At 16.14 2012.06.16, you wrote:
>Electron wrote:
>> NOTE: all I really need is to know the inclination of my vehicle (dirt b=
ike)
>> but as I said the vibrations are extreme, and the mounting point can't b=
e
>> chosen/changed (it's on the frame).
>
>I figured as much. But in that case, you really only care about the
>orientation of the acceleration vector, not its magnitude, right?

Yes, and a gyro would be the answer, but the gyro alone wouldn't work, as i=
t
still needs a fixed reference. I cannot use a magnetometer, because the dev=
ice
is mounted on a steel frame. And the accelerometer cannot cope with the (HU=
GE!)
vibrations coming from the big bore two stroke engine.

I thought, since it's a vehicle after all, about slowly "drifting" the gyro=
 to
assume that gravity is below, i.e. bends, accelerations, decelerations, hil=
ls
up and down will cancel out in the long period. But then again it wouldn't =
be
reliable, e.g. if one uses it on a ring circuit.


>Vibration will affect that calculation, too, but it should still be a usab=
le
>result.

I thought that too, but the vibrations are huge (a few g's) and after solvi=
ng
clipping issues (which by the way lowers the final resolution) and aliasing=
,
it still looks like those vibrations don't cancel out but, like a hammer, g=
ive
a contribute to raise the average magnitude.

It puzzles me how can this may even be theoretically possibile, it's probab=
ly
due to errors in the accelerometer, i.e. when the "hammer" hits, it quickly
integrates a lot of energy, but this energy is then released very slowly an=
d
the low resolution of the accelerometer isn't able to integrate in the oppo=
site
direction. I.e. maybe if it was a perfectly analogic, infinite resolution s=
ystem,
vibrations would cancel out, but given their highly impulsive nature, on a =
real
system the errors are so big that it becomes unusable.

There's also the issue of aliasing: although I took care of it, it doesn't =
mean
that there aren't vibrations that escape the resonance frequency of the MEM=
S.


>Or are you using the magnitude value to try to decide whether or not
>the bike is moving?

My goal was to detect the lean angle of the bike, if it was on a hill, and =
how
fast it was accelerating or braking. Of course I expected to do it with lar=
ge
errors, as gravity adds a lot of problems by itself (but I thought that any=
way
the artificial forces would cause the magnitude to differ from 1.0g, so tha=
t
would have been of help). In the end, the vibrations predominate a lot over=
 any
other force, including gravity. And I'm even talking about a non moving bik=
e,
idling, when we add rocks, jumps, etc.. it would even go worse.


>
>-- Dave Tweed
>--=20
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