Velocity feed forward

Velocity feed forward 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 Velocity feed forward is off)
  • The gain is given in per mil [1]. 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.
[1]1000 means the target position signal will be routed directly, 500 means scaled to half

Note

It is highly recommended to use an encoder with a resolution of more than 14 bit per revolution. Using encoders with a lower resolution will result in computed velocity quantization and torque spikes during the acceleration of the motor. Using a dual loop control might result in a similar effect if the gear reduction ratio is high and the resolution of the driving shaft encoder is small. In these cases, the feature still can be used with a very low cut-off frequency (5-10 Hz). Alternatively, a master-generated feed forward velocity can be sent to the slave via the velocity offset object.

Using a velocity offset additionally

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 Velocity feed forward

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