SENSOR DEFINITIONS AND CONVENTIONS COPYRIGHTED …

Position Sensors, First Edition. David S. Nyce. © 2016 John Wiley & Sons, Inc. Published 2016 by John Wiley & Sons, Inc. Companion website: www.wiley.com/go/Nyce/PositionSensors

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

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2 SENSOR DEFINITIONS AND CONVENTIONS

converted from the input energy. A transducer that does not require external applica-tion 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 trans-ducer. 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 pro-duces 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 resis-tance 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 descrip-tion of both a transducer and a sensor. This can be confusing, and many specialized

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IS IT A SENSOR OR A TRANSDUCER? 3

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 trans-ducer, 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 sen-sors for commercial, factory automation, medical, automotive, military, and aerospace industries, as follows:

An input transducer produces 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 “trans-ducer” 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 device that 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 litera-ture 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 pro-posed 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 addi-tional 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

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has found, for instance, that some actual manufacturers of pressure sensing devices that include internal voltage regulator, amplifiers, filters, and other signal condi-tioning 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.

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).

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.

Pressure port

LVDT

Signal conditioning

Pressure capsule

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.

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POSITION VERSUS DISPLACEMENT 5

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 trans-mitter 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 sen-sors). 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.

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.

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.

Lead wiresLVDT

Reference port

CoreCapsule

Pressure port

Pressure housing

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

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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).

Cable

Mounting flange

Read head

Encoder scale(inside housing)

Movable

FIGURE 1.4 Incremental magnetic linear encoder.

Measured positionMeasuring range

Position magnet

Null zoneSensing rod

Dead zoneElectronics

module

FIGURE 1.3 Magnetostrictive linear position sensor with position magnet.

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CONTACT OR CONTACTLESS SENSING AND ACTUATION 7

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 prob-ably 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 non-contact (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

Mounting foot rail

End cap

Movable mounting feet

End cap with bearing and wipe

Motion axis

Actuator rod with nuts

FIGURE 1.5 A linear potentiometer.

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is inside the housing and extends over most of the housing length. The potentiom-eter 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 indi-cation of the measurand. A more complete description of the linear potentiometer is provided in Chapter 4.

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 contact-less 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).

Core

Motion axis

(Core is normally inside LVDT, butshown outside so that it can be seen)

Cable or wire leads

LVDT

FIGURE 1.6 LVDT linear position sensor with magnetically permeable core.

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CONTACT OR CONTACTLESS SENSING AND ACTUATION 9

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.

Contactless sensors are generally more complicated than linear or rotary potenti-ometers and typically require signal conditioning electronics. The use of an LVDT requires signal conditioning electronics that comprise AC excitation, signal demodu-lation, 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 uti-lize 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.

Contactless actuation, obviously, does not utilize any rubbing parts that can wear out and reduce life or accuracy of the measurement. Conversely, contacting actuation may be used with an inherently contactless sensor, as when a toolholder presses the plunger of an LVDT gage head, for example. See Figure 1.7 for a pictorial represen-tation of an LVDT gage head sensor, similar to several models designed by the author.

(Note: the spelling of gage head, as opposed to gauge head, has been the standard in the industry for many decades, and therefore gage head is the accepted spelling and used in this book. This is similar to the accepted spelling of strain gage, another type of sensing element that is sometimes replaced by an LVDT gage head.)

Even though the LVDT itself operates as a contactless sensor, the contact actua-tion of the plunger leaves the system somewhat open to reduced life and varying accuracy due to wear. In this example, repeated rubbing of the gage head shaft against its bushings will eventually result in wear, possibly affecting performance

Dust cover(�exible bellows)

CableCase

Mounting threadPlunger Stylus

tip

Motionaxis

FIGURE 1.7 Contacting actuation in an LVDT gage head.

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through undesired lateral motion of the shaft, or increased operating force. To combat this possibility, LVDT gage heads are often constructed with high‐quality ball bearings and a hardened stylus tip that serve to provide long life even with contact actuation.

1.5 LINEAR/ANGULAR CONFIGURATION

Linear and angular position transducers and sensors operate by utilizing any of a large number of technologies, some of these being resistive, cable extension (resistive), capacitive, inductive, LVDT, distributed impedance, Hall effect, mag-netoresistive, magnetostrictive, and optical. Each of these technologies can be utilized in the design of both linear and angular sensors. For example, a resistive type of linear position sensor shown in Figure 1.5 operates in much the same way as a resistive angular position sensor constructed to measure an angular or rotary measurand (see Fig. 1.8).

A resistive angular position sensor is also known as a rotary potentiometer. The rotational nature of the sensor dictates the addition of a rotating shaft to hold the wipers, and the resistive element is circular in shape. Other than that, the basic theory of operation is the same as with a resistive linear position sensor. The resistive element and electrical terminals are stationary with the housing. The wipers rotate with the shaft. A highly conductive layer (usually metallic) is placed in the same plane with and around the resistive element. Some wipers rub onto the resistive element, and some wipers rub onto the conductive layer. Since the wipers are connected together, a voltage is picked up from the resistive element and placed onto the conductive layer. The conductive layer is then connected with the center terminal of the sensor. The two end terminals are connected with the two ends of the resistive element. A power supply voltage is connected with the two end terminals to apply a voltage across the resistive element.

Terminals

Resistive element and common conductor

Shaft

Shaft seal and ball bearing

HousingShaft insulator

Wipers Cover

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

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POSITION, VELOCITY, AND ACCELERATION 11

1.6 POSITION, VELOCITY, AND ACCELERATION

In addition to measuring a position, it is sometimes necessary to measure how fast a position is changing (velocity) or how fast a velocity is changing (acceleration). Separate types of sensors are available to measure velocity and acceleration. Figure 1.9 is a linear velocity sensor designed by the author using an inductive principle.

A rod magnet is movable within the bore of a sensing element. Two coils are arranged coaxially along the bore. Moving the magnet induces a voltage across each coil, according to Faraday’s law:

e N

d

dt (1.1)

in which a voltage, e, is induced across a coil having a number of turns, N, of a mag-nitude proportional to a change in magnetic flux, Ф, with respect to time. This also means the voltage induced across a coil is proportional to the strength of the magnet, the number of coil turns, and the velocity of movement of the magnet.

If only one coil was used, the voltages induced from the north and the south poles (ends) of the magnet would cancel each other out. Having two coils connected in series opposing polarity as shown allows the two voltages to add, producing an output voltage that has a linear relationship with the velocity of magnet motion.

As Figure 1.9 showed a magnetic induction velocity transducer, Figure 1.10 shows two types of accelerometers designed by the author: spring mass and force balance.

In the spring‐mass type of accelerometer, a small mass (called the seismic mass) of ferromagnetic material is held in place by a spring. The mass is suspended bet-ween two sensing coils. As the mass experiences acceleration, it moves against the spring force by an amount proportional to the amount of acceleration. The coils are driven by an AC excitation, a signal is demodulated, and an output signal is devel-oped that is indicative of the magnitude and direction of the acceleration (such as +5 V to –5 VDC for an acceleration of +1 to –1g).

Movable magnet Two coils Housing

SN

Voltage output

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.

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Similar accelerometers are available that use a micromachined sensing element, providing for a very small sensor. These often utilize diffused strain gage sensing elements to sense force or motion of the seismic mass. Accelerometers of ±1g or less are sometimes called inclinometers, for indicating the angle of inclination from a horizontal attitude.

In the force‐balance type of accelerometer, the seismic mass is suspended near a detector. Any acceleration that tends to move the mass closer to or farther from the detector is counteracted by a force generated in response to an error signal from the detector. The force‐balance accelerometer of Figure 1.10 uses a system similar to a taut band analog meter movement. A needle on the taut band has a flag at one end, which forms the seismic mass. An oscillator coil nearby detects the presence of the flag. Changes in the oscillator coil signal are amplified and used to drive the meter coil, keeping the flag in the original position. The amount of current in the meter coil needed to maintain the flag position is conditioned to produce the accelerometer output signal.

The signals provided by velocity sensors and accelerometers can sometimes be replicated by a position or displacement sensor. Velocity is the first derivative of displacement, and acceleration is the second derivative. So, when position or displacement information is available to a microcontroller, the velocity and/or acceleration can be calculated. Alternatively, an analog differentiation circuit can be used, as shown in Figure 1.11.

With input voltage Vin, capacitance C, and resistance R, the circuit of Figure 1.11

will provide an output voltage Vout

according to the following formula:

V RC

dV

dtoutin (1.2)

So, for example, if a position sensor has an output of 0–5 VDC, adding the circuit of Figure 1.11 could provide a velocity output of 0 V when there is zero velocity, 0 to –5 V for velocity in one direction, and 0 to +5 V for velocity in the other direction. Adding a second stage of the same circuit could provide an acceleration output.

Whether the velocity or acceleration signal output is obtained from a position or displacement sensor by using an analog circuit or by calculation within a

FIGURE 1.10 Accelerometers: spring‐mass type on the left and force‐balance type on the right.

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OPERATIONAL LIFETIME 13

microcontroller, the signal integrity may be limited by the presence of ambient electrical noise. In such cases, it is sometimes better to use a velocity sensor or an accelerometer rather than deriving those functions from position or displacement sensor signals.

1.7 APPLICATION VERSUS SENSOR TECHNOLOGY

Linear and angular position sensors can be designed which are based on one or more of a wide variety of technologies, as mentioned previously and presented later in this book. When determining which sensor type to specify for use in a specific applica-tion, it may be important to match the technology of the sensor to the requirements of the application.

If the sensor will undergo continuous repetitive motion, as in constant vibration, contactless sensing and contactless actuation may be required to eliminate parts that could wear out. In this case, magnetic, inductive, distributed impedance, or optical coupling to the sensor can be used, for example.

If it is desired to use the same linear position sensor type for short strokes (tens of millimeter) as well as long strokes (several meters), then a sensor technology with this operating range capability may be required. Magnetostrictive or distributed impedance technology can be used in this case.

Advantages and disadvantages for each technology are listed in the respective chapters, but Table 1.1 provides general information on application suitability.

1.8 OPERATIONAL LIFETIME

The rated lifetime of the sensing element can be an important consideration in the application of a contact‐type linear potentiometer in the presence of continuous vibration. A typical lifetime rating for a potentiometer is 20 million cycles. If the motion system has a constant dithering or vibration at 10 Hz, for example, this

Output

R

U1

C

Input

+

–

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

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number of cycles can be accumulated at a small spot on the element within 2 months. Many motion systems have two primary positions in which they operate over 90% of the time. The number of cycles of the example in each of these two positions is represented, per month, by Equation 1.3:

10 2 59 10 50 90 11 6 106 6Hz month cycles position mons. / % % . / /dc tth. (1.3)

where dc is duty cycle and 50% is for the two positions.This assumes that the two primary positions are used about equally. Accordingly,

a contact‐type resistive sensor (potentiometer) exposed to 10 Hz dithering in two positions can wear out within weeks. See Chapter 4 for more details on resistive sensing.

1.9 QUESTIONS FOR REVIEW

1. A distance measurement between the present and a previous position of a target is:

a. Displacement

b. Differential transformer

c. Acquisition

d. Double precision

e. Slow

TABLE 1.1 Application Suitability of Various Sensor Technologies

Technology Absolute Noncontact Lifetime Resolution Range Stability

Resistive Yes No Low Medium Medium MediumCET Yes No Low Low to

mediumHigh Medium

Capacitive Yes Some models

High Low to high Low Low

Inductive Yes Yes High Medium Medium LowLVDT Yes Yes High High Medium MediumDistributed

impedanceYes Yes High High High High

Hall effect Yes Yes High High Low LowMagnetoresistive Yes Yes High High Low LowMagnetostrictive Yes Yes High High High HighEncoder Some

modelsSome

modelsMedium Low to high Medium High

Optical triangulation

Yes Yes High Low to medium

Low Medium

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QUESTIONS FOR REVIEW 15

2. Circuitry to power the sensing element, filter, and amplify its signal is called a: a. Multiplier

b. Divider

c. Function circuit

d. Signal conditioner

e. Signal pattern

3. Position sensor type that indicates with respect to a constant, or reference, datum:

a. Incremental

b. Absolute

c. Integrating

d. Interval

e. Index

4. This type of analog circuit can provide a velocity output from a position input: a. Gyrator

b. Peltier

c. Cyclotron

d. Kelvin

e. Differentiator

5. An unusable area near the end of the measuring range is called the: a. Keep out area

b. Partition

c. No crossing zone

d. Twilight zone

e. Dead zone

6. Faraday’s law shows a voltage induced across a coil is proportional to a change in:

a. Capacitance

b. Elevation

c. Magnetic flux

d. Phlogiston

e. Light waves

7. An analog potentiometer usually has this many terminals: a. Eight

b. Two

c. Two for power supply plus four for signals

d. Three

e. Four

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8. An LVDT utilizes this type of coupling between its primary and secondary coils:

a. Resistive

b. Inductive

c. Capacitive

d. Conductive

e. DC

REFERENCES

[1] E. Herceg, Handbook of Measurement and Control. Pennsauken, NJ: Schaevitz Engineering, 1976.

[2] J. Fraden, Handbook of Modern Sensors. New York: Springer‐Verlag, 2010.

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SENSOR DEFINITIONS AND CONVENTIONS COPYRIGHTED …