differention

[Mike26]

I have a data acquisition of position sensor. When in LABIEW, this data is in number form. I wish to take the derivative with respect to time of the position. How to you take the slope of two adjacent points. I know the time step but I dont know how to subtract two adjacent points to get the slope

[Dr._Imad]

pass the data array to a while loop
the use the index array function
DY=Data_index(i+1)-Data_index(i)

divide Dy by the time step

there has been a recent thread on differentiation
have a look at this thread
http://forums.ni.com/ni/board/message?board.id=170&message.id=109985

[Mike26]

I tried that but when I tested it for an sine input i just got back a sine waveform of different amplitude. I'm not sure if I am using the index array functions or the while loop correctly. Distinguishing between the index (i) and index (i+1) i used the shifting (?) property of the while loop to get the previous value for (i).

[Dr._Imad]

Sure, the derivitive of a sine wave, is also sine wave (actually a cosine). so you would still get a sinosoid, but with a phase diffrenece of 90 degrees

[Mike26]

I know I should get a sinusoid but I'm getting a phase shift of 180 degrees

[SciWare]

Hi Mike

Try the Savitzky Golay filter available from my website.

www.sciware.com.au

It allows you to smooth and differentiate your data.

Regards

Kurt

[Dr._Imad]

Attached is a vi that does differentiation. Hopeit helps

[Mike26]

I have LabView 6i so i cannot view this vi.

[Mike26]

For the Savitzky Golay filter, the x and y values are arrays and my input values are numbers. How do I modify the Savitzky Golay filter for my system?