When you want to use an LVDT to measure distance but just
don't have space, consider using a half-bridge LVDT, which uses two coils
instead of the three in the LVDTs described in earlier columns (see sidebar). The two coils form half of a Wheatstone bridge circuit (see
diagram). You apply the ac excitation signal across both coils and measure the
output from the connection between these coils. By comparing this output with
the excitation signal an instrument can provide a voltage proportional to
position. Note that measurement of the excitation signal occurs at the coil and
not at the excitation-signal output.
Because a half-bridge LVDT has only two coils, you don't observe
a null position - the point at which a 3-coil LVDT produces either a minimum- or a zero-voltage output when centered
between the coils. But you still get
a linear variation from
signal-conditioning instruments and can easily determine absolute position.
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You might wonder how fast an LVDT can respond to distance
changes. Much depends on the mass of the core and its connecting linkages, the
friction of the core in any sleeve inside the LVDT coils, and the friction in
mechanical joints. Using a small low-mass core reduces inertia but at the
expense of distance range.
Response also depends on how quickly signal-conditioning
instruments can handle changes in signal levels. Keep in mind a rule of thumb
that the frequency of core movement should not exceed one-tenth the frequency
of the excitation signal.
Unlike some sensors, an LVDT exhibits negligible hysteresis. So
you can accurately determine the core's position regardless of whether it moved
from left to right or vice versa. You might see some hysteresis arise from
loose mechanical linkages, though. Even though you should not move a core
outside the LVDT body during measurements, over-travel will not damage the core
or coils. So you don't need to protect an LVDT with "hard stops" that mechanically
limit core movements. Forcefully rotate a potentiometer beyond its stops and
something will break.
You also can buy rotational variable differential transformers
(RVDTs) that use a curved coil and a hollow toroidal core. This configuration
lets you measure rotational displacements, but it
requires hard stops.
The metal used to package LVDT coils reduces the
effects of noise from external magnetic fields. But you must choose a
non-magnetic rod to connect with the core. Several series-300 stainless steels
have an austentite (non-magnetic) structure that makes them a good choice and
they resist corrosion, too. Keep an eye on the coefficient of linear thermal
expansion (CLTE) of coupling materials. Delrin, for example, has a coefficient
of about 5 x 10
-5 in/in/°F, but series-304 stainless steel comes in
at about 1 x 10
-5 in/in/°F.
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