MTS Series 793 - Manual (Page 29)

About Tuning
MTS Series 793 Tuning and Calibration Introduction 29
Note Excessive negative (–) D Gain can cause your system to become
unstable.
Feed Forward (F or F2) Gain
Feed Forward is a gain adjustment not needed for many systems. It is more likely
needed on systems where you need to move a lot of oil fast to get the actuator
moving. So it is more likely to be found on systems with large actuators, massive
grips, or moving load cells. For our car example, and if you remember how
carburetors work, feed forward is equivalent to the accelerator pump—that is the
gizmo that gives a quick slug of gas when you suddenly floor the gas pedal.
This mode is like Derivative mode, except that it anticipates changes rather than
reacts to them.
Feed forward watches the command side of the summing junction, and provides
a derivative of the command. Remember that a derivative is proportional to the
rate of change of a signal; therefore, the faster the command is changing (like
during the leading/trailing edges of a squarewave), the greater the signal is.
So look at feed forward as a form of a “predictor” of where the actuator should be
going. The signal gives the servovalve an early indicator to tell it that it needs to
open faster than would be expected from the existing error signal.
Feed forward gain looks at the command
side of the summing junction. The output is
the derivative of that signal, indicating how
fast the command is changing.
Inadequate feed forward. Correct feed forward.
Command
Feedback
Error