Nonlinear Curve Fitting with string formula

[Nav21]

Dear Forum Members,

I am working on a nonlinear curve fitting vi using formula string. The fitting formula is quite simple (Y0+A*(X-X0)^1.5). I can provide Y0 and X0 values for initial parameters quite good (from graph) but initial value of A, is a rough guess. The fit turns out well, but it doesn't iterate the initial parameter "A" at all. Rather it iterates X0 and Y0 parameters and returns the same value of A, as mentioned in initial parameters.

The vi and a set of data is attached. The best fit parameters I found from Origin Software are A=11310, X0=0 & Y0= -4e-6. With this as initial parameters one can get perfect fit. But if one changes the initial parameter "A" value to 14000, it still returns good fit, but retuns the same value for A in best fit parameters.

I am wondering if I am doing something wrong. Can someone have quick look at my vi??

[Daniel_Fulop · Accepted solution]

Dear Nav21,

I looked at your VI. If you set the tolerance in the termination cluster to a lover value (1e-14 for example), the resulting 'A' will be different, closer to your optimal value.

Daniel

[Nav21]

Dear Daniel,

Thanks..it did help.

Best,