How do scale I from strain units to mV in MAX?

Question

I am trying to setup a strain channel with 1/4-bridge configuration in Measurement and Automation Explorer. I want the channel output to be in Volts (or mV), not in strain units. I am going to try to setup a custom scale to convert from strain units to volts with this equation:
Vo = Vex (GF*strain/4*(1/1+GF*strain/2))
Will this equation provide the correct results in Volts?
I know I could also try to measure the strain gauge signal through a "custom voltage with excitation" channel in MAX in order to obtain the desired voltage output, but this option does not provide offset nulling and shunt calibration like it does in the strain channel option (as far as I can tell). Therefore I would rather use the strain channel function in MAX instead of the voltage option.
Any comments or suggestions are greatly appreciated.
Thank you.

Answer 1

Hi Mclarge,
I think there is a couple ways to do this. A straight forward conversion of this would be to use the following formula (derived from an older T-DAQ VI that took voltage and converted it to strain).
Where S=strain value, Gfactor=gauge factor, Rl=line resistance, Rg=gauge resistance, Vsig=the signal from the strain gauge, Vo=the voltage from the strain gauge with no load, and Vex=the excitation voltage applied to the gauge.
The way that I would recommend is to yes, set up a custom voltage with excitation task and add the null offset and shunt calibration vis. Note that the shunt calibration vi is newly introduced with DAQmx 8.7 and the most recent version is DAQmx 8.7.1. This is available for free at ni.com/downloads. Please refer to the following article for information regarding load cells that could very easily be adapted to strain gauges.
Load Cell Data Acquisition with Null Offset and Shunt Calibration
http://zone.ni.com/devzone/cda/epd/p/id/5902
Message Edited by PBear on 05-02-2008 09:59 AM
PBear
NI RF
Attachments:
Strain to Voltage.gif ‏3 KB