Parker Hannifin 88-021610-01G - Manual (Page 83)

Parker Hannifin
Chapter 5 Tuning 83
Servo System Gains
Proportional Feedback Control (SGP)
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—see TPER command). The
proportional gain is set by the Servo Gain Proportional (SGP) 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 (SGP 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 (SGP) to zero, except when
open-loop operation is desired.
Velocity Feedback Control (SGV)
Using velocity feedback control, the control signal is proportional to the
feedback device's velocity (rate of change of the actual position). The Servo
Gain Velocity (SGV) command sets the gain, which is in turn multiplied by the
feedback device's velocity to produce the control signal. Because the velocity
feedback acts upon the feedback device's velocity, its control action
essentially anticipates the position error and corrects it before it becomes too
large. Such control tends to increase damping and improve the stability of the
system.
A high velocity feedback gain (SGV) can also increase the position tracking
error when traveling at constant velocity. In addition, setting the velocity
feedback gain too high tends to slow down (over-damp) the response to a
commanded position change.
Integral Feedback Control (SGI)
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 (SGI) 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 (SGP), then there is no need to use
the integral feedback control.