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The LVDT is a Linear Variable Displacement Transformer and is commonly used for precise non-contacting measurement of linear displacement. When in the rotary form, it is termed an RVDT.
The LVDT and RVDT are identical in terms of concept, and differ only in terms of implementation. Both use three coils around a single ferromagnetic core.
Application
The LVDT and RVDT operate on the basis of transformer coupling. One of the coils is a primary, and then the two adjacent cores are secondaries. The primary is excited with AC current at a frequency of between 1 and 10 kHz. The central core is made from ferromagnetic material and is in direct contact with the moving item.
As the core moves, the mutual inductance between the coils changes and the combined output from the secondary coils can be measured. Additionally, the direction of movement can be determined by examining the phase difference between the primary and the secondary waveforms.
The LVDT/RVDT is highly linear and very reliable due to the absence of contacting parts. The fact that the zero or home position occurs when there is complete balance between the two secondary coils is an important feature that contributes to the excellent stability of the measurement.
The LVDT/RVDT requires a carrier frequency amplifier with appropriate detection circuitry in order to convert the output from raw form to a more conventional form such a 0 +/- 10V or 4~20mA.
