Optical rotary encoder using dual frequency beat difference

Abstract

A rotary encoder includes a laser device which emits a laser beam having beats caused by superposition of a parallel wave and a vertical wave, which have mutually different frequencies and whose oscillating surfaces are normal to each other. A part of the beating laser beam is led to a first detector where the beat frequency of the original laser beam is detected. The remaining laser beam is split into the parallel and vertical waves by a splitting/irradiating device, and the parallel wave is irradiated onto a predetermined position of the rotational member and is scattered or reflected upon Doppler shifting attendant to the rotation of the rotational member. The scattered or reflected parallel and vertical waves are led to a synthesizer where both waves are synthesized to provide a laser beam with beats. The synthesized laser beam is led to a second detector where the beat frequency of the laser beam after being Doppler shifted is detected. The thus detected beat frequency and the beat frequency detected in the first detector are supplied to an arithmetic unit. The arithmetic unit computes a rotational angle of the rotational member in accordance therewith.