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From: nagle@well.sf.ca.us (John Nagle)
Newsgroups: comp.robotics,comp.lsi,sci.electronics
Subject: Re: velocity sensing for robotic joints
Message-ID: <19481@well.sf.ca.us>
Date: 9 Aug 90 15:42:41 GMT
References: <1990Aug7.205751.21206@ecf.utoronto.ca>
Lines: 26


      This is a worthwhile area for exploration.  We have direct-drive arms,
so we should have direct-drive sensors, ones which produce useful position
and velocity data while being rotated through small angles.  DC tachometers
are not useful at low angular rates, and adding a geartrain to support a
tachometer is a messy solution; among other things, you would need antibacklash
mechanisms.  Shaft encoders have similar problems.

      Potentiometers seem an obvious choice, but they aren't a good solution.
They tend to become noisy as they wear, and this can result in violent arm
motions in a servo loop.  Differentiating the output of a pot to get velocity
is even worse; any noise indicates a big velocity change, and adding a low-pass
filter introduces a phase lag.

      Variable capacitors are a good choice.  These are the usual sensors
in laser mirror deflection systems.  Since there's no physical contact between
the plates, the noise and wear problems of pots are eliminated.  The
usual circuts for sensing capacitor values produce a variable frequency
as an output.  This can be counted and used digitally with little
difficulty.  You can use a high frequency, megahertz if necessary, so that
you can obtain frequent counter readings and difference for velocity data.
Remember to use an enclosed variable capacitor with a grounded, conductive
case, so that the readings aren't affected by nearby moving conductive
objects.

					John Nagle