Patent ID: 8513589

Claim:
An indexing method for use with an optical encoder, the indexing method comprising: providing an encoder disk, comprising: a signal track comprising a diffraction grating formed as a ring on the encoder disk; and an index track formed as a ring on the encoder disk, the index track comprising an index mark provided at an index angular coordinate; providing an illumination system structured to direct light to the encoder disk; providing a detector structured to detect light diffracted from the encoder disk, the detector comprising: two offset detectors structured to detect light diffracted from the signal track and output a quadrature signal; and an index detector structured to detect light reflected from the index track and output an index pulse; calculating an estimated state count k est of quadrature states from a rising edge of the index pulse to a middle of the index pulse; calculating Q kest , wherein Q kest is the quadrature state at k est and corresponds to the quadrature state at an approximate center of the index pulse; and determining an offset correction to be applied to the optical encoder based on a value of Q kest .