Effects of Temperature Changes on LVDT Sensors

Date:2021-09-15 00:00

  For more than 50 years, LVDT linear position sensors have been reliable linear position feedback tools in a variety of laboratory, industrial, military and aerospace applications. LVDT displacement transducers are inherently reliable devices capable of providing high-accuracy linear displacement measurements from microinches to 2 feet,

  And suitable for a wide range of operating temperature. While the output stability of an LVDT linear displacement sensor is suitable for many applications, it is also affected by temperature changes. In some specific applications, such as aircraft, subsea equipment and turbomachinery, temperature effects can be significant.

  The effect of temperature changes on the output signal

  Changes in temperature 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 winding.

  The net effect is to signal false core motion, resulting in zero migration errors. Temperature may also alter the LVDT's primary input current or the magnetic properties of the core material, thereby affecting the LVDT's electrical properties. This produces scale factor changes or trip migration errors.




  AC output LVDT

  AC and DC LVDT output signals

  AC powered LVDT displacement sensors have a maximum operating temperature of up to 300°F due to the remote mountability of the sensor companion electronics. On the other hand, DC powered LVDTs that contain electronics inside the sensor body are limited by the properties of the materials in the electronic signal conditioning module.

  DC LVDTs can operate down to –40°F if the temperature is near constant.

  ambient temperature

  Ambient temperature changes can have predictable effects on the operation of both AC- and DC-powered LVDTs. While signal conditioning helps compensate for primary current variations in AC LVDTs, DC LVDTs cannot use this method due to space constraints.

  The effect of temperature on LVDT materials

  Temperature changes have little effect on the magnetic properties of the LVDT core material, and have negligible effects on transformer operation within the normal operating temperature range. To counteract the influence of the thermal expansion coefficient of the LVDT material, the LVDT expands symmetrically from the center to the two ends in structure.

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

  High temperature ratings are achieved through the use of special fabrication materials and special high melting point soldering for the linear displacement sensor.

  Resistance change due to temperature

  Elevated transformer temperatures increase the resistance of the copper wire commonly used for the primary and secondary coils. The most immediate consequence of this resistance increase is an increase in the primary impedance.

  Primary Current Stability

  A constant current excitation source is an obvious but not always practical solution to temperature effects. 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.