Hi,
I have an encoder and a voltage source I need to measure, synchronously (I need to know the position at every time I sample the voltage) for 25 seconds straight. Although I need to perform a couple checks on the data as it comes in, I can use microchips outside the PCI board (a PCI-6280--single-channel sampling frequency 625 kS/s and multichannel sampling 500 kS/s, onboard AI buffer is 4096 bytes) to do this instead and use the PCI-6280 for only reading the measurements as they come in if necessary. I've already thought of a couple ways to do this, and I want to know whether I'd be implementing them properly:
- Make a task with 4 AI's (voltage/A/B/index), all sampling at the same frequency (somewhere between 6 and 60 kHz, as above). Start the task, and then use NI-DAQmx's 'Read' VI to read periodically from the buffer while sampling occurs continuously. LabVIEW apparently will make a buffer 100kS in size for this frequency of continuous sampling, so the Read VI would need to read out 60kS worth of data every second (I figure I can use a timed loop in which the Read VI is called on a 1-second period to pull 240000 samples out of the buffer--or, 460 kB of data? Obviously the 'Read''s retrieval frequency isn't too constrained, and I can make it go faster so it doesn't need to pull out as much data at a time. Any clocking suggestions?). Note that three of these AI's are for the quadrature encoder, which seems pretty wasteful given that all I need to know is the position when the voltage is read.
- Use the encoder as an external clock for the measurements and sample 1 AI in pretty much the same way as above when it comes to the way 'Read' VI is used. One potential problem here is that I don't want one measurement per external clock tick--I want 5-50 measurements at each encoder tick (x2 resolution). This (encoder) external clock input would have a frequency close to 1.2 kHz (although it would vary, I would guess, between 800 and 1.4 kHz). Instead of using it as an external clock per se (unless LabVIEW can be configured to take several measurements at each clock tick), can I wire the encoder input in as a digital trigger, and tell the PCI board to take n measurements every time it triggers? And after I've done that, could I reliably recover the encoder measurement by counting the number of times the PCI board has sampled n times after I've retrieved the data using 'Read' VI (ie, take all the measurements the board made and divide it by n)? Is there any way I could monitor the index value as well so that the PCI board could tell when it hit an index point and somehow put that into the data (I can always grab a simple microchip counter to do this outside the machine, but I'd really prefer to do it in the PCI board)?
- As an alternative to using the 'read' VI, since I only want to log data can I use TDMS on the 4 analog inputs? Would it be a faster way of draining the buffer so that I don't miss any of my measurements, or will 'Read' VI work best for this purpose?
Thanks for any help you guys can offer. I'm sorry I don't have any VIs to show you yet--this is more of a theory problem right now.
