Servo Tuning Advice
- Todd Zuercher
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01 Oct 2012 13:34 #24800
by Todd Zuercher
Servo Tuning Advice was created 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:
Have you tried setting the "offset" to match the steady-state output required to support the weight of the head?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.
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01 Oct 2012 13:57 - 01 Oct 2012 14:02 #24802
by Todd Zuercher
Replied by Todd Zuercher on topic Re:Servo Tuning Advice
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
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
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
Replied by Todd Zuercher on topic Re:Servo Tuning Advice
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.
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
Replied by Todd Zuercher on topic Re:Servo Tuning Advice
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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)
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
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
Replied by Todd Zuercher on topic Re:Servo Tuning Advice
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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