Transducer 1s682k

UNIT 7 TRANSDUCERS

TRANSDUCERS  It’s a device which convert one form of energy to another form  Non electrical quantity is converted into an electrical form by a transducer.  Another name is pick up

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Advantage of Electrical Transducers  Electrical amplification and attenuation can be done easily.  Mass inertia effects are minimized.  The effect of friction is minimized.  The electrical or electronic systems can be controlled with a very small power level.

Conti..  The electrical output can be easily used , transmitted and processed for the purpose of measurement.  Telemetry (aerospace – remote indication / recording)  Miniaturization on of use of IC’s.

Two parts/element of transducer  Sensing or detector element. A detector or a sensing element is that part of a transducer which responds to a physical phenomenon or a change in a physical phenomenon.  Transduction element. A transduction element transforms the output of a sensing element to an electrical output. The transduction element in a way acts as a secondary transducer.

Classification of Transducers  On the basis of transduction form used.  As primary and secondary transducers  As ive and active transducer.  As analog and digital transducer.  As transducers and inverse transducers.

Classification based upon principle of transduction  Resistive  Inductive  Capacitive etc Depending upon how they convert the input quantity into resistance, inductance or capacitance respectively. Eg piezoelectric, thermoelectric, magneto restrictive, electro kinetic and optical

Primary and Secondary Transducers LVDT (Linear Variable Differential Transformer) Primary- Pressure to displacement (bourdon tube) Secondary-Displacement into analogous voltage (LVDT).

ive and Active Transducer  Active Transducer: Also known as self generating type, develop their own voltage or current from the physical phenomenon being measured. Velocity , temperature , light intensity and force can be transduced with the help of active transducer.

Conti..  ive Transducer: Also known as externally powered transducers, i.e., derive the power required for energy conversion from an external power source. e.g. POT (Potentiometer)-used for the measurement of displacement .

Analog and Digital Transducer.  Analog Transducers : It converts the input quantity into an analog output which is a continuous function of time.  E.g. LVDT, Thermocouple or a thermistor (gives output which is continuous function of time)

Conti..  Digital Transducer: Converts input quantity into an electrical output which is in the form of pulse.

Transducers and Inverse Transducers  Transducer: Non electrical to electrical quantity  Inverse transducer: Electrical quantity into non electrical quantity.

Characteristics and Choice of Transducer  Input Characteristics  Transfer Characteristics  Output Characteristics.

Input Characteristics  Type of Input and Operating Range  Loading effect. Type of Input :The type of input, which can be any physical quantity, is generally determined in advance . Operating Range : Choice of transducer depends upon the useful range of input quantity.

Conti..  Loading Effect : The transducer, that is selected for a particular application should ideally exact NO force, power or energy from the quantity under measurement in order that is measured accurately.

Response of transducer to environmental influences.  It should not be subjected to any disturbances like stray electromagnetic and electrostatic fields, mechanical shocks and vibrations temperature changes, pressure and humidity changes, changes in supply voltage and improper mechanical mountings.

Output Characteristics  Type of Electrical Output.  Output Impedance  Useful Range.

Type of Electrical Output.  The type of output which may be available from the transducers may be available from the transducers may be a voltage, current , impedance or a time function of these amplitudes.

Output Impedance  Ideally the value of output impedance should be zero if no loading effects are there on the subsequent stage.  Since zero output impedance is not possible , it should be kept as low as possible, since it determines the amount of power that can be transferred to the succeeding stages of the instrumentation system.

Useful Output Range  The output range of a transducer is limited at the lower end by noise signal.  The upper limit is set by the maximum useful input level.

Factors Influencing the choice of Transducer. 1. 2. 3. 4. 5. 6. 7.

Operating Principle Sensitivity Operating Range Accuracy Cross sensitivity Errors Transient and frequency response

Conti.. 8. Loading effects. 9. Environmental compatibility 10. Insensitivity to unwanted signals 11. Usage and Ruggedness 12. Electrical aspects 13. Stability and Reliability 14. Static characteristics.

1. Operating Principle: The transducer are many times selected on the basis of operating principle used by them. The operating principle used may be resistive, inductive, capacitive , optoelectronic, piezo electric etc. 2. Sensitivity: The transducer must be sensitive enough to produce detectable output. 3. Operating Range: The transducer should maintain the range requirement and have a good resolution over the entire range.

4. Accuracy: High accuracy is assured. 5. Cross sensitivity: It has to be taken into when measuring mechanical quantities. There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan. 6. Errors: The transducer should maintain the expected input-output relationship as described by the transfer function so as to avoid errors.

7. Transient and frequency response : The transducer should meet the desired time domain specification like peak overshoot, rise time, setting time and small dynamic error. 8. Loading Effects: The transducer should have a high input impedance and low output impedance to avoid loading effects.

9. Environmental Compatibility: It should be assured that the transducer selected to work under specified environmental conditions maintains its input- output relationship and does not break down. 10. Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals.

11. Usage and Ruggedness: The ruggedness both of mechanical and electrical intensities of transducer versus its size and weight must be considered while selecting a suitable transducer. 12. Electrical aspects: The electrical aspects that need consideration while selecting a transducer include the length and type of cable required. 13. Stability and Reliability : The transducer should exhibit a high degree of stability to be operative during its operation and storage life.

14. Static Characteristics :Apart from low static error, the transducer should have a low non- linearity, low hysteresis, high resolution and a high degree of repeatability.

Resistive Transducers R

L

,

A R  resistance;  L  length of conductor; m A  cross- sectional area of conductor;m   resistivity of conductormaterial;  - m 2

 Any method of varying one of the quantities involved in the above relationship can be the design basis of an electrical resistive transducer.  The translational and rotational potentiometers which work on the basis of change in the value of resistance with change in length of the conductor can be used for measurement of translational or rotary displacement.

 Strain gauge work on the principle that the resistance of the conductor or a semiconductor changes when strained. This property can be used for measurement of displacement, force and pressure.  The resistivity of the material changes with change of temperature thus causing a change of resistance. This property may be used for measurement of temperature.

Potentiometers  POT  Resistive potentiometer used for the purposes of voltage division is called POT.  Resistive potentiometer consist of a resistive element provided with a sliding .  Sliding -Wiper

POT  It’s a ive Transducer.  Linear Pot –Translational Motion  Rotary Pot-Rotational Motion  Helipots- Combination of the two motions (translational as well as rotational).  In Electrical Measurement , Standard potentiometer are used to measure the unknown voltage by comparing it with a standard known voltage.

Resistive potentiometer

Translational, rotational and helipots

Strain Gauges  If a metal conductor is stretched or compressed , its resistance changes on of the fact that both length and diameter of conductor change. Also there is a change in the value of resistivity of the conductor when strained and this property is called piezoresistive effect. Resistive strain gauges are also known as piezoresistive gauges.

Types of strain gauges       

Unbonded metal strain gauge Bonded metal wire strain gauge Bonded metal foil strain gauge Vacuum deposited thin metal film strain gauges. Sputter deposited thin metal film strain gauge. Bonded semiconductor strain gauges. Diffused metal strain gauge.

Unbonded metal strain gauge

 Used almost exclusively in transducer applications.  At initial preload , the strains and resistances of the four arms are normally equal, with the result the output voltage of the bridge, e0=0.  Application of pressure produces a small displacement , the displacement increases tension in 2 wires and decreasing the resistance of the remaining 2 wires.  This causes an unbalance of the bridge producing an output voltage which is proportional to the input displacement and hence to the applied pressure.

Bonded metal wire strain gauge

 It consist of a grid of fine resistance wire of diameter of about 0.025mm.  The wire is cemented to a base.  The base – thin sheet of paper or bakelite.  Wire is covered with a thin sheet of material so that it is not damaged mechanically.  The spreading of wire permits a uniform distribution of stress over a grid.

Bonded metal foil strain gauge

Extension of the bonded metal wire strain gauge.

The bonded metal wire strain gauge have been completely superseded by bonded foil strain gauge.

Metal foil strain gauge

Semiconductor strain gauge.

The semiconductor strain gauge depends for their action upon piezo resistive effect. i.e. the change in the value of the resistance due to change in resistivity.

Rosettes

Requirements of a conductor material to be used in RTDs are  The change in resistance of material per unit change in temperature should be as large as possible.  The material should have a high value of resistivity so that minimum volume of material is used for the construction of RTD.  The resistance of material should have a continuous and stable relationship with temperature.

Thermistors  “thermal - resistors”  It is composed of semiconductor materials.  Used in applications which involve measurements in the range of -60oC to 15oC  The resistance of thermistors ranges from 0.5ohm to 0.75 mega ohm

Thermistors

Composed of sintered mixture of metallic oxides such as manganese, nickel , cobalt, copper, iron and uranium.

Thermocouple  When two metals having different work functions are placed together, a voltage is generated at the junction which is nearly proportional to the temperature. This junction is called a thermocouple.

Variable Inductance Transducer  Change in self Inductance  Change in Mutual Inductance.  Production of eddy currents.

Transducers working on principle of change of Self Inductance.  Self inductance of a coil

L  N2 R where N  Number of turns R  reluctanceof the magnetic circuit  l μA

N2 N 2μA A  InductanceL    N 2μ  N 2μG l μA l l A  geometric form factor l A  Area of crosssection of coil; m 2 l  length of coil; m   permeability whereG 

Transducers working on principle of change of Mutual Inductance.  Uses multiple coils.  The mutual inductance between two coils is M  K L1L 2 where L1 and L2  self inductances of two coils K  co  efficient of coupling

Transducers working on principle of production of eddy currents  If a conducting plate is placed near a coil carrying alternating current, eddy currents are produced in the conducting plate.

Linear Variable Differential Transformer (LVDT)  The transformer consists of single primary winding P and two secondary windings S1 and S2 wound on a cylindrical former.  The secondary windings have equal number of turns and are identically placed on either side of the primary winding.  The primary winding is connected to an alternating current source.

LVDT

 A movable soft iron is placed inside the former.  The displacement to be measured is applied to the arm attached to the soft iron core.  Since the primary winding is excited by an alternating magnetic field which in turn induces alternating current voltages in the secondary windings.  The output voltage of secondary , S1is Es1 and that of secondary, S2is Es2

Capacitive Transducer  The Principle of operation of capacitive transducer is based upon the familiar equation for capacitance of a parallel plate capacitor. C

A

d where



 r o A d

A  overlapping area of plates, m 2 d  distance between two plates, m    r  0  permittivity of medium ; F/m

 r  relative permittivity ,  r  Permittivit y of free space.

Photoconductive cells

Photoconductive material Electrode

Electrode

Base pin

Metal casing Base pin

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Ceramic substrate

Photovoltaic cells(solar cells)

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