The LVDT is an electromechanical device that produces an electrical signal whose amplitude is proportional to the displacement of the transducer core.

The LVDT consists of a primary coil and two secondary coils symmetrically spaced on a cylindrical former.

Schematic of an LVDT

A magnetic core inside the coil assembly provides a path for the magnetic flux linking the coils. The electrical circuit is configured as above with the secondary coils in series opposition.

When an alternating voltage is introduced into the primary coil and the core is centrally located, then an alternating voltage is mutually induced in both secondary coils. The resultant output is zero, as the voltages are equal in amplitude and in 180º opposition to each other.
When the core is moved away from the null position the voltage in the coil, toward which the core is moved, increases due to the greater flux linkage and the voltage in the other primary coil decreases due to the lesser flux linkage. The net result is that a differential voltage is produced across the secondary tappings, which varies linearly with change in core position. An equal effect is produced when the core is moved from a null in the
other direction but the voltage is 180º different in phase.

 
Core displacement characteristics

The LVDT can be operated where there is no contact between the core and extension rod assembly with the main body of the LVDT housing the transformer coils.
This makes it ideal for measurements where friction loading cannot be tolerated but the addition of a low mass core can. Examples of this are fluid level detection with the core mounted on a float and creep tests on elastic materials. This frictionless movement also benefits the mechanical life of the transducer, making the LVDT particularly valuable in applications such as fatigue life testing of materials or structures.This is a distinct advantage over potentiometers which are prone to wear and vibration.

The principle of operation of the LVDT, based on mutual inductance between primary and secondary coils, provides the characteristic of infinite resolution. The limitations lie within the signal processing circuitry in combination with the background noise.

The principles of operation of the LVDT enables the transducer to be configured in a variety of housings depending on the degree of mechanical protection required. The use of rod end bearings, linear rolling element bearings and flexible conduit, helps the LVDT to survive even the most severe environments.

The LVDT principle can also be applied to the measurement of angular position; an RVDT (Rotary Variable Differential Transformer) converts the rotation of a shaft into a proportional electrical output signal.
Although the transducer is capable of continuous rotation, a plot of angular rotation against magnitude and phase of the output signal over 360º would result in a complete sinusoidal wavelength response and therefore two null positions. To avoid ambiguity, just one of the null positions is chosen during calibration, providing a typical measurement range of ± 60º. Some key characteristics of the Sensonics LVDT range are as follows.

• Range : 2.5mm to 600mm
• Core type : Sprung, guided or free
• Signal connection : End or side exit connector or cable with option of conduit.
• Mechanical conn : Rolling, ball or rod end
• Operating temp : -40ºC to +220ºC
• Specials : Submersible (.100m depth), Short body to stroke ratio

 
A range of Sensonics LVDT devices