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From: Michael Harte on 3 Jun 2010 05:46
Hello,
So I want to use FFT & IFFT to solve a simple SDOF system (so that i can move on and use a freq domain method to solve more complex dynamic models). I've solved a simple SDOF system with a sin wave forcing function in time domain using ode45 and then was looking to get the same results using freq domain approach using a FFT and IFFT method,
Im aware of the method, transform the input time history
p(t)-FFT-P(w)
Calculate the response of the SDOF system in the frequency domain using the transfer (frequency response) function
U(w)=H(w)P(w)
Use the inverse FFT to obtain the response of the SDOF system in the time domain
U(w)-IFFT-u(t)

And my code is as follow

%frequency domain

function freqsdof ()

m = 2;
k = 10;
c = 0.26;

wn = sqrt(k/m); %nat freq
I = c/(2*m*wn);

t = linspace(1,100,10000);
dt = t(1,2) - t(1,1);
% dt = 1;
f = sin(t);

ff = (f/m) ; %load divided by mass

Qi = fft((ff)) ;

N = length(f);
fs = 1/(dt);
Q_hertz = ( 0:(N-1) )*fs/N; %Freq in Herz
Q_w = 2*pi*Q_hertz ; %Cir freq in rads per second


Qf = Qi(1: ceil(N/2));

wQ = Q_w(1: ceil(N/2));


m = length (Qf);


for n = 1:m


X1(n,1) = ( ( (wn^2 - wQ(1,n)^2) + (i*2*I*wn*wQ(1,n)) )^-1 *(Qf(1,n)) );

%Conjugates of above line
X2(n,1) = ( ( (wn^2 - wQ(1,n)^2) - (i*2*I*wn*wQ(1,n)))^-1 * conj(Qf(1,n)));

end



x (1:m,1) = X1(:,1);
x ( ((m+1):(m*2)),1 ) = X2(:,1);


x1 = ifft( x );

tspan = linspace(1,100,(2*m));

figure
plot ( tspan, real(x1) );
title('Freq domain')


end


However im not getting the right results,
I think the problem is where i have to get the complex conjugate of the values.

Can anyone please help!
Thanks in advance!
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