idea for ultra fast power level setting

Electronics related to CNC

Re: idea for ultra fast power level setting

Postby lasersafe1 » Thu Oct 27, 2011 11:39 pm

Call me Dream Smasher. While you might spin a chopping wheel at a high rate, momentum will prevent you from changing that rate at any significant rate. Also, just for fun, calculate the wheel diameter that would be required to have a slit opening edge give you a 100us rise time. I think you'll be surprised.
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Re: idea for ultra fast power level setting

Postby r691175002 » Fri Oct 28, 2011 1:06 am

dirktheeng wrote: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.

You must be using a different power supply, check the manual for the light object one:
http://www.jinlantrade.com/eBay/40W%20L ... Manual.pdf
In the sample circuit you must use both the ttl and analog input at the same time. To use it as described in the manual you must be providing both a PWM signal and setting its power with a potentimeter at the same time.

dirktheeng wrote: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.

I don't understand why a PWM signal would be slower than analog. The TTL signal (which is where PWM should be applied) is not getting filtered, its likely just controlling a giant transistor somewhere in there. From an electrical perspective it would be silly for whatever analog control they have to respond faster than the TTL (especially considering the TTL is rated to 50KHz which is 50-100 times as fast as the power supply can respond).

lasersafe1 wrote: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.

http://www.synrad.com/Manuals/v40_v3.1.pdf
Look at page 3-6 and you will see that rf lasers are only on or off. There is no grayscale on the synrad firestar lasers. In fact, the recommend PWM frequency is only 5KHz.
synrad page 3-7 wrote:Firestar lasers cannot be controlled by analog or current signals
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Re: idea for ultra fast power level setting

Postby dirktheeng » Fri Oct 28, 2011 2:56 am

lasersafe1 wrote:Call me Dream Smasher. While you might spin a chopping wheel at a high rate, momentum will prevent you from changing that rate at any significant rate. Also, just for fun, calculate the wheel diameter that would be required to have a slit opening edge give you a 100us rise time. I think you'll be surprised.


Ok, so I did some quick calcs and it isn't completely unfeasible.

say the disks spin at 72000 rpm. Thats 1200 rev/s. If there are 100 slats in the chopper, that works out to be 120kHz. if the diameter is 3 inches (76.2 mm) the velocity at the edge is 287,267 mm/s. if the beam diameter 3mm, then the rise/fall time is 10.4 microseconds.

The rub here is that the circumference is 239.4mm. If the flags and slots are the same width, that means that they are 1.196mm, so the beam wont completely fit through an open slot, so even at full open with 2 disks, the power will be less than 50%.

120kHz gives you 3.6 on/off cycles per pixel. The disks only need to move 1.8 degrees relative to eachother to transition from full on to flull off.

Anyhow, this could work, but it would be quite a challange... plus we would loose quite a bit of laser power... so much that I don't know if we could really do much with it.
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Re: idea for ultra fast power level setting

Postby mattrsch » Fri Oct 28, 2011 3:30 am

i'm playing around with the idea of making them from 72,000 rpm hard drives


Do you mean 7,200 RPM hard drives? I have a feeling that at 72,000 rpm or 7,200 rpm it will take a fairly powerful motor (and great control) to make minute adjustments in the phase angle between the two motors at any useful sort of frequency. It has always seemed to me like my HDD's take a couple seconds to spin up when I turn on the computer.
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Fri Oct 28, 2011 9:47 pm

r691175002 wrote:http://www.synrad.com/Manuals/v40_v3.1.pdf
Look at page 3-6 and you will see that rf lasers are only on or off. There is no grayscale on the synrad firestar lasers. In fact, the recommend PWM frequency is only 5KHz.
synrad page 3-7 wrote:Firestar lasers cannot be controlled by analog or current signals


I stand completely corrected. Still, I love the 3D engravings from the rf lasers. Much better than DC tubes.

... and I think I'll shut up about 72000 rpm before I get myself in deeper. My 2.2Kw CNC spindle takes about 8 seconds to reach 24000 rpm.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Sat Oct 29, 2011 2:15 am

lasersafe1 wrote:There are no acousto optic or electro optic devices for 10.6um.


I knew that there were all kinds of optics/qswitches for the co2/IR lasers... they use them at work. I found a few and they are pretty common. They use a Germanium crystal.

Here's a link to a company http://www.isomet.com/FinalWebSite/AOHo ... ameset.htm

They come up on ebay every now and again for a couple hundred dolars they have rise/fall times of around 500ns. The actual cell is probably the least expensive part. You need an RF driver, which you can find for $500-1000. This is probably the cheapest and most reliable way to get truely greyscale results if you can find the components used on ebay or something. You would have to use this with a beam dump which is easy to build and fairly cheap (could be as simple as a copper pipe coated with soot from a candle).
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Re: idea for ultra fast power level setting

Postby dirktheeng » Sat Oct 29, 2011 2:24 am

There's actually a complete unit on ebay right now with PSU, diver, and cell for $999 if anybody is interested... it will work with up to 100W lasers.

http://www.ebay.com/itm/ISOMET-1207B-6- ... 19b1281826
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Sat Oct 29, 2011 3:20 am

I stand corrected again. Might be worth a look. Perhaps it will even still be capable of being driven from a DSP or the Retina Engrave, but at a much higher pixel count.
See, that's my plan... shoot down ideas and make people work to prove me wrong. Just kidding. I was wrong.
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Re: idea for ultra fast power level setting

Postby lasersafe1 » Sat Oct 29, 2011 4:17 am

Still, I think you still have a problem with this plan. I believe that AOM and EOM will require polarization of the beam along the fast axis because they rely on an acoustic or electro birefringence shift of the optic to divert the beam. DC CO2 tubes are unpolarized, so you will need a polarization optic and will need to throw away half your beam to get the linear polarization you will need. I've only worked with lower wavelength AOM and EOM, and even then it is a lossy process within the device itself. Bummer.
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Re: idea for ultra fast power level setting

Postby dirktheeng » Sat Oct 29, 2011 5:34 am

I am not 100% confident in this answer as my background is not in optical engineering or optical physics, but my understandining is that the acoustic waves are transverse to the bem of light which can set up a bifringant phase shift in the crystal beause it introduces anisotropic properties. That means that it could potentially diffract light more or less strongly based on polarization relative to the direction of the sound waves. Basically, this means that when the sound is active a nonpolarized point light source may be spread out into a 2 point refracted beam based on the polarization. But in this case it doesn't matter because when the sound waves are off, the effect goes away. We con't care about the consistancy of the point source when we are bending it to get it into a beam trap. Put another way, I don't expect that the polarization effects will be there when the AOM is turned off and the beam proceeds basically strait through the crystal and I expect the bending to be significant enough in both the fast and slow axis to take the light out of the path to the laser so it should work regardless. I will talk to one of the few experts we have at the lab on monday if I have the time. We have some of the worlds fore most experts in laser optics, raman spectroscopy, and laser induced internal combustion at our lab and they are my close friends. I will ask what they think as they work with these devices all the time. Im just a lowely chemical/biochemical engineer who does computational science, materials research, and biochemical decompositions in support of advanced gasification. I'm not a true, blue laser jock... though I use them to get data and information.

With regards to lossyness... there is some, but the coatings they have these days are pretty good... most of the optics have less than 10% refraction loss which is good.. though rest assured these units need cooling but most of that is due to the high intensity sound waves.
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