PMAC/PMAC2 V1.17C A DDENDUM J UL -2005 Reserved Setup Word : The second setup line of this entry type is reserved for future use, and should be left at 0. Active Bias Correction Setup Word : The third setup line of the five-line entry contains the sine and cosine bias terms that are used in the sum-of-squares calculations. Two signed 12-bit bias terms are combined in a 24-bit word. The sine bias-correction term is in the high 12 bits (bits 12 – 23); the cosine bias-correction term is in the low 12 bits (bits 0 – 11). These terms match the high 12 bits from the corresponding A/D converters.
This word does not necessarily match the bias “result” term derived from using this entry to determine a suggested bias correction, or the bias correction used in the “feedback” table entry for the encoder or resolver. Reserved Setup Words : The fourth and fifth setup lines of this entry type are reserved for future use, and should be left at 0. Result Word (Sum of Squares): When bit 16 of the first setup line is 0, the final (fifth) result word contains the sum of squares of the biased sine and cosine measurements for the most recent servo cycle.
Result = (SineADC + SineBias) 2 + (Cosine ADC + CosineBias) 2 The values SineADC and CosineADC are read from the A/D converters at the address specified in the first setup line. The values SineBias and CosineBias are read from the third setup line. To understand the scaling of the result word, it is best to think of all four of the values as being normalized, that is, as having a valid range of -1.0 to +1.0. With small bias terms, the sum of squares result would have a possible normalized value of 0.0 to +2.0. When read as an unsigned integer, this register has a range of 0 to 16,777,215 ($FFFFFFF), corresponding to a normalized range of 0.0 to 2.0.
When the encoder and interpolator circuitry, or the resolver and excitation circuitry, are working properly, the sum of squares should have a normalized value of +0.25 to +0.9999 (2,097,152 to 8,388,607, or $200000 to $7FFFFF). If the resulting normalized value is greater than or equal to +1.0 (8,388,608, or $800000), meaning that the most significant bit (bit 23) is set to 1, at any point in the cycle, this indicates that saturation has occurred in at least one of the readings due to either too large a signal or a significant bias. This should be corrected before using this sensor in actual operation.
If the result has a normalized value of less than +0.25 (2,097,152, or $200000), meaning that bits 23, 22, and 21 are all 0, at low sensor frequencies, the signals are too small to get full resolution from the result, and this should be corrected before using this sensor in actual operation. Many sinusoidal encoders do have a reduction in signal magnitude of up to one-half at their highest frequencies, reducing the magnitude of this square term by three-quarters, and this is acceptable. It is possible to monitor this term in the actual application to check for loss of the encoder.
If the inputs are no longer driven externally, for example because the cable has come undone, the positive and negative input pair to the ADC will pull to substantially the same voltage, and the output of the ADC will be a very small number, resulting in a small magnitude of the sum of squares in at least part of the cycle. (If both signals cease to be driven externally, the sum of squares will be small over the entire cycle). The high four bits (bits 20 – 23) of the sum-of- squares result can be monitored, and if the four-bit value goes to 0, it can be concluded that the encoder has been “lost”, and the motor should be “killed”. V1.17C Firmware Version 25