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Thread: Will this burn up galvos?

  1. #1
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    Default Will this burn up galvos?

    I have a question on 2 signals:: Will the attached signals burn up any motors?

    These signals are 'pre-buffered'...meaning it is not centered on 0V, and the gain is not correct. The current output is 0V to +4V.

    I am asking for help on the timing and the 'noise' on the signals.

    I am creating a new laser controller, and I want to make sure that A) The Speed of the switching signals and B)Noise on the lines are not going to blow the motors.

    I have the first signal set up to output at 20k.

    If anybody could please let me know if this signal is going to work with Galvo amps, I would really appreciate it. I do not want to burn up anybody's motors...

    As far as the second signal...It looks like it is outputting at about 60k, but they are fairly small steps...

    The Second attachment is a test for Rastering an image...Is this too much for galvos?

    Thank you for any help.
    Shaun
    Attached Thumbnails Attached Thumbnails 0004.jpg  

    0006.jpg  


  2. #2
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    I think rastering an image might be out, I don't think any galvos can cope with that, except the vertical scan.

    The ILDA test pattern uses fast jumps in the inner square but the amplitude isn't a full scale deflection. Most galvos can cope with one or two arbitrarily fast full scale deflections I think, so long as they aren't repeated enough to hammer them. WideMoves are forgiving, CT scanners probably not so. I don't know if you can be sure a designed scan is safe for all galvos, it might be better to put a slew rate limiter on the input of a specific drive board to do that in case of extremes in the scan signal.

  3. #3
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    Are you talking a 20khz square wave (first pic) or 20k points per second?

    The scanner will not even think about doing a 20khz square wave forget about 60. We are lucky if we can get a diode laser to modulate correctly at 10k. A really good well tuned pair of scanners might be able to do a two or three k sine at a couple of degrees. Remember we are moving little mirrors that have inertia.

    The noise does look a little bit high. Depending on what freq you are trying output you might want to think about a low pass.

    I understand that the test output is unipolar but to play nice with everyone else you need to plan on a balanced bipolar output.

    Chad


    When the going gets weird, the weird turn pro.


  4. #4
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    I think rastering an image might be out, I don't think any galvos can cope with that, except the vertical scan.
    Galvos can raster, granted, its not tv quality, but it does work and can be a pretty neat effect. Is this what you were refering to or am I missing something?
    Attached Thumbnails Attached Thumbnails raster1.JPG  


  5. #5
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    That's raster. It depends how fast you want to go, frame rate, line count. Whenever the subject comes up on alt.lasers the usual answer is that a spinning polygonal mirror is best for the horizontal scan, and a galvo or slow polygon mirror is best for vertical.

    If you want TV quality, the vertical galvo can cope with the sawtooth scan but even if using triangle instead of sawtooth for the horizontal one you can't take the strain off to bring it in range of a current galvo, you'd still need nearly 8 KHz of triangle wave for 25 fps and 625 lines.

  6. #6
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    Cool Raster scaning

    Doc;

    Actually, if you *really* want to get good raster images, there's another solution for getting the high-speed horizontal scan rate... Use an AOM!

    The problem is that you only get 1 to 2 degrees of optical scan , but it's damn fast!

    NTSC video has a horizontal scan frequency of 14.7 Khz. No galvo on the planet comes close to that, but a multi-faceted spinning mirror can get close. But AOM's can reach that speed with no problem. You just need a really long throw!

    And as you said, the vertical scan is easy. Even at 60 frames per second you're only talking about a 60Hz sawtooth wave. Your average 30Kpps galvos can manage that.

    DZ:

    I did my best to count the lines in that raster image... I came up with 48 lines, so at 15 frames per second that equals 720 hz on the horizontal, which is within the small signal bandwidth for 30Kpps galvos. I'm guessing that you can probably display that at around 3-4 degrees optical, right? Not exactly NTSC quality, but cool nonetheless.

    Adam
    Last edited by buffo; 09-01-2007 at 14:04.

  7. #7
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    No reason to use sawtooth just draw every other frame from top to bottom.

  8. #8
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    Cool

    You need to study how video signals are formed... Even if you draw every other frame you still want to use a sawtooth.

    You use a sawtooth wave because when you're drawing the picture you need to be slow enough to keep up with the color signal, but on the return scan you can move as fast as you want. All video, whether it's NTSC, VGA, or High-def, uses a sawtooth for both the horizontal *AND* the vertical sync.

    In a standard monitor (or a TV, for that matter), you have two oscillators running. One controls the electron beam moving left to right. It's a sawtooth waveform, so it moves smoothly until it gets to the right, then while the beam is shut off it is zipped back to the starting position on the left. The other oscillator controls the vertical steering of the beam. It moves from top to bottom, again very slowly to give the horizontal beam time to draw all the way across the screen. This means that the lines that make up the screen are actually drawn on a slight slope from left to right. When the beam reaches the bottom, the vertical oscillator ramps back up to the top. The funny thing is that during this vertical retrace the horizontal oscillator is still running, so if you invert the intensity signal to the electron gun you can see the retrace line zig-zag it's way up the screen!

    This is why a polygonal mirror works so well for raster scanning. The trace from left to right is at a set speed, but the beam returns nearly instantly as the corner passes the beam and the next facet comes into alignment. It's just about the fastest sawtooth you can get.

    AOM's will work even better, since they are several orders of magnitude faster than standard video signals. (Even VGA only runs at 31.4 Khz on the horizontal.)

    Adam

  9. #9
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    Nope. You don't want to use a sawtooth in this case. A saw tooth is used with TV because there are no physical limits and the video stream comes in the correct order. But if you are going to send the images to a scanner you don't have to do it that way because you can capture each frame, digitize it, and ship it to the galvos any way you please. It would be silly to start at the top each time because of the second frame you can just draw the image from bottom up.

  10. #10
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    The vertical scan rate isn't the limiting factor, the horizontal scan rate is. Even if you wanted to do 60 frames per second, you could still scan from top to bottom just like a TV does. So scanning top-to-bottom-to-top doesn't buy you anything.

    Futhermore, there is a good reason for scanning top to bottom. Persistance of vision. Since the image will be displayed at a greatly reduced frame rate (because of the limits on the horizontal scan frequency) you want to always scan the same way. If you do frame # 1 top to bottom and frame # 2 from bottom to top, then you can develop flicker at the top edge of the picture because it will be dark for a longer period of time than the bottom will. (This will be reversed when you start displaying frame # 3.) The end result will be flicker at the top and bottom of the image.

    This is a fundamental part of video; after all, there's no electronic limit that would prevent a TV from scanning top to bottom to top... It's all about consistency... About maintaining a constant brightness to the eye to limit flicker.

    Adam
    Last edited by buffo; 09-01-2007 at 17:38.

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