Distance-measuring sensors range from precise laser
interferometers to simple potentiometers, but for linear distance measurements
in industrial equipment and testing apparatus, a linear variable differential
transformer (LVDT) proves difficult to beat. This versatile sensor measures
distances in valves, robotic actuators, dispensing equipment,
dimensional-measuring instruments, production testers and so on.
LVDTs can measure distances on the order of fractions of
millimeters up to several feet with a linearity of about ±0.25 percent over the
specified measurement range and with a repeatability error of about 0.01
percent of full-scale output (FSO). And unlike some sensors, an LVDT provides
absolute-position information. So, when you start equipment you know the core's
position and don't have to return an actuator to a known reference point.
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A typical LVDT relies on three coils wound coaxially but
separately around a hollow cylinder. A piece of highly permeable metal serves
as a core that moves through the hollow cylinder (see figure). External electronics
pass an ac sine-wave signal through the center, or primary, coil. The moveable
core magnetically couples to the two outside sensing, or secondary, coils. The
position of the core determines how much of the ac signal couples into each
secondary coil. Connecting these two coils in series but with opposite polarity
creates an ac output that varies in amplitude as the core position changes. The
phase of the ac output signal - often around 3 kHz - shifts by about 180
degrees as the core moves through a "null point" at which the output drops to a
minimum.
LVDT manufacturers produce cores threaded
at each end for attachment of a mechanical link. Although an LVDT offers good
linearity, if a core moves beyond its specified limit, the output begins to
flatten, so response looks like a "bow tie." Engineers might use a look-up
table or a linearization algorithm to obtain results in this extended region,
but in my opinion you're better off finding a more suitable LVDT.
Equipment designers can buy LVDT signal conditioners that
create a dc voltage proportional to the position of an LVDT core. Or, they can
design their own circuit around an IC such as the AD598 from Analog Devices.
John Matlack, global business-development manager at Macro Sensors, an LVDT
supplier, says if someone needs fewer than 10 or so sensors, they usually buy
off-the-shelf electronics or LVDTs with internal electronics. If they use more
LVDTs, they often design their own signal-conditioning circuits.
The use of internal electronics limits the temperature range
an LVDT can experience to the range tolerated by the electronic components,
about -40 to 85C. And according to Matlack, if an LVDT fails, it's usually the
electronics that have died, not the sensor.
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