# Accelerometer wiki page erroneous axes ?

François TOURDE fr-om at tourde.org
Thu Apr 9 01:20:36 CEST 2009

```Le 14342ième jour après Epoch,
rixed at happyleptic.org écrivait:

> Well, some confusion here.

Yes, probably because of mixing gravity, force, acceleration, inertia,
etc... :)

> OK, let's imagine a _perfect_ accelerometer.
> This device would report only acceleration. So, when the phone lies on the
> table, it would report 0 (neglecting the rotation of earth here
> :-)).

Please, don't stop earth rotation, because my weight will increase too
much :(

The _perfect_ accelerometer is measuring 2 forces: The gravity, and
the table reaction against the phone fall. These 2 forces are in
opposite directions, then accel value is 0.

If you shift left (or right) your _perfect_ accelerometer, it will
(probably) indicate a X acceleration value. But only if it has no mass
so no inertia :)

The Freerunner accelerometers are able to compare these 2 forces, and
so it compares gravity vs reaction, or force vs inertia.

> Now, lets suppose that this perfect accelerometer is not calibrated correctly,
> and offset every values by 900. It will then report first a value >900,
> then 900, then a value <900.

Your idea works only for the accelerometer Z, in case of the phone is
left horizontaly, screen up. X and Y accelerometers don't have the
"calibration error" in this case.

And if your phone is falling, the accel values will turn to zero, even
in the earth gravity field. If you will try this, please consider
using a pillow (for example) to avoid crash ;)

> (*): our accelerometer is not perfect, but not because of a random
> offset. It is not perfect because it has no mean to distinguish between
> an actual acceleration from the effect of gravity.

> This is not a
> defect and there is no need to send the phone back to OpenMoko :-)

I agree ;)

> because
> it is just how nature work (http://en.wikipedia.org/wiki/File:Elevator_gravity.svg)
> The effect of gravity (downwards) is the same as an acceleration
> (upwards!!)

Not the same, but we can't make any differences just looking at the
trajectory.

```