is the FFT right?

I am trying to analyze data that I recorded from the Force Sensitive Resistors (FSRs) coded on Arduino through data streamer on excel. I used four FSRs, so there are 4 columns of data over a set of time (1200 rows, about 15 seconds). I seperated time from the FSR data so that I can graph the FFT of the FSR data against the time.

I have made a different question which has been answered, but I was wondering if this method works as well.

Here is the code:

X = importdata('botdata.csv');
t = importdata('botdatatime.csv');
subplot(2,1,1);
plot(t,X);

xft=fft(X,[],1);
xabs = abs(xft);
subplot(2,1,2);
plot(t,xabs);

This is what was displayed.

Now I don't know if it makes sense or not.

What is the meaning of the y axis?

Is this correct?

Answer 1

[Star Strider]

‘What is the meaning of the y axis?’

It is necessary to normalise the fft result by the length of the vector it uses as an argument. (This is nin the fft documentation.) Once that is done, the y-axis units make sense.

‘Is this correct?’

Almost. I made a few changes to eliminate the constant offset from the fft result, normalise it, and plot a one-sided Fourier transform. The multiplication-by-2 corrects the one-sided Fourier transform amplitude, since the energy at each frequency is divided evenly betweek the ‘positive’ and ‘negativeÄ fft frequencies.

X = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/606440/botdata.csv');
t = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/606445/botdatatime.csv');
subplot(2,1,1);
plot(t,X);
grid
xlabel('Time')
ylabel('Amplitude')

L = numel(X);
Ts = mean(diff(t))

Ts = 0.0128

% Tsd = std(mean(diff(t)))
Fs = 1/Ts;
Fn = Fs/2

Fn = 39.1012

xft=fft(X-mean(X),[],1)/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
xabs = abs(xft);
subplot(2,1,2);
plot(Fv,mag2db(xabs(Iv)*2))
grid
ylim([-75 50])
xlabel('Freqency [units]')
ylabel('Power [dB]')

.