Velocity Feedforward (Vff)

Determines the contribution in the 16-bit DAC command output that is directly proportional to the instantaneous trajectory velocity. This value is used to minimize following error during the constant velocity portion of a move and can be changed at any time to tune the PID loop.

Using velocity feedforward is an open-loop compensation technique and cannot affect the stability of the system. However, if you use too large a value of Vff, following error during the constant velocity portion can reverse, thus degrading rather than improving performance.

Velocity feedforward is typically used when operating in PIVff mode with either a velocity block amplifier or substantial amount of velocity feedback (Kv). In these cases, the uncompensated following error is directly proportional to the specified velocity. You can reduce this following error by applying velocity feedforward. Increasing the integral gain (Ki) also reduces the following error during constant velocity but only at the expense of increased following error during acceleration and deceleration and reduced system stability. For these reasons, increased Ki is not the recommended solution.

Velocity feedforward is rarely used when operating in PID mode with torque block amplifiers. In this case, because the following error is proportional to the torque required (not to the velocity), it is typically much smaller and velocity feedforward is not required.