> That's how I would imagine it to work, but I'm intrigued to learn how
> thaey have managed to remove any non-linearities in the system.  I may
> be miles off the mark, but I'd have thought that the output from a hall
> sensor would not be proportional to the angle of a magnet, and
> furthermore that the output would be heavily dependant on the relative
> mounting  points of the magnet and sensor.  Trying to get a totally
> repeatable system that dosen't require a lot of calibration must be
> tricky, at least with 4096 counts per revolution. 

I just went through all of this on a much smaller scale, with GMR sensors
instead of Hall effect.  Repeatability with Hall effect is much more doable;
GMR sensors have nasty levels of hysteresis.

At any rate, if you keep a magnet at the same distance from a Hall effect
sensor and change the angle of the magnet field (say, by rotating the
magnet), you'll find that the field varies in a manner which would easily
allow you to measure the relative position of the magnet- with one caveat.

The measurement is going to vary sinusoidally from full scale to 50% of
full scale.  However, with only one sensor, your readings will be identical
at either side of the circle.  This can be accounted for by checking whether
the reading is increasing or decreasing- first derivative test, IIRC.

I guess I should also mention that this is assuming that the axis of the
magnetic field is perpendicular to the axis of rotation.  From the design
of this product, I'd say they intend for you to bore a hole (with a lathe,
probably) in the center of the shaft of your motor, then put a magnet in
that hole which is cylindrical.  The magnet should be polarized not from
flat face to flat face but along some axis perpendicular to the center.

Those aren't impossible to find; wondermagnet.com sells quite a few
sizes of those, and in quite powerful magnetic alloys (?proper term?)
as well.

I think I'll put some math-heavier stuff in another message...

Mike H.
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