5.4 instrumentation systems

8/12/2019 5.4 Instrumentation Systems

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Topic 5.4 Instrumentation Systems

Learning Objectives:

At the end of this topic you will be able to;

describe the use of the following analogue sensors:

thermistors and strain gauges;

describe the use of the following digital sensors:slotted discs (for sensing rotational speed),

encoded discs (for sensing angular position);

recall the Gray code (3 bit) and explain its use in encoded discs;

design and analyse sensor sub-systems which incorporate thermistors

and strain gauges in bridge circuits;

recall the adantages of a bridge circuit compared to a simple oltage

diider circuit;

recall, and explain the significance of, the ideal properties of an

instrumentation amplifier ! high input impedance and high common-

mode re"ection ratio;

analyse and design instrumentation amplifiers based on the op-amp

difference amplifier circuit;

select and use the formula:

#$%&' #**(+* +);

design a logic system to process the output of slotted and encodeddiscs to meet a gien specification.

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Module T5

lectronic Systems !pplications.

!nalogue sensing units:

An analogue signal can hae any oltage alue, limited only, usually, by theoltages of the power rails.

/ome deices hae a resistance which responds to changes in their

surroundings. *or example, a 0+ (0ight ependent +esistor) has a resistance

which decreases when more light falls on it. &here are two 1inds of

thermistor (temperature dependent resistor). &he ptc (positie temperature

coefficient) thermistor has a resistance that increases as its temperature

rises. &he ntc (negatie temperature coefficient) thermistor has a resistancethat decreases as its temperature rises. &his course considers only ntc

thermistors.

&he simplest form of sensing unit is made by connecting

one of these deices in series with a resistor.

&he output signal is ta1en from the point where the

resistor is connected to the deice.

&he diagram shows this arrangement used in a

temperature sensing unit. &he analogue output signal

changes as the temperature changes.

*or example, suppose that:

the thermistor has a resistance of 1. at a temperature of 24;

the ariable resistor is set to a resistance of 21;

the supply oltage #/' 2#.

&he output oltage is obtained from the oltage diider formula:

#$%&' #/(+ +5 +2)

n this case, at 24,

#$%&' 2 x (2 2 5 ) ' 6#.

"ercise # (&he solutions are gien at the end of the topic.)

At

4, a thermistor has a resistance of .71. &he ariable resistor isunchanged. 4alculate the output oltage of this temperature sensing unit at

64.

2

#

$ u t p u t

3 #

/

& h e r m i s t o r

# a r i a b l er e s i s t o r


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! strain gauge

8hen truc1s drie oer a bridge, or someone stands on bathroom scales, the

structure is s9uashed slightly. /train is defined as this change in length

diided by the original length, and strain gauges are used to measure it.

&he layout of a typical strain gauge is shown in the

picture. t is often glued to the structure, and so

is distorted when the structure is distorted.

&his changes the resistance of the strain gauge.

*or example, when a straight wire is stretched, it gets longer and thinner. Asa result, its resistance increases.

y measuring the change in resistance, we can monitor the strain thatproduced it.

&he strain gauge could be incorporated into a oltage

diider circuit, shown opposite, which behaes li1e the

one "ust considered for a temperature sensor.

oweer, there are problems with this 1ind of sensor:

. &he resistance of the sensor may change because of some factor otherthan the one you are trying to measure. *or example, the resistance of a

strain gauge changes if the strain gauge gets hot. &his has nothing to do

with any forces applied to it.

2. 0oo1 at the oltage diider formula again:

#$%&' #/(+ +5 +2)

As this shows, the output oltage depends on the supply oltage.

n many situations, the supply oltage will fluctuate.

&he system may be battery-powered and using batteries that aregoing flat.

3

#

$ u t p u t

3 #

/

/ t r a i ng a u g e

# a r i a b l er e s i s t o r


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Module T5


&he system may be using a mains power supply, which does not

hae good line regulation.

&he power supply cables might be sub"ect to electrical noise,which changes the instantaneous alue of the supply oltage.

n other words, you cannot 1now whether a change in the output indicates

a change in the factor you are trying to monitor, or is the result of a

change in some other factor in its surroundings.

! better sensing circuit:

oth of the problems outlined aboe can be oercome or reduced by using abridge circuit, (though this title does not refer to the bridges that truc1s

drie oer. $ne is sub"ected to

the temperature changes under inestigation. &he other is not. &hat is the

only difference. $therwise, both thermistors are exposed to the same

conditions.

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$ull measurement tec%ni&ue:

%sually, the bridge is ?balanced@ initially. n other words, the ariable resistoris ad"usted until #$%&is ero.

n this condition:

+esistance of = ' +esistance of >

+esistance of ariable resistor +esistance of +

Bxercise 2 will show that in this condition, the power supply oltage ma1es no

difference at all. Any alue can be used, but two conflicting issues need to be

considered:

the higher the supply oltage, the more sensitie the output oltage is

to changes in temperature (for the temperature sensing bridge circuit.)

the higher the supply oltage, the greater the self-heating effect of all

the resistors in the circuit.

Any changes in the condition being monitored, e.g. temperature, ma1es the

bridge unbalanced, meaning that #$%&is no longer ero. %sing this null

measurement ma1es it possible to detect ery small changes in conditions, by

connecting the bridge circuit to a high gain oltage amplifier, as outlined

below.

! strain gauge bridge circuit:

As in the temperature sensing bridge circuit, there are two sensing deices.

n this case, they are labelled ?/train gauge@ and ?ummy strain gauge@, thoughtheir positions in the bridge can be reersed.

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Module T5


&he ?/train gauge@ is glued to the structure in such a way that it is distorted

by moement of the structure. &he ?ummy strain gauge@ is glued nearby so

that it is exposed to the same conditions, except for the distortion.

$ften, the two strain gauges are

formed on the same substrate,

as shown in the diagram.

"ercise ' (&he solutions are gien at the end of the topic.)

A thermistor bridge circuit is shown opposite.

&he power supply oltage #/' 2#

&he ariable resistor is ad"usted until the bridge

is balanced, i.e. the output #$%&' #. t then has

a resistance of exactly 2.71.

&hermistor > is found to hae a resistance

of .21.

(a)4alculate the resistance of thermistor =.

(b)&he power supply oltage is changed to #.

4alculate the new output oltage #$%&.

"ercise ( (&he solutions are gien at the end of the topic.)

nitially, the strain gauge bridge circuit is

balanced by ad"usting the ariable resistor,

which then has a resistance +' C.

0ater, a force is applied to strain gauge /,

and its resistance becomes 3DD.

&he dummy strain gauge, , is unaffected,

and still has a resistance of 37.

+2is a fixed resistor, with resistance C.

%se the oltage diider rule twice to calculate

the oltages at A and when the force is applied, and hence calculate the

output oltage #$%&.(All your calculations should be 9uoted to two decimal places.)

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Instrumentation ampli)iers:

&he output from a bridge circuit is usually only a few milliolts. t is usuallyamplified by a high gain oltage amplifier 1nown as an instrumentation

amplifier.

deal characteristics:

igh input impedance ! &his ensures that as much as possible of the

oltage signal from the bridge circuit is transferred to the

instrumentation amplifier. &he current flowing in the wires connecting

the two sub-systems is ery small, and so the oltage dropped acrossthe output impedance of the bridge circuit, (and so not transferred to

the amplifier) is 1ept to a minimum.

igh common-mode re"ection ratio !on both inputs. &here will be steady

4 oltages on the outputs of the two oltage diiders that ma1e up the

bridge circuit. (Eou calculated these earlier.) =art of this 4 oltage will

appear on, ( be commonto) both outputs. A high 4F++ ensures that the

instrumentation amplifier ignores (re"ects) these, and amplifies only thedifference between these oltages signals.

&he diagram shows the circuit for an

instrumentation amplifier, used in the

8B4 specification. t is based on an

op-amp difference amplifier. 8ith

careful design, it can match the ideal

characteristics ery well. t consists of

two pairs of resistors, labelled +and +*

and an op-amp. n practice, the circuits

for instrumentation amplifiers are more complex.

8hen used to amplify the output of a bridge circuit, input is connected to

output A, and input 2 to output of the two oltage diiders that ma1e up

the bridge circuit.

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&he next section is H$& examinable< t shows you how to analyse the difference amplifier

circuit, using the rules for op-amps deeloped in earlier modules. f you can follow this, it

will help you to understand the way the circuit wor1s.

*+ules, )or op-amp be%aviour:

f the output is not saturated, the two inputs sit at the same oltage;

&he input impedance of the inputs is so big, they draw negligible current;

&he circuit shown will be used for

this analysis. &he sies of the

input oltages are much greater

than we would expect from abridge circuit, but will ma1e the

arithmetic easier

5.4 instrumentation systems

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