topic Re: DAQ Assistant Curve fitting . in LabVIEW

DAQ Assistant Curve fitting .

Archer99 — Thu, 09 Jan 2014 04:26:56 GMT

Hello,

I am trying the monitor data from my DAQ device, the signal is oscillating with a Damped-Sinusoidal rule: x(t) = A.Exp(-µt).Sinus(wt+phi) •
So, I want to fit the Real-Data using this equation but I dont really know how.
The VI attached read real waveforms from my DAQ-Assistant, and when I stop monitoring by buttom STOP, the waveform must be fitted using the equation of x(t).
So how can i fit this data using the equation x(t) ?
The goal of fitting is to return the Amplitude A !
And another question, I would like to save the waveform in .CSV file but when I saved, I only see in the CSV file the Voltage values ( Y ) and #### in the time values ( X ).
So how can I save both of X times and Y voltage in .csv file ?

The attached VI is enough simple to help me.
Thanks for helping.

Re: DAQ Assistant Curve fitting .

johnsold — Thu, 09 Jan 2014 22:07:58 GMT

Please run the VI until you have some typical data shown on the LVDT graph, select Edit >> Make Current Values Default, then save. Post that VI. This will put one set of data into the graph where it will be available for us to see what is happening. Posting a measurement file would also work.

Note that the way you have your program set up, it will be the next acquistion after you press STOP which gets analyzed.

Usually #### simply indicates that the column width in the spreadsheet where you are viewing the data is too narrow to display the entire value. It may also represent a format difference but I would expect that to throw an error when the spreadsheet program tries to open the file. What does the file look like when you open it with a text editor?

Lynn

Re: DAQ Assistant Curve fitting .

Archer99 — Mon, 13 Jan 2014 04:45:03 GMT

Hello,

This is the new VI attached with a sample experimental curve, and cleaned too.
The second attachement is a sample of .excel file from the experimental curve (Problem of saving .excel file solved, thank you).

Before labVIEW, I was fitting curves using MATLAB and this takes me times, now, I would like to fit the curve and get the a0 ( Amplitude which I look for) just when I stop the program.

When I stop the program, it is supposed to fit the curve but I dont know how that Fit function works.
Would you like to help, Thank you again.

Re: DAQ Assistant Curve fitting .

johnsold — Tue, 14 Jan 2014 18:34:57 GMT

Your new VI does not seem to have any data in it. Unfortunately charts do not keep data as default. Try replacing the chart with a graph or post one of the data files.

Lynn

Re: DAQ Assistant Curve fitting .

Archer99 — Wed, 15 Jan 2014 12:03:11 GMT

Hello,

I tried to save data as you told me but I dont know why it is not saving, I just attached the data which I saved into an excel file, Anyway, the attached VI is a very simplified one, It read data from an lvdt and after pushing stop button, I want to fit the data with the equation mentionned before and output me the Amplitude a0 and the fitted data.
I dont know how to use the Fit Curve VI either plotting the data graph with its fitted function.
the VI is simple, would you like to help please ?
Thanks .

Re: DAQ Assistant Curve fitting .

johnsold — Wed, 15 Jan 2014 14:11:21 GMT

OK. I will take a look at it.

One thing which is immediately obvious is that the model does not match the data! You need at least one additional parameter. When I plot your data I see a significant offset from the X-axis and your model equation does not have any such offset.

Note that the mean value appears to be approaching 3.5 to 4.0, not zero.

One thing I learned from altenbach, who is probably the most expert participant on the Forums on curve fitting, is to always plot the data so you can see what is going on.

Lynn

Re: DAQ Assistant Curve fitting .

johnsold — Wed, 15 Jan 2014 14:39:51 GMT

It fits quite well with an appropriate model.

I prefer to use the Nonlinear Curve Fit.vi rather than the Express VI. It requires a bit of extra work but usually gives good results.

Lynn

Re: DAQ Assistant Curve fitting .

johnsold — Wed, 15 Jan 2014 14:45:48 GMT

Here is a version with the model equation in the Express VI fixed.

Lynn

Re: DAQ Assistant Curve fitting .

Archer99 — Thu, 16 Jan 2014 05:41:49 GMT

Hello;

Thanks a lot for your VI, It works perfectly, and helped me

But, when I run my VI with logging from the LVDT for real measurement ; there is no fitting, there no saved data either when I push the stop buttom.
I atteched the new sample .
Could you help please
Thanks again for your help I appreciate.

Re: DAQ Assistant Curve fitting .

johnsold — Thu, 16 Jan 2014 16:47:25 GMT

I do not have the DAQ Assistant, so I cannot see what it is doing. The Saved Data graph has only two points. If you are only collecting a small number of points on each iteration of the while loop, you will need to accumulate the data in some way, such as a shift register. The file you posted yesterday had 1000 points.

You may want to look at the Producer/Consumer Design Pattern. It would allow you to keep the program running and do repeated analyses and saves in one run. This will not solve your immediate problem but may be useful in the future.

Lynn

Re: DAQ Assistant Curve fitting .

Archer99 — Fri, 17 Jan 2014 03:00:39 GMT

Thank you for the answer, but how can I at least save the data which I want to fit ? how to accumulate the wanted data as you said with shift register ?
At least, if i fail for doing that I woul like at least to save the wanted data in Excel using " Export to Excel " in the Graph window, and then I just load them by LabVIEW and do the fitting as you did.

But I tried several time to load the excel file which I uploaded to fit it as you did, but It didn't worked for me.
Can you tell me how did you readed my Excel file and ploted it in the "raw" window? a simple VI can be a good help for me .

I thank you again for your support, I appreciate.

Re: DAQ Assistant Curve fitting .

Archer99 — Fri, 17 Jan 2014 06:58:27 GMT

I tried to save the data with shift register but it only save few points
I attached the program

Re: DAQ Assistant Curve fitting .

johnsold — Fri, 17 Jan 2014 14:33:56 GMT

You need to append the new data to the data already in the shift register. This is a basic LV concept. If you have not done so, I recommend that you spend a little time on the on-line tutorials for Getting Started with LabVIEW.

The attached VI shows one way to accumulate the data. This is not the most efficient way and can lead to memory allocation problems if the array gets large, but it is conceptually simple. In the True case the data is converted to an array from the Dynamic Data Type (DDT). This step is not essential but the DDT makes it so hard to know what is going on that I avoid it where possible (which is always!). The Build Array primitive is modified (context-click) to Concatenate Inputs. If this is not done, the result is a 2D array. In the False case a Wait is added so that the loop does not try to run millions of times per second while waiting for the user to push the sv button. An event structure is even better but let's not complicate things learning that now. I moved the Waveform Graph outside the case structure so it is updated every iteration.

Lynn

Re: DAQ Assistant Curve fitting .

Archer99 — Sat, 18 Jan 2014 10:35:03 GMT

Hello JohnSold

Well I begin to get the idea , i thank for your help I fixed my VI succeded to save the wanted waveform; the VI is attached with the fittin functions
Only one problem which I cant fixed is that the saved waveform is totaly saved as the inverse of the real one , I attached two picture of the real waveform and the saved one.

lvd: is the real waveform.
svd: the savd waveform.

I attahed also my VI
I ca't understand why it's saved inversly
Can you help please ? I appreciate your support and help again

THANK YOU

Re: DAQ Assistant Curve fitting .

johnsold — Sat, 18 Jan 2014 17:44:04 GMT

Ah! I messed it up for you. Sorry. By prepending the new data at Build Array, it reverses the array. The image below shows the correct way.

Other questions: It appears that you are reading 10 data points on each iteration of the loop but only appending one to the array. Why are you throwing away the other 9 points? It also appears that you need to read the DAQ Assistant and process the data 50 times per second to keep up. As the array grows larger, it will become more difficult to do that. I suggest reading larger chunks less often and looking at managing the array size appropriately.

Lynn