Introduction. Physical parameters to be measured are most of the time nonelectrical.

Transcription

1 Note-6

2 TRNSDUCERS

3 Introduction Phyical parameter to be meaured are mot of the time nonelectrical. Non-electrical quantitie are converted into electrical quantitie for better meaurement. Thi i becaue electrical method of meaurement (ening, ignal conditioning, tranmiion, indication) are much more advantageou than other method (e.g. mechanical and chemical). Proce of converting one form of energy into another i called Tranduction, and the device that perform thi function i called a Tranducer. 3

4 The device which convert other form of energy into electrical form are called Electrical Tranducer However, with reference to thi coure, when we ay tranducer, it refer to electrical tranducer. nice definition: tranducer i a device which, when actuated by energy in one form, produce an electrical output energy 4

5 dvantage of Electrical Tranducer The advantage of converting phyical quantitie into electrical quantitie are:. Electrical amplification and attenuation can be done eaily and with tatic device. The ma-inertia effect are minimized. 3. Effect of friction are minimized 4. Can be controlled with a very mall power level 5. Output can be eaily ued, tranmitted, tored and proceed 6. Telemetry i ued in almot all ophiticated meaurement ytem. The entire aeropace reearch and development i baed upon telemetry and remote control. Mechanical tranmiion would not make thi poible. 7. Miniaturization i poible by the ue of IC 5

6 Claification of Tranducer Can be claified according to their application, method of energy converion, nature of output ignal and o on. claification can be done baed on the electrical circuit parameter that i meant to be changed due to the preence of the meaurand:. Reitive tranducer. Capacitive tranducer 3. Inductive tranducer 4. Voltage and Current tranducer 6

7 nother Claification Primary tranducer Tranducer that ene a phyical phenomenon at the firt tage of detection and convert it to electrical ignal; e.g. thermocouple Secondary tranducer The phyical phenomenon i firt ened by a non-electrical tranducer. It output i then converted into an electrical ignal by an electrical tranducer. Thi econd tranducer i called econdary tranducer. For example: in one type of preure meter, a mechanical tube (Bourdon tube) firt convert the preure to be meaured into an analogou diplacement. It i then ened by an electrical tranducer which i called econdary tranducer. 7

8 Reitive Tranducer Tranducer that employ change in reitance of an ening element i called reitive tranducer. Reitive tranducer are preferred over tranducer employing other principle. The well-known equation of reitance of a reitor element: Working bai: any combination of the 3 quantitie are affected by the meaurand pplication: R For example: temperature can change ; train can change, and, altogether Meaurement of temperature, train, diplacement, force, preure etc. 8

9 Example: Strain Gauge If a metal conductor i tretched or compreed, both and change, o it R change. lo can change. Thi property i called piezo-reitive effect. So, reitance train gauge are alo known a piezo-reitive gauge. Theory of Strain Gauge: Reitance of an untrained wire, R et a tenile tre be applied to the wire. Thi caue a poitive longitudinal train and negative lateral train. et, Δ= change in length, Δ=change in area, ΔD=change in diameter and ΔR= change in reitance 9

10 0 d dr R we have,, w.r.t Differentiating D D d dr R can be written a, Now, equation () ) ( d dr R R, by both ide Dividing ) ( D D, D D. π., D π, Or 4 So, 4 rea

11 Now Poion' ratio, ν ateral Strain ongitudinal Strain D/D, / Or, D / D / R dr d ν For mall variation, the above relation can be written a : ΔR R Δ ν Δ Δ ΔR/R Gauge Factor, G f Δ/ We Know, Δ/ longitudin al train G f ν Δ/ Δ/ ε

12 If the piezo-reitive effect i negligible, the gauge factor i, G f ν The Poion ratio for all metal i between 0 and 0.5 Unit of train i microtrain microtrain = µm/m Many type of train gauge are there. bonded wire train gauge i hown here. Meaurement of reitance: can be done by bridge (e.g. Wheattone bridge)

13 pplication For example:. Structural tre analyi - e.g., mart bridge. In order to keep bridge in good condition, engineer ue train gauge on bridge to monitor tree on weak point of a bridge. - bridge can have hundred of enor to upervie it tre level at hundred of point of the bridge. Meaurement of force, preure, flow etc. - indirectly meaure the force/preure through the material train. 3