Simple way to measure rpm thru FFT

Radio, Telemetry, FTDI/USB, ESCs, Motors, etc.

Simple way to measure rpm thru FFT

Postby afernan » Sun Dec 15, 2013 5:50 pm

Using your phone or tablet and some program for sound analysis (like "FFT" = Fast Fourier Transform) you can simply put the terminal close to the propeller rotating and "heard" the noise. The software performs the FFT of the noise and shown main vibration mode frequency, that is just the one of the two blades rotating.

Image below shows the FFT of a motor rotating at 5700rpm, giving 193 Hz = 193 Hz x 60sec/min / 2 blades = 5700rmp.
In other words, for a two blade propeller, simply multiply by 30 the Hz value.

Hope you like this tip

Angel
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Re: Simple way to measure rpm thru FFT

Postby joebar.rc » Sun Dec 15, 2013 7:14 pm

There have been a number of apps that can do that yes, I have used some of them before the telemetry era :) almost all of them are not even close to the real values in my experience. An 8 euro simple IR rpm counter used for helicopters or planes is very precise and will last a lifetime. The esc32 has it's internal rpm counter.
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