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Need Solution for Peculiar Ring/Pointer Problem

Discussion in 'General Electronics' started by Maddy, May 22, 2004.

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  1. Maddy

    Maddy Guest

    I have a peculiar problem. I have a circuler ring(1 metre dia, marked
    from 0 to 360 in degrees) which is free to rotate. There is a pointer,
    not connected to the ring which can be rotated seperately around the
    ring, touching the ring. I want to digitally check which part of the
    ring the pointer is touching(with an accuracy of about 0.1 degrees).
    The ring cannot have any power source on it since it rotates. The
    pointer can be powered from its rotation assembly. Can anybody give
    any suggestions? (one suggestion was to put a wire on the ring and use
    the wheatstone principle, but I am not sure how)
  2. Well, you could put a bunch of magnets on the ring, evenly spaced, and a
    pickup coil on the pointer, and then use a counter to count how many magnets
    the pointer had moved past... or you could make an optical encoder to do the
    same thing. The electronics for both are very well established, it would
    simply require designing the physical aspects of it. Say, an optical
    encoder ring just below your ring, attached so it rotates with the ring, and
    a photodiode / LED pair hanging from the pointer to read it. You can get
    the encoder ring custom made for you (sorry, can't provide a source as I
    have never needed the service, but I have heard that there are those who do
    such for a fee....) or you could take aluminum or steel sheet and make holes
    at the appropriate intervals to make it yourself. The reader can be ripped
    out of almost any printer, many disk drives, etc. As for the magnet
    solution, the evenly spaced magnets could be done with a length of magnetic
    cassette tape, and use the read/write head to write the evenly spaced spots
    onto the tape, and then to read them during operation.

    If you don't want something hanging from the tip of your pointer, you could
    just use the encoder ring to measure how far the ring had turned and in
    which direction, and another to measure the movement of the pointer, and add
    or subtract the two values appropriately. There're lots of tutorials on
    encoders on the web that will teach you how to get direction and distance
    information out of an encoder.
  3. Robert Baer

    Robert Baer Guest

    If i remember correctly, ther are afew rotary optical encoders
    available intheDigiKey catalot (switch section).
    Also try Mouser.
  4. JeffM

    JeffM Guest

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