Dear Lab View discussion group members, I would like to request some advice about how to stop sampling of two different signals simultaneously. I have been able to confirm they start sampling at the same time (based on time stamp on loop first call), but the waveform time for my two signals is vastly different. The AE signal always samples for the correct time, but the load signal always shows about 1.5 to two seconds longer acquisition time. it must have to do with the vastly different sampling rates ( 500 Khz vs. 1000 Hz). I can't get my head around why the load acquisition loop would run so much longer though even though each loop iteration should be about .001 + 10 micro seconds (acquisition rate plus buffer that NI adds). I tried to use timed loops, but the result is the same. in this case I wired the loop iteration value (dt) to be... (signal acquisition rate)*(number of samples to acquire) I also tried to use notefiers and master slave configuration ( where load loop was the slave and a notefier was sent every time 500 iterations of the AE loop were accomplished). This did not results in simultanous stops either. My ultimate objective is to get two files and then compare spikes in my AE signal to when a specific load occurred. I will then count the number of spikes occuring within certain load ranges and generate a histogram. I will be out of the office next week, returning July 6. I will reply to any posts at that time. thank you Alfonso

I can not see that you need two separate loops. Also skip the software timing. I guess that is causing your trouble. Just put the the NI-6221 reading into the loop running the NI-9221 read. Perhaps put them into a frame so both are read before data is written to file. I think also are pulling data from your NI-9221 buffer to often. You read the buffer 500 times pr second. Perhaps lover this to 50 times pr second. So each time you read the NI-9221 pull 10000 samples from the buffer, and 40 from the NI 6221 buffer. And for the timing just skip it. Stop your loop then you have run it 100 times. Assuming you read 10000 samples from the NI-9221 on each iteration you will then have read 100x10000 samples equal to two second of data

ni.com is currently undergoing scheduled maintenance.

Some services may be unavailable at this time. Please contact us for help or try again later.

LabVIEW

Parallel sampling - different rates

Solved!

Author

NAKeel,

you are very astute. I have recently changed what the NI 9221 is docked into. I switched from a compact DAQ chassis to a USB single module port.

However, I did switch the source in my VI.

never the less, I did got back into MAX and reset the device and restarted the whole system. Lo and behold it now works.

thanks for your help very much.

I also must thank CoqRouge for suggesting a single loop and pointing out how the read operations affect timing.

A. Lopez

Message 11 of 11

(369 Views)

LabVIEW

Parallel sampling - different rates

Re: Parallel sampling - different rates