June 18th, 2009, 07:39 PM  #1 
Newbie Joined: Jun 2009 Posts: 1 Thanks: 0  FFT on sample data
Hi I am trying to perform Fast Fourier Transforms on sample data coming out of a load cell. (Load cells are weighing at 10items a second) I have got the data in excel (256 values = 3 items). I am using Matlab to do this. Code below: x=Load_Cell_Data(:,2) nfft= 2^(nextpow2(x)); % Use next highest power of 2 greater than or equal to length(x) to calculate FFT fftx = fft(x,nfft); % Take fft, padding with zeros so that length(fftx) is equal to nfft % Calculate the numberof unique points (half) NumUniquePts = ceil((nfft+1)/2); %ceil rounds up to nearest integer % FFT is symmetric, throw away second half fftx = fftx(1:NumUniquePts); % Take the magnitude of fft of x mx = abs(fftx) % Evenly spaced frequency vector with NumUniquePts points. f = (0:NumUniquePts1) % Generate the plot, title and labels. plot(f,mx); title('FFT'); xlabel('Frequency (Hz)'); ylabel('Amplitude'); Comments added to show train of thoughts. I get a graph which doesn't look right. What am I missing in the code? I am not getting distinct frequency components which one would expect to see with such application. Thanks for your help in advance. Ramyy 
June 29th, 2009, 04:49 PM  #2 
Newbie Joined: Jun 2009 Posts: 5 Thanks: 0  Re: FFT on sample data
Looks wrong how? Shape or amplitude? I notice that you're not dividing the number of samples. fftx = fft(x,nfft)/nfft; Also, maybe try windowing? fftx = fft(x .* hann(nfft),nfft)/nfft; 

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