Shaft angle encoder with a symmetrical code wheel

Abstract

A shaft angle encoder is provided with a symmetrical code wheel and pattern of photodetectors for providing an index pulse indicative of shaft angular position. The code wheel has two concentric index tracks each of which is symmetrical in a circumferential direction and are symmetrical with respect to each other and also with respect to a data track for indicating increments of angular position. In an exemplary embodiment there is a pair of windows of unit width in the one index track with a spoke therebetween with a width of at least two units. A pair of lateral windows extend away from each other more than a unit width along the second track. An opaque area at least two units wide is provided between the lateral windows in the second track. The photodetectors are arranged so that when the code wheel is in the index pulse position, a pair of photodetectors for the inner track are illuminated through the windows in the inner track, and two photodetectors for the outer track are both occulted by the opaque area between the lateral windows in the second track. Such an arrangement provides symmetry of the code wheel so that it can be inverted either accidentally or intentionally during assembly and yet side lobes of photocurrent from the photodetectors on the separate tracks are not coincident, thereby avoiding false index pulse triggers.