I probably missed the whole point but my first read on this sounds a lot 
like a track ball.

John Ferrell
http://DixieNC.US

----- Original Message ----- 
From: "Jake Brownson" <jbrownson@gmail.com>
To: <piclist@mit.edu>
Sent: Wednesday, February 22, 2006 7:33 PM
Subject: [EE] Accelerometer to measure rotation?


> I'm trying to measure relative rotation within a sphere. I have an ACA302
> (http://www.star-micronics.co.jp/eng/products/sensor/se01.htm).
>
> So, imagine holding a pole with an accel attached at the end, you can
> spin in place (horizontal rotation), and raise the pole up and down
> (vertical rotation), but the accelerometer stays a fixed radius
> (approximately) from a central point. (Note that the accelerometer is
> not always level)
>
> Let's establish some terminology. The Z axis is pointing up from the
> accelerometer, the X axis is pointing out from the middle of the
> sphere and the Y axis is normal to the plane made by Z and X. The
> origin is at the accel, and moves as the accel moves.
>
> okay, I hope I've painted a good picture... here's how I'm trying to do 
> it...
>
> I'm assuming that the person's rotations will be _relatively_ slow and
> smooth, so the vertical rotation can be measured by looking for an
> acceleration of 1G between the Z and X axes. technically there are
> some problems with this, but this will be close enough.
>
> The horizontal rotation will be a little more tricky... theoretically
> I can do a numerical 'integration':
>
> 1. Get the acceleration along the Y axis
> 2. Linear Velocity += sample period * acceleration
> 3. Angular velocity = linear velocity * 360/(2 * 3.14 * r) (or
> something like that)
> 4. Horizontal rotation = sample period * Angular velocity
> 5. Repeat as fast as possible
>
> I've been looking at the accel on the scope and it seems like for the
> slow movements that would be used there is just a lot of noise. Also
> I'm concerned that if the zero point for the accelerometer isn't
> calibrated _exactly_ the drift will add up very quickly.
>
> So is this idea insane? Do I need a more accurate accelerometer? Is
> there another sensor that would be better?
>
> I'm aware of digital compass modules, and that is a possibility,
> however those need to be level to operate, and this is a portable
> system so that might be a bit of trouble. I also have a very low
> budget for this, and anything over $50 might be a problem.
>
> By the way this is for a student project at Oregon Institute of
> Technology if anybody is curious.
>
> ~Jake B
>
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