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 Finding good initial values for fitting		  New Topic  Reply to Topic
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MurmelMaster

Germany
2 Posts
Posted - 03/02/2009 :  04:25:20 AM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
Im working with Origin 6.0.

I have some troubles finding the right initial values to do a fitting with a script. I get the error message: "Error: 28036. Try setting parameter 't1' as fixed". When i simply used the fitting ExpDec1 and iterate i got this error quite frequently. So i tried to initialize the parameter with the more robust simplex method. But even that doesnt always work. The function doesnt seem to be able to find the optimum then and produces huge values for the parameter.

Here is a section of my code.


nlsf.init();
nlsf.func$=ExpDec1;
nlsf.constr$="";
nlsf.v1=0;
nlsf.P1=0.63;
nlsf.v2=1;
nlsf.v3=1;
nlsf.fitdata$=exp_kWerte;
nlsf.funcx$=exp_Conc;
nlsf.dataBegin=1;
nlsf.dataEnd=N+2;
nlsf.simplex(100000);
nlsf.iterate(2000);
nlsf.xMode=2;
nlsf.funccol$=exp_FITFINAL;
nlsf.makecurve(func);


I have no idea how i can come around this. If i copy the data to be fitted into a seperate work sheet and use the plot-> scatter function and then do a manual fitting with the analyze -> fit exponential decay -> first order everything works just fine. Any help would be appreciated.

Thx,
Mathias

MurmelMaster

Germany
2 Posts
Posted - 07/09/2009 :  10:46:36 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Doesn't anybody has an idea how to intitialize the fitting parameter to a given dataset with Origin 6.0? I worked around this problem but i'm still lacking an full automatic solution for this problem. Any help is appreciated.

thx!
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easwar

USA
1965 Posts
Posted - 07/11/2009 :  2:35:11 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi,

In your code you are initializing only P1, is there some part of the code you did not paste here?

Also move the param assignment etc after all other properties are initialized, such as:


nlsf.init();
nlsf.func$=ExpDec1;
nlsf.constr$="";
nlsf.fitdata$=exp_kWerte;
nlsf.funcx$=exp_Conc;
nlsf.dataBegin=1;
nlsf.dataEnd=N+2;
nlsf.v1=0;
nlsf.P1=0.63;
nlsf.v2=1;
nlsf.v3=1;
nlsf.simplex(100000);
// Call iterate multiple times, equivalent to clicking
// fit button multiple times
nlsf.iterate(500);
nlsf.iterate(500);
nlsf.iterate(500);
nlsf.iterate(500);
nlsf.xMode=2;
nlsf.funccol$=exp_FITFINAL;
nlsf.makecurve(func);


You could then download the version 8.0 demo from our website and try with same data in this new version, we have made improvements to convergence etc since version 6.0

You can view the following topic for examples on fitting from script in 8.0:
http://wiki.originlab.com/~originla/wiki/index.php?title=LabTalk:Curve_Fitting

Easwar
OriginLab
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