Abstract: A rotary encoder for measuring a rotary angle and the number of rotation, which consists principally of a rotary shaft, a rotary slit disk fitted to the rotary shaft and a light emitting element and detectors opposing each other while interposing the rotary slit disk between them and fixed to a part of a housing in the non-contact arrangement with respect to the rotary slit disk. The present invention is characterized in that the rotary encoder is equipped with at least two pairs of detectors, each consisting of two detectors, and each detector pair is arranged in such a fashion that the resultant value of the detecting signals of the detectors forming each pair can be regarded as being detected at one point of resultant center which is substantially predetermined on the rotary slit disk.

Abstract: This invention relates to a rotary encoder for use in measuring the number of revolutions per minute or measuring the angular phase of a rotary shaft of a rotary machine. The encoder is provided with a fixed slit disc having slits therein, and a rotary slit disc disposed so as to be spaced from the fixed slit disc at a predetermined distance, having a plurality of slits arranged radially at regular angular intervals, and adapted to be rotated with the rotary shaft. The encoder is shipped to a user after the rotary slit disc has been fastened to the fixed slit disc on the part of the manufacturer.