Sensor not appearing in DAQ Assistant

[dflynn]

I am new to LabVIEW, trying to set up a test bench. My current equipment is as follows:

3 Dynisco PT 130 Pressure Transducers

Fox FT2 Mass Flow Meter

Omega Thermocouple

Strain Gage

NI 9239 High-Speed Input Module

NI 9207 Voltage/Current Module

NI 9211 Thermocouple Module

NI 9148 Eithernet RIO Expansion Chassis

LabVIEW 2013

Because I am so new to LabVIEW I am looking to start with baby steps and just get one pressure transducer taking measurements first before trying to manage the entire system. 

My problem: I can get data by using the scan mode and manually selecting a channel (FGPA is available but scan was more straight forward as far as programming goes to test my system), but the DAQ Assistant will not find my pressure transducer. I would stick with scan mode but unless there is a way to set the input voltage, output units, etc I do not have a way to quantify my data. Browsing the forums it appears as if my transducer should be listed under the Voltage tab, but it says no device can be found for any of the tabs. 

I have also read to make the measurements in MAX, but cannot figure out where in the program to make measurements. 

Any suggestions on how to make my sensor appear in DAQ Assistant? Or have a suggestion for a better method of acquiring a pressure measurement that I can later add more sensors to?

[crossrulz · Accepted solution]

The Ethernet Expansion Chassis does not use DAQmx.  The DAQ Assistant uses DAQmx.  Instead, you need to read the variables from the expansion chassis.

Do you really need the FPGA in the chassis?  The only thing I can think of for needing the FPGA is for control, but I do not see anything that outputs signals.  You should really get a cDAQ chassis, which uses DAQmx.  If you want to keep with Ethernet, go with a cDAQ-9189.  Or you could go with a USB (cDAQ-9174 for 4 slots, cDAQ-9178 for 8 slots) for a more direct connection.

[dflynn]

As far as I can tell there is no need for output and there will never be a need in the foreseeable future, so the FPGA is unnecessary. The equipment I was given was grandfathered in from a prior engineering group so I am unsure of why they chose what they did. Luckily I can purchase new hardware if necessary. Just to ensure I make the correct purchase do you suggest that I replace the 9148 with a cDAQ-9189 or cDAQ-9174, which will allow me to take advantage of the DAQ Assistant and simplify the setup? If so I would choose the 9174/8 to change over to usb connectivity.

Also do you know of any resources that could walk me through reading variables from the expansion chassis? I looked into it quickly and it appears as though I could get the data I need from the current setup, but it would greatly complicate the code

[natasftw]

I'd ask the exact opposite question of Crossrulz.

If you already have the hardware, it's zero cost to use the FPGA.  While it may be overkill for what you're doing, it's oddly enough cheaper than the DAQmx route as that route means you need to purchase new hardware.

Do you need to use the DAQmx library for this or are you able to use the FPGA/Scan Mode options successfully?  If you can use either of these, I'd lean towards working with the device you have and learning that skill. 

The complication you're looking at is you'll have to view things as multiple devices.  With the cDAQ, you'll be plugging in the device to your PC (either through Ethernet or the USB option) and it'll be an extension of the PC.  With the Expansion RIO chassis, you'll have a second PC running autonomously.  You'll need to do some work to transfer the values from that device back to your Host PC.  With the Scan Mode, it'll abstract away a lot of that for you.

[Michael_78]

I remember trying scan mode when I first got an expansion chassis.  I always pick fpga expansion over cdaq and it is actually very easy to program the fpga and access the fpga from your PC so you might leave scan mode behind quite quickly.  The learning curve is much easier than you might be thinking.  That said, if you frequently change the measurement devices and use the chassis for portable daq, then cdaq may be better.  My test rig choice is usually fpga expansion  and a PC,  I rarely need the real time option of CRIO but I often need more flexibility (tight control) than cdaq.  Fpga supports more modules than cdaq too.

I hope this helps you with your choices, and we are here to help.

[dflynn]

Thanks for the help! Natasftw, currently I am able to get a reading using scan mode (I have not been able to get a reading from FGPA mode yet, most likely due to my programming ability), but I am unsure how to quantify the reading I am getting from the pressure transducer. Please correct me if I'm wrong but just setting up a numerical output from the transducer would output a voltage difference across the transducer which I would later need to convert into psi/Pa/etc? But even a voltage output would be skewed by not properly setting the correct input voltages for the individual pressure transducers? I hooked up my digital flow meter and took readings from each channel, which seemingly gave me completely random outputs. 

Looking at the chassis as a second autonomous PC is a great analogy for helping me to understand how the program would work. The question would be how quickly could I learn and debug the more complicated program, and pass the knowledge on to others who will also be using the test bench before buying a new chassis (cDAQ-9174 is around $900 at the moment) becomes the cheaper option. For my setup simplicity and correct data of course are more important than a relatively small startup cost, but I need to be certain that the setup will work more simply if I order the chassis.

[dflynn]

Michael78 - Can you please expand upon what you mean by the FPGA having tighter control than cDAQ?

My bench only needs to take in the three pressure readings, flow, temperature, and strain gauge data and overlay all of the data onto one graph. I do not see the function adapting or needing to change any time soon, as long as I can press a record button and gather data my bench will fulfill its permanent function. 

Also would the cDAQ make gathering readings from my strain gauge and digital flow meter more simple as well?

One more thing, I noticed that my PT 130 sensors are designed to measure the pressures of hydraulic systems. I am measuring air pressure, does anyone know if my readings will be skewed as a result of measuring a different fluid than the transducer is intended for?