low mesurement frequency in Data acquisition

[HamzaSI]

Hello, (for some back ground)... I am working with LabVIEW and and NI cDAQ 9179 to acquire voltage signals from sensors that can mesure force and torque on three axis. I am using a timed loop to read the data from the physical channels and a sample clock to define the rate and number of samples. I want to read the data continiously and extract the value on the sensors in each timed loop iteration.

(the problem) I can only read with a rate of 2 Hz, once I try to raise the rate of data acquisition (I need multiple readings per second around 100 Hz or higher) the value I get keeps freezing for 3 to 5 iterations and I can't get enough significant information. The same problem occurs when I try things with a N finite sample reading the rate doesn't go over 2 Hz even using the DAQassistant. I read all the articles I could find on rate, number of samples and buffers but I still can't solve my issue.

I am working with a Labview student licence if that could give you more insight. I also joined a snapshot of my code. https://imgur.com/a/ZISGwJ8

Did anyone have a similar issue? Can anybody help me with this?

Thank you very much.

[crossrulz]

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@HamzaSI wrote:

I also joined a snapshot of my code. https://imgur.com/a/ZISGwJ8

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Sorry, but that is useless.  For one, my corporate network blocks it.  Second, we can't debug pictures.  Attach your actual code.

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@HamzaSI wrote:

I am working with LabVIEW and and NI cDAQ 9179 to acquire voltage signals from sensors that can mesure force and torque on three axis.

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The cDAQ-9179 is just a chassis.  What modules do you have in it that you are trying to use?

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@HamzaSI wrote:

I am using a timed loop to read the data from the physical channels and a sample clock to define the rate and number of samples. I want to read the data continiously and extract the value on the sensors in each timed loop iteration.

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The Timed Loop is a major red flag to me.  Just use a normal While loop.  When you perform a read, just tell the DAQmx Read to read ~100ms worth of samples.  So if you are sampling at 100Hz, read 10 samples at a time.

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@HamzaSI wrote:

I can only read with a rate of 2 Hz, once I try to raise the rate of data acquisition (I need multiple readings per second around 100 Hz or higher) the value I get keeps freezing for 3 to 5 iterations and I can't get enough significant information.

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This could be a limitation of one of your modules.  If only we knew what modules you were using...

[HamzaSI]

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@crossrulz  a écrit :

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@HamzaSI wrote:

I also joined a snapshot of my code. https://imgur.com/a/ZISGwJ8

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Sorry, but that is useless.  For one, my corporate network blocks it.  Second, we can't debug pictures.  Attach your actual code

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I sincerely appologize for the image link, this my first time posting on a NI forum.

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@HamzaSI wrote:

I am working with LabVIEW and and NI cDAQ 9179 to acquire voltage signals from sensors that can mesure force and torque on three axis.

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The cDAQ-9179 is just a chassis.  What modules do you have in it that you are trying to use?

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I am using 8x NI 9219 input modules to acquire data from multiple channels. How can I read about it's limitaion?

I have joined my code as well

Thank you

[Kevin_Price · Accepted solution]

I don't have any experience with that particular device, but the spec sheet shows a max sample rate of 100 Hz when *not* measuring any thermocouples, 50 Hz if there are any.  See pages 8-9.

You probably have to program it for high speed operation to get that rate though.  The "high resolution" mode limits you to 2 Hz, so that's likely the mode you're operating in, probably because it's  the default.

The good news is that since the modules perform simultaneous sampling, you should be able to support that same rate for whatever number of (identical) modules and channels you use.

-Kevin P

[HamzaSI]

Thank you very much Kevin P! Your tip was on point. I was operating on high resolution mode that is limited to 1.95 Hz. In order to change that setting we need to add a "DAQx channel" node and select "AIADCTimingmode" in input mode. This allows to chose between High resolution (limited to 2 Hz), high speed (Up to 100 Hz for a NI9219 module), and other modes. From now on I'll make sure to always read the specs sheet. Thank you very much for your answer.

- Hamza S. I.