Stepper motors and linear scale closed loop setup?

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30 Jul 2018 23:07 #115185 by andypugh

Hakan wrote: I will eat up the remaining error. But not in this machine. While idle and with a tad of f-error I can hear the motor taking a step say every second. But the motor doesn't seem to be strong enough to move the table just one step,


This is where I-gain comes in, to take away the steady-state error.
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31 Jul 2018 12:15 #115217 by Hakan
I know, it was just that the motor never stopped reposition. I thought it should stop doing that once inside the deadband, but it just kept on although not very often, one microstep every 5-20 seconds or so. Wasn't used to that, I guess.

When using the I-parameter it happened that the position was 1 µm off (seen on the DRO). I just let it do its thing and yeah eventually the DRO switched to 0. So it does its work. It is very sensitive to large I-values, it is easy to get the axis to oscillate.

I now have two sets of parameters that work fine.
P=250, I=0, D=0
P=300, I=10, D=0.5
I really can't see much of a difference in the axis response. Had to lower P from earlier post, jogging very short distances 0.01 mm was a bit sensitive.

Also made a quick milling test, milled in the X axis direction. Following error was a bit noisy like in the figure before, but inside 0-10 µm. So load doesn't seem to be a big factor. Admittedly light load.

Also noticed that the f-error from joint.0.f-error is different from "my f-error" which is encoder.00.position - pid.x.command. "my f-error" is larger than joint.0.-f-error (can be seen in prev figure), I wonder if "my f-error" is wrong.

Why do we use velocity mode? I don't know so I switched to position mode and tried to tune the PID loop. Got worse result, the best result f-error was at least double the f-error with velocity mode. So I run velocity mode again. For servo motors I see the point using velocity mode, but for stepper motors at least it felt more logical to use position mode. But that didn't work out in reality.
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01 Aug 2018 16:55 #115327 by Hakan
Tuned and tested in the video.



Now the Y axis.
The following user(s) said Thank You: Clive s
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07 Aug 2018 09:30 - 07 Aug 2018 09:32 #115736 by Hakan
Finishing off the X axis. Aligning the reading head, mounting the cover.
Another of these un-polished videos that show every detail.

Last edit: 07 Aug 2018 09:32 by Hakan.
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08 Aug 2018 15:02 - 09 Aug 2018 01:35 #115811 by RayJr
Hi Peter.
Re: PID command-deriv pins on all stepgens
Could you elaborate a bit more on this?
Does it apply to open loop PID setups, or both open & closed?

I am on LCNC V. 2.7.14

Thank you!

Ray
Last edit: 09 Aug 2018 01:35 by RayJr.
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15 Aug 2018 23:24 #116106 by Hakan
The Y axis is now done. The Y axis is in poor mechanical shape with a backlash of 0.2-0.3 mm.
Nevertheless I test to see if the encoder can improve things.



I think the result was interesting, although not perfect. The Y axis will be rebuilt later on.
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