Help with ... stepper lag?

Hello all.

I’ve built a fairly large laser engraver 1120x780 mm and it uses LED laser. I’ve been using Marlin and SKR1.4 turbo board. This was really just a warmup for actual CO2 laser build and I’ve decided to start with the controller so I picked up Ruida RDC6445G. Here’s the problem:

When I do a shape fill with faster speed (100mm/s), the stepper(s) for the axis that’s doing the scan seems to lag:

You can play back on 25% speed to even further slow down the video. You can see that laser comes on before the first line and also goes off before the second. Same when it travels backwards. Here’s the kicker: I get identical result when I switch to the x axis to do scanning, it’s just rotated 90 degrees. Same result when I lower the x-y acceleration to ridiculously low number and any/all jerk and gantry bend is eliminated. Clearly the actual position during movement is not where controller thinks it is.

I have placed the logic analyzer on the direction/pulse/laser/light sensor, went through the steps and everything looks spot on. There is no backlash in the system, it is belt driven and laser is very stiff. It is not losing steps, it will bulls-eye the target pattern before and after the fill operation.

I don’t recall seeing this problem using the gcode board, but it’s possible I wasn’t running it fast enough. This is not obvious at 30mm/s.

I am out of ideas, if anyone has any, I’d appreciate the help.



Hey Miro,

I think you should read this: Scanning Offset Adjustment - LightBurn Software Documentation

That is most probably the issue that you are facing (the delay of your Diode laser to actually emit the laser beam after getting the signal from the controller board). And luckily if that’s it, there is also the description for the solution.

Good luck and cheers,

Hi Brian and thanks for the reply. That has indeed solved the issue. However, it is not clear to me how. In the slow mo video I linked you can see that laser is firing too early. Scanning offset adjustment, as I understand, will make laser fire even earlier. How doesn’t that make things worse?


… bump ?

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