# Understanding accelerometer output readings in general (though using LSM303DLHC)

I am using a LSM303DLHC accelerometer to measure vibration. I had thought I understood something … but now am not sure.

Assume the device is sitting still. The only physical impact on the device being the rotation of the earth (that is, ignoring my central AC, overhead fan, the cat …). Assuming the ‘steady state’ output values are all 0.0000 (I know it would not be, but for purpose of understanding …). If an object is dropped onto the surface where the device is, the outputs all begin to oscillate, going above and below 0.0 for a period of time until the vibration stops.

If this is the data for one of the axes is graphed like this …

Would it be correct to say that at time …

… N0 - the device was in its steady state position
… N1 - the device has been accelerating, physically moving in one direction
… N2 - the device has reached its peak acceleration - but NOT the farthest distance from its steady state position
… N3 - the device has begun to slow down, but is still moving in the same direction
… N4 - the device has stopped accelerating and has moved the farthest away from its steady state position
… N5 - the device begins to accelerate again and is moving in the opposite direction, back towards its original location
… N6 - the device has reached its peak acceleration
… N7 - the device has begun to slow down
… N8 - the device has stopped accelerating and is back to its original location

Would appreciate any feedback !

Not true. The accelerometer will measure the acceleration due to gravity, 9.8 m/s/s, along any axis pointing straight up or down. There will also be a component of the 9.8 m/s/s acceleration due to gravity along any axis that is not perfectly horizontal.

The comments associated with N4 and N8 are not correct. An object accelerating according to the graph, e.g. along the horizontal X axis and starting from rest, will not reach its maximum distance from the start point at time N4. In fact it is moving at maximum speed at that time. It will be farthest from the start position at time N8.

Consult your favorite high school physics book!

So, on the graph of acceleration versus time, at N4 the device begins to slow down ?

Acceleration is the change in velocity.
If the acceleration is positive, velocity is increasing.
If acceleration is zero, the velocity is not changing.