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Thread: Galvo details for beginners?

  1. #1
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    Default Galvo details for beginners?

    Hi,

    I wonder if there are some resource available somewhere that describe how a galvo works in detail.

    The theory is clear for me: there are two lines for power where a +- voltage is feed into, depending on the polarity and voltage the galvo moves into one or the other direction.

    And there are some feedback-channels that show at what position the galvo really is. And that's the interesting part - how is the current position sent back? Via encoder pulses? Via an analogue signal? Something else?

    Happy if someone could shed some light on this or point me to some useful information sources.

    Thanks you :-)

  2. #2
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    This will teach you everything you ever wanted to know about galvos. Laser Scanners; Technologies and Applications by William Benner

    It is a great read for the low, low price of US$0.99 using the Kindle app.

    -David
    "Help, help, I'm being repressed!"

  3. #3
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    Feedback is typically from a differential pair of photodiodes. There is an (IR) LED in the rear housing of the galvo that shines on a pair of photodiodes, and in between is a flag attached to the galvo shaft. As the shaft rotates, the flag moves to obscure one photodiode, the other, or some combination of the two. The photodiode produces a current proportional to the amount of light incident on it, so as the flag moves from covering more of photodiode A to covering more of photodiode B, the photocurrent from A increases and the photocurrent from B decreases. The difference between those two photocurrents is converted to a voltage that represents the position of the galvo. Typically there is an AGC circuit that monitors the combined photocurrent from both photodiodes and regulates the LED drive current to a level that produces a useful signal range from the photodiodes.

    The attached picture of a cheap galvo shows the flag attached to the shaft in the center, the LED at the bottom, and the two photodiodes top left and top right. In the position shown, the flag is obscuring more of the left photodiode than than the right photodiode. Other galvos use photodiodes mounted flat on the PCB, with a flag mounted normal to the shaft axis, and an LED above. The exact arrangement chosen is a balance of manufacturability, mechanical performance, feedback accuracy, patents, etc.
    Attached Thumbnails Attached Thumbnails IMG_20200716_121522.jpg  

    Last edited by aberry; 07-16-2020 at 08:36.

  4. #4
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    I'll reply shortly with some detailed images. I'm driving right now,

    The data is out there if your looking for it.

    Aberry just typed most of my reply nearly identically...

    I'll cover the capacitive sensor used in high precision systems and the PID loops.

    Steve
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  5. #5
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    For closed loop (position detection) galvos, there is feedback from the galvos to the amp. The feedback is analog. Some of the early galvos used capacitance feedback. Most, if not all, modern galvos use optical feedback. An infrared emmiter and a photodiode detector(s) . The rotor of each galvo axis will have a flag or some interrupter to attenuate the infrared from the emitter to the detector. Some even use mirrors or other reflection means.

    Effectively, galvos are an optical servo. With the position detection in the galvo, the amps will try to match the feedback signal from the galvos to the signal coming from the source. As far as amp design, it's basically a tunable comparator circuit and the complexities can go from basic to exotic!

    Looks like I was beat out while typing!
    If you're the smartest person in the room, then you're in the wrong room.

  6. #6
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    See attached

    More later when I'm home...

    ~
    J1 is the cable going to the Galvo.
    ~
    Photodiode bridge of either a pair or quad of detectors go on J1(5) and J1(9) with common to J1(4) Resulting in currents IA and IB flowing from the sensor to the Galvo Driver board.
    ~
    R19 gets very carefully calculated as in most cases the LEDs are pushed very hard.
    ~
    U4(A) an U4(c) are the transimpedance amplifiers that convert the PD currents (IA,IB) to voltages.
    ~
    Points "A" and "B" connect the detector to the LED current source and the linearization feedback.

    ~
    The term "AGC" is a holdover from the classic General Scanning G120 which used a capacitive position sensor that needed feedback from the amplifier to ensure accuracy.

    ~
    You know you have repaired too many galvo amps for people when you keep the older basic schematic on your phone..

    ~


    Steve
    Attached Thumbnails Attached Thumbnails feedbackistani map.png  

    SENsor 6850  AMP SNIP.png  

    Last edited by mixedgas; 07-16-2020 at 09:07.
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  7. #7
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    15 chars, duplicate post
    Last edited by mixedgas; 07-16-2020 at 09:52.
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  8. #8
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    Please note that modern galvos do not self center except for a few Scannermax products that have a magnetic field designed for centering. . This means that if you apply power to the drive coil (1.2 to 7 Ohms, Model Dependent) without a working feedback loop, the Galvo slams to one side and stays there. In many cases it can then heat up and blow the fragile coil.
    ~
    Older open loop and some closed loop General Scanning products have a torsion rod that acts as a centering force. New generation designs, ie post 1990, do not.
    ~
    Steve
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  9. #9
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    I thought it was some sort of black magic?
    Sincerely,
    Ryan Smith
    ScannerMAX Mechanical Engineer
    ryan {at} scannermax.com

  10. #10
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    Hey, I hold back from publishing "All" the details, least an incompetent person sells a chopped and Muntzed* scanner amp on Tindie for 5$, leaving a vast trail of expensive scanner destruction.

    * https://en.wikipedia.org/wiki/Muntzing
    `
    Lesson learned from long ago... Bad G120 amps from a major manufacturer escaped into the Fleabay river…. Oodles of innocent G100s and G120s died a needless death, from being connected to amps that should have been Euthanized in a PCB shredder.
    `
    I spent a lot of bench time repairing those amps, which paid for my text books for two semesters... If you know how to remove the outer can, 50% of the time you can repair blown sensors on G120s by installing three selected and matched transistors.
    The problem is the zero set was highly dependent on the sensor parts electrically matching. Others started doing that repair, without knowing that, and the resulting magic smoke releases made the EPA top ten most wanted list.
    `
    NO I was not the reseller... One major guy on the East Coast was sending me a pair of amps to repair every two weeks.
    `
    Those boards left a trail of tears back when used G120s cost a small fortune.
    ~
    The point being, Galvo Amps should be built by professionals.
    `

    Steve
    Last edited by mixedgas; 07-16-2020 at 10:39.
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