Conceptual problem in using a PCI-6602?

Question

Hi, All You Wild and Crazy NI/LabVIEW Types --
I have a problem that's been close to sending me off the deep end for more than a month now, and I think that I'm so enmeshed in it that I can't see the forest for the trees anymore. I'd like to apologize in advance for the length of this post, but there are some details that might or might not be important, and I'd rather make the mistake of giving you too much information than not enough.
We do impact-cratering experiments, using a gun to launch (usually) spherical projectiles at a variety of targets. One of our big efforts is to measure the velocities at which material is ejected from growing craters. We do that with a line-generating laser, oriented so the "sheet" of laser light is perpendicular to the target's surface and through the impact point; we strobe the laser at a programmed rate (with our PCI-6259 board) and take a time exposure of the scene with a Nikon D100 DSLR. When target material -- usually grains of sand -- is ejected, the laser illuminates a "slice" through the curtain of ejecta, illuminating a small portion of the fragments numerous times in their trajectories. Since we know the scale of the photograph and its orientation relative to the laser's illumination, and we also know the rate at which the laser is flashing, we can easily calculate the velocities of the illuminated particles. I'm attaching a picture (file 4044, cropped.jpg) of an example taken with an older camera to give you an idea. In the picture, the laser's illuminating the scene from the right of the frame and the projectile flew into the picture from the top of the frame, traveling along the left-hand edge. The brightest portion is the flash from the impact, and the rest of the parabolic trajectories are the grains of sand in flight. The target bucket, full of coarse sand, is the elliptical looking cylinder at the bottom of the picture. It's roughly 28 cm in outside diameter, if you'd like a scale.
As you might imagine, with our projectiles moving anywhere between 0.7 and 3 km/s, sequencing everything is pretty important. We're using (or trying to use) LabVIEW to do all of the sequencing, instrument control, and data storage. Things are going pretty well so far, except for what I'll describe in more detail below. A couple more things, though, first. It's important that we measure the projectile's speed in each shot, as that controls impact energy and momentum, which are critical to know in a given experiment. We do that with simple laser-photodetector pairs (which we normally call "velocity stations") arranged along the projectile's trajectory. As the projectile passes between a laser and its detector, its shadow is detected and a TTL pulse is sent to our PCI-6602 board. Depending on the experiment, we use three or four such laser-dector pairs. They use counters 0, 1, 2, and 3. We know the distances between the laser stations and, once we get the times between detections, it's s simple matter to calculate the projectile's velocity.
We also use LabVIEW to fire the gun, and we do that because opening the camera's shutter has to be synchronized with the firing pulse, which currently is sent via P1.1 on the 6259. Here's the problem: when we test the laser-detector arrays in a "standalone" mode (that is, without any other tasks or operations being done with LabVIEW), they work infallibly. It's when we try to use LabVIEW to fire the gun that we get either very erratic results from the velocity stations/6602 or no results at all.
I've tried a range of things, from starting the two-edge measurement task before the firing signal is sent, to trying to force things with a timed sequence, to doing things with brute force via a seuqence structure. When I try to start the two-edge measurement task first, though, the firing signal isn't sent until the counters time out. This of course, wrecks the experiment, because all of the timing is then messed up. The VI that I've attached (Version 1.vi) is a HIGHLY simplified version of the initial attempt I made at doing this, with all sorts of background stuff removed just so I can cut to the chase. (Only one two-edge measurement task is shown, for instance.)  I think that the VI is pretty self-explanatory (and embarrassingly primitive), so it probably doesn't need much in the way of explanation. Counter 7 and PFI 0 on the 6602 are used to accept the signal from the firing button and trigger the event structure, which contains the two-edge separation and gun-firing tasks. (In reality, I use a separate VI to have three to four concurrent two-edge separation tasks running concurrently, one for each velocity station.) I start the two-edge separation tasks first so the detectors and counters are ready for the projectile. It's not necessary here, but I kept the 500 ms wait frame in this example because that's why the sequence structure exists -- to allow the shutter of the Nikon to open completely before the gun fires. After those 500 ms, the firing signal is sent to the circuit that actually fires the gun.
What happens in this configuration is that the second frame of the sequence structure doesn't execute until the 5-s timeout transpires in the two-edge separation task. I've also tried this using a line on the 6602 to fire the gun instead of P0.1 on the 6259, but that ends up with the same result. (Both counters are used on the 6259 to strobe the main illumination laser, so they're unavailable, if you're wondering. In any case, we need four counters for the four velocity stations.)
FINALLY, my question: What am I doing wrong, here? If I put the two-edge separation tasks in the same frame of the sequence structure as the firing task, the gun fires when it's supposed to, but we get no velocity measurements. I've also tried to force the timing with another version of a sequence structure; I'm attaching another very simplified version below as Fire and speed example.vi.
After you recover from your violent fits of laughter, I'd really appreciate hearing what you might recommend. (And no, surrender isn't an option.)
Thanks for taking all of your valuable time to read this huge post -- I really appreciate it!
Mark
Attachments:
4044, cropped.jpg ‏197 KB
Version 1.vi ‏35 KB
Fire and speed example.vi ‏30 KB

Answer 1

I agree with all 3 of Kevin's points.  His first suggestion will probably fix your problem (see below).  The 2nd and 3rd suggestion would improve efficiency and responsiveness, but #2 might not be possible since independently triggering four outputs in hardware would require the use of 4 counters (on the 6602 anyway) which might be busy doing other tasks in your application (although if you don't need the stations to trigger independently then you could implement this with a single counter).
I think I have an explanation of the problematic behavior you are seeing based off of the following bits of information from your post:
1.  Running the small example code by itself works flawlessly, but adding other simultaneous functionality fails.  You mentioned you are doing this on 4 stations, so I'm assuming 4 counter input tasks running in parallel.
2.  The behavior you are seeing is that the 2nd sequence does not execute until after the read times out (note that the sequence is supposed to be executing in parallel with the read).
It sounds like the problem is coming from a combination of calling into DAQmx Read before data is available (this consumes one of the threads that LabVIEW has allocated to your application until DAQmx Read finishes executing) along with the fact that LabVIEW allocates 4 threads per execution system per priority by default.  Since all of your threads (from what I can tell) are executing on the same priority, the 4 reads you are calling will block anything else from executing until they have completed.  By then it's too late and the firing of your gun happens after the counter task has already timed out.
You *may* increase the number of threads allocated to your application by using a VI that is included with LabVIEW (vi.lib\Utility\sysinfo.llb\threadconfig.vi) and this would also probably remedy the behavior you are seeing.  However, rather than throwing more threads at this application I think the better solution would be to change the sequencing of your tasks like Kevin suggested ("create and start the 2-edge task before entering the sequence structure, and defer the 2-edge *reading* until *after* firing the DO")--in doing this you would now expect to see data immediately upon calling DAQmx Read and you avoid the situation where Read is blocking indefinitely and consuming an application thread.  You could take this a step further by checking the Available Samples per Channel property (or using the DAQmx EveryNSamplesAcquiredIntoBuffer event) to ensure that data is actually available before calling Read.
Best Regards,
John Passiak