idea for ultra fast power level setting

Electronics related to CNC

idea for ultra fast power level setting

Postby dirktheeng » Tue Oct 25, 2011 11:15 pm

All,

I was thinking about how fast we coudl get the laser on and off and realized that we are basically limited to binary (ie dithering) application for greyscale under the current configuration. We have a choise, either use a pwm signal to charge an rc circuit, or use a potentiometer to set a given value. The pwm signal change is too slow to really be of use for engraving because it takes so long to get to a steady state voltage. As of now, the pot is kindof out of the question because we have been using manually set poteniomenters. Has anybody thought about a digital potentiometer??? They make Ic's that can set a new resistance value within a fraction of a microsecond. Why haven't we explored that as an option to set the values? I would expect that we could get a digital potentiometer to behave just as fast as an on/off signal. We don't have to wait for a PWM signal to settle after 20 pwom periods, we can set a digital pot and transfer the information alost instantaniously. In fact, I woul expect that we can transfer the information via the digital pot faster than the laser can respond. What are the thoughts on this idea? If it were to work out, we could do ture multi-bit greyscale instead of dithering.
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Re: idea for ultra fast power level setting

Postby bdring » Tue Oct 25, 2011 11:25 pm

I have heard that the power supply does not react too fast to power level changes. You should do a quick change in power and see how fast the current changes on a scope. I have used a digital pot to set power before, but not at a really high rate.
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Re: idea for ultra fast power level setting

Postby BenJackson » Tue Oct 25, 2011 11:26 pm

I read an article about CO2 lasers that suggested the frequency response of the power setting was about 3kHz. I don't remember what factors went into it.
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Re: idea for ultra fast power level setting

Postby r691175002 » Wed Oct 26, 2011 1:04 am

My opinion is that regardless of what we try the Chinese tube will always be just a Chinese tube.

There is a lot of research to be done if you want to get a real answer on how to get the best performance out of the tube. A manufacturer specs it as 1ms rise time: http://laser-cn.en.made-in-china.com/of ... -40W-.html

If you take a close look at the manual you will see that they also hold the 1ms rise time:
http://www.jinlantrade.com/eBay/40W%20L ... Manual.pdf

We have a choise, either use a pwm signal to charge an rc circuit

I never figured out why people started using rc + pwm to control laser power since it is a very suboptimal solution. If you have a digital signal to begin with and the laser is designed to take a digital signal why put a low pass filter between the two?

You could alternatively use the power supply the way it was designed, namely use the potentiometer to set the maximum current and then provide a 20-50KHz PWM signal through the TTL input as specified in the manual. If you do the math, with a 1ms rise time you can shove a crapload of pulses into the power supply and even a single pulse would be enough since hopefully one would have 8 bit or higher control.

I have no reason to believe this isn't the best way since the retina engrave uses a 20KHz PWM signal and can vary power while engraving no problem (my potentimeter always stays at maximum and when doing a low power engraving the PWM signal just reduces the duty cycle of the dots that need to be engraved).

The retina is fully capable of grayscale engravings and they have addressed it in the past:
http://www.fullspectrumengineering.com/ ... rt=30#p568

The short version is that nobody does grayscale engraving since materials don't respond as consistently and to your eye there would be no advantage (any form of printed media is dithered). Inkjets use dithering (its not like you can apply a "lighter" dot of colour, the droplet is either there or it isn't) and the visual quality is fine.
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Wed Oct 26, 2011 2:11 am

Not correct. The big guys do grey scale, but they use RF lasers. The deep wood engravings that are seen on the GantryCo web site are done with 400W laser in power control mode. A pulsed mode 400W laser would burn clear through 1/2" of wood on every pulse. This was confirmed to me by the man who designs the controllers for Universal lasers. Unless he was outright lying to me, they don't dither.
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Re: idea for ultra fast power level setting

Postby r691175002 » Wed Oct 26, 2011 4:16 am

You are right, I expressed it wrong although I believe fullspectrum is still correct with what they are saying.

None of the RF-excited lasers seem to have analog inputs so all power control is PWM. Since the laser technically has only two states - on and off, doing a "greyscale" engrave would essentially be a result of breaking up the grays into discrete white and black sections.

I guess when you are doing digital control at some point the image is going to have to be dithered in some way or another regardless of if dithering is done by you, the software, or a PWM generator in hardware somewhere.

Although it might be fair to say if the pulses are faster than the laser rise time then the power output won't be discrete. In that case, however, the chances are we are already doing some grayscale engravings without realizing it since the Chinese tubes are so slow. Faster engraves at 1000DPI will be faster than the tube.
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Wed Oct 26, 2011 5:40 pm

You can't see 1000 dpi, so why would anyone need to engrave at 1000dpi? With most of us having 40W lasers, really high scan speeds are also worthless. It will barely burn anything if you go too fast.
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Re: idea for ultra fast power level setting

Postby TLHarrell » Wed Oct 26, 2011 8:11 pm

For my unit, the difference between 250DPI, 500DPI and 1000DPI is primarily raster cutting depth due to overlap. I don't usually use 1000DPI as it takes forever. 500DPI I use for heavier raster engraving into plexiglass and MDF. 250DPI if I'm just etching the surface to frost it.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Wed Oct 26, 2011 9:50 pm

From the discussion, it sounds like it is largely unnecessary to use an actual greyscale approach. That is it is possible to go to resolutions which are higher than the eye can generally percieve so putting multiple dots in place via simple on/off commands is more than enough to do most anything we want via dithering the image.

So if I understand this correctly, it is possible to do a 3D relief by dithering at high DPI's. 1000DPI means that each pixel is only 0.001" wide. The laser beam is about 0.003" wide at a good focus, so a beam will overlap several pixels and this will result in more power being apolied to areas where there are many black pixels (if black means laser on). Also affectin depth will be the raster speed and power set level.

I also understand that the 3d qualit could be bad if the material isn't uniform in the way it responds to the laser.

At lower DPI's say 250, the laser focal diameter is about the same width of a pixel... now it is like a printer because there is no overlap and the dot density acts like a shade of color or grey even though the image is binary.
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Re: idea for ultra fast power level setting

Postby TLHarrell » Wed Oct 26, 2011 10:32 pm

Actually, for the materials that can handle it, having greyscaling would be a great feature to have. Even maybe having a selection of 4 or 8 user definable depths (power levels) would be fantastic. If I wanted to do 3D carving right now, I'd have to do many successive passes at different power levels. This would take far longer than being able to dynamically change power level within the cutting cycle.

DPI setting, at least where my machine stands, doesn't really have any effect on pixel size per se. It seems a little counter intuitive to me until I got used to it. The laser beam still focuses to approximately 0.003". You are correct. At 1000DPI, this means it's basically overlapping like 3x. 500DPI, there is some overlap. 250DPI is roughly no overlap, give or take a little for focus.

I have done rastering at 250DPI and 500DPI in plexiglass (you would figure it to be very uniform), and both methods leave striations which look a little like the effect of brushed aluminum (coarse like if you ran it through a belt sander). I actually like the effect, especially when painted.

If we could come up with a way to dynamically set the laser power, it would open up some really cool options for carving.

I also ran into an issue last night while cutting in MDF that I might go experiment with. I did a dot hatch pattern (in Autocad) which was supposed to be shading on a part. It was run at 4% power. The desired effect was some dots, but they were too closely grouped and resulted in a fairly heavy cut with significant darkening in the hole, the bottom of the hole being heavily textured. Since I have several different colors to work with, I may do some experimenting with various dot patterns and densities at differing power levels to see if I can produce a 3D carved effect. It would take some time, but may well be worth the effort if it produces a desirable result.
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