>>Hi, I'm working on location sensing by using 1 accelerometer and 1
>>magnetometer only (so called "dead reckoning") but now I realize that
>when
>>I move my device, it is always possible to tilt the device a littl
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 given
>by
>>the accelerometer, the result is largely inaccurate.
>
>It seems doubtful. Why would you only have 1 accelerometer? (If I ha
to
>guess, you're on some sort of wheeled ground vehicle that's onl
"supposed"
>to move in 1 local direction? If so, consider odometry instead, or in
>addition.) If you've got any significant angular velocity and your
>accelerometers are not at the center of mass, you will have other
>components you need to remove, too, not quite as significant as gravity,
>but potentially swamping platform acceleration.
>

Hey, Thank you very much for your answer! Actually, I'm not dealing with
wheeled vehicle...I'm working on a cellphone(Android phone, to be specific
that has 1 accelerometer and 1 magnetometer built in (also 1 temerapur
sensor which I dont think it helps in any way here). I just read abou
"odometry" on wiki, and figured it seemed only applicable to a wheele
device, so it isnt really suitable for my case here, am I correct?

>But even with a 3-axis magnetometer, as you mention later, you only ge
a
>single vector (magnitude and direction) in 3-space. Magnetometers are
>fraught with all sorts of issues, regarding disturbances, both platfor
and
>environmental.

Yeah...I'm already aware of this...the magnetometer is used in th
cellphone together with the accelerometer to determine/calculate th
orientation.

Ignoring those (a potentially serious source of error), if
>your magnetometer outputs something of the same magnitude (t
oversimplify
>again) as your local magnetic model, but rotated, you're generally still
>short of knowing how it was rotated. Now if you can stop your platform
>completely, and your accelerometer is not mounted in an unfortunate
>orientation, then *maybe* you can get something. But, even if tha
works,
>that doesn't really help with "separating gravity": that's just usin
what
>is known to be gravity (barring all sorts of acceleromete
nonidealities)
>to compute static orientation. I suspect it's a waste of your time to
>try.
>
>You'd really be MUCH better off getting 3 accelerometers and 3 gyros, or
>re-thinking the strategy altogether. You're really just making things
>difficult on yourself, and if your precision requirements are such tha
a
>few cheap sensors won't do, then I think you're headed for serious
>problems.

Thanks for your advice. Then if I had 3 accelerometers and 3 gyros, ho
would I solve my problem, getting the orientation or offsetting G? Coul
you roughly let me know the approach? I'm quite new to this field. Actuall
I've been thinking of the similar problem: how the aircraft determines it
orientation...

>
>
>>So I have to offset G first to get the real motion acceleration. I
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 my
>case
>>here.
>>So do Kalman filter help? Thank you very much!
>
>If your platform model is accurate enough (is it? do you even hav
one?),
>it should help, relative to nothing at all (after all, it should b
better
>than open-loop dead-reckoning based on actuator commands, and a goo
enough
>model can make it better than just looking at the sensors at each time
>instant). But it may or may not be ENOUGH to do what you want.

Well...what do you mean by "platform model"? Is it possible you sugges
any? and I will do the further reading on them. I only heard Kalman filter
got something to do with the model...yeah, would appreciate your
suggestions!
>
>
>