Threshold detector PtByPt - negative threshold

[LaBoris]

Hello, I am using Threshold detector PtByPt to detect a signal that goes from 0 to -90. I set my threshold to -50 but the detector doesn't detect it, is there a way to negate my input signal or change the "internals" of the Threshold detector PtByPt .VI so it can detect?

It is a waveform signal that consists of Y, dt and t0. I  tried to negate but nothing happened. 

Thank you for any advice.

[Kevin_Price]

When you tried to negate your signal (so it would go from 0 to +90), did you *also* negate your threshold value (so it would change from -50 to +50)?

It's been over a decade since I posted with the same question about detecting a threshold crossing from above to below.  I think this reply may be the most relevant one to adapt into a pt-by-pt evaluation.

(I solved that original issue via binary search, which operated on a large array after the whole thing had been accumulated.   I was only able to do so because of special circumstances that led to my dataset being inherently sorted.)

<time passes>.   Actually, I just opened up Threshold Detector PtByPt.vi.   That's probably an even easier starting point.  Make a copy under a different name in one of your personal folders.   There's a >= comparison between the input value and the threshold value.   It sure looks like all you need to do to make a threshold detector for descending data is to change that to a <= comparison.

I'd probably put the comparison operator inside a case structure so you could provide a boolean input to specify either ascending or descending data.

-Kevin P

[LaBoris]

Thank you Kevin for your comprehensive reply, that's exactly what I Did. I changed the comparison sign inside the threshold block and it works like a charm now.

I am still working on my project and I encountered another issue which is how to save the position (index) of the threshold detected with the input data to the xls. file.

If I should open new topic for this problem, let me know. In any case, thank you for your help.

Boris

[Bob_Schor]

@LaBoris:

I would recommend starting a new Topic for "Adding additional Data to an Excel Worksheet".  I gather that your question about negative threshold arose from analyzing a row (or column) of data from Excel.  You now want to update the Worksheet to add an entry for the Threshold -- do you want to indicate the Threshold value, the Threshold location (within the Array), or both?  Depending on how your Worksheet is organized, you could write additional rows/columns with, say, a 2x2 Cell array, Threshold, Threshold Location, and two values.  You could also write these data on a separate Worksheet.  

If you do post such a query, consider posting an example Excel file so we can make "recommendations that work" for your data.

Bob Schor