Abstract: The absolute encoder utilizes two or more sensors per track and the output of each sensor is modulated by the track to generate a multiphase analog signal from each track. An analog to digital converter converts each analog signal to a digital value. The multiphase cyclic digital values are combined into a single linear output per track that increases linearly from zero to a maximum over the full span of the measuring scale. Corrections are made for zero drift in the analog signals, for changes in amplitude (gain) of the analog signals and for deviations in the linear values from a perfect straight line. When measuring rotary displacement the methods disclosed may be used to combine the outputs of the geared scales.

Abstract: An incremental encoder comprises a rotatable disc having an axis of rotation and a plurality of circumferentially arranged slots formed in the disc at equal angular increments. Light source is so positioned on one side of the disc so as to illuminate the slots. A first light sensor is so positioned on the other side of the disc as to receive light from the source means transmitted through the slots. A second light sensor is so positioned on the other side of the disc farther from the axis than the first sensor so as to receive light from the source transmitted through the slots out of phase with the light transmitted to the first sensor. A third light sensor is so positioned on the other side of the disc as to receive light from the source transmitted through the slots for control purposes.