idea for ultra fast power level setting

Electronics related to CNC

Re: idea for ultra fast power level setting

Postby dirktheeng » Thu Oct 27, 2011 9:30 pm

bdring wrote:My power supply is analog or TTL PWM on the "input". I assume it does have a PWM on that circuit. The TTL enable is definitely a digital (on or off) circuit.


Do you mean "I assume it does have a rc filter on that circuit" in the second sentance? If so, that makes sense from what we have talked about before. My power supply lets you switch to use a pot or the PWM but not both at the same time. The only way it could do that effectively is to use an rc ciruit. That said, the on/off is definately a digital gate signal.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Thu Oct 27, 2011 9:43 pm

r691175002 wrote:The main differentiator here is the speed of the pulses vs the rise time of the laser. Naturally if you are pulsing faster than the rise time the pulses will blend together. I would argue that we are already pulsing faster than the rise time on our chinese tubes so technically we are already grayscale engraving anyways.


If you sent really fast PWM signals to the laser you would be able to set the grey level. However, a PWM signal control like that makes the laser SLOWER to respond than it already is unless you have some kind of feedback method and a very fast PID controller. Getting a good greyscale out of these tubes is going to be almost hopeless because the transients are not nice to deal with. It would be one thing if the tube had a very predictable on/off cycle and played nice, but it doesn't. The problem is that the tubes responce changes with a lot of variables and the on response is not nice... it goes to about double max power and then decays down.

I am now coming back to my original conclusions that the only way to get reliable, quick performance out of these tubes on some reasonable engraving speed is to use a beam chopper and/or an AOM or EOM which is fairly pricy and leave the tube on all the time durning an engraving and let the chopper do the work of splitting the beam into very small pwm segments
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Thu Oct 27, 2011 10:23 pm

1. No mechanical chopper at that speed exists.
2. There are no acousto optic or electro optic devices for 10.6um.
3. Enjoy the DSP or Retina Engrave. They work pretty darn well.
4. Look at Craigslist and see the number of poor souls that are selling their trophy and engraving businesses because they can't make a profit while trying to pay off their $25,000 laser. We are darn lucky to have what we have.
5. Almost all pro systems are moving to fiber lasers. Different wavelength, but fantastic lifetime and precise power control.
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Thu Oct 27, 2011 10:37 pm

r691175002 wrote:My point is that grayscale and dithering are really the same thing.

Consider: All higher end lasers (such as the synrad firestarters) use exclusively digital control. Using a grayscale control program, a 128 gray would be produced by pulsing with a 50% duty cycle. This is identical to the behavior one would see with dithering.

Regardless of what kind of grays you feed into any controller at some point in the process that grey is becomming a series of pulses.
The main differentiator here is the speed of the pulses vs the rise time of the laser.


Where in the world are you reading this or learning this? I personally think you are way off base. Just as you can control the power of a radio broadcast, you can instantly change the RF power to a laser tube. All within one wave of the RF frequency.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Thu Oct 27, 2011 10:41 pm

There are plenty of AOM's out there that work for 1064nm

here's one company that makes a bunch http://www.quanta-tech.com/Acousto_opti ... lators.php

you can get choppers that work to 125khz

you can also use a Peckels cell off a ndyag laser.
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Thu Oct 27, 2011 10:56 pm

Yes. 1.064um (1064nm) is common, but I thought we were talking about C02, which is 10.6um (10640nm). Exactly 10 times higher and will not transmit through 1um optics. Choppers are an on_off_on_off process. Useless for engraving. The fastest shutters in the business are Uniblitz, but they won't handle power.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Thu Oct 27, 2011 11:05 pm

dang it... i always get those mixed up in my head! I do that in the lab all the time. I realized it just after I posted it then my internet went down!
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Re: idea for ultra fast power level setting

Postby dirktheeng » Thu Oct 27, 2011 11:06 pm

lasersafe1 wrote:Yes. 1.064um (1064nm) is common, but I thought we were talking about C02, which is 10.6um (10640nm). Exactly 10 times higher and will not transmit through 1um optics. Choppers are an on_off_on_off process. Useless for engraving. The fastest shutters in the business are Uniblitz, but they won't handle power.


couldn't you use 2 choppers and adjust the phase between them to get the effective open area.
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Thu Oct 27, 2011 11:11 pm

No, that will only give you sub-harmonic frequencies that are still on off at the difference of the spin rates.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Thu Oct 27, 2011 11:25 pm

I'm not sure you understand what I am saying. If you use 2 choppers spinning in the same direction at exactly the same speed (say they are servo controlled which a lot of them are). The sections are open, closed so if you have both of them on the same phase the pattern would be

111110000011111
111110000011111

here 0 open. This would be exactly the same as if there were 1 wheel.

so if they are 180 out of phase they would be

111110000011111
000001111100000

That is like a complete block

now you could put them at a position inbetween

111110000011111
001111100000111

Now that is partially open.... this is like creating a PWM pulse that spans from 50% off to 100% off... granted the above example is only a 5 bit, but a real situation doesn't have to be like that... you could even use multiple choppers to create a wider range of off to on ratio

You would need an optical encoder and a good controller, but this could be done. The "PWM" frequency is set by the rotation speed and the number of choppers in the wheel. I think you can get high speed choppers up to 125khz... i'm playing around with the idea of making them from 72,000 rpm hard drives because they can be servo controlled.
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