Pedersen VP2000 810M retrofit, severe servo hunting

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05 Jan 2019 14:12 #123541 by Juckix
Hey all. I am doing a retrofit of a milling machine that ran a Siemens 810M system previously using a Mesa 7i92 and a 7i77 for running all the I/O. We have gotten to the point where movement mostly works but we are having an issue where the Z axis specifically hunts back and forth about 0.02mm. This is unlike the other axes which hunt an expected 0.001mm since we are using 1 micron linear scales and analog velocity mode servos. Can anyone think of why this is happening? It decreased after adding shielding to the signal cable but is still not satisfactory.

The original system never showed any significant hunting (not noticeable). I suspect it might have disabled individual axes after reaching the set point, but it seems that all the analog enable signals only show up as a single pin in HAL. This is just speculation though. Thoughts?

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05 Jan 2019 14:17 #123543 by PCW
Can you post your hal and ini files.

Z axis are often harder to tune because they typically have a static load

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05 Jan 2019 14:25 - 05 Jan 2019 14:25 #123545 by Juckix
Here you go.

File Attachment:

File Name: vp2000_2019-01-05.hal
File Size:11 KB

File Attachment:

File Name: vp2000_2019-01-05.hal
File Size:11 KB
Attachments:
Last edit: 05 Jan 2019 14:25 by Juckix. Reason: Linked twice

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05 Jan 2019 14:55 #123552 by PCW
Looks like you have 2 hal files and no .ini file

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05 Jan 2019 15:23 #123556 by Juckix
Crap, sorry.

File Attachment:

File Name: vp2000_201...05-2.ini
File Size:3 KB
Here's the INI.
Attachments:

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05 Jan 2019 22:26 #123571 by tommylight
Use a smartphone with whatever software for sound analysing, check if the frequency of that hunting is 50Hz ( or 60 Hz if you are in USA ). If yes, there is something wrong with the cabling or shielding. If any other frequency, do some tuning to the Z axis, namely lower the P value a bit and check if it subsides.

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06 Jan 2019 14:16 #123605 by Juckix
Okay thanks. The frequency is more like a fraction of a hertz. I'll try some further tuning.

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14 Apr 2019 17:37 #130860 by jmelson
OK, fraction of Hz definitely sounds like gravity pulling the Z down. For machines with heavy spindle heads, you need a counterbalance for the weight. So, check this first, it may need adjustment of air pressure or something. (On a Mazak, could even be hydraulic.) If not counterbalance, then you need to adjust the BIAS in the axis tuning.
This adds a steady force in the servo to counter the head's weight.

Jon

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14 Apr 2019 17:45 - 14 Apr 2019 17:46 #130862 by PCW
If these are velocity mode drives, its actually better to add some offset to the torque
loop in the drive itself (to counteract gravity) if this is possible.
Last edit: 14 Apr 2019 17:46 by PCW.

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14 Apr 2019 21:22 #130874 by jmelson

PCW wrote: If these are velocity mode drives, its actually better to add some offset to the torque
loop in the drive itself (to counteract gravity) if this is possible.

Yes, if this is available in the drive, and IF the pots can be set fine enough for this, then that is quite right.
But, some servo amps just have one-turn pots, and you can't get anywhere near fine enough adjustment.

Jon

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