How to create a FRF graph of the real and imaginary parts of a signal

Hello,

I ran a hammer test on a structure that gave me acceleration and force data. I know MATLAB has an inbuilt function, modalfrf, that outputs a FRF in terms of magnitude and phase, but I'm trying to get FRF plots in terms of the real and imaginary parts of the data. Does anyone know how I would go about getting this?

As a bonus question, I know from the modalfrf plot that I can pick out natural frequencies. Is there a way for me to get matlab to calculate the damping ratio of the specific natural frequencies I want?

Thank you so much for your time!

Answer 1

[Star Strider]

It appears that is its default behaviour if you request outputs —

load modaldata

figure
subplot(2,1,1)
plot(thammer,Xhammer(:))
ylabel('Force (N)')
subplot(2,1,2)
plot(thammer,Yhammer(:))
ylabel('Displacement (m)')
xlabel('Time (s)')

winlen = size(Xhammer,1);

figure
modalfrf(Xhammer(:),Yhammer(:),fs,winlen,'Sensor','dis')

[frf,f] = modalfrf(Xhammer(:),Yhammer(:),fs,winlen,'Sensor','dis');

figure
semilogy(f, real(frf), 'DisplayName','Real')
hold on 
plot(f, imag(frf), 'DisplayName','Imag')
plot(f, abs(frf), 'DisplayName','Magnitude')
hold off
grid
legend('Location','best')

Warning: Negative data ignored

[Otto Randolph]

Awesome, thank you so much! I had to change the graphs to not be log scale since there were negative data points but it worked!

[Star Strider]

Thank you! As always, my pleasure!

I completely understand. I debated on that, and finally decided on a logarithmic axis because I used an example from the documentation, and the default modalfrf plot it produces uses a logarithmic magnitude scale. (I used the logarithmic y-axis here to demonstrate that it produced the same plot.)