Study & Calibration of LVDT Transducer for Displacement measurement

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Aim: To calibrate Linear Variable Differential Transformer (LVDT) for the performance using Micrometer.
Apparatus: LVDT, Digital Indicator, Micrometer.
Theory:LVDT is an inductive transducer used to translate the linear motion into electrical signal LVDT consists of a single primary winding ‘P’ and two secondary windings (S1 & S2) wounds on a cylindrical armature. An AC source is connected to the primary winding. A movable soft iron core attached with an arm placed inside the armature.

The primary winding produces and alternating magnetic field which includes alternating voltage in the secondary windings. Single voltage is obtained by connecting the two secondary windings in series. Thus the output voltage of the transducer is the difference of the two voltages.
When the core is at null position, the flux linking with both the secondary windings is equal. Since both the secondary winding have equal number of turns, M the induced emf is same in them. The output voltage is the difference of the two emf say e1 & E2. When they are equal, the voltage is zero at null position.
When the core is moved to the left side from null position more flux links with S1. The output voltage is V=E2-E1, is greater, the V value is –ve. Means the voltage is read in terms of mm length on the display board indicates the negative value. When the core is moved to the right side of the null position, more flux links with S2 induces voltages which is +ve. The display board indicates the +ve value in mm of length.
The voltage output is linear and is depending on the position of the core. Hence LVDT can be conveniently used to measure the thickness ranging from fraction of a mm to a few cm’s. normally LVDT can give better result up to 5mm.
PANEL DETAILS: POWER ON: Rocker switch which switches on the supply of the instrument, with red light indication.
ZERO: Ten turn potentiometer. The display can be adjusted to read Zero when no force is applied.
CAL: Single turn potentiometer. The output of the amplifier is adjusted by this potentiometer such that the display gives full scale for given range of sensor.
TO SENSOR: Sensor is connected to the indicator through a five core cable with 5 pin respective colour connectors.
MAINS INPUT: Power cable. Power cable to be connected to the mains supply of 230V 50Hz.
FUSE: 500 mA cartridge fuse with holder located on the rear side of the instrument to protect the instrument from internal electrical shorting.
CAUTION: Do not remove the fuse cap with power cable plugged to the mains supply.

PROCEDURE: The experiment can be carried out for both +ve and –ve sides.

1. Connect the power cable to 230V 50Hz to mains and switch on the instrument.
2. Make the display to read zero (000) by using zero knob.
3. Connect the LVDT cable pins to the instrument with proper colour code.
4. Make the display to read zero by rotating the micrometer. This is called null balancing and note down the micrometer reading.
5. Give the displacement of 5mm by rotating the micrometer from the null position either clockwise or anticlockwise.
6. Then display will read 5.00mm. if not adjust the display by using Cal knob. Now the instrument is calibrated.
7. Again rotate the micrometer to null position and from there take down the reading in steps of 1mm. that is both the sides.
8. Plot the graph micrometer reading v/s display reading ( Actual reading v/s Measure reading ).
   • X axis micrometer reading (in mm)
   • Y axis display reading (in mm)

Observation Table

Sno	Actual reading $R_a$ (micro meter reading) in mm	Measured Reading $R_m$(LVDT shown reading) in Vots	Measured Reading $R_m$(LVDT shown reading) in mm (Output Volt.*Multiplication factor)	% Error $\frac{R_m - R_a}{R_a}$
1
2
3
4
5

Graphs: Actual reading v/s Measured reading
Conclusion:

• Range of Micrometer.
• Least count of Micrometer.
• Linearity Range of LVDT.
• Least count of LVDT.
• Initial reading of Indicator (null position).
• Micrometer reading at null position.