LVDT Ratios and Mounts

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

V_out (V_s2 - V_s1)/(V_s2 + V_s1)

wherethe subscripts refer to the two secondary LVDT coils. Measuring a voltage ratiorather than a voltage difference provides better accuracy, better response to temperaturechanges and less dependence on a constant current through the primary LVDTcoil. But you need an extra connection to the LVDT and more sophisticatedsignal-conditioning circuits than in a 4-wire configuration.

Temperature can greatly affect LVDT performance due to expansionor contraction of mechanical components. Temperature changes in the LVDT itselfcauses the coil characteristics to change slightly because the resistance ofcopper increases with temperature. Also, the dimensions of mechanical links towhatever you must measure will change. But according to John Matlack, globalbusiness-development manager at Macro Sensors, although thermal effects seemonerous, you can predict them and use known thermal-expansion coefficients to calculate corrections. Of course, you mustmeasure temperature accurately, too.

You might wonder how you can suspend a moving core accuratelywithin an LVDT's axial opening. Normally the clearance between a core and theLVDT's inner surface amounts to 0.02 to 0.03 inches. In some cases, engineerssimply suspend the core within a stationary LVDT and let it move up or downfreely. In other situations, engineers can specify a Teflon sleeve that keepsthe 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 movesin or out.

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

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

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