counter triggering, synchronising period measurements etc...

[TimWay]

Hello,
I have a PCI6602 card which I want to use to measure instantaneous angular speed from 3 different sensors (a quadrature encoder, magnetic pickup+gear and optical sensor – with conditioning to give a TTL output). I think this means I need to connect each of the signals to the GATE on three of the counter channels, with the sources set as the internal time base, so that I can do continuous period measurements on all three channels.

My question is, is it possible to configure the counter to start timing simultaneously on all three channels upon receiving a reference signal, such as the “Z” signal from the encoder? If so, how do I do this and where should the reference signal be connected to?

Also, I have been trying to write a program in LABVIEW to acquire period measurements on three channels, but the DAQ assistant won’t let me have more than one channel in the task (when I run it, it says I need the multi channel version of DAQmx read but I cant find this). I could have 3 separate tasks to acquire each channel but I’m not sure if they would be synchronised if I did it that way? (I tried it with the execution highlight thingy on and it seemed to do one measurement after the other).

Thanks, 🐵
Tim

[gus]

That error message is a little misleading. You cannot have multiple input counters in a single task, so you will need to use a separate task for each counter. In order to synchronize measurements on the 6602, you should configure each counter to use an Arm Start Trigger. There is a duty cycle measurement example that uses the Arm Start Trigger that ships with NI-DAQ. For all three measurements, set the Arm Start Trigger Source to the same signal and you should be set.

I hope this helps!
gus....

[TimWay]

Thanks!
I've got 2 seperate period measurement tasks working properly now, I'll have a look for the duty cycle measurement example... any idea where you would normally connect the signal for the arm start trigger
on the timer board?
🙂
Tim

[gus]

There is no default for the Arm Start Trigger, so you can select any PFI line as the trigger source. Most every PFI line will have a direct connection to the arm start trigger.

I hope this helps!
gus....

[TimWay]

Thanks!

I had a look at the DAQmx duty cycle example today and it seemed to make sense!

Tim 🙂