Hello, I've used NI DAQ hardware before, but I've never used a counter. I'd like to use a counter with the quadrature encoder output from a stage to send a trigger pulse every time the stage travels a certain distance (e.g. 5um). I plan to start the counter, move in a single direction, then stop the counter. The encoder has a resolution of 1nm, which I understand means I will get a rising edge on Ch A every 4nm. If the stages are moving at 100mm/s, this means a rising edge every 40ns, corresponding to a frequency of 25MHz. I've looked into Counter Output Events, and it seems like they can do what I want. I would set the initial count to correspond to my output trigger interval. (e.g. 5um / 4nm = 1250 initial count). I would set the action to count down, and select the output terminal and behavior (pulse). Then I expect that every time the counter measures 1250 pulses it will output a pulse, reset, and continue counting. I plan to run the stages ~10mm with the task running, and would expect 2000 output pulses (10mm / 5um = 2000). Then I would stop the task. Before I run off and buy some hardware, I wanted to check my understanding with the experts here.1. How do I pick hardware that is capable of counting a rising edge every 40ns? What is the relevant spec to look for?2. Is my understanding of Counter Output Events correct? I can count down from initial count to output a trigger and it will continue resetting until I stop the task? Please let me know what information I've left off, and thanks for your help!

Good questions, and you're definitely on the right track. There are a couple subtle differences between my suggestion for pulse mode and low/high/init = 50/50/50 and your question about (presumably) toggle mode and low/high = 45/5. First off, the ever-troublesome (but also useful) "initial delay". Oft-ignored and frequently with little consequence, there are certain times it's important to get right. And your situation sounds like one of them. A standard pulse is defined by a time in idle state (low by default) followed by a time in "active" state (that's what I call it, sorry for confusion if NI's terminology differs). HOWEVER, there's a special behavior for the very first pulse. The time spent in the very first idle state is defined by the "idle time" rather than the low time. I usually wire the same value to both the low time and idle time inputs to make every pulse have the same timing. But that wouldn't be the right thing to do in your particular case. Read on. Here's how your proposed config would work: 1. you configure for low/high = 45/5, leave the counter task in "output toggle mode" (the default) and start it 2. if unwired, the default idle time is 0 ticks. Sneakily however, DAQmx will coerce that to the smallest legal value of 2 ticks instead. 3. So after 2 encoder edges (your initial idle time), your counter output will go high. 4. It will stay high for 5 total edges, then revert to low again on the 7th edge overall. 5. Thereafter it will follow your desired 45 / 5 pattern. So it'll go high on edge 52 and low again on edge 57. And so on... 6. Note: you could get the kind of timing you really want if you set low/high/idle to 45/5/50. That'd produce your first output transition to high after 50 idle time edges. Then all subsequent low->high transitions happen after 5 high and 45 low edges. Here's how my proposed config would work: 1. you configure low/high/idle for 50/50/50, set the counter task to "output pulse mode" and start it 2. after 50 encoder edges worth of initial delay, the counter output will pulse. I *think* the pulse width will be 1 cycle of the device's internal master timebase -- so 12.5 nanosec for the 6602, 10.0 nanosec for the 6612. 3. after another 50 encoder edges worth of "high time", the counter output will pulse again. 4. Thereafter it follows the low/high timing of 50/50, pulsing briefly at the end of each interval. So either approach can be made to work. I kinda locked in on pulse mode since you seemed ready to use it in the original post. But your 45/5/50 approach allows you to lengthen and choose your pulse width, which might be important for whatever equipment needs to react to the pulse. To your direct question: when output mode is set for "pulse", the terminology low time, high time, and initial delay are no longer really meaningful. The output is just always in idle state except for a brief pulse high at the end of each of those intervals. [The internal behavior of the counter is to load one those values into its count register, then decrement on each "source of ticks" edge until reaching 0, aka terminal count. Upon reaching terminal count, the output mode determines whether the output state is toggled or whether a brief pulse happens during the next master timebase cycle. In the default toggle mode, low ticks and high ticks are good terms for what really happens. In pulse mode, not so much.] -Kevin P

Counter/Timer

Use Counter to Trigger based on Quadrature Encoder input

Solved!

Yeah, the way scaling gets applied to the initial position has bothered me before too. It's been quite a while since the last time I connected an encoder to a data acq device, so I haven't ever gotten around to investigating the little oddities more thoroughly. I just remember to *beware*, and then do some early simple tests to observe *actual* behavior.

-Kevin P

ALERT! LabVIEW's subscription-only policy came to an end (finally!). Unfortunately, pricing favors the captured and committed over new adopters -- so tread carefully.