When to use offset null and shunt calibration for strain gages?

[nevmo12]

Hello,

I am measuring the strain on a PCB using gages. I am using the DAQ NI 9236 and LabView to acquire and view the data. I used an example program called "strain- contuinous input.vi". Both the shunt calibration and offset null are enabled

I noticed that when am running the program to measure strain I cannot stop it and then start the program and get the same strain measurements. If I restart the program then the output is back to zero even though I did not change my system.

If I turn off offset nulling than the program cannot perform shunt calibration because the gain adjustment is negative. How do I account for this so that if I stop the LabView program the actual strain is always being measured?

[Kodester]

Hi Nevmo,

1) How are you stopping the example?

2) So every time you close and open the VI, does it work the first time you run it?

3) Have you seen this community page? https://forums.ni.com/t5/NI-DAQmx-Examples/Continuous-Acquisition-Strain-Samples-with-Calibration-wi...

[nevmo12]

Hi Kodester,

1. I stop the VI but pressing the stop button in LabView.

2. I am trying to figure out a way of measuring strain from 4 gages while it is in a stress state. I am measuring strain in a PCB vs temperature and would like to measure each gage without having to redo the temperature soak. The first time I run the program is when the system is relaxed. After this the system is becoming stressed as temperature increases.

Is is possible to measure strain from a gage that is already stressed initially?

Thanks.

[nevmo12]

Also shunt calibration will not work unless I enable offset nulling. I get a negative gain correction factor if I disable offset nulling.

Any idea as to why this is the case?

[Kodester]

Hi Nevmo,

1. I didn't do a very good job of wording my first question. Are you using the stop button on the front panel of your VI, thus allowing the VI to correctly perform it's cleanup phase? Or are you hitting the abort button in the tools panel at the top of LabVIEW?

I believe your last question about shunt calibration is explained by the first note in this online example
http://www.ni.com/example/30816/en/

"Note:You should perform an offset null compensation just before you perform a shunt calibration. Performing a shunt calibration before an offset null compensation causes improper gain adjustment because the offset signal voltage is compensated multiple times."

[Kodester]

Nevmo,

It sounds like you aren't going to be able to do shunt calibration without offset nulling. The way I see it you have two options to go with from here.

1. Build a state machine in the example. Instead of running the VI multiple times, use a state machine to stop recording values but leave the VI on. This way, you can maintain the shunt calibration from your initial strain gage readings, and not experience this offset which makes your later readings invald

2. Try doing neither the offset null or calibration and seeing if the readings are within expected range the second time you run this VI.

[nevmo12]

Hi Kodester,

I am actually using the abort button. However the stop button appears to only stop the waveform from displaying strains but does not stop the acquisition.

Thanks

[Kodester]

Hi Nevmo,

Can you elaborate on what you mean by this? I tried the example myself, and after hitting the stop button the task performs it's clean up (stop & clear the task) and the VI completes.

Did you see my last message? For the functionality you desire you will need to change the example a bit

[nevmo12]

Disregard my last message. I now see that the Stop DAQmx task subVI is executed after the stop button is pressed. But the VI stops completely when you press this button.

I have seen your last comment and will work on making a idle state where the VI is running but there is no acquisition.

Thanks Kodester.

[nevmo12]

I got a task error when trying to stop the acquisition. A picture of my block diagram is below.

I used a event statement to start/stop the acquisition using a single button(stop 3). LabView somehow did not like that.