Appendix A
79
Gain Stability Damping
Disturbance
Rejection
Steady
State Error
Tracking
Error
Proportional (SGP)
Improve Improve Improve Improve Improve
Integral (SGI)
Degrade Degrade Improve Improve Improve
Velocity Feedback (
SGV)
Improve Improve ------------- ------------- Degrade
Velocity Feedforward (
SGVF)
------------- ------------- ------------- ------------- Improve
Acceleration Feedforward (
SGAF)
------------- ------------- ------------- ------------- Improve
Proportional Feedback Control (SGP)
Proportional feedback is the most important feedback for stabilizing a servo
system. When the APEX615n 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, as well as reduce the steady state position error.
Since the control is proportional to the position error, whenever there is any disturbance
forcing the load away from its commanded position (such as torque ripple or a spring load), 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,
underdamped, or in some cases unstable.
NOTE
The proportional feedback gain (SGP) should never be set to zero, except when
open-loop operation is desired.
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 can be 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.
In the task of reducing position error, the integral gain (SGI) works differently than the
proportional gain (SGP); this is because the magnitude of its control signal is not dependent
on the magnitude of the position error as in the case of proportional feedback. If any position
error persists, then the output of the integral term will ramp up over time until it is high
enough to drive the error back to zero. Therefore, even a very small position error can be
eliminated by the integral feedback control. By the same principle, integral feedback control
can also reduce the tracking error when the system is commanded to cruise at constant
velocity.
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