Why doesn't my non-linear curve fit finish?

I am working with an experiment that gives me X- and Y- data that I need to fit to a theoretical model (somewhat involved). I use to do this in matlab using the "lsqcurvefit", which works fine,
but for varous reasons I need to do this in LabView instead.
So, first thing I did was to make a vi which is my theoretical model. I throw in my X-data and get the Y data out of it.
This vi works fine of it's own, but now I want to do my fitting by calling the model-vi from the Nonlinear curve fit.vi. I enter the same initial parameters, termination conditions, upper- and lower boundaries as in the matlab code, but no matter what, after some 100 function calls, the curve fit interrupts and tells me:
"Error -20041 occurred at NI_Gmath.lvlib:Nonlinear Curve Fit LM.vi
Possible reason(s):
Analysis: The system of equations cannot be solved because the input matrix is singular."
I have tried to locate the problem at varoious levels but I get confused when I start to look into the nonlinear curve fit vi, I just lose myself in tons of block-diagrams.. Can anyone see what my problem is? I'd sure appreciate it! I added the necessary files. Main vi: langevin fit. vi, then open the data.txt.
Thanks
Attachments:
langevin fit.zip ‏94 KB

Your model function does not produce anything resembling your data. As a first step, graph the data using your initial guesses, here's how it would look.
Here' the data and your initial guess on the same graph. You need to find initial guesses so the output resembles your data.
My best guess is that your model is faulty. It is very hard to debug, because it is a mess: deeply stacked sequence structures and unneeded local variables. Also the model parameter constants should be provided via the "data variant" input instead of a global variable. You have a weird mix of datatypes. Why is half of the stuff EXT instead of DBL for no reason?
I would also highly advise NOT to color your own VIs light yellow. Don't make them look like system tools!
I am busy today, but maybe I can have a stab at it tonight. Good luck!
Message Edited by altenbach on 07-06-2009 08:59 AM
LabVIEW Champion . Do more with less code and in less time .
Attachments:
langevinFit.png ‏21 KB

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