> I have a need to know the position of a shaft, within one degree
>accuracy, that rotates at a speed of one revolution in two minutes.
>The shaft can only make one turn.

Quick answer, find an old daisy wheel printer from the 80's, and look at the
daisy wheel position sensor. Most of these were an inductive transformer
type of sensor, with a driven coil, and two pickup coils, a bit like a
circular LVT, but giving an inductive equivalent of a rotary encoder.
Because the resultant signal is a pair of 90 degree amplitude shifted
waveforms, it is possible to measure the actual angle extremely accurately.

However as you are setting out to do an "after market fit" you may like to
try a slightly different way of doing a similar thing. Fit a semicircular
sheet to the shaft, but insulated from the shaft, and have some means of
making an electrical connection to it. Now each side of it fit a pair of
same sized semicircular sheets, which are 90 degrees rotated from each
other. Now drive the moving vane, and pick up the signal from the fixed
vanes, which will give different amplitudes. However a better way is
probably to drive the fixed vanes with 90 degree phase shifted signals, and
pick up the signal from the moving vane. A phase shift detector should then
give you the shaft angle directly from the sum of the two signals.

As others have pointed out using a hall sensor is not likely to get you a
very good signal. If you really wish to follow that route then I suspect you
will need an actively driven coil and detect the damping of the signal
amplitude by the gear teeth moving past.

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body