Good Day! I would like to ask what does x y z from the triaxial

accelerometers refer to? What is the displacement, velocity and

acceleration.

I am currently having a project of vibration monitoring using adxl345 and

arduino. I want to know what axis is needed to undergo FFT to determine the

vibration frequency sensed by the accelerometer.

Thank you for your post. The ADXL345 measures linear acceleration on all three axes (x, y, z) of the classic Cartesian coordinate system, which are mutually orthogonal to each other. Unless you are certain of the direction of vibration in the platform you are monitoring, I would probably start with performing spectral transforms (FFT) on all three axes. Some eventually determine that they can combine all three for their long-term tracking algorithms, while others determine that they need each axis to properly diagnose each condition of concern in their application. I hope that this helps.