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 Nonlinear curve fitting with multiple equations

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T O P I C    R E V I E W
cweb Posted - 04/07/2011 : 11:45:35 PM
Origin Ver. and Service Release (Select Help-->About Origin): Origin 6.0
Operating System: Windows XP

I have been trying to fit a curve with multiple intermediate variables using the following code (in Equations mode):

y=(IP/x)*deltaip+(2*D/x)*deltad+(I/x)*deltaf;
D=(1+4*Kd*(x-I)-((1+8*Kd*(x-I))^0.5))/(8*Kd);
I=x-IP-2*D;
IP=(1+2*Kip*(x-2*D)-((1+4*Kip*(x-2*D))^0.5))/(2*Kip)

Where Kip,Kd, deltaip, deltad and deltaf are parameters and D, I, IP are the intermediate variables.

Whenever I try to fit using this function I initially get an error saying too few data points or the data step is too big. If I click the iteration button again then I get an error claiming my fitting function is not generating values and to check that my dataset has values (All I have done at this stage is click the iteration button again!).

Are there any ways around this? I have a feeling it has to do with defining initial values for D, I and IP but I'm not sure how to go about this? Thanks!!
2   L A T E S T    R E P L I E S    (Newest First)
cweb Posted - 04/18/2011 : 10:28:02 PM
Thanks for that. I couldn't get it to work on Origin 6.0 but 8.5 worked like a charm.
easwar Posted - 04/14/2011 : 3:43:47 PM
Hi,

The order of your statements is wrong, the statement with y= should be the last, so try it this way:


D=(1+4*Kd*(x-I)-((1+8*Kd*(x-I))^0.5))/(8*Kd);
IP=(1+2*Kip*(x-2*D)-((1+4*Kip*(x-2*D))^0.5))/(2*Kip);
I=x-IP-2*D;
y=(IP/x)*deltaip+(2*D/x)*deltad+(I/x)*deltaf;


Also try the 8.5 demo.

We now have a Fitting Function Builder tool, where you can put in some initial values and test the function to see if it is generating values, while you are creating it. So that makes it easy to check if function is okay, BEFORE you try fitting with it.

See: http://originlab.com/www/helponline/Origin/en/Category/The_Fitting_Function_Builder.html

Easwar
OriginLab

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