Chapter 7 Sensors and Transducers

7/31/2019 Chapter 7 Sensors and Transducers

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Chapter 7

SENSORS AND

TRANSDUCERS


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Sensor and Transducer

A transducer is defined as a device that receives energy from one system

and transmits it to another, often in a different form.

A sensor is a device that responds to a physical stimulus (as heat, light,sound, pressure, magnetism, or a particular motion) and transmits aresulting impulse (a signal relating to the quantity being measured).

For example, certain sensors convert temperature into a change inresistance.


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Active and Passive Transducers

Active transducer

generates an electrical signal directly inresponse to the physical parameter anddoes not require an external power

source for its operation.self generating devices, which operate

under energy conversion principle andgenerate an equivalent output signal(for example from pressure to chargeor temperature to electrical potential).

Typical example of active transducers

are piezo electric sensors (forgeneration of charge corresponding topressure) and photo voltaic cells (forgeneration of voltage in response toillumination).

Passive transducer

operate under energy controllingprinciples, which makes it necessary touse an external electrical source with

themdepend upon the change in an

electrical parameter (R, L and C).

Typical example are strain gauges (forresistance change in response topressure), and thermistors (forresistance change corresponding to

temperature variations).


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Electrical Transducer

An electrical transducer is a sensing

device by which the physical,mechanical or optical quantity to bemeasured is transformed directly by asuitable mechanism into an electricalvoltage/current proportional to theinput measurand.


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Use of Electrical Transducers

Electrical transducers are used mostly to measure non-electricalquantities.

For this purpose a detector or sensing element is used, which convertsthe physical quantity into a displacement.

This displacement actuates an electric transducer, which acts as asecondary transducer and give an output that is electrical in nature.

This electrical quantity is measured by the standard method used forelectrical measurement.

The electrical signals may be current, voltage, or frequency; theirproduction is based on R, L and Ceffects.


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Transducer Components

A transducer which converts a non-electrical quantity into an analogelectrical signal may be considered as consisting of two parts, the sensingelement, and the transduction element.

The sensing or detector element is that part of a transducer whichresponds to a physical phenomenon or to a change in a physicalphenomenon. The response of the sensing element must be closely relatedto the physical phenomenon.

The transduction element transforms the output of a sensing element toan electrical output. This, in a way, acts as a secondary transducer.


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Transducer Category

Transducers may be further classified into different categoriesdepending upon the principle employed by their transductionelements to convert physical phenomena into output electrical signals.

The different electrical phenomena employed in the transductionelements of transducers are as follows.

Resistive Photo-emissive

Inductive Photo-resistiveCapacitive Potentiometric

Electro magnetic Thermo-electric

Piezo-electric Frequency generating


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Resistive Transducers

Resistive transducers arethose in which the

resistance changes due toa change in some physicalphenomenon. The change

in the value of theresistance with a change in

the length of the

conductor can be used tomeasure displacement.

Strain gauges work on theprinciple that the

resistance of a conductoror semiconductor changeswhen strained. This can beused for the measurementof displacement, force and

pressure.

The resistivity of materialschanges with changes in

temperature. Thisproperty can be used for

the measurement oftemperature.


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Resistive Potentiometer

A resistive potentiometer (pot) consistsof a resistance element provided with a

sliding contact, called a wiper. Themotion of the sliding contact may be

translatory or rotational.


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Resistance Pressure Transducer

Measurement in the resistive type of transducer is basedon the fact that a change in pressure results in a resistance

change in the sensing elements. Resistance pressuretransducers are of two main types. First, theelectromechanical resistance transducer, in which achange of pressure, stress, position, displacement or othermechanical variation is applied to a variable resistor. Theother resistance transducer is the strain gauge, where thestress acts directly on the resistance. It is very commonlyused for stress and displacement measurement ininstrumentation.


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Resistance Pressure

Transducer


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Resistive Position Transducer

A common requirement in industrialmeasurement and control work is to be able to

sense the position of an object, or the distance ithas moved.

One type of displacement transducer uses aresistive element with a sliding contact or wiper

linked to the object being monitored ormeasured. Thus the resistance between the sliderand one end of the resistance element dependson the position of the object.


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Resistive Position Transducer


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Strain Gauges

The strain gauge is an example of a passive transducer that uses the variation in electrical resistance inwires to sense the strain produced by a force on the wires.

It is well known that stress (force/unit area) and strain (elongation or compression/unit length) in amember or portion of any object underpressure is directly related to the modulus of elasticity.

Since strain can be measured more easily by using variable resistance transducers, it is a commonpractice to measure strain instead of stress, to serve as an index of pressure. Such transducers arepopularly known as strain gauges.

If a metal conductor is stretched or compressed, its resistance changes on account of the fact that boththe length and diameter of the conductor changes. Also, there is a change in the value of the resistivityof the conductor when subjected to strain, a property called thepiezo-resistive effect. Therefore,resistance strain gauges are also known aspiezo resistive gauges.


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Strain Gauges

A tensile stress tends to elongate thewire and thereby increase its lengthand decrease its cross-sectional area.

The combined effect is an increase inresistance, as seen from thefollowing equation


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Resistance Thermometer

The resistance of a conductor changes when its temperature is changed. This property isutilised for the measurement of temperature. The resistance thermometer is aninstrument used to measure electrical resistance in terms of temperature, i.e. it uses the

change in the electrical resistance of the conductor to determine the temperature.

The main part of a resistance thermometer is its sensing element.

The sensing element may be any material that exhibits a relatively large resistance changewith change in temperature.


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Resistance Thermometer


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Thermistor

Thermistor (THERMally sensitive resISTOR) arenon-metallic resistors (semiconductor material),

made by sintering mixtures of metallic oxides suchas manganese, nickel., cobalt, copper and uranium.

Thermistors have a Negative TemperatureCoefficient (NTC), i.e. resistance decreases astemperature rises.


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Thermistor


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Capacitive Pressure Transducer

A linear change in capacitance withchanges in the physical position ofthe moving element may be used toprovide an electrical indication of theelement's position. The capacitance is

given by

where

K= the dielectric constant

A = the total area of the capacitor surfaces

d = distance between two capacitive surfaces

C= the resultant capacitance.


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Capacitive Pressure

Transducer


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5k

5k

12V10k

Vout

A

conducti

ng strip

shaft

resistive position

transducer

3

cm

20k

R2

R3 R4

R1Vout

VinA B

10k

10k

(a) (b)

Quiz

A resistive position transducer with a resistance of 20 kand a shaft stroke of10 cm with a bridge circuit is used to display the height of a staircase bymoving it to the right as shown in the figure. The initial position to be used asa reference point is when the shaft is at midstroke like in figure (a). Theequivalent circuit of the system is shown in figure (b)

Derive the formula for Vout in terms of the value of resistors in the circuit

What will the value of Vout be if the shaft has reached point A?

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Chapter 7 Sensors and Transducers