## 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

Greg Locock

## RE: Obtaining mode shapes from velocity FRF

## RE: Obtaining mode shapes from velocity FRF

## RE: Obtaining mode shapes from velocity FRF

Greg Locock

## 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.

Greg Locock

## 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

Greg Locock