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Position Measurement

Discussion in 'Electronic Design' started by [email protected], Feb 6, 2007.

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  1. Guest

    The problem at hand is a relatively accurate (1/8") measurement of a
    cart's position for introductory level physics course. The sampling
    rate would be in the area of 5 s/sec. The measurement will go up to
    about 4 feet (no more than 5). After doing some googling, it seems
    that the best options I have are:

    --String Pot
    --Laser Measurement

    The string pot seems like the best approach, however, may be out of my
    budget. I'm fairly electronically savvy and was wondering if it was
    possible to build one of these myself. I figured just getting the
    distance translated into a voltage would be sufficient for porting
    into the data acquisition setup I have available. I saw Spaceage
    Controls Series M Position Transducer looked pretty good, but I'm not
    sure over how much one of these might cost.
  2. It may not be so simple. By your definitions you want to measure
    1/(5x12x8) units = 1/480 ~ 0.2% of full scale. To do it properly your
    transducer should be at least certified five times more accurate than
    the reading obtained.

    Good hunting

  3. Guest

    A long-long time ago in a galaxy far-far away my physics teacher used
    the rotary encoder hacked off a mouse for this.
  4. Rich Grise

    Rich Grise Guest

    By "string pot", do you mean a long "string" of resistance wire, with
    the wiper on the cart, or just a string that turns a pulley on a pot?

    I can't imagine either being outside the budget for a class - you'd
    have the kids help build the thing, and a precision pot can probably
    be had for less than $20.00.

    Getting that kind of precision with ultrasonics might be a project
    in itself, albeit not for beginners. :)

    Good Luck!
  5. jure

    jure Guest

    you don't specify the actual experiment you have in mind,
    but here I offer a Copernican twist.

    place optical interrupters at different locations, and this way you
    know the
    time when your object was at a specific place.
    Then you need to time tag the transitions on a computer.

    this would apply, for example, to the case of balls running down a

    Jure Z.
  6. Tim Williams

    Tim Williams Guest

    I don't think that's what he wants. Not simple enough and universal enough.

    What the physics department uses at my school is the ultrasonic detectors.
    They plug into an USB recorder thing and the lab's laptops run some software
    that graphs things. Appears to be all commercial stuff, and likely quite a
    chunk of change too (no wonder all the gear in the department is 30+ years
    old :^) ).

    I imagine laser rangefinders would work, too.

    For home-built, on-the-cheap and easy to interface, I like the suggestion of
    a rotary encoder. You'd want it either spring loaded (which will affect
    measurements!) or looped on a pulley to maintain tension in the string, and
    of course, oiled for a minimum of friction (ball bearings are a must).
    Unfortunately this is as fixed (or moreso) as the resistance wire Rich
    mentioned, as far as portability and setup.

  7. John Fields

    John Fields Guest

    Me too.
    Doesn't have to be...

    Why not just use a mouse and a spring-loaded non-rotating plunger to
    keep it in contact with the floor when the table's being lifted and

    That way you'd have both X and Y (so you could correct for any
    lateral movement) and if you sent the mouse's output via IR or RF
    the portability issues go away.
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