Hi all, here’s my situation. I have a rotary encoder putting out 4304 pulses per revolution for 500 revolutions on a device rotating at approximately 500 rpm (all of these numbers are arbitrary). I want to take data on 4 channels every time the rotary encoder puts out a pulse. What is the best way to do this? When I have much less data (was using 800 pulses per revolution yesterday) my current method works fine, which is to have a DAQ Read inside a for loop that gets called once for every pulse I’m expecting, i.e. I wire 800 pulses/rev * 500 rev into the N terminal. PFI8 is the rotary encoder pulse. PFI 3 is the rotary encoder's index pulse, which fires once every revolution, it's just used to tell my acquisition when to start. My DAQ knowledge is pretty small, so I don't know if I'm trying to do something that's fundamentally flawed, or some of my settings are just wrong. Any insights welcome. I'm using an NI PCI-6229 DAQ card.

Tim, You're absolutely correct... I guess I should get my afternoon coffee before I attempt basic math 🙂 OK, we'll run with the assumption that what you're seeing is a buffering error. M Series devices like your 6229 have 32 bit wide AI counters, so the hardware can handle finite records of up to 4 G Samples in size. As your record size increases, you'd likely see issues handling the data in LabVIEW, or allocating buffers with DAQmx long before your hardware would have issues. If you use a finite acquisition, DAQmx will by default attempt to allocate a buffer that is big enough to hold your entire record. For your VI, it would allocate a buffer that was 2.152 M * 4 samples in size (16 MB since each sample is two bytes wide). I would not expect to see an issue with this. Read (DAQmx Read - N Chan N Samp) would then return you a very large 2 D array of data (similar to what you're doing now with the auto-indexed output of your for loop). If you're familiar with dealing with large data sets in LV (carefully avoiding memory copies), then this shouldn't be too much of an issue for you. Breaking this up into multiple reads with a smaller buffer should also be a reasonable approach. If you were to attempt to read iteration worth of data at a time, I would likely do the following: DAQmx Timing VI: Rate: 143kHz - This parameter isn't really used for much since you're clocking your data externally... If you weren't specifying the buffer size, DAQmx would pick a default based on this. Additionally DAQmx would use this for dt information if you were using the waveform data type Sample Mode - Continuous Samples Per Channel - 100,000 - Just taking a guess here... This would give you buffer space for nearly 25 revolutions... seems like plenty to me I would then use the DAQmx Read (N Chan N Samp) VI, and specify that you read 4303 samples at a time. My first recommendation would be to attempt to set up your acquisition as a finite acquisition. Perform a single read to retrieve all of your data. If you run into issues with this, then I would look into breaking this down into multiple reads. Dan

Multifunction DAQ

error with rapid data acquisition

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So, as you've been suggesting, from what I can tell, it's an encoder issue.

4E-6 should be the limit, given 250kS/s. We regularly dip below that, meaning the pulses are coming too fast. Right?

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Tim Elsey
Certified LabVIEW Architect

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Tim,

Thanks for getting that data. I believe that you are reading the data correctly. Your encoder is issuing pulses that are just a little too fast for the ADC on your 6229.

Dan