This page describes SDC's (Symmetric Differential Capacitor) Sensors.
These are a relatively recent development of the traditional use of
capacitance to measure position changes.
SDC sensors are stunningly intuitive (once you get the idea :-) ) and offer
potentially very high resolution and accuracy at relatively low cost.

SDC lends itself to amateur and low cost implementations (unlike some
methods which are applicable only to advanced or high technology users.) A
basic measurement system could be produced from scrap material and off the
shelf parts for a few dollars.

        http://physics.mercer.edu/petepag/sens.htm

This page is well worth exploring if you are interested in low cost or
accurate position sensing. SDC applications extend into the sub micron range
(eg Scanning Squid Microscope), high accuracy (Cavendish tortional balance),
linear, rotational & multi dimensional sensing and more. Applications
covered include seismograph, accelerometer, pressure transducer and a range
of position measurement systems. SDC MAY even offer a solution to a present
requirement of mine :-).

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Ref supplied by Ken Mardle.

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