Backlash causes erratic servo movements

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Backlash causes erratic servo movements

Postby mrtned » Wed Feb 28, 2018 9:15 pm

I have used Mach3 and Mach4 for years and decided to switch to UCCNC. When the backlash is enabled, the servo motors lose their position and become erratic when jogging. I have read the other posts regarding others complaints and have adjusted comp accel and accel with very little improvements. I never had this problem with MACH 3 or 4. Is there a fix to this issue? I have also downloaded the latest version of UCCNC and it behaves the same.
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Re: Backlash causes erratic servo movements

Postby cncdrive » Wed Feb 28, 2018 10:55 pm

There is no fix for this issue, because this issue is yet unknown.
Can you please check if the same issue happens with the same tests you do (e.g. jogging what you mentioned) when you disable the backlash?
Also can you post your profile file and tell me which axis or axes has this issue?

And yes, comp.accel is what sets the acceleration for the backlash compensation and it has to be higher than the normal acceleration parameter to work properly,
however the UCCNC automatically adjusts the comp accel higher than the normal acceleration parameter, so it takes care that it can't be a problem that the comp.accel is too low.

Mach3 backlash compensation works different, it stops the axes when doing the compensation while the UCCNC doing it without stopping the axes.

I'm wondering though how can the servomotors loose their positions? I mean even if the UCCNC would do the compensation incorrectly servos are closed loop, so they should not loose position except if the frequency goes too high. So, one thing to check is the kernel frequency, maybe it is set too high and the signal can't go through which would explain jerky and erratic movements and also lost steps.
Please note that if you used mach3 previously with LPT port for example that could only do 100kHz as maximum while the UCCNC can do 400kHz with the ethernet controllers, while they are adjustable to even as low as 25kHz. The default is 100kHz...
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Re: Backlash causes erratic servo movements

Postby mrtned » Thu Mar 01, 2018 3:50 am

I'll try posting the profile tomorrow. All Axis exibit this issue. I have the same results whether I use 25KHz or 100KHz, I normally run 25KHz. The condition can be readily seen when jogging, for example, in step mode .01 resolution. I tested it using a backlash of .03 When I increased the comp accel from 240 to 300 it did seem a little less erratic in a sense. What I mean is, instead of the axis just randomly jerking each time the axis + was it, it would advance in the correct direction for several times before randomly reversing direction. Keep in mind, this all happens each time the axis is jogged in the SAME direction, not just when changing directions. Meanwhile, the DRO readout advances in the correct direction.

The backlash adjustment should only occur when reversing. This jerking effect happens VERY frequently even after the axis initally reverses. In other words, when backlash is enabled, the selected axis will jerk and go in random directions even when the same jog button is hit sequentially over and over..... ex: if I jog y axis in the + direction and keep hitting the + direction button the axis will randomly go in the - and the + direction. So there's actually 2 problems that I can initially see, first is It's like the program is trying to apply faulty backlash moves each and every time the axis moves in the same direction instead of only applying it when the axis initially reverses direction. Second problem is the backlash isnt always moving the axis in the correct direction. I hope this helps.
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