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Re: Limb Position Measurement

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  • Re: Limb Position Measurement

    A response to my angle measurement query.
    ------------------------------- Original Message -------------------------------
    Dear Warren,

    I have just read your mail with much interest - I am actually
    right now preparing for a PhD study that seems to be somewhat
    related to your work.

    I do have some (limited) experience with position transducers.

    So far I have been using high-quality potentiometers - they
    are available in encapsulated form and even with ball bearings.
    The encapsulation prevents dust and moisture from sticking to
    the resistive surface, thus preventing excessive wear and
    measurement noise. Potentiometers are even available as resistive
    circle segments (arcs) without any encapsulation, which makes them
    less space consuming but also more vulnerable. This solution is used
    in the new Boston elbow.

    Another applicable techlology is using optical or electrical
    encoders. My experience indicates that encoders with _absolute_
    measurement of angle are too large for use in prostheses (if you
    think or know otherwise, let me know!!) _Relative_ encoders give out
    pulses that have to be counted continously, so that the value stored in
    the counter register at any time indicates the absolute position. of the
    joint. This solution thus introduces the extra cost of a digital counter,
    but this cost will probably be negligible - and no additional
    analog-to-digital conversion is needed, as this conversion is an
    intrinsic property of the transducer.

    The durability of optical digital encoders will far exceed that of
    analog potentiometers. Still, potentiometers are available with
    guarranteed durability of more than 10 million revolutions. The weight
    of the two solutions will probably be of the same order of magnitude.
    Optical encoders often use LEDs, which may cause the current consumtion
    to become "large" - the LED has to be conducting ALL the time for the
    counter to keep track of all joint movements.

    I hope this will help answering your questions.

    Now back to your project: have you made any publications yet? I would
    be very interested in having some more information on your work - and
    I am looking forward to reading the summary of the answer you receive!

    Oyvind Stavdahl
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