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. > > Ortwin > > On 6/5/07, Tim Newsom <[EMAIL PROTECTED]> wrote: > > On Tue, 5 Jun 2007 7:52, Mauro Iazzi wrote: > > > On 05/06/07, Tim Newsom <[EMAIL PROTECTED]> 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 > > > > _______________________________________________ > > OpenMoko community mailing list > > community@lists.openmoko.org > > http://lists.openmoko.org/mailman/listinfo/community > > _______________________________________________ > OpenMoko community mailing list > community@lists.openmoko.org > http://lists.openmoko.org/mailman/listinfo/community _______________________________________________ OpenMoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community