# Neo1973 Update!

David Duardo dduardo at gmail.com
Tue Jun 5 19:21:10 CEST 2007

```Why the Neo is going to have 2 tri-axis accelerometers is beyond me. The
only reason you would want to use 2 3d accelerometers is if you want
higher accuracy in rotation measurements, but for the type of
application I see little gain in the extra accuracy. The Nintendo
Wiimote only uses 1 3d accelerometer and the sensitivity is good enough.

Here is a pdf that shows you what I'm talking about:

http://kionix.com/App-Notes/AN005%20Tilt.pdf

Notice how in figure 6 the angle of the different axis effect the tilt
sensitivity.

- David

Mauro Iazzi wrote:
> On 05/06/07, Tim Newsom <cephdon at gmail.com> wrote:
>> If your tracking movement with 2 3D accelerometers... What would another
>> one provide.
>> As far as I can tell (I am not an expert...)
>> Tracking all 6 vectors will tell you absolute movement in space.  I.e,
>> when 2 vectors point in the same direction with the same magnitude at
>> approximately the same acceleration as gravity.. Its probably laying or
>> positioned flat on that side.
>
> "Probably" is the key here. with two 3d (linear) accelerometers you
> cannot sense rotation around the axis between the two in an inertial
> frame of reference.
> Moreover you cannot distinguish if the Neo is laying face down or
> pushed downwards with 2mg force. This example is somewhat artificial,
> but means that you can probably find more realistic (though
> complicated) movements that are not distinguishable with only two
> accelerometers.
>
> You must then consider the errors which sum up, if you try to track. A
> rough mental estimate gives that you can sum up as much as 1 meter of
> error in ten seconds if you have a precision of 10^-3g over
> acceleration measure. (it does not mean that you are 1 meter away from
> the real position, it means that you can only be sure that you are at
> most 1 meter away from that).
>
> Most of the time you will need good assumptions to get any information
> from raw data:
>
> http://www.wiili.org/index.php/Motion_analysis
>
> can be of some help. No linear accel, no rotation, no tilt, are
> assumptions which can give some meaning to the data and can be done
> for single application, where you can assume the user will have some
> particular behaviour (or you require it).
>
> Still absolute tracking won't probably be anyhow realizable.
>
> --mauro
>
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