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Discussion in 'Electronic Design' started by Ebi, Jan 9, 2006.

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  1. The problem then becomes one of dust and/or grease obscuring any track unless
    enclosed. Or wires and pulleys and rotating stuff on bearings etc

    I'm starting to think about using concentric insulated metal cylinders and
    measuring the capacitance. Very similar to Bill's system. All they have to do is
    slide in and out, plus (and this is just a moderately educated guess...) they do
    not have to maintain a constant concentricity to maintain constant capacitance
    for a given length. As a side effect I might be able to use one of the tubes as
    a structural support.


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  2. They are capacitive, not magnetic, just as Paul wrote, using two
    multiple-plate electrodes with fairly wide spacing and interpolating
    to get the required resolution. The mass-market calipers use an ASIC
    to do most of the work. The original patents are from a Swiss company
    IIRC, and the above type of Chinese clones are slightly different from
    the Swiss and Mitutoyo designs.

    If they are really moving at 1500mm/second you'll need thousands of
    readings per second to get 1mm accuracy.

    Can you just use a simple optical linear quadrature pickup? Neither
    the 1mm nor the speed are anything like a problem. You could add an
    index channel to keep it pseudo-absolute rather than incremental.

    Best regards,
    Spehro Pefhany
  3. Sjouke Burry

    Sjouke Burry Guest

    Google for wire potentiometers, they also come equiped
    with up/down pulse digital potentiometers,an can be put
    inside a save/clean housing.
    They come with a travel path of 10cm to 3 meters.
  4. And cost <$20?
    Also, they are not reliable AFAIK beyond 10m cycles.


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  5. Sjouke Burry

    Sjouke Burry Guest

    I dont think that you will find any solution for
    10m cycles and less than 20$ (maybe not even for
    2000+ dollars).
  6. The capacitative one looks good.
    I only need a resolution of around 1mm
    Interestingly, nobody makes the kind I'm thinking about that I can discover.


    The Consensus:-
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  7. Joseph2k

    Joseph2k Guest

    First, martin griffin suggested the LVDT. Second, IME an LVDT is a "cheap"
    part <us$30. Third it does have the accuracy requested, but maybe not when
    all the requirements are known, or the cost may go up.
    The OP seems to have rejected the suggestion out of hand without
    investigating. I wanted to know why.
  8. Because I looked up manufacturers of LVDTs and saw prices of $60+, without
    signal conditioning. Anyway, I now think I can see a way to include position
    measurement into the structure of a linear motor, using the same uP controller
    that does the control loop.


    The Consensus:-
    The political party for the new millenium
  9. Guest

    You weren't asking the OP - "Ebi" <> - but Dirk
    Bruere at Neopax, who is a completely different person in a completely
    different country, asking for a solution to a rather different problem.

    Do pay attention ...
  10. Any old "ball" mouse should be fine for ~1mm accuracy.... re-mount and
    interface to the wheels and IR LED interface on the platform of choice, and
    hey presto... a position sensor. PIC should easily cope with reading. The
    old serial (D9) mice possibly better, since more 'hobbyist' info available.
    Very cheap and simple.
    With a simple driver / rx chip, you could skip mouse interface IC , and go
    from PIC direct to LEDs yourself.

    Dave M.
  11. An optical mouse might be an option.
    However, I'm a bit wary of using optics in such a potentially dirty and
    inaccessible environment. BTW, what sort of surface is best for an optical mouse?


    The Consensus:-
    The political party for the new millenium
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