I had an idea that pulses caused by the brushes on the commutator could be
conditioned to digital levels and used as an encoder. Connecting a scope
to a small dc motor while running shows a pulse of 20mv whose frequency is
in proportion to the speed. It seems like capacitive coupling and amplifier
to the motor and maybe using a schmitt trigger at the amp out would be a working
alternative to opto sensors.

Hope this helps

James M

At 11:51 AM 8/26/02 -0700, you wrote:
>Hello,
>
>I'm trying to build an underwater tachometer for an AUV (autonomous
underwater vehicle) being built at the University of Victoria by a group of
engineering students.  What we hope to do is the standard shaft encoder
scheme where one determines speed by counter the number of strips that pass
by a photoreflector for a given time period.  As such, we are looking for an
IR photoreflector that is contained in a waterproof casing (maximum depth we
will be going is 30 feet).  Can anyone recommend such a photoreflector or an
alternative way to match the speed of two motors?
>
>Regards,
>Donovan
>
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