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Thread: Somebody needs to get this kid into lasers

  1. #31
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    Your both half right

    you need some terms defined and to learn a few new rules.

    If your sending out 30K samples per second from a sound card your max output frequency before aliasing and distortion set in is a little below 15 kilohertz, from sampling theory. A fellow named Nyquist and a fellow named Shannon worked that out. You can look up Nyquist and Shannon on the web. Modern sound cards that do 48 or 96 or even 128K samples per second have theoretical bandwidth of 24 khz, 48 khz or 64 khz, but they actually are oversampling, ie they take each data point a few times and interpolate that waveform back in the audio frequency range, they actually have output filters that limit them to 15-20 kilohertz audio. You can also undersample, but you loose resolution and can shift the frequency of the recorded signal, thats a discussion for another day.

    so for a scanner with 2.5 kilohertz of bandwidth theoretically you need to have a sound card with a 5 khz sampling rate. that same galvo maxes out small step at about 750 max points in a frame, go figgure.So James Lehman (laserboy creator) can oversample about 6 times with a standard card, and work some real magic in the color control area with dithering etc if he so chooses.

    Now there is vector, and there is point by point. Traditional laser uses point by point, and uses the galvo's low frequency bandwidth as a mechanical low pass filter, smoothing out curves etc. You can go a little faster and smoother using vector, where you oversample and fill in signal between the points that would make up a point by point image, that is known as "vector" in the LS industry
    Pangolin can do both , ie you can designate a frame or animation as point or vector. vector has some advantages in a given jump size area over point by point, and animations can look smoother with vector, but a lot depends on how the galvos are tuned. the ILDA test pattern is for point by point.


    Now you two kiss and make up and respectfully agree to disagree on terms. then go look up Dr Nyquists law as homework or I will write you both a detention and put big mirrors on your scanners . :-) for bonus points, look up ballistic motion and inertia on a servo web site like da Dr says.

    PS, btw, it looks like scope boy is cheating, and is adjusting the persistance control mid show on a STORAGE scope, ie the scope screen has the ability to keep a image up for a controlled amount of time after it has been scanned by the electron beam. He's be really dissapointed by a laser's short persistance.

    Steve Roberts
    Last edited by mixedgas; 09-04-2007 at 10:37. Reason: I'm a assaholic perfectionist.

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    No hard feelings on my side... I think we are both ex navy nukes so are argumentative by nature. You kind of lost me with the vector talk. I might have to go back and try to figure that out at some point. I am writing my own software and my DAC expects a series of points and a point rate and it does the rest. A sound card works pretty much the same because you have to set the sample rate (point rate) and points in the WAV files that you give to it. In both cases, once you give it the points and point rate it is out of your control so you have to make sure that the points you feed to them can be achieved by the galvos within the time slice.

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    Red face Mea Culpa

    I agree - ex Navy nuc's love to argue, and I took it too far. I read back over my last few posts, and realized that I was getting personal. There's a fine line between being passionate and just being a jerk. Looks like I crossed that line about three posts ago. My bad... I apologize Carmangary...

    Steve;

    My third post in the thread talks about Nyquist's axiom (sample rate vs frequency response), though I didn't name him specifically. And I did mention ballistic motion in a later post. Do I get a homework pass, or are you still going to put big mirrors on my scanners?

    I wasn't aware of the difference in the way Pangolin handles Point and Vector frames. That's great information! (Which is not documented in the help files, as far as I can tell.) I knew there was a difference, but didn't know the significance of that difference. Thanks!

    One final question: What would it take (besides a big, honkin' amplifier, stronger mirrors, heavier shafts, and really good cooling) to operate a theoretical set of galvos ABOVE their resonant frequency all the time?

    Adam

  4. #34
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    One final question: What would it take (besides a big, honkin' amplifier, stronger mirrors, heavier shafts, and really good cooling) to operate a theoretical set of galvos ABOVE their resonant frequency all the time?

    Adam[/QUOTE]

    They exist, they are called resonant scanners, I have a 1/2 video frequencey one in a desk drawe (no driver damn it!) , trouble is they only have a narrow range of frequencies they wanta operate in.

    lets see, to get above the ballistic, hum...... gotta think about that, probably a dsp to make fast corrections and water cooling, and NO torsion bars. I dont think you'll get that high. I've heard rumors that LFI had a water cooled cambridge setup, but getting LFI to talk about what they have done is hard, they have real good security on their IP. I've also heard rumors of PWM galvo intensity/color control at LFi. I've also heard rumors of a laser media (now dead company) prototype that put +/- 60V across the galvos and ran them in current, not voltage control. in reality its a subtle difference in the control loop whether your I or V, but galvos are I sensitive devices.

    I'd still like to have one of the new DSP galvo driver pairs for next selem, perhaps we should invite cambridge ?

    Nest step is DMD, I have two DMD test boards in a desk drawer, but TI wont give me the pinouts as theya re old, and I dont have 13K for a new evaluation board.

    can I get you guys to fart some radon into one of my plasma tubes (estimate need 6 foot of shielding around laser) ?
    Would like to see some new lines. Just kidding, I'm a twidget wanta be (Buffo knows the story) Although my ex navy physics teacher did shorten it to twidge in the familar, so I guess that was somewhat endearing. damn, that was 20 years ago.
    Steve
    Last edited by mixedgas; 09-04-2007 at 14:20.

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    Quote Originally Posted by mixedgas View Post
    can I get you guys to fart some radon into one of my plasma tubes (estimate need 6 foot of shielding around laser) ?
    Would like to see some new lines. Just kidding, I'm a twidget wanta be (Buffo knows the story) Although my ex navy physics teacher did shorten it to twidge in the familar, so I guess that was somewhat endearing. damn, that was 20 years ago.
    Steve

    WHAT???

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    Just be glad that you don't know what he is talking about.

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    Wink

    Quote Originally Posted by carmangary View Post
    Just be glad that you don't know what he is talking about.
    Then how am I to effectivly talk shitte or add the appropriate smiley...

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    mixedgas wrote
    I'd still like to have one of the new DSP galvo driver pairs for next selem, perhaps we should invite cambridge ?

    Nest step is DMD, I have two DMD test boards in a desk drawer, but TI wont give me the pinouts as theya re old, and I dont have 13K for a new evaluation board
    You reminded me of what i saw at the confrence in Italy on the LOBO stand, they had Projectors that contained "general scanning G120s" now what they were doing with them was amazing it looked 40k + they were doing raster images, flicker free grafix, awsome stuff ..... on G120s, again they would not give much away about how they achived this, but one guy mentioned it was down to the inhouse designed DSP amplifiers, they can make the 120s out perform the original specs, they had one on the stand with a perspex cover on , a 2U 19" rack full of processors and digital power supplys, and this was only the galvo Amplifier not the D to A

    they had obviously got a Budget that allows for perfection

    link here ....
    http://lobo-electronic.de/fileadmin/...roduct_pdf.pdf



    all the best ... Karl
    Last edited by Banthai; 09-05-2007 at 05:32.

  9. #39
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    Karl;

    I've heard of people getting incredible performance out of G-120's using the Turbo-Track amps... (BTW, you do know that Bill Benner was largely responsible for the development of those amps, right?) Have a look at this thread for more details on just how far you can push a set of G-120's with a really good amp. (Can you say 50Kpps! ) So yeah, if someone has taken the Turbo Track amp and tweaked it even more... (Or reverse-engineered it!) I can believe those numbers.

    Mixed Gas;

    You won't need much shielding if you fill the tube with Radon. Radon is an alpha emitter, so the glass will stop the particles. What you need the shielding for is the gamma and neutron flux from the operating fission reactor that you need to PUMP the Radon with! (Actually, Radon can also be excited electrically just like Neon; my guess is they were syphoning the radon from the decay producs of the fuel and decided to see how much extra pumping they could get by using the reactor flux directly...)

    Here's an interesting thought: I wonder what lines you would get from radon using a hard pump with N-16 gammas (figure 6 mev nominal) from a BWR? Hard X-rays, maybe? Think about this... You could position the plasma tube outside the primary shield and use the primary coolant to pump your laser... N-16 has a half-life of around 8 seconds, so it would last just long enough to decay in the jacket around your laser tube, pumping the gas. Of couse, you still couldn't get near the tube, as it would be inside the secondary shield, but I bet you'd be able to pump the hell out of that gas... (The N-16 gammas are otherwise wasted in an operating reactor; the secondary shield soaks them up.)

    The only other problem is that Radon has a half life of just under 4 days, so after a month you woulnd't have any gas left in the tube! (Bummer) Maybe it's not such a hot idea after all. (Plus I doubt the Navy would allow anyone to mount their science project inside the reactor compartment for a patrol or two...)

    If your teacher shortened Twidget to Twidge, then perhaps it was a term of endearment. Given the alternative, I'd say that's the preferable way to remember it!

    Re: above-resonance operation. I'm just thinking that if speaker systems can do it, there's got to be a way that you could accomplish the same thing with a mirror. (Granted, were talking more mass and non-sinuosidal motion, but still...) Damnit - I want 210Kpps! (Or, to put it another way, I want a small signal bandwidth limit of at least 15Khz.) I know, I know... Use a piezoelectric micro-actuator and you can get there. But then your optical scan angle is .00002 degrees or something... (Yeah, I'm kidding.)

    Aaron;

    Feel free to talk sh*t anyway. Let's face it, by the time it gets down to the point where you have to talk sh*t, you're not being effective anyway, right? And as for smileys - I'm sure you've got several to choose from that would be appropriate. (You do for every other topic!) What, you mean you don't have a smiley for a nuclear reactor-pumped laser? For shame!

    Adam

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    Question status update

    does anyone know what happened with the kid? you know... the original topic of this thread... did Laserben find him and they lived happily ever after?
    i wonder if he's got to do stuff with lasers

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