Temperature Considerations When Specifying LVDT Linear Position Sensors
- Created on Sunday, 01 December 2013
- Macro Sensors, Pennsauken, New Jersey
LVDT linear position sensors offer significant mechanical life for industrial process control and factory automation systems.
The LVDT linear position sensor can operate successfully over a wide range of temperatures, depending upon its design and the application in which it is operating. While the output of the LVDT linear position sensor is not immune to temperature variation, proper design and construction for environmental effects can produce a sensor that provides a relatively stable, highly linear output over a wide temperature range. Depending upon materials and construction techniques, modern LVDT linear position sensors can be built to withstand temperature extremes from -195 to +600 °C.
In general, the stability of the LVDT output is adequate for many applications without specific concern about temperature effects. This is especially true when the LVDT is used in laboratory and industrial plant environments. On the other hand, temperature effects are generally significant in aircraft and military applications, and in certain specific industrial uses with extreme environment conditions that affect temperature.
The typical maximum operating temperature of an AC-operated LVDT is 150 °C. Hermetically sealed sensors, with housings constructed entirely of stainless steel, can operate reliably in temperatures as high as 200 °C (see Figure 1).
For applications where sensors must operate in extreme temperatures, the sensing element of an AC-operated LVDT is segregated from the electronic circuitry. Connected by long cables up to 31 meters (100 feet), AC-operated LVDTs work with remotely located electronics that power the sensors, amplify, and demodulate their output. Output is then displayed on a suitable readout and/or input to a computer-based data acquisition system for statistical process control, or automated process control.
The DC-operated LVDT, on the other hand, is limited by the properties of the materials in the electronic signal- conditioning module that is contained within the sensor. DC-LVDTs typically operate at temperatures from –40 to 125 °C.
Ambient temperature variations, if great enough, affect LVDT characteristics in either or both of these ways:
1) The sensitivity, or change in output voltage for a unit core displacement, may increase or decrease. This is equivalent to an error produced by multiplying the calibrated output signal value by a scale factor that is slightly larger or smaller than unity.
2) The actual core null or zero position may shift in one direction or the other by a distance that is the same when measured at any position within the range. This effect is equivalent to an error produced by adding or subtracting a small constant to or from the calibrated output signal value — an effect commonly referred to as zero-shift error. This kind of error does not affect sensitivity (volts out per unit core travel).
A rise in ambient temperature with a resultant rise in LVDT temperature increases the resistance of the copper wire used in the primary and secondary coils of the LVDT. The most direct consequence of this increased resistance, particularly at lower frequencies, is an increase in primary impedance, with an associated reduction in primary current. This, in turn, affects output and sensitivity levels. This resistance change results in errors similar to 1) above.
Resistance variations in the secondary circuit are not an important factor if a high-impedance load is used, but will factor into power transfer for a low-impedance load. For example, a change in secondary resistance of +50% would result in a voltage output decrease of approximately 1%, if the load impedance to secondary resistance ratio is 50 to 1.
Temperature or temperature variation can affect the output signal of the LVDT in two ways: mechanical expansion and change in electrical properties. Mechanical expansion causes relative motion between the LVDT core and the LVDT windings, with the net effect being a false core motion signal that produces a zero-shift error. Changes in the electrical properties of the LVDT caused by effectively changing its primary input current or magnetic properties of the core materials produces a scale-factor change or span-shift error. Both temperature effects must be considered separately when attempting to use LVDTs in high-temperature areas or when attempting to compensate for temperature effects.