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Position "Sensing"

Discussion in 'Electronic Design' started by Jon Slaughter, Feb 8, 2008.

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  1. Anyone know of any reference that discusses the methods of measuring
    distances both small and large and the electronics behind them? I want a
    method to measure with mil to sub mill accuracy over a few inches. My
    initial thought was to use high freq sound pulses since this would be a
    cheap way but after doing some calculations it seems that it might be
    impossible, or at least very difficult without precise sensors(piezo's might
    work but still a problem of determining when the sound is "heard"). Speed in
    measuring is not really an issue either(can take a few seconds if
    necessary).

    I'm a bit afraid of using light because it seems it would require relatively
    expensive devices and techniques. BTW, I need a wireless method but the
    device itself can be wired(so it can compare the speed with that of a wired
    EM signal).

    Any ideas where I can look for more info on this? The main reason for this
    is to get around having to manufacture very precise positioning parts.
    Obviously if the cost of "remote" position sensing is more than that of just
    manufacturing the parts then its not worth it.

    Also note that I suppose that I could have many sensors(I was thinking 4 to
    6) that have super-mil accuracy but take some statistical average to get a
    better result.

    In fact I don't care much about the relative position but only the change in
    position.

    I just found this site:

    http://electronics.sensorsmag.com/s...Sensing/ArticleStandard/Article/detail/320838

    Which seems to say that the ultrasonic method might work... It also gives
    two other methods. I'm going to read more about it but a book would probably
    be much nicer.

    Thanks,
    Jon
     
  2. Charles

    Charles Guest

    Laser interferometry?
     

  3. I'll look into it but my guess was that it would be too complicated for my
    setup. Maybe for the accuracy I need it can be done easier than I think.
    I'll see what I can learn about it. (if it can be done easy and cheaply then
    it probably would be the best bet. I really know nothing about it except for
    the basic physics behind it so I guess its time to learn more.

    Thanks,
    Jon
     
  4. Charles

    Charles Guest

    Yes, it probably is too expensive but I am amazed at the positioning
    accuracy that it can achieve. I had some experience with it, 12 years ago,
    and hoped that the cost has come down. Good luck.
     
  5. Guest

    As Paul Hovnanian says, you will have to be a lot more specific about
    the items whose separation you want to measure. Heidenhain has a lot
    of postion-measuring products

    http://www.heidenhain.com/index.php?WCMSGroup_2238_177=479

    but it is by no means obvious that you can fix a moving sensor to the
    object whose position you want to monitor, and have it travel along a
    fixed scale, reading off where it is as it goes.

    High frequency sound pulses may not be a good idea - the speed of
    sound in air does vary.

    For precision interferometry, the speed of light in air can vary
    enough to disturb very accurate measurements.
     
  6. Non contact position sensing, I assume (otherwise the problem becomes
    trivial). Can you attach a target to the object? What are its optical
    and acoustic properties? Also the characteristics of the environment in
    which the measurement will be performed.

    How many degrees of freedom must be monitored? X, Y, and Z position?
    Orientation of the object as well?
    --
    Paul Hovnanian mailto:p
    ------------------------------------------------------------------
    It's easier said than done.
    .... and if you don't believe it, try proving that it's easier done than
    said, and you'll see that it's easier said that `it's easier done than
    said' than it is done, which really proves that it's easier said than
    done.
     
  7. Eeyore

    Eeyore Guest

    Since the speed of sound isn't constant it would be a lousy method.

    Graham
     
  8. hrh1818

    hrh1818 Guest

    Take a look at linear incremental optical encoders made by Gurley
    Precision instruments.
    See http://www.gpi-encoders.com/lininc.htm
    Scale length to 125 inches, resolution to 0.5 micrometers with
    incremental interpolators and non-contacting friction free.

    If you do a Google search on "linear optical encoder" you will find
    other companies also make them.

    Howard
     
  9. hrh1818

    hrh1818 Guest

    Also take a look at linear optical encoders made by Renishaw.
    See http://www.renishaw.com/en/6433.aspx
    and also click on their non-contact position encoder brochure.
     
  10. James Arthur

    James Arthur Guest

    1) The sensors and encoder strips used in ink jet printers are
    economical & hi-res, made by one of HP's orphans.
    2) For small sale use, Chinese digital calipers / (dremel + cutoff
    wheel) = cheap digital scale.

    Cheers,
    James Arthur
     
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