fsolve with symbolic variables

Hi everyone,

I am solving a nonlinear system of 4 equations with fsolve. out of the four results, i want to use one as input for another system of equation.

i can do it manually in 2 scripts, but i would like to combine then

I tried to define a global symbolic variable

syms y_pivot;
global y_pivot;

then i calculatd y_pivot, and substituted it

subs(y_pivot,x(4));

then i called the function, inside the function i defined the global symbolic variable as well

    function F = constraintequation_vollhub(x)

    syms y_pivot;
    global y_pivot;
    %  

       F = [cos(x(1))* 18.144 - sin(x(2))*(19.667+10.5) - 14.070 + x(3);
           sin(x(1))* 18.144 - cos(x(2))*(19.667+10.5) + x(4) - 15;
           cos(x(1))* 18.144 + cos(pi+x(1)-3.2217382) * 19.789 + x(3) -  y_pivot * sin(0.296705972) - 14.070
           sin(x(1))* 18.144 + sin(pi+x(1)-3.2217382) * 19.789 - 15  -  y_pivot * cos(0.296705972) + x(4) + 36.769];

then i tried to solve it with

x0_voll = [0; 0; 0; 0];  % Make a starting guess at the solution
[x_voll,fval] = fsolve(@constraintequation_vollhub,x0_voll) % Call solver

i get the error

??? Undefined function or method 'isfinite' for input arguments of type 'sym'.

Error in ==> trustnleqn at 27
if any(~isfinite(Fvec))

Error in ==> fsolve at 366
    [x,FVAL,JACOB,EXITFLAG,OUTPUT,msgData]=...

when i replace the symbolic variable manually, everthing works fine, so i guess it is only the symbolic variable in the function

i tried to find solutions in the forum, but i am not that used to matlab and i couldnt find anything suiting, so i am sorry if the topic is already around..

Thank you in advance

Answer 1

[Star Strider]

You need to use either the Symbolic Toolbox solve function or matlabFuncton to convert your expression into a form fsolve can use.