How do I Measure the Dynamic Resistance of the Strain Gauge?Hardware: SCXI>>Modules>>SCXI-1520
Problem: I have a strain gauge setup and I would like to measure the dynamic resistance of my strain gauge resistor. Is there any way in which I can do that in LabVIEW? Solution: Strain gauges are used to measure strain on a system and they use metals whose resistances change depending upon the strain applied externally. Hence, in order to compute dynamic resistance of a strain gauge, we need to delve into the physics of the situation. A strain gauge measurement is usually achieved by constructing a Wheatstone Bridge. Hence, we could have a Quarter Bridge (just one Strain Gauge), a Half Bridge (two Strain Gauges) or a Full Bridge Configuration (four Strain Gauges). There are variations in these configurations that would be dependent on which direction you want to measure strain in. When you begin the experiment, the strain gauge is balanced and outputs a zero voltage -- under this condition, the resistances on the opposite arms of a Wheatstone's bridge are equal to each other and the resulting voltage is zero. However, when stress is applied to the strain gauge, it undergoes strain and this in turn changes its resistance resulting in an output voltage. The equation that is used to compute this voltage is :
R1, R2, R3 and R4 are the static resistances of the bridge and for our purpose, we will assume that R4 is the static strain gauge resistance. Vex is the excitation voltage that is supplied to the Wheatstone Bridge and Vo is the output voltage that we measure from either LabVIEW or any other program. If R1/R2 = R3/R4, then Vo is zero but if one of the resistances changes then we would get a non-zero voltage. That being the case, we can now solve the equation for R4, as R4 is the resistance that changes. Rearranging this equation to compute R4, we get:
The solution to this equation will give the dynamic resistance of the strain gauge for a given output voltage. In order to use this in LabVIEW, one can construct this function and include it in the same while loop that is used to read the voltage from the strain gauge and it should compute the dynamic resistance as you read in the strain. Related Links: Developer Zone Tutorial: Measuring Strain with Strain Gauges Attachments:  
Report Date: 09/15/2004 Last Updated: 07/27/2007 Document ID: 3DEC7H3F |
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