Frf,Modal analysis, Naturel frequencies

Hi;

I have experiment data for time depending force and displacement values.

I need to plot Displacement (dB) - frequency grapf. what should be my x-axis?

B1=xlsread('frf.xlsx',1);
a = B1(1:50001,5:5);
ffa = fft(a);
F = B1(1:50001,6:6);
fff = fft(F);
H=ffa./fff;
aH =abs(H); 
t_step=0.0002;
ts=1/t_step;
        
semilogy(aH)

[Mathieu NOE]

hello

your method for converting from time to frequency is not the best

consider using tfestimate

[Txy,F] = tfestimate(X,Y,WINDOW,NOVERLAP,NFFT,Fs) returns the transfer

function estimate as a function of physical frequency. Fs is the

sampling frequency specified in hertz. If Fs is empty, it defaults to

1 Hz.

you will get the frequency vector F that is your x axis vector

Txy is a complex (vector) that you can convert to magnitude and phase like a Bode plot

Answer 1

[Gloria]

I appreciate your help, thank you!

It didn't work. I guess I have a mistake.

Txy values seem as a Nan+Nani

It is also good to know Fs can be 1 Hz. I also have data with 1 Hz

B1=xlsread('frf.xlsx',1);
Y = B1(1:50001,5:5);
X = B1(1:50001,6:6);


Fs=2500;

[Txy,F] = tfestimate(X,Y,50001,[],[],Fs) ;

% to plot the magnitude plot you need to take the magnitude
semilogy(F,abs(Txy))

[Mathieu NOE]

hello

can you share the file 'frf.xlsx' ?

[Gloria]

Thanks again, I am not familiar with frf. If you can help me for both sample , I appreciate ;

first column is force , second column acceleration and Fs is 1 Hz. I wonder this experiment sample because I always obtain frequency range from time_step^-1/2. If Fs is 1 Hz, how can I see acceleration for range of frequency?

For the second sample; 6. column is displacement and 7. column is force time step=0.0002; Length of data is=50001