Usable Feed-Forward Gains
Feed-forward gains have been somewhat of a dilemma in the motion control industry. Typically, the use of velocity and acceleration feed-forward gains is helpful in improving tracking accuracy. Unfortunately, when these gains are turned up to their optimum levels, the axis gets extremely noisy because the quantization noise in the internal velocity and acceleration variables is pushed directly into the torque loop (sometimes refer to as "torque chatter"). In addition to wear on the mechanics, this torque chatter causes the motor’s RMS output to go up dramatically, reducing the usable continuous output of the motor.
The SSt servo drives with RAS enabled, create accurate, smooth internal representations of the velocity and acceleration so the feed-forward gains can be turned up to their optimum settings without generating torque chatter. This increases tracking accuracy, and when tuned correctly, actually reduces the RMS torque usage because the motor is less likely to waste torque as it oscillates around the command profile. The oscilloscope screen-captures below illustrate how effective the feed-forward gains become once the RAS is enabled. Notice the improved tracking accuracy and improved settling time:
The RAS feature makes feed-forward gains usable and productive. Notice the reduced torque chatter and improved tracking. Also notice that the RMS torque used with the RAS engaged is 38% less in this example.
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- A) Without Feed-forward gains
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- B) With Feed-forward gains
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- C) With Feed-forward gains and RAS
Because RMS torque/force usage is reduced when using feed-forward gains the motors will run cooler or can be reduced in size, enhancing your design options.