bst handout e04

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ROYAL SCHOOL OF ARTILLERYBASIC SCIENCE & TECHNOLOGY SECTION

GUNNERY CAREERS COURSES

Sensors

DWR Sensors

E04-1 E04_Sensors.QXD

INTRODUCTION

The human body is equipped with a variety of sens-es that communicate information from the outsideworld to the brain. The ability to detect heat, light,

sound, position and our orientation is essential for sur-

vival. Most military hardware requires similar types of

information for correct operation.

When equipment is controlled by a computer then

the computer requires information back from the equip-

ment so that the computer knows where the equipment

is pointing, how fast it is moving, etc. This information isprovided by sensors (transducers) that convert informa-

tion about the environment into an electrical signal.

The main task of a sensor is to convert the required

information, e.g. angle of elevation of a launcher, into

an electrical signal that can be further processed by the

electronic circuits that control the system. This handout

covers the basic operation of position and angle sen-

sors used in military equipment.

DETECTING VISIBLE AND INFRA-RED LIGHT

M

ost detectors of light use the energy of light pho-

tons to release electrons from the atoms of thematerial in the detector. When the light falls on the

material then its energy frees electrons and the materi-

al allows a current to flow. For this to work well then a

number of conditions must be met:

The amount of energy needed to release an elec-

tron must be less than the energy in the photons

that we are trying to detect.

The material must allow the released electrons to

move through it so that they can reach the output.

The electrons must be mobile.

The released electrons must remain free for suffi-

cient time to be detected. The electrons must notimmediately return to an atom.

The material must not have many free electrons

when there is no light. The material must be a poor

conductor or semi-conductor.

There are a few materials that happen to have the

right properties for use in practical detectors. Examples

of materials that can be used to detect heat or light are:

Silicon: used in photo-diodes and photo-transistors

(e.g. at the back of the missile, to detect the laser

grid in HVM). Fast operation is obtained from thesedevices.

23 Jul 04

Cadmium Mercury Telluride: used to detect infra-red

radiation in ADAD and other thermal devices.

Cadmium Sulphide: used in photo-conductive cells,

these give a very large variation in resistance for a

given change in light level. They are relatively slow

to operate.

The need for cooling: The energy in the longer

infra-red waves, such as those emitted by objects at nor-

mal temperatures, is comparable with the thermal ener-

gy of room temperature. This means that any detectorthat responds to these wavelengths will be blinded by its

own heat energy. Consequently, it is necessary to cool

these detectors to around 190 C (80 K) so that the

thermal energy in the detector does not cause the

release of a significant number of electrons.

Another reason for cooling a detector would be to

reduce noise levels in order to receive weak signals.

Thermal noise power is proportional to the Kelvin tem-

perature: room temperature is about 300 K so reducing

the temperature to around 100 K (or 173 C) will

reduce the thermal noise power by a factor three.

Integration : when the light or thermal intensity islow then the detector might take a longer time to accu-

mulate sufficient free electrons to record a response.

This is called integration - the accumulation of a signal

over a period of time. Devices such as television cam-

eras, which produce 25 pictures per second, must use

detectors that can respond to the light within 1/25 sec-

ond - the available integration time.

Circuit Symbols: Devices that respond to light and

infra-red radiation are shown in circuits as having two

arrows pointing towards them, to represent the arrival

of light. This is illustrated in Figure One.

Figure 1: The Circuit Symbols for a Photo-Resistor (left) and a Photo-Transistor (right)


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DETERMINING ANGULAR POSITION

In order to determine the position or orientation of anobject, we need to convert information into an electri-cal signal. If object involved is a gyroscope in a missile

then we might want information about the orientation ofa missile so it can be steered towards the target. If it is

a joystick then we might want to know whether it has

been moved left or right. One simple device that can do

this is the potentiometer.

As illustrated in Figure Two, the shaft of the poten-

tiometer is connected to the moving object, in this case

a joystick. As the joystick is moved then it moves the

slider of the potentiometer. The ends of the potentiome-

ter are connected to +9 V and 9 V dc, as shown. In

the central position, the slider makes contact with the

mid-point of the track of the potentiometer which will be

at Zero volts - exactly between +9 V and 9 V.When the joystick is moved upwards then the slider

is moved downwards, towards +9 V. You can probably

see from the diagram that when the joystick has moved

up by 45 then the voltage output will be +4.5 V. When

the joystick is moved downwards then the slider is

moved towards 9 V. As before, when the joystick has

moved down by 45 then the voltage output will be

4.5 V, as illustrated in Figure Three. We now have a

sensor, or transducer, that gives an output of one volt

for each ten degrees of angular motion and a positive

output for an upwards movement.

The magnitude (size) of the voltage is determined

by the number of degrees of rotation and its polarity is

determined by the direction of rotation.Linear Potentiometers may also be used for posi-

tion sensing. These have a straight track and the slider

moves along it. They are sometimes used in graphic

equalisers to adjust the settings of each channel.

Where an object has to move back and forth by a few

centi-metres then attaching the slider of a linear poten-

tiometer to the object produces a voltage that depends

on the objects position. A linear potentiometer is

attached to the actuator that moves the control sur-

faces of a Rapier missile so that the servo system can

monitor its movement. (See Figure Four.)

Limitations: Some of the limitations of the poten-tiometer as a sensor are:

The maximum amount of rotation that can be

accommodated by this type of sensor is about 320

and it cannot register around an entire circle as

there has to be a gap between the plus and minus

connections.

There is some friction at the sliding contact and the

track of the potentiometer is also subject to wear

and contamination by dust, etc. (A potentiometer is

also used as a volume control in Hi-Fi systems and,

when it is worn or contaminated, you can often hear

loud crackles from the loudspeakers when youadjust the volume.)

Changes in the voltage supply to the ends of the

track will cause corresponding changes in the out-

put, so a stable supply might be required.

Advantages: The potentiometer gives a large out-

put voltage and is very simple in operation.

STRAIN GAUGE

When the distance to be moved is very small orwhen the movement is fairly rapid then the poten-tiometer is not very effective owing to its mechanicallimitations. A strain gauge is a very effective sensor

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23 Jul 04E04 Sensors.QXD

Input

Shaft

+9 V

Output

-9 V

0 V

Figure 2: Use of a Potentiometer to ConvertAngular Posit ion to a Voltage.

Input

Shaft

+9 V

Output

-9 V

-4.5 V

Figure 3: Use of a Potentiometer to Convert45 Down to a Vol tage of 4.5 V

Figure 4: Potentiometer connected to ControlSurface Actuator to Determine its Position


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when small distances or rotations have to be deter-

mined as it has no moving parts.A strain gauge is made from a thin strip of insulating

material on which is deposited a grid of a conductor,

made from an alloy (e.g. Copper/Nickel). The resis-

tance across the grid of conductor is, typically, about

100 . When the gauge is subjected to strain - a

change in length, a bend or a twist - then the dimen-

sions of the grid will change slightly and the distance

between its atoms will change slightly and its resis-

tance alters. The change in resistance is usually quite

small (e.g. from 100 to 100.2 for a small bend) but

it is proportional to the amount of strain so it increases

or decreases in a linear way.When an emf is connected across the strain gauge

then bending it causes the current to decrease by an

amount that depends on the amount of bending.

Because the change is very small, the signal requires

processing and amplification before it becomes use-

able. However, the advantage of the strain gauge is

that it has no moving parts and can respond quickly to

any changes.

One domestic use for strain gauges is in electronic

weighing machines (scales). The scale pan is support-

ed on a lever connected to a strain gauge and the

amount by which it bends under the weight of an object

is used by the electronic circuits to derive its weight.Strain gauges have even been placed under roads to

measure the weight of passing lorries.

One military use of the strain gauge is in the aiming

unit of J avelin/HVM. To scan the laser guidance grid, a

semi-conductor laser is made to move a few milli-

metres from side to side. The amount and timing of the

movement is monitored using a strain gauge connected

to the moving arm that supports the laser.This ensures

that an accurate grid is produced to guide the missile(s)

to the target. Strain gauges are also used to monitor

the movement of the motors that move the mirrors in

the unit (see Figure Five).

RESOLVERS

Both the potentiometer and the strain gauge sensorsare limited to small movements and rotations. Tomeasure over the full range of 360 a different system

is needed and one such system is the resolver. Its pur-

pose is to produce an electrical output over the full 360

of rotation that can be used to determine its orientation.

The resolver is a type of transformer - so it operates

on alternating current. The rotor is the primary of the

transformer and it is connected to an ac supply (e.g. 24

V and 400 Hz). There are two secondaries and they are

arranged at right-angles to each other, with the primary

in the middle, as shown in Figure Six. The dot at the

centre represents the axis of rotation.

For a transformer to operate, there must be a mag-

netic flux that links from the primary to the secondary.

When the current changes in the primary then the flux

changes also and that change reaches the secondary

where it produces an emf. For the orientation shown in

Figure Six, it will be secondary coil S1 that has maxi-mum flux linkage and, therefore, maximum emf. The

magnetic flux linkage (during one half-cycle of the ac

supply)is indicated by the grey lines in the Figure.

Coil S2 is oriented at right-angles to coil S1 and the

flux linkage is zero because the two parts of the flux

cancel out - look at the arrows on the flux linking

through coil S2 to see this.

Applying some Mathematics to this situation, gives

the results that the emf from coil S1 depends on the

Cosine of the angle of rotation whilst the emf from coil

S2 depends on the Sine of the angle of rotation, mea-

sured clockwise from the vertical.In the position shown in Figure Six, the angle is

Zero because the rotor is vertical. The emf in coil S1 is

V Cos 0 = V 1 = V whilst the emf in coil S2 is

V Sin0 =V 0 =0.

Connections to the rotor coil are made through car-

bon brushes and slip rings. These require little mainte-

nance because they carry only the small current to pro-

duce the magnetic field.

When the rotor has turned by 45 then the situation

is like that of Figure Seven. The flux linkage through

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E04 Sensors.QXD23 Jul 04

Figure 5: Strain Gauge, Used to Monitor theMirrors Movements, in Javelin Aiming Unit

S1

S2V Sin( )x

V Cos( )x V - acsupply

Rotor

Figure 6:Schematic Diagram of a Resolver


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coil S1 has reduced whilst that through coil S2 has

increased. Applying the Sine and Cosine factors,

Sine 45 =0.707 and Cos 45 =0.707 so the emfs from

the two coils are equal and about 71% of V.

For each position of the rotor, there will be a corre-

sponding set of Sine and Cosine values - some of

which will be negative. The negative values arise when

the flux direction reverses. This will occur in coil S1

once the rotor has turned through 90, when the flux

will be oriented down (previously it was up).

Output Waveforms: the outputs from the coils of

the resolver are alternating current. As the rotor isturned then the amplitude of these outputs changes as

described above and their phase changes too - when

the flux direction reverses. A set of waveforms for vari-

ous angles of orientation is shown in Figure Eight. Note

that the output from coil S1 reverses phase when the

rotor has turned past 90

In military systems, resolvers are generally used to

determine such things as the angle of elevation or

azimuth of a missile launcher. The signals from theresolver are passed to a computer which processes

them and calculates the value of the angle. In effect,

the resolver operates by splitting (resolving) a vector

(the angle of orientation of the rotor) into its horizontal

and vertical components. This is a similar process to

that used to convert range and bearing to Eastings and

Northings and is also called polar to rectangular con-

version.

Once the computer has the two voltages from the

resolver then they are divided to give the Tangent of

the angle required. This is illustrated in Figure Nine.

The advantage of dividing the two components to cal-culate the tangent of the angle is that this will cancel

any variations in the ac supply that is used to energise

the rotor. Any variation will affect both the top and bot-

tom parts of the division and have no effect on the

result.

The two signals, S1 and S2, are processed using a

circuit called a Phase-Sensitive Rectifier. This con-

verts the alternating signal into a direct voltage whose

magnitiude is proportional to the amplitude of the sine

wave and whose polarity depends on the phase of the

sine wave. You will encounter this circuit in more detail

during the Radar Theory part of the course, as these

circuits are used as part of the radar receiver.

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S1

S2V Sinx

V Cosx

V-ac

supply

45

45

Figure 7:Schematic Diagram of a Resolver withthe Rotor Turned Through 45

S1

S2

0

S1

S2

45

S1

S2

135

Figure 8: Resolver Waveforms at 0, 45 and 135

V Sin

V Cos

V

Figure 9: Resolving V into Sine & Cosine Components

45

45


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SYNCHROS

Asynchro is a device similar to a resolver and it isused in servo systems to determine when thealignment of an object has reached a pre-determined

setting. Note that this is different from the task per-

formed by a resolver - the resolver measures the align-

ment itself (e.g. elevation angle of 23 mils) but the syn-

chro is used to measure how far away from 23 mils ele-

vation, for example.

When a servo system is used to control the position

of objects such as missile launchers. The synchro also

provides information as to how far the object is mis-

aligned to enable the servo system to apply the correct

amount of force that would be required to drive the

object into alignment.

As with the resolver, the synchro does not have any

ends and can indicate continuous rotation. The signals

produced by a synchro, when used to detect misalign-

ment, are as follows:

When the object is correctly aligned then the electri-

cal output of the synchro is zero - in simple terms

this means that the servo does not need to move

the object any further.

When the object is misaligned clockwise then the

synchro might produce an alternating signal whose

amplitude increases with the amount of error.

When the object is misaligned anti-clockwise then

the synchro would produce an anti-phase signal

compared to that which it produced for a clockwise

error.

The construction of the synchro is illustrated

schematically in Figure Ten. Alternating current is con-

nected to the three stationary coils (stators) S1, S2 and

S3 to produce a magnetic field in the central region. By

varying the direction and amplitude of each current a

magnetic field can be produced at any orientation. (See

sidebar, right). The computer that is controlling the sys-

tem calculates what these currents should be and uses

HOW THE MAGNETIC FIELD (H) IS PRODUCED

BY THE CURRENTS (I) IN A SYNCHRO

To produce a magnetic field that points verticallydownwards, as shown in the Figure, above, analternating current is passed through coil S1 so that,

for one half-cycle, the current flows towards the cen-

tre of the synchro and for the next half-cycle then

the current flows away from the centre. That pro-

duces a magnetic field vertically downwards for the

first half-cycle and an upwards field in the second.

At the same time, an anti-phase current is

passed through coils S2 and S3. In the first half-cycle, these currents both flow away from the cen-

tre. Since the currents must balance, the currents in

coils S2 and S3 are each half the current in coil S1.

These currents produce a magnetic field oriented

down and to the left and right, as indicated in the

Figure.

The magnetic field at the centre of the synchro is

the vector sum of these three fields and it is oriented

as required, vertically down.

By adjusting the relative amplitude and direction

of the three currents, a field with any orientation can

be produced. Any increase in one current will cause

a corresponding decrease in one or both of the oth-ers and this keeps the strength of the field at a con-

stant value even though its orientation changes.

The rotor coil, which turns at the centre of this

field, acts as the secondary of a transformer. When

it is at right-angles to the field then there is no emf

induced in the rotor and that is taken as correct

alignment. At any other angle, the induced emf

forms an error signal that is amplified, processed

and fed to an electric or hydraulic motor to turn the

system back into alignment.

Minor variations in the voltage of the supply do

not affect this system because the rotor is driven tothe zero point and this is not affected by the supply.

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S1

S2S3

R

Figure 10: Schematic Arrangement of Rotor (R)and Stator (S) Coils of a Synchro

S1

S2S3

I

II

1

1

23 23

H

H H

Producing a Magnetic Field from the Three HComponents at the Centre of a Synchro


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digital to analogue converters to produce them. This

sets the reference direction for the synchro.

When a synchro is used to detect alignment errors

then it operates as follows:

An alternating magnetic field is established at right-

angles to the required direction of alignment.

The synchro rotor is connected to an amplifier that

operates a motor. Any emf that is induced in the

rotor will cause the motor to turn. When the rotor has moved to a position at right-

angles to the field then the emf induced in it is zero

and the motor stops turning.

Once the computer has set the reference direction,

as described above, then the servo system can be left

to move the sytem to that orientation. This leaves the

computer free for other taasks.

INDUCTOSYN

The potentiome-ter, resolver andsynchro are all

devices that mea-

sure the position of

an object so that it

can be controlled.

They are especially

suited for elevating

and traversing

weapons systems

where the system

has to be moved to a position and then halted there.

The antenna of a surveillance radar has to be con-

trolled in a different way because it keeps moving all

the time. The control system must keep it rotating at a

constant rate and also know where it is pointing at any

time - to a high degree of precision.

The Inductosyn is a device suited for measuring theposition of a radar antenna. The inductosyn uses a cir-

cular magnetic track on the rotating object and a sta-

tionary read head that detects changes in magnetism

during the rotation. (See Figure Twelve, above.)

A pattern of thousands of pairs of North/South magnet-

ic poles is formed on the track (during manufacture) in a

similar way to that used for storing data on video tape and

computer disks. As this pattern passes the read heads,

which are small coils of wire, an alternating emf is induced

in the coils. The computer can analyse this emf and deter-

mine the position and speed of the rotating object. The

read heads do not touch the track, although they are veryclose to it, so there is no friction or wear. Dust has no effect

because the magnetic field passes right through it.

A marker that indicates a reference point is usually

added to the track (on Rapier systems this will be in

line with the towing eye of the trailer) and this gives an

additional signal each time the antenna aligns with it.

Alternatively, two tracks can be laid down, side-by-side,

with one track having one set of poles fewer than the

other (N/N-1). The poles of the two tracks will only align

at one point around the circumference and this is used

as the reference point.

Thus, to monitor the position of the antenna, the

computer waits for it to pass the marker point and thencounts the number of North/South poles that it passes.

To monitor the speed of rotation of the antenna, the

computer simply counts the number of North/South

poles that it passes per second.

The synchro and resolver have only one pair of

magnetic poles per revolution whereas the inductosyn

can have several thousand. This enables the induc-

tosyn to measure angular position to the high accuracy

that is necessary to obtain an accurate azimuth for a

radar target. By placing the magnetic track on the mov-

ing object and the read heads on its base, there is no

need to have any slip rings connected to the induc-tosyn because there is no electrical connection to the

magnetic track.

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Figure 11:Synchro & Tacho

Used on Rapier B2Azimuth Gearbox

Figure 12: Inductosyn Track


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TACHO-GENERATORS

For effective control of a moving object, especiallywhen it is being moved from one position to anoth-er, it is necessary to measure the speed of the object in

addition to its position. This allows the control system to

move the object quickly when it has a long way to go to

its new position and to move it slowly as it approaches

the correct position. A device that produces an electri-

cal signal proportional to speed (more correctly - angu-

lar velocity) is called a tacho-generator.

DC Tacho-generator: The direct current tacho-gen-

erator contains a coil that rotates in a magnetic field

(usually a permanent magnet). This has the same basic

construction as any dc generator and it generates an

emf that depends on the rotational velocity. If the rota-

tional speed is doubled then the emf doubles. However,

the polarity of the emf will reverse if the generator is

turned in the opposite direction. Thus, the following out-

puts would be obtained from a tacho-generator that

gave 5 V for each 500 rpm:

500 rpm, clockwise = +5 V

500 rpm, anti-clockwise = 5 V

200 rpm, clockwise = +2 V

1000 rpm, anti-clockwise = 10 V

AC Tacho-Generator: This is a type of rotary trans-

former where the output is an alternating current at the

supply frequency. As the generator is turned faster then

the amplitude of the alternating current increases - but

not its frequency. This means that the ac tacho-genera-

tor is different from the alternator, because the outputfrequency of an alternator increases if its speed

increases whereas the tacho-generators does not. If

the ac tacho-generator is reversed then the phase of

the output is inverted. Thus, the following outputs would

be obtained from a tacho-generator that was energised

from a 24 V, 400 Hz supply:

500 rpm, clockwise =5 V, in-phase with supply.

500 rpm, anti-clockwise =5 V, anti-phase with supply.

200 rpm, clockwise =2 V, in-phase with supply.

1000 rpm, anti-clockwise =10 V, anti-phase with supply.

The tacho-generator is used in Rapier systems toenable the computer to monitor the speed of rotation of

the launcher in azimuth. (See Figure Eleven.)

MET SENSORS

When making measurements of meteorologicalconditions, it is necessary to measure tempera-ture, pressure, windspeed/direction and humidity. The

devices used might have to be small and lightweight,

so that they can be carried aloft in a balloon. Genreally,

the sensor is designed so that a change in the quantity

being measured will produce a change in the electrical

properties of the sensor.

Wind Speed: the cup anemometer uses the wind to

turn a small turbine and this, in turn, is connected to a

tachometer. The output voltage of the tachometer

depends on the wind speed. The drawback of this

method is that it has rapidly moving parts that are dun-

ject to wear and require maintenance. An alternative

form of anemometer allows the airflow to flow over a

heated wire - the amount of cooling depends on the

flow of air. Yet another form of anemometer uses a

pitot tube to measure the pressure exerted by the mov-

ing air as it is colected in a tube. When using a balloon,the balloon itself is used to obtain the windspeed as it is

blocn along by the wind and being tracked by radar or

some other means.

Wind Direction: this can easily be measured using

a weather vane attached to a resolver. Alternatively, a

pair of hot-wire or pitot-tube anemometers may be set

at right-angles to measure the easterly and northerly

components of the wind. The true direction can then be

calculated by combining these components, using

trigonometry.

Temperature: the thermistor is a small piece of sili-

con (or similar material) whose resistance decreaseswith temperature. This is easy to measure and to con-

vert into a temperature reading. Other temperature sen-

sors use the thermal expansion of a material to move

the plates of a variable capacitor.

Pressure: by making a baromter capsule in two

halves, with an insulating material separating top and

bottom, this acts as a variable capacitor. Changes in

atmospheric pressure change the distance between the

top and bottom surfaces of the capsule and, hence,

change the capacitance. Measuring the capacitance

gives a measure of pressure.

Humidity: some materials (e.g. extract of geranium

root, extract of seaweed) swell-up when moist andshrink when dry. This change in dimension can be used

to sense humidity. A mixture of seaweed extract and

carbon particles will have a low, electrical resistance

when dry, because the particles of carbon are close to

each other and the current can flow from one grain to

another. When the sensor encounters increased

humidity then the seaweed extract expands and this

increases the distance between the carbon grains. The

electrical resistance increases and this is easily mea-

sured. An alternative technique is to put the extract

between the plates of a capacitor. In this case, the

swelling of the material causes the separation of theplates to increase and this changes the capacitance.

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SUMMARY OF TERMS AND FORMULAE

Conversion between Celsius and Kelvin:

Kelvin Temperature =Celsius Temperature +273

Celsius Temperature =Kelvin Temperature - 273

The degree sign is NOT used with Kelvin temperatures.

Resolver:

Two outputs, S1 and S2. Arbitrary reference point:

when the rotor is parallel to S1 then the angle is zero

and:

emf from S1 = V Cos

emf from S2 = V Sin

The angle is clockwise. Negative values of this angle

arise when the rotor is turned anti-clockwise.

MORE INFORMATION

You can find out more about the devices that aredescribed in this handout by using an InternetSearch Engine, such as Google, and searching for

the name of the device. All these devices are used in

civilian systems and their manufacturers include much

technical data in their sales material.

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SELF-TEST QUESTIONS

1. A material to be used as a detector of infra-red (heat)

radiation must :

a. have a large number of free electrons.

b. operate at a high temperature.

c. be made from a material that does not allow

electrons to pass though.

d. have electrons that can be released by infra-red pho-

tons.

2. When a photon detector is used at low light levels

then integration is often used. This process:

a. cools the system.

b. accumulates the signal over a period of time.

c. gives a faster response over a shorter time.d. makes the electrons move faster.

3. When a component that is sensitive to light is shown

in a circuit diagram then it can be identified by the:

a. circle around the symbol.

b. square around the symbol.

c. two arrows pointing at the symbol.

d. two arrows pointing away from the symbol.

4. The potentiometer shown in Figure STQ 1 is being

used to monitor the position of an object that can rotate

through a maximum angular range of:

a. 270

b. 180

c. 90

d. 360

5. When a strain gauge sensor is bent or twisted then

the affect on its electrical properties is to:

a. increase its frequency.

b. increase its resistance.

c. decrease its wavelength

d. decrease the induced voltage.

6. The potentiometer shown in Figure STQ 1 is being

used to monitor the position of an object that can

rotate. As drawn, the object is oriented horizontally, to

the left. In this position, the output from the potentiome-

ter will be:

a. +15 V

b. 15 V

c. Zero Volts.

d. +5 V

7. One advantage of using a strain gauge instead of a

potentiometer to determine the position of an object is

that the strain gauge has:

a. a bigger output.

b. the ability to rotate over 360

c. no moving parts.

d. no need for amplification.

8. A strain gauge sensor, when used to determine the

position of an object is suitable for objects that:

a. move through small distances.

b. move through large distances.

c. rotate continuously.

d. do not move quickly.

9. A device that can be used to determine the orienta-tion of an object at any angle would be a:

a. potentiometer.

b. strain gauge.

c. tacho-generator.

d. resolver.

10. In Figure STQ1, to make the output zero, rotate by:

a. 90, clockwise.

b. 45, clockwise.

c. 180.d. 45, anti-clockwise.

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Input

Shaft

+15 V

Output

-15 V

FigureSTQ 1

1.Theremustbeenoughenergyinaphoton(d)2.Integrationaccumulatestheelectrons(b)

3.Light-sensitivewhentwoarrowstowards(c)4.Potentiometerslimitedtoabout320(a)5.Straingaugesincreaseresistance(b)

6.Byproportion,outputis5V(d)7.Straingaugehasnomovingparts(c)8.Straingaugessensitivetosmallmovements(a)

9.Resolvercandofull360(d)10.45,clockwise-midpointoftrack.(b)


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11. The diagram of Figure STQ2 shows a resolver ori-

ented at 45 to the vertical. In this position:

a. coil S1 has maximum output and coil S2 has

zero.

b. coils S1 and S2 have equal, but not zero, outputs.

c. coil S1 has zero output and coil S2 has maxi-

mum output.

d. both coils have zero output.

12. In the diagram of Figure STQ2, which shows a resolver, if

the rotor were turned another 45 clockwise then:

a. the voltage from coil S1 would increase.

b. the voltage from both coils would increase.

c. the voltage from coil S2 would become zero.

d. the voltage from coil S1 would become zero.

13. When the rotor of a synchro transformer is correctly

aligned then its output voltage is:

a. zero.

b. in-phase with the supply.

c. in anti-phase with the supply.

d. a maximum value.

14. When a synchro transformer is used to determine

the error in a control system then the direction of the

error is obtained from the signals:

a. phase.

b. frequency.

c. amplitude.

d. current.

15. An inductosyn differs from a resolver in that the

inductosyn has:

a. thousands of magnetic poles.

b. only one pair of magnetic poles.

c. stator coils set at right-angles to each other.d. only one set of slip rings.

16. A sensor that would be suitable for measuring the

azimuth of a rotating antenna would be a:

a. potentiometer.

b. strain gauge.

c. inductosyn.

d. synchro transformer.

17. A dc tacho-generator produces +5 V when turned

at 1 000 rpm in a clockwise direction. If this device is

turned at 500 rpm in an anti-clockwise direction then its

output voltage would become:

a. 2.5 V

b. +2.5 V

c. 5 V

d. +5 V

18. An ac tacho-generator produces 8 V at 200 Hz

when turned clockwise at 200 rpm. When its signal is

2 V at 200 Hz and in anti-phase with the original then it

is being turned:

a. clockwise at 800 rpm.

b. anti-clockwise at 200 rpm.

c. anti-clockwise at 50 rpm.

d. anti-clockwise at 100 rpm.

Sensors


S1

S2

V-acsupplyFigure STQ 2 11.At45,bothareequal&non-zero(b)

12.ZeroVwhenastatorcoilat90torotor(d)

13.ZeroVwhensynchrorotoratzeropoint(a)14.Phaseofsignalindicatesdirection(a)15.Manypolesoninductosyntrack(a)

16.Inductosyngoodforcontinuousrotation(c)17.Halve&reversepolarityto2.5V(a)

18.Reducebyfourtimes&reversedirection(c)


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bst handout e04

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