Servo Tuning

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16 May 2024 02:01 #300710 by B.Reilly01
Servo Tuning was created by B.Reilly01
I've followed John Thorntons servo tuning guide, and my X and Y are okay, but I don't know what to do with the image on my Z.  How do i shift the whole ferror plot up?  If I change FF1 then my FError positive and negative movements move away from each other.  Also the spike at the beginning...
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16 May 2024 09:48 #300719 by zmrdko
Replied by zmrdko on topic Servo Tuning
Can you please share a link with John Thorntons tuning guide?

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16 May 2024 11:01 #300723 by B.Reilly01
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16 May 2024 11:36 #300728 by tommylight
Replied by tommylight on topic Servo Tuning
FF1 should not be over 1, at least i never ever had it over 1.
Spikes can be corrected with a bit of FF2.
As for f-error, that looks like a normal Z axis without counterbalance, so try adding just s bit of Bias.
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16 May 2024 13:42 - 16 May 2024 13:43 #300740 by PCW
Replied by PCW on topic Servo Tuning
The FF1 value is directly influenced by the output scale.
FF1 can be over 1 if the outputs scale is greater than machine units per second
at full output. This is why I suggest for analog system tuning, that the output scale
(and limits) be set to machine  units per second at 10V. This means FF1=1.0.

I would not change this now if the tuning is close.
Last edit: 16 May 2024 13:43 by PCW.
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16 May 2024 16:35 #300764 by tommylight
Replied by tommylight on topic Servo Tuning
OK that explains it, thank you PCW.

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20 May 2024 19:16 #301026 by B.Reilly01
Replied by B.Reilly01 on topic Servo Tuning
I can't seem to get rid of the spike with FF2, is there anything else I can do?

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20 May 2024 21:46 #301037 by PCW
Replied by PCW on topic Servo Tuning
Can you plot the current error along with velocity?
(position is not really useful for this kind of tuning)

FF2 is use to reduce the error during acceleration
if the "spike" duration is the same as the period of
acceleration you should be able to tune this out.
(if you add too much FF2 you can make the spike reverse)

If the spike is only at the beginning and end of the acceleration
phases, this represents delays in the command --> feed back
in the system and the physical limitations in the drive/motor
hardware (its not possible to have a step acceleration function)

Delays may be cured by drive setup (for example any input filtering
should be minimized)

The delay and drive/motor physical limit issues can be worked
around a bit by lowering acceleration. A better solution is to use a
jerk limited trajectory planner. There is some ongoing work on this:

forum.linuxcnc.org/38-general-linuxcnc-q...ner-using-ruckig-lib
 

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28 May 2024 23:14 #301708 by B.Reilly01
Replied by B.Reilly01 on topic Servo Tuning
I reduced the acceleration, then could tune out the spike with FF2. Makes sense the head needs a lower accel than the table.

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29 May 2024 02:29 #301719 by PCW
Replied by PCW on topic Servo Tuning
Yes, it may be that you did not have enough torque
for the original acceleration settings, given the Z axis
mass and unbalanced weight.
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