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Thread: Position detector

  1. #1
    Join Date
    Dec 2007
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    Default Position detector

    Hello
    Who can help me ? I made a pair of scanners (home made) like the cambridge model and I use the servoamp from CT8XX servoamp.pdf
    After many tuning I can see the lasermedia test (some squares in a rhombus in one square) perfect at 35kpps (using a lumax dac) but I can't see the standard ilda test 8degree 12kpps over 4kpps scaning speed.
    The text is bad too.
    Can you help me with some adevices ?
    What photodiode can I use in my positions detector ? What infrared leds can I use too ? I tryed with BPW34 and SLCD 6N1 as photodiode and the OP293 for the infrared leds.
    Please help me to improve my scannning speed.
    Thank you
    gigel

  2. #2
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    before Allthat can do it... I've always wanted to use this image.



    Allthat, I figured I would take some weight off your shoulders by doing your job for once

    As far as the actual galvo sensors. This is not my area of expertise. I have always wanted to rip apart a scanner just to get a better idea of its internal workings. But I have yet to run into a decent one I could afford to destroy.

    I can, however, draw on my personal experience with a type of robotic optical sensor. In my line of work with moving lights (specifically Vari*lite line of fixtures from the VL-5 to the VL-3500) almost every motor has a position sensor. The precision is controlled by a clear plastic disc which has black marks equally spaced around the ring (there are others which use magnets on the bulkheads which do not need to be so precise). The optical sensor sees that (just like how a barcode reads) and therefore know the exact position at all times. Of course in some CT scanners or other high end scanners this would be completely useless since it would increase the rotational inertia dramatically and at those speeds I doubt it would work. But maybe on a small scale (proof of concept) this might be a place to start.

    A way to overcome the rotational inertia would be to make the rotor shaft the ratating disc by marking black lines around the surface of the shaft. Same concept, with a negligible added weight. This might even be how its done, but again I am not sure (just brainstorming here, heck maybe we'd stuble on something new).

    My 2 cents.
    -Max

  3. #3
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    Quote Originally Posted by mliptack View Post
    I can, however, draw on my personal experience with a type of robotic optical sensor. In my line of work with moving lights (specifically Vari*lite line of fixtures from the VL-5 to the VL-3500) almost every motor has a position sensor. The precision is controlled by a clear plastic disc which has black marks equally spaced around the ring (there are others which use magnets on the bulkheads which do not need to be so precise). The optical sensor sees that (just like how a barcode reads) and therefore know the exact position at all times. Of course in some CT scanners or other high end scanners this would be completely useless since it would increase the rotational inertia dramatically and at those speeds I doubt it would work. But maybe on a small scale (proof of concept) this might be a place to start.
    What you are referring to is not a position sensor, but more a rotary encoder. Only relative position information can be obtained from such a encoder, but not absolute values like in a set of scanners. This type of sensor is only good for slow speeds....
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  4. #4
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    Right, that's what I was saying. But since it was all homemade up to this point I figured I would give him info that would be applicable to something that would be feasible to test and get results - they may not be perfect, but results nonetheless.

    This type of sensor, knows its position and with fairly good precision based upon a simple start up calibration.

    Really before we get into all of this it would be a good idea to wait for pictures so we can see what it is that he is working with.

    Just out of curiosity, do you know how the feedback works with a scanner? Like I said, I've never taken one apart, but I assume you have, since it seems you have more experience with scanners.

  5. #5
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    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Its not the photodiode or solar cell you choose, but how you bias and load it.
    In photovoltaic mode, most silicon devices need a 400-2000 ohm load across them. In photocurrent mode, most photodiodes want 9-15V of reverse bias across them to make them Linear. Some just need a bias voltage and the input of the scanner amp is the transimpedance device.

    google "photodiode transimpedance amplifier"

    Also overdriving the leds oversaturates the cell. Keep the cell area small if possible.

    Also, drive the leds with a constant current source.

    Some papers here might help:

    www.electrooptical.net Dr Phil Hobbs. In fact if you email Phil he will probably help you.

    However do NOT use a phototransistor.

    Ask Norm what he uses.

    http://www.ctrnd.com/NormLaserShow/LatestNews.htm

    Elm chan uses simple RF.

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

    Good Luck,

    Steve
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