Re: Accelerometer + Tilt compensation

On Thu, 21 Aug 2008 09:51:02 -0700 (PDT), jdhar <jai.dhar@xxxxxxxxx>
wrote:


Start with asking what you are trying to measure.  If you don't need the
DC, then high pass filtering is your easiest solution.

Thanks Scott... I don't need DC, I'm concerned only with the delta in
acceleration over time. I'm not sure if highpass will do exactly what
I need however. If I simply just highpass filter all 3 axes, it won't
help the fact that moving in one TRUE axis (say movement in true X)
will couple into 2 axes on the accelerometer due to mounting tilt,
right?

I may be smelling a conceptual error here. Accelerometers do not
measure displacement or velocity. It sounds like you want a
navigational instrument based on accelerometers only. It might work
but a magnetic compass and dead reckoning works better. A handheld
coupled to the magnet compass and a velocity sword will produce much
better results. Then again if your issue is making sure just which
way is "up" the 3d accelerometer is a good choice.


Since gravity is just like any acceleration, once you start thinking
about your problem, you'll see there are some ill-posed aspects.  If you
can find a situation where you can be assured that the platform has no
acceleration other than gravity, you should be able to correct, but if
you're talking about a ship, it will be rolling and heaving, so the
problem is difficult.  Consider moving up to a 6-degree of freedom
system, that has three accelerometers and three gyros, if you can't just
high-pass filter to remove the low frequency.


It seems like 6-degree is where it's going, but I need to fully
understand why first. The vessel is a boat, but this is for a sports-
performance based application, so it's not a big ship that is in waves
or anything. So in short, I could find a situation where there is
"virtually" no acceleration other than gravity. The problem is I can't
guarantee the situation where the sensor will be mounted such that
gravity is only applied on the Z-axis.

--
Scott
Reverse name to reply
.

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