LO DSP/LaserCAD PPI question

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LO DSP/LaserCAD PPI question

Postby JakeS » Sat Nov 10, 2012 4:17 pm

Hi all,

I was trying to dial in the focus by cutting a 50 PPI (the lowest LaserCAD allows) line at 40% and 200 mm/sec on a paper ramp. While I was able to find the focus ok, I noticed some other stuff:


The lines start and end heavy, like you may expect for doing unpulsed cutting while the head is accelerating and decelerating. But I thought that PPI explicitly addressed that issue. Is there a LaserCAD setting or LCD interface setting that I'm missing to fix this?


(Also, the lines are consistently wavy and I'm going to bet that the distance of the periodicity is about the circumference of the v-wheels, which I think I can fix by adjusting the eccentric nut.)
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Engraving problem

Postby JakeS » Fri Dec 07, 2012 4:24 pm

I was engraving a portion of the periodic table (I have a 2.x with a LO DSP) and I noticed this:


This is with a bidirectional x scan. The source vertical lines are perfectly straight. The waviness in the vertical lines seems like it would be backlash, but the period of the waviness is much longer than the width of the scan line. Also, I have engraved stuff before and not had this problem. One new thing here is that this was the widest horizontal engrave I have done. The problem went away almost entirely when I switched to a y scan direction. (But it was still there a little bit, which is really confusing!)

Is this just a mechanical issue? This doesn't really match my understanding of backlash. Also, I don't have any noticeable backlash when vector cutting.

Anybody have any ideas?
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Re: LO DSP/LaserCAD PPI question

Postby bdring » Fri Dec 07, 2012 5:01 pm

That is weird. I don't think it is backlash because the vertical distance between scan lines is unrelated to what you are seeing. What is the distance between waves. Look for something related to that. It might be related to a shaft or belt. It looks like a sine wave. Look for something that rotates
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Re: LO DSP/LaserCAD PPI question

Postby TLHarrell » Fri Dec 07, 2012 5:29 pm

I get similar on my laser at high engraving speeds. It is greatly reduced at lower speeds. I too would be interested to find out the source of this so I can eliminate it from my system as well.
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Re: LO DSP/LaserCAD PPI question

Postby cvoinescu » Fri Dec 07, 2012 6:30 pm

It's just a shot in the dark, but it could be a form of quantization error or interference (as in waves in general, e.g. Moiré patterns, not necessarily electrical interference). If the DSP's rastering clock was a frequency of the same order of magnitude as the step pulses for the X axis, this could happen. However, I think that that's unlikely, because a DSP is plenty fast and should run at a much higher frequency. Another source could be the high-frequency switching taking place in the laser power supply. That frequency is not that high: it's in the tens of kHz, which is comparable to the frequencies involved in stepping. So either the laser is amplitude-modulated with that high frequency, or the control signal is sampled at discrete intervals of that frequency. The latter seems more likely: it is entirely conceivable that the laser can not be turned on or off except in quanta of one period of the oscillator in its power supply. Staying with the same idea, you may also be seeing the mechanical effect of the stepper drivers being choppers, also working at a finite frequency.

Then again, I may be way off base and it could be something else entirely.
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Re: LO DSP/LaserCAD PPI question

Postby Techgraphix » Fri Dec 07, 2012 7:30 pm

What's the interval (distance) between two dips or peaks? What is the tooth-to-tooth distance of your belt?
If this is somewhat the same ask yourself:
How tight is your belt (is it too tight)
Is it the right belttype for the wheels?
What type are your guides?
Is the effect gone when you engrave Unidirectional?

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