How to Get Strain Gage Voltage Output

[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.  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.

[OKors]

SE will output a strain value if you set up the device as Strain and calibrate using the Strain Gage Wizard. Sounds like you need to set up your load cell as a Custom Voltage with Excitation. This will output a voltage; however, if you want it "calibrated" you'll need a mV/V sensitivity or slope to determine the curve. If it's a manufactued load cell this should be included in the documentation.

However, you said you're trying "make and calibrate" a load cell. Is this in-house built? I'm pretty familiar with custom built load cells so if you describe your setup and need further guidance let me know.

[jgh@AET]

Thank you very much for the reply!  I have a fixed pin inside of a fixture that I want to mount strain gages to in a full bridge configuration.  I then will apply a known load (with hydraulic test machine) and I want to just know what the voltage output is so that I can get a load vs. output voltage curve.  I'm not concerned about what the strain is, I just want to know for a given exitation voltage (I think the NI9219 is 2.5V for full bridge) and load, what the output voltage is.  Also, I'll have to look into the Custom Voltage with Excitation you mentioned - I'm not familiar with how to do that.

Thanks again!

[OKors]

I work with custom load cells all the time and we either calibrate the part in a load frame or in a cal fixture out on the bedplate (all hydraulic controlled). Normally we calibrate our in-house load cells as force vs strain so we can calculate a shunt value. Sounds like you're looking for a mV/V sensitivity.

Custom Voltage with Excitation is pretty straight forward just define your Input Range, enter the Excitation voltage (in your case it looks like you’re limited to 2.5v), Bridge Type = Full Bridge, no scaling necessary since you’re acquiring voltage.

From there you’ll want to add your load cell signal to SE. Normally, for our testing purposes, we condition the mfg load cell within the hydraulic controller which outputs a +/-10V signal equal to the full scale range of the transducer (i.e. 10v = 5kip, -10v = -5kip). That being said, I’ll add a Voltage channel based on the controllers output and create a custom scale to define the load curve.

I don't want to get too detailed as to our process because it may be different than yours and I don't want to add any confusion. As mentioned, we normally treat our in-house load cells as a full-bridge device since its strain we're after, but you'll want Custom Voltage with Excitation. How you condition your commercial load cell may be different also.

If you do need more details or further clarification just let me know.

[jgh@AET]

Thanks a lot.  Yes, i am looking for a mV/V sensitivity.  We will not use these internally, but it sounds like my setup is the same as yours with the +/-10V output on the hydraulic load frame.  I really appreciate your help!

[OKors · Accepted solution]

Then it's pretty simple.Two channels:

Ch01-  DAQmx Acquire>Analog Signal> choose Voltage: enter the actuator load cell's min/max range in the Input Range of the Voltage setup (+/- 5k for instance), then create and apply a Custom Scale (choose Map Ranges) where +/- 10V = full scale range of the commercial load cell. Once applied "Scaled Units" should automatically update to show the custom units you defined in in the scale.

Ch02- Custom Voltage with Excitation: Signal Input range (can probably leave default), Scaled Units= Voltage, Ex Source=Internal, Ex Value= 2.5v, Use Ex for Scaling unchecked, Terminal Configuration dependant on setup, Custom Scaling= none.

Optional: I normally add an Amplitudes and Levels (Analysis> Time-Domain Measurements) step, drag/drop the "DC" signal of both channels into the graph, and create a Scalar XY graph so I can see my plot in real time.

Add a Save to ASCII step and save both signals to excel so you can plot your curve.