Copy of Transducers.pdf
- 1. Unit:03 Transducers By Prof.Mayur M.Sevak Electronics and Communication Engg. Department BVM Engineering College
- 2. Introduction ● It is device that converts one energy form into another form. ● I.e Electrical,Mechanical,Chemical,Opti cal or thermal ● The Transducer that gives electrical energy as output is called electrical transducer. ● For measuring non electrical quantity, a detector is used which usually converts physical quantity into displacement and then to electrical energy. ● Displacement transducer take help of force summing device to covert force into displacement. Active Transducer: ● Own voltage or current as output ● Energy is obtained from physical phenomenon Passive Transducer: ● Externally Powered Transducers ● They may also absorb some energy during conversion 2
- 3. Active and Passive transducers Passive Transducers ● Resistance ● Potentiometric Device ● Resistance Strain Gauge ● Resistance Thermometer ● Thermistor ● Photoconductive Cell ● Inductance ● LVDT ● Capacitance ● Voltage and Current ● Device using hall effect ● Photoemissive cell ● Photomultiplier tube 3 Active Transducers ● Thermocouple ● Piezoelectric Transducer ● Photovoltaic Cell ● Moving Coil Generator ● Photoelectric Cell
- 4. Basic Requirement of Transducer 4 Linearity i/o characteristics should be linear Ruggedness It should withstand overload with measures for overload protection Repeatablity It should produce identical o/p for same i/p at different times under same environmental condition High Stability and Reliability Output must not be affected by temperature and other variations and minimum error in measurements Good dynamic response Input may be dynemic i.e changing in nature. It must respond to changes inputs quickly as possible Convenient Instrumentation and Good Mechanical Characteristics It must produce high analog o/p with high SNR so that it can be measured directly. It is subject to mechanical strain and hence it must not be deformed or performance must not be affected.
- 5. “ Capacitive Transducer ● When a force is applied to diaphragm,which also acts one plate of capacitor,distance between diaphragm and static plate is changed. ● Change in capacitance can be measured with a.c bridge or oscillator circuit that detects change in frequency and hence measured by electronic counter. ● In capacitor microphone this principle is used. ● It can measure static and dynamic changes ● It is sensitive to temperature variations
- 6. Inductive Transducer ● When force applied to ferromagnetic armature,the air gap is changed ● Hence reluctance is changed ● So, applied force is measured by change of inductance in a single coil ● This transducer enables static and dynamic measurements. ● It has limited frequency response 6
- 7. 7 Linear Variable Differential Transformer ● It consists of primary and exactly two similar secondary coils with rod shaped magnetic core at centre. ● An alternating current is fed into primary and hence voltages Vo1 and Vo2 induced in secondary coils. ● Vo=Vo1- Vo2 ( Series opposition of secondary coils) ● Look at the figure. If core is at centre the net output voltage Vo will be zero. The voltage will increase towards which core moves and on other side voltage decreases. ● The output voltage is linear over some range and after it will be flatten.Voltage phase changes by 1800 ● Repeatablity is excellent due to low hysteresis and cont. Resolution. ● Less friction and less noise due to no sliding contacts. ● It is sensitive to vibration and temperature and compatible with receiving a.c instrument.
- 8. ● Force summing device is used to change the distance between parallel plates and thereby changes the capacitance. ● Hence in LC circuit,the frequency is changed that can be measured by counter. ● So applied force can be measured by o/p electronic counter value. ● It measures both static and dynamic phenomena and used in telemetry systems. ● Limited range,poor thermal stability and low accuracy. Oscillation Transducer 8
- 9. Potentiometric Transducer ● Resistance element is contacted by movable slider. ● A force summing member is used to move the slider. ● That reflects change in output resistance and hence change in output voltage. ● Same principle is used in bridge circuits. ● High electrical efficiency ● It provide sufficient output to permit control operation without amplification. 9
- 10. Electrical Strain Gauges 10 ● If metal conductor is stretched or compressed, it’s resistance changes because of (L or A) ● If wire is under tension and increases its length from l to then its resistance is change by R to ● Sensitivity of strain gauge is measured by Gauge Factor. I.e Unbonded Strain Gauges ● Measuring forces are transmitted to platform containing unbonded structure by force rod. ● When external force is applied to gauge then armature moves in direction indicated. A and D are stretched,B and C are compressed. ● Change in length is resistance is measured by wheatstone bridge. ● Unbalance bridge produces output voltage.
- 11. Contd. ( Bonded Wire Strain Gauge) ● Bonded wire strain gauge consists of grid of fine resistance wire of diameter of 25 micrometer. ● Wire is connected to base.The base may be thin sheet of paper or thin Bakelite sheet. ● Wire is covered with thin sheet of material so that it can’t be damaged mechanically. ● Base is bonded to structure under study with adhesive material. 11
- 12. Thermocouple ● A thermocouple is junction between two dissimilar metals or semiconductors that generate small voltages.with coefficient range of 50 µV/0 C ● In figure it consists of two junction, reference and sensing are maintained at different temperature. ● Each junction is made by welding two dissimilar metals. ● The reference junction is maintained at fix temperature i.e 0 0 C and o/p voltage is depending upon temp. of sensing junction ● It is necessary to amplify output for calibration. ● Iron-constantan- 760 0 C ● Chromel-Alumel-1370 0 C 12
- 13. Resistance Thermometer ● The resistance of most electrical conductor varies with temperature according to ● R=R0 (1+𝞪T+βT2 +...) ● R0 is resistance at temperature T0 at 00 C ,R=resistance at T and 𝞪,β =constants ● R=R0 (1+𝞪T) ,𝞪 is temperature coefficient of resistance. ● Important properties of materials used for resistance thermometers are ● 1. High temperature coefficient of resistance ● 2. Stable properties (not be drifted by rapid heating or cooling or mechanical strain) ● 3. High resistivity to permit construction of small sensors. ● From figure it can be seen that tungsten has suitable temperature coefficient of resistance but it is brittle and difficult to from. Copper has low resistivity and size is flexible 13
- 14. Contd. ● Both platinum and nickel are widely used because easy to obtain in pure state. ● Resistance temperature relationship for platinum resistance is given by ● Rt is resistance at temperature T,R0 is resistance at temperature T0 at 00 C ● R100 is resistance at temperature 1000 C d is calendar constant 14
- 15. Thermistor ● It is two terminal device whose resistance decreases with temperature. Oxide of cobalt,nickel,copper ,iron,uranium and manganese are used to manufacture it. ● It has very high temperature coefficient making it an ideal temperature transducer. ● The temperature coefficient of resistance is negative.the resistance at any temperature T is given by ● 15
- 16. Contd. ● Look at the characteristics the cure is non linear and drop in resistance from 5000Ω to 10Ω occurs for increase in temperature of 200 C to 1000 C. ● An increase in current through device will raise the temperature and resistance will be dropped. ● Any externally applied heat source will do the same action. ● Three useful parameters to characterize thermistor are ● 1. Time Constant: Time for thermistor to change its resistance by 63% of its initial value for zero power dissipation. 1 s to 50 s values. ● 2. Dissipation Factor: it is power necessary to increase temperature of thermistor by 10 C.(1 mW/0 C to 10 mW/0 C). ● 3. Resistance Ratio: it is ratio of resistance at 25 0 C to that 125 0 C ● They are used to measure temperature,flow,pressure,liquid level,voltage or power level,vacuum etc. 16
- 17. Piezoelectric transducer ● It works on principle that if the dimension of asymmetrical crystalline material,such as quartz,rochelle salt or barium titanite are changed by mechanical force ,the crystal produces emf. ● Here Crystal is placed between solid base and Force-summing member. ● Externally force produces emf across crystal which is proportional to applied pressure ● It is having good frequency response and its used in high frequency accelerometer. ● It is self generating transducer. ● It can not measure static condition and o/p voltage is affected by temperature variations. 17
- 18. Photoelectric transducer ● It uses phototube and light source separated by small window. ● Aperture of window is controlled by force summing device. ● The quantity of incident light on photosensitive cathode is varied in accordance with externally applied force thereby changing anode current. ● It is compatible with static and dynamic measurements. ● It is not responding to high frequency light variation. 18