TRANSDUCERS: VARIABLE RESISTIVE/CAPACITIVE/ INDUCTIVE 5/9/2012 PUNJAB EDUSAT SOCIETY 1 PRESENTED BY:...

TRANSDUCERS:VARIABLE RESISTIVE/CAPACITIVE/INDUCTIVE

5/9/2012PUNJAB EDUSAT SOCIETY1

PRESENTED BY:Er. SANJEEV GOYALSr LECTURER Govt Polytechnic CollegeBATHINDA

Introduction Of Transducers


““ENERGY CAN ENERGY CAN NEITHER BE CREATEDNEITHER BE CREATEDNOR BE DESTROYED, NOR BE DESTROYED,

IT CAN ONLY BE TRANSFORMED IT CAN ONLY BE TRANSFORMED FROM FROM

ONE STATE TO ANOTHER”ONE STATE TO ANOTHER”



Transducer is a device, usually electrical, electronic, or electromechanical, that converts one type of energy to another for various purposes including measurement or information transfer.

It must produce the output which depends on some way on the input quantity/measurand.



Its output will depends on its input if it processes upon the actual input.

So transducer extracts some energy from the measured medium.

Some energy is required to convert the in input to output.

This energy can be supplied from outside or by the measurand only.



Transducers are divided into two types:Active TransducerPassive Transducer

Active Transducers


An active transducer does not require an external power supply to provide an output signal.

These transducers usually rely upon magnetic inductance or piezoelectric effects to produce their output signals.

Ex. Piezoelectric crystal used for acceleration measurement.

Active Transducers


Measurand is converted into output without any other form of energy requirements.

They are also called self generating type transducers.

Passive Transducers


Passive transducers need an external power supply in order to amplifies the input and generate an output signal.

All passive transducers change one of, or a combination of, the three general circuit parameters; resistance, inductance, or capacitance.

Ex. Potentiometer is used for measurement of pressure, displacement and position.

Basic Requirements Of Transducer


Linearity: Linearity of any transducer is the prime requirement. A transducer having linear input output characteristics is a big plus.

Repeatability: A transducer having this quality produces the same result again and again when the same input signal is applied repeatedly under same environmental conditions ex. Temperature, pressure, humidity etc.



Ruggedness: A transducer should be robust in construction. It should be mechanically rugged, so that is capable of withstanding overload.

High Signal To Noise Ratio: The quality of output signal of transducer should be good; it should be free from the internal and external noise.



Highly Reliable: Output of the transducer should be highly reliable and stable it should be precise. It should give minimum error in measurement for temperature variations, humidity vibrations.

Good Dynamic Response: A transducer may be called upon to respond to either slowly varying or dynamic signals. Its output should be faithful to input when taken as a function of time.



No Hysteresis: A good transducer is free from hysteresis. It should not introduce any hysteresis during measurement while input signal is varied from its low value to high value and vice versa.

Residual Deformation: There should be no deformation of testing material after the removal of any pressure after long period of application.

Variable Resistance Type Transducer


The variable resistance transducers are one of the most commonly used types of transducers. The variable resistance transducers are also called as resistive transducers or resistive sensors.

They can be used for measuring various physical quantities like temperature, pressure, displacement, force, vibrations etc.



These transducers are usually used as the secondary transducers, where the output from the primary mechanical transducer acts as the input for the variable resistance transducer.

The output obtained from it is calibrated against the input quantity and it directly gives the value of the input.

Principle Of Working Of Variable Resistance Type Transducer


The variable resistance transducer elements work on the principle that the resistance of the conductor is directly proportional to the length of the conductor and inversely proportional to the area of the conductor. Thus if L is the length of the conductor (in m) and A is its area (in m square), its resistance (in ohms) is given by: R = ρ L/A.



L is the length ,A is cross sectional area and p is resistivity of the resistance material.

So resistance can be changed if any of these value p ,L or A is changed.

Measurand is connected to the resistance in such a way that it varies any one of its parameters.

A change in the value or R is proportional to the measurand.

Thus the measurand can be measured by measuring the change in resistance.



Where ρ is called as resistivity of the material and it is constant for he materials and is measured in ohm-m.

The resistance of some materials also changes with the change in their temperature. This principle is primarily used for the measurement of temperature.



Some of the variable resistive transducers are:

Sliding wire(potentiometer)Strain gaugeLoad cellRTDThermistorHot wire anemometerCarbon microphone Humidity sensor

Potentiometer


A potentiometer informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat.

Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment.

Potentiometer


The potentiometer is a displacement transducer.

This is a passive transducer.It consists of resistive material whose

resistance is proportional to its length.Slider displacement x is proportional to

the output voltage.The most common problem is dirt under

the slider.

Potentiometer


One end of conductor is fixed.The position of the other end is decided

by the slider/brush. This contact can move along the whole

length of the conductor. When the body moves the slider also

moves along the conductor so its effective length changes.

Due to which its resistance also changes.

Potentiometer


The effective resistance value is measured between the fixed position of the conductor and the position of the slider.

The resistance is calibrated against the input quantity, whose value can be measured directly.

Potentiometer


Thermistor


Thermistors are thermally sensitive resistors, generally composed of semiconductor materials. All resistors vary with temperature, but thermistors are constructed of semiconductor material with a resistivity that is especially sensitive to temperature. This resistance changes predictably with temperature.

Thermistor


Although Positive Temperature Coefficient(PTC) units are available, most thermistors have a Negative Temperature Coefficient(NTC); that is, their resistance decreases with increasing temperature.

The thermistor is an extremely non-linear device.

Thermistor


The price we pay for this increased sensitivity is loss of linearity.

These are usually connected to an electronic circuit that reads out temprature digitally.

Thermistor


The resistance temperature relation is generally of the form:R = R0 exp[β(1/T – 1/T0)]

R = Resistance at temp. T, Ω R0= Resistance at temp. T0, Ω β = Constant, Characterstics of

materialT, T0 Absolute tempratures, K

Thermistor


NTC thermistors operates over a range of -200ºC to +1000ºC.NTCs should be chosen when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with a high degree of sensitivity.

PTC thermistors are temp dependent resistors manufactured from Barium Titanate and should be chosen when a drastic change in resistance is required at a specific temperature or current level.

Thermistor


Advantages Of Thermistor


InexpensiveRuggedReliableRespond QuicklyHighly SensitiveManufactured in a wide range of

shapes, sizes and values.

Humidity Measurement


Humidity is the amount of water vapour in the air and Humidity Measurement is a measure of relative amount of water vapour present in the air or a gas.

The humidity can be expressed in different ways:

Absolute HumidityRelative HumidityDew Point



Devices that indirectly measure humidity by sensing changes in physical or electrical properties in materials due to their moisture content are called hygrometers.

The three major instruments used for measuring humidity in industry are:

The Electrical HygrometerThe PsychrometerThe Dew Point Meter

Resistance Hygrometer


This is an electrical hygrometer.It is an active transducer.These instruments are suitable for

measuring moisture levels between 15% and 95%.

It has typical measurement uncertainty of 3%.

Atmospheric contaminates and operation in saturation conditions both cause characteristics drift.

Principle Of Resistance Hygrometer


Some Hygroscopic Salts exhibit a change in resistivity with humidity. Resistive hygrometer humidity sensors use the change in resistance of a hygroscopic material between two electrodes on an insulating substrate.

The hygroscopic salt is deposited between two electrodes. The resistance of the element changes when it is exposed to variations in humidity.

Resistance Hygrometer


The Resistance Hygrometer should not be exposed to conditions of 100% humidity as the resulting condensation may damage the device.

These are accurate to within ± 2.5 % or ± 1.5 % in some cases.

Response times are typically of the order of a few seconds.

APPLICATIONSAPPLICATIONSHumidity sensors can be used not only

to measure the humidity in an atmosphere but also to automatically control: -> Humidifiers-> Dehumidifiers-> Air conditioners for adjustment.

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Strain Gauge Pressure Transducer


When the tension is applied to the electrical conductor, its length increases while the cross section area decreases.

So its resistance changes.This change can be measured to

measured. Used for the measurement of force,

stress and strain.



A strain gauge is a passive type resistance pressure transducer whose electrical resistance changes when it is stretched or compressed

A pressure transducer contains a diaphragm which is deformed by the pressure which can cause a strain gauge to stretch or compress. This deformation of the strain gauge causes the variation in length and cross sectional area due to which its resistance changes.



The wire filament is attached to a structure under strain and the resistance in the strained wire is measured.



Construction and Working



Strain gauge pressure transducers are used for narrow pressure span and for differential pressure measurments

Available for pressure ranges as low as 3 inches of water to as high as 200,000 psig

Inaccuracy ranges from 0.1 % of span to 0.25 % of full scale

RESISTANCE TEMPERATURE DETECTOR (RTD)


Resistance temperature detector (RTD) devices are conductors used for temperature sensing.

They can be used in bridge method as well as ohmmeter method to take the output.

The change in resistance of material per unit change in temperature should be as large as possible.

RESISTANCE TEMPERATURE DETECTOR (RTD)


The material should have high value of resistivity to get required value in less space.

Resistance and temperature relation should be continuous and stable.

Platinum, nickel and copper are the most commonly used.

Tungsten and nickel alloy are also used.

APPLICATIONS OF RTDAPPLICATIONS OF RTDThey can be used in average and

differential temp. measurement.Differential temp. sensing to an

accuracy of 0.05º have been accomplished in a nuclear reactor coolant heat rise application.

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Variable Capacitance Type Transducer


A capacitor is a pair of parallel plates with some insulating dielectric material between them, which can be used to store an electrostatic charge.

Capacitive transducer is a passive transducer used for sensing displacement. It is a primary transducer.



Capacitive Transducers/Sensors are extensively used in industrial and scientific applications.

They are based on changes in capacitance in response to physical variations.

These sensors find many diverse applications but main are related to the measurements of displacement.

Principle Of Capacitive Type Transducer


Capacitive Transducers measure the displacement by measuring the change in capacitance.

Where capacitance is given by, C= €A/D.Where C= Capacitance in Farads A=Area of cross section of plates in square

meters D=Distance between two plates in meters €=Dielectric Constant



Means change in capacitance occurs due to:

Change in overlapped area of platesChange in distance between the two

platesChange in dielectric medium. Capacitance may be changed either

mechanically or electronically.



o The variable capacitance used to convert physical phenomena into electrical signals is called a variable capacitance transducer.

o As there are no moving is no wear and the life expectancy quoted is 200 years.

o The major problem with these transducers is their high impedence which makes them very suspectible to noise.

Advantages Of Capacitive Transducer


Low HysteresisGood linearityHighly sensitiveStabilityRepeatabilityResolution as small as 0.003mm can be

obtainedStatic pressure measurement capability.

Disadvantages Of Capacitive Transducer


High impedance output.High temperature sensitivity.Need for complex electronics.Dirt may vary the output.

Capacitive Transducer(For Thickness Measurement)


Capacitive transducer can also be used to measure the thickness of the material. This method of measuring thickness is used in case the material to be tested is an insulator.

Material whose thickness is to be measured is placed in between the two metal plates forming a parallel plate capacitor.



Capacitance of the parallel plate capacitor is given by C = €A/D

where C= Capacitance in farads A=Area of cross section of plates in

square meters D=Distance between two plates in

meters €=Dielectric Constant

Dielectric Gauge(For Liquid Level Measurement)


Capacitive transducers can be used in liquids and free flowing solids for continuous level measurement.

Materials placed between the plates of a capacitor increase the capacitance by the factor € known as the dielectric constant of the material.



When two capacitor plates are partially immersed in a nonconductive liquid, the capacitance CD is given by:

CD = (Ca*€*d + Ca )/rWhere Ca = Capacitance with no liquid € = Dielectric constant of liquid between

the plates r = Height of the plates d = Depth or level of the liquid between

the plates



There are large variations in dielectric constant with temperature, so that temperature correction may be needed. The liquid level is given by:

d = (Cd-Ca)r/€CaThis type of sensors are widely used for

chemical and petrochemical industries; and can be used for a wide range of temperature(-40 to 200 0C) and pressure variation (25 to 60 kg/cm2).

Variable Inductance Type Transducer


Inductance is another property of electric circuits which can be used in transducers. If a length of wire is wound into a coil and an electrical current passes through it a magnetic field is generated.

If the current varies the magnetic field varies and this induces other currents in the coil which tend to oppose the original.

Variable Inductance Type Transducer


Self inductance is a measure of this effects and is given by:

L= N2u0urA/l henriesWhere, N is the number of turns in the coil A is the cross section area of the coil in

metres2

l is the length of the coils in metres u0 is the permeability of free space

(vacuum)=4∏*10-7 H/m

Linear Variable Differential Transducer (LVDT)


LVDT is used for the measurement of displacement.

It is the passive transducer.Supply is provided to the middle coil

and outputs is taken across the other two coils.

Differential amplifier is used to measure the differential output from the windings.

The output is an AC waveform.

Linear Variable Differential Transducer(LVDT)


Why use LVDT?


Friction – Free OperationNO mechanical contact between core

and coil (usually) Infinite Mechanical Life

Infinite ResolutionElectromagnetic couplingLimited only by electrical noise

Low risk of damageMost LVDT’s have open bore holes

Why use LVDT?


Null Point RepeatabilityZero displacement can be measured

Single Axis SensitivityEffects of other axes are not felt on

the axis of interestEnvironmentally Robust

Stable/Strong sensors – good for structural engineering tests.

Underlying Principle Of LVDT


In the null position, the magnetic field generates currents of equal magnitude in both secondary coils.

When the core is moved, there will be more magnetic flux in one coil than the other resulting in different currents and therefore different voltages.

This variation in voltages is linearly proportional to displacement.

Underlying Principle Of LVDT


Advantages Of LVDT


Measurement resolution is almost infinite.

Negligible hysteresis.High signal to noise ratio and low output

impedance.Relative to low cost.No permanent damage to the LVDT if

measurements exceed the designed range.

Disadvantages Of LVDT


Presence of harmonics in the excitation voltage.

Stray capacitance also affects output.Non zero output of low magnitude when

the core is in the null position.The core must contact directly or

indirectly with the measured surface which is not possible or desirable.

Bourdon Tube Pressure Gauge


Perhaps the most common device around today is the pressure gauge which utilizes a bourdon tube as its sensing elements.

Bourdon : A bourdon tube is a curved, hollow tube with the process pressure applied to the fluid in the tube. The pressure in the tube causes the tube to deform or uncoil. The pressure can be determined from the mechanical displacement of the pointer connected to the Bourdon tube. Typical shapes for the tube are “C” (normally for local display), spiral and helical.

Bourdon Tube Pressure Gauge


Bourdon tubes are generally are of three types;1. C-type2. Helical type3. Spiral type

SUMMARY


There are some other types of transducers also , such as:

Solid state temperature sensorPWM SensorsThermocouplePiezoelectric TransducerPhotoelectric DevicesBimetallic ThermometerUltrasonic Flow-meters

TRANSDUCERS: VARIABLE RESISTIVE/CAPACITIVE/ INDUCTIVE 5/9/2012 PUNJAB EDUSAT SOCIETY 1 PRESENTED BY:...

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