The term accelerometer is a bit misleading. It's probably better to think
of them as force meters, since they are actually measuring the force on a
known mass that is required to keep the mass from moving relative to the
rest of the system. Ok, that sounds a little confusing, but I think it
makes sense :) The force is then divided by the value of the known mass to
give an acceleration. Each accelerometer is ideally sensitive along only
one axis, so if it is stationary in the earth's gravitational field with the
axis of the sensor along the gravitational field, it will read 1g, since
that is the force needed to keep the mass stationary divided by the mass.
If the axis of the sensor is perpendicular to the gravitational field, it
will read 0g, since there is no force required to keep the mass stationary
in the direction that the sensor is sensitive. If the sensor is in free
fall it will also read 0g because even though the sensor is accelerating the
mass is being accelerated at exactly the same rate and again no force is
required by the sensor itself to keep the mass stationary with respect to
the sensor.
The problem is that there is no way to measure the difference between an
object in a gravitational field of 9.8 m/s^2 and one that is in an
elevator being accelerated in free space at a rate of 9.8 m/s^2. This is
known as the equivalence principle and is at the heart of general
relativity. All inertial navigation systems have to have some way to
subtract the earth's gravitational field in order to give the right answer.
I hope that made some amount of sense.
Mark
On Mon, Mar 9, 2009 at 12:15 PM, Chris Barker <ftog@xxxxxxxxxxxxxx> wrote:
> No, that doesn't persuade me, Piers :-). The position, such as lat
> and long, are not important, but the level/not level position is. And
> the accelerometers won't give any information when there is no
> movement, whatever their relative positions or orientations. You need
> a gyro-stabilised platform to give level.
>
> I'm about to buy an iPhone which has accelerometers; perhaps that will
> persuade me.
>
>
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