A new capacitive speed transducer

The theory and fabrication of a new capacitive transducer suitable for accurate measurement and control of angular speed is described. The transducer lends itself to an essentially contactless method of speed measurement, in which only a thin slotted metal disc need be attached to the rotating member. The slotted disc, which rotates in the gap of a parallel plate air capacitor alters, through its shielding action, the capacitance of the latter in a cyclic manner at a rate proportional to the angular speed. The variation of this capacitance is sensed with the aid of a unity-ratio transformer bridge set-up and the output of the bridge is amplified, demodulated and filtered, giving rise to a nearly sinusoidal signal whose frequency is a measure of the angular speed.