[Python] Compute FFT after reading accelerometer data

I'd like to detect abnormal vibrations on a motor I have. I'm using the following devices:

• Raspberry PI 5
• Adafruit accelerometer ISM330DHCX

I've successfully managed to read the sensor at a specific rate (2000Hz) and I'm using it's absolute value (although it's not changing that much):

def read_sensor():
global data
target_duration = 1 / SAMPLE_RATE

# Connect to sensor
...

while True:
loop_start_time = time.perf_counter()

x, y, z = accelerometer.acceleration
acceleration = np.sqrt(x**2 + y**2 + z**2)
data = np.roll(data, -1)
data[-1] = acceleration

# Try to get exactly SAMPLE_RATE per second
loop_duration = time.perf_counter() - loop_start_time
sleep_time = max(0, target_duration - loop_duration)
end_time = time.perf_counter() + sleep_time
while time.perf_counter() < end_time: pass


Then, I use numpy.fft to compute the FFT on the above data and save it to a CSV file:

n = len(data)
fft = abs(np.fft.fft(data * np.blackman(n)))[:n//2]
amplitudes = np.abs(fft) / n

# Identify max frequencies
max_amplitude = np.max(amplitudes)
max_index = np.argmax(amplitudes)
frequencies = np.fft.fftfreq(n, d=1/n)[:n//2]

# Generate summary
result = {
'mean': round(data.mean(), 4),
'std': round(data.std(), 4),
'max': round(data.max(), 4),
'min': round(data.min(), 4),
'range': round(data.max() - data.min(), 4),
'fft_mean': round(np.mean(fft), 4),
'fft_std': round(np.std(fft), 4),
'max_freq': round(frequencies[max_index], 4),
'max_amp': round(max_amplitude, 4),
'speed': duty_cycle
}


It produces lines like these ones:

mean,std,max,min,range,fft_mean,fft_std,max_freq,max_amp,speed
9.9587,0.1261,10.599,9.5467,1.0523,15.6502,308.3268,0.0,4.1798,47
9.9609,0.1287,10.599,9.5467,1.0523,16.2121,308.5345,0.0,4.1826,47


I've attached a propeller to the motor and used a piece of soft plastic to hit the propeller. I known the Nyquist frequency theorem saying that I can only detect frequencies below SAMPLE_RATE/2 Hz.

My questions are the following:

• Why isn't the acceleration changing that much when I add the piece of plastic ?
• Why is my FFT always showing 0.0 frequency ?
• Where did I make a mistake in my code ?

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