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Watson W
02-03-1995, 08:32 AM
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!


Regards,
Oyvind Stavdahl
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