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TRANSDUCERS
By LOKESH KOLHEMechanical Engg
Revision 01 2
Transducers Definition: Technically… A device that converts one energy form to
another (eg, mechanical to electrical). Any device or component that converts an input
signal of one form to an output signal of another form
An element or device which receives information in the form of one quantity and converts it to information in the same or an other quantity or form.
A device for translating the magnitude of one quantity into another quantity.
Revision 01 3
Transducers (Briefly)
Transducer(conversion)Any measureable
quantity in
Anythingout
eg. any measurable quantity:• energy: sound, electrical, mechanical,
light, chemical, • pressure, level, density, temp, pH, flow,
temperature• position, distance, mass, time• etc, etc.
eg. any measurable quantity:• energy: sound, electrical, mechanical,
light, chemical, • pressure, level, density, temp, pH, flow,
temperature• position, distance, mass, time• etc, etc.
This allows for a VERY broad interpretation...
Revision 01 4
Transducers
Definition: Practical and realistic…• A sensor that converts one energy form to
another (eg. mechanical to electrical). Things that AREN’T generally referred to as transducers:• Valves• Motors• Solenoids• Alarms• Contactor• Heater• Power transformer• Hydraulic cylinder
eg. • Microphone• Thermocouples• Thermistors• Tacho-generators • a diode can be used
to measure temperature.• pH probe• Ultrasonic level detector• etc, etc.
Revision 01 5
Types and applications
Some common transducers and common uses Thermistor/thermocouple temperature
eg;motors LDRs/LEDs flame or smoke Opto-coupler data transfer Speaker/microphone acoustic/sound Magnetic pickup stylus/vibration Strain guage tension Hall effect magnetism Peltier effect device temperature Piezzo stress/pressure
Revision 01 6
Quantities and Units
AAmpereCurrent
sSecondTime
T (Wb/m2)Tesla (Webers per metre squared)
Magnetic Flux Density
dBDecibelSound level (relative)
PaPascal (Newton per square metre)
Pressure
NNewtonsForce
lxLuxIlluminance
cdCandelaLight Intensity
m/s2 (m/s/s)Metres per second squaredAcceleration
CCelsiusTemperature (Alt)
KKelvinTemperature (SI)
m/sMetres per secondVelocity
mMetreLength / Displacement
kgKilogramMass
SymbolName
Unit – NB : Shaded boxes indicate a base SI unit.Parameter
AAmpereCurrent
sSecondTime
T (Wb/m2)Tesla (Webers per metre squared)
Magnetic Flux Density
dBDecibelSound level (relative)
PaPascal (Newton per square metre)
Pressure
NNewtonsForce
lxLuxIlluminance
cdCandelaLight Intensity
m/s2 (m/s/s)Metres per second squaredAcceleration
CCelsiusTemperature (Alt)
KKelvinTemperature (SI)
m/sMetres per secondVelocity
mMetreLength / Displacement
kgKilogramMass
SymbolName
Unit – NB : Shaded boxes indicate a base SI unit.Parameter
Revision 01 7
Classification of transducers
There are many ways to classify transducers: By what they are measuring
General classification. Specific classification.
By the output signal type. By whether or not they produce their own
supply. (Active or Passive) Input to output. Contact type or not Direct or indirect. Method used to sense input.
Revision 01 8
Transducer parameters
Transducer operating characteristics are usually defined by a number of parameters.
Some of the main parameters to be considered are: Sensitivity – Range – Span Linearity – Hysteresis – Accuracy Precision (Reproducibility, Repeatability) And others.
Revision 01 9
Hysteresis
A transducer should produce the same output whether the value has been reached due to a continually increasing input or a continually decreasing input.
Out
put
Input
Hysteresis
Ideal –Negligible Hysteresis
Revision 01 10
Accuracy
Accuracy can be expressed as a comparison of the static error of the transducer compared to the actual value (at full scale) expressed as a percentage of full scale. (Accuracy may also be expressed in other ways.)
(Measured value – Actual value) x 100
Actual value% Accuracy =
E.g. A temperature transducer that reads 102 C at full scale, when the temperature is 100 C, has an accuracy equal to 2% of full scale.
Revision 01 11
Precision (Reproducibility, Repeatability)
Poor AccuracyPoor Precision
Poor AccuracyGood Precision
Good AccuracyGood Precision
The ability of the transducer to produce the same output each time the same input is applied.
Revision 01 12
Sensitivity
Sensitivity is the ability of the transducer to generate an output for a given change in input.
Change in output
Change in inputSensitivity =
E.g. A thermocouple that increases output voltage by 3mV per degree Celsius temperature change has a sensitivity of 3mV/ C
Revision 01 13
Range
The highest and lowest values that the transducer is designed to measure.
E.g. A Temperature transducer may have a range of –50 C to +50 C
Revision 01 14
Units we need to know.
Revision 01 15
Revision 01 16
Measuring temperature
Thermocouple
Thermistor
As the junction temperature increases a small voltage is created in the loop. The voltage produced at the junction of the dissimilar metals is due to a phenomenon called the “Seebeck Effect”.
• The higher the temperature at the junction, the greater the voltage produced by that junction.
• The relationship between voltage and temperature is constant and therefore will graph as a linear line.
Thermocouples
Revision 01 18
Thermistors
Thermistors are made from semi-conductor materials.
Semi-conductor thermistors have a Negative Temperature Coefficient (NTC). i.e. as temperature increases, the resistance decreases.
Res
ista
nce
Temperature
Revision 01 19
Optical devices
Many measurement and control systems utilise light and light-intensity as a way of detecting other physical properties.
Using direct or reflected light can provide an ideal non-contact sensing mechanism.
Photoelectric Transducers
Photoelectric transducers are devices that produce an electrical variation in response to a change in light intensity, or produce a light intensity variation due to a change in applied electrical energy. Photoelectric transducers operate in three classifications, they are:
• Photoconductive, • Photovoltaic,
• Photoemissive.
Acoustic Transducers Acoustic transducers are devices that convert a variation in electrical energy into a change in mechanical energy, (physical vibrations or oscillations, i.e.. sound waves). Or conversely, convert a variation in sound wave energy into electrical energy.
Common examples of acoustic transducers are the:
• Acoustic speakers,
• Acoustic microphone,
• Piezoceramic transducers, and
• Magnetostrictive transducers
Inductive Proximity Sensors Inductive proximity sensors rely
on the effect of a magnet approaching a high turns ratio coil that produces a voltage proportional to the relative distance of that magnetic source from that coil. Another variat ion is to have the inductive source coupled via the proximity of the magnetic field. The sensor generates a magnetic field and as the magnetic conductive material approaches the magnetic field, it provides a decreasingly reluctant path to magnetism. This effect is proportional to the distance of the object from the sensor and produces an increasing output, the closer the object gets to the sensor.
Position and displacement measurement Potentiometers
Measurement of displacement with a potentiometer relies on the fact that the resistance between the sliding contact and the reference end of the resistance element is proportional to the distance between the two points.
Linear Variable Differential Transformer (LVDT)
Using AC instead of DC, we are able to avoid sliding contact between parts if we use a variable transformer instead of a potentiometer. Devices made for this purpose are called LVDT’s, which stands for Linear Variable Differential Transformer. The design of an LVDT looks similar to the layout in the diagram at Figure below
Revision 01 25
Tachogenerator
Permeant magnet tacho- generator
Shaft mounted tacho
Revision 01 26
Tachogenerator
Thank You
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