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frep2tf Scilab Group Scilab Function frep2tf
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frep2tf - transfer function realization from frequency response
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[h [,err]]=frep2tf(frq,repf,dg [,dom,tols,weight])
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frq : vector of frequencies in Hz.
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repf : vector of frequency response
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dg : degree of linear system
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dom : time domain ('c' or 'd' or dt)
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tols : a vector of size 3 giving the relative and absolute
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tolerance and the maximum number of iterations (default values
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are rtol=1.e-2; atol=1.e-4, N=10).
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weight : vector of weights on frequencies
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h : SISO transfer function
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err : error (for example if dom='c' sum(abs(h(2i*pi*frq) -
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Frequency response to transfer function conversion. The order of h is a
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priori given in dg which must be provided. The following linear system is
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solved in the least square sense.
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weight(k)*(n( phi_k) - d(phi_k)*rep_k)=0, k=1,..,n
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where phi_k= 2*%i*%pi*frq when dom='c' and phi_k=exp(2*%i*%pi*dom*frq if
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not. If the weight vector is not given a default penalization is used
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(when dom='c'). A stable and minimum phase system can be obtained by
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using function factors.
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h=syslin('c',(s-1)/(s^3+5*s+20))
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frq=0:0.05:3;repf=repfreq(h,frq);
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clean(frep2tf(frq,repf,3))
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frq=logspace(-3,2,200);
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[frq,rep]=repfreq(Sys,frq); //Frequency response of Sys
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[Sys2,err]=frep2tf(frq,rep,10);Sys2=clean(Sys2)//Sys2 obtained from freq. resp of Sys
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[frq,rep2]=repfreq(Sys2,frq); //Frequency response of Sys2
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xbasc();bode(frq,[rep;rep2]) //Responses of Sys and Sys2
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[sort(trzeros(Sys)),sort(roots(Sys2('num')))] //zeros
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[sort(spec(Sys('A'))),sort(roots(Sys2('den')))] //poles
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dom=1/1000; // Sampling time
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h=syslin(dom,(z^2+0.5)/(z^3+0.1*z^2-0.5*z+0.08))
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frq=(0:0.01:0.5)/dom;repf=repfreq(h,frq);
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[Sys2,err]=frep2tf(frq,repf,3,dom);
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[frq,rep2]=repfreq(Sys2,frq); //Frequency response of Sys2
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xbasc();plot2d1("onn",frq',abs([repf;rep2])');
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imrep2ss, arl2, time_id, armax, frfit