Constrained nonlinear curve fit can properly handle the function like ln(1+b(x-xc)/a) ?

Hi all,
I met some problems about using constrained nonlinear curve fitting vi. It seems to me that this vi can't properly deal with the function like ln(1+b(x-xc)/a), a,b, and xc are the parameters, and b is in the range of 0 and -1. The reason I said that is as I used the other nonlinear fitting function in the other software, like Originlab, the fitting function can work properly.
The error message from LV occurs as 1+b(x-xc)/a is less than zero.
However, due to a,b are both adjustable parameters, how could this situation happen ?
Now I attach the vi files. The attachment includes main vi, mathmatical formula, and an input XY txt file.
I appreciate any help or suggestion from you!!
Here is the error message
Joy
Solved!
Go to Solution.
Attachments:
LN-fitting.vi ‏21 KB
LN-fittingmodel.vi ‏18 KB
Book2.txt ‏1 KB

Thanks for reply. whitenoiz
Actually I have more than 100 XY data set to test my vi. Some of the data set can be fit flawlessly. And the best fit parameters obtained from my vi are exactly the same as those from other analytical software, like OriginLab. In this case, I know my vi is valid. And in this scenario, I also found 1+b(x-xc)/a is always larger than zero with the best fit parameters (a and b). However, for certain XY data set, the error message will pop up if I use LV built-in vi, but I still can obtain the best fit parameters with OriginLab. And then I realized that error message always pops up as 1+b(x-xc)/a is less than zero, which it means to me that as LV built-in vi handles this type mathematical function, this built-in vi will encounter some difficulties.
I will step through my code, and also focus on the Matrix Left DivisionMV.vi, try yo find where the problem is.
Best regards,
Joy

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