LVDT Wrap Up

When you want to use an LVDT to measure distance but justdon't have space, consider using a half-bridge LVDT, which uses two coilsinstead of the three in the LVDTs described in earlier columns (see sidebar). The two coils form half of a Wheatstone bridge circuit (seediagram). You apply the ac excitation signal across both coils and measure theoutput from the connection between these coils. By comparing this output withthe excitation signal an instrument can provide a voltage proportional toposition. Note that measurement of the excitation signal occurs at the coil andnot at the excitation-signal output.

Because a half-bridge LVDT has only two coils, you don't observea null position - the point at which a 3-coil LVDT produces either a minimum- or a zero-voltage output when centeredbetween the coils. But you still geta linear variation fromsignal-conditioning instruments and can easily determine absolute position.

You might wonder how fast an LVDT can respond to distancechanges. Much depends on the mass of the core and its connecting linkages, thefriction of the core in any sleeve inside the LVDT coils, and the friction inmechanical joints. Using a small low-mass core reduces inertia but at theexpense of distance range.

Response also depends on how quickly signal-conditioninginstruments can handle changes in signal levels. Keep in mind a rule of thumbthat the frequency of core movement should not exceed one-tenth the frequencyof the excitation signal.

Unlike some sensors, an LVDT exhibits negligible hysteresis. Soyou can accurately determine the core's position regardless of whether it movedfrom left to right or vice versa. You might see some hysteresis arise fromloose mechanical linkages, though. Even though you should not move a coreoutside the LVDT body during measurements, over-travel will not damage the coreor coils. So you don't need to protect an LVDT with "hard stops" that mechanicallylimit core movements. Forcefully rotate a potentiometer beyond its stops andsomething will break.

You also can buy rotational variable differential transformers(RVDTs) that use a curved coil and a hollow toroidal core. This configurationlets you measure rotational displacements, but it
requires hard stops.

The metal used to package LVDT coils reduces theeffects of noise from external magnetic fields. But you must choose anon-magnetic rod to connect with the core. Several series-300 stainless steelshave an austentite (non-magnetic) structure that makes them a good choice andthey resist corrosion, too. Keep an eye on the coefficient of linear thermalexpansion (CLTE) of coupling materials. Delrin, for example, has a coefficientof about 5 x 10^-5 in/in/°F, but series-304 stainless steel comes inat about 1 x 10^-5 in/in/°F.