Question
The inclinometer works very well: if I incline the device, I can measure the
angles correctly. But, even if I move the device in a plain PARALLEL to the
inclinometer axis, I have some variations in the inclination data! In other
words, if I apply a force parallel to the X or Y axis, the inclination outputs
should be equal to zero, but they aren't. In the data sheet I can read: "The
ADIS16201 inclinometer output data is linear with respect to degrees of
inclination and is dependent on no forces, other than gravity, acting on the
device".
Answer
The incline angle is derived from the acceleration measurement. Any external
force will impact the accuracy. In order to do what you are doing, you will
likely need to fuse multiple sensors to isolate the actual angle. A classic
approach is to use a gyro with a product like the ADIS16201. Then, the work is
in determining what the fusion algorithm should be for your intended purpose.