Optical encoder for optically measuring displacement of moving body

Abstract

In order to improve signal modulation characteristics and detect a position at a higher precision, collimated light of wavelength .lambda. and beam radius D enters a fixed diffraction plate and a movable diffraction plate comprising a grating of pitch 'p'. The grating has main diffraction components of orders .+-.1. The plates are arranged in parallel with a distance 'g' between them and generally perpendicular to an optical axis of the beam. The beam diffracted by the diffraction plates and in parallel to the optical axis of the beam is condensed with a condenser lens for limiting an incident pupil size to be within D-2 g.lambda./p. The amount of the light condensed by the condenser lens is detected by a photosensor. In a different example, a collimated light beam enters a first fixed plate and a rotary diffraction plate successively to transmit diffracted light of orders .+-.1. Then, the transmitting light is guided to a symmetrical point with respect to a rotation center of the rotary plate. Then, the beam is transmitted through the rotary plate and a second fixed plate which transmits only diffracted light of orders .+-.1. Then the beam is detected by a sensor. Then, even if the rotary plate has an eccentricity on the rotation thereof, fluctuations of the optical intensity of the beam are small, and a rotation angle can be detected with no accumulated errors. In order to detect a position of a moving diffraction plate precisely, a condenser lens is attached to the moving diffraction plate and first and second photosensors are provided on a trace of light transmitted through the condenser lens. A gap between the tow photosensors is smaller than a beam size of the condensed beam while a width of each of the two photosensors is larger than the beam size. A signal processor processes output signals of the photosensors to determine a position of the plate without being affected by the amount of light from a coherent light source.