Curve Fitting

Hi
I have a set of measurement data that i wish to fit to a complex curve.
After the fitting i need to find the constants from the curve and use them for further calculations.
equation for my curve is
x= -a* integral(p^n dt) + c.,
where x,p are known data sets at time t. 'a' and 'n' and 'c' are constants. after the fitting i have to find best values for a and n.
somebody please explain how i can accomplish this.
Waiting For the inner calling

Inorder to use the Levenberg-Marquart method, it is neccessary to specify the formulae string. but how can i specify a formula containing integral function as a labview string.
Warm regards
Deepu Jacob
DFGray wrote:
You have two main options:
Convert the integral to an actual equation, then use a non-linear fitting routine, such as the Levenberg-Marquart, to do your fit. You will need approximate values for your parameters before you start.
Use one of LabVIEW's numeric integrators to do your integral, and use a non-linear fitting routine to find the parameters. This will be slower.
Before you go any further, I would strongly recommend you read the chapters in Numerical Recipes, 10 and 15, about fitting data to parameters. Nonlinear curve fitting can be very finicky, so you need to know what you are doing. LabVIEW has all the algorithms (if you have a student, full, or pro version), so you won't have to code much more than your actual function.
Good luck! Let us know if you run into problems.
Waiting For the inner calling

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How can I apply curve fitting to simple BER measurement data?

I am collecting simple BER data from a communications receiver for modulations such as PSK, CPM, etc. The data looks good but has the usual measurement noise, which I would like to spiffy-up for customer presentations.
I have tried virtually all of the vi's in the LV Fitting palette, with little or no success. One of the NI AE's provided me with a GP Fit approach, but it has problems when you introduce error. I have attached a vi which demonstrates this, and also shows an attempt on my part to use the Least squares vi, with various functional estimates of the erfc vi. The vi also includes sample data.
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Q(x) = 1/2 erfc (x/sqrt(2)), where erfc (x) is the LabView vi block.
Thanks,
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Hi Eric,
Well, BER is just data measured at certain input levels to a receiver. For digital communications systems, we typically use Eb/No to specify the input signal strength, and measure the BER at that level. This is analagous to signal-to-noise ratio (SNR) measurements in the analog domain.
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Nonlinear Curve Fit LM Formula String in Labview 8

hi, guys. I write a vi which use Nonlinear Curve Fit LM Formula String. When I run it ,the program says "Syntax error of parser." and I don't know what's wrong with it, so help me,Please. By the way ,you can fill the arrays as you wish.
帖子被trinight在02-11-2006 07:51 PM时编辑过了
Attachments:
curve fitting.vi ‏16 KB

You MUST wire initial estimates for the parameters. Attached modification works just fine.
LabVIEW Champion . Do more with less code and in less time .
Attachments:
curveFittingMOD.vi ‏17 KB

How to use the parameter "data" in the vi of Nonlinear Curve Fit

I used "Nonlinear Curve Fit" to do state estimation. If I make the number of unknows a constant number, it is fine to code the fitting model for f(x,a). But if I want to code the number of unknows as a variable, how can I do this?
Maybe "data" should be used here. But I just could not find any examples talking about how to use "data" here.
Thanks.

Hey hilary,
Have you looked at the LabVIEW help for the Nonlinear Curve Fit VI? Which version of LabVIEW are you using? In mine, the VI is polymorphic, and you can declare the formula to be a formula string. Then it provides pins to enter the input parameters programmatically. Perhaps you could post a screenshot of how you are currently using the VI, and we could help more.
Brian B
Field Sales Engineer
Tennessee/Southern Kentucky
National Instruments

How to create curve fitted line chart?

Line chart is formed with straight lines, but how to create curve fitted line chart?

Jasper wrote a blog on curve fitting charts

Curve Fit of ODE, should I use Lev-Mar or Constrained Nonlinear Optimization?

I wanna do a curve fitting of differential equations, following is the equations:
dx/dt = u*x-a*x
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and u = um*s/(km+s+s^2/ki)
I've tried "nonlinear curve fit( Lev-Mar)", but I met difficulties when writing the f(x,a) of Lev-Mar.
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in the "labview\examples\math\curvefit.llb" was similar to the fitting I tried to do, but there was only one parameter
need to estimate in this example,but what I need is to estimate 3 parameters.
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How can I achieve this? is it possible to do this fit only using Labview VIs or need Matlab Script Node?
Any help will be appreciated.

I forgot to mention that in the equations above, a,b,c,d,e,f is constant,
and the data of x,s,p corresponding with the data of time is already known,
and which parameters I wanna identify is um,km,and ki
帖子被myafu在12-22-2006 01:33 AM时编辑过了

Gaussian Peak Fit and Nonlinear Curve Fit on small data differ from Origin Peak fit

Hi all,
I am developing a program in which I have to fit Gaussian curve on only 4 or 5 data points. When I am using Gaussian Peak Fit or Nonlinear Curve fit, it linearly connects all the points while other fitting software like Origin is fitting Gaussian curve on the same set of data I have attached two images One is from Labview with Gaussian Peak Fit and NonLinear Fit and other is from Origin.
Data is
X                       Y
799.09857      257
912.25256      641
1026.00366   1516
1137.92871   1118
1253.43713   329
Labview user
Solved!
Go to Solution.
Attachments:
Origin_Fit.PNG ‏21 KB
Labview_fit.PNG ‏15 KB

That looks like a plot of the input data.
When I run it with your data I get a Gaussian curve which looks much like your Origin plot.
The Plot 2 curve is from 128 points equally spaced along X and Y calculated from the Gaussian formula in the LV help using the output parameters from the fit. Plot 1 is the Best Gaussian Fit output from the fit VI. It only has five points.
I used default values for everything except X and Y.
Lynn

Constrained nonlinear curve fit can properly handle the function like ln(1+b(x-xc)/a) ?

Hi all,
I met some problems about using constrained nonlinear curve fitting vi. It seems to me that this vi can't properly deal with the function like ln(1+b(x-xc)/a), a,b, and xc are the parameters, and b is in the range of 0 and -1. The reason I said that is as I used the other nonlinear fitting function in the other software, like Originlab, the fitting function can work properly.
The error message from LV occurs as 1+b(x-xc)/a is less than zero.
However, due to a,b are both adjustable parameters, how could this situation happen ?
Now I attach the vi files. The attachment includes main vi, mathmatical formula, and an input XY txt file.
I appreciate any help or suggestion from you!!
Here is the error message
Joy
Solved!
Go to Solution.
Attachments:
LN-fitting.vi ‏21 KB
LN-fittingmodel.vi ‏18 KB
Book2.txt ‏1 KB

Thanks for reply. whitenoiz
Actually I have more than 100 XY data set to test my vi. Some of the data set can be fit flawlessly. And the best fit parameters obtained from my vi are exactly the same as those from other analytical software, like OriginLab. In this case, I know my vi is valid. And in this scenario, I also found 1+b(x-xc)/a is always larger than zero with the best fit parameters (a and b). However, for certain XY data set, the error message will pop up if I use LV built-in vi, but I still can obtain the best fit parameters with OriginLab. And then I realized that error message always pops up as 1+b(x-xc)/a is less than zero, which it means to me that as LV built-in vi handles this type mathematical function, this built-in vi will encounter some difficulties.
I will step through my code, and also focus on the Matrix Left DivisionMV.vi, try yo find where the problem is.
Best regards,
Joy

Constrained Nonlinear Curve Fit VI: Termination Revisited (LabVIEW 8.5)

Hello:
In the Constrained Nonlinear Curve Fit VI, the Help for the VI states that if the number of iterations exceeds max iterations specified in the termination control, the fitting
process terminates. Would this not indicate that if the above condition is met, the number of iterations recorded (an output of this VI) should be equal to the max iterations specified (since the VI is exits at this point). I believe I have seen situations where this condition is met, yet the number of iterations recorded is greater than the max iterations I have specified in the termination control. Has anyone else seen this?
Thanks,
Don

Don,
Just wanted to clarify the termination criteria for the nonlinear curve fitting routines. There are two criteria used to terminate the fitting process:
"max iteration" and "tolerance". The main while loop terminates when either of these conditions is met, so:
IF( (current_iteration > max_iteration) OR (current_tolerance <= tolerance) ) THEN (terminate loop)
"current iteration" is just the current loop counter(starts at 1 for the first iteration). This term is mainly a guard against having the fitting process run too long.
tolerance is computed as the relative change between the current and previous weighted least squares values.
Let wls = SUM_OVER_ALL_X( weight(x)*(f(x,a)-data(x))^2 )
current_tolerance = ABS(current_wls - previous_wls)/(ABS(current_wls) + machine_epsilon)
where ABS indicates absolute value. Adding machine_epsilon to the denominator is just a guard against division by zero.
For the bound nonlinear curve fit VIs, if the "method" input is chosen to be LAR or bi-square, then wls is defined as:
wls = SUM_OVER_ALL_X( weight(x)*reweight(x)*(f(x,a)-data(x))^2 )
where reweight(x) is a term reducing the weight of high-leverage data points.
For both the constrained and unconstrained fitting routines, if the weight input is unwired then all points are considered to have a weight of 1.
Christian has nicely covered the difference between the number of iterations and the number of function calls. If your
When the fitting process terminates the best result obtained so far is returned. As you noted, this may be a good result, although the algorithm may not have completely converged yet.
-Jim

Nonlinear curve fit

Hi everybody,
I am trying to fit a custom nonlinear curve to my data. I want to fit a curve to the data twice. After the first fit, I want to use the coefficient a (6.57962) in my second curve fit as my coefficient b. When I try to use this and create a custom equation by string concetanete, everything seems fine but I do not get what I expect to get. To double check I used curve fitting express VI and I added 6.57962 as a coefficient in my equation and at the end a turns to be 6.752. However, this does not happen in my second curve fit. Does anybody have any idea what might be wrong? I cannot find my mistake, could you please help me?
Thank you very much
Attachments:
Read.vi ‏129 KB
read.png ‏66 KB

The errors you get provide some clues, although they are not very explicit.
The -23001: Syntax error of parser is usually a sign that your string describing the function is ill-formed or that there is a parameter mismatch. You have both.
The string array constant feeding into the second Lev-Mar VI has two elements, the second one being an empty string. Even though the string is empty, the parser expects two elements in the Initial Parameters array. But since your equation has only one parameter (a), the proper solution is to remove the blank element from the constant array.
The other problem is that the first Initial Parameters array is empty. Maybe you always enter two values but I did not know what to put in.
The second fit is somewhat sensitive to the initial Parameter value. Negative values do not work. 1 returns 6.753... 2 through 6 and 8 (integer values only) return 6.246... 7 returns the same result as 1. Values 10 and above return the value entere with error -20041 (singular matrix) appearing at >=12.
The initial b parameter in the first fit does not matter over a fairly wide range.
I also noticed that you seem to have a severe allergy to straight wires. Wires running all over the diagram and particularly behind other objects makes it very hard to read your code. Dataflow along the wires should generally be from left to right as much as possible. The code works the same regardless of the number of bends in the wires but straighter wires are much easier on the programmer.
Lynn

Nonlinear Curve Fit and using static data

I am trying to fit data to a 2D function using the Nonlinear Curve Fit VI in Labview 8.2.1, but I would like to also include 2 parameters that I don't want to be optimized. I cannot directly use constants since every time I run the fitting I need to change the 2 parameter values. I tried to modify the example by Altenbach according to my own application in the link below.
http://forums.ni.com/ni/board/message?board.id=170&message.id=230831
I think the problem is that I don't completely understand how to use the static VI reference with the Nonlinear Curve Fit VI? It looks like there is a static data control that I could use to add additional parameters, but I am not sure? I can get Altenbach's code to work if I change the function to something different, and even if I change the number of inital fitted parameters. I just can't figure out how I can add parameters using a control that will not be optimized. I can attach an example, but it is very similar to Altenbach's code except the function is different, and I am trying to add additional parameters that are not optimized. Thanks for any help.
Kevin Baker

Thanks Trey. I am sorry I didn't make it clear what I was talking about before. I am attaching an example that will hopefully clarify what I would like to do. When I try to pass the static reference to the Nonlinear Fit VI it says gives me a broken wire and says "function conflict". I am assuming the problem is that I inserted another control to the VI when I wasn't supposed to. I just don't know of any other way to pass data into the VI, and still have the Nonlinear Fit VI still work? What is the "data" control in the Nonlinear Fit VI used for anyway? It seems that that it was want I need, but I don't know how to use it? Thanks again for any help.
Attachments:
Fit equation.vi ‏15 KB
general lorentzian.vi ‏26 KB

For "Nonlinear Curve Fit.vi", what does -20068 mean and how can I avoid it?

Hey guys,
I am trying to fit an oval in a set of 2D data points, which represents a partial oval.
So I am trying to see if "Nonlinear curve fit.vi" would work in the situation.
However, sometimes that math VI would give me an error -20068 and would return incorrect fitting parameter.
I can't figure out what condition would return that error so can you tell me
what the cause is and how I can avoid that error?
You can run the attached LabVIEW 2013 VI and see the error.
Attachments:
ellipse fit data.zip ‏41 KB

jcyth wrote:
The model is a result of solving the ellipse equation for y,
(x^2)/(a^2) + (y^2)/(b^2) = 1
So I don't really see a simple way to make the negative values less likely...
but maybe I can use "Constrained Nonlinear Curve Fit" where I can bound the parameters then?
This is slowly drifting away from a LabVIEW problem but any help is appreciated!
The solution is to re-parameterize the problem and solve for something much less pathological
You can directly fit for the above formula by providing x&y for all points to the model and optimize the four ellipse parameters until you get an array of all ones.
I quicky coded this in the attached example and it seem to be quite stable, even with lots of noise.
See if this works for you.
LabVIEW Champion . Do more with less code and in less time .
Attachments:
Ellipse FittingMODCA.zip ‏40 KB
FitEllipse.png ‏26 KB

Why doesn't my non-linear curve fit finish?

I am working with an experiment that gives me X- and Y- data that I need to fit to a theoretical model (somewhat involved). I use to do this in matlab using the "lsqcurvefit", which works fine,
but for varous reasons I need to do this in LabView instead.
So, first thing I did was to make a vi which is my theoretical model. I throw in my X-data and get the Y data out of it.
This vi works fine of it's own, but now I want to do my fitting by calling the model-vi from the Nonlinear curve fit.vi. I enter the same initial parameters, termination conditions, upper- and lower boundaries as in the matlab code, but no matter what, after some 100 function calls, the curve fit interrupts and tells me:
"Error -20041 occurred at NI_Gmath.lvlib:Nonlinear Curve Fit LM.vi
Possible reason(s):
Analysis: The system of equations cannot be solved because the input matrix is singular."
I have tried to locate the problem at varoious levels but I get confused when I start to look into the nonlinear curve fit vi, I just lose myself in tons of block-diagrams.. Can anyone see what my problem is? I'd sure appreciate it! I added the necessary files. Main vi: langevin fit. vi, then open the data.txt.
Thanks
Attachments:
langevin fit.zip ‏94 KB

Your model function does not produce anything resembling your data. As a first step, graph the data using your initial guesses, here's how it would look.
Here' the data and your initial guess on the same graph. You need to find initial guesses so the output resembles your data.
My best guess is that your model is faulty. It is very hard to debug, because it is a mess: deeply stacked sequence structures and unneeded local variables. Also the model parameter constants should be provided via the "data variant" input instead of a global variable. You have a weird mix of datatypes. Why is half of the stuff EXT instead of DBL for no reason?
I would also highly advise NOT to color your own VIs light yellow. Don't make them look like system tools!
I am busy today, but maybe I can have a stab at it tonight. Good luck!
Message Edited by altenbach on 07-06-2009 08:59 AM
LabVIEW Champion . Do more with less code and in less time .
Attachments:
langevinFit.png ‏21 KB

Want to do curve fitting !!

Hi,
I'm new to Java but want to do curve fitting codes using Java 2D or Java 3D but don't have the time to read a whole bunch of tutorials / books.
I've managed tto create a basic class within my objects, then I get rather tired. wonder any one got some hints for me to finish the job.
I hope to create an applet that accept coordinates in a large text box,
as ::
x1, y1 <--|
x2, y2 <--|
x3, y3 <--|
x4, y4 <--|
x5, y5 <--|
x6, y6 <--|
then the applet draws a fitting curve that goes through all the points
given as input.

Am i right, that you write the code for calculating the code, and you now want to know how to get in on the screen?
if so, the easiest way would be, not to use Java3D at all.
Just build up a Windows (e.g. JFrame), attach a JPanel for example and use the paint(Graphics g) method, that comes with every class derived from Component or JComponent.
With the given Graphics-Object you can put lines, single pixels and more onto the component.
Just have a look at here, what posibilies you have:
http://java.sun.com/j2se/1.4.2/docs/api/java/awt/Graphics.html
I hope i could help you.
Sash

Curve fitting problem: simultaneously fitting two datasets to two different model functions with shared parameters

Hello! As stated above, I have the following problem: I have 2 sets (different systems) of measurement data (Voltages measured as functions of applied frequency). I would like to make curve fits with Levenber-Marquardt-method to both datasets (the datasets have the same x-values = frequencies), fitting the 2 datasets to their own separate model functions. However, the different model functions should have same values for the shared parameters (this prevents me from doing 2 totally separate curve fittings).
I have LabVIEW version 7.1, if anybody could help me with the problem, I would appreciate it very much. Thank you!

Would you like to try this:
http://forums.ni.com/t5/LabVIEW/levenberg-Marquardt/td-p/668782
http://zone.ni.com/reference/en-XX/help/371361G-01/gmath/nonlinear_curve_fit/
http://digital.ni.com/public.nsf/allkb/BACF6FDF1B40993686256CC300657BA4
With LV 7.1, I don't know if we have a such feature with that version. Please check out for the latest LV versions.
BR,
Make Nguyen
NI Finland Technical Support

Curve Fitting

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