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In a feed-forward system, the control variable adjustment is not error-based. Instead it is based on knowledge about the process in the form of a mathematical model of the process and knowledge about or measurements of the process disturbances.
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What are PID gains and feed-forward gains?
Feed-forward gains
Where PID gains are reactive—based on error that has already occurred—feed-forward gain is proactive. Feed-forward gains, which include velocity feed-forward and acceleration feed-forward, predict the commands needed to achieve zero error and inject them into the control loop.
Velocity feed-forward (Vff) works against viscous friction (friction that’s proportional to velocity) and minimizes error during the constant velocity portion of the move.
Acceleration feed-forward (Aff) works against inertia in the system to minimize error during the acceleration and deceleration phases of the move.
reactive-被动的；proactive-前瞻的，主动的，有前瞻性的，先进一步的；
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Lecture 5 - Feedforward 这个课件挺好的
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Feed-forward in position-velocity loops
Motion control systems use feed forward gains to speed up the response to rapidlychanging position commands. Feed forward is often used in the cascadedposition/velocity loop; here a faster velocity loop is enclosed within a slower positionloop. The feed forward path speeds response by taking the command around the slowerposition loop, directly to the velocity loop.
With or without feed forward, servo systems rely on high loop gains to achieve goodperformance. High loop gains provide two functions: they help the controller respond tothe command and they help reject disturbances. Unfortunately, as we have seen, servogain values are limited because, when they are too high, they cause instability. Sincefeed-forward gains work outside the loop, they do not cause instability. Feed-forward, inparallel with high loop gains, can greatly improve the dynamics of a servo system.