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Abstract

An optical encoder for sensing angular position includes a rotatable optical medium having a major surface and an axis of rotation perpendicular to the surface, with a plurality of uniformly sized angular sectors defined on the surface. An annular absolute track is defined on the surface concentric with the axis, a plurality of digital bits being formed on the surface in sequence along the absolute track within each sector. Each such bit is optically readable as a binary 1 or a binary 0, with each plurality of bits defining a binary word. A first series of optically detectable incremental marks are evenly spaced around a first incremental track on the surface. The binary word in a sector is optically read to identify the angular position of that sector and indicate the approximate angular position of the medium, while the first series of incremental marks can be optically detected to refine the approximate angular position indication. A source of light illuminates the surface along a fixed radius from the axis, while a digital track detector senses the illumination reflected from the absolute bits and a first incremental track detector senses the illumination reflected from the incremental marks. Signal processing and decoding means convert the output of the absolute track detector into a binary word and interpolate the precise angular position of the medium from the output of the first incremental track detector. An input absolute track optical fiber is connected between a absolute track optical coupler and proximate the surface of the medium, while a first input incremental track optical fiber is connected between a first incremental track optical coupler and proximate the surface. Output absolute track and first incremental track optical fibers are connected between the couplers and the track detectors for conveying reflected light from the tracks to the detectors.

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