Servo Tuning detailed How To.

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06 Aug 2019 08:41 #141490 by tommylight
I think you would be better off with a new config if the feedback is detached somewhere.
Yes that is the vibration i am refering to, try to reduce that by also setting the deadband, it helps minimise the vibration when stationary.
Do not try to tune anything until you get to the point where you can see the DRO in Linuxcnc change when you move the motors by hand, then check if they are counting in the right direction, then you can try to enable the drives and check if you need to reverse the output.
The main thing is when you enable the drives in Linuxcnc they should stay put, try to jog them very slowly after setting the jog speed to a reasonable value (50 - 100mm/m) and see if they stop ( some drift is ok at this point or hunting ).
Only after checking all that proced to tune the P value till you get some vibration while stationary and some more vibration after jog and stop, then lower that value till you get barely noticeable vibration, tune the deadband, test, see if you can increase P by just a bit and still have barely noticeable vibrations. Now try to move the motors by hand while enabled, they should fight back quite vigorously the more you try to move them.
Now the tuning can start.
If you notice that the motors can move a lot before they fight back, deadband should be adjusted some more.
Now you can try ading some I or/and D till you get very stiff motors that fight back as soon as you try to move them. It is much better to have I and D set to zero than to have them at wrong values, this can be tuned even after everything else.
To tune the FF1 there are two methods, the one i use and explained here, and the other that PCW recommends by setting the output scale to maximum velocity in machine units so the FF1 is always 1 ( please do check this as i might have missed something as i never used this method ).
If using the method described here, start with 0.1, then 0.2 and so on all this while the velocity is set to 50-100mm/m, check the feedback for the least error while jogging, then add another decimal like 0.25 and test again for the lowest error, then do 0.255 and check, then 0.2555 and so on till you get a flat feedback line. Increase the jog speed a bit and test again, add decimals, increase speed, and so on. Those values are just example, but do not worry if you end up with something like 0.345678.

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06 Aug 2019 11:37 #141513 by Type_Zero_Design
Thanks for the feedback Tommy!

So what's strange is I do believe that I have done everything you mentioned I should up to the point of tuning. I'll have to verify tonight but I believe that my DRO was counting manually moving the motor by hand. I know for sure that when enabled that the motor will fight you to try to stay in position.

If the DRO is changing when rotating by hand without enabled is there anything else I should check to get the joint.N.f-error reporting something on the screen?

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06 Aug 2019 11:37 #141514 by Type_Zero_Design
Thanks for the feedback Tommy!

So what's strange is I do believe that I have done everything you mentioned I should up to the point of tuning. I'll have to verify tonight but I believe that my DRO was counting manually moving the motor by hand. I know for sure that when enabled that the motor will fight you to try to stay in position.

If the DRO is changing when rotating by hand without enabled is there anything else I should check to get the joint.N.f-error reporting something on the screen?

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06 Aug 2019 13:10 #141523 by Todd Zuercher
If the DRO is working, the joint.N.f-error will be working as well, They are calculated from the same input.

You may need to adjust the vertical gain in Halscope to actually be able to see what the f-error is doing. It is entirely possible (quite probable in fact) that the f-error is so small that it looks like a flat line in Halscope until you adjust the gain. I usually have the gain set to 1-10u/div (0.001-0.01) when tuning a servo.
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06 Aug 2019 14:14 #141530 by Type_Zero_Design
Thanks Todd,

I will check that out when I get back to the garage. It is completely possible that I mis-remember the fact that the DRO was reading with manual rotation.

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07 Aug 2019 02:07 #141559 by Type_Zero_Design
Tommy,
I just got a chance to take a look at this. So you were correct it is not counting on the DRO when moved by hand... I can go into Hal config and look at the encoder count under: pins/hm2_7i76e/0/encoder

That shows the the encoder counting.

So what's the Disconnect here and how do I resolve?

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07 Aug 2019 02:10 #141560 by HueyHQ

Type_Zero_Design wrote: Lastly, will my accel/decell and velocity affect the tune. Should they be solidified prior to running? I'm not to sure on how to decide those numbers yet.

Just to add my experience about tuning with regards to acceleration values!

On my retrofit, I had a lot of trouble tuning following all the advice I had found because I had entered an arbitrary value for the acceleration parameter (way too high). It wasn't until I bumped it down to something more in line with what I had seen in others config files, that I could even start to tune my servos. My machine works fine now, but doesn't move as fast as it did before the retrofit, so I could go back and refine the tuning with higher accel params.

So, yes - in my experience, acceleration values will affect the tune!
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07 Aug 2019 09:38 #141596 by tommylight
@Type 0
You are better of by doing a new config than chasing gremlins at this stage.
You can post the config files here, you might have already but am on the phone so not much use for now from me.
I'll have a look when i get my hands on one of my laptops.
Regards,
Tom.
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07 Aug 2019 09:40 #141597 by tommylight
@Huey
Thank you, i missed answering that.

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07 Aug 2019 17:49 #141626 by Type_Zero_Design
So Tommy, is there something I need to do different in setting up the config this go round to make sure that happens? is there some option I need to enable in PNCConfig?

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