MATLAB: Why can’t I fit a formula to a data set? How to fix this

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could someone please help me fix this problem.

I'm trying to fit my function to the data set. The data is from a tensile test. I'm trying to determine the constants k and n and i don't have any initial parameters for k and n. However when i run it, i get an error. I've tried this before with a different data set and it worked fine.

the fit doesn't have to be super accurate, just one that follows the plot with a smooth curve, so that i can predict the strain at a certain point of stress. (stress-strain curve)

Best Answer

Have you tried Curve Fitting Tool? Here is a 3rd order polynomial for your given data set:

y=1.267e+04*x^3-1.351e+04*x^2+4252*x+0.1923

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MATLAB: Weighted least square fit

Yes. There is LSCOV.

MATLAB: What exactly is a StartPoint for a Curve Fitting Tool, its significance and how it works

curve fitting works by trying to find the parameters of your model, lets say you are trying to fit a gaussian to a single peak curve:

then your model has three parameters a for amplitude, mu for average and sigma for standard deviation

least squares fitting works by iteratively guessing those parameters then improving the guess each iteration according to how well it fits the data, that is how large the margin from the data is in each point of the curve.

start point is the initial guess, it can help by narrowing down to something close to the data.

So now lets go back to the gaussian model,

fit(x, y, 'gauss1', 'StartPoint', [1, 2, 0.5]);

Now, i'm telling matlab to start from an amplitude of 1, average 2 and standard deviation 0.5

If you don't use start point matlab will use something random

Best Answer

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MATLAB: Weighted least square fit

MATLAB: What exactly is a StartPoint for a Curve Fitting Tool, its significance and how it works

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