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Any designs for a cheap x-y position sensor?

Discussion in 'Electronic Design' started by Robert Macy, Sep 17, 2012.

  1. Robert Macy

    Robert Macy Guest

    need a cheap, low power [read that as don't want to pay much, and need
    it portable] x-y position sensor that tells me where I am when I ask..

    specs
    1. RELATIVE position once START is ok. Then can RESET and start again
    from another marker.
    2. over an area of approx 100 ft by 100 ft
    3. update when requested, every 50, up to maybe 100mS. just need to
    mark where I'm at on a grid.
    4. absolute accuracy [over relative field] 'gentle' distortion of over
    6 inches be ok.
    5. relative accuracy for adjacent readings, better than +/- 1/4 inch,
    probably get by with +/- 1 inch, but should be monotonic.

    Note: absolute accuracy means a gentle stretching of the grid relative
    to the perimeter is ok. but needs to be reproducible. Like exactly
    where a mark is can be off by 1 foot, but it is always possible to
    find the EXACT location within the tighter relative accuracy.
    Monomotonic is like no backlash.

    I could get away with wheels like those used by the realestate agents,
    but the less paraphernalia/clutter the better plus likely to be in
    rough terrain.

    Be nice to simply stick a post in the ground and know where I am in
    polar coordinates away from that stick when I ask.

    I thought of GPS, relative, but expensive? power hungry? and updates
    only every second ...sometimes. I thought of simple g force MEMS
    devices double integrate, but their drift may preclude using that.

    Anybody have a brilliant approach?
     
  2. Guest

    Laser scanner, I believe that is what they use at building sites
    these day

    Laser and rotating mirror measuring angles and maybe distance to
    reference points I assume

    -Lasse
     
  3. hamilton

    hamilton Guest

  4. Ecnerwal

    Ecnerwal Guest

    In a controlled environment (evidently not applicable) large scale
    (billboard+) printers have been built using a spark gap (for acoustic
    generation) on the print head and microphones at the edges. Read about
    it, don't know the details, I'd assume the controller makes a spark and
    only counts the first impulse at each microphone. IIRC the use of a
    spark was in part to get a nice "sharp" sound that was not easily
    confused with background.

    Some variant on laser rangefinding would do the job, but cheap and 100ms
    might not coincide too well. Park some retroreflective prisms at the
    corners and go to town (look at what surveyors use these days - a "total
    station" - good to 1/100th of a foot anyway at moderate ranges.)
    Depending on available optics, moving the prism around and parking the
    station would also work. For actual available tech, two bodies, and much
    longer than 100mS updates (well, you have to walk the prism and park it
    to get a reading - I don't know how fast the TS can track it as it
    moves), one guy to point the TS at the prism and one guy to walk the
    prism around the field is a typical approach, at least until the TS
    needs to move in order to see the prism. That also gets you 3D.

    I don't have a TS to play with, but TS and GPS are the reason I have a
    transit that "used to be" reasonably nice and is now (and has been for
    decades) "obsolete."
     
  5. Owen Roberts

    Owen Roberts Guest

    Optical version of a Aircraft VOR. Diode lasers are cheap. Mount them
    at the corners and sides of the field on a rotating stage. Fan out the
    beam with a cylindrical lens. You transmit a sync pulse, either
    optically or by other means, when the laser is at true north. Look
    for the laser light using a bandpass filter. Find the leading edge of
    the beam as it passes over a detector. This gives you the angle at a
    known time. Any two of them gives a fix. There are plenty of
    papers on the net on multilateration.

    If you have the leading and trailing edges of the beam, and its
    diverging, you also have a measure of the distance in the far field of
    the source.

    Cheaper then time of flight.

    Steve
     
  6. Jasen Betts

    Jasen Betts Guest

    Scanning lasers are only used for level (altitude) AFAIK
     
  7. Jasen Betts

    Jasen Betts Guest

    Subpixel resoulution should be possible, so it may be doable with
    lower resolution cameras.
     
  8. Martin Brown

    Martin Brown Guest

    Two surveyed wooden posts along a ~100' baseline and an ultrasonic tape
    measure. You will have to put a repeater on top of the posts to send
    back the pulse since in clear air there will not be enough echo.

    Realistically you would probably want something to do the maths for you.
     
  9. Robert Macy

    Robert Macy Guest

    Acoustics is interesting, but I've got the soundcard tied up. Didn't
    think of it, but could back off and go after 8 channel 44100 rates.

    Interesting approach, a 'mechanical' solution. I can picture the
    'tether' method. Like two tape measures attached to two poles.
    Accuracy when directly between the two poles is the pits, but best
    around 45 degrees from each. The 'field' easily extends past the two
    poles also. Sadly, being in a remote area precludes lugging a lot of
    weight up into the region. Plus, being tethered seems fraught with
    physical breakage problems. But still, a somewhat unusual approach,
    which might have applications for another project - inspection sites,
    where the whole thing could be a fixture.
     
  10. Robert Macy

    Robert Macy Guest

    I think you might be right to just use a robust surveying tool.
    Doesn't Zircon out of Campbell, CA make a laser tape measure for cheap?
     
  11. Robert Macy

    Robert Macy Guest

    Wow! didn't think of that at all! Thought of a mouse-like rolling
    thingy, but the approach in that URL is much more self contained and
    actually fits our existing software!

    We have a prize winner!
     
  12. Robert Macy

    Robert Macy Guest

    Great approach. Firt thing I thought of, too. but alas, I have some
    fields already in existence around the system that the introduction of
    any new field is likely to cause problems.

    Just a footnote to readers: Distance measuring using magnetic fields
    is really cheap and fairly accurate [relative terms]. I designed a
    portable distance measuring system for a Medical Product which
    essentially measured displacement along 16 axes, simultaneously. We
    built a simple two axis unit for measuring displacement from 6 inches
    to over 27 inches. Cost less than $5. No coil winding. Power
    consumption less than 5 mA per axis at 5Vdc. The exciting thing was
    that the MEASURED noise at a displacement of 8 inches was a mere 1 mil
    rms!!
     
  13. Robert Macy

    Robert Macy Guest

    Acoustics, I haven't considered very thoroughly, but could have some
    effectiveness.Will think about that one.

    The prism idea is interesting for its accuracy. But I think Ineed this
    to be 'single' operator system.
     
  14. Robert Macy

    Robert Macy Guest

    Hadn't considered optics much. But there is a lot of power in using
    'many' cheap units and sorting things out in SW
     
  15. tm

    tm Guest

    Wow! didn't think of that at all! Thought of a mouse-like rolling
    thingy, but the approach in that URL is much more self contained and
    actually fits our existing software!

    We have a prize winner!

    =============================================

    Good luck getting it to work over rough ground.
     
  16. Robert Macy

    Robert Macy Guest

    Sheer genius! I love USING the weakness - the divergence of the beam,
    as a feature, rather than fighting it!

    Now if I can just figure out a way to make a non-mechanical rotating
    beam. pulse when goes by north, time around to find angle divergence
    finds distance voila! polar coordinates! plus the resulting position
    data should be monotonic. have some distortion have some error, but
    never overlap. perfect.

    Single pole stuck in the ground don't get too far away. battery
    consumption to power a beam to overcome sunlight? snow light?
    This technique is worth thinking about for the lawn robot!
     
  17. Robert Macy

    Robert Macy Guest

    I don't know how often a camera supplies an image. But, stitching
    shouldn't be a problem. I've done spline fit and stitching software
    for some really 'distorted' images caused by views from two different
    positions and it is surprising the results possible. - if you're
    willing to stretch the final image, too.
     
  18. Robert Macy

    Robert Macy Guest

    Luckily, x-y should be all that's necessary. If I get into x, y, z
    I'll also get into 'tilt' and end up with 6 axes to deal with! x-y
    simulates the 'flat' earth, so should be sufficient. Better than not
    knowing where you're at or how fast you're moving.
     
  19. Robert Macy

    Robert Macy Guest

    good catch.

    It would be easy to put a 'marker' on the unit and as the Operator
    moves around, spot it in the camera's field of view. Live with the
    camera rep rate and simple interpolate to find where the system is at
    WHEN the system asks.
     
  20. Robert Macy

    Robert Macy Guest

    starting to lean that way, too. gross, numbers but getting pretty
    cheap. Plus have a record of EXACT terrain for archives.
     
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