How do I get this Knee?

[Gaudier]

How can I get the value at the saturation point shown in the attached picture?

[Collin M]

Have you already captured data and do you have access to it in LabVIEW? In what form is the data, an array?

To determine the saturation point, I think you first need to define exactly what the saturation point is, and then translate that into LabVIEW. For example, the definition may mean the slope of a tangent line is within some range of values.

[LuI]

Gaudier,

if you differentiate your waveform Y(x), you'll get Y'(x)=(Y(i+1)-Y(i))/(x(i+1)-x(i)), which is a curve with a local maximum at the point you called saturation point.
You can detect local extrema of a curve by differentiating it and searching for a value of zero. So calculate Y''(x)=(Y'(i+1)-Y'(i))/(x(i+1)-x(i)) and search for the index i, where  Y''(i)=0.

There is, however, a pitfall with numerical differentiation: In the formulat for Y'(x), you do not get Y'(i+1)=(Y(i+1)-Y(i))/(x(i+1)-x(i)), but Y'(i+0,5)=(Y(i+1)-Y(i))/(x(i+1)-x(i)). In other words, numerical differentiation calculates the medium slope of the curve between the both base points of the calculation. In order to get the proper slope at the index i, you need to take the curve values at I-0.5 and i+0.5. This leads to Y'(i)=(Y(i+1)-Y(i-1))/(X(i-1)-X(i+1)).

Based on your data, you would find the local max of Y' at about X= 2.49 and Y=2.1

HTH   and Greetings from Germany!<br>-- <br>Uwe

[Gaudier]

This is the VI I'm using to get data. I'm using "Extract portion of signal" to get the part I need for calculations but I know this is not the right way to do it. How can I tell this progam to get X and Y values at that crossing point? For X would be the plasma potential and for Y the saturation current.The slope of the line below the knee is the inverse of the temperature of plasma in eV.

[Anonymous]

you can try to using the "Savitzky-Golay smoothing/differentiation" to
do it.
you either find the maximum of first derivative or the zero of second
derivative.
www.sciware.com.au -
http://www.sciware.com.au/_software/OrthogSmooth6.0.zip
i had try it, very useful and suitable for langmuir probe analysis.
besides that, you can easily find the EEDF/EEPF by "druyvesteyn
formula", savitzky-golay sommthing give the second derivative term that
is needed for EEDF computation.

[Gaudier]

I need the( ln I ) of mi graph It should be 3 lines . From  the line in the midle I need the slope because the inverse of the slope from that line is the plasma temperature. How do I tell the program to give me those 3 lines ,identify the midle one and calcuate the mean?

[AESulzer]

Gaudier,

I'm not sure what 3 lines you are talking about -- the image you posted has only one plot. I'm assuming by "ln" that you are talking about a natural logarithm? You can accomplish a natural logarithm in LabVIEW by using the natural logarithm function. If you could perhaps give a concise example of what you want, we might be able to better help you.

Kind Regards,

[Gaudier]

See if this attachment helps.

[AESulzer]

Gaudier,

I'm still not quite sure what you are trying to do. First of all, I do not know what you mean by "taking the mean of a line" -- are you talking about the midpoint? Secondly, it still looks like there is only one line (plot) on the graph, but if this is actually the combination of multiple lines (plots) and if you are looking for an intersection point, then I have addressed this issue before on this post. If you still need assistance, feel free to post again with questions and a more detailed explanation.

[Gaudier]

Here is a beter explanation.  Using matlab and mathcad I can process my data as shown in this attachment. Using LabVIEW I can get the reading shown. Now the question is : Can LabVIEW get this parameters(Vf,Vp,Ies) and calculate density(ne) and temperature(kTe)?

Thanks for your patience.