>Hi, I'm working on location sensing by using 1 accelerometer and 1
>magnetometer only (so called "dead reckoning") but now I realize tha
when
>I move my device, it is always possible to tilt the device a little bit,
>such that the gravity is decomposed onto 3 axes, which causes that the
>real
>motion acceleration and the gravity cannot be told apart. Thus, if I
>perform integration(dead reckoning) on the acceleration directly give
by
>the accelerometer, the result is largely inaccurate.

It seems doubtful. Why would you only have 1 accelerometer? (If I had t
guess, you're on some sort of wheeled ground vehicle that's only "supposed
to move in 1 local direction? If so, consider odometry instead, or i
addition.) If you've got any significant angular velocity and you
accelerometers are not at the center of mass, you will have othe
components you need to remove, too, not quite as significant as gravity
but potentially swamping platform acceleration.

But even with a 3-axis magnetometer, as you mention later, you only get
single vector (magnitude and direction) in 3-space. Magnetometers ar
fraught with all sorts of issues, regarding disturbances, both platform an
environmental. Ignoring those (a potentially serious source of error), i
your magnetometer outputs something of the same magnitude (to oversimplif
again) as your local magnetic model, but rotated, you're generally stil
short of knowing how it was rotated. Now if you can stop your platfor
completely, and your accelerometer is not mounted in an unfortunat
orientation, then *maybe* you can get something. But, even if that works
that doesn't really help with "separating gravity": that's just using wha
is known to be gravity (barring all sorts of accelerometer nonidealities
to compute static orientation. I suspect it's a waste of your time t
try.

You'd really be MUCH better off getting 3 accelerometers and 3 gyros, o
re-thinking the strategy altogether. You're really just making thing
difficult on yourself, and if your precision requirements are such that
few cheap sensors won't do, then I think you're headed for seriou
problems.


>So I have to offset G first to get the real motion acceleration. Is this
>possible to be done by only using the accelerometer and one more
>magnetometer (since my device has it built in too)?
>
>I read an old thread on this web,
>http://www.dsprelated.com/showmessage/39145/1.php
>
>It talks about using Kalman filter to filter out G and sort of
>things...but I couldn't really see any straight answers related to m
case
>here.
>So do Kalman filter help? Thank you very much!

If your platform model is accurate enough (is it? do you even have one?)
it should help, relative to nothing at all (after all, it should be bette
than open-loop dead-reckoning based on actuator commands, and a good enoug
model can make it better than just looking at the sensors at each tim
instant). But it may or may not be ENOUGH to do what you want.