How to Capture NI9219 Full Bridge Voltage Output with Signal Express

[jgh@AET]

Hello.  I am trying to make and calibrate a load cell with full bridge strain gage setup.  I am using a NI9219 module with a cDAQ chassis.  Is there a way to capture the actual output voltage?  Signal Express gives me a strain value, but I really need to know the output voltage.  Where do I look.  I only need two channels for the full bridge.  I think could connect some wires to the remaining two channels and read the voltage output from the strain gages which would be connected as a voltage input to the 9219, but I would think Signal Express could give me the excitation voltage and output voltage directly.  Any input would be appreciated. Thanks!  P.S. I only use this equipment on occasion and am not the most familiar with it, so keep it simple for me.  Thanks again.

[BenDAE]

Hey there,

Here is an article that shows how to convert strain back into voltage using the guage factor (specific to the device). http://zone.ni.com/devzone/cda/tut/p/id/4172#toc7

If you are hoping to measure the voltage directly, the express VI could be set to voltage rather than strain. 

Thanks,

[Brad_K · Accepted solution]

Hi jgh@AET,

The NI 9219 measures the voltage ratio of full-bridge sensors in hardware, which allows any variation in the excitation voltage to cancel out. You won't be able to measure the sensor's output voltage independently of the excitation voltage without additional channels, but you can measure the raw voltage ratio using the Bridge (V/V) measurement type. You can also use the Force (Bridge) measurement type to perform a load cell measurement with appropriate engineering units (N, lb, kgf, not strain).

This screenshot shows where both Bridge (V/V) and Force (Bridge) can be selected in the DAQ Assistant:

These measurement types were added sometime around DAQmx 9.1, so if you have an old version of NI-DAQmx, your DAQ Assistant might not have them. The latest version is currently NI-DAQmx 9.4. Before NI-DAQmx 9.1, the recommend approach for measuring load cells was to use the Custom Voltage with Excitation measurement type and a custom scale. However, Custom Voltage with Excitation does not support bridge calibration in the DAQ Assistant.

Brad

[jgh@AET]

Thanks a lot guys.  I really appreciate the help!  I looked into it today and I think I can do the Custom voltage with excitation, at least it appeared to be an option.