Neo1973 Update!

[Mauro Iazzi]

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