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

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  1. Ebi

    Ebi Guest

    hi experts,

    i am doing project in motion control using microcontroller, i am moving
    a plate linearly using a motor, and i have to move the plate according
    to the length size generated by the pc every 10 secs.. so i'm an a need
    of position sensing... potentiometer is one solution to get the
    feedback of the position and direction. is there any other posibility
    for the position and direction sensing to interact with the
    microcontroller..

    thanks in advance

    regards
    Ebi
     
  2. google LVDT for stuff like this
    http://www.sensotec.com/lvdt.asp


    martin
     
  3. Guest

    http://www.heidenhain.com/product.html

    Sony make a system which depends on a magnetically coded rod, which
    works better under really messy conditions, and you can go to a Zygo
    laser interferometer if you want real precision, but a Heidenhain
    Moire-fringe optical scale is most likely to be the right way to go.

    Potentiometers were used commercially in cheap systems when I was young
    - and back then I worked up a capacitive position sensor which was
    cheaper and better, but sadly bulkier - but today they'd be strictly
    for very cheap and nasty gear.
     
  4. Tim Wescott

    Tim Wescott Guest

    Or encoders for longer strokes and (possibly) less precision.
     
  5. Terry Given

    Terry Given Guest

    I had to troubleshoot an interesting problem once. It was an automated
    sawmill, that used a fancy travelling-salesman type algorithm (8 dual
    pentium boxes) to optimise cuts based on current market prices. very
    fancy laser/optical doodads for measuring knots in logs etc, monstrous
    rams (it chopped up 25yo pine trees), a 12' circular saw blade on a ram
    driven by a 600kW machine etc.

    problem was, whenever any of the drives turned on, it went apeshit. rams
    slammed up and down, sawblades came alive, that sort of thing. All up,
    very bad. plant commissioning did *not* go well at all :)

    they were using some fancy position measuring transducers, that were
    basically a piston inside a tube. The piston had a magnet on the end,
    and an RF signal was pumped into one end (IIRC). some of these were 20m
    long, and absolute accuracy was a fraction of a mm. a most excellent
    transducer, but they did *not* like stray H fields, especially huge HF ones.

    ultimately the problem was unshielded motor cables. Well actually they
    were shielded, but an instrumentation guy insisted the shield be
    connected at one end only. big mistake. fixing that fixed the problem.

    Of course if the s/w guys had run a bullshit detector past the
    transducer data, it would have been far less problematic. the resultant
    tantrums were quite impressive though, but scary if standing nearby.

    Cheers
    Terry
     
  6. Rich Grise

    Rich Grise Guest

    Heh. "Bullshit detector". I like that. The closest I've ever come to that
    was a 68HC11 in an industrial universal battery charger: 24V, 40A, with
    SCR phase control, and I sampled the output 16 times per mains cycle,
    and averaged the voltage and current readings to decide on the timing
    for the next cycle. (it did both half-cycles symmetrically - I tried
    to do it a half-cycle at a time, but couldn't get them balanced), but
    if the current ADC had one reading that was out of range, (indicating
    a short), it immediately shut down and refused to trigger the SCRs
    again until the short had been cleared and the processor was reset.

    It's kind of a good feeling, when you've got a 24V supply pumping out
    40A, and when you short it, the box goes, "pup" and quits. Then starts
    up again when the short is gone. ;-)

    Cheers!
    Rich
     
  7. I'm looking for something linear, cheap, possibly nasty, extremely reliable and
    only need an accuracy of about 1mm. Any ideas?

    --
    Dirk

    The Consensus:-
    The political party for the new millenium
    http://www.theconsensus.org
     
  8. Joseph2k

    Joseph2k Guest

    And you ignored the LVDT why?
     
  9. Guest

    Depends how long the stroke was, but my capacitative position sensor
    compared two capacitances between two lumps of printed crcuit board. On
    obe side I had a common "receiving" electrode (over the top of a solid
    ground plane) and on the other side I had two transmitting plates being
    driven in anti-phase. One saw 100% of the receiving plate all the time,
    and was driven with a controllable amplitude square wave, and the other
    was screened from the receiving plate by a moving grounded lump of
    metal (a ground plane inside a flexy in my case) and driven by a
    constant amplitude square wave.

    The controllable voltage was adjusted to keep the net demodulated
    output from the receiving plate as close as possible to zero - which
    reverses what we were actually doing. In fact we adjusted the amplitude
    of the controllable squre wave, and used a PID loop to drive an
    electric motor to drive the grounded in and out to screen more or less
    of the constant amplitude square wave transmitter.

    All the switching and demodulation was done in CMOS. The only cute bit
    of the circuit was the high impedance amplifier on the receiving plate
    which was FET-bipolar pair whose AC gain was set by a 0.1pF capacitance
    between the broken ends of a printed track (we cut the track on the
    first prototype, but had neatly round ends on the next layout).

    E-mail me if you want more detail (at bill dot sloman skip this
    anti-spam insert at ieee dot org).
     
  10. Guest

    There are a couple of possible explanations. For one thing, an LVDT
    isn't cheap.
    For another, the range over which it is linear (or even useful) is a
    lot shorter than the transformer doing the measuring, or the moving
    core whose position is being measured.

    And he was asking for suggestions, and you've phrased your suggestion
    in a pretty unfriendly fashion.
     
  11. OK. Here's what I want as a preliminary spec.
    Reliable non contact. The movement could be up to 5Hz over a max 30cm stroke for
    weeks ie tens or hundreds of millions of measurements before failure.

    Cheap - <$20 1000up

    Accuracy to +/- 1mm over 300mm

    --
    Dirk

    The Consensus:-
    The political party for the new millenium
    http://www.theconsensus.org
     
  12. Would this work over a 300mm stroke?
    See my other post for a few more details.

    --
    Dirk

    The Consensus:-
    The political party for the new millenium
    http://www.theconsensus.org
     
  13. Guest

    My capacitative sensor could fill the bill. The original was accurate
    to about 0.1% - measured with a 6-decidmal digit DVM and a milling
    machine bed.The flexy that ran through the position sensing part of the
    sensor also ran through the compensating part, which helped, but my
    system wasn't non-contact because the flexy wasn't that stiff. A
    system devused for you application could be reliably non-contact, but
    lot depends on what your moving electrode can look like, and how you
    can support the sensing and driving electrodes

    The price may be attainable - but that depends on what you'd be paying
    for the double-sided printed circuit boards for sensing and driving
    electrodes.
     
  14. Slidepot with a boot?

    The thing from an old dot-matrix printer?

    I saw a linear system with toothed belt and whatever you call those
    toothed wheels that the belt rides on, with a 10-turn precision pot
    as one of the wheel axles.

    Good Luck!
    Rich
     
  15. Guest

    My example worked fine over 100mm. The interesting questions about
    getting a 300mm stroke would be all mechanical, and would depend on the
    details of your mechanism - the system should scale up nicely.
     
  16. What's that thing for?

    robert
     
  17. Paul Burke

    Paul Burke Guest

    Have you looked at those capacitive linear sensors- like the working
    part of electronic vernier calipers? Simple, robust, cheap, I can buy a
    300mm scale for £25 in ones, so 1000s should be no problem. Typically
    0.1mm resolution. Downside: slow, stiffish (probably doesn't have to
    be), fairly bulky in the commercially available ones. But some variation
    on the theme might do it, even if you have to design it yourself and up
    the excitation rate to get the speed. Buy one and take it apart- it's
    just PCB, the electronics is pretty simple.

    Paul Burke
     
  18. Not seen one of those.
    Can you post a URL please?

    --
    Dirk

    The Consensus:-
    The political party for the new millenium
    http://www.theconsensus.org
     
  19. It would wear out in a matter of hours, if not minutes.
    Anything mech complex would not be reliable enough over 100m repetitions

    --
    Dirk

    The Consensus:-
    The political party for the new millenium
    http://www.theconsensus.org
     
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