Hello. Any help would be greatly appreciated. I have done my best to learn from other examples in the forum, but I am still struggling to find a solution. My vi is attached below. Thank you! My goal is to generate a continuous signal. Then by pressing the "Collect Data" button, this signal is recorded (amplitude vs time) for n seconds. After n seconds, the user is prompted to save the n seconds of data. Throughout all of this, the signal must continue to be generated. My vi has 2 issues: 1. The "Elapsed Time" resets to 0 after the 1st data collection, but it continues to repeatedly count from 0 to n even when "Collect Data" is false. As a result, the "Elapsed Time" is not at 0 when the "Collect Data" button is pressed again. The good news is that once the desired "Elapsed Time" is reached, the "Collect Data" button is turned off. 2. Only 0.1 seconds of data is recorded and not the desired n seconds. Once the "Collect Data" is pressed, I wanted the signal to start being loaded into an array. Once the desired "Elapsed Time" is reached, then this array (with n seconds of data) is then saved. At least that was my idea.

OK, there are multiple lessons here, and the easiest way to do this is to make a simple Demo that illustrates the major points. Here's a Picture (this one is not a Snippet, so I'm attaching the VI) and a bunch of points. Please pay attention to each one (unlike your last attempt, where you followed some of them and ignore the others). Block Diagram of P-C Demo Notice that this VI has a (simple) Icon. This takes less than a minute to make, and really helps when you use them. All of the Controls and Indicators on the Front Panel are no longer shown as Icons (right-click, turn off the check mark on "View as Icon"). This makes the Block Diagram neater and more compact. I got rid of all of the DAQmx stuff and replaced it with a Waveform Generator that generates a sinusoid. This lets me test the code without worrying about hardware. I set Sampling Rate at 1000 (Hz), and Collection Time at 5 (s). The Producer Loop generates 1000 points at a time, so it "ticks" at 1 Hz which I simulated by putting a 1000ms Wait inside the Producer Loop. Note that the Consumer Loop runs "full speed". There are two little pink boxes, one going into the Obtain Queue function, one going into the final Enqueue function. This is a Waveform Constant, which is a special kind of Cluster. Go ahead and right-click the edge of this symbol (but step back ...) -- a LabVIEW Feature allows Cluster Constants (which can take a lot of Real Estate on the Block Diagram) to be replaced with an Icon. The three Inputs (Collection Time, Sampling Rate, and File Save Path) are connected by wires to the place they are used. It is much easier to follow a wire (and see its branches) than to see a Local Variable, a Global Variable, or a Value Property and have to ask "What is this, where did it come from, has it been modified?". There is one (1) Error Line that is branched in two for the Two Loops, then brought back together with Merge Errors. The Producer Loop has a Waveform (Palette) "Waveform Generator", generating a sinusoid. The cluster of inputs wired into the bottom of the Generator contains Fs (Frequency for samples) and #s (# of samples). I wired Sampling Rate into both, which gives me (1000 (samples)/1 kHz) * 1000 ms/s , which results in 1000 ms that I wired (without "showing the math) into the Wait (ms) function. This "times" the Producer, making it generate 1000 points at a time, running 1/sec. The Cluster is put on the Producer, and then the Producer is done! It has Produced, and that's all it is supposed to do. Just for fun, I put an indicator on the For Loop index so you can want it "count to 4" (why 4?). When the Producer exits, I put a Sentinel, a "default" Waveform constant (which has an Empty Array, a unique signal that you don't have to worry about) on the Queue. So the Producer has put 5 "Waveforms with 1000-point arrays" and one (Sentinel) Waveform with an Empty array, and now is really done. Now the Consumer. When we dequeue, we can use a function on the Waveform Palette to "get Components", the most important of which is Y, the Data array. We test for an Empty Array -- if True, it's the Sentinel. So the False branch (shown) has the data. So what do we do? We plot it on a Chart, and send it to an Export Waveform to Spreadsheet File function. Note I did not pass this Waveform through Build Array, even though the default form for this function wants an array of Waveforms (appropriate if you have multiple channels of data). You obviously didn't do the experiment I suggested of removing the Build Array and watching the (polymorphic) Function "adapt" to its input. Use the Help function to read how this function works. I'll admit the Help isn't the clearest I've ever seen (I got the input wrong the first time I tried it, and it is, in fact, "backwards", but too late to fix now ...). You want to wire a False when the Loop fires the first time (True means "Append to File", which means False means "Open and write to file"). How do you get True every time except the first time? See the Shift Register? There's another Index for the Consumer. Why does the Consumer Loop run one more time than the Producer Loop? When the Consumer exit, it can safely release the Queue, and both loops are done. Try this out. I put a constant name in for the File Name -- that kind of makes this a "run one time" VI, which is why I suggested you make a sub-VI that asks the User for a File Name, makes sure the Name isn't already holding data, and then lets the Program proceed. Note you want to do this before the Producer starts. Play with this. Make sure you understand the parts. This simple(r) VI requires no hardware, you can play and change things to see how all the parts work. Oh, yes, one more thing -- I changed the Graph to show "Relative Time" (meaning "Time from beginning of recording"), since having the date and time of day on the graph isn't particularly useful. I also changed the format to HH:MM:SS so I could see the 5 seconds of data (otherwise the time labels would be 00:00, 00:00, 00:00, boring). Bob Schor

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How to collect data for n seconds after pressing button

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Re: How to collect data for n seconds after pressing button

Bob_Schor

Knight of NI

‎02-14-2019 08:40 PM

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It always helps to read the "Help" for DAQmx functions, especially those that you don't choose to wire (many have "default values" that may or may not be what you want (but if you leave it blank, it will be what you get).

See if fixing these omissions helps.

Bob Schor

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