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Hello,
I am trying to fit a curve using this function: t is the time vector, x the force measurement, d0 and nu are constants. I want to extract Er, tau1 and tau2.
function [Er,tau1,tau2] = viscoelasticcone(t,x,d0,nu)
fitfunctionconical=sprintf('Er*2*tan(35*pi/180)/(pi*(1-%d^2))*%d^2*(1+(tau1-tau2)/tau2*exp(-t/tau2))',nu,d0);
ftconical=fittype( fitfunctionconical, 'independent', 't', 'dependent', 'F');
[fitresult,R,~,CovB,MSE] = nlinfit(t,x, ftconical,[1e3 0.001 0.001]);
fit=ftconical(fitresult,t);
[Ypred,delta] = nlpredci(ftconical,t,fitresult,R,'Covar',CovB,'MSE',MSE,'SimOpt','on');
confidence{k}=delta;
lower = Ypred - delta;
upper = Ypred + delta;
end
If I use this function however, i get the following error:
Undefined function 'func2str' for input arguments of type 'fittype'.
Error in nlinfit (line 204)
m = message('stats:nlinfit:ModelFunctionError',func2str(model));
Any help ?
Michael

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Walter Roberson
Walter Roberson 2015-8-11

0 个投票

You need to call fit() on fit objects returned from fittype(), not nlinfit()

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Michael Lherbette
Michael Lherbette 2015-8-12
Hello Walter,
What about the nlinfit ?
I use fit with option equal to NonLinearLeastSquares but I have a real trouble with the fitting.
s = fitoptions('Method','NonlinearLeastSquares');
fitfunctionconical=sprintf('Er*2*tan(35*pi/180)/(pi*(1-%d))*%d^2*(1+(tau1-tau2)/tau2*exp(-t/tau2))',nu,d0);
ftconical=fittype( fitfunctionconical, 'independent', 't', 'dependent', 'F','coefficients',{'Er','tau1','tau2'},'option',s);
fitresult = fit(t',x, ftconical);
coeffvals = coeffvalues(fitresult);
I get wrong values for the coefficients and the curve is not at all fitted.
Michael Lherbette
Michael Lherbette 2015-8-12
All right,
Here my final code:
ftVisco= @(b, t) b(1)*2*tan(35*pi/180)/(pi*(1-nu))*d0^2*(1+(b(2)-b(3))/b(3).*exp(-t./b(3)));
[fitresultnlinfit, R,~,CovB,MSE,Errorinfo] = nlinfit(t',x, ftVisco, [1e3 0.001 0.001]);
fitresultnlinfit
[Ypred,delta] = nlpredci(ftVisco,t',fitresultnlinfit,R,'Covar',CovB,'MSE',MSE);
confidence=delta;
lower = Ypred - delta;
upper = Ypred + delta;
It seems to work well.
Thank you Walter for pointing me out that we can't use fittype for nlinfit.
Cheers

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