Encoder self-callibration apparatus and method

A processing apparatus for use with an optical encoder, the apparatus comprising:

• a. a digital estimator for generating samples of phase, Φi, according to the equation
• 7 Φ i = A ⁢ ⁢ TAN ⁡ [ Y i X i ] ,
• Xi and Yi being samples of quadrature signals received from the encoder, i being an integer having values from one to an integer n;
• b. a fringe counter for generating a count, the count increasing by one every time the phase, when measured modulo two pi, crosses from a fourth quadrant of a unit circle to a first quadrant of the unit circle, the count decreasing by one every time the phase, when measured modulo two pi, crosses from the first quadrant of the unit circle to the fourth quadrant of the unit circle, the fourth quadrant extending from angles 3/2 pi to 2 pi, the first quadrant extending from angles zero to pi/2;
• c. a state generator that receives the count and the samples of phase Φi and generates in response thereto two burst output signals in A quad B format, each of the burst output signals being characterized at any point in time by either a high value state or a low value state, comprising:
• an interpolator that generates an integer number representative of the count and the phase Φi;
• an accumulator that generates a running sum by counting transitions in the A quad B burst output signals;
• a comparator for generating a signed difference value representative of a difference between the integer number and the running sum;
• a burst generator that generates transitions in the A quad B burst output signals until the signed difference value is zero.

A method of processing signals generated by an optical encoder, comprising:

• a. generating samples of phase, Φi, according to the equation
• 8 Φ i = A ⁢ ⁢ TAN ⁡ [ Y i X i ] ,
• Xi and Yi being samples of quadrature signals received from the encoder, i being an integer having values from one to an integer n;
• b. generating a count, the count increasing by one every time the phase, when measured modulo two pi, crosses from a fourth quadrant of a unit circle to a first quadrant of the unit circle, the count decreasing by one every time the phase, when measured modulo two pi, crosses from the first quadrant of the unit circle to the fourth quadrant of the unit circle, the fourth quadrant extending from angles 3/2 pi to 2 pi, the first quadrant extending from angles zero to pi/2;
• c. generating two burst output signals in A quad B format by:
• generating an integer number representative of the count and the phase Φi;
• generating a running sum by counting transitions in the A quad B burst output signals;
• generating a signed difference value representative of a difference between the integer number and the running sum;
• generating transitions in the A quad B burst output signals until the signed difference value is zero.

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 the 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 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 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 is substantially equal to the phase index.