Acceleration Feedforward

In addition to proportional feedback of the following error nearly all position controllers contain a proportional acceleration feedforward: The K_a factor. This acceleration feedforward should normally only be used in association with the proportional component of the position controller: The K_v factor. Acceleration feedforward control requires to adjust the axis for strict symmetry:

• When stationary, the following error is symmetrical about 0 (DAC offset).
  Confer section Automatic DAC Offset Adjustment.
• When moving steadily, the following error is symmetrical about 0 (reference velocity).
• Set K_v.
• Measure the extreme value of the acceleration a_+max and the associated following error d_+max in the middle of the acceleration phase. Measure the extreme value of the deceleration a_-max and the associated following error d_-max in the middle of the braking phase.
• K_a+ = K_v * d_+max / a_+max,
  K_a- = K_v * d_-max / a_-max,
  K_a = (K_a+ + K_a-) / 2.