# Accelerometer(s), Camera, and Memory

Ian Stirling OpenMoko at mauve.plus.com
Mon Jul 21 22:02:38 CEST 2008

```Dee Ayy wrote:
> I just heard of OpenMoko and Neo FreeRunner today.  Awesome!
>

>
> I noticed the Hardware Highlights lists "3axis Motion Sensors (2)" and
> would hope that the developers included 2 accelerometers for the
> purpose of sampling the rotation as well as translation.  The
> "closedPhone" and "closedPod" only sample translation via their sole
> accelerometer.
>
> So my question is the same as Francesco Cat's.  Where exactly are the
> accelerometers positioned on the device?
> Would someone please confirm why there are 2 accelerometers?

As far as I can tell, because someone decided it would be a good idea,
without doing the numbers.

From some basic numbers, you can do little with the fact that there are
two accellerometers.
The fundamental problem is that the accelerometer chips have a
resolution of some 16mg.
If the accels are 10cm apart - on the periphery of a circle 5cm in
diameter, then as accelleration is proportional to the speed squared

A=v^2/r
0.016 = v^2/.05
.32 = v^2
v = .56m/s.

This is a rotational speed of .56m/s / circumference, or .56m/s / .3m =
1.5 revs/second.
And this is for one LSB, so it will be very noisy.

The accels can - if positioned appropriately - I don't know where they
are - pick up taps on the case, and their relative distance from each accel.

```