I am in need of some help trying to fit a set of data that requires the use of nonlinear curve fitting. I have attached a txt file containing the data that needs to be fitted. The first row contains the x-axis values while the second row contains the y-axis values. The model for this set of data can be described by a single parameter, A, for the first five data points. The remainder of the data points can be described by the model A+A1*exp((0.4-x)/A2), where A, A1, and A2 are all parameter. As you can see the two models share the parameter A. I have determined some initial values for each of the parameters: A=176.32, A1=2133.4133, and A2=1.4936. I have spent the last couple of days trying to figure out how to use the Lev-Mar nonlinear curve fit vi but have had little success. I have been doing LabVIEW programing for many years now and I have never felt as lost as I do now trying to understand how the Lev-Mar nonlinear curve fit vi works. Any help you can provide with my data fitting would be greatly appreciated. Thanks. -Alonzo

I think the problem is that the first 5 points are not part of the data sent to Lev-Mar. Because of this the fitting process fits the remaining data very well, but the constant term is able to freely move. I modified the model function to output A for the first 5 terms, and the complete function for the rest. The offset found is now more reasonable. Also, your quote your model to be A+A1*exp((0.4-x)/A2), but what is implemented in the model VI seems to be A+A1*exp((x-0.4)/A2). -Jim

LabVIEW

Nonlinear curve fit help needed

Solved!

Your post has really nothing to do with the existing thread (except for the fitting), so please start a new thread next time. Posting in a thread that has already been marked as answered will prevent you from marking an answer once we solve your problem. Not good! 😞

There is no way to see what's going on unless you show us some code. Are you still running LabVIEW 6 on the 2GHz computer (GHz does not mean much. What is the exact processor name?) or did you also upgrade the LabVIEW version?

LabVIEW 6 is ancient and the nonlinear fitting VIs have received a major overhaul in LabVIEW 8.0.

What is the model function? How many datapoints? Do you use analytical or numerical partial derivatives? How many fitting parameters? Who wrote the program? How do you measure the timing?

If the time/iteration is independent of the computer power, The first suspicion is a small misguided wait statement inside the model VI.

Please show us your VIs. Also include some typical data.

LabVIEW Champion.

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Nonlinear curve fit help needed

Re: Nonlinear curve fit help needed