Drive and Tach Sign Correction

Two optional sign inversions can be introduced in the velocity servo loop: one for the tachometer input and one for the drive output. The inversions must be set so that:

• Overall servo is stable.
• Positive requested velocities results in the positive tachometer velocities.

If the first condition is not met, then flipping either sign makes it stable. If this leads to a violation of the second condition, then you must flip both signs. Therefore, both conditions can be met by a suitable choice of multipliers.

The need for the stability condition is obvious but the need for the correct tachometer sign results from the requirements that the position servo imposes when it is running.