Detect slope/various points on my graph

[RSA]

Hi,

I am running LabVIEW FDS 8.0 on XP.  Please see the attached JPEG and simplified vi.  I am trying to do a few things:  1.) Detect at what time I switched my current on:  The signal from my hall probe takes a big jump from 5V.  Call this T1.  2.)  Detect the voltage at T1 right after the jump, at the start of the curve (usually around 5.45V).  3.) Detect at what time (usually after 12 seconds or so) my voltage reached a maximum (T2) and what that maximum voltage is (usually 6.09V).  4.)  Calculate the difference between T2 and T1.

Additional notes:

1.)  The signal I am trying to analyse is in white on the attached JPEG and you can see faint red circles at the points of interest.

2.)  The "start" voltage is always < 5.1V, usually jumps to approx 5.45V as I switch on the current, follows a very similiar curve to that as shown and reaches a maximum after approx 12sec where it stays until I switch the current off.

3.)  I have tried to use a basic level trigger vi but it would be better (not a must for starters) if I could detect those points independant of the voltage levels - the shape of the graph will remain the same though - amplitudes and total time may vary slightly.

I quite the novice so any modifications to my attached vi would be appreciated.

Thanks

Andreas 

[sarahk]

Hi, Andreas.

There's no plug-in solution to trigger when a particular slope occurs in your signal, but it shouldn't be too difficult to implement. You can use the Curve Fitting Express VI to find the slope of the closest linear fit to your signal and then compare that to your trigger value.

I've included an example that does just that. It does this very slowly so you can see how it works; notice that changing the number of samples (in the Simulate Signal Express VI) determines what segment of the signal is evaluated.

Good luck! If you have any more problems, let me know.

[RSA]

Thanks!  A worthwhile method to look at, although I ended up using another method - I was going to struggle a bit to impliment this method in my application.

Cheers