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exponential fit - revisited.

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Hi - I keep thinking I should be getting a lot better fit results on my decaying exponential for both exponential models and all calculation methods possible in the attached VI (modified from LabVIEW example, so all subVIs should be available within LabVIEW). 

 

I don't necessarily want to have to provide initial parameter bounds.

 

Any thoughts?

 

Sincerely,

 

Don

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Here is what I obtain using Excel - which is closer to what I would expect.

 

exp-excel.PNG

 

 

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Hi DonRoth, can you post an screenshot of what are our results in LabVIEW?

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Your curve is not a decaying exponential, but something entirely different. What are you trying to get out of all this?

 

Your data covers an extremely small Y range, thus you need to adjust the termination condition accordingly.

 

Also the paramters seem to differ bu huge orders of magnitude, making the fit somewhat ill conditioned.

 

 

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Accepted by topic author DonRoth

Things come out looking very different if you graph the input (white) and curve fit (red) on an XY plot.  Maybe the X-axis values aren't evenly spaced?

 

 

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Wart - You nailed it.  I forgot I had unevenly-spaced X data.

 

The exponential + offset works if I supply a very large number for estimate of a (amplitude in the model a*exp(bt) + c).

 

exp.png

 

exp+offset.png

 

 

Christian - I actually agree with you that exponential model will not provide the best fit.  However, on this project, there is literature in which this model is used for some experimental data and depending on the value of b, some conclusions seem to be able to be drawn.

 

Sincerely,

 

Don

 

 

 

 

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