Jerky arc moves with pressure advance



  • I read through a couple related threads about this issue before posting, mainly these ones:

    Pressure Advance idea for arcs
    More strange pressure advance behaviour

    I haven't been able to fix my issue, which is that at higher speeds on arcs with pressure advance enabled, the extruder chatters enough to skip steps or ruin the finish.

    The model I'm primarily using for testing is exported with 72 segments per circle. Firmware is 1.20RC2.

    Movement settings:

    M201 X2000 Y2000 Z100 E10000 ; Accel
    M566 X1200 Y1200 Z30 E2000 ; Jerk
    M572 D0 S0.5 ; Pressure advance
    
    

    From looking around, it seems that the following things should reduce chatter:

    • Lowering speed

    • Increasing arc segments

    • Increasing XY jerk settings

    Lowering speed definitely works, but I have to reduce speeds from 60 to 80 to below 20 mm/s to get the chattering to stop.

    Increasing the XY jerk settings doesn't seem to have any effect at all. I've tested from 1200 to 1,000,000, and accel up to 1,000,000 also. There's gotta be a maximum below that? The upside to this is that I found that my printer is way faster than I thought.

    I haven't been able to test increasing segments. I can export the model with an arbitrary number, and the STL file size increases as expected, but Cura seems to do some downsizing because the resulting gcode size actually decreases. The gcode files Slic3r outputs are actually shorter than Cura's with the same settings, so it seems that's a no-go too.

    Lowering pressure advance helps, of course, but the relationship seems linear. The chattering doesn't disappear like it should with adjusted jerk settings, it just gets quieter.


  • administrators

    When using high values of pressure advance, the extruder jerk tends to be one of the limiting factors.



  • I tested with an E jerk value of up to 1,000,000. It didn't seem to have any effect either.

    I was under the impression that the goal with arcs was to have the resolution and the jerk high enough that the tool doesn't have to slow down when completing a segment, meaning that pressure advance shouldn't take effect at all. Is this right?

    I checked the M122 report for buffer underruns, because deckingman's thread mentioned it, but I didn't see any. I'm only using one extruder anyway.

    My config file is here: config. Its a simple CoreXY setup.



  • I reverted to an earlier firmware version and did some comparisons using the same gcode and config files.

    Left side is 1.19.2 and right is 1.20RC4

    Left side is 1.20RC4, and right is 1.19.2

    In 1.20RC4, the extruder chatters at about the same volume as 1.19.2, but if you look closely at the nozzle you can see it moving unsteadily around the curves. 1.19.2 seems to move at about the same speed on the curves as on the straights, but 1.20RC4's speed drops down significantly.

    In the firmware changelog the only related thing I can see is in 1.20b2, which is "The start and end speeds of short segmented moves in accelerating or decalerating sequences are more closely matched where possible, which should give smoother motion". Is that referring to arcs or to SCARA movement?



  • That's interesting. Can you confirm that those two parts printed with pressure advance and if so, what value you used?



  • @deckingman:

    That's interesting. Can you confirm that those two parts printed with pressure advance and if so, what value you used?

    Pressure advance was 0.5; I have a pretty long bowden cable.



  • Issue is still present with the latest firmware, 1.20.1RC2. I have to switch back to 1.19.2 to print curves with any quality.


  • administrators

    Please share the gcode file and your config.g.



  • The test print I showed in the pictures is here: https://drive.google.com/open?id=1doV6srMX5rTykufwo9jgSxIBOiGrGZas

    Printer config can be found here: https://pastebin.com/Zh8gesXR

    I took a video of part of the test print with both firmware versions. The outer curves of the print are where the issue is most noticeable, like in the pictures I posted earlier. In the post-1.19.2 firmware video, you can see the printer move more slowly on those curves, with louder extruder jitter, compared to the 1.19.2 firmware.

    1.19.2: https://youtu.be/vjKV19ek8iM

    Post-1.19.2: https://youtu.be/Q5ceiCHCYeA

    The test prints in the videos turned out identically to the earlier pictures in terms of surface finish.

    Edit: Is there any other info I can post that might be helpful?


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