Convert Strain Gauge Reading VI Output Off by Half

[sethmurphy]

Hello,

I am working on writing code using the Scan Engine on my EtherCAT  cRIO. One of the measurements I need to take is strain, and when using the "Convert Strain Gauge Reading" VI, my value is always half of what it should be(i.e.. I simulate a strain of 100 ue, I read 50 ue). I have tried using the same card on a cDAQ in MAX and that worked fine. I have attached my code so that you can see what values I am using.

The only issue that I can think of is with the shunt resistor on the 9235. I know that when you calibrate the 9235, the default resistance is 100k, when it should be 50k. If this is not changed, your values will be off by half(much like I'm experiencing with the cRIO). I know that I'm not addressing the shunt resistor at all, so I'm a little confused.

Thank you for your time,

Seth

[David-A]

Hi Seth,

Would you be able to post the portion of your code that you use to setup the module on the cRIO?  I suspect that you've configured your module and scaling with different resistance values that may be causing you to return unexpected values.

David A

[David-A]

After digging a little deeper it appears that this module cannot have gage factor and gage resistance values specified in scan mode.  I would recommend taking a look at this Knowledge Base article to confirm that your Convert Strain vi is configured properly.

David A

[JAPI81]

Hello,

I already have had the same problem. And I think the solution is changing the Vexc input to value one. When the VI calculate the Vr is not necesary to be divided by Vexc. I have the NI9236 module with Vexc 3,3V and my error factor was about 3. In my example my value should be 300ue and I read 100ue. I had the Vexc input to value 3.3 and when I have changed by 1 the result is ok.

I hope the labview developer expert could confirm this issue.

[sethmurphy]

Japi81,

The reason my value was incorrect was that I was using the wrong VI. I was using 'Convert Strain Gauge Reading' when I actually needed convert ratio to microstrain. If you are using either a 9235 or 9236, your output is not going to be in strain, it's going to be in V/V. Try replacing the VI with that one to see if it resolves your issue.

You can find the example in: National Instruments\Labview X\Examples\Compact RIO\Module Specific\NI 9235

There is a sub-VI there which will correctly calculate your output.

-Seth