Experiment 3: Measurement of displacement using LVDT and study its characteristics.

    Setup:

     The setup has two units

1. Sensor Module
2. Signal conditioning and processing unit.

• The sensor module consists of a spring loaded LVDT and a screw gauge.
• The LVDT (three coils and a core) has been placed inside a case.
• Among the three coils one is primary and other two are secondary. The secondary coils are not connected in series opposition. The differential amplifier does this work.
• The core of LVDT has been connected with a rod (shaft) internally and is taken out of the case.
• The connecting wires of the LVDT has been connected with a D9 connector.
• There is 0-20mm screw gauge. The shaft of the screw gauge and shaft of LVDT touches each other. So that the screw gauge gives displacement to the LVDT core by these shafts.

• The signal conditioning and processing unit has a D9 male connector through which the LVDT is interfaced with the board.
• There is an in-built sine wave generator which supplies a constant 2V peak to peak voltage at 4KHz frequency. It acts as an excitation source for the LVDT.
• There are two knobs in the front panel of the setup. One for span adjustment and other is for zero adjustment.
• There is a four digit seven segment display unit to show the displacement in mm.
• There are some testing point sockets also given.

Working Principle:

• The screw gauge gives a known displacement to the core of LVDT. When the core is displaced we get two voltages at the secondary coil of the LVDT.
•  These two voltages are 180 degree out of phase to each other. We can measure these voltages at TP2 and TP3 w.r.t ground.
• The two voltages are fed to the differential amplifier. The output of the differential amplifier is given to the phase sensitive demodulator circuit. It is used to detect the voltage magnitude and phase angle changes and condition the secondary voltage output to provide pure DC output signal. The low pass filter is designed to filter the noise and allow only DC signals.
• Using zero adjustment POT we can nullify the output voltage at 10mm displacement in the micrometer. And span adjustment is used to adjust the extreme point displacement (+10 and -10).

Apparatus Required:

• Multimeter
• Dual trace Oscilloscope
• Trace paper / pen drive of 4gb or less

Experiment 1: To study the characteristics of an LVDT position sensor with respect to the secondary output voltage. And measure the voltage due to residual magnetism.

   Procedure:

• Connect the D9 connector of LVDT with the Signal conditioning setup.
• Switch ON the unit
• Connect the multimeter (in AC mV mode) and CRO at T2, T3, T4, and T7.
• Adjust the micrometer to 10mm displacement and tune the zero adjustment POT to zero mm displacement on display.
• Adjust the micrometer to 20mm displacement and tune the Span adjustment POT to +10mm displacement on display.
• Adjust the micrometer to 0mm displacement and tune the Span adjustment POT to -10mm displacement on display.
• After completion of the calibration give the displacement from the micrometer to the core of the LVDT sensor.
• Gradually decrease the micrometer displacement from 10mm to 0mm, 2mm in step, and note down the forward core displacement from 0 to 10mm. Increase the micrometer displacement from 10mm to 20mm, 2mm in step, and note down the backward core displacement from 0 to -10mm.
• Measure the two secondary voltages at T2 and T3 w.r.t T7 (ground). And also measure their difference at T4 by multimeter (in AC mV mode) and observe the wave form by CRO at each position of the micrometer.

  Observation:

Sl no	Screw gauge micrometer displacement (mm)	Core displacement (mm) on Display board	Voltage at T2 (mV)	Voltage at T3 (mV)	Voltage at T4 (mV)

Plot the graph between core displacement (mm) along X-axis and secondary output voltage at T4 (mV) across y-axis.

Experiment 2: To study the characteristics of an LVDT position sensor w.r.t signal conditioned output voltage.

Procedure :

• Calibrate the system in the same way as in expt.1
• After completion of the calibration connect multimeter (in DC V mode) at T6.
• Give the displacement to the core of LVDT sensor and measure the voltage across T6 and T7.

Observation:

Sl no	Screw gauge micrometer displacement (mm)	Core displacement (mm) on Display board	Voltage at T6

Plot the graph between core displacement (mm) along X-axis and signal conditioned output voltage at T6 (V) across y-axis