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Submitted by BrianR on Wed, 01/27/2010 - 16:07

I am new to closed-loop servo controls and I still have a lot to read and learn about tuning PID loops, but I wanted to get these questions out here to make sure I'm heading in the right direction.

I'm using the PWM outputs of a DMC-2123 to drive coils that control hydraulic valves that control the motion of the motors. 1 valve for forward and 1 valve for reverse. I use the DIR output to decide which valve gets the PWM signal. The motors don't start moving until the duty cycle gets to about 33-38% in either direction creating a significant deadband.

Will a properly tuned servo be able to compensate for the deadband and still make smooth moves and transitions in direction? Or will I need to make some other adjustments? What might they be? (I was thinking the offset command might help, but it would only adjust in one direction and not the other)

Comments 1

Galil_AndyH on 01/28/2010 - 14:07

The setup of this system is definitely not ideal for closed loop control. The non-linearity in the response of the valves along with the swapping from one drive to the next will make it difficult to tune.

Ideally the system would bypass the hydraulic valves and control the motors directly. If this is possible it should be considered.

To tune the system, you will not be able to use any integrator (KI). The system will need to be tuned with KD and KP exclusively. If KI is used or KP is set too large, then the system will oscillate between the deadband of the forward and reverse motors.

If you have encoder feedback that is on the motors themselves, then you may be able to run in dual loop mode.

You are correct that the offset command is not going to help in this situation.

I would recommend getting the system up and running and use GalilTools to get some scope captures of the response of the system. Once you have some basic data, give us a call (800-377-6329) and we can work with you to improve the performance of the machine. If you would like to discuss in more detail the mechanics of the system, e-mail support@galilmc.com and ask for the e-mail to be forwarded to Andy.