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Thread: Laser Slowly Loses Steps in X Axis throughout project or "drifts"

  1. #1

    Laser Slowly Loses Steps in X Axis throughout project or "drifts"

    Hi all,

    This issue has been haunting me for about a year... I have a Redsail X700 with a Ruida controller, running with RDWorks.

    When I do large engrave job, the x axis slowly starts to drift to be offset, as it progressively moves down the Y. You wouldn't be able to tell, until you go to cut it out. The cut is accurate at the bottom of the project (closest to the last engraving) then slowly becomes terribly offset toward the top (furthest from the last engrave). The X


    I've worked through a lot of possibilities and I think it has to be a stepper, voltage or software issue... this is also happening to a couple of machines of other people I know, who run various versions of rdworks. It is certainly not mechanical, as I have checked all belts, wheels, made sure no movement is happening on the table, and the beam is focused everywhere. I've also ran the program in many different ways, running the engrave and cut as two separate jobs, running it as Udisk, all one layer, all one jpeg, multiple layers, putting it all in a rectangle "frame", etc. Same result. The same thing also happens on other large projects.


    I have attached images so you can see what I am talking about.







    Does anyone think it could be a voltage problem? I see some potentiometer dials on the drivers that are connected to the x, y, and z stepper motors. Image below:



    Maybe I need to just replace a stepper driver or stepper motor? Anyone solve a similar problem to this? Any idea how to fix it?

    Thank you for any help you can provide!

    Sincerely,
    Jon
    Last edited by Keith Outten; 11-07-2018 at 6:52 AM.

  2. #2

    Anybody out there?

    Bump. Does anyone have experience with a similar issue?

  3. #3
    Does the offset happen during shorter jobs?

    From a gantry style CNC machine, I would assume it's one of a few things if this cropped up on my machine:
    1) Wrong steps to inches conversion in the software (but then this should happen for every movement. If you tell it to move 10" does it, or does it move 9.7"?)
    2) Slippage-- are all of your set screws locked down (is there a pulley or coupler that is barely loose but feels firm?), if you have belts, how's the tension?
    3) Stepper driver current. Probably not an issue for a laser cutter (you aren't moving a lot of mass or hitting resistance), but for a CNC if you don't have enough current or force, the machine can lose steps over the course of a job.
    4) Stepper driver temperature: A lot of electronics will shut down if they get too hot to save themselves. Often it's for brief periods here and there that is hard to notice, but after along job, the missed steps start to add up. Add a small fan to the driver or even set a box fan blasting across all of your electronics to see if if changes the behavior (at least for troubleshooting)
    5) Communication issue: I doubt this since it is a gradual issue and not more sporadic or erratic in it's behavior.

    Best of luck!
    Licensed Professional Engineer,
    Unlicensed Semi Professional Tinkerer

  4. #4
    in your settings somewhere are the basic speed parameters of your machine. What the parameters are named depends on who wrote the engraving program. Mine is PHCad/Lasersoft, and this is (one of) my parameter lists (these are defaults from my demo version)
    acc's.jpg
    to be honest, I don't know what the numbers actually represent or what half the meanings are, and your software will likely use different but similar meanings, but--
    'space speed' is the speed the machine travels in the 'dead space' when moving between one object and another. Default here is 300, that's kinda fast and can be slowed down to 200..

    'space acc' is..? Not sure what 2400 represents, but I do know lower numbers means slower drive train acceleration, which equal less shock to the drive train-

    'space acc acc', 50,000? Again, lower the number, see what happens...

    'initial speed', makes sense, 10 sounds slow enough but 8 might be better?

    --etc etc-- all the speeds shown can be adjusted and tested. IF your problem is the controller is trying to move the gantry faster than it wants to in certain circumstances, the stepper(s) can skip. Lowering the speed numbers may help smooth everything out. While my machine has never lost minor steps like yours seems to be doing, it WAS producing wobbly curved lines at medium-high speeds, and lowering some of these numbers helped immensely-
    in this pic, there's 2 sets of identical graphics, all were engraved using the same speed and power settings, but note the lower-most arcs in each pair of graphics is much smoother that the one above it- this was the result of reducing some of the parameter speed settings.
    arcs.jpg


  5. #5
    In addition to what the other guys said, it sounds like a rounding error to me. I wouldn't think this is an issue in an assembled machine, but if there was some small conversion somewhere that was off, it could add up over lots of small moves.

    Try running the machine with much fewer things to engrave and seeing if it gets out of whack. Maybe it's the number of X axis moves, or the number of Y moves, or something similar/related.

    Does the laser return to its exact same starting spot at the end of each run? Fire a test dot, run the whole program, and fire another dot. If they don't line up, you can measure the actual change in position over time.

    Does the problem persist across all resolutions? What resolutions are you running at?


    As a side note for those more familiar with these controllers: if the steps per millimeter isn't an integer multiple of the DPI, what happens to the "lost" steps? For example with made up numbers: if you have a system that runs at 0.1 mm per step, and you have an image at 300 DPI that is 16.2" wide, you have an image that is 411.48 mm wide. The machine can't do part of a step, so it can either run the image as 4114 steps in width or 4115 steps in width. Ideally, it would keep track of things, but if there was some glitch somewhere I could see it running 4115 steps right, then 4114 steps left, and so on, accumulating a lost single step every time it moves. Has this been a possibility with any standard drivers?

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