| T O P I C R E V I E W |
| merrick |
Posted - 01/29/2013 : 4:49:17 PM
Hi
I've started to use origin (8.0) and I need to try to fit my data with a function
y=a*(b*exp(-c*x)+d*exp(e*x))
with following constrains:
a<=1
b+d=1
The problem is that when I create this function using as base exp1dec2 I have issues parameters initialization.
Can someone help me with this issue!
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| 1 L A T E S T R E P L I E S (Newest First) |
| Hideo Fujii |
Posted - 01/29/2013 : 5:31:45 PM
Hi merrick,
I suppose you have already defined your user-defined fit function with the linear constraints.
Surely, you need to find and start fitting with a set of good initial parameters. I suggest the following:
1) Run NLFit. When it starts, go to the "Parameter" tab, and enter a set of parameter values in the "Value" fields (double-click the value cell to enter the value), which satisfies your constraints, and was guessed from your field knowledge. (If you run NLFit on a plot, you can see the initial red curve on the graph; If NLFit runs on a worksheet data, the curve can be seen in the "Fit Curve" tab in the lower panel.)
2) If the curve is not good enough, adjust any parameter value, and press "x^2" (Chi-Sq.) button. The curve will be updated accordingly.
3) Until you satisfied the closeness of the initial curve to the data points, repeat 2).
4) Once you satisfy the curve close enough to the data points, press the button of "1-iteration" as many times as you like, or "Fit until converged" to let NLFit go to the point of converged state by itself.
Hope this helps.
--Hideo Fujii
OriginLab |
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