LVDT (Linear Variable Differential Transformer) [specifications]

• comprises of one primary coil and one secondary coil: an AC current is fed into the primary coil, while the voltage difference between both ends of the secondary coil is measured (see figure)
• works based on Farady's Law - voltage induced due to change of magnetic field
• for small deviation of position off the equilibrium position, the voltage output varies linearly to the position of the metal core
• applied frequency to the primary coil - 50 to 25K Hz; for dynamic displacement the carrier frequency should be 10 times greater than the highest frequency anticipated
• cousin: RVDT

• calibration procedure:

- Check and adjust the zero and gain settings of the signal conditioner:

(a) first zero the signal conditioner at zero displacement,

(b) traverse the micrometer to a maximum displacement that you anticipate in the calibration experiment, (c) adjust the gain setting to attain a transducer "high" output value,

(d) record the "reference values" of the zero, maximum displacement and the corresponding transducer output.

In future experiments, you may set the zero and gain of the signal conditioner to these "reference values" and use the calibration results or curves obtained from this laboratory for displacement measurements.

• LAB assignments:

(1) Take at least five different settings to cover the displacement range of 0 to 1 in. For each displacement setting, repeat five times for increasing displacement (or, low to high) and repeat five times for decreasing displacement (or, high to low) to check for hysteresis.

(2) Establish a spread sheet and calculate the average voltage for each displacement.

(3) Plot the averaged voltage output of the signal conditioner versus displacement, and obtain a linear correlation between the averaged voltage output versus displacement, using a least squares technique (for example, in the format of y = a + bx ± c).