error message: DOUBLE cannot convert the input expression into a double array. If the input expression contains a symbolic variable, use VPA.

Hello. I need your help. I am trying to use the optimazation function 'fminsearch' to find 8 unkown values. The function M file which I made is,

function y_1=opti_real(x)

global Ra Rb La Lb Ga Gb Ca Cb Zc Zc_A Gamma Z Y wm R L G C Frequency

wm=2*pi*Frequency;

syms Ra Rb La Lb Ga Gb Ca Cb real

wm=2*pi*Frequency;

R=[Ra, Rb, Rb; Rb, Ra, Rb; Rb, Rb, Ra;]; L=[La, Lb, Lb; Lb, La, Lb; Lb, Lb, La;]; G=[Ga, Gb, Gb; Gb, Ga, Gb; Gb, Gb, Ga;]; C=[Ca, Cb, Cb; Cb, Ca, Cb; Cb, Cb, Ca;];

Z=R+j*wm(1,1)*L; Y=G+j*wm(1,1)*C;

Gamma=sqrtm(Z*Y); Zc=Gamma*inv(Y);

Ra=x(1); Rb=x(2); La=x(3); Lb=x(4); Ga=x(5); Gb=x(6); Ca=x(7); Cb=x(8);

y_1=abs(real(Zc_A)-real(Zc));

where Ra, Rb, La, Lb, Ga, Gb, Ca, Cb are unkown parameters. Frquency and Zc_A are calculated values from main code. Zc_A is a 3 by 3 matrix that contains complex numbers. The main code to find the values of Ra, Rb, La, Lb, Ga, Gb, Ca, Cb is,

x=[R_open(1,1), 0, L_open(1,1), 0, G_open(1,1), 0, C_open(1,1), 0]; bb=fminsearch(@opti_real, x);

where, R_open(1,1)... are calculated real values in main code. After I run the main code, the error message is displayed as follows:

DOUBLE cannot convert the input expression into a double array. If the input expression contains a symbolic variable, use VPA.

can you explain the problem and fix my code please?

Answer 1

[Alan Weiss]

It would be much easier to read your code if you would use the {} Code button to format your code.

fminsearch expects a function that takes a single real initial vector and gives a single real scalar output. I think that you are making a mistake by using symbolic variables, but I cannot read your code to say exactly what is going on.

I suggest that you ensure that your function accepts a real input vector, say your x0, and returns a single real scalar output. I further suggest that you do not use any symbolic variables, as I am pretty sure that you can use MATLAB commands for all your calculations.

Alan Weiss

MATLAB mathematical toolbox documentation

[YoungGon Ryu]

Thank you for your answer. I rewrite by code by using {} Code button. What do I fix in the code? Can you advise more please..? The function code is,

function y_1=opti_real(x)

global Ra Rb La Lb Ga Gb Ca Cb Zc Zc_A Gamma Z Y wm R L G C Frequency

wm=2*pi*Frequency;

syms Ra Rb La Lb Ga Gb Ca Cb real

wm=2*pi*Frequency;

R=[Ra, Rb, Rb; Rb, Ra, Rb; Rb, Rb, Ra;];
L=[La, Lb, Lb; Lb, La, Lb; Lb, Lb, La;];
G=[Ga, Gb, Gb; Gb, Ga, Gb; Gb, Gb, Ga;];
C=[Ca, Cb, Cb; Cb, Ca, Cb; Cb, Cb, Ca;];

Z=R+j*wm(1,1)*L;
Y=G+j*wm(1,1)*C;

Gamma=sqrtm(Z*Y);
Zc=Gamma*inv(Y);

Ra=x(1);
Rb=x(2);
La=x(3);
Lb=x(4);
Ga=x(5);
Gb=x(6);
Ca=x(7);
Cb=x(8);

y_1=abs(real(Zc_A)-real(Zc));

And the main code is,

x=[R_open(1,1), 0, L_open(1,1), 0, G_open(1,1), 0, C_open(1,1), 0];
bb=fminsearch(@opti_real, x);

Please check my code again and advise me. I really appreciate your help Thank you.

[Alan Weiss]

I believe that things will work much better for you if you follow my suggestion and remove all symbolic variables from your code.

Also, your code makes no sense to me, because the output y_1 does not depend on the input variable x, it only depends on some external global variables. So you need to rethink your entire code.

Alan Weiss

MATLAB mathematical toolbox documentation

[YoungGon Ryu]

Thank you for your answer and advise. After revmoving all symbolic variables, I could get the result. :)