Differential Amplifier for the galvos

I’ve made some research on how to convert a single ended analog signal to a differential output to drive the galvos for the scanner. The requirements are:

  • Galvos input voltage range is +/- 15V (and there is a +-15V power supply connector available on the driver board to pick)
  • Signal bandwidth is limited to audio range, so no need to use very high speed products
  • DIP package would be great for breadboard prototyping
  • The sound card level has probably a limited voltage output so some amplification would be good to get a nice mirrors angle deviation
  • Cheap and in stock in local store

I’ve found quite a few full differential amplifiers but the best match so far is the DRV124 from TI. Not only it seems simple to use and compatible with my available power source, but the DIP package is convenient. It’s not available locally, but you can request free sample on TI website, so that’s perfect to try 🙂

Companies like TI are publishing very nice white papers on the theory of fully differential amplifiers (Here and there). I’ve added a few more amplifiers in the sampling cart so I might be able to use some of this information.

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2 thoughts on “Differential Amplifier for the galvos

  1. polytechnick

    The way I understood galvos (as in what’s called “galvo” in laser show equipment) is that it is a proto/quazi brushless DC motor, and perhaps even an actual BLDC with 4 cogs / 2 poles or some such simple configuration. I actually have a pair of galvos but have yet to try to use them or even just stick an ohmmeter in them to see what shows 🙂

    Anyhow, I was just curious why you have started to look into driving them by using a sound card as a frequency source and then figure out how to split the signal into two phases (if I understood your idea correctly) as opposed to driving them as an actual motor that they (at least in part) are? Was that because of a planned audio modulation of the laser beam position? And by driving as a motor I mean just digitally walk through the winding commutations according to the required rate/direction by using either a specialized BLDC controller IC or an MCU with two/four half-h bridges.

    Galvo newb here, by the way – like I said, still waiting for the right time to play with the ones I’ve got but very interested in them in general, so if my question makes no sense, please point me in the right direction.

    Cheers!

    Reply
  2. damienideas Post author

    These are all good questions and thanks for your interest.

    I’m also new to these galvanometers anyway, and for me they are more like servo-motors than the historical galvanometer, especially as the one I have include a position feed back loop.

    The reason I was looking at single ended->differential converters was because my kit (see http://wp.me/p3gI7M-27) is intended to be used for a laser show and the standard interface to the controller are differential inputs. I guess that’s probably to increase the wiring distance between the ADC and the amplifier itself. So to build the scanner, my first view was to use a PC sound card to generate the stereo driving signals and convert the output to differential.

    The only short coming here is I might have to battle against the output filters that can destroy my driving signal… The other drawback is that sound output level calibration might be tricky as the OS can change the amplification without noticing the software. So my first try will be using the differential amplifier and an arbitrary signal generator to make sure that my frames can be rendered in the best condition possible…

    As for your questions about driving and sensing/position feedback, the best page I found on the subject was : http://elm-chan.org/works/vlp/report_e.html were the blogger is building his own ‘galvo’ system. Maybe you can use some of his findings?

    Cheers
    Damien

    Reply

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