Servo Tuning Advice

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01 Oct 2012 13:34 #24800 by Todd Zuercher
I'm having a little trouble dialling in the z axis. It is a vertical axis run by a zero backlash lead screw with no counter balance. The Screw and nut have a fair amount of friction. Maybe I'm just hoping for to much, but I just can't seem to get the following error as good as I think it should be. The part of the move that is giving me the most trouble is the last half of the deceleration.

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01 Oct 2012 13:36 #24801 by andypugh
Replied by andypugh on topic Re:Servo Tuning Advice
Todd Zuercher wrote:

I'm having a little trouble dialling in the z axis. It is a vertical axis run by a zero backlash lead screw with no counter balance.

Have you tried setting the "offset" to match the steady-state output required to support the weight of the head?

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01 Oct 2012 13:57 - 01 Oct 2012 14:02 #24802 by Todd Zuercher
I ment to attach a picture but it was a little to large to be accepted


When you say "offset" do you mean the Bias? Yes I did try that, but the I term seems to do be having the same effect.
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Last edit: 01 Oct 2012 14:02 by Todd Zuercher.

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01 Oct 2012 14:41 - 01 Oct 2012 14:49 #24803 by PCW
Replied by PCW on topic Re:Servo Tuning Advice
A negative -I term is just wrong (it cannot be a different sign than the P term)
Not only is it adding positive feedback, but if it winds up to the point that it bounds
it will cause a run-away due to an assumption in the PID comp that "I" is always positive
Last edit: 01 Oct 2012 14:49 by PCW.

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01 Oct 2012 14:57 - 01 Oct 2012 14:58 #24805 by BigJohnT
Replied by BigJohnT on topic Re:Servo Tuning Advice
Todd,

Following Peter's advice when I tuned my BP mill I created a servo tuning tutorial for velocity drives. Just so I don't forget how to do it...

John
Last edit: 01 Oct 2012 14:58 by BigJohnT.

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01 Oct 2012 15:11 #24808 by Todd Zuercher
Here is another try, with the acceleration turned up a little more to try to make things a little more obvious. I think I will have to reduce it by at least 1/3 on this axis for real life. Also these are torque mode drives.
This plot has a little more FF2 which seems to realy bring out some oscilations at the transitions.

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01 Oct 2012 15:52 #24811 by Todd Zuercher
This is about as good as I can get it any suggestions?

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01 Oct 2012 16:01 #24812 by andypugh
Replied by andypugh on topic Re:Servo Tuning Advice
I assume the max f-error is 500u" (0.0005") not 500umm (0.0005mm, or 0.00002")?

0.0005" f-error in rapids might be acceptable, but you also seem to have something of an offset steady-state, with no apparent attempt by the controller to correct it. (and it seems to be rather larger than the deadband)

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01 Oct 2012 16:38 #24813 by PCW
Replied by PCW on topic Re:Servo Tuning Advice
Yes, heres a static error (250 uInch) in the middle that the Integral term should pull in
I would make the integral term larger until it gets unstable and then use about 1/2 to 1/3 of that amount. Seems like the deadband could be smaller as well just so it does not interfere with interpretation of the plots

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01 Oct 2012 16:41 #24815 by Todd Zuercher
Yes it is inches (not mm). I am not sure what to try to do about the steady state error. Turning off the dead band doesn't seem to affect it. Nothing seems to make much of a dent in it except more P, and I already have an awful lot of that. Is this a problem that is just inherant to torque mode setups? It just seems like I'm missing something. I do have another set of screws laying around with twice the pitch, any chance that could help?

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