Obtaining mode shapes from velocity FRF
Obtaining mode shapes from velocity FRF
(OP)
Hello,
Recently i have been tasked to do the modal analysis of the data collected from a test. I was able to obtain its natural frequencies, damping coefficients(viscous and structural) and the modal constant using a SDOF descomposition of the FRF. I am supposed to obtain 4 mode shapes ( since i focus on 4 natural frequencies ) but i have not seen it explained well in any of the references i have looked up to. I know it has something to do with the imaginary part of the FRF but i can't get the complex mode shapes to progress any further in my work.
Help with be appreciated, Thank you.
Recently i have been tasked to do the modal analysis of the data collected from a test. I was able to obtain its natural frequencies, damping coefficients(viscous and structural) and the modal constant using a SDOF descomposition of the FRF. I am supposed to obtain 4 mode shapes ( since i focus on 4 natural frequencies ) but i have not seen it explained well in any of the references i have looked up to. I know it has something to do with the imaginary part of the FRF but i can't get the complex mode shapes to progress any further in my work.
Help with be appreciated, Thank you.
RE: Obtaining mode shapes from velocity FRF
Otherwise refer to this
http://ziaeirad.iut.ac.ir/sites/ziaeirad.iut.ac.ir...
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Obtaining mode shapes from velocity FRF
RE: Obtaining mode shapes from velocity FRF
RE: Obtaining mode shapes from velocity FRF
Thank you in advance.
RE: Obtaining mode shapes from velocity FRF
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Obtaining mode shapes from velocity FRF
RE: Obtaining mode shapes from velocity FRF
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Obtaining mode shapes from velocity FRF
RE: Obtaining mode shapes from velocity FRF
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Obtaining mode shapes from velocity FRF
RE: Obtaining mode shapes from velocity FRF
RE: Obtaining mode shapes from velocity FRF
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Obtaining mode shapes from velocity FRF
O is the origin, C is the centre of the fit, and F may be the resonant frequency
For a modal amplitude you take the radius, abs(CF), and the phase is the angle between CF and the x axis
BUT
If you were to use a peak pick method your amplitude would be abs(OF) and phase would be the angle between OF and the x axis
So long as you use the same method for the driving point and all the others, the factor of 2(roughly) in amplitude will all come out in the wash when you come to resynthesise your FRFs.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Obtaining mode shapes from velocity FRF
Could you link me the file of that circle fit Greg? I went with a least square approach to reconstruct the FRF (with success, getting all 4 peaks i selected very well, just missing the complex mode shapes) and i would like to learn about that other method for modal analysis, since i am fairly new to this process.
RE: Obtaining mode shapes from velocity FRF
close all
clear
load H.mat
load f.mat
ufl=length(f);
freq=f;
df=max(freq)/length(freq);
for i=1:21
subplot(4,1,2)
plot(freq(1:ufl),phase(H0(1:ufl,i)))
grid on
hold on
subplot(2,1,2)
plot(freq(1:ufl),log(abs(H0(1:ufl,i))))
hold on
grid on
end
lof=floor(1540/df);
hif=floor(1570/df);
resfreqguess=floor(1550/df)-lof;
for chan=1:21
figure
XY=[real(H0(lof:hif,chan)) imag(H0(lof:hif,chan))];
plot(XY(:,1),XY(:,2),'o')
hold on
Par=CircleFitByPratt(XY);
a=Par(1);
b=Par(2);
R=Par(3);
%Par = [a b R] is the fitting circle:
for i=1:361
fitxy(i,1)=cos(i/360*2*pi)*R+a;
fitxy(i,2)=sin(i/360*2*pi)*R+b;
end
plot(fitxy(:,1),fitxy(:,2),'r')
plot([a,0,XY(resfreqguess,1),a],[b,0,XY(resfreqguess,2),b],'k')
grid on
text(0,0,'O')
text(a,b,'C')
text(XY(resfreqguess,1),XY(resfreqguess,2),'F')
end
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?