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?
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.
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.
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'.
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.
