An error is reported when plotting the Nyquist curve with matlab. Please help me to see the problem and how to modify it?

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Shuai Wang 2020-1-15

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https://ww2.mathworks.cn/matlabcentral/answers/500338-an-error-is-reported-when-plotting-the-nyquist-curve-with-matlab-please-help-me-to-see-the-problem

评论： Walter Roberson 2020-1-15

The code show as below:

clc;clear;
%The relevant parameters of series part
H=6.4;
Dp=1140;
K=5;
Dq=150;
wn=2*pi*50;
T1=1.59e-5;
T2=1.59e-6;
P=10e3;
Q=0;
Id=2/3*10e3/220;
Iq=0;
Ud=220;
Uq=0;
Ug=220;
L=4.5e-3;
w0=2*pi*50;
X=w0*L;
syms y;
y=vpasolve(220*220*y*cos(y)==90*pi);
delta=0.0058419042066544734259150800059222;
Em=Ug*cos(delta);
%The relevant transfer function of series part
Fpq=[tf(1,[H Dp 0]),tf(0,1);tf(0,1),tf(1,[K Dq])];
Fpq_u=[tf(1.5*Id,1),tf(1.5*Iq,1);tf(-1.5*Iq,1),tf(1.5*Id,1)];
Fpq_i=[tf(1.5*Ud,1),tf(1.5*Uq,1);tf(1.5*Uq,1),tf(-1.5*Ud,1)];
K=[tf(1,[T1*T2 T1+T2 1]),tf(0,1);tf(0,1),tf(1,[T1*T2 T1+T2 1])];
F1=[tf(-Em*sin(delta),1),tf(cos(delta),1);tf(Em*cos(delta),1),tf(sin(delta),1)];
F2=[tf(0,1),tf(Em/Ug*sin(delta),1);tf(0,1),tf(-Em/Ug*cos(delta),1)];
FL=[tf([L 0],[L*L 0 L*L*w0*w0]),tf(L*w0,[L*L 0 L*L*w0*w0]);tf(-L*w0,[L*L 0 L*L*w0*w0]),tf([L 0],[L*L 0 L*L*w0*w0])];
E=[tf(1,1),tf(0,1);tf(0,1),tf(1,1)];
A= FL * F1 * Fpq * Fpq_i * K + E;
B= FL * (F2 - F1 * Fpq * Fpq_u * K - E);
C= inv(B);
%The output impedance of series part 
Zout = - C * A;
%The output impedance of shunt part
Lg=2.3e-3;
Zg = [tf([Lg 0],1),tf(-w0*Lg,1);tf(w0*Lg,1),tf([Lg 0],1)];
L=Zg/Zout;
[V,D]=eig(L);
D
nyquist(D)

The error message is as follows:

Error using DynamicSystem/eig

Too many output arguments.Too many output arguments.

Error in test2 (line 53)

[V,D]=eig(L);

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Answer 1

Walter Roberson 2020-1-15

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Your Zg is a transfer function. Your Zout is a symbolic array. You cannot do operations between a transfer function and a symbolic array.

The 's' of the transfer function is not the same as the symbolic variable s in your Zout array.

You can use sym2tf from https://www.mathworks.com/matlabcentral/fileexchange/10816-ovivero-mimotoolbox to convert Zout into a tf .

[V,D]=eig(L);

That will not work on the transfer function L: the eig() function that applies to transfer functions does not support multiple outputs. You can use

P = eig(L); %poles

but the result will not be a transfer function and so you will not be able to nyquist(P)

On the other hand you can directly

nyquist(L)

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Shuai Wang 2020-1-15

But I need to sovle the eigenvalue of the transfer function matric L, and draw a nyquist diagram for the eigenvalue function.

Walter Roberson 2020-1-15

Well, the eig() function that is available for transfer functions is really just a call to pole()

https://www.mathworks.com/help/control/ref/pole.html

If you need something else, then you are probably going to need to convert the transfer function into a symbolic system and try to find the eigenvaues . After that you would have the task of figuring out how to create a nyquist diagram out of the eigenvalues.

It seems odd to me that you cannot simply ask to nyquist(L) which will draw the nyquist diagram directly.

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