How can I get the FFT (Amplitude - Frequency) from Time (s) - Acceleartion(m/s^2) data?

clc;
clear all;
close all;
Time = xlsread('Data','A2:A513'); % Time (s)
Acceleration = xlsread('Data','B2:B513'); % Acceleration (m/s^2)
n = length(Acceleration); 
Ts = Time(2)-Time(1);            % Sample Time
Fs = 1/Ts;                       % Sampling Frequency
NFFT = 2^nextpow2(n);            % Next power of 2 from length of data
Y = fft(Acceleration,NFFT)/n;
f = Fs/2*linspace(0,1,NFFT/2+1);
plot(f,Y) 
xlabel('Frequency (Hz)')
ylabel('Amplitude (m)')

I am trying to get the FFT from Time Acceleration Data.

Error using plot
Vectors must be the same length.

P.S I have attached the Data.xlsx

Answer 1

[Star Strider]

In your code, ‘f’ is half the length of ‘y’ so you need to create an ‘index vector’.

Try this:

... CODE ...
Y = fft(Acceleration,NFFT)/n;
f = Fs/2*linspace(0,1,NFFT/2+1);
Iv = 1:length(f);                   % Index Vector

plot(f,abs(Y(Iv))) 
xlabel('Frequency (Hz)')
ylabel('Amplitude (m)')

That should work.

[Bob]

Thanks it works.

One more question, is it normal the straight line from 4 hz to 12?

[Star Strider]

My pleasure.

If there are essentially no significant data in your signal with frequencies above 4 Hz, there would be none in your Fourier transform. There clearly is some information above 4 Hz, but it appears to be broad-band, likely a sort of low-amplitude ‘impulse’ noise.

[Bob]

Ok, thanks for your answer.

[Star Strider]

My pleasure.