User guide

26 6250 Servo Controller User Guide
Servo Control Techniques
To ensure that you are tuning your servo system properly, you should understand the tuning
techniques described in this section.
The 6250 employs a PIV&F servo control algorithm. The control techniques available in this
system are as follows:
P .......... Proportional Feedback (controlled with the SGP command)
I .......... Integral Feedback (controlled with the SGI command)
V ......... Velocity Feedback (controlled with the SGV command)
F.......... Velocity and Acceleration Feedforward (controlled by the SGVF and SGAF commands,
respectively)
The block diagram below shows these control techniques in relation to the servo control
algorithm configuration. The following table presents a condensed summary of each control's
effect on the servo system.
Analog
Control Signal
Servo Control Algorithm
Velocity Feedforward
(SGVF)
Acceleration Feedforward
(SGAF)
+ +
+
-
+
-
+
+
Drive
Motor
Load
Encoder
-10V
+10V
Digital-to-Analog
Conversion (DAC)
Current, Torque, or Velocity
Control Signal
Servo Drive System
Variable Integral Limit
(set with SGILIM)
Proportional Feedback
(SGP)
Integral Feedback
(SGI)
Velocity Feedback
(SGV)
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 6250 uses proportional feedback, the control signal is linearly proportional to the encoder
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 (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,
underdamped, or in some cases unstable.
NOTE
The proportional feedback gain (SGP) should never be set to zero, except when
open-loop operation is required.