Potentimeters, LVDTs and miscellaneous Sensors

Review of Analog Sensors/TransducersMuhammad Zaigham Abbas ShahSensorMeasures some physical quantity, converts it in to electrical

Transducer

Transforms a signal from one form in to another form.

Temperature Thermocouple

An active sensor which works on the Seebeck effect

When two wires with dissimilar electrical properties are joined at both ends and one junction is made hot and the other cold, a small electric current is produced proportional to the difference in the temperature

Image: www.labfacility.com

Resistance Temperature DetectorWork on the principle that the electrical resistance of a conductor changes with temperature.

According to the relation R = Ro(1 + t) where is the temperature coefficient

Image: www.globalspec.com, www.labfacility.com

ThermistorsMade from semi-conductor material and produce a large change in resistance for a small change in temperature

Resistance increases or decreases depending on type

Image: www.sowparnikatthermistors.comResponse curves for Thermocouples, RTDs and Thermistors *

* http://digital.ni.com/public.nsf/allkb/C50FA55B3B2F85D9862572D00083350E, Image: www.globalspec.comComparison chart for Thermocouples, RTDs and Thermistors *

PotentiometersAlso called Pots for shortConsist of a slider terminal and two terminalsThe resistance between the slider terminal and any of the other two terminals can be varied by rotating the slider Two ways in which they are constructed Wire wound- Linear Coated- Can be linear, Logarithmic etcRotory pots have a varying degree of rotation(usually 270)Used for Volume control and regulating current

Linear Variable Differential Transformer(LVDT):A linear displacement/position transducer that works on the phenomenon of electromagnetic inductionConsists of a primary coil and two secondary coils which are wound 180 out of phase(one clockwise and the other anti-clockwise. The coils are usually wound on a hollow form and a core of moveable material.

At the mean/null(center) position, the output is zero since an equal amount of magnetic flux cuts each of the secondaries.As the core is moved from one secondary to the other the phase of the output voltage changes.LVDT Operation

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Transducers for rate of flow, speed, pressure, forceTwo things to understand, stress and strainStress- The force applied on an object. Mathematically, Stress()= Force(F)/Area(A)Strain- The deformation that results from that external force. Mathematically, Strain()=Change in Length(L)/Original Length(L)Although Mathematically unit less, it is often represented in inch/inch or mm/m, also measured as microstrain(x10-6)Three kinds of strainTensile- Results from a stretching stress(also called tension)

Strain Gauge

Compressional Strain- Results from a contracting/compressing stress

Shear Strain- Results from a shearing stress

A strain gauge is a sensor that can be used to measure strainIt changes its resistance in response to a strainWorks on the principle R=L/AwhereR= Resistance()= Resistibity(.m)A= Area(m2)

The resistance change is given byR= (2R0L)/L0Gauge FactorRelates resistance change and strain, it is the ratio of the fractional change in resistance upon the applied strain(fractional change in length)GF= (R/R)/strain=(R/R)/(L/L)Pressure Sensitive EffectsTwo kinds of pressure effectsPiezo Resistive EffectPiezo Electric Effect

Piezo Resistive Effect:Change of the resistance of a semiconductor when it is applied by a mechanical pressureThe pressure causes a change in the band gap of the shells thus varying the resistanceMathematically,= (/)/= Piezo Resistivity= Change in Resistivity= Actual Reistivity= Applied Stress

Piezo Electric Effect:Application of pressure produces a voltageObserved in crystalline materials such as Quartz, Barium titanate, tourmalineWhen mechanical stress is applied, ions in the asymmetircal crystal structure are displaced thus polarizing the unit cellThe individual effect of each unit cell adds up and polarizes the whole crystal thus establishing a potential difference across the crystal facesFor a Rectangular piece of crystal, the output voltage is given by V=kFd/AF= Force in g, k= piezo electric constant, d= thickness, A=area in mmUsed in force, displacement,pressure measurementTutorialsDefine Cold junction compensation for a thermocouple, why is it important?

Name few applications of the sensors, transducers discussed.

Check out the operation of the LM35 temperature sensor.

Find (just name and mention purpose) of other analog sensors that you have come across.