Potentiometers are widely used as position transducers, although the degree of repeatability and accuracy they deliver typically can't match digital encoders. But traditional mechanical potentiometers offer adequate resolution and good linearity, making them suitable for many applications over a wide temperature range. The drawbacks to using potentiometers are also well known. For example, poor contact and long-term drift that accompany mechanical wear reduce the life of potentiometers and for some applications require frequent replacement.
An alternative to traditional resistive potentiometers is a noncontacting device that uses capacitive coupling instead of moving mechanical parts. It consists of a fixed resistive track of conductive plastic and a moving probe. An ac voltage is applied to the track and the probe picks off a current, called a displacement current, by capacitive coupling. To avoid the need for a trailing contact cable, the displacement current is transmitted to a parallel collector track. The displacement current is proportional to the position of the probe along the two tracks and is expressed as
An integrator sums these currents, and its output voltage (which is proportional to the position of the probe) is then defined by the sum of the displacement currents over a fixed period.
Although the basic principle of this system is not new, the fact that the coupling capacitances vary with track position has never been taken into account before. Three capacitances are involved; the capacitance between the probe and the fixed resistive track, between the probe and the collector track, and the stray ground capacitance. All three capacitances vary with the position of the probe. This means that in the equation for current, the capacitance, C, is not constant, making the equation nonlinear.
The potentiometer uses feedback to eliminate or compensate some variability in capacitance by monitoring the applied voltage on the fixed resistive track. To do this, an adjustable ac voltage is applied simultaneously to both ends of the resistive track. This produces a position-independent output signal that relies only on the signal amplitude and the probe's coupling capacitance. The signal is then fed into a PI controller where it is compared with a fixed reference signal. The controller then adjusts the track voltage accord-ingly. In this way the capacitance fluctuations that produced nonlinearities are compensated by adjusting the supply voltage to the track.
Compared with mechanical resistive potentiometers that use standard contacting wipers, the noncontacting potentiometer gives more accurate readings. Improved resolution is an added benefit that results from the integrating effect of the probe. The output smoothness characteristic, which is a measure of the spurious electrical variations in the output present in contacting potentiometers, is also vastly improved since it has no mechanical contact.
Information for this article was contributed by Rainer Utz, Novotechnik U.S. Inc., 155 Northboro Rd., Southborough, MA 01772, (508) 485-2244, Fax: (508) 485-2430, www.novotechnik.com