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Chapter 4 Velocity and position transducers 121

with displacements as small as 1 mm up to 600 mm in a variety of linearities and sen-
sitivities. Because there is no physical contact between the core and the coils, the main
mechanical components of the LVDT will not degrade with use. If precision bearings are
used in the design, an almost infinite resolution, with zero hysteresis, is possible. The
small core size and mass, and the lack of friction, mean that LVDTs have a high-response
capability for dynamic measurements (for example, measurement of vibrations). Due to
their rugged construction, LVDTs are capable of operating in extreme environments, for
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example, ambient pressures up to 10 Pa and temperatures up to 700 C.

4.3.3 Resolvers

Resolvers are based on similar principles to LVDTs, but the primary winding moves
relative to the two secondary windings rather than having a moving solid core, as shown
in Fig. 4.13A. As the relative positions of the primary and secondary windings change, the
output varies as the sine of the angle. By having two windings ninety electrical degrees
apart and considering only the ratio of the outputs (Fig. 4.13B), the variations due to the
input voltage and the frequency changes become unimportant. The signals from the
resolver are therefore relatively insensitive to an electrically noisy environment, and they
can be transmitted over considerable distances with little loss in accuracy. In order to
dispense with the need for sliprings, a separate rotary transformer is used to provide

FIG. 4.13 The construction and wiring of a conventional housed resolver. (A) Internal construction, showing the
brushless construction. (B) Internal wiring.