Patent ID: 7075057

Claim:
A method of generating an index signal for an optical encoder, the encoder generating quasi-sinusoidal quadrature signals indicative of a position of a scale relative to a sensor head, the encoder also generating a window signal, the window signal being characterized by a high value whenever an index mark of the scale is aligned with the sensor head, the window signal being characterized by a low value whenever the index mark is not aligned with the sensor head, the method comprising: a. setting a first number equal to the value of a measured phase when the window signal transitions from a low value to a high value; b. setting a second number equal to the value of the measured phase when the window signal transitions from a high value to a low value; c. if a difference between the first number and the second number is greater than pi and less than 3 pi, then setting a phase index equal to a value that is between the first and second numbers; d. generating the index signal whenever the window signal is characterized by a high value and when the phase value is substantially equal to the phase index.