Appendix C: Tuning Your System
85
Figure C-1. Insufficient Proportional Gain
In the above plot, the motor (actual position) is not keeping up with the command position, yet
the output voltage is not saturated. Clearly, more gain is required.
Figure C-2. Excessive Proportional Gain
Excessive proportional gain is characterized by oscillation. In some situations, damping
(derivative gain) can be increased to help compensate.
C.2.2 Derivative Gain (K
d
)
This term provides damping and stability to the system by preventing overshoot as the error
changes. A low value for the Derivative Gain causes the system to have very fast response to
changes in position error but may have overshoot or “ringing” after a step change in position.
Large values of Derivative gain have slower step response but may allow higher Proportional
Gain to be used without oscillation.
Typical values for Derivative Gain are roughly 2 times the proportional gain for velocity
(voltage) controlled servos , i.e. 200-1000 and roughly 4 times the proportional gain for torque
(current) controlled servos, i.e. 1000-8000.
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