LabVIEW
Save Sequence Images with a given timing
Solved!
Hi there,
I see a few things that could be improved.
First of all, right now you don't have any kind of timing in your while loop at all, so you'll need to add some. In the "programming" palette is a "Timing" palette. You'll find timing VIs in there that you can put in your loop. That will help control your timing. However, I must point out that the timing VIs such as "Wait(ms)" and "Wait Until ms Multiple" will run in parallel with the rest of your code. Therefore, if it takes longer to capture your image than it does for the timing VI to execute, your loop won't run at the speed you might expect.
(For instance: if you put a "Wait(ms)" function in your loop and set the delay to 200msec, but it takes 500msec to capture your image, your loop won't iterate at 200msec. It will wait until everything inside it is finished executing, then iterate. Does that make sense?)
The second thing I see is your architecture. I'd direct you towards a Producer-Consumer, where you capture images in one loop and write them to file in another, separate loop. Transfer the image data from your capture loop to your file loop with a queue. To look at an example of a Producer-Consumer architecture, in LabVIEW click "File...New..." then under "VI" choose "From Template...Design Patterns...Producer/Consumer Design Pattern (data)". Changing your architecture will allow you to capture images at one rate, then write them to file at a different rate...so that writing the images to file doesn't slow down your acquisition the way it does now.
Does that give you something to get started with?
Hi Emanuele,
You use the queue to transfer the data. That's the whole idea. 
Look at the help files for Enqueue, Dequeue, etc.
I've modified your VI slightly, added some comments for you, and set up your queue. I have a question and a comment or two.
The question: why do you have a "False" constant wired to your case structure? That will ensure that the "False" case executes with every loop iteration -- and the "False" case doesn't contain any code. Either that constant needs to be replaced by a button, or you need to eliminate the case structure. With the way it's written now, you'll never capture any images at all, because your image capture code is inside the "True" case, and that case will never execute.
The comments:
Please straighten out your wires. It really does make your block diagram much more readable, and that's very important. I've straightened all of your wires for you. It may feel like an unimportant detail, but it's actually quite important.
I don't know if you'll be able to capture images at 10Hz with the way you have things written now. All of that code might very well take more than 100msec to execute. You may have to streamline things somewhat. I believe you only need to configure your buffer list once, so you can do that outside the loop. (Forgive me if I am wrong about that -- you'll have to play with it a bit.) Then configure each buffer before you acquire the image.
Ok, take a look at this, and have a look at those help files for the queue functions.
Edit: I forgot to attach the error wire from your property node to your "IMAQ Create" function!
Hi Emanuele,
You're welcome. I'm glad it works now. I'm not convinced that the timed loop is going to ensure that you capture images at the rate you want -- you'll have to try it and see. I don't use timed loops, so I don't know how reliable they are. You're still dependent upon the Windows processor for your timing. As I understand it, timed loops are mainly for use with RIO systems, where they're using a hardware clock (and therefore have a reliable timing source). But if it works for you, great!
Of course you can save images at a different rate from which you acquire them, and doing so at a sub-multiple of your capture rate is a very good idea. I'd use a simple counter. See attached to give you an idea. This is something I'm pretty sure you could have figured out yourself -- it's not a hard question and you are clearly smart -- so next time I'm going to ask you to try to solve the problem yourself first and then show me what you've done. Fair? You're doing a good job of putting the pieces together from different sources, but I do feel like you're sometimes asking for help earlier than you need to!
Finally -- remember what I said about straight wires? It's important! Straighten out your wires!
Keep up the good work!
