Continuous Auto-Calibration of sin/cos-Encoders

Triamec encoder auto-calibration on a 20μm scale: improvement by a factor of 7
Triamec encoder auto-calibration on a 20μm scale: improvement by a factor of 7
  • Noise reduction
  • Better absolute accuracy
  • Higher axle stiffness
  • Temperature compensation

Everyone knows the problem: Alignment of encoders of linear axes is very difficult. Often it is not possible to achieve good alignment over the entire travel range. Once a workable compromise is achieved, temperature effects, vibrations caused by transport or aging can partially ruin the work. A complex re-adjustment is necessary.

 

Triamec solves this problem very elegantly with an algorithm running continuously in the servo drive, which automatically eliminates ellipse errors of a not perfectly aligned sin/cos encoder - without any user interaction, even the previously unique calibration is no longer necessary.

Auto-Calibration compensates for all five parameters of the ellipse error (first-order error): two offsets, two amplitudes and one phase, or the angle between the sine and cosine signals. This is done continuously in the FPGA of the Servo Drive at 100kHz intervals. The result is an almost perfect encoder signal over the entire travel range, which has multiple positive effects:

  • Improved absolute accuracy of the position and thus machine precision
  • Reduces acoustic noise
  • Allows a significant increase in controller damping ("D" component)
  • Mechanical expansion of the encoder mounting due to temperature fluctuations is compensated. Drift errors in the evaluation electronics are also corrected.

The robustness of auto-calibration is proven on the one hand by continuous filtering of the calculated calibration data and on the other hand by numerous very successful applications on customer machines.