.1 LINEAR AND ROTARY

13.2 HISTORY OF ENCODERS

13.3 CONSTRUCTION

13.4 ABSOLUTE VERSUS INCREMENTAL ENCODERS

13.5 OPTICAL ENCODERS

13.6 MAGNETIC ENCODERS

13.7 QUADRATURE

13.8 BINARY VERSUS GRAY CODE

13.9 ELECTRONICS

13.10 ADVANTAGES

13.11 TYPICAL PERFORMANCE SPECIFICATIONS AND APPLICATIONS

13.12 MANUFACTURERS

13.13 QUESTIONS FOR REVIEW

REFERENCE

14 OPTICAL TRIANGULATION

14.1 LINEAR SENSING

14.2 HISTORY

14.3 CONSTRUCTION

14.4 LIGHT SENSOR

14.5 ELECTRONICS

14.6 LASER

14.7 ADVANTAGES

14.8 TYPICAL PERFORMANCE SPECIFICATIONS AND APPLICATIONS

14.9 MANUFACTURERS

14.10 QUESTIONS FOR REVIEW

BIBLIOGRAPHY

GLOSSARY OF SENSOR TERMINOLOGY

INDEX

END USER LICENSE AGREEMENT

List of Tables

Chapter 01

Table 1.1 Application Suitability of Various Sensor Technologies

Chapter 02

Table 2.1 Calculating Repeatability

Table 2.2 Example of an Excel Spreadsheet with Reference Position versus Sensor Output

Chapter 03

Table 3.1 A Listing of Some CiA Application Profiles

Table 3.2 CANopen Transmission Rate versus Cable Length

Table 3.3 CANopen Identity Object

Table 3.4 Time to Receive Updates from All Devices on a Network

Chapter 04

Table 4.1 Specification of a Typical Resistive Linear Position Sensor

Chapter 05

Table 5.1 Typical Specification of a Heavy Industrial CET with Voltage Divider Output

Chapter 06

Table 6.1 Dielectric Constant of Several Common Insulating Materials

Chapter 12

Table 12.1 Magnetostrictive Linear Position Sensors versus Other Linear Position Sensing Technologies

Table 12.2 Applications of Magnetostrictive Position Sensors

Chapter 13

Table 13.1 Decimal Equivalents of Hexadecimal, Binary-coded Decimal (BCD), Natural Binary, and a Gray Code

Table 13.2 Typical Specification of an Incremental Optical Linear Encoder

Table 13.3 Typical Specification of an Incremental Optical Rotary Encoder

Table 13.4 Typical Specifications of an Absolute Optical Rotary Encoder

Table 13.5 Typical Specifications of an Absolute Magnetic Rotary Encoder

Table 13.6 Typical Specifications of an Incremental Magnetic Linear Encoder

Chapter 14

Table 14.1 With a Known Base Length,

BC

, the Measured Distance Varies as the Tangent of the Measured Angle

Table 14.2 Typical Specification of an Incremental Optical Linear Encoder

List of Illustrations

Chapter 01

Figure 1.1 Example of a pressure sensor, on the left. Partly disassembled at the right, to show a pressure capsule that transduces pressure variation into movement of the core of an LVDT.

Figure 1.2 Pressure capsule and LVDT, some of the components of a pressure sensor.

Figure 1.3 Magnetostrictive linear position sensor with position magnet.

Figure 1.4 Incremental magnetic linear encoder.

Figure 1.5 A linear potentiometer.

Figure 1.6 LVDT linear position sensor with magnetically permeable core.

Figure 1.7 Contacting actuation in an LVDT gage head.

Figure 1.8 A resistive angular sensor (aka a rotary potentiometer).

Figure 1.9 Velocity sensor utilizing a magnetic core that is movable within a pair of coils, all inside of a nickel–iron alloy housing that provides magnetic shielding.

Figure 1.10 Accelerometers: spring-mass type on the left and force-balance type on the right.

Figure 1.11 Analog differentiator circuit to provide velocity output from position input.

Chapter 02

Figure 2.1 Amplifier circuit that has interactive zero and span controls.

Figure 2.2 Amplifier circuit that has noninteractive zero and span controls.

Figure 2.3 Nonlinearity, an ideal characteristic (a straight line) compared with the real sensor characteristic.

Figure 2.4 Finding the best straight line (BSL) and maximum nonlinearity error.

Figure 2.5 Zero-based nonlinearity.

Figure 2.6 End-point nonlinearity.

Figure 2.7 Graph of the sensor data and the least-squares straight line of Table 2.2.

Figure 2.8 Hysteresis shown in a plot of measurand versus sensor output.

Figure 2.9 Remanent magnetic field in a magnetic material. Line (

a

) is an initial nonmagnetized state.

Figure 2.10 Wiper flexing may cause different upscale and downscale readings.

Figure 2.11 Calibration error.

Figure 2.12 Linear encoder (magnetic strip type).

Figure 2.13 Laser interferometer.

Figure 2.14 Environmental test chamber with heating and mechanical refrigeration.

Figure 2.15 Pressure gauge, rotameter, and desiccant provide final drying near chamber.

Figure 2.16 Diagram of complete air-drying system.

Figure 2.17 Input signal and output signal versus time, showing lag time, time constant, and stabilization.

Figure 2.18 Sensor output stabilization versus damping.

Figure 2.19 Low-pass filters: (a) single-pole and (b) double-pole.

Figure 2.20 Cross axes in (a) A Hall effect angular or rotary sensor. (b) An LVDT linear sensor.

Figure 2.21 (a) Sensor mounted to a vibration forcer by using a mounting fixture and (b) source and amplifier.

Figure 2.22 A drop test type of shock tester, with accelerometer attached.

Figure 2.23 Example of current profiles for different ESD models.

Figure 2.24 Burst test: one burst cycle lasts 10 s. One test cycle lasts 20 s. Test duration is six cycles for 110 s total from start of first burst to end of final burst.

Figure 2.25 A pulse in accordance with the surge test.

Figure 2.26 Protection of power input and signal output lines of a sensor.

Figure 2.27 The combustion triangle.

Figure 2.28 Intrinsically safe (IS) barrier devices installed between a sensor located in a hazardous area and associated equipment located in a nonhazardous area.

Figure 2.29 A typical intrinsic safety passive Zener barrier (and is about 1/2 in. thick).

Figure 2.30 Schematic of an IS single-channel passive zener barrier.

Figure 2.31 Explosion-proof housing and flame path.

Figure 2.32 Purging systems: type Z (a) and type X (b).

Figure 2.33 MTBF is the arithmetic mean value of operational time between two failures.

Figure 2.34 Bathtub curve includes infant mortality, useful life, and wear-out.

Figure 2.35 Basic example of a design FMEA (DFMEA).

Chapter 03

Figure 3.1 Voltage output sensor with load resistor.

Figure 3.2 4–20 mA current output sensors: (a) two-wire (or loop powered); (b) three-wire, having a separate positive power supply voltage.

Figure 3.3 Safe operating area chart.

Figure 3.4 SSI hardware functional diagram. The hardware connection utilizes optical isolation and RS 422/485 parameters.

Figure 3.5 Transmission rate versus cable length.

Figure 3.6 SSI clock and data timing sequence.

Figure 3.7 Optocoupler connection diagram.

Figure 3.8 OSI seven-layer reference model, as used with CANopen and DeviceNet protocols.

Figure 3.9 The Sequence of Fields in a CANbus Data Frame, in Order from Top to Bottom. Shading shows data that are grouped together as fields.

Figure 3.10 Two example PDOs that could be sent from a CANbus-enabled string pot, in order from left to right.

Figure 3.11 Connecting devices on CANbus (each device is called a node).

Figure 3.12 CANbus hardware circuit functions.

Figure 3.13 Nondestructive bitwise arbitration. SOF is start of frame. Message B has lower priority and loses arbitration when it changes to recessive (high) at bit 10. The numbers 1 through 10 represent the starting points of successive bit times.

Figure 3.14 Dominant versus recessive bit; closed switch at node B is dominant.

Figure 3.15 NRZ compared with Manchester bit representation.

Figure 3.16 HART leader and field device (followers) connections in a multidrop configuration.

Figure 3.17 Main and secondary buses plus additional devices.

Figure 3.18 FSK signals, loop current parked.

Figure 3.19 Total number of bits to transmit one character.

Figure 3.20 The structure of one complete HART message.

Figure 3.21 Components of the HART short form and long form address frames.

Chapter 04

Figure 4.1 A two-resistor voltage divider circuit (a) and a potentiometer circuit (b).

Figure 4.2 A pressure sensor that utilizes a wirewound resistive element and wiper to form a potentiometer. The black disk is the housing cover, incorporating the three terminals for electrical connection.

Figure 4.3 Rod-type resistive linear position sensor construction.

Figure 4.4 A miniature version of a resistive linear position sensor.

Figure 4.5 Rodless-type resistive linear position sensor.

Figure 4.6 Connections to the wiper and to the left and right ends of the resistive element.

Figure 4.7 A resistive position sensor circuit with low output impedance.

Figure 4.8 The shape of a sensor output versus measurand characteristic including backlash, which is normally considered to be a part of hysteresis.

Chapter 05

Figure 5.1 Construction of a cable extension transducer.

Figure 5.2 A cable extension transducer having a plastic housing and with a 317 mm full stroke range. Cable fully retracted at (a). Cable partly extended at (b).

Figure 5.3 Internal components of a 10-turn wirewound potentiometer.

Figure 5.4 Resistance element and wiper of a 10-turn potentiometer.

Figure 5.5 A close-up view of the resistive element reveals that it comprises a thin resistance wire that is wound onto a flexible, insulated cable core.

Figure 5.6 Nongrooved drum at (a) and grooved drum at (b).

Figure 5.7 Various cable end terminations: (a) eyelet, (b) split ring, (c) scissor snap, (d) slide snap, (e) snap swivel, (f) bead, and (g) button.

Figure 5.8 A heavy industrial CET model.

Figure 5.9 The extension cable may resonate when subject to wind, shock, or vibration.

Chapter 06

Figure 6.1 Capacitive position sensor and target.

Figure 6.2 Construction of a simple parallel plate capacitor.

Figure 6.3 A rudimentary capacitive sensor capacitance changes with plate separation distance,

d

.

Figure 6.4 Several plate configurations for variable capacitance in response to a variation in linear position. (a) Variable spacing. (b) Variable area. (c) Variable dielectric.

Figure 6.5 Rotor and stator configurations for variable capacitance in response to a variation in angular position. (a) 180° measuring range with semicircular target. (b) 270° measuring range with spiral target.

Figure 6.6 A capacitive linear position sensor, with user-supplied target, includes a sensing element, electronics, and a housing with mounting and connection means.

Figure 6.7 An improved plate design, with overlap, to reduce sensitivity to motion in the cross-axis.

Figure 6.8 Adding a shield around a variable spacing sensor to prevent unwanted error signals from nearby conductors or electric fields.

Figure 6.9 One-shot circuit in which the timed period depends upon the variable value of sensor capacitance,

C

s

.

Figure 6.10 A free-running pulse generator to drive the one-shot circuit of Figure 6.9.

Figure 6.11 A free-running oscillator in which the sensing capacitance,

C

s

, determines the operating frequency.

Figure 6.12 Frequency-to-voltage converter circuit block diagram.

Figure 6.13 (a) Single-element circuit with variable voltage amplitude. (b) A dual-element circuit where one capacitance increases while the other decreases.

Figure 6.14 (a) A dual sensing element with a single-diode demodulator. (b) A dual sensing element with a dual-diode demodulator.

Figure 6.15 An op-amp differential amplifier circuit.

Figure 6.16 When using a demodulator diode provided with leads, thermally induced stress can be reduced by forming the leads into loops.

Figure 6.17 A synchronous demodulator derives a DC amplitude signal from a variable AC waveform by operating switches synchronously to the AC waveform.

Figure 6.18 Driving a guard electrode with a voltage similar to that of the movable sensing plate reduces the sensitivity to electrical noise in the environment.

Chapter 07

Figure 7.1 (a) Linear. (b) Angular. An inductive position sensor includes a sensing coil, a magnetically permeable core, an electronic circuit, and a housing. The electronic circuit can be included within the housing.

Figure 7.2 Construction of a simple inductor, with coil and core.

Figure 7.3 One of many possible configurations for an inductive angular position sensor. The shaft and core are mechanically coupled to rotate together.

Figure 7.4 Circuit representation of the inductance, parallel capacitance, and series resistance of a real inductor.

Figure 7.5 Inductive linear position sensor with rod ends.

Figure 7.6 The basic elements of a single coil winder.

Figure 7.7 A change in the variable inductance,

L

, results in a change in the amplitude of the differential output voltage.

Figure 7.8 Variable inductance sensor circuit with a DC output voltage.

Figure 7.9 Sensing circuit with compensating coil and DC output voltage.

Figure 7.10 A dual-coil variable inductance linear position sensor with semiconductor switches and DC output.

Figure 7.11 Inductive sensing element with a ferromagnetic target tube and remotely mounted electronics module. The target tube slides over the sensing element.

Chapter 08

Figure 8.1 Cutaway view, left, and side view, right, of an LVDT.

Figure 8.2 Pictorial representation of the coil configuration of a typical LVDT.

Figure 8.3 A simple LVDT signal conditioner with diode demodulator.

Figure 8.4 One type of core and coil configuration that can be used in an RVDT.

Figure 8.5 LVDT or RVDT series-bucking connection schematic.

Figure 8.6 LVDT or RVDT with diode demodulator and differential amplifier circuit.

Figure 8.7 Square wave oscillator with synchronous demodulator using two transistors.

Figure 8.8 Frequency and amplitude stable low distortion sine wave generator, having phase-shifted outputs.

Figure 8.9 (a) The steps of the digitally generated staircase are weighted according to the sine function. (b) The filter circuit smoothes the waveform.

Figure 8.10 An LVDT with internal signal conditioner (disassembled for illustration).

Figure 8.11 An external signal conditioner with digital display, analog output, phase adjust, and alarms.

Figure 8.12 A 4–20 mA two-wire current loop LVDT/RVDT signal conditioner.

Figure 8.13 A DIN rail mounting signal conditioner, with RS-485 communication, analog and digital outputs, alarms, push-button calibration, and other features.

Figure 8.14 A high-stability signal conditioner with selectable excitation frequency and fixed amplification.

Figure 8.15 Various configurations of leader and follower oscillators to eliminate beat frequencies: (a) designated leader, (b) daisy chain, and (c) one-wire sync.

Figure 8.16 Fixture for accurately adjusting an LVDT core position.

Figure 8.17 Fixture for accurately adjusting an RVDT core position.

Figure 8.18 Cutaway view of a gage head LVDT.

Chapter 09

Figure 9.1 Inductive sensors have a lump capacitance in parallel with each coil: (a) shows a lump capacitance in parallel with each of the three coils of a typical LVDT, while (b) shows the lump capacitance in parallel with the coil of a single-coil LVIT (but some LVDTs and LVITs may have different numbers of coils).

Figure 9.2 Distributed inductance and capacitance of a distributed impedance position sensing element having a dual helix configuration (helices).

Figure 9.3 One configuration of a distributed impedance linear position sensor is a dual-helix set of conductors that may be wound onto a dielectric cylindrical substrate material.

Figure 9.4 Photo of a distributed impedance linear position sensing element (without target tube).

Figure 9.5 Magnetic field lines of inductive and distributed impedance sensors. (a) Widely-spaced magnetic field lines of an inductive sensor. (b) Closely-spaced magnetic field lines of a distributed impedance sensor.

Figure 9.6 Electric and magnetic fields (

E

,

H

, respectively) of a dual flat spiral configuration of a distributed impedance position sensor electrodynamic element. (a) Electrodynamic element (top view) and (b)

E

and

H

fields (edge view).

Figure 9.7 Angular sensing element (left) has electrodynamic conductor patterns on top and bottom of a fiberglass substrate. A metal target is shown to the right.

Figure 9.8 A transmission line, formed by a distributed impedance electrodynamic element, such as a dual helix, has capacitance as well as inductance distributed along its length.

Figure 9.9 Pierce oscillator circuit to drive the sensing element.

Figure 9.10 Block diagram of complete position sensor circuit to provide a conditioned output signal.

Figure 9.11 A distributed impedance linear position sensor, with stainless steel housing and aluminum target tube (sealing o-ring not installed).

Figure 9.12 A distributed impedance angular position sensor, with anodized aluminum housing and mounting plate, with stainless steel shaft.

Figure 9.13 A test fixture for calibration of a distributed impedance linear position sensor.

Chapter 10

Figure 10.1 The Hall effect: Mutually perpendicular current,

I

, and magnetic flux,

B

, in a conductor, result in generation of the Hall voltage,

V

H

.

Figure 10.2 Concentration of carriers is forced to one side of the Hall device by magnetic force and produces the voltage differential,

V

H

.

Figure 10.3 A simple position sensor based on a Hall device, together with a typical curve of the output voltage versus position.

Figure 10.4 Alternate configuration of a Hall effect position sensor with longer stroke and bipolar output voltage.

Figure 10.5 Hall effect position sensor with improved configuration for greater stability.

Figure 10.6 Angular or rotary Hall sensor.

Figure 10.7 Block diagram of an analog circuit for a single Hall device or for a bridge configuration of Hall devices.

Figure 10.8 Block diagram of a digital circuit with a microcontroller.

Figure 10.9 Expanded range linear position sensor employing many Hall devices in a linear array.

Figure 10.10 A pressure sensor configuration utilizing a Hall device to measure the diaphragm position as a function of an applied pressure (sectional view of the diaphragm).

Chapter 11

Figure 11.1 A magnetoresistive sensing element with position magnet.

Figure 11.2 The resistance of a nonmagnetic conductor, in ohms (Ω), varies in response to the magnetic flux density,

B

, but not polarity (north or south).

Figure 11.3 Resistance of an AMR magnetoresistor decreases with application of a perpendicular magnetic field.

Figure 11.4 A single AMR element, with a magnetic field

B

, applied at a right angle to the current flow,

I

.

Figure 11.5 In a nonferromagnetic conductor, charge carriers are moved to one side by a magnetic field.

Figure 11.6 Magnetic domains tend to align in one direction when a ferromagnetic wire is drawn. Their rotation by a perpendicular magnetic field decreases resistivity.

Figure 11.7 Anisotropic magnetoresistive effect in a drawn ferromagnetic wire: magnetic domains align along the linear axis of a drawn Ni–Fe wire. When a perpendicular magnetic field is applied, this alignment is changed, and this changes the electrical resistance.

Figure 11.8 Placing conductive strips at 45° to improve linear response when sensing low magnitude magnetic fields.

Figure 11.9 A GMR sensing element comprises alternate layers of ferromagnetic and nonferromagnetic conductors sandwiched together.

Figure 11.10 A set of magnetoresistors connected in a Wheatstone bridge configuration.

Figure 11.11 A linear array of magnetoresistors may be used to form a linear position sensing element that is longer than is possible with a single magnetoresistor and having a position magnet movable along the sensing axis.

Figure 11.12 A magnetoresistive linear position sensor, with sensing element, housing, mounting feet, cable, and position magnet.

Figure 11.13 Configuration with a longer position magnet provides a longer measuring range.

Figure 11.14 MR angular position transducer.

Figure 11.15 Wheatstone bridge circuit with meander pattern of four MR legs to form one sensing element.

Figure 11.16 The MR sensing elements of a linear array should be spaced at one-half of their linear range.

Figure 11.17 A single MR element position sensor with analog output.

Figure 11.18 A linear array with microcontroller and digital or analog output.

Figure 11.19 A long-range linear position sensor with coded magnets.

Chapter 12

Figure 12.1 Magnetostrictive linear position sensor with position magnet.

Figure 12.2 Positive magnetostriction: Magnetic domains align with magnetic field,

H

, and cause stress, inducing an increase in mechanical dimension, Δ

L

.

Figure 12.3 The Wiedemann effect: A torsional force occurs at the location of an axial magnetic field (position magnet) when current is applied to a ferromagnetic wire, forming a torsional wave.

Figure 12.4 A serial memory device using a magnetostrictive wire delay line.

Figure 12.5 Signal pulse and reflection, measured as voltage across the pickup coil of a magnetostrictive linear position sensor.

Figure 12.6 Two or more position magnets may be implemented on one waveguide.

Figure 12.7 Position magnet shapes.

Figure 12.8 A C-shaped magnet with the same characteristics as a ring magnet.

Figure 12.9 Sensor with track to guide a sliding position magnet.

Figure 12.10 Types of pickup devices.

Figure 12.11 Some examples of damping elements.

Figure 12.12 Two examples of waveguide suspension system components.

Figure 12.13 Electronic functions block diagram without a μC (a) and with a μC (b).

Figure 12.14 Oscillator and interrogation circuit.

Figure 12.15 Timing of the interrogation pulse, set, and reset.

Figure 12.16 A magnetostrictive angular/rotary position sensor.

Figure 12.17 Test fixture for accurately locating the position magnet of a magnetostrictive linear position sensor.

Figure 12.18 A magnetostrictive linear position sensor installed into a hydraulic cylinder.

Chapter 13

Figure 13.1 Binary pattern of a brush type of absolute linear encoder.

Figure 13.2 An optical type of linear encoder.

Figure 13.3 Incremental magnetic linear encoder with separate magnetic scale.

Figure 13.4 Magnetic rotary encoder.

Figure 13.5 Binary pattern for an absolute rotary encoder (simplified).

Figure 13.6 Pattern of an incremental linear encoder.

Figure 13.7 An incremental optical encoder with LED and two phototransistors arranged to provide quadrature output.

Figure 13.8 An incremental optical rotary encoder.

Figure 13.9 An incremental magnetic linear encoder with magnetoresistive pickups spaced to provide quadrature output.

Figure 13.10 A quad B outputs are separated by 90°. Transitions 1 through 4 occur during each count cycle.

Figure 13.11 A Gray code pattern corresponding to Table 13.1.

Figure 13.12 LED and phototransistor connection circuit with Schmitt trigger.

Figure 13.13 Circuit schematic for converting gray code to natural binary.

Chapter 14

Figure 14.1 Triangulation method to determine the distance to a target by using a known angle, a known distance, and a measured angle.

Chart 14.1 Measured distance versus measured angle.

Figure 14.2 A laser diode, lens, light sensor, and electronics module comprise the main active components of an optical triangulation position sensor.

Figure 14.3 Alignment of the lens and focal plane may be adjusted to improve focus of the spot image over the measuring range.

Figure 14.4 Construction of a position-sensing detector.

Figure 14.5 Pictorial of a single CCD cell.

Figure 14.6 Construction of a CMOS light-sensing cell (pixel).

Figure 14.7 Packaging of three types of light sensor arrays: (a) PSD, (b) CCD, and (c) CMOS.

Figure 14.8 Circuit to connect with a PSD.

Figure 14.9 Circuit to connect with a CCD array.

Figure 14.10 Single-axis CMOS array with electronics integrated on chip.

Figure 14.11 (a) Digital data controls a D/A converter. (b) A digital potentiometer is used instead of a D/A.

Figure 14.12 An optical triangulation position sensor configuration that is representative of some products commonly available on the market.

Guide

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POSITION SENSORS

 

DAVID S. NYCE

Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in Canada

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permissions.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

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Library of Congress Cataloging-in-Publication data applied for

ISBN: 9781119069089

 

 

 

 

To my lovely and talented wife, Gwen, and our three wonderful children, Timothy, Christopher, and Megan, whose unconditional love and support helped me to complete this project. To friends, colleagues, and readers who might find this book helpful in their continuing effort to understand sensing technology, thus helping to advance this exciting and growing field of engineering. And thanks to my parents, Jonathan and Emma Nyce, who allowed me to build and operate a lab in the basement of our home where I grew up. Much was learned by experiments in the lab involving electronics, chemistry, physics, mechanics, aerodynamics, propulsion, and other areas of science (although sometimes the experiments proved to be a little bit on the dangerous side).

ABOUT THE AUTHOR

David S. Nyce has developed sensors of many types for over 30 years. He has worked as an electronics, mechanical, and chemical engineer as well as technical manager, operations manager, technical consultant, and business owner. His duties have ranged from development engineer and project engineer to chief engineer, director of technology, general manager, and vice president. He founded the Revolution Sensor Company in 2003 and has developed industrial, commercial, medical, military, and automotive products, including an automated production line for onboard automotive sensors.

His experience comprises the design of transducers, sensors, and instrumentation for many types of measurement, including temperature sensing using a thermocouple, RTD, thermistor, or semiconductor; pressure sensing with LVDT, resistive, strain gage, and diffused semiconductor sensing elements; resistive and strain gage flow sensors; linear and angular position, velocity, and acceleration transducers and sensors with LVDT, Hall effect, inductive, capacitive, optical, magnetostrictive, and distributed impedance technologies; resistive, inductive, and force-balance accelerometers; liquid level sensors based on ultrasonic waves, magnetostriction, and capacitive, LVDT, and distributed impedance technologies; densimeters; gas analysis using flame ionization, chemiluminescence, infrared, paramagnetic, zirconia, and electrochemical techniques; and intrinsic safety, explosion proof, purging, and inerting safety systems.

Nyce holds a Bachelor of Science degree in Electrical Engineering and Master of Business Administration. He is the inventor on 29 US patents.

PREFACE

Since the publication of Linear Position Sensors, Theory and Application in 2004, another book by this author, society and industry worldwide have further continued to increase their reliance on the availability of accurate and current information from sensors. Timely access to information is critical to effectively meet the indication and control requirements of industrial processes, manufacturing equipment, household appliances, onboard automotive systems, and consumer products. A variety of technologies are used to address the specific sensing parameters and configurations needed to meet these requirements.

Since the advent of smart phones, the general public has begun to understand that sensors are involved in their daily lives. Many personal device applications rely on the onboard accelerometer, GPS capability, ambient light sensor, microphone, touch sensor, camera function, and other sensors that may be added in future designs. Sensors are used in cars to measure many safety- and performance-related parameters, including throttle position, temperature, exhaust gas composition, suspension height, pedal position, transmission gear position, and vehicle acceleration. In clothes washing machines, sensors measure water level and temperature, load size, and drum position variation. Industrial process machinery requires the measurement of position, velocity, and acceleration, in addition to chemical composition, process pressure, flow rate, temperature, and so on.

More recent sensor designs often include the capability of communication over a shared bus, wireless communication, and communication via the Internet and personal devices. This will likely continue the trend toward greater ease of installation, additional performance features, and facilitation of routine and preventive maintenance.

Linear and angular/rotary position measurement comprises a large portion of the worldwide requirement for sensors. And so the original book on linear position sensors has been rewritten to include angular/rotary position sensors and update and expand the information on linear position sensors. Some chapters address position sensing technologies that have not been adequately described in any previous book. This book explains the theory and application of the technologies used in sensors for the measurement of linear and angular/rotary position, providing information important to sensor design in general. A chapter on sensor outputs and communication protocols is included and is applicable to all types of industrial sensors. Also included is information on electromagnetic interference, electrostatic discharge, and intrinsic safety.

There is often some hesitation in selecting the “proper” word—sensor or transducer—since the definitions of the terms are somewhat overlapping in normal usage and any distinction is becoming less important. Chapter 1 presents a working definition of these and other, sometimes confusing, terms used in the field of sensing technology.

Chapter 2 explains how the performance of linear position transducers and sensors is specified. Chapter 3

TRADEMARKS

The following are the owners of the trademarks, as noted in the book:

17-7 PH

AK Steel, West Chester, OH

Beldsol

Belden Corporation, Chicago, IL

CANbus

Robert Bosch GmbH, Stuttgart, Germany

DeviceNet

Rockwell Automation, Milwaukee, WI

Excel

Microsoft Corporation, Redmond, WA

HART

HART Communication Foundation, Austin, TX

Loctite

Henkel AG & Company KGaA, Düsseldorf, Germany

Manganin

Isabellenhütte Heusler GmbH & Co. KG, Dillenburg, Germany

Ni-Span C

Huntington Alloys, Incorporated, Huntington, WV

Norsorex

Startech Advanced Materials, GmbH, Wien, Austria

NyceWave

Revolution Sensor Company, Apex, NC

Permalloy

B&D Industrial and Mining Services, Inc., Macon, GA

Pomux

Sick-Stegmann, Minneapolis, MN

PowerPoint

Microsoft Corporation, Redmond, WA

Profibus

Profibus International, Karlsruhe, Germany

Ryton

Phillips Petroleum Company, Houston, TX

Solidon

Superior Essex Corporation, Atlanta, GA

SSI

Sick-Stegmann Corporation, Minneapolis, MN

Teflon

The Chemours Company, Wilmington, DE

Temposonics

MTS Systems Corporation, Eden Prairie, MN

Terfenol-D

ETREMA Products, Inc., Ames, IZ

Thermaleze

Phelps Dodge Copper Products, Norwich, CT

Torlon

Amoco Performance Products, Inc., Alpharetta, GA

All photos were taken by the author. All drawings were drawn by the author, using Microsoft PowerPoint™.

ABOUT THE COMPANION WEBSITE

This book is accompanied by a companion website:

http://www.wiley.com/go/Nyce/PositionSensors

The website includes:

Answer guide

1SENSOR DEFINITIONS AND CONVENTIONS

1.1 IS IT A SENSOR OR A TRANSDUCER?

There has been an ongoing evolution of the accepted use of the words transducer and sensor and the differentiation between them, especially since “smart sensor” (and not smart transducer) has become a very common term. So, this text will address the terms as they are becoming more commonly used, which relies partly on the original definition of “transducer”—A transducer is a device that changes energy from one form into another—and relies partly on the widespread acceptance of “smart sensor,” and rarely, if ever, a smart transducer. This may conflict with some other treatments of the subject but will clarify the further use of these terms within this book.

As mentioned, a transducer is generally defined as a device that changes energy from one form into another or, more specifically, a device that converts input energy into output energy. Typically, the output energy may be in a different form from the input energy but is related to the input. This includes converting mechanical energy into electrical energy as well as converting from one form of mechanical energy into another form of mechanical energy. For example, a convoluted thin metal diaphragm converts a differential pressure change into a linear motion change with a force, and a bimetal strip converts a temperature change into a motion with a force. Besides electrical and mechanical energy, forms of energy also include heat, light, radiation, sound, vibration, and others. Sometimes, there is no external application of energy in addition to the input energy that is being transduced or changed. The transducer output is often, but not necessarily, in the form of a voltage or current directly converted from the input energy. A transducer that does not require external application of energy (other than the energy that is being transduced) in order to produce a desired output is called an active transducer [1, pp. 2–4]. Some active transducer examples include the following: a loudspeaker converts a varying electrical energy input into a varying pressure wave output, a piezoelectric microphone converts a varying pressure wave input into an electrical output, a thermocouple pair converts a temperature difference into a voltage and current, a stepper motor converts an electrical input into a change of rotary position with a force, and an antenna converts an electromagnetic field into a voltage and current.

A transducer that requires an external supply of energy is called a passive transducer. A passive transducer produces an output signal that is usually a variation in an electrical parameter, such as resistance, capacitance, and inductance. For example, a photocell responds to a variation in light level by producing a relative change in the electrical resistance across two terminals (this is different from a solar cell that produces an electrical output from a light input). An external power supply can be used to convert this resistance change into a change in voltage or current. Other examples of passive transducers include a coil with a movable core so that moving the core further into the coil causes an inductance increase, a thermistor has a changing resistance with temperature, and others.

A sensor is generally defined as an input device that provides a usable output signal or information in response to a specific physical quantity input. The physical quantity input to be measured is called the measurand (such as the measured pressure, temperature, or position) and affects the sensor in a way causing an output that is indicative of the input quantity. The output of most modern sensors is an electrical signal but, alternatively, could be a motion, pressure, flow, or other usable types of output. Some examples of sensors include the following: a pressure sensor typically converts a fluid (gas or liquid) pressure into an electrical output signal indicative of the amount of pressure, a magnetostrictive position sensor converts a position into an electrical output signal indicative of the measured position, and many other types of sensors are in common use.

A sensor may incorporate several transducers [2, p. 4, fig. 1.2]. In the general case, a sensor is the complete assembly required to detect and communicate a particular event, while a transducer may be the element within that assembly that accomplishes a detection and/or quantification of the event.

For example, a diaphragm may be the transducer that changes a differential pressure into a linear motion or force, but a pressure sensor would include that plus additional transduction and circuit elements as needed in order to provide a desired electrical output, such as an output of 0–5 VDC.

In the example of a bimetal strip temperature transducer, adding a needle and a calibrated scale can form a complete sensor. Or adding a linear variable differential transformer (LVDT) and signal conditioning could make it a sensor having an electrical output.

Obviously, according to these definitions, a transducer can sometimes be a sensor and vice versa. For example, a microphone or a thermocouple can each fit the description of both a transducer and a sensor. This can be confusing, and many specialized terms are used in particular areas of measurement (e.g., an audio engineer would seldom refer to a microphone as a sensor, preferring to call it a transducer).

Although the general term “transducer” refers to both input and output devices, we are concerned only with input devices in this book. Accordingly, we will use the term transducer to signify an input transducer (unless specified as an output transducer, such as a speaker or a stepper motor).

So, for the purpose of understanding sensors and transducers in this book, these terms will be more specifically defined as they are typically used in developing sensors for commercial, factory automation, medical, automotive, military, and aerospace industries, as follows:

An input transducerproduces a usable output that is representative of the input measurand. Its output would then typically be conditioned (i.e., amplified, detected, filtered, and scaled) before it is suitable for use by the receiving equipment (such as an indicator, controller, computer, or PLC). The terms “input transducer” and “transducer” can be used interchangeably, as will be done in this work. So, for example, a pressure sensing diaphragm could be the input transducer that becomes part of a complete pressure sensor. An input transducer is sometimes called the sensing element, primary detector, or primary transducer.

A sensor is an input devicethat provides a desired electrical output in response to the input measurand. A sensor provides a signal that is conditioned and ready for use by the receiving equipment. A sensor is sometimes able to send its signal over long distances by wire or sometimes can transmit the signal information wirelessly.

One final note on transducers and transducers versus sensors: a review of the literature by the author revealed some sources that agree with the definitions presented here regarding active and passive transducers. But a significant number of the works proposed the opposite meanings, with the same definitions but the words active and passive switched. The same thing happens when checking the definitions of a sensor versus a transducer: some say a sensor is the complete device that includes a transducer plus conditioning electronics, while others say the opposite (switching the words transducer and sensor). So, a reader would be well advised to avoid getting caught up with the discrepancies. But the definitions presented here will be utilized in this book.

A smart sensor is a term commonly used since the mid-1980s referring to sensors that incorporate one or more microcontrollers in order to provide increased quality of information as well as additional information. This may include such functions as linearization, temperature compensation, digital communication, remote calibration, and sometimes the capability to remotely read the model number, serial number, range, and other information.

Intelligent sensor is the term commonly used when a smart sensor includes additional functionality, such as self-calibration, self-testing, self-identification, adaptive learning, and taking a particular action when a predetermined condition is present.

The usages of the smart sensor and intelligent sensor monikers also reinforce our working definition of the word “sensor,” since these are not called smart transducers or intelligent transducers.

Sometimes, common usage will have to override our theoretical definition in order to result in clear communication among engineers in a specific industry. The author has found, for instance, that some actual manufacturers of pressure sensing devices that include internal voltage regulator, amplifiers, filters, and other signal conditioning electronics call their product a transducer. That is, the term transducer is sometimes used to name what our definition defines as a sensor. In any case, we will rely upon the definition presented here, because it now seems to apply to most modern uses.

An example of a transducer as part of a sensor may help one to understand our present definition of a sensor being a device that provides a desired electrical output (such as 0–5 VDC) in response to the input measurand (such as 0–5 PSIG) and a transducer being a device that changes (or transduces) energy from one form into another. Figure 1.1 is an example of a pressure sensor (designed by the author) on the left, and at the right is the same sensor with the top and bottom covers removed.

Figure 1.1 Example of a pressure sensor, on the left. Partly disassembled at the right, to show a pressure capsule that transduces pressure variation into movement of the core of an LVDT.

The pressure capsule (i.e., a set of circular convoluted thin metal diaphragms welded together at their edge) acts as a transducer to change the potential energy of a pressure difference into the mechanical energy of a linear motion at a force. The motion is internal to the housing and also moves a ferromagnetic core within an LVDT (LVDTs are explained in Chapter 8). The pressure capsule expands with an increasing internal pressure and compresses with increasing external pressure. In this example, the pressure to be measured is introduced through a pressure port into the inside of the capsule. The outside of the capsule is exposed to atmospheric air, thereby enabling the sensor to respond to gauge pressure (see Fig. 1.2).

Figure 1.2 Pressure capsule and LVDT, some of the components of a pressure sensor.

The pressure capsule is mechanically coupled with the core of an LVDT. The LVDT also acts as a transducer so that movement of its core affects the inductive coupling among its three internal coils. An associated electronics module powers the LVDT, demodulates the LVDT coil output voltages, and provides an electrical output signal indicative of the measured pressure.

One type of sensor has its own descriptive term: a 2-wire current loop transmitter, also called a loop transmitter, or often just shortened to transmitter. Popular transmitter types include temperature, pressure, flow, and position, among others. Such a transmitter has two wires over which both power and signal are transferred. This is explained further in Section 3.1.

1.2 POSITION VERSUS DISPLACEMENT

Since linear and angular position sensors are presented in this work, the difference between position and displacement should be understood. A position sensor measures the distance between a reference point and the location of a target. The word target is used in this case to mean that element of which the position or displacement is to be determined. The reference point can be one end, the face of a flange, or a mark on the body of the position sensor (such as a fixed reference datum in an absolute sensor), or it can be a movable point, as in a secondary target, or programmable reference datum.

As an example, consider Figure 1.3, showing the measuring range components of a magnetostrictive linear position sensor designed by the author. This is an absolute sensor, measuring the location of a position magnet with respect to the face of the mounting flange (Chapter 12 presents more detail on magnetostrictive position sensors). Some position sensors may have an unusable area near the end of their measuring range, called a dead zone, as may be noted in Figure 1.3.

Figure 1.3 Magnetostrictive linear position sensor with position magnet.

A magnetostrictive sensor may have another unusable area near the other end of its measuring range, called the null zone. This is not to be confused with the null of an LVDT, in which the null normally falls within the measuring range (as LVDT null is explained in Chapter 8 on LVDTs).

A displacement sensor measures the distance between the present position of a target and the previously recorded position of the target. An example of this would be an incremental magnetic linear encoder as shown in Figure 1.4.

Figure 1.4 Incremental magnetic linear encoder.

Displacement sensors typically send their data as a series of pulses, or sets of pulses on two lines, in which the pulses are time related so that both the amount of displacement and its direction can be encoded. This is known as square wave in quadrature (or just quadrature) and is explained further in Chapter 13 on encoders.

Position sensors can be used as displacement sensors by adding circuitry to remember the previous position and subtract the new position, yielding the difference as the displacement. Alternatively, the data from a position sensor may be recorded into memory by a microcontroller, and differences calculated as needed to indicate displacement. Unfortunately, it is common for many manufacturers of position sensors to call their products displacement sensors or transducers.

To summarize, position refers to a measurement with respect to a reference datum, while displacement is a relative measurement indicating the amount (and sometimes the direction) of movement from a previously noted location.

1.3 ABSOLUTE OR INCREMENTAL READING

An absolute-reading position sensor always indicates the measurand with respect to a constant, or reference, datum. The reference datum is usually one end, the face of a flange, or a mark on the body of the position sensor. For example, an absolute linear position sensor may indicate the number of millimeters from one end of the sensor, or a datum mark, to the location of the target (the item to be measured by the sensor). If power is interrupted, or the position changes repeatedly, the indication when normal operation is restored will still be the number of millimeters from one end of the sensor, or a datum mark, to the location of the target. If the operation of the sensor is disturbed by an external influence, such as by a power outage or by an especially strong burst of electromagnetic interference (EMI), the correct reading will be restored once normal operating conditions return.

To the contrary, an incremental-reading sensor indicates only the changes in the measurand as they occur. An electronic circuit in the receiving equipment is used to keep track of the sum of these changes (the count) since the last time that a reading was recorded and the count was zeroed. If the count is lost due to a power interruption, or the sensing element is moved while power has been interrupted, the count when normal operating conditions are restored will not represent the present magnitude of the measurand. For example, if an incremental encoder is first zeroed, then moved upscale 25 counts, followed by moving downscale 5 counts, the resulting position would be represented by a count of 20. If there are 1000 counts per millimeter (mm), the displacement is 0.02 mm. If power is lost and regained, the position would probably be reported as 0.00 mm. Also, if the count is disrupted by an especially strong burst of EMI, the incorrect count will remain when normal operation is restored. When an incorrect count is suspected, it becomes necessary to re-zero the sensor.

1.4 CONTACT OR CONTACTLESS SENSING AND ACTUATION

One classification of a position sensor pertains to whether it utilizes a contact or noncontact (also called contactless) type of sensing element. With contactless sensing, another aspect is whether or not the sensor also uses contactless actuation.

In a contact type of linear position sensor, one or more parts of the device making the conversion between the physical parameter being measured (such as a movable arm or a rotating shaft) and the sensor output incorporate a sliding electrical and/or mechanical contact. A primary example is the linear potentiometer (see Fig. 1.5). An actuator rod is connected internally to a wiper arm. The wiper arm incorporates one or more flexible metallic contacts, which press against a resistive element that is inside the housing and extends over most of the housing length. The potentiometer is powered by applying a voltage across the resistive element from one end to the other. Changing position along the motion axis causes the wiper(s) to rub against the resistive element in respective positions along the resistive element, thus functioning as a voltage divider circuit and producing an output voltage as an indication of the measurand. A more complete description of the linear potentiometer is provided in Chapter 4.

Figure 1.5 A linear potentiometer.

The movable mounting feet shown in Figure 1.5 can be moved to their respective positions, as desired, along the mounting foot rail for mounting of the potentiometer. Then they lock into place on the rail when tightened.

It is because of the rubbing contact between the wiper and the resistive element that a linear potentiometer is called a contact sensor. The primary advantages of using a linear or rotary potentiometer as a sensor are their simplicity and that they often do not require signal conditioning. This sensing technology is also generally thought of as a low-cost solution for many linear and angular sensing applications, although automation of manufacture of other types of sensors continues to close any cost gap that may still exist.

The disadvantage of a contact sensor is that there is a finite lifetime associated with the rubbing elements. Further explanation of the lifetime limitation and the design features implemented to optimize operating life are presented in Chapter 4.

In a contactless position sensor, the device making the conversion between the measurand and the sensor output incorporates no physical connection between the moving parts and the stationary parts of the sensor. The “connection” between the moving parts and the stationary parts of the sensor is typically provided through the use of inductive, capacitive, magnetic, or optical coupling. Examples of contactless position sensing elements include the LVDT, inductive, optical, Hall effect, distributed impedance, magnetostrictive, and magnetoresistive sensors. These are explained further in their respective chapters later in the book, but as an example, we consider the LVDT here briefly.

An LVDT linear position sensor with core is shown in Figure 1.6. The core is attached to the movable member of the system being measured (the target). The LVDT housing is attached to a stationary member of the system. As the core moves within the bore of the LVDT, there is no physical contact between the core and the remainder of the LVDT. AC power is applied to the LVDT primary coil. Inductive coupling between the LVDT primary and its secondary windings, through the magnetically permeable core, affords the noncontact linkage.

Figure 1.6 LVDT linear position sensor with magnetically permeable core.

Contactless sensors are generally more complicated than linear or rotary potentiometers and typically require signal conditioning electronics. The use of an LVDT requires signal conditioning electronics that comprise AC excitation, signal demodulation, amplification, filtering, scaling, and an output amplifier or other circuit to provide the desired type of output signal. This is explained further in Chapter 8.

In addition to contactless operation within the sensor, a sensing system may utilize contactless actuation when there is no mechanical coupling between the sensing element and movable physical element (the target) of which the position is being measured. As an example of magnetic coupling, a permanent magnet can be mounted to a movable machine toolholder, and a magnetostrictive position sensor (as shown in Fig. 1.3) can be mounted along the motion axis of the toolholder. The measurement of tool position is thus made without any mechanical contact between the toolholder and the sensing element.