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building a digital optical protractor

Discussion in 'Electronic Basics' started by [email protected], Apr 26, 2007.

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  1. Guest

    i am needing to measure the angle of an arm. I would like the readout
    to be very accurate to .001 degrees. Has this been done. I thought
    of measuring the optical pulses through a digital encoder like used in
    a digital mouse. thanks in advance for the help.

    Rick Wells
  2. Jamie

    Jamie Guest

    Yes, that would be the way to do it how ever, you need to get a
    high resolution type for that and maybe shieve it up to increase
    the ratio mechanically..
    to keep the slack out of it, use a mini steel pulley with tension
    spring in there. this will give you a positive response. Or, you
    can use a direct contact rubber grip roller, large one on the
    arm side and small one on the encoder shaft..
  3. John Larkin

    John Larkin Guest

    There are super-high-resolution rotary optical encoders, numbers like
    5 million counts per revolution. But they are mechanically precise and
    definitely not cheap. Google "high resolution encoder."

    Why do you need this resolution?

  4. Eric R Snow

    Eric R Snow Guest

    Greetings Rick,
    You can do this mechanically but the system will need to be very rigid
    and virtually lash free. Any slop will need to be less than the
    resolution and accuracy desired. You will also need to account for
    hysteresis in the moving parts. Assuming you can do this then listed
    below are some links that may help.
    Anti-backlash gears from SDP/SI, about $100.00 US for 72 tooth gear
    that might work.
    Metal spur gears from SDP/SI, about $12.00 US for a 20 tooth gear that
    might work with the above gear.
    A digital readout from US Digital that will read rotary encoders in
    quadrature. About $104.00 US.
    A 2500 count digital encoder from US Digital that will return 10,000
    pulses when read in quadrature with the above readout.

    360/10,000=.036 degrees. With the above gears and a 10 tooth gear you
    can reduce further the system to attain .001 degree resolution. It's
    up to you now to build something accurate enough. Maybe your best bet
    is to instead figure out what the range is going to be, how many
    degrees total you will be moving, and use a laser micrometer to
    measure the position of the end of the arm. A 1 inch long arm rotated
    1 degree will raise one end about .017". This means that you can use
    the arm length as a reducer, similar to a gear reduction.
  5. kell

    kell Guest

    Hmm, the job calls for you to measure rotational movement with an
    accuracy of one ten thousandth of an inch over a radius of six
    inches. This poses mechanical challenges as much as electronic, seems
    to me...

    I used one of those electronic protractors for carpenters the other
    day. The protractor has a resolution (ostensibly, accuracy) of a
    tenth of a degree.

    These protractors cost on the order of a hundred dollars which
    reflects economy of scale in mass production, and the amortization of
    initial development costs over many thousands of units manufactured.
    Wonder how many kilodollars went into engineering it?
    You want to build something with an accuracy a couple of orders of
    magnitude greater.
    I say if you can lick this thing on your own with nothing much more
    than some advice you solicited over the internet, you will have my
    profound admiration.
  6. Guest

    This arm is a spindle that holds a gem stone against an abrasive
    disk. The facets of the gemstone are cut this way. I was wrong on
    the .001 and .01 would be fine. I have studied the encoders. My
    hangup is I don't understand what to do with the output or pulse count
    from the encoder. thanks
  7. Guest

    Thanks for the response. I mis stated my accuracy need. It only
    needs to be .01 not .001.
  8. Eric R Snow

    Eric R Snow Guest

    Greetings Rick,
    Did you see my post about the devices from US Digital? They sell the
    counter you need.
  9. kell

    kell Guest

    Okay, if I couldn't afford to make a mistake and had to grind a
    something valuable like a gemstone with great accuracy I wouldn't try
    to do it all in one throw by measuring the position of the spindle.
    I'd grind some material off the stone, measure the stone, then grind
    some again, in several steps. Grind a few stones, you will adjust
    pressure and time empirically. Then after you've got it down, you can
    shave your stones within a few thousandths in only a couple passes if
    you're lucky.
    Hmm, "Shave your stones," that don't sound right...
  10. kell

    kell Guest

    Now that I've had some time to think about it, I'd advise against
    using a feedback loop of any kind in an attempt to get the accuracy
    you need. Instead, just use a mechanical stop on the action. The
    screw from a micrometer will make an adjustable set-stop with
    micrometer accuracy.
    Using controlled force from some pneumatic cylinder or the like to
    advance it, the stone holder keeps moving until a lever or boss on it
    comes in contact with the micrometer set-stop, and ceasing travel.
    Maybe you can work out the design. If your budget allows, perhaps a
    mechanical engineer. Or hire a very talented mechanic.
    If your app requires automation, use a stepper motor to turn the screw.
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