Linear Variable Differential
The linear variable differential transformer is a mechanical displacement transducer. It gives an a.c. voltage output proportional to the distance of the transformer core to the windings. The LVDT is a mutual-inductance device with three coils and a core. (Figure 10) An external a.c. power source energizes the central coil and the two phase opposite end coils are used as pickup coils. The output amplitude and phase are dependent on the relative positions between the two pickup coils and the power coil. Theoretically there is a null or zero position between the two end coils, although in practice this is difficult to obtain perfectly.
Figure 10 Schematic of a linear differential transformer
A typical representation of a core displacement to output voltage is shown in Figure 11. The output voltage on either side of the null position is approximately proportional to the core displacement. The phase shift that occurs on passing through the null position can be sensed by a phase sensitive demodulator and used to detect the side that the output voltage is from.
Figure 11 Output for a differential transformer
The sensitivity of a LVDT can be determined by the following equation:
Sensitivity = (output x input) / (excitation voltage x displacement)
The typical range for LVDT sensitivity is 0.4 - 2.0 mV/V.10-3cm. LVDT‚s are typically used in force, displacement and pressure measurement. They offer the advantages of being relatively insensitive to temperature changes, and providing high outputs without intermediate amplification. The appreciable mass of the core is a disadvantage in the area of dynamic measurements.
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Support for the development of this module was provided by the National Science Foundation and The Cooper Union for the Advancement of Science and Art.
Please send questions or comments to Professor Ron Adrezin or Professor Daniel Raichel.