Fall time measure at Labview

[arielsl]

Hello,

I triggered my scope (Tektronix TDS2024B) using Labview.

I got with that trigger a capicator discharge formwave.

Now I want to measure the time when the discharge started until a spesific voltage that I can determine.

I know how to do it with my scope using the cursors (manualy),  but I don't know how can I do it when I trigger the scope with Labview and then measure it automaticly.

Best Regards,

Ariel.

[Mathan]

This is a very simpler one depending on your waveform. Please post the vi with a waveform sample and all the necessary criteria in measuring the fall time measurement. I ensure people here are very much expert in solving this.

[arielsl]

Thank you for your answer.

I realy sorry for asking this, but how do I post here a VI?

[Peñata]

Hi Ariel,

You can attach a VI by clicking the Add Attachments link that is located below the Post and Cancel buttons when you reply to a post.

Aaron P

National Instruments

Applications Engineer

http://www.ni.com/support

[arielsl]

Thank you for your answer.

This is my graph.

I want to measure the time from the start of the fall until 4.5V.

Thank you for helping.

[arielsl\]

I have on more question except the first one...

I know I can save data that I get from a graph to a file (spreadsheet and excel), but can I save to a PDF file that I create and save it on a specific place at that file?

[Peñata]

Hi Ariel,

Regarding your initial issue, I am a little confused as to what your ultimate goal is. What exactly do you mean by "I want to measure the time from the start of the fall until 4.5V."

Regarding your second question, I do not believe there is a way to write directly to a PDF file. I apologize for the inconvenience. If you would like to file a product suggestion, you can do so by going to http://sine.ni.com/apps/utf8/nicc.call_me and clicking the link under Product Feedback.

Aaron P

National Instruments

Applications Engineer

http://www.ni.com/support

[arielsl]

Thank you for your answer...

I'll try to express my self better.

You can open my VI and see that my wave is start from something like 18V and then (after one second) it's start to fall until it gets to 0V.

I want to measure the the of the start of the fall (someting like 1 second), until 4.5V (someting like 1.15 second).

I just need it exactly and not an assumption.

I hope I expressed my self better this time.

Ariel.

[tbob]

To measure fall time from a waveform, you must extract the dt element and the array of data from the waveform.  Then search through the array for the index of the element where the voltage is less than 4.5 in your case.  Take the index of that element and multiply it by dt.  This gives the time to where the data fell below 4.5, or in simple terms, this is the fall time in seconds.  here is a snippet that shows how its done.  I used the waveform from your attached vi and I came up with a fall time of 1.147 seconds.

Message Edited by tbob on 03-12-2010 09:28 AM

Forgot to mention that this may be a case for an infinite loop if the array of data never contains an element less than 4.5v.  You should add code to stop the loop if i becomes greater than the number of elements in the array.  In this case, make the fall time NaN or something like that.

Message Edited by tbob on 03-12-2010 09:31 AM

[Peñata]

To expand on tbob's post, you can also avoid an infinite loop by using a For Loop instead of a While Loop:

NOTE: You can add a conditional terminal to a For Loop by right-clicking the border and selecting Conditional Terminal.

As tbob noted, you should probably still implement some code to change the Fall Time indicator to a specific value (i.e. NaN or -1) when the array did not contain a value less than your desired threshold. Something like this may work:

Hope this helps!

Aaron P

National Instruments

Applications Engineer

http://www.ni.com/support