Problem with non-linear fit

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R7 DR 2015-10-9

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评论： Star Strider 2015-10-9

R7 DR Matlab.xlsx

在 MATLAB Online 中打开

Hi I am trying to fit the data and find the constant values. my code is...

[T]=xlsread('Matlab.xlsx','sheet2','C5:C2685')
[alpha]=xlsread('Matlab.xlsx','sheet2','I5:I2685')%time
[dalpha_dt]=xlsread('Matlab.xlsx','sheet2','J5:J2685')
    F=ones(size(alpha));
    F=(1-alpha) 
    coeff0=[2.17E+22, 5] % initial guess
    coeff=nlinfit([T,F],dalpha_dt,@model,coeff0);
    A=coeff(:,1)
    n=coeff(:,2)

Function:

function dhat=model(coef,a)
% par_fit = [a b]
  a=coef(1);
  b=coef(2);
  T=a(:,1);
  F=a(:,2);
  % predicted model
  dhat=a.*exp(208000./(8.3147*(T+273))).*(F^b);
  return

I need to find the new coefficient values, but it is showing following error..Please tell me how to resolve this.

Error using nlinfit (line 205)
Error evaluating model function 'model'.
  Error in fitting (line 16)
coeff=nlinfit([T,F],dalpha_dt,@model,coeff0);
Caused by:
    Attempted to access a(:,2); index out of bounds because numel(a)=1.

Thanks

回答（2 个）

Star Strider 2015-10-9

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/247714-problem-with-non-linear-fit#answer_195353

You defined ‘a’ as a scalar (appropriately) so it is by definition a (1x1) ‘array’ (taking liberties with the concept here). It doesn’t have a second dimension.

How do you want to define ‘T’ and ‘F’?

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R7 DR 2015-10-9

Thank for the information.

I will try to find the solution using genetic algorithms. If I need any help I will come back to you.

Thanks for your time. Have a nice weekend.

Star Strider 2015-10-9

My pleasure.

You, too!

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R7 DR 2015-10-9

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/247714-problem-with-non-linear-fit#answer_195374

在 MATLAB Online 中打开

The idea is that, I have data generated from the experiment. I can also calculate same kind of data from mathematical equation which is depended on two coefficients. Now I need to find these coefficients by fitting the mathematical data with experimental data.

The equations is...dalpha_dt= A*exp(-208000/(8.3417*(T+273)*(F^n)

                          T=loaded from the excel.
                          F=loaded from excel.
                          A & n are the coefficients I want to find.

I made the same calculation using excel with an initial guess of 'A & n' and the curves look like the following image.