errore too many output

Quando eseguo lo script fino al plot delle due prime figure funziona, per la parte che segue riguardante la funzione fft mi da errore: " Error using fft Too many output arguments.", come risolvo?

close all
format longE
clear 
clc
%definition of the file path
data_folder_path= 'l2_CFRP/';
file='l2_CFRP.txt';
file_path= strcat(data_folder_path, file);
%load data
% column 1: sampling period [mV]
% column 2: time between samples 
% column 3: acceleration z-axis [mV/m/s^2]
% column 4: displacement z-axis [mV/ms/^2]
accelerometer = readmatrix(file_path);
%time vector
N=size(accelerometer,1);
samples= accelerometer (:,1);
time= accelerometer(:,2)*10^4;
acc_z = accelerometer(:,3);
displacement = accelerometer (:,4);
dt= round(time(2)-time(1),2);
%sampling frequency
for n= 1:738
    fs(n)=n/dt;
end
fs(n)=n/dt;
disp('sampling frequency:');
disp(fs(n));
figure
plot( time, acc_z)
xlabel('time')
ylabel('acc_z [m/s^2]')
xlim ([0,samples(end)])
grid
figure
plot( time, displacement)
xlabel('time')
ylabel('displacement')
xlim ([0,samples(end)])
grid
%FFT
[norm_sp,f_vector]=fft(acc_z,fs(n));
figure 
plot(f_vector,abs(acc_z), LineWidt=1.5)
xlim([0,f_vector(end)])
xlabel('Frequency [Hz]')
ylabel('amplitude [m/s^]')
grid
figure
semilogy(f_vector, abs(acc_z))
xlim([0,f_vector(end)])
xlabel('Frequency [Hz]')
ylabel('amplitude [m/s^]')
grid
function [norm_sp, freq_vec]=fft_n(acc_z,fs)
dim=size(l2_CFRP);
if dim(2)>dim(1)
    'l2_CFRP=l2_CFRP';
end
N=length(l2_CFRP.txt);
df=fs/N; 
end

Answer 1

[Les Beckham]

You are trying to do this

[norm_sp,f_vector]=fft(acc_z,fs(n));

If you look at the documentation for fft you will see that there is only one output.

Y = fft(X)

Y = fft(X,n)

Y = fft(X,n,dim)

[Dyuman Joshi]

The question for OP is, why are you using that? What are you expecting or what do you want to do?

[Silvia]

Io ho i dati nel dominio del tempo e ho la necessità di trasformarli ed effettuarne la lettura nel dominio delle frequenze. in modo da avere i picchi massimi delle mie oscillazioni in relazione alla frequenza e non più al tempo.

[Les Beckham]

Comment translation:

I have data in the time domain and I need to transform them and read them in the frequency domain. in order to have the maximum peaks of my oscillations in relation to the frequency and no longer to the time.

Response:

Yes, and that is a good reason to use the fft function. You need to use it in accordance with its documentation, however. It will not work if you try to get it to return information that it was not designed to -provide. Read the documentation.

I suggest that, in addition to reading the documentation, you read this answer, by an excellent contributor to Matlab Answers, to a previous question: https://www.mathworks.com/matlabcentral/answers/460152-fft-for-the-given-data-to-find-the-dominant-frequency#answer_373524

You should be able to adapt that to your problem.

Response translation:

Sì, e questo è un buon motivo per usare la funzione fft. Tuttavia, è necessario utilizzarlo in conformità con la sua documentazione. Non funzionerà se provi a fargli restituire informazioni che non è stato progettato per fornire. Leggi la documentazione. Suggerisco che, oltre a leggere la documentazione, leggi questa risposta, di un eccellente collaboratore di Matlab Answers, a una domanda precedente:

https://www.mathworks.com/matlabcentral/answers/460152-fft-for-the-given-data-to-find-the-dominant-frequency#answer_373524

Dovresti essere in grado di adattarlo al tuo problema.

Buona fortuna.