Wednesday, August 18, 2010

Wheatstone Bridge Based Measurements with myDAQ and LabVIEW

This document explains using Wheatstone Bridge to make a precise measurement for sensors such as strain gauges and load cells. The data will be acquired using the DAQ Assistant that is installed into LabVIEW with the NI DAQmx driver as a voltage and converted to a strain, force, or other engineering unit using basic programming in LabVIEW.

The user interface we created shows the Voltage Drop, VO, the measured Excitation Voltage, Vex, and the ratio of the two.  
Figure 3: LabVIEW Front Panel

In LabVIEW we need to measure the voltage drop across the terminals, as well as the excitation voltage applied to the bridge. The voltage drop is then divided by the excitation voltage to find the ratio between the two.  This value is very useful in bridge-based measurements and more logic can be built in to tailor the application as needed.  Finally, we will output the results to numeric indicators on the front panel.

Figure 4: Coding Block Diagram

The LabVIEW block diagram looks very similar to the coding block diagram
Figure 5: LabVIEW 2009 Block Diagram
(The attached LabVIEW code snippet can be dragged-and-dropped to a LabVIEW block diagram, use attached PNG file.  After locating the PNG file, just drag the file icon onto a blank block diagram, as if you were dragging the file onto your desktop.)

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