Help with measuring the frequency of a pulsing signal

Hi,
I am trying to use LabView to measure the flow from my ultrasonic flow meter. The meter sends a pulse for evey 10 litres of flow: I am picking up the signal in LabView without any trouble - it's basically a square wave that pulses at 10V and goes back down to 0V for the same duration.
All I need to do is measure the frequency or period of the wave, but I have had no luck with any of the vi's. They seem to focus on the wave when it's either at 10V or at 0V and return the period as infinite or 0.
I then attempted to program a sequence which returned true if the signal was =1 (the rising edge of the pulse), if true it would go to the next step, check again if =1 (falling edge), if true go to the third step and check if signal =1, and time that sequence, giving the period of the wave. Unfortunately this didn't work either.
Any suggestions?
Cheers,
Liam

Hi, I have the same problem - my LabView edition is Student so I don't think I can access them.
Unfortunately the problem is still persisting. I have been working with someone who is quite experienced with LabView and he can't solve the problem either.
There was some progress when I used the Trigger and Elapsed time .vi's to measure the time between rising edge pusles. Since the pulses were quite slow, by putting the period-to-flow conversion equation in, the flow could seen reducing to a minimum value as the elaspsed time increased to the full period. I have fitted new, more accuarate flow meters into he pipes which now send 100 pulses for each litre of flow. These flowmeters now pulse so quickly that it's impossible to see what the minimum flow value is to get the flow.
If anyone knows of a way to select the maximum time value (i.e. the full length of the period of a square wave) that would be one solution to the problem. Any other suggestions on the best way to measure square waves would also be gratefully received.
Thanks in advance
Liam

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Attachments:
autotrain2.vi ‏19 KB

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NI-5122 measure trigger frequency of TTL trigger signal

I have two signal channels and a TTL trigger signal on my NI-5122. The frequency of the tigger signal is between 5 and 30 Hz. Before I want to sample the two signal channels I need to know the actual trigger frequency to calculate and set sampling rate and number of sampling points. I use LabView 7.1
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many thanks for your help.
Markus

Jason V,
thanks for your help. So there is no programable counter on the 5122...
My freuqency is constant and I need an accuracy of <1%.
What I did meanwhile:
I make a loop of e.g. 10 triggerevents.
I record the times when the trigger events occur.
After the loop I fit a line to the time vs. iteration number.
This gives a pretty good value for the period, hence I get the inverse frequency.
I need 10-30 trigger events to be accurate.
For 10 Hz this makes 1-3 seconds init at the start of each measurement.
I can reach a relative error as low as 1e-4. That's o.k.
I did some more tweaking to allow for missed trigger events.
The whole VI works fine to retrieve frequencies up to 100 Hz.
For higher frequencies I am limited due to the millisecond resolution of LabViews 'Tick' vi.
====
For higher frequencies I would program a counter:
Start a measurement for 1000 scans and measure the time for that.
However, this is not accurate enough for low frequencies and short recording times (10-30 trigger events).
====
I also thought about Fourier transforming the signal itself and locating the peak frequency.
Same as above: This is only accurate when I record many periods.
====
The fastest and most accurate solution would be to sample the trigger signal itself.
Then I would need only one period and could detect the trigger slopes with microsecond resolution.
However, I need both input channels for recording simultaneous signals.
Conclusion:
(1) The NI-5122 does not has a programable counter on board
(2) For my application I found a solution tht works.
MarkusMessage Edited by Nepu on 03-09-2005 02:04 AM
Message Edited by Nepu on 03-09-2005 02:07 AM

Help with measuring the frequency of a pulsing signal

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