For over 50 years, LVDT linear position sensors have been a reliable tool for linear position feedback in a variety of laboratory, industrial, military, and aerospace applications. LVDT position sensors are inherently reliable devices that provide high-precision linear position measurements from microinches to 2 feet,

and are suitable for a wide range of operating temperatures. While the output stability of LVDT linear position sensors is suitable for many applications, it can be affected by temperature changes. In certain specific applications, such as aircraft, subsea equipment, and turbomachinery, the temperature effects can be significant.

Effects of Temperature Changes on Output Signal

Temperature changes can affect the output signal of an LVDT in two different ways, including mechanical expansion and changes in the electrical properties of the LVDT. Mechanical expansion causes relative motion between the LVDT core and the LVDT windings.

The net effect is a false signal of core motion, producing a zero shift error. Temperature can also affect the electrical properties of the LVDT by changing the primary input current of the LVDT or the magnetic properties of the core material. This can produce scale factor changes or travel migration errors.

AC Output LVDT

AC vs. DC LVDT Output Signals

AC-powered LVDT position sensors have a maximum operating temperature of up to 300°F because the sensor electronics can be remotely mounted. On the other hand, DC-powered LVDTs, which contain the electronics within the sensor body, are limited by the material properties in the electronic signal conditioning module.

DC LVDTs can operate as low as –40°F if the temperature is kept close to constant.

Ambient Temperature

Variations in ambient temperature have predictable effects on the operation of both AC-powered and DC-powered LVDTs. While signal conditioning can help compensate for changes in primary current for an AC LVDT, this approach is not possible with a DC LVDT due to space limitations.

Effects of Temperature on LVDT Materials

Temperature changes have little effect on the magnetic properties of the LVDT core material and have negligible effects on the transformer's operation over the normal operating temperature range. To compensate for the effects of the LVDT material's coefficient of thermal expansion, the LVDT's structure expands symmetrically from the center to the ends.

New manufacturing techniques and materials also allow LVDTs to operate in harsh environments, including extreme high and low temperatures. Custom LVDTs can be designed to operate at continuous temperatures up to 400°F.

High temperature ratings are achieved by using special materials of manufacture and special high melting point solders for the linear position sensor.

Resistance Changes Due to Temperature

Increasing transformer temperature increases the resistance of the copper wire commonly used for the primary and secondary windings. The most direct consequence of this increased resistance is an increase in primary impedance.

Primary Current Stability

A constant current excitation source is an obvious but not always practical solution to the effects of temperature. If a constant current source is not available, some stabilization of the primary current can be achieved by placing a large external resistor in series with the primary.