module ii transducers

8/6/2019 Module II Transducers

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Module II

Transducers


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Transducers

An electronics instrumentation system consists of a number ofcomponents to perform a measurement and record its result.

A measurement system consist of three major components

1. An input devices,

2. A signal conditioning or processing device,

3. An output devices.


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Contd

Transducer: a device which converts one form ofenergy to another

Sensor: a transducer that converts a physicalphenomenon into an electric signal. an interface between the physical world and the computing

world.

Actuator: a transducer that convertselectric signal to a physical

phenomenon


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Contd

Transducer

Transducers as which, when actuated transforms energy

from one form to another. A device which convert a physically parameter such as heat ,

intensity of light , flow rate , liquid level, humidity and pH

value into electrical form by means of transducers.


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Contd

These transducers provide an output signal when stimulated

by a mechanical or non-mechanical input

A photoconductor convert light intensity into change of

resistance A thermocouple convert heat energy into electrical voltage

A force produces a change of resistance in strain gauge

An acceleration produces a voltage in a piezo-electrical

crystal and so on.


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Electrical transducers

In electrical system to measure non-electrical quantities a

detector is used which usually convert the physical quantity

into displacement , this displacement actuates an electrical

transducer, gives an output that is electrical in nature. The electrical signal may be a current be a current or a voltage

or a frequency and production of these signals is based upon

electrical effects


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Advantage ofElectrical transducers

Electrical amplification and attenuation can be done easily

The mass-inertia effects are minimized

The effects of friction are minimized

The electrical system very less power

Telemetry is used in almost all sophisticated measurement

systems

Miniaturization on account of use ofICs


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Contd

Note:

1. The reason for transforming a physical phenomenon into

electrical form is that the electrical output can be easily used,

transmitted and processed for purpose of measurement.

2. Another name of transducer is pick up.


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Contd

The transducer may be consist two part

1. Sensing or detector element: It is that part oftransducer which respond to a physical phenomenon or

change in a physical phenomenon.

2. Transduction element : A transduction elementtransforms the output of a sensing element to an electrical

output. The transduction element , in a way acts as a

Secondary transducer


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Classification oftransducers

The transducers can be classified

(i) On the basis of transduction form used,

(ii) as primary and secondary transducers,

(iii) as passive and active transducers,

(iv) as analog and digital transducers and

(v) as transducers and inverse transducers


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(1) Classification based upon principal of Transduction: Basis

of principal of transduction as resistive, inductive,

capacitive.

Example: piezoelectric, thermoelectric, potentiometer devices,photoconductive cell etc.


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Contd

(ii) Primary and secondary transducer:

Example: Convert pressure into displacement by help of

Bourdon's tube and linear variable differential transformer

(iii) Passive and active

Example :resistive, inductive and capacitive transducers

( Passive)

Example: Thermocouples, photovoltaic cells etc( Active)


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Contd

(iv) Analog transducers: Convert input quantity into analog

output

Example: Thermocouple, thermistor, strain gauge, LVDT etc.

Digital Transducer: convert input quantity into electrical pulse

(v) Inverse transducers: Convert an electrical quantity into non-

electrical

Example: Oscilloscopes, Convert electrical signal intomechanical movement


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Selection ofatransducers

The following factor should be considered while selecting a

transducer:

1. Operating range : Chosen to maintain range requirements

and good resolution2. Sensitivity: The transducer must be sensitivity enough to

produce detectable output.

3. Accuracy:


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Types of transducer based on principal

Resistive transducers

Capacitive transducers

Variable inductance transducers


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Resistive Transducer

Resistive transducers are those in which resistance the

resistance changes due to a change in some physical

phenomenon.

The change in the value of the resistance with a change in thelength of the conductor can be used to measure displacement.

strain gauge used for measurement of displacement, force and

pressure. Its work on the principal that the resistance of a

conductor or semiconductor change when strained.

The resistive of materials change with change in temperature.

This property can be used for the measurement of temperature.


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Contd

Resistive transducer:

Potentiometer

Strain gauge

Rosettes

Thermistors

RTD ( Resistance Temperature Detector)


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Capacitive transducers

Measurement of liquid level by change in variationof dielectric constant


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Inductance Transducers

Self Generating type and passive type

Piezoelectric Transducers

Photoelectric transducers

Digital transducer


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Potentiometer

A resistive potentiometer (pot) consists of a resistanceelement provided with a sliding contact, called a wipe.

The motion of the sliding contact may be translatory or

rotational. The potentiometer is a passive transducer

since it requires an external power source for its operation

Note : On the basis of motion these are three types

(i) Translatory Type (ii) Rotation Type (iii) Helipot


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Advantage of potentiometer

They are inexpensive Simply to operate

They are useful fore the measurement of large amplitudes of

displacement.

Electric efficiency is very high, and they provide sufficient

output to allow control operation.


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Disadvantages

When using linear potentiometer, a large force is required tomove the sliding contact

The sliding contact can wear out, become misaligned and

generate noise.


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Numerical

A displacement transducer with a shaft stroke of 4.0 inch isapplied to the circuit. The total resistance of the potentiometer

is 5Kohm. The applied voltage Vt is 5V. When the wiper is 1.2

inch from B, What is the value of the output voltage?


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Solution

R2= (1.2 inch/4.0 inch) * 5 K= 1500 ohm

Then Vo= Vt* / R2+R1=5V*15000hm/5K=1.5


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Strain Gauge

Strain

Definition: Strain is the definition of how much a material has

been stretched (or compressed) when compared to its original

length. The most common measure of strain in metals is calledengineering strain, calculated as the change in length divided

by the original length. For example, a 2.0" titanium bar that

has been stretched to 2.2" is said to have experienced a tensile

strain of 0.1, or 10%.


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Types ofstarin

Linear Strain

Lateral Strain

Volumetric Strain Shear Strain


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Contd

Strains

Strain is defined a the ratio of change in dimension to original

dimension of a body when it is deformed. It is a dimensionless

quantity as it is a ratio between two quantities of samedimension.

module ii transducers

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