The LVDT or Linear Variable Differential Transformer is a well established transducer design which has been used throughout many decades for the accurate measurement of displacement and within closed loops for the control of positioning. So, how does an LVDT work? In its simplest form, the design consists of a cylindrical array of a primary and secondary windings with a separate cylindrical core which passes through the centre. (Fig A).
The primary windings (P) are energised with a constant amplitude A.C. supply at a frequency of 1 to 10 kHz. This produces an alternating magnetic field in the centre of the transducer which induces a signal into the secondary windings (S & S ) depending on the position of the core.
Movement of the core within this area causes the secondary signal to change (Fig B). As the two secondary windings are positioned and connected in a set arrangement (push-pull mode), when the core is positioned at the centre, a zero signal is derived.
Movement of the core from this point in either direction causes the signal to increase (Fig C). As the windings are wound in a particular precise manner, the signal output has a linear relationship with the actual mechanical movement of the core.
The secondary output signal is then processed by a phase-sensitive demodulator which is switched at the same frequency as the primary energising supply. This results in a final output which, after rectification and filtering, gives D.C. or 4-20mA output proportional to the core movement and also indicates its direction, positive or negative from the central zero point (Fig D).
The distinct advantage of using an LVDT displacement transducer is that the moving core does not make contact with other electrical components of the assembly, as with resistive types, as so offers high reliability and long life. Further, the core can be so aligned that an air gap exists around it, ideal for applications where minimum mechanical friction is required.
The LVDT design lends itself for easy modification to fulfill a whole range of different applications in both research and industry.
Some typical variations include:-
- Complete sealing for part or full submersion in liquids and gases
- Heavy construction build for tough industrial areas
- Miniature and low cost models for price-conscious OEM usage
- Internal electronic circuitry eliminating the need for additional instrumentation