FP-IND-PREDMNT1 amplitude value of the FFT

[ABent.1]

I would like to know how is the amplitude of the FFT calculated by the FP-IND-PREDMNT1 firmware related to the acceleration measured by the accelerometer found on the STEVAL-STWINKT1 board.

It is expressed in an absolute value but I need to know the relation to the intensity of the vibration/acceleration.

[Eleon BORLINI]

Hi @ABent.1​ ,

you can check this FP-IND-PREDMNT1 guide, that might explain you (from page 32).

From there, data should be expressed in m/s^2.

The conversion from g to m/s^2 accounts of the standard 9,81m/s^2 factor, while for the LSB to physical units (milli-g, or mg) conversion you can refer to the IIS3DWB datasheet sensitivity of 0.061 mg/LSB in the 2g FS case (p.5).

Please let me know if this answer your question and, in case, please select it as "best".

-Eleon

[ABent.1]

Hi @Eleon BORLINI​ 

thanks for the answer but it doesn't explain what I was asking, I'll show you what value of the FFT I need to evaluate so you can help me further.

The image attached is the representation of the FFT points acquired by the log of the PREDMNT firmware. I would ask you how are the amplitude values that you can seein the image related to the acceleration sampled by the accelerometer.

Thanks for your time and patience.

Andrea

[Eleon BORLINI · Accepted solution]

Hi Andrea @ABent.1​ ,

Thanks for sharing the data for better comprehension.

As written before, the value in AmplitudeX, AmplitudeY and AmplitudeZ columns are expressed in m/s^2, meter on second square, the standard unit for the acceleration. They refer to the FFT values for each of the frequency points (in Hz). To convert these into "g", you have to divide them by 9.81m/s^2, and to get back the LSB you have to multiply them for 1/1000 * 1/sensitivity in mg/LSB.

Is it clearer now?

-Eleon

[ABent.1]

Hi @Eleon BORLINI​ ,

thanks for the fast and kind reply, now it is clear and you have solved all of my doubts!

Best,

Andrea