Motion System Design
MSD 101: Sensors and signals

MSD 101: Sensors and signals

LVDTs (linear variable differential transformers) have been around for nearly 100 years in one form or other. They are among the most precise and rugged linear position sensors.

LVDTs measure linear displacement as a function of the mutual inductance between magnetic coils. Most consist of a primary (input) coil flanked by two secondary (output) coils symmetrically positioned on either side. A magnetic core travels back and forth through the center of the coils and, depending on its position, diverts more or less magnetic flux from the primary to the two secondaries.

The output signal is usually the difference between the secondary voltages, and is thus zero when the core is at the center or null position. Here, an equal amount of flux flows to each secondary, inducing identical ac voltages. As the core moves in either direction, the voltage on one coil increases, while that on the other decreases. The amplitude of the difference varies linearly with core position, while the phase, which changes abruptly as the core passes the null position, indicates the direction of motion.

LVDTs can last indefinitely because their sensing elements do not make contact. By contrast, sensors based on sliding or bending mechanisms are prone to wear and subsequent degradation. Another advantage for LVDTs stems from the nature of their measuring principle. The resolution of magnetic flux is theoretically infinite and, with suitable signal processing, can reveal increasingly small movements.

Questions & answers

Q: How precise can an LVDT be?

A: LVDT sensitivity is defined in terms of mV/V/0.001 in., where "mV" represents the differential secondary output voltage, "V" is the excitation voltage on the primary, and "0.001 in." is the "per unit" displacement. Sensitivities generally range from 0.05 to 10 mV/V/0.001 in.

Q: What's the typical measuring range?

A: The stroke can be anywhere from a fraction of an inch to around ten inches or so. Short stroke devices are usually more sensitive.

Q: Is dc better than ac?

A: Some LVDTs have built-in signal conditioning electronics. For a dc input, they produce a fairly linear dc output that can be read from almost any voltmeter. Though these "dc" LVDTs are easier to use than ac devices, they cost more, are somewhat larger (because of the integrated chips), and have a lower temperature range. For anything above 858C, an ac LVDT would be better.

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