Why am I getting a "shark finn"-like signal?

[matteo_ianna]

Hello! My collegue and I are having hard time to understand what is happening. Basically we have a tapping device that work with Arduino and a pneumatic pump. The pump allows the tapping of two surfaces which perform triboelectric effect. We are connecting to those a logic analyzer (Seleae), to detect the signal and eventually quantifying the voltage produced. When we do it, we occour in this "shark finn" shape of the spikes, which pattern repeats all the time after n taps (see highlighted area in the picture). Do you have any idea why we are getting this pattern and no constant spikes?

Thank you so much in advance, really!

 

[berkeman]

Welcome to PF.

Ca you zoom in more on the signal? It may just be that you are getting some sampling aliasing in the displayed waveform.

Also, do you have a schematic and links to the datasheets for your components that you can share with us? Thanks.

 

[Rive]

logic analyzer

What does your (desktop) oscilloscope says?
What's your grounding setup?

Ps.: for me this looks like noise from regular USB communication.

 

[Tom.G]

Is that trace of
1) one device
1) both of them electrically combined
2) both of the mechanically (acoustically) combined

Try putting a drop of water on one of the triboelectric devices to change its resonant frequency. That may well give clues to the cause.

Post the screen shot here so we can take some more wilda*s guesses.