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
Vout (Vs2 - Vs1)/(Vs2 + Vs1)
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.