Measuring Strain

[JoseR94]

Hello all, 

I am am trying to take strain measurements for experiments I am running.  Currently I have my strain gauges hooked to a NI 9945 (quarter bridge 350 ohm) then that connects to at NI 9237, which is sitting in a cDAQ NI-9188.  It seems that I have everything set up properly but the results I am getting seem to be a bit large. In terms of the code I am using the DAQmx VIs and not DAQ assist.  How are these VIs converting Voltage into strain? Is there some where I can find the equation it uses? 

Thank you,

Jose 

[$agar]

*but the results I am getting seem to be a bit large*

What do you mean by large?

So currently you are not using any equation? 

From your question i understood that your device is communicating properly but final data is not correct. Am i right?

[JoseR94]

So members of my lab have used the same type of strain gauge using a different DAQ, and different code.  In their code they measure the out put voltage and then using the some equations calculate what the strain should be.  I told lab view to measure strain using the DAQmx AI Strain VI. I am getting values in the milli strain range while they are getting micro strain. Currently I have no equations hard coded, I assumed the VI would do that for me. Everything does seem to be communicating properly I am just questioning the results, so I wanted to understand how the VIs worked before I jump to conclusions.      

[Salazar.Slytherin]

Generally the strain would be coming in as a ratiometric measurement with the voltage returned from the sensor being divided by the excitation voltage that's been set.

[Yamaeda]

Have you set up the Strain Gauge correctly?

/Y

[JoseR94]

So are you saying that the output of the VI is simply Vo/Vex? 

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[JoseR94]

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@Yamaeda wrote:

Have you set up the Strain Gauge correctly?

/Y

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I believe I coded it correctly.  The only difference seems to be that I am using a quarter bridge set up and you have a Full Bridge.  See Attached

[Ben]

When dealing with a new type of Acq. I will often start by using MAX to create a task fo the type I need and then "poke at the settings" until I get a good set of readings.

Then save the task and then got to the BD of a new VI, drop a DAQmx Task constant on the diagram and THEN right-click and select "generate config and example".

LV will script-up the code to do what you saw when poking around in MAX.

The other thing to investigate is if you can do a "null'.

Ben

[Yamaeda]

On the strain gage I used there was calibration values printed on it that I needed to enter/use in the code, the default didn't cut it.

/Y

[JoseR94]

I emailed engineers at National Instruments apparently it may be a hardware issue, although I am still waiting a response.  I did find a similar VI, Convert Volts to Strain, that provides the proper equation.  If I can't find a solution I may switch to this VI.