Tech Briefs

Temperature Considerations When Specifying LVDT Linear Position Sensors

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, visit http://info.hotims.com/45611-122.