LVDT Ratios and Mounts

The basic linear variable differential transformer (LVDT) described in the previous column (see references below) used a 4-wire connection with signal-conditioning electronics that took the differential ac signal and produced a dc voltage to indicate position. Some applications might use a 5-wire circuit in which the "center" tap between the 2-coil secondary coils (pick-up coils) in the differential transformer connects to ground and the other end of each secondary transformer connects to a signal-conditioning circuit. In this ratiometric mode, signal conditioning equipment uses the formula:

V_out (V_s2 - V_s1)/(V_s2 + V_s1)

where the subscripts refer to the two secondary LVDT coils. Measuring a voltage ratio rather than a voltage difference provides better accuracy, better response to temperature changes and less dependence on a constant current through the primary LVDT coil. But you need an extra connection to the LVDT and more sophisticated signal-conditioning circuits than in a 4-wire configuration.

Temperature can greatly affect LVDT performance due to expansion or contraction of mechanical components. Temperature changes in the LVDT itself causes the coil characteristics to change slightly because the resistance of copper increases with temperature. Also, the dimensions of mechanical links to whatever you must measure will change. But according to John Matlack, global business-development manager at Macro Sensors, although thermal effects seem onerous, you can predict them and use known thermal-expansion coefficients to calculate corrections. Of course, you must measure temperature accurately, too.

You might wonder how you can suspend a moving core accurately within an LVDT's axial opening. Normally the clearance between a core and the LVDT's inner surface amounts to 0.02 to 0.03 inches. In some cases, engineers simply suspend the core within a stationary LVDT and let it move up or down freely. In other situations, engineers can specify a Teflon sleeve that keeps the core in position. Because the core includes threaded holes at both ends, you can devise other mounting arrangements, too. Most often, the coil assembly remains fixed in place and the core moves in or out.

During a conversation with Matlack, he noted engineers must carefully mount an LVDT without distorting its package and thus the internal coils. Some manufacturers will offer mounting blocks that gently but solidly clamp an LVDT in place. "You can't use a wrench to position an LVDT body or a set screw to hold it in place," says Matlack. "You'll crimp the coils."

Engineers might think that the nature of an LVDT means they can move a core outside the LVDT during measurements, but they cannot. The core must remain within the sensor.

I'll wrap up LVDTs in February's column.