measurement strategy AI at multiple rates with C-Series modules

[Olaf_Stetzer]

Hi forum,

in our lab we have an experiment which we want to control using C-Series modules possibly all connected to the same 9217 chassis.

Now the problem is: Th experiment consists of two more or less independent parts (instruments) which have already their own Labview programs.

What we wanted to do now is to merge these two by using C-Series modules on the same chassis. What makes it even more complicated: For one instrument we would ideally need two sampling rates: 10 kHz for three AI channels with 14 or 16 bit to sample signals, and 3 more channels which could be sampled with only 1 Hz because it is housekeeping data which does not change too often. Our idea was to use a 9205 module but it seems to be impossible to sample with two rates from the same module.

For the second instrument all (thermocouples, AI/AO DO) could be read/written at 1 Hz because it is all housekeeping data.

From what I have learned now this is probably not possible at all: Using one 9217 chassis for both independant instruments and even using different sampling rates. Unfortunately we already purchased the 9205 module so what strategy would ypu recommend for what we need?  

Thanks,

Olaf

[A_Person]

Can you verify which chassis you are using?  The 9217 is another C series module, not a chassis, so I am guessing that you are using an NI-9172 CompactDAQ Chassis.  Is this correct?  Also, do you have any other modules available, maybe a digital I/O module?

[A_Person]

I ran out of time to edit my message but wanted to add some more information.

If you are in fact using the 9172 chassis, you cannot specify an analog sampling rate for more than one task on the chassis.  What you can do is sample all of your inputs at the highest desired sample rate and decimate or average some of your channels to reduce the amount of data.

If you are using different hardware, let us know and we'll see what other options you might have.

[John_P1]

To further elaborate a bit, the new 9174 and 9178 chassis actually have three separate timing engines for analog input.  You still can't sample at multiple rates on a single module module, but you can use different sample rates on different modules.

Other than that I don't see any problem with sampling all of the channels at max rate and throwing away samples that you don't need so you don't have excessively long log files.  If the max rate is 10 kHz, you can get up to 25 channels on each 9205 at this rate since it has an aggregate rate of 250 kHz.

One way you can do this is to set the Number of Samples to Read input of DAQmx Read to 10000.  This would give you a loop rate of 1 second (assuming 10 kHz sample rate), so you could just index out the a single value from the channels that you only want 1 sample/second.

Best Regards,

John

[Olaf_Stetzer]

Thanks for all the answers so far. I had a closer look at which hardware we really have and what tasks we want to accomplish. First, we want to have two almost identical systems, but we have one old 9172 chassis and just purchased a new one type 9178. If we could re-use the old one it would be nice.

What we want to do in addition to sampling three 10 kHz signals is the housekeeping data as I already told. This is thermocouples with 9211 TC modules some other AI and also AO modules to control the hardware. 

One of my direct questions is: Since the express vi for DAQ is capable fo measuring AI plus TC in one taskhow can this be done with standard programming (I don't like the express VI's so much).

Second question: Since the housekeeping data is only needed once per second and the timing is not so importnat here, is it possible to read these values with software timing without a timing engine of the chassis. In this way it should be possible to re-use the old chassis as we only need the timing engine for the 10 kHz signals.

Olaf

[Olaf_Stetzer]

One more thing: We have one 9215 (currently plugged in a USB singe carrier) which we used with an older verision of the setup to sample the 10kHz signals. We could use this and buy one  more for the second setup. Again, this would only work with the 9172 carrier if we could use some kind of software triggering or timing for the 1Hz housekeeping data.

Olaf

[John_P1]

Hi Olaf,

Even on the 9172 you shouldn't need any additional hardware (USB Sleeves etc.).  Since the 9211 is considered a slow sample module, you can actually run it in a task with a faster module without compromising the sample rate.  The 9211 will simply repeat data at the faster rate.  See the following KnowledgeBase for more information:

Specifying Different Sample Rates for Multiple CompactDAQ Modules

As far as defining multiple AI Channel types in a single task in LabVIEW, you can simply call multiple instances of AI Create Channel using the same task handle.

-John