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The letters L.V.D.T stand for Linear Variable Differential Transformer.
Principle of operation
The LVDT is an industry established position sensor that has been used for many decades for accurate measurement of displacement. The sensor design consists of a cylindrical array of one primary and two secondary coils wound on a hollow tube. A separate cylindrical core passes through the centre of the tube.
The primary winding is energised with a constant amplitude A.C. voltage at a frequency up to 12 kHz. This produces an alternating magnetic field in the centre of the coil assembly (see fig 2), which induces a voltage signal in the secondary windings dependant on the position of the core in the hollow tube. Movement of the core within the tube assembly causes the secondary signal to change (see fig 1 & 3). As the two secondary windings are positioned and connected in push-pull mode, when the core is at the centre (fig. 2), a zero signal is derived (null voltage). Movement of the core from the centre in either direction causes the signal to increase (Fig 4). The precision wound coils create an output signal with a very linear relationship to the mechanical movement of the core.
The advantage of using a LVDT position sensor is that the moving core makes no electrical contact with the electrical components of the assembly (coils). Also, the core can be aligned with an air gap around it and is ideal for applications where minimal mechanical friction is required. The LVDT sensor is ideally suited in position sensing applications where complete sealing for part, or full or immersion in liquids and gases is required. Additionally, LVDT sensors can be constructed from materials designed to withstand temperatures up to 200°C (400°F).
Because LVDT sensors are inductive devices the primary coil is energised from an AC signal source (oscillator circuit), which then creates the induced voltages in the secondary coils. An additional circuit then converts (demodulates) the secondary voltages to a usable measurement signal. LVDT sensor models are available with integral or separate signal conditioning circuits (electronics) dependant on the measurement application.
| AC/AC LVDT sensors | DC/DC LVDT sensors | Electronics for LVDT's |
LVDT sensors are available in various housing diameters, from a micro-slim 6.00mm to robust 19mm (3/4"). Mounting options include body clamp, body flange and spherical bearings (rod-ends). The measurement range can be a few microns to about one metre (40"). Modern manufacturing techniques plus Active Sensors highly evolved coil winding technology have enables us to design very compact stroke-to-body length ratios which make the LVDT sensor suitable eevn for the tightest mechanical envelopes.
Active Sensors have a team of engineers with specific LVDT design and manufacturing capabilities and would welcome the opportunity to discuss your application in more detail. If you have a technical question regarding the suitability of LVDT sensors for your measurement system, please contact us.
