I use SMAC electromagnetic actuators. Up till now all the actuators had 5 micron optical encoders, and they worked great. Recently, we obtained an actuator with a 1 micron encoder. Initially I left the PID settings the same, but the actuator was unstable. Then, I played around with the PIDs but couldn't stabilize the motor. Would changing the TM constant help in this case? Which direction should the TM constant be adjusted? The motor was tested on a different non-Galil controller and ran fine. The encoder is working properly as well. Thank you,
Nick
EricK on 01/12/2006 - 11:45
Due to the fact that the system resolution has been increased by a factor of 5, the stability will become more difficult to obtain with the standard servo update rate of TM1000. For high resolution system we often recommend a faster servo rate, which will allow for an increase in range and resolution of the KD (damping) term. I would suggest starting with TM500 or even TM250.
Note changing the TM rate will affect all time based function. Meaning if TM500 is set then the speed SP should be halved and the AC values should be reduced by a factor of 4 to have the same effective velocity profile as TM1000. Additionally since the KD and KI term are time dependent you would have to double KD and halve KI to allow for similar PID filter parameters. This is in addition to the scaling of PID that would accompany the increase in gain due to the greater resolution.
For help with tuning and analysis, I would highly recommend our WSDK software to aid in this task. This software allows you to easily view the system step response while trying different PID values. This software also features some auto tuning algorithms that may help.