Calipers
Measurement method

Many digital calipers contain a capacitive linear encoder. Inexpensive Chinese models have 56 narrow emitter plates and one long receiver plate etched on the sliding display's printed circuit board, which intersect with a repeating pattern of T-shaped plates in the longer "stator" board. The top of the "T" plates intersect with the receiver plate, while the vertical bars of each "T" intersect with the emitter plates. The pitch of each "T" in the stator is slightly less than 8 times the pitch of each emitter plate, so their intersecting capacitive area is not perfectly aligned but rather forms an interference pattern. As the slider moves, these variable capacitances change in a repeating linear fashion. The slider's circuitry counts these repetitions as it slides and achieves finer resolution using linear interpolation of the capacitances. One model sends 8 periodic pulse-width modulation voltage signals (which appear identical but out of phase by 1 ⁄ 8 of the period), each connected to 7 emitter plates, and the resulting analog signal is read through a single receiver plate. The 1983 German patent DE3340782C2 (see figure) is said to describe the workings.

Other digital calipers contain an inductive linear encoder, which allows robust performance in the presence of contamination such as coolants. Magnetic linear encoders are used in yet other digital calipers.
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