Servo Tuning Advice

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01 Oct 2012 17:13 - 01 Oct 2012 17:22 #24821 by PCW
Replied by PCW on topic Re:Servo Tuning Advice
For a torque loop I would add P and D until it cannot be made stable
(more D will allow more P up to a point) then back off on both proportionally until its stable again. (do this with no I term)

Then as I said before, add I until its unstable and then use 1/2 to 1/3 of that amount of I
I suspect you need 10 to 1000 times more I
Last edit: 01 Oct 2012 17:22 by PCW.

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01 Oct 2012 17:21 #24822 by Todd Zuercher
PCW wrote:

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


When I make "I" large the whole thing starts shifting up (like adding more bias). Is it normal for this to happen? Then should I use the bias to pull it back to 0?

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01 Oct 2012 17:27 - 01 Oct 2012 17:42 #24823 by PCW
Replied by PCW on topic Re:Servo Tuning Advice
No that makes no sense. Something is wrong if 'I' term behaves this way. it should at the least start to pull in the static error to 0

A plot would help
Last edit: 01 Oct 2012 17:42 by PCW.

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01 Oct 2012 18:31 #24825 by Todd Zuercher
But most of the tuning threads on the forum that I've looked at said that a lot of I was bad.

After about 5000 or so I stopped moving the error away like the bias, and started to bring the steady state error in line.

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01 Oct 2012 19:22 #24830 by PCW
Replied by PCW on topic Re:Servo Tuning Advice
You _may_ be able to reduce the width of the error spikes at the
beginning and ending of motion by increasing 'I' further

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03 Oct 2012 13:50 #24906 by Todd Zuercher
I am a bit frustrated again. Made some different length moves at different speeds and the steady state error came back with a vengence. I am not sure where the solution lies. But right now I'm to buisy with other things to mess with it. :unsure:

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30 Oct 2012 19:33 #26055 by Todd Zuercher
I've finally had a little time to play with it again. I was not able to able to add enough I to cause instability. However having an I of 50K did result in a very interestingly odd shaped f-error plots in hal-scope. I'll post a pictuer of it later. Also I don't understand why adding "I" is messing with the bias number so much.

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30 Oct 2012 19:50 #26056 by andypugh
Replied by andypugh on topic Re:Servo Tuning Advice
"I-term windup" may be the problem. The I term increases all the time there is an error, then a small overshoot causes it to reduce again, but very slowly.

Some controllers zero out the I term when the error term changes sign. (But sometimes that is very much the wrong thing to do)

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30 Oct 2012 22:58 #26073 by Todd Zuercher
Replied by Todd Zuercher on topic Servo Tuning Advice
Here is the hal scope for I @50000
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30 Oct 2012 23:09 #26076 by Todd Zuercher
Replied by Todd Zuercher on topic Servo Tuning Advice
Are there any simple tricks to make the ouput scale positive? I wonder if I have a positive/negative problem screwing with the PID math. I ran it backwards a little the other day and I thought things seemed a little better, so I switched 2 encoder wires to reverse that, but I haven't found a simple way to reverse the motor, (I don't want to deal with swiching a power wire and re figuring out the hall sensors (that was much to painful the last time). I will have a look at the drive software I might be able to do it there. I know it can for step/dir, but I don't know about analog torque mode.

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