Newport ESP6000 User Manual page 213

Section 7 — Servo Tuning
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
Following Error Too Large
This is the case of a soft PID loop caused by low values for Kp and Kd. It is
especially common after performing the procedures described in para-
graph 7.2.2.
First increase Kp by a factor of 1.5 to 2. Repeat this operation while
monitoring the following error until it starts to exhibit excessive ringing
characteristics (more than 3 cycles after stop.) To reduce ringing, add
some damping by increasing the Kd parameter.
Increase it by a factor of 2 while monitoring the following error. As Kd is
increased, overshoot and ringing will decrease almost to zero.
Remember that if acceleration is set too high, overshoot cannot be
completely eliminated with Kd.
If Kd is further increased, at some point oscillation will reappear, usually
at a higher frequency. Avoid this by keeping Kd at a high enough value,
but not so high as to re-introduce oscillation.
Increase Kp successively by approximately 20% until signs of excessive
ringing appear again.
Alternatively increase Kd and Kp until Kd cannot eliminate overshoot and
ringing at stop. This indicates Kp is larger than its optimal value and
should be reduced. At this point, the PID loop is very tight.
Ultimately, optimal values for Kp and Kd depend on the stiffness of the
loop and how much ringing the application can tolerate.
The tighter the loop, the greater the risk of instability and oscillation
when load conditions change.
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.
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.
If intolerable overshoot develops, increase the Kd factor. Continue
increasing Ki and Kd alternatively until an acceptable loop response is
obtained. If oscillation develops, immediately reduce Ki.
NOTE
NOTE
7 - 3