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:
- 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.
- 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.
The effect of temperature variation on the magnetic properties of the LVDT core material is small and has negligible influence on transducer operation over the ordinary operating temperature range. To offset the effects of the thermal coefficient of the expansion of the LVDT materials, LVDTs are constructed to expand symmetrically from the center toward either end. Materials of construction are selected having similar or complementary coefficients of thermal expansion.
A constant-current excitation source is an obvious, but not always practical solution to scale factor temperature effects. If a constant-current power source is not available, primary current may be stabilized somewhat by connecting large external resistance in series with the primary.
An alternative method of translating the LVDT output is to measure the secondary voltages independently to generate a ratio of the difference divided by the sum of the secondary voltage values. Since the secondary output voltages are functions of temperature (temperature coefficients of sensitivity for each coil are assumed equal), the effects of environmental temperature fluctuations on sensor output are immunized. Other improvements for ratiometric operation include: less sensitivity to supply voltage and frequency variations, primary and secondary phase response, and common-mode noise.
In operation, the LVDT’s primary winding is energized by alternating current of appropriate amplitude and frequency, known as the primary excitation. The LVDT linear position sensors’ electrical output signal is the differential AC voltage between two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic circuitry to high-level DC voltage or current for convenient use by a computer or other input device. Because there is normally no contact between the LVDT’s core and coil structure, no parts can rub together or wear out. This means that an LVDT linear position sensor features unlimited mechanical life. This factor is highly desirable in many industrial process control and factory automation systems.
This work was done by Lee Hudson, Application Engineer at Macro Sensors. For more information, Click Here .