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Abstract

A multi-turn shaft position encoder employs a plurality of encoder discs with successive speed reductions of 1:2.sup.n and the generation of n+1 bits of binary data representative of the angular position of the respective discs as the input shaft rotates. Specifically, a high-speed disc is coupled to an input shaft and low-speed disc means is coupled to the input shaft with a speed reduction of 1:2.sup.n relative to the high-speed disc. At least one group of n+1 bits of binary data representative of the angular position of the high-speed disc are generated as the input shaft rotates. A group of n+1 bits of data representative of the angular position of the low-speed disc means is generated as the input shaft rotates. Thus, the low-speed disc is divided in to 2.sup.n units per revolution but produces 2.sup.n+1 bits of data. The data is generated by the encoder in unit distance code and is then converted into fewer bits of data in natural binary code. The encoder has a high-speed disc in connection with which two groups of data bits are generated, one being shifted relative to the other. In addition, there is cumulative bit shifting from disc to disc. The n+1.sup.st bit is generated on the same track of each encoder disc as the n.sup.th bit by placing the corresponding data bit-generating sensors at angularly spaced apart positions relative to the same track.

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