Parker Hannifin
Chapter 5 Tuning - 55 -
Servo System Gains
Proportional Feedback Control (PGAIN)
Proportional feedback is the most important feedback for stabilizing a servo
system. When the controller uses proportional feedback, the control signal is
linearly proportional to the position error (the difference between the
commanded position and the actual position). The proportional gain is set by
the Servo Gain Proportional (PGAIN) command. Proportional feedback can
be used to make the servo system more responsive (stiff), as well as reduce
the steady state position error.
Because the control is proportional to the position error, whenever there is
any disturbance (such as torque ripple or a spring load) forcing the load away
from its commanded position, the proportional control can immediately output
a signal to move it back toward the commanded position. This function is
called disturbance rejection.
If you tune your system using only the proportional feedback, increasing the
proportional feedback gain (PGAIN value) too much will cause the system
response to be oscillatory, under-damped, or in some cases, unstable.
Note: Do not set the proportional feedback gain (PGAIN) to zero, except
when open-loop operation is desired.
Derivative Feedback Control (DGAIN)
Using derivative feedback control, the control signal is proportional to the rate
of change of the position error. The Servo Gain Derivative (DGAIN)
command sets the gain, which is in turn multiplied by the feedback device's
rate of change of position error to produce the control signal derivative rate of
change of position error. Such control tends to increase damping and
improve the stability of the system.
A high derivative feedback gain (DGAIN) can also increase the position
tracking error when traveling at constant velocity. In addition, setting the
derivative feedback gain too high tends to slow down (over-damp) the
response to a commanded position change.
Integral Feedback Control (IGAIN)
Using integral feedback control, the value of the control signal is integrated at
a rate proportional to the feedback device position error. The rate of
integration is set by the Servo Gain Integral (IGAIN) command.
The primary function of the integral control is to overcome friction and/or
gravity and to reject disturbances so that steady state position error is
minimized or eliminated. This control action is important for achieving high
system accuracy. However, if you can achieve acceptable position accuracy
by using only the proportional feedback (PGAIN), then there is no need to
use the integral feedback control.
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