What is the computation formula for |XYZ| Modulus in Freescale Vibration Monitoring?

[roberto70]

I'm trying to use a FRDM-KL26Z as data reference for my own sensor (based on a FXOS8700 as well). I do need the vector magnitude of the acceleration. To get the data I use the Freescale Vibration Monitoring software. In that software there is a |XYZ| Modulus which according to the help file is giving the "magnitude of the combined channels". Why does the software still show negative values? Does anyone know based on which formula the values are computed?

[JackITB]

Hi Roberto,

Formula is the classical "square root of the sum of square": |XYZ| = sqrt(X²+Y²+Z²)

Could you provide plot or screenshot of your observation.
Make sure you don't have the DC notch or similar HPF engaged.

Regards,     Jacques.

[roberto70]

Hi Jacques,

thanks for your prompt answer.

I've been expecting that formula but was wondering about the negative values I got. I do have the DC notch engaged (See attached picture). I assumed that this check box enables the HPF within the FXOS (as I do in my application). I understand that the Freescale Vibration Monitoring applies the DC correction after calculating the magnitude and is not using the build in HPF of the chip. Correct?

Thanks again,

Regards

Roberto

[JackITB · Accepted solution]

Hi Roberto,

Thanks for the picture, it's cristal clear and your understanding is correct: we run the DC notch in the MCU, just before the FFT and not at the sensor level. Rational is to remove the DC content of the waveform vs time (1g for |XYZ|) that would otherwise appear as a significant power in the 0Hz bin of the FFT plot, hidding the smaller frequency content nearby.

Regards,     Jacques.

[roberto70]

Jacques, thank you very much for your help.

Roberto