Developing analog signal input/output

[Yatsco]

Let me start by saying the I am a new user to LabVIEW. My experience with LabVIEW is limited to an information session in which we covered the material in the manual, "Introduction to LabVIEW and Computer-Based measurements Hands-On Customer Manual".  As for what I am trying to accomplish:

I am using LabVIEW 8.6, NI cDAQ-9172, and module numbers NI 9205 and NI 9264.  I have a load cell that requires a 10V constant power supply in order to work.  I have no idea how to generate this signal or the signal, in mV, that is sent from the load cell back to the cDAQ-9172.  I have tried using DAQ-Express for the input and output signals.  Once I create the two DAQ assistants, I am unsure of what to do next.  Also, the load cell has four wires: green, white, red, and black.  Green = +SIG, red = +EXC, white = -SIG, and Black = -EXC.  The red wire is connected to ao0 and the black wire is connected to the COM of NI 9264.  the Green wire is connected to ai18 and the white wire is connected to ai26 of NI 9205.  

Any help on this matter is greatly appreciated!

Yatsco

[RavensFan]

With load cells, you don't actually generate a voltage to make them work.  The voltage is supplied by the appropriate signal conditioning module.  It doesn't necessarily need to be 10V.  A lower voltage source could work as well.

It's theoretically possible to use a combination of an analog output to be the source to your load cell and an analog input to be the measurement for it.   But that is going to be more trouble than it's worth.  You should really be looking a module designed for full bridge sensors such as the CDAQ-9219.

[Dan_K · Accepted solution]

Hello Yatsco,

Ravens Fan is correct that you would be more successful using an NI 9219 instead of the AI/AO module combination. However, it could be possible to use the modules you have to use the load cell you are attempting to use, but more information on the sensor would be required to say for certain. A link to the data sheet would be best.

Assuming that everything would work out with the sensor itself I would do something like the following:

1. Configure your analog output DAQ Assistant to output on ao0 of the 9264, keep all settings default except for the Generation Mode, which you should change to 1 Sample (On Demand).
   
2. Configure your analog input DAQ Assistant to input on ai18, keeping all settings as default again except for the Timing Settings, which should look something like the following:
   
3. Your block diagram should look something like this:
   
   

If you do this and run into any difficulties with your sensor or it's response, post the datasheet (or at least the manufacturer and part number), and I will look into it further.

[Yatsco]

Ravens Fan and Dan_K,

Thank you very much for your help and input.  I would have spent numerous hours trying to figure this out if it was not for the two of you.  I set-up the block diagram and DAQ assistant like you said, Dan_k and I am getting a signal from my load cell! Now the only thing I have to do is set-up a scale so that I can get readings in lbf instead of amplitude.  Thanks again!

Much thanks,

Yatsco

[Dan_K]

Hello Yatsco,

I would recommend using the custom scaling within the DAQ Assistant. By doing this you can perform all of your scaling in the DAQ Assistant and have scaled data ready for further logic, logging to a file and/or displaying on a front panel chart. This can be done either within the DAQ Assistant itself or by going into Measurement and Automation Explorer (MAX). A shortcut labeled Measurement & Automation should be on your desktop.

I would recommend creating the scale within MAX, for full instruction on how to do this see KnowledgeBase 4XC8EAN8: How do I make a custom scale in the Measurement and Automation Explorer (MAX... Once you create the scale you can select it in the DAQ Assistant custom scale pull down menu.

[Yatsco]

Hi Dan_K,

I am running into problems after I scaled the data to my liking.  The problem occurs when I put a load on the load cell.  When a load is put on the load cell, very little change occurs in the waveform chart.  It isn't until after 15 seconds, or so, after starting the program that it starts to register a load.  However, there is very little fluctuation during the time that the load is applied and the time when there is no load applied.  Also, here is a link to the data sheet for the load cell;

http://www.futek.com/files/pdf/Product%20Drawings/lcf300.pdf

The capacity of the load cell is 500 lbs. 

If any more information is needed, let me know and I will get you what you need.

Sincerely,

Yatsco

[Brandon_B]

Hello Yatsco,

Dan is out of the office but I would be more than happy to help you.  So I'm assuming that you have set your program up the way Dan mentioned in the early post.  If so, I have some questions about your application.

1. What is your sample rate and number of samples for your analog input task?
2. What type of custom scale are you using and what are your scaling coefficients?
3. How heavy of a load you are applying?
4. What voltage (excitation) are you using in the analog output task?
5. Are you experiencing the delay in signal response when not using the custom scale in your DAQ assistant?

If you don't mind posting your code I would be happy to take a look at it.

Regards,