Accelerometer wiki page erroneous axes ?

giacomo "giotti" mariani giacomomariani at
Wed Apr 8 16:26:02 CEST 2009

>> > I beg to disagree. If you are talking about acceleration of the sensor
>> > itself then you'll see that: 1. it's not actually accelerating if you
>> > attach a reference frame to the Earth (the freerunner is laying
>> > still).  2. The gravitational force applied to the sensor is directed
>> > to the center of the Earth,
> OK untill here.
>> > so the reported "acceleration" should be positive too.
> The reported acceleration is positive, this is a fact (cat /dev/input/event3).
> The question is : is a positive acceleration accelerating downward or upward ?
> The wiki tells that a positive acceleration is downward (Z axe pointing down).
> This is not true, in my opinion.
> When standing still, the accelerometer must report an acceleration directed in the
> _oposite_ direction the the gravitational force. As it is reporting a positive
> acceleration, then the Z axe goes up.
> And I believe the other axes are inverted as well (which, as suggested by Michael,
> is easily provable by turning the FR around so that each axes points down).
What is drawn in the wiki is a reference system; in that reference
system the gravity acceleration take the right sign, being  in the same
direction as the z axis.
If you accelerate upwards your phone (with the screen pointing up) with
an acceleration a  you see a greater acceleration a_tot (along the z
direction) that is the sum of gravity acceleration g and your imposed
acceleration a (a_tot=a+g). That means that the force acting on the
phone is equal to the mass of the mobile m time the total acceleration
and you can verify it with a dynamometer.


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