Simultaneous analog and counter acquisition in CVI

[RobertoBozzolo]

Hi all,

I'm trying to perform two separate acquisition simultaneously: an analog acquisition on some channels and a frequency measurement on a counter. The developing environment is CVI 7.1, using a PCI-6281 DAQ board, NI_DAQ 8.5.0f5

My problem is that the frequency comes from an encoder on a motor which is performing a speed ramp, so the period of the signal is not constant and I must syncronize speed values with analog measurements.

I have no access now to the device so I tried to develop a solution on a simulated device, my code is the following, on the simulated device it terminates with no error (but no measurements on Cdata but this is probably due to the board being a simulated one):

 // Create the analog task
 DAQmxChk (DAQmxCreateTask ("Analog", &taskAI));
 DAQmxChk (DAQmxCreateAIVoltageChan (taskAI, "/Dev1/ai0", "Voltage", DAQmx_Val_Cfg_Default, -5.0, 5.0, DAQmx_Val_Volts, ""));
 DAQmxChk (DAQmxCfgSampClkTiming (taskAI, "OnboardClock", 1000, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps, size));

 // Create the counter task
 DAQmxChk (DAQmxCreateTask ("Counter", &taskCI));
 DAQmxChk (DAQmxCreateCIFreqChan (taskCI, "/Dev1/ctr0", "Counter", 2.0, 100.0, DAQmx_Val_Hz, DAQmx_Val_Rising, DAQmx_Val_LowFreq1Ctr, 0.001, 4, ""));
 DAQmxChk (DAQmxCfgImplicitTiming (taskCI, DAQmx_Val_FiniteSamps, size));

 // Set counter task to share start with analog task
 DAQmxChk (DAQmxSetTrigAttribute (taskCI, DAQmx_ArmStartTrig_Type, DAQmx_Val_DigEdge));
 DAQmxChk (DAQmxSetTrigAttribute (taskCI, DAQmx_DigEdge_ArmStartTrig_Src, "/Dev1/AI/StartTrigger"));

 // Start the tasks
 DAQmxStartTask (taskCI);
 DAQmxStartTask (taskAI);

 // Wait until task done
 while (!doneAI) {
  DAQmxChk (DAQmxIsTaskDone (taskAI, &doneAI));
 }

 // Read measurements
 DAQmxChk (DAQmxReadAnalogF64 (taskAI, DAQmx_Val_Auto, 0.0, DAQmx_Val_GroupByChannel, Adata, size, &sRead, 0));
 DAQmxChk (DAQmxReadCounterF64 (taskCI, DAQmx_Val_Auto, 0.0, Cdata, size, &sRead, 0));

Question is: does this make sense at all? Can I expect it works on a real board the same as on the simulated one? And am I guaranteed that Cdata[n] was acquired exactly in the same moment as Adata[n]?

Since I am acquiring at very high rates, I can imagine that there isn't a new frequency measurement for every analog sample, so I suppose I will get an array of frequencies like that

[ 100.0, 100.0, 100.0, 110.0, 110.0, 110.0, 115.0, 115.0 ..... ] and so on, that is frequency measurement is repeated if there is no new value ready: is this true or am I doing wrong assumptions?

[RobertoBozzolo]

Is there anybody that can help me with this question? I would like to have a second look at it.

Message Edited by Roberto Bozzolo on 03-11-2008 09:01 AM

[AndreaN]

Hi Roberto,

I'm sorry for my late reply.

I've tested your code with a PCI 6251. I've used a BNC-2120 and I've acquired a sine wave on channel 0 and I've sent a TTL square wave into the CTR0 gate (PFI 9).

I was able to get values on Adata and Cdata. Your assumptions about Cdata were correct, the array has equal values since the frequency changes slowly.

I hope this can help you.

Ciao,

Andrea N.

[RobertoBozzolo]

Ciao Andrea,

wow! This is *really* a good answer I can now proceed to develop final application.

Just to clarify me some item:

• My signal sweeps from 100Hz to 2kHz: I'm assuming the measure with one counter is enough to achieve a good precision for this range
• In this case "Measurement time" parameter of DAQmxCreateCIFreqChan is not used (I suppose I will find a new value in Cdata as far as it is acquired, that is at variable time while the speed is varying)
• Can I force the choice of timebase frequency to 20MHz or DAQmx automatically chooses it? In case I can, which parameter should I pass to DAQmxSetChanAttribute (task, "", DAQmx_CI_CtrTimebaseSrc, ???, NULL); instead of "???"

Thank you

[AndreaN]

Ciao Roberto,

the Low Frequency measurements is good for your range of frequencies. Since the measurements have an error of +1/-1 source cycle, with a 20 MHz timebase you would get 10000 cicles for a frequency of 2 KHz. This means that you get a frequency of 2000,2 - 1999,8  Hz.

With the DAQmxCreateCIFreqChan the "Measurement time" parameter is not used as stated in the help:

The length of time to measure the frequency or period of a digital signal, when measMethod is DAQmx_Val_HighFreq2Ctr.

I used the DAQmxGetTimingAttribute (taskAI, DAQmx_SampClk_Timebase_Rate, &TBrate)  to get informations about the timebase rate.

For the taskAI I've gotten 20 MHz for my NI PCI 6251.

Ciao,

Andrea N.

[RobertoBozzolo]

Ok, many thanks to you

Ciao
Roberto