I know this question is not fully related to aviation, but I assume the people on this subsection will also have some idea.
Suppose I have an accelerometer resting on a table(flat surface, horizontal). When I look from directly above, I see its reference coordinate system(it is labeled on it) as follows:
(from this site)
When I monitor the accelerometer readings (Ax, Ay, Az) ,
X and Y values are 0 initially(as they should be), and when I push the accelerometer(well, I am pushing the entire breakout board indeed) in the +x direction let's say, there happens an increase first and a decrease when I stop pushing it( I guess because of inertial acceleration ) in the Ax value(in terms of g) which is not surprising. Same for the y axis movements.
So this makes me conclude that the g in here is pure 9.8 m/s^2 without the sign.
But when it comes to z axis I am getting confused. In the above setup - in which positive z axis is pointing through the ground - when horizontal and no external force is applied(other than gravitational); Az value is -1, i.e -g or -9.8 m/s^2.
There the question, what does this accelerometer output actually? In such a standing on a surface scenario, there is only a gravitational force on the device and it is pointing towards +z direction. So should not the sensor give + 1g instead of - 1g ?
I guess I have misunderstood some concepts about these sensors(accelerometers).
Please explain me, thanks.