Newport MM4006, User Manual

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MM4006 S e r v o Tuning
7.2.2 I n c r e a s i n g Performance
If your system is stable and you want to improve the performance, start
with the current parameters. The goal is to reduce the following error dur-
ing motion and to eliminate it at stop.
Depending on the performance starting point and the desired outcome,
here are some guidelines for further tuning.
Following Error Too Large
This is the case of a soft loop. It is especially common if you just performed
the steps in 7.2.1. The proportional gain Kp is probably too low and Ki and
Kd are zero.
Start by increasing Kp by a factor of 1.5 to 2. Continue this operation while
monitoring the following error until it starts to exhibit excessive ringing
characteristics (more than 3 cycles after stop.) To reduce the ringing, add
some damping by increasing the Kd parameter.
Start with a Kd value one order of magnitude smaller than Kp. Increase it
by a factor of 2 while monitoring the following error. As Kd is increased,
the overshoot and the ringing decrease almost to zero.
NOTE
Remember that if the acceleration is set too high, the overshoot cannot
be completely eliminated with Kd.
If Kd is further increased, at some point the oscillation will reappear, usual-
ly at a higher frequency. Avoid this by keeping Kd at a high enough value,
but not so high as to reintroduce oscillations.
Next add more gain. Increase the Kp value by 50% at a time until signs of
excessive ringing appear again.
Alternatively increase Kd and Kp until Kd cannot eliminate the overshoot
and ringing at stop. This indicates Kp is larger than its optimal value and
should be reduced.
Ultimately, optimal values for Kp and Kd depend on the stiffness of the
loop and how much ringing the application can tolerate.
Errors At Stop (Not In Position)
If you are satisfied w i t h the dynamic response of the PID loop but the
motion device does not always stop accurately, modify the integral gain
factor Ki. As described in the Motion Control Tutorial Section, this term of
the PID reduces the following error to near zero. Unfortunately it can also
contribute to oscillation and overshoot. Always change this parameter
carefully and in conjunction with Kd.
NOTE
Ks (0 to 1) controls the saturation level of Ki integral factor of the PID
position
closed loop. A excessive value of Ks implies the delayed effect
on the controller reaction towards processes to command. Conversely, a
too little value eliminates the integrator action. The optimal value varies
between 0.5 and 0.9.
Start, if possible, with a value for Ki that is at least two orders of magnitude
smaller than Kp. Increase its value by 50% at a time and monitor the over-
shoot and the final position at stop.
N e w p o r t 7 . 5 E D H 0 1 8 1 E n 1 0 2 0 — 03/03