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Position measurement.

Discussion in 'Electronic Design' started by Dirk Bruere at Neopax, Feb 3, 2006.

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  1. I'm looking for a small(ish) non contact solution to measure an oscillating
    surface to within a few tens of microns over a stroke of some 50mm at a
    frequency of around 100Hz. I need to take measurements at 100uS intervals. Any
    ideas? Cost as always is a factor so anything under (say) $300 in mass production...


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  2. Guest

    Angled laser, optics and a CMOS image sensor?
  3. Tim Wescott

    Tim Wescott Guest

    - Better than 1ppm accuracy
    - Better than 100Hz bandwidth
    - Measurements at 10kHz


    - price < $300

    Oh dear.

    Keyance makes a non-contact laser distance measuring instrument that
    gets your 10um accuracy over less than 1mm stroke with about 1/10th your
    rep rate and your full bandwidth -- they charge about $10k.

    Mostly I mention the Keyance's product because you may be able to copy
    some of it's salient features for your application.

    While you're asking your question, you may be able to significantly
    lower the price of your solution if

    - You have control over the surface, and can either prepare it in a
    certain way or (better) embed something into it to make the job easier.
    - The oscillating part is significantly less than 50mm stroke (I'm
    thinking a 2-part sensor like a capacitive or inductive proximity sensor
    on the end of a movable probe).
  4. I'm thinking of that.
    Maybe some kind of interferometer setup with either fringe counting or convert
    the fringe rate to a velocity measurement.


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  5. I'm wondering whether I can get away with measuring velocity.
    Still, this is some time away as a project so I'm just scouting round for
    options. Maybe someone will come up with a really clever idea I have not even
    thought of.


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  6. Ian Stirling

    Ian Stirling Guest

    Is the stroke strictly one-dimensional?
  7. Pretty much so as far as I can tell.


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  8. Guest

    I can get you the resolution and about half the stroke, using a
    capacitive measurement.
    Our prototype here at the University is a two opamp triangle generator,
    a 25 mm diameter pair of disks to make the capacitor , a one opamp
    preamp/clipper, a two opamp precision rectifier and a passive low pass
    filter. Its crude but we can see distance changes on the order of 5 nM
    with it. A properly shaped linear capacitor could do it with our
    circuit, which is by no means a origional concept. Adding a
    synchronous rectifer as a sort of rude lock-in would really improve
    things but we havent got around to that yet. Would you like a scanned
    diagram emailed to you?

    Steve Roberts
    Research Associate,
    Polymer Science, Akron U.
  9. John Fields

    John Fields Guest

  10. Guest

    opps, forgot, we need a cap plate on each side, so I guess we are not

  11. John Perry

    John Perry Guest

    50mm / 50um = 1000

    He needs around 0.1% accuracy. Does that make it any easier?

  12. Yes, <$300 he said.

  13. Not the idea, but maybe the implementation.


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  14. Tim Wescott

    Tim Wescott Guest


    Why yes, it does.
  15. Not really since I'd already done the calcs for a 12 bit converter.


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  16. I don't even see accuracy requested. It might just be precision and
    repeatability, which could be calibrated for accuracy if needed (but
    not necessarily.)

  17. That's very impressive.
    I had thought about capacitative measurement but the distance seemed too great.
    How would increasing the dia of the disk affect the spacing limits?

    Also, this has given me a different idea. Would it be possible to put a thin
    film electret on the device and sense its proximity with the plate?

    Anyway, I would definately like to see the circuit as I have other position
    measurement stuff to do that may be related. Email as above.


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  18. production...

    Total travel per second = 100 * 100 => 10.000 mm. Travel between
    100uS samples => 1mm. Accuracy 0.02 mm while object is moving...
    You say 'tens' not 'tenths'. I assume 'tens' not 'tenths'.

    Doesn't sound as a real time control of something, but just a
    recording/measurement apparatus. If it were a real time control,
    the slightest delay in processing could cause problems.

    I'd try a linescan camera with ((1/0.02) * 50) => 2500 pixels,
    perhaps 2048 or 4192 pixels. Don't know if you can get those
    for <$300. Optics could be a simple pinhole, no lens needed.
  19. Guest

    a bigger set of disks increases sensitivity.
    could also be used in bridge mode with a little thinking to see your
    parts count = LM412 X 1 Lm318 X 1
    2 diodes 7 caps 12 Rs 1 pot runs off +-12 V

    we use a 40 Khz 5V p/p triangle as excitation

    NASA used to hand out data in NASA TECH BRIEFS on something they
    called capaciflector which was a scaled up multiple electrode sensor
    that could have one side "blind" as you need,

  20. Thanks - that is definately something I have not considered.


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