topic Re: Search point in waveform in LabVIEW

Search point in waveform

leocavadas — Tue, 09 Sep 2008 14:16:09 GMT

I have 2 signals (square and sine) in same time range, I need get a trigger point in sine wave (i did it with Basic Trigger Level VI) and I need now search the last falling point in square wave before of sinewave trigger point.

Logic Steps:

1- Search trigger point index in sine wave;(i did it)

2- Go back in index until find the first (or last if look since start index - index 0)fall edge in square wave. How i do it?

Figure in attach

Re: Search point in waveform

smercurio_fc — Tue, 09 Sep 2008 14:26:27 GMT

I would suggest using the Transition Measurements VI (Signal Processing -> Wfm Measure) to get your fall times. Then it's a simple matter of finding which array element is the first that's less than your trigger point.

Re: Search point in waveform

leocavadas — Tue, 09 Sep 2008 18:16:21 GMT

How I get all fall times? the Transition Measurements return only to 1 cycle (1rs, 2rs, 3rs...), dont return index, dont return fall times array....I dont understand.

Thx for help.

Re: Search point in waveform

smercurio_fc — Tue, 09 Sep 2008 18:27:18 GMT

Just use a loop. When the start time and end times are the same (i.e., zero) you've reached the end.

Message Edited by smercurio_fc on 09-09-2008 01:27 PM

Re: Search point in waveform

leocavadas — Wed, 10 Sep 2008 17:36:56 GMT

Your Help is great, but i have a problem yet...my START and END TIMES in measurement info always show me 3,3E9....don't interest if i put first, second or other EDGE number or other sample wave...always give me result like START TIME = 3,3E9 and END TIME = 3,3E9. What did I wrong?

I have a square wave with aproximated -18 and 5 (low and high lvl) and 48 transitions in 3 seconds sample.

The example Pulse and Transtitions Measurements.VI work fine, i ran it.

Thx for help again...

Re: Search point in waveform

smercurio_fc — Wed, 10 Sep 2008 18:21:43 GMT

There is a wiring error in the example I posted. The "reference levels" cluster is wired to the wrong input terminal on the VI. It just so happens, though that in this case it doesn't affect the results. I tried it using a square wave between -18 and 5 with about 46 pulses in 3 seconds and it seemed to work just fine. Can you upload your data?

Re: Search point in waveform

leocavadas — Wed, 10 Sep 2008 18:58:37 GMT

Data in attached....thx

Re: Search point in waveform

smercurio_fc — Wed, 10 Sep 2008 20:06:57 GMT

Unfortunately, you did not save the waveform control with the actual data. Rerun the parent VI so the control is populated. Note: make sure this subVI's front panel is open. Then right-click on the waveform control and select Data Operations -> Make Current Value Default. Then save the VI and upload it again.

Message Edited by smercurio_fc on 09-10-2008 03:06 PM

Re: Search point in waveform

leocavadas — Thu, 11 Sep 2008 11:12:45 GMT

I'm sorry....Data saved in attach

Re: Search point in waveform

smercurio_fc — Thu, 11 Sep 2008 14:45:58 GMT

Hmmm... That's quite peculiar. Off-hand I don't see anything particularly odd about your waveform, so I don't see why that function would not work. I'll need to look at this in more detail...

Re: Search point in waveform

smercurio_fc — Thu, 11 Sep 2008 16:14:09 GMT

OK. Found the problem. I had not noticed this at first, but your waveform had a starting time. The way the Transition Measurements function works is that it adds t0 to the values it calculates as the falling edge starting time and ending time. The problem is that it converts the timestamp to a DBL, and this DBL is a very large number. Hence the huge numbers that you're seeing. If you just set t0 to zero you will get the right values:

Message Edited by smercurio_fc on 09-11-2008 11:14 AM