Feedforward control

Feedforward control allows to increase performance as the target position signal gets routed directly to the velocity controller so that the velocity signal can be generated faster. After differentiation, the signal is filtered by a first-order low-pass filter and multiplied by a configured gain.

../../../../../../_images/feedforward_control_structure.png

Note

The filter is used to smooth the derivative of the target position. This is needed to soften communication lags (if present), attenuate noise in the target position signal and diminish the influence of target position quantization.

Usage

  • Default value of gain is 0 (This means Feedforward is off)
  • The gain is given in per mil *. A value of 1000 is a good starting point.
  • Reduce the cutoff frequency if periodic torque spikes are observed during motion. The default value is 1000 Hz.
  • Adjust gain and filter cut-off frequencies until torque distortions during motion are acceptable.
  • To switch off the filter enter a very high cut-off frequency.

*1000 means the target position signal will be routed directly, 500 means scaled to half

Note

This feature is very sensitive to distortions in target position signals (e.g. communication lags). This can lead to torque spikes.

Additionally, a velocity offset can be added to the control loop for even faster movements with high dynamic.

../../../../../../_images/feedforward_vel_offset_structure.png

Click here for details for using the velocity offset.

Object used for Feedforward control

In this object the cut-off frequency of the low-pass filter and the gain are configured.