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Thread: Galvo details for beginners?

  1. #11
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    Elmi, If you ask Ryan nicely, he may send you a manual covering the C506 and the Mach DSP amplifier installation, which I found much to my delight, covered the basics of the control loop theory VERY well. I received the Shipment Ryan, and I can't wait till this weekend for the installation. Nice tutorial on XY-100 in there too...

    Steve
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    Quote Originally Posted by mixedgas View Post
    Elmi, If you ask Ryan nicely, he may send you a manual covering the C506 and the Mach DSP amplifier installation, which I found much to my delight, covered the basics of the control loop theory VERY well. I received the Shipment Ryan, and I can't wait till this weekend for the installation. Nice tutorial on XY-100 in there too...

    Steve
    I'll have to send you the new one Bill just finished with more XY2-100 detail, and FB4 direct communication protocol as well.

    Let us know how the install goes!
    Sincerely,
    Ryan Smith
    ScannerMAX Mechanical Engineer
    ryan {at} scannermax.com

  3. #13
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    Steve, didn't some of Selwyn's scanners use a linear photodetector array? I seem to recall the Laserworks SS500's using a mini array. Not really relevant to the topic, I was just thinking about the different feedback methods.
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    This laser projector build takes scratch-builds to a whole new level, down to DIY galvos with capacitive position sensors. Includes a bit of control theory as well:

    http://elm-chan.org/works/vlp/report_e.html

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    Attached is the classic 6850 by Cambridge... All is well until you realize I did not include the rather large look up table for the many different models of galvos and what you need to change for each model.

    Problem with this one, is if you don't know how to calibrate it for the correct level of volts per degree on the position sensor, performance can be quite bad at large angles or it will oscillate badly during some phases of tuning. That is why I stress DO NOT BUILD, Buy! That initial constant carries through all the math functions but one on the board, and if it's off calibration, well that fuse is there for a reason. Not to mention the coil temperature calculator chip has been out of production for like 15 years. Same for the output stage amp chip.

    One of the funky things on this one is in a laser show or marking system, the I-V characteristics on
    the clipping diodes somewhat matters. I know it shows 4148, but it is often not a 1n4148...

    This is outdated, modern galvo amps use a bridge driver amp to get more swing across the coil.



    Steve
    Attached Thumbnails Attached Thumbnails galv amp.gif  

    Last edited by mixedgas; 07-16-2020 at 15:24.
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  6. #16
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    Here is a partial diagram of a capacitive sensor bridge.
    I actually have galvos based on this design, and the drawing is close enough to understand, but missing some things on the actual implementation..
    Excitation oscillator not shown. It would be better to use a synchronous detector, too..

    Steve
    Attached Thumbnails Attached Thumbnails partial cap sensor.png  

    Last edited by mixedgas; 07-16-2020 at 15:21.
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    I knew I had some guts laying around. Who remembers these beasts?? (please excuse the wife's shoes).

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    Great, thanks for all the fast and detailed information!

    So the position feedback is done via two analogue signals which (I assume) are both at the same level when the mirrors are centred perfectly.

    Just an other thing: there are also digital galvo drivers out there now which no longer implement the PID-loop with discrete components but via a DSP controlling the whole process. Do these drivers expect different galvos with an other method of position feedback or are the galvos still the same? I'm just curious, sampling analogue data is a time-consuming process and I could imagine this causes problems for such a digital DSP...

    Thanks :-)

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    Quote Originally Posted by Elmi View Post
    Do these drivers expect different galvos with an other method of position feedback or are the galvos still the same? I'm just curious, sampling analogue data is a time-consuming process and I could imagine this causes problems for such a digital DSP...
    I assume you're talking about the ScannerMax DSP driver. The feedback from those galvos is also analog, using 4 photodiode detectors. Those amps will ONLY work with ScannerMax galvos.
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  10. #20
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    Unless you are talking about some very exotic, beasts that use digital encoders or rotary interferometers, most galvos are analog. They do exist, but the cost is enormous.

    In more then 30 years of doing this, no one has ever came to me and said
    "I need an all digital scanner with an absolute encoder for a laser show".
    The fluid motion and subtle errors generated by an all analog loop actually has aesthetic and artistic value in a laser show synced to music.

    Analog can get to 16 bits or greater with care. The average human eye/brain combination in a laser show situation gets very happy with 10-12 bit resolution according to tests done in the past. So we use 16 bit DAC chips for laser shows...
    But nobody in their right mind uses eight bit laser show systems any more.

    I did, about 15 years ago, go to an ILDA conference and in the advanced technical concepts meeting, say roughly something along the lines of "I want my DSP Scanner Amp, Why are you not making them?". I heard how the laser show market was too small and the demand too low. Funny how one just arrived here via Fedex... I'll keep the name of the two executives, from two different companies that said that confidential. But you KNOW who you are..

    Most digital scanners have large diameter shafts, placing them in the slow but accurate category.

    The mechanical speeds we are talking about here may be far faster then most stepper or analog motion systems, but do not tax the input systems of a modern DSP chip in the least.

    For an individual doing marking on a home or small company basis, Analog sensing is more then enough. If you have one of the few apps that needs more then 12- 16 bits, your probably going digital, if you can afford it. Think NEW CAR in terms of pricing for a new, low end, truly all digital system.

    There is one new market that may drive demand up for galvos with encoders in the near future. I'm not seeing it yet, because those who are entering that market, right now want accurate, reliable, small, and cheap..

    At work, I, on a daily basis, take four megapixel microscope images with an analog galvo based imaging system with sub micron depth resolution. The Z axis is digital piezo, the x and y are galvo. The course sample positioning is stepper. That system is ten years old.

    Steve
    Last edited by mixedgas; 07-17-2020 at 02:50.
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