Why 2 accelerometers?

[Christoph Mair]

Am Freitag 18 September 2009 18:18:14 schrieben Sie:
> On Friday 18 September 2009, RANJAN wrote:
> > On Fri, Sep 18, 2009 at 8:35 PM, Al Johnson
> >
> > <openmoko at mazikeen.demon.co.uk>wrote:
> > > On Friday 18 September 2009, RANJAN wrote:
> > > > Hi,
> > > >
> > > > I was wondering why FR has two accelerometers.They could have one 3
> > > > axis accelerometer and a single angular rate gyro.How is a
> > > > combination for 2 accelerometers more useful than one 3 axis
> > > > accelerometer and a gyro?
> > >
> > > Or, better, use a 3-axis magnetometer instead of a gyro.
> > >
> > > At the time the FR was designed, gyros were significantly more
> > > expensive and
> > > less available than accelerometers, and magnetometers were large and
> > > even more
> > > expensive. Spaced accelerometers sounded like a good way of getting
> > > most of the way there while still being affordable. In practice it
> > > doesn't work so well. In the meantime gyro and magnetometer prices have
> > > tumbled.
> >
> > I think when FR was designed the Gyros costed around USD 17 .Is there a
> > next version of Openmoko phone being designed?
> 
> Openmoko are designing something that we don't know the details of, but
>  that is supposed not to be a phone.
> 
> The gta02-core project is designing a phone, but this is more about the
> community development process and tools than the design itself. The aim is
>  to produce a handful of boards, and if they're lucky they might even work.
>  http://wiki.openmoko.org/wiki/Gta02-core
> 
> There's a project to add a magnetometer to existing FRs using a little PCB
> above the SIM.
> http://wiki.openmoko.org/wiki/Talk:I2C_Compass
> 
> There's also the Flow which is fairly open and potentially fully featured,
>  but rather expensive.
> http://www.gizmoforyou.com/e107_plugins/content/content.php?content.32

I have a working prototype with the HMC5843 three-axis magnetometer. It is 
part of a project which will include two gyros (XY, Z), the magnetometer and a 
MSP430 microcontroller which allows to add other features (currently planned: 
ambient light sensor which regulates the backlight without CPU intervention)
The whole PCB will fit in the empty space under the GPS connector.

Some pictures, the board layout and the schematics as pdf can be found here: 
http://chonyota.net/freerunner/
All sources (kernel driver, schematics and layout) are available from 
http://gitorious.org/freerunner-navigation-board
The application which processes the measurement data is not online yet because 
it's still a quick hack which needs some improvements. I hope to get this done 
in the next few days.

Christoph