openmoko at mazikeen.demon.co.uk
Tue Jun 5 19:24:58 CEST 2007
I think the reason for using accelerometers not solid state gyros is cost.
Sean quoted $3 for accels on the list a while back, while 3 axis gyros with
SPI were closer to $20 last I looked, and that was for one that wasn't
available yet. Production will ramp up, prices will fall and a future
OpenMoko will probably use an accel and a gyro, but for now 2 accels gets you
most of the way there for several $ less.
Compass would be interesting, to me at least.
On Tuesday 05 June 2007 17:54, Ortwin Regel wrote:
> Including more than one accelerometer doesn't really make sense I
> think because there are rotation sensors for detecting rotation. I
> have one in my DS Motion ( ndsmotion.com ) which can detect rotation
> around one axis. Include 3 of those (or probably a combined one) and
> you are good to go. In addition it would also make sense to include an
> electronic compass. Combined with the GPS, you should then always know
> where on earth you are and which direction you are facing.
> On 6/5/07, Tim Newsom <cephdon at gmail.com> wrote:
> > On Tue, 5 Jun 2007 7:52, 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
> > So are you saying that 3 3d accelerometers in a line with 2 on the end
> > and 1 in the middle will allow you to distinguish between rotation
> > around the center axis, etc?
> > It would seem to me that there are some realistic assumptions which can
> > be made to reduce error under normal usage. In addition, in a
> > navigation sense it would seem that you can use gps to provide error
> > correction and thus be at least as precise (or maybe not far from it) as
> > the gps between times when you are out of gps signal (I.e. Tunnel) etc.
> > Other than a navigation use, accelerometers will be useful for
> > manipulating applications, but without a compass module, pointing or
> > other types of "external" information apps might not be possible
> > anyway. If that's true, then each program will have some assumptions
> > built in for normal usage.
> > Errors can be mostly ignored since what will usually matter will be the
> > differences between vectors in very short timeframes OR the difference
> > between the start vectors and the current vectors. If the phone is
> > suddenly dropped or thrown that's probably detectable as an extreme
> > motion and maybe ignorable. /shrug
> > --Tim
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