Duration of a zero crossing

Hi all,
I m a newbie to LabVIEW and I really need some help with some zero
crossing issues.
I have a wave generated from the values output by a pressure sensor. I
differentiated this wave to obtain a wave with peaks, valleys and zero-
crossings. Then I used the sign generating vi to generate another
square wave depending on if the value is positive negative or zero.
What I need to do now is to find the time duration between each zero
crossing.i.e. How much time elapses before the next zero crossing
occurs. Any ideas how to do this ?
Thanks and regards.

Here's a small VI which does part of that
Have a nice day!!!
LabVIEW | LabWindows/CVI | TestStand
Certified LabVIEW Architect
Attachments:
Duration between peaks.vi ‏18 KB

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How to measure time difference between zero crossing of a sine wave and rise time of a pulse on a same graph?

I have a 50Hz sine wave and a pulse signal on a same graph. The phase difference between two is between 0-90 degrees.
Now I need to calculate the time difference between (when the sine wave crosses zero volts) and (when the pulse rises). The frequency will stay approximately same for both signals.
The application is for a three phase generator. In simple words, when the time difference between the zero-crossing of sine wave and the pulse rises increases, that means that the load on the generator has increases.
I am a beginner user of LabView (version 9, 2009), maybe it is a very simple problem but I've been pulling my hair out for the last few days and coudln't figure anything out. Any help would be greatly appreciated. I am using DAQ USB-6008 to measure these voltages and pulse from the generator and a sensor
I have attached a jpg (a graph that i just made with excel to explain). The time 't' is what I am trying to measure
Cheers
Awais
Message Edited by awais.h on 03-30-2010 11:20 PM
Message Edited by awais.h on 03-30-2010 11:21 PM
Solved!
Go to Solution.

Hi
Thanks for the code but I'm afraid it won't work. Like you said the probability of choosing a value that is on both graphs may not happen. This is something that I would like the code to do automatically.
But I did use the previous code posted and made some modifications to it and it seems to work perfectly. Now the next thing is to be able to get just that one value that tells you the first time difference.
Here is what I get using that code.
As you can see from the t Values. I only need the one that is highlighted. If there is a way to filter out the rest it would be great.
I was thinking of a while loop and as soon as the value is higher than 3E-5 it would store the number and stop the loop, but I'm not too familiar with arrays in labview.
Here is the the code modified.
As you can see, it wasn't that big of a modification and it still is your code.
I will keep trying.
Thanks for the help
Attachments:
FinalShockSpeed.vi ‏55 KB

Zero crossing, "Triggering" and XY-Graphs

I'm trying to measure the electrical hysteresis of a sample. The way in which I do this is by measuring two voltages within a circuit, sending them to my DAQ, and then plotting them in an XY-Graph to view the shape of the curve. I can somewhat see the hysteresis, but II need to measure the "zero-crossings"; that is to say, the value of one function, when the other is 0, and vice versa. So when V1 = 0, I want to know what V2 is, and when V2 = 0, I want to know what V1 is. Seems simple enough, and I have included my VI.
First I tried the ZeroCrossingPtbyPt, along witha Relay, which never seemed to trigger properly.
Second, I tried using Trigger and Gate, but that seems to give inflated values.
This VI can be tested by removing the DAQ assitant and replacing it with two "Generate Signal"(s), with the same frequency, different amplitudes, and a small phase difference, and this will produce a similar effect without the real signals.
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Please help! Thanks in advance.
Attachments:
hysteresis.vi ‏176 KB

As it turns out, the issue was that I was not collecting enough samples at a high enough sample rate from the DAQ. This is why it worked with "Generate Signals" and did not work with the DAQ Assistant. Because there were not enough samples, the apparent zero crossing was occuring either too early or too late, leading to an incorrect value. Therefore, the best solution when it comes to finding zero crossings is make sure your # of samples, and sample rate are high enough to get an accurate measurement.
As a result, this is why my XY Graph was showing blocky hysteresis loops, rather than nice smooth curve hysteresis loops. The data being sent to it was not quick enough, and therefore the XY- Graph connected the dots between the points that it was sent, yielding a truncated loop, which appeared blocky or sketchy.
On the issue of triggering, this can apparently be set within the DAQ Assistant in the "Triggering" tab; the key is chosing the correct channel to trigger from, and a reasonable trigger level. This was my problem, and this is why my Graph appeared to jitter. It was not the graph itself, but the DAQ Assistant that did not tell it the proper triggering timing.
So, in short, I solved my own problems, but I leave this as a record for those who encounter similar problems in the future.
Thanks.

Record time of zero crossing

I am trying to record the time for zero crossing in order to get the phase angle and inevitably get the power facto . soi was trying out a few ideas to see if this is achievable and i came up with this . can someone tell me where i am wrong . or what to do get the right values ..
Solved!
Go to Solution.
Attachments:
zero crossing in waveform.vi ‏17 KB
Zero Crossing.PNG ‏21 KB

1. You need to pass the waveform into the Get Waveform Time Array.
2. No need to wire up the N on the FOR loop since you are using Autoindexing tunnels.
3. The Conditional Indexing tunnel is your friend here.
There are only two ways to tell somebody thanks: Kudos and Marked Solutions
Unofficial Forum Rules and Guidelines
Attachments:
zero crossing in waveform_BD.png ‏20 KB

Zero crossing and syncronizing

I use 2 programs.I have atachetted them below.
In "Line zero cross sync" I detect the crossings through zero of a sine and then for every crossing I try to write on a analog output an impulse.
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How can I delay the moment between detecting a zero crossing and writing the impuls, with the value calculated with the second program.
The second problem: If I use a loop to delay the time how can I make sure that the two operations will be syncronized (if I modify the frequncy of the signal during the measurement I want the second program to modify the delay time simultaneously,not with a delay).
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Attachments:
Line zero cross sync.vi ‏85 KB
sincronizare cu reteaua subvi.vi ‏32 KB

I solved the problem.Thank you anyway

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Hello,
I have Amplitude vs. time data (wave).
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Try something like this, maybe? See if it makes sense.
(It currently fails if a value is exactly zero and the function does not switch sign afterward. It just needs a bit more code to fix that. Modify as needed.)
Message Edited by altenbach on 04-13-2008 12:52 PM
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Attachments:
ZeroCrossings.png ‏33 KB
ZeroCrossings.vi ‏57 KB

I wanted a zero crossing detector for a a acquired wave form.

it shoul detect the zero crossing of a wave

The technique for finding a zero crossing is basically to detect a change in sign between two consecutive numbers. You need to evaluate the previous number sign with the current number.
A common way to do so is by multiplying the previous number and the current number. If the product is positive, both has the same sign and thus there was no zero crossing. If the product is negative, there was a zero crossing.
Make sure you are sampling you signal fast enough to accurately detect when the zero crossing occur. If you know your signal, you may easily calculate the exact moment based on your aqcuired information.
Best regards;
Enrique Vargas
www.vartortech.com

Phase angle measurement using zero crossing

I am using zero crossing to measure the phase angle between (voltage and current) which is represented as two input in my vi to eventually get the power factor. But however the reading i got for the power factor remain at 1 all through no changes. I am using an SCT- 013 -000 current transformer for the current and a power transformer for the voltage 240 - 6V . below is my vi . Can anyone help me troubleshoot why my result is showing 1 for different loads including resistive , capacitive and inductive loads .
Attachments:
Phase Angle Measurement.vi ‏39 KB

Are you working with Nigel91 who posted a similar question in the Electrical Power Suite board?
You have the same problem that he has. For dt = 0.02 (equivalent to a 50 Hz sampling rate) you cannot measure anything about a signal with a frequency higher than 25 Hz. And you could not get meaningful phase information at frequencies above a few hertz.
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Zero-crossing function

Hi,
I found this zero-crossing function in one of the old forums and I'm trying to understand how it works. In my case I have two sine waves, instead of one square pulse but I can't really figure out how this function works. I know that first, the waves are generated then you get their attributes, the data values go into the zero-crossing detector that controls the case statement but I don't know how the trigger time and time interaval are used to build the array, I think the timestamp outside the 'for' loop is for storing the previous itteration of the zero-crossings but I'm not really sure.I have no clue what happens in the final step before the zero-crossing times are subtarcted, as in the threshold array and so on. I've read how each function individually but I can't add the blocks together.
So I was wondering if someone could please explain how this works?
Thanks
Solved!
Go to Solution.
Attachments:
Zerocros.png ‏131 KB

Adding to what crossrulz said, it helps to understand what the code you're looking at is.
It would appear you don't quite understand what t0 and dt are. t0 is the time at the initial point in the waveform. This can be a wide variety of times rather than simply 0. dt is the amount of time between samples. If we multiply the iteration by dt, we know how much time has passed since the first sample. After 0, 0dt is the same as 0s. From there, 1dt, 2dt, 3dt, etc can all be calculated to a value measuring time. But, it's only measuring "time since the first sample." If it's only measuring that, the timestamp could be meaningless. If you want to convert the time since first sample to an actual time, you need to add the initial time to it. As an example, if dt is 1s and t0 is 4:00:00pm, after 3 samples we are at 2dt so 2 seconds. Do we care about 2 seconds or 4:00:02?
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Splitting up plots on XY Graphs depending on zero crossing

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I posted this question earlier and thought I got a solution. Please read the old thread: http://forums.ni.com/ni/board/message?message.uid=1098899#U1098899
Basically, my problem now is that the way its being done now, whenever it crosses the zero axis with a negative slope, it temporarily creates another plot and as soon as it crosses it with a positive slope, it merges those 2 graphs together. So somewhere between the slope going from -'ve to +'ve and crossing the zero, I have an extra plot, and it distorts the colors of my plots, which seems like a bug to the user and may confuse them. I have attached a VI below where you can see this behavior in the 2D array indicator.
Attachments:
XY Example VI.vi ‏18 KB

I don't know what your intentions are with this VI, or what you mean by "temporarily creates another plot".
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Solved!
Go to Solution.

Sorry for not responding earlier about my progess.
I was able to extract the timepoints using your advice but now want to go back to the original data and extract the y-values. For some reason, my for loop below completes but only writes no-values? Extracting the x and y values in parallel won't work because I generated by x-value subset using the differential of the original dataset.
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FOR i=1 TO InitContrLength
   ChDx(i, InitContrY) = ChDx(ChDx(i,InitContr)/Interval, Filtered)
   Next
InitContr is an array of discontinuous time values which represent a subset of the original time series. My for loop is designed to iterate through the InitContr, converting them to an integer representing the appropriate row in my original dataset (Filtered) by dividing the time value by the step interval, and then writing the value from Filtered (which is an array of y-values) that corresponds to that row to InitContrY.
Definitions in case the above is confusing
InitContr = Channel containing a subset of time values
Filtered = Channel containing all y-values from the original waveform
Interval = step width of the original waveform
InitContrY = New Channel that I want to write y-values to that correspond to InitContr x-Values
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Duration of a zero crossing

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