> I'm going to assume for the sake of this discussion that a person is
> holding a stick with the three axis accelerometer on the far end.  The
> user is spinning around and pivoting the near end of the stick so the
> far end (with the accel) can 'touch' any point on the surface of the
> imaginary sphere the user is in.

This is exactly right, how concise :)

> Your description will work, but as you know you'll have a lot of error
> over time for the "RPM".

Actually, the RPM will be about 0. This isn't meant to be a continuous
rotation, but just somebody moving the stick around the sphere in a
realtively smooth, slow way. I didn't make that clear.

> Also note that centripetal force is going to drastically affect the Y
> and Z accelerometers depending on the angle of the stick relative to
> the angle of the spin.  However, this may be to your advantage since
> the outward force of those two combine will also give you some
> information about how fast the user is spinning.

I don't think this will be a problem because the rotation isn't
actually continuous.

> If you want to add to your workload and still keep it cheap, I'd
> consider adding a phototransistor to the tip of the stick.  Put a
> diffuse screen over it, and low pass filter it to drop frequencies
> above 5Hz.  As the user spins around this will output a signal whose
> lowest major frequency is very likely to be the spin rate of the user.
>  The diffuse screen helps give the same signal while the user "raises"
> and "lowers" the stick.  Since you already know the ballpark spin rate
> due to the accelerometer you can use that to digitally filter the
> signal and give yourself a synchronizing pulse to correct the
> accelerometer's error.

I really like that idea, I'll have to keep it in mind, but again, as
the rotation isn't continuous...

~Jake B

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