Y Axis Shortening

Bearings/Motors/Belts/Gears/Etc.

Y Axis Shortening

Postby twehr » Mon Dec 20, 2010 2:58 pm

In another thread I mentioned that running at lower power levels for simple engraving is generally OK. Of course lower power also tends to mean slower speeds. And running at slower speeds causes an issue, for me, that I am having trouble wrapping my mind around. Hopefully, someone here will have some thoughts on it.

Issue - Engraving (scanning) at speeds of 110 mm/s or less causes the Y axis to be shortened to about 80% of the original output. Vectors work fine at any speed in my machine's normal speed range (0-400mm/s), but the Y axis when scanning only comes out correctly at 115 mm/s or more. (See attached illustration - each25mm box was first done in vector outline, then scanned to fill.) I believe it is losing steps, because after the shortened engraving, it vectors back to the orig, but the origin is now offset (negative) by the same amount that is failed to engrave. (Doing actual photos you can see the Y compression.)

My thoughts - Intuitively, that seems backward. If the Y axis were losing steps, it would more likely that it was at higher speeds, which is why this is so confusing to me. I have adjusted the acceleration speeds, max speeds, etc and nothing seems to change. Regardless of what I do, it shortens when running 100 mm/s or less and at 115 mm/s or more, it works perfectly.

I am assuming this is a mechanical issue. If there are other DSP (from LightObject) users here, I would love to have you test to see if you have the same symptoms, or if you already have and found it to be a setting that you could share with me, that would be great as well.

Thoughts, well-wishing, and sympathy all welcome. Magical solutions are highly desired.
Attachments
Y-Axis Shortening.png
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Re: Y Axis Shortening

Postby bdring » Mon Dec 20, 2010 4:02 pm

I have no idea how the controller works, but normally the y move would be a rapid and not depend on the engrave speed. Does it run at rapid speed when it returns to 0,0? I agree, it has the classic symptoms of lost steps.

I wish you well, you have my sympathies and maybe Santa can bring you a magic solution.
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Re: Y Axis Shortening

Postby twehr » Mon Dec 20, 2010 4:16 pm

bdring wrote: Does it run at rapid speed when it returns to 0,0? I agree, it has the classic symptoms of lost steps.


Yep! It runs rapid to the origin. (Not necessarily 0,0 - I have software set to return to where project started rather than machine 0,0 - saves time when doing multiple runs of same project.)

Unfortunately, I handle a lot of the tech support questions over at LightObject as I have a lot of experience with the controller and software and I can't figure it out. Next step is to have Marco get a hold of his engineers. But if it IS a hardware issue, then they won't be able to help much with that.

Thanks for pondering the question.
tim
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Re: Y Axis Shortening - the solution

Postby twehr » Fri Dec 24, 2010 9:27 pm

With the help of another DSP user who had a similar issue, I have solved the y shortening issue at speeds < 110mm/s. :D

The DSP software allows you to set the distance for each step, which is how you dial in your system to produce exactly 1" output for a 1" vector. For my system, that is 12.67 micro meters (.01267 mm).

When scanning a vector object, you can set the scan interval (distance between each scan line). By default, this is .100 mm. Setting the scan interval to some multiple of the step size causes the step size and the scan interval to be in sync. for my system, setting the scan interval to .038 mm (3 x the step size) makes it all work perfectly.

While it was certainly losing steps, I am guessing it was because with scan interval of .100, it was (about 20% of the time) asking the head to move a distance that was less than one full step, thus losing a step.

None of this explains why it only happens at the slower speeds, but does solve the problem. It may be that the software adjusts the way it calculates scans for different speeds, I don't know.

When scanning bitmaps, the scan interval setting is disabled. However, ensuring that the bitmap resolution (dpi) is set at some multiple of the conversion between mm and inches (25.4 mm/inch), like 127, 254, 508, etc, works.

Like most problems, the answer appears to be obvious and easy, once you know what it is.

tim
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