Errors At Stop (Not In Position)
If you are satisfied with the dynamic response of the PID loop but the
stage does not always stop accurately, modify the integral gain factor
Ki. As described in the Motion Control Tutorial section, the Ki factor
of the PID works to reduce following error to near zero.
Unfortunately it can also contribute to oscillation and overshoot.
Change this parameter carefully, and if possible, in conjunction with
Kd.
If intolerable overshoot develops, increase the
Kd factor. Continue
increasing
Ki, IL and Kd alternatively until an acceptable loop
response is obtained. If oscillation develops, immediately reduce
Ki
and
IL.
Remember that any finite value for
Ki will eventually reduce the error
at stop. It is simply a matter of how much time is acceptable for the
application. In most cases it is preferable to wait a few extra
milliseconds to get to the stop in position rather than have overshoot
or run the risk of oscillations.
Following Error During Motion
This is caused by a Ki, and IL value that is too low. Follow the
procedures in the previous paragraph, keeping in mind that it is
desirable to increase the integral gain factor as little as possible.
• Use the Windows-based "ESP_tune.exe" utility to change PID
parameters and to visualize the effect. Compare the results and
parameters used with the previous iteration.
• The ESP300 controller uses a servo loop based on the PID with
velocity and acceleration feed-forward algorithm.
• Use the lowest acceleration the application can tolerate. Lower
acceleration generates less overshoot.
A summary of servo parameter functions is listed in
Table 6.2.1.
Start with the integral limit (
IL) set to a high value and Ki value at
least two orders of magnitude smaller than
Kp. Increase its value by
50% at a time and monitor overshoot and final position at stop.
6.2.4 Points To Remember
• Use the default values provided with the system for all standard
motion devices as a starting point.
• Use the minimum value for
Ki, and IL that gives acceptable
performance. The integral gain factor can cause overshoot and
oscillations.
6-4 Section 6 – Servo Tuning
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