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Thread: Wavy-line distortion (resonance?) with DT-40pro scanners at 18Kpps speed...

  1. #11
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    No disagreement here.

    I figured, worst case, letting out the magic smoke on some phoenix dragon tiger galvos is much less painful than the original cambridges... My first set of cambridges cost over $3k! (though it included a 3rd galvo set for blanking)... but at the time I felt everything about them was nice since I no longer had to deal with the stupid torsion bars (yuck)... The fragility of GSIs made me want to guard the stupid scanheads with a ruler and smack every finger that went near the unpowered galvos and shout "No ger-finger-poken!" in a bad german accent. What is with people's fingers and attraction to moving mirrors? I figure there's a phd paper in there somewhere...

    IMHO as long as you're sane and follow the instructions exactly, you won't put too much wear on the galvos... In fact the DT-40's have a 3rd hidden protection mechanism... the provided power supplies will let out the magic smoke long before you achieve a full aggressive tune on the scanheads... thus saving you from having to replace the fuse. The newer DT drivers implemented temperature stuff, but I always felt it was a bandaid... if your galvo gets too hot to touch you have bigger problems.
    IIRC Cambridge's max rated coil temperature was hotter than boiling water! So my Mk.IV fingers made for a great temp guage...

    In all seriousness, If the previous post wasn't clear, the scanhead will audibly complain when it hits the resonant frequency... that point is right when you *really* want to stop scanning... and setting the notch filter should be done with the scanner muted...

  2. #12
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    Holy shit! Thanks for all the info guys!

    I had no idea this process was so involved, or so risky. So basically you drive the scanners open-loop (no feedback) and intentionally put them into resonance, but hopefully with a low enough input signal that they don't run away and destroy themselves before you can kill the signal... Then you mute the output and adjust the notch filter (with the same signal applied) until you get the lowest output...

    And this is only to correct the first resonance (which, presumably, is for the entire rotor), correct? So if there is a second resonance in the system (say, of just the mirror, for example), then there's no way to deal with that, is there?

    Sheesh! I'm conflicted here. On the one hand, I really want to try this. On the other hand, I'm nervous about blowing up someone else's scanners. (And they are used scanners at that...) And for sure I'd need to buy a good function generator first. (I have a couple dual-channel 'scopes.)

    Let me think on this a bit. Maybe it would be something to try at SELEM?

    Adam

    PS: Yadda mentioned torsion-bar scanners being broken by people playing with them... Interestingly, DZ and I had a discussion about this recently. He's been using G120s with Turbo-Track amps for quite a while now, and the output is flawless (largely due to those awesome amps). But then I asked him what happens when the torsion bar breaks. To find out, he dug out an old set of G120s that had broken torsion bars and hooked them up. Amazingly, they still scanned the ILDA test pattern correctly! I don't remember what the scan angle was (likely less than 8 degrees), but it sure looked good to me... My guess is that at wider scan angles there would be problems though.

  3. #13
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    PS: Yadda mentioned torsion-bar scanners being broken by people playing with them... Interestingly, DZ and I had a discussion about this recently. He's been using G120s with Turbo-Track amps for quite a while now, and the output is flawless (largely due to those awesome amps). But then I asked him what happens when the torsion bar breaks. To find out, he dug out an old set of G120s that had broken torsion bars and hooked them up. Amazingly, they still scanned the ILDA test pattern correctly! I don't remember what the scan angle was (likely less than 8 degrees), but it sure looked good to me... My guess is that at wider scan angles there would be problems though.[/QUOTE]
    ~

    Yeah, Turbotracks will do that with the G120. Nothing else I'm aware of can.

    Torsion bars are nitinol in some cases, and spring steel in others. Galvos used to be so expensive that I would press the G120 shaft out and silver solder or braze a new T-bar in. Repositioning the shaft then takes an hour with the O'scope. Then they redesigned the lower bobbin to make a one piece assembly and that did not work any more.
    ~
    ----------------------------------
    If you have a proper bench, a good Bench PSU set with programmable current limiting , a scope, a good signal generator, a Pangolin setting there, the manual, and patience, tuning from scratch is not that difficult for a trained ET.
    ~
    Thinking you can just waltz in and twiddle knobs however will result in loosing money. It is like learning to fly a plane. You have to break your pre-conceptions, forget much of what you learned on the PC Flight Simulator, and look at the procedures and instruments. This won't make a good Youtube video either, too easy to loose your place talking and tweaking at the same time.
    ~
    Steve
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  4. #14
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    Yah as easy as that "Just watch out for the alligators and you'll be fine... "

    If you have a second resonance band, you can either get a cambridge dual-notch filter (6744 series) or home brew your own version. If you have the 678x instruction manual, the schematic should be in the appendix somewhere...

    Finding the second resonance is exactly the same as the first... only you start at the previous discovered frequency and sweep up from there.

    Just use your ears as a guide and you should stay safe...

    Torsion bars (along with tuned weights) were a trick to get higher natural resonant frequencies out of the older galvo technologies... I'd imagine it would work without them, but you'll see the resonance popping up earlier which can kill your overall performance.

    P.S. also there was a one-armed bandit's chance the mirror starts up in the wrong place and the whole thing shakes itself to oblivion while trying to stabilize itself... Thats one of the reasons the greek scanners (got branded eyemagics) impressed me... you could clamp
    the mirror and shaft live without detonating anything... though I'd imagine you were probably degaussing the *** out of the magnets.
    Last edited by yaddatrance; 07-10-2018 at 06:50.

  5. #15
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    Buffo is ordering the low distortion digital wave gen I suggested. It has the least bells and whistles of anything on the market, but the very nice waveforms and dead simple user interface are worth it.
    ~
    Alligators? More like a dozen pack hunting, hungry Humboldt Squid who are tired of cannibalism.
    ~
    L. Michael Roberts of Laser-FX and Mike Thanos From Cambridge walked me thru all this years ago, Thanks Gents if your reading this. Playing with what was then (still is one of ) the world's fastest analog servomechanisms has served me well in my career.

    ~
    Steve
    Last edited by mixedgas; 07-10-2018 at 07:14.
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  6. #16
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    Lol, don't judge, but I'm still using my "vintage" analog HP generator ever since I got burned by a digital generator I had bought while doing impedance testing many moons ago... I did pick up a fancy looking siglent one a little bit ago but haven't really used it outside of oohing at the pretty graphics on the screen.

  7. #17
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    Buffo, when you get your new toy, plot a few graphs.
    ~
    Calibrate your test wall with pencil marks (very important tool) for say 5 degrees full angle, 8 degrees, then 10, 15 and 20 degrees.
    Hook up your favorite projector.
    Ground the Y axis, bias on a laser somehow and bypass the interlocks if installed. Generate a DOT.
    Ground the negative side of the x axis on the ILDA connector and hook the sig generator to the positive side.
    ~

    Start at 30 Hz sine wave and adjust the line on the wall for the width of the 8 degree marks.
    ~
    Ramp up the frequency slowly until the line collapses to 50% of its width. Note the frequency. Listen to the scanners for strain.

    ~
    Do the same for each increasing angle.
    ~
    Your measuring the angle specific 3 Db point for your scan pair, and you can plot the response graphs for each starting angle.
    ~
    If your brave switch to square waves. A 30 Hz square wave will teach you much about your damping. Note that square wave performance will be less then 0.7X sine performance.
    ~
    This is a eye opener the first time you do it. Especially when you see the line width go up and then back down some times at modest angles.
    Bonus points if you graph the input power while you do this.
    ~
    Don't be like the synthesizer fellow who was on here a few years ago, who fried his galvos pushing the waveforms too high in frequency.. Work in moderation.
    ~
    The ScannerMax folks have a similar plot for each mirror size in the current datasheet for C506. Neat graph to study.

    ~
    Too bad there is no way on Earth to relate this test to KPPS.
    Oh wait, the slope of the 3 db points on the right kind of graph.. Hum... Not an exact relationship, but interesting.
    ~
    Steve
    Last edited by mixedgas; 07-10-2018 at 08:00.
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  8. #18
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    The Instek SFG-1003 works fine for me.. Should be good for Buffo.
    Old technology now, not that cheap, but is a solid beast that won't die easily.
    Not an HP by a long ways.

    Steve
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  9. #19
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    Quote Originally Posted by mixedgas View Post

    Don't be like the synthesizer fellow who was on here a few years ago, who fried his galvos pushing the waveforms too high in frequency.. Work in moderation.

    Steve
    Ah... if only I could have collected a dollar for every scanset demolished by people making lissajous... little did they know that bad values of oscillators would treat the galvo mirrors like orbital slingshots...

  10. #20
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    Quote Originally Posted by yaddatrance View Post
    Ah... if only I could have collected a dollar for every scanset demolished by people making lissajous... little did they know that bad values of oscillators would treat the galvo mirrors like orbital slingshots...
    You just made me laugh.
    ~
    Ironically he was on here trying to pick our brains trying to make the universal galvo protecting filter block... So he could sell his product as abstract consoles.
    Not so easy to instantaneously protect a galvo set and still get good graphics. At least not easy if you do not tap into the feedback signals.
    He really did not want to hear about a 1.5 KHz roll off.
    ~

    I could retire early if I had 20$ for every scanset destroyed by ravers on drugs. Some day ask me about repairing scan amps every few weeks for the guy who ALWYS retuned his scanners mid rave on acid.... I finally stopped taking his money, sometimes you need to fire the customer. At least to ease your mind, and for the good of the industry. Ex child actor you would all recognize. He swore up and down that tuning while high made for the best shows ever. That was in the early 2000s...

    Steve
    Last edited by mixedgas; 07-10-2018 at 11:14.
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