The LVDT Operating Priciple
The Linear Variable Differential Transformer (LVDT) is an electro-mechanical transducer that converts the linear motion of the object to which it is coupled mechanically in a corresponding electrical signal.
The basic design consists of a cylindrical array with a primary winding centred between two identically-wound secondary windings. The coils are wound on a hollow glass-reinforced polymer (GRP) former, which is surrounded by a high permeability magnetic shield. This assembly is then secured into a stainless steel tube that is sealed at both ends. This tubular element is the static part of the LVDT construction.
The moving part of an LVDT is a separate short rod of magnetically permeable material called the core. The core is free to move within the hollow bore of the coils and and is mechanically coupled to the item being displaced (Figure A). One of the major benefits of the LVDT design is that there is no physical contact between the core and the coils, giving the LVDT a hugely extended operating life.
The primary winding (P) is energised with a constant amplitude a.c. supply at a fixed frequency anywhere between 1kHz and 10kHz. This produces an alternating magnetic field in the central winding which induces a signal into the secondary windings S¹ and S² depending on the position of the core (Figure B).
The output signal is the differential a.c. voltage between the two secondary windings which varies according to the position of the core within the coil
(Figure C). When the core is positioned in the centre the output signal is zero, this is known as the null position. As the windings are precisely wound, the signal output has a linear relationship with the physical position of the core (Figure D).
Separate instrumentation is available to externally process the resultant a.c. voltage from the secondary coils into a d.c voltage or current and can also include a digital display. Alternatively, LVDTs can incorporate a circuit that provides oscillation, demodulation and signal conditioning. These are referred to as DC/DC LVDTs as they require a simple DC supply and output either a d.c. voltage or current proportional and in-phase with the position of the core.
The LVDT design can be easily adapted to fulfil a broad range of applications in research and industry. Some typical variations include:-
- Complete sealing for partial or full submersion in liquids and gases.
- Heavy duty construction for use in harsh environmental conditions.
- Miniature and low-cost models for price-sensitive OEM applications.