Abstract: A code track on a rotable member (e.g., a disc) defines a pseudo-random sequence of binary digits consisting of ones and noughts. The pseudo-random sequence extends continuously around the rotatable member and includes a given number of n consecutive noughts. The binary digits of the sequence are arranged so that each group of n adjacent digits is different from each other group of n adjacent in the pseudo-random sequence. A reading mechanism is operatively positioned relative to the code track for reading n consecutive digits of the pseudo-random sequence, wherein each n consecutive digits of the pseudo-random sequence define a position of the rotatable member.

Abstract: An encoder for accurately determining the position of one body relative to another consists of an optical mask carried by one body which moves through the focal point of a concave reflector carried by the other body. The reflector produces an image of the mask in the same plane of the mask and the mask and its image together produce positional information which is utilized by a photo-sensitive detector. The optical pattern of the mask is arranged such that the detector receives constant variations in the levels of illumination as the mask moves.