Full-Bridge Circuit

When feasible, use of the full-bridge circuit offers several advantages, including a better signal-to-noise ratio. Typical applications are: beams in bending, shafts in torsion, and axially loaded columns and tension links. Although the simple examples described here do not incorporate the circuit refinements characteristic of commercial transducers, it is common practice to infer the magnitudes of mechanical variables such as bending moment, torque, and force from the full-bridge strain measurements.

Bending & Torsion
The circuits shown below (for bending and torsion) have essentially the same characteristics, and can be grouped together for shunt-calibration purposes.

Bending beam, with Poisson gage orientation: .

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Bending or torsion: = 4.

In each of these circuits, the bridge output voltage varies linearly with strain, since equal and opposite resistance changes occur in arms 1 and 2, and in arms 3 and 4. The nonlinearity of shunt calibration must be accounted for, however, to achieve exact strain stimulation at large strains. The proper calibration resistor to simulate a given surface strain (e.g., the longitudinal strain, in the case of the beam) can be calculated from the following:

     (514.29)

Once calibrated according to Eq. (514.29), an accurate instrument will register the correct strain at any other strain magnitude. As in the case of the half-bridge circuit, the leadwire resistance is present in the calibration relationship, but does not need to be known with high precision.

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