Signal Conditioning
Gauge Factor of strain gauges in a Wheatstone bridge
If you have strain gauges with a Gauge Factor of 2.07, and these are arranged in a Wheatstone bridge configuration, what would the resulting Gauge Factor be? Can it be extrapolated like parallel/series resistors?
We are getting low readings from our SCXI-1520/PCI-6221, such as 33% low, and I was wondering if it's due to having 2.07 as a Gauge Factor while reading a bridge.
@Josh B wrote:
Hi Broken Arrow,
Typically the strain gauges themselves are based on the concept of a Wheatstone Bridge and come in quarter, half, or full-bridge configurations. The configuration of quarter, half, or full-bridge is based on the number of elements in the bridge. The following developer zone discusses this topic in more detail.
http://zone.ni.com/devzone/cda/tut/p/id/4172
Do you have four resistors that you are setting up in a full bridge configuration? If this is the case what type of full-bridge are you configuring? If you go to page 2-6 of the SCXI-1520 user manual you will find information on the different types of full-bridges and how to wire them to the SCXI-1520.
http://www.ni.com/pdf/manuals/372583e.pdf
Another question I have for you is what excitation voltage are you supplying to the strain gauge?
Regards,
Josh B
Thanks Josh. I have since discovered that LabVIEW is smart enough to know what "happens" to Gauge Factor at the various settings. In other words, set to Full Bridge, the software/hardware knows that the gain is a parallel/series combination of gains. At least that's what my further testing has shown.
My excitation voltage varies with application. 1 volt to 10 volt.
Hi Josh,
Well, if you really want to know the back story: We have a sensor with 4 active gauges in it. I have ran it as Full Bridge III and have gotten very stable, temperature-compensated data from it. However, the mechanical engineers deduced that the output was low. If I set it to Quarter Bridge II, I get the output that they seem happy with - the numbers match an input that they supply (which is only applied in one axis, hence, Quarter bridge makes sense). However, I am not convinced that the real world loads occur in one axis. This is when I started thinking about the Gauge Factor, but have sense scratched that idea.
Even though my numbers are "good" now, I am concerned about temperature effects and side loading of the sensor when set to Quarter Bridge. I am going to monitor ambient temperature and correlate sensor drift that way.
So, yes, the low signals are resolved, for now.
Since you're probably wondering, I perform Offset Bridge Nulling but not Shunt Calibration because our cables are very short. I use the 100Hz hardware filter. 1000S/s.
