function [z,nf,a,com]=VTB7_4(f,TF,b)%[z,nf,a,com]=VTB7_4(f,TF) Curve fit to SDOF FRF.% f is the frequency vector in Hz. It does not have to %    start at 0 Hz.% TF is the complex transfer function.% z and nf are the damping ratio and natural frequency (Hz)% a is the product of the residues of the coordinates the % transfer function is between. (For example, in the example % below, a1 times a2 is returned. See equation 7.42)% If com is returned as a real number, then it is the % compliance between the two coordinates. % Only one peak may exist in the segment of the FRF passed to % VTB7_4. No zeros may exist withing this segment. Otherwise, % curve fitting becomes unreliable.%% EXAMPLE:% M=eye(2);% K=[2 -1;-1 2];% C=.01*K;% [Freq,Recep,Mobil,Inert]=vtb7_5(M,C,K,1,2,linspace(0,.5,1024));% figure(1)% n=250;% f2=Freq((1:n)+450);% R2=Recep((1:n)+450);% R2=R2+.1*randn(n,1)+.1*randn(n,1)*i;% Poorly Simulated Noise% [z,nf,a,com]=vtb7_4(f2,R2)%% Note that by changing the parts of Freq and Recep used% We can curve fit to other modes.% Copyright Joseph C. Slater, 10/8/99% Updated 11/8/99 to improve robustnessglobal XoFif nargin==2	[y,in]=max(abs(TF));	lf=length(f);	f(in);	a0=abs(TF(1))*(2*pi*f(in))^2;	z=.0005;	a0=-sign(imag(TF(in)))*abs(TF(in))*2*z*(2*pi*f(in))^2;TF(in);	x=[a0;z;2*pi*f(in);0;0;0];%sign(real(TF(1)))*	%x2=x;%	%cost=vtb7_4(x,f,TF)	if in-3<1|in+2>length(f)		disp('The peak response must be near the middle of the data')		disp('Please center your peak and try again')		break	end			x		x=fmins('vtb7_4',x,[],[],f(in-3:in+2),TF(in-3:in+2));    x	cferr(x,f,TF);	%x	%cost=vtb7_4(x,f,TF)	x=fmins('vtb7_4',x,[],[],f,TF);	x	%cost=vtb7_4(x,f,TF)	x=fmins('vtb7_4',x,[],[],f,TF);	x	%cost=vtb7_4(x,f,TF)	x=fmins('vtb7_4',x,[],[],f,TF);	x	%cost=vtb7_4(x,f,TF)	x=fmins('vtb7_4',x,[],[],f,TF);	x	%cost=vtb7_4(x,f,TF)	%x=x2	z=x(2);om=x(3);	%z,om	if f(1)==0		k=x(4)+x(1)/om^2;	else		k=sqrt(-1);	end	com=k;	nf=om/2/pi;%*2*pi;	a=x(1);	%plot(f,20*log10(abs(XoF)),'g',f,20*log10(abs(TF)))	%grid on	%zoom on	if 1==1	  Fmin=min(f);	  Fmax=max(f);	  phase=unwrap(angle(TF))*180/pi;	  phase2=unwrap(angle(XoF))*180/pi;size(phase);		%size(XoF)	  subplot(2,1,1)	  plot(f,20*log10(abs(XoF)),f,20*log10(abs(TF)))	  as=axis;	  zoom on	  legend('Identified FRF','Experimental FRF',0)	  axis([Fmin Fmax as(3) as(4)])	  xlabel('Frequency (Hz)')	  ylabel('Mag (dB)')	  grid on	%  Fmin,Fmax,min(mag),max(mag)	%  axis([Fmin Fmax minmag maxmag])	  while phase2(in)>50		  phase2=phase2-360;	  end	  phased=phase2(in)-phase(in);	  phase=phase+round(phased/360)*360;	  phmin_max=[floor(min(min([phase;phase2]))/45)*45 ceil(max(max([phase;phase2]))/45)*45];	  subplot(2,1,2)	  plot(f,phase2,f,phase)	  xlabel('Frequency (Hz)')	  ylabel('Phase (deg)')	  legend('Identified FRF','Experimental FRF',0)		  grid on	  axis([Fmin Fmax  phmin_max(1) phmin_max(2)])	  gridmin_max=round(phmin_max/90)*90;	  set(gca,'YTick',gridmin_max(1):22.5:gridmin_max(2))	  zoom on	  endelse%	global  XoF%f,TF,b    x=f;	f=TF;	TF=b;	w2=f*2*pi;	lx=length(x);	x(3)=abs(x(3));	x(2)=abs(x(2));	XoF=x(lx-2)+x(lx-1)*i*w2-x(lx)*w2.^2;%	for j=1:(lx/3)-1	XoF=XoF+x(1)./(-w2.^2+2*x(2)*w2*i*x(3)+x(3)^2);%	end		vtb74=norm(XoF-TF);	z=vtb74;end%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%function cferr=cferr(x,f,TF)global  XoFw2=f*2*pi;lx=length(x);XoF=x(lx-2)+x(lx-1)*i*w2-x(lx)*w2.^2;for j=1:(lx/3)-1XoF=XoF+x(3*j-2)./(-w2.^2+2*x(3*j-1)*w2*i*x(3*j)+x(3*j)^2);endcferr=norm(XoF-TF);%pause