Fitting a custom function to my data

For each value of t, I have a measured datapoint T_measured. Now, I have a custom function ils(x1,x2,a,b,t) with two variables (x1,x2), two constant coefficients (a,b) and time array (t), that models T_model

function T_model = ils(x1,x2,a,b,t)
alpha = x1/x2; Fo = alpha*t/a^2;
x1 = 1./(4*Fo); g1 = 0.25*expint(x1)/pi;
T_model = b*g1/(x1);
end

One solution, that I´ve tried (lsqcurvefit) did not work (I get an error:

"Error using lsqncommon ... Objective function is returning undefined values at initial point. lsqcurvefit cannot continue. Error in lsqcurvefit (line 295) lsqncommon(funfcn,xCurrent,lb,ub,options,defaultopt,optimgetFlag,caller,..."

Here is my attempt:

% Coefficients 
a = 75/1000; 
b = 98.2;
% Initital guess 
x1 = 2.0; 
x2 = 0.5*10^6;
x0 = [a b]; 
F = @(x,t) ils(x0(1),x0(2),a,b,t); 
[x0,resnorm,~,exitflag,output] = lsqcurvefit(F,x0,t,T_measured);
T_model = ils(x0(1),x0(2),rb,t,q); 

The goal would be to get the best fitting values for the parameters x1 and x2. The result should then look something like this:

Fig. 1: red: my measured data T_measured(t), green: the fitted function T_model(t)

Thank you very much for your help! :)

Answer 1

[Image Analyst]

T_model is not being returned from your function. You need to assign T_model to something before you exit the ils() function.

And check if it's nan before you return it and set it to zero or something if that is the case.

[Michael]

Sorry, I changed some variable names for this question to make things easier to read and forgot to change one variabl name. I edited it. The problem remains.

[Michael]

In fact, I have one value within the array that is NaN.

I added this line of code at the end of my function, and now everything works fine! Thanks a lot! :)

T_model(isnan(T_model))=0;