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Thread: wtd full schematics of typical analog scanner servo system

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    Default wtd full schematics of typical analog scanner servo system

    After the excellent TEXLEM workshop about scanner alignment, I would like to study the servo system, and understand the gain compensation loops, alignments for high and low frequency, drive methods, etc. Is there a full technical or engineering manual available free or posted somewhere?

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    Hi,

    google for "678xx schematic" and you will find a manual and schematics for the cambridge 678xx Drivers, which is basically the design most others are copied from.

    Andreas

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    Quote Originally Posted by opcom View Post
    After the excellent TEXLEM workshop about scanner alignment, I would like to study the servo system, and understand the gain compensation loops, alignments for high and low frequency, drive methods, etc. Is there a full technical or engineering manual available free or posted somewhere?
    Skywise hosts this for me:

    http://www.skywise711.com/lasers/scanner/scanner.html

    Note the amps shown are way out=dated. Some of us still use them, though.

    LM22SDA schematic is not shown in that link, because its single ended feedback sensor circuit melts galvos if not set up correctly.

    Notch filters are not always needed, in fact some times they get in the way.
    One set of "clones" out there has a needless notch filter.


    MiniSax manual probably covers the "engineering" tuning tradeoffs best:

    http://cambridgetechnology.com/archi...X_Manual_G.pdf

    A NOTE to the novice who may find this post in the future, messing with the scale factor and or notch filter, when it was most likely already set can lead to oscillation and distruction of your galvos. Its not something you mess with, unless you have the manual, have a test fixture set up, and have a oscilloscope and square wave generator. Resist the temptation to turn the linearity/scale factor trimmers.


    Steve
    Last edited by mixedgas; 02-18-2012 at 09:48.

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    Thanks, those will keep me busy a little while!

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    Thank you Andy, Thank you Steve!, you both had new links for me, my turn to contribute too!

    http://www.lasershowforums.com/viewt...hp?f=21&t=1035
    http://lasershowparts.com/files/Arti...20Tutorial.pdf
    http://www.stanwax.plus.com/lw/scananglechart.pdf
    some general concepts:
    http://www.onestoplasershop.com/info.htm

    EDIT:
    be sure to check out Elm Chan's experiments
    https://www.google.com/#hl=en&sugexp...c826770a6d954c
    Last edited by LaserCo; 02-19-2012 at 04:53. Reason: so inspiring

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    I'm looking at the G120 schematic and I see the current feedback (transconductance arrangement common to all drivers of reactive loads) but I do not see how it detects position on the fly. 'corrects' it to a calibrated current level, maybe. IC52, the remote input amp?
    Attached Thumbnails Attached Thumbnails G120_LstudioUKampSchematic.jpg  


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    mixedgas's Avatar
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    Quote Originally Posted by opcom View Post
    I'm looking at the G120 schematic and I see the current feedback (transconductance arrangement common to all drivers of reactive loads) but I do not see how it detects position on the fly. 'corrects' it to a calibrated current level, maybe. IC52, the remote input amp?

    Position detection signals on a G120 are a few microamps of current output on two leads as differential. The signal is riding on 3-5V RMS of nasty RF noise from the 300 volt, 2 Mhz oscillator that drives the capacitance sensor in the galvo. The feedback signal comes in on that LC filter in front of the transimpedance opamp. (IC57, IC58)

    Inside the G120 is a three transistor circuit, one transistor is the blocking oscillator and drives a step up coil, the other two form a simple diff amp that provides some isolation and drive from the capacitance ratio bridge that senses position. The galvo rotor is grounded, has a unique shape, and is the "plate" of 4 small capacitors, two of which are mechanically 90 degrees out of phase with eacth other. This gives a rotor position signal.

    Some galvo amps just use the position sensor in the galvo, others use the position sensor and a resistance-inductance ratio bridge that takes a in a difference signal.

    If you go to the Cambridge/General Scanning web site and look at the A102 schematics, you can see the R-L bridge circuit for damping on a open loop amp. Then look at the CX660 for what a full blown G120 amp looks like.

    The difference between the amp driving a resistitve load and the galvo "complex" component load is a damping signal. Some amps use it, some do not.

    Cambridge also uses that method some times to get the LF damping signal.


    Pump 3959673 and 4142144
    Into Google patents to see what your dealing with.

    Steve
    Last edited by mixedgas; 02-21-2012 at 06:15.

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