External trigger for PCI-6602 and quadrature endocer Encoder Data Acquisition

[leean]

Hello,

I am new to using PCI 6602 board with quadrature encoders and would much appreciate some help.  

I have 2 quadrature encoders at a fairly low rate (~250Hz) that should be triggered with an external TTL signal.  I am also trying to record the trigger channel to verify the synchronization in post processing. I started with a Labview example code but the vi just executes without waiting for the trigger.

Would anyone have insight to this problem?

[Kevin_Price]

The "Arm Start" trigger you show in the screencap is the correct way to set up a counter input task for a triggered start.  Counting (in this case, quad encoder measurement) won't begin until after you've first called DAQmx Start *and* then seen the Arm Start Trigger signal assert.

Since you perceive no delay before measurements start, I suspect that the triggering signal is an already-present clock.  Try specifying a different PFI pin that you can choose to assert manually.

I *further* suspect that you're confusing the idea of "triggering" with the idea of "sampling".   Under DAQmx, a trigger is a one-time event in the life of a task, most typically serving the function of starting the task.  Sampling is the thing that acquires and accumulates measurement data throughout the life of a task.

So here's what I suspect you probably *need* to do:

1. Call DAQmx Timing to configure your tasks to use that external 250 Hz signal as the 'source' for the sample clock.  You probably want to configure for Continuous Sampling.

2. Create your own trigger signal with a simple DO task and configure both encoder tasks to use it as their "Arm Start" triggers.  Then make sure you start both encoder tasks before asserting the trigger signal.  This is what synchronizes your measurements.

3. Read multiple samples per call to DAQmx Read.  The rule of thumb is ~100 msec worth, so at 250 Hz that'd be 25 samples.

4. Don't bog down your reading loop by accumulating these samples indefinitely.  Two main options:

- Write the data to file every iteration.  If you do this, you should open the file just once before the loop and make sure the Write function takes in a file reference, *not* a file path.  Then close it just once after the loop is done.   This is probably the simplest option.

- Set up a Producer / Consumer pattern with a parallel consumer loop.  This takes a little more work to set up, but will be worthwhile to learn about in the long run.

-Kevin P

[leean]

Hi Kevin,

I do not have access to the external clock sadly.  The trigger signal is coming in to PCI6602 as digital input when another system starts - hence my reason for recording the trigger channel. In this case, would you still create DO task from PCI6602 and use it to trigger encoders?

Thanks for the suggestions #3&4 - I will incorporate it.

[Kevin_Price]

From the discussion and code I've seen so far, I would for sure do at least *some* things differently, but I don't know your system and the requirements of your data collection enough to identify exactly which ones.

I very much doubt I'd use any DI task at all and I strongly suspect I'd call DAQmx Timing to use a hardware clock for sampling the encoder values, even if I had to generate the clock myself using one of the other counters.

The Arm Start trigger may or may not be important, though it's unlikely that it's hurting you any.

-Kevin P