An integrated capacitive position sensor

Capacitive position sensors prevail in small battery-powered instruments. They "read" a scale (consisting of a row of floating electrodes) with a row of driver electrodes and a pickup electrode, shielded so that mutual coupling may occur only via the scale. The novel sensor described here has only one row of identical multipurpose electrodes, acting in turn as driver, pickup and shield. A theoretical model for inter-electrode capacitance optimizes sensor geometry at a size suitable for integrating the complete sensor on a 7.5 by 3.9 mm die. Drive and control signals scan the electrodes, and the position-tracking phase information derived from the pickup signal is digitally filtered on-chip. Measurements made with a 1.016 mm scale electrode pitch and a sensor-to-scale gap of 0.2 mm yield a pitch-to-pitch periodic nonlinearity of 0.002 mm.