Automating Manufacturing Systems - Process Control and ...

continuous sensors - 23.17
Aside: Changes in strain gauge resistance are typically small (large values would require
strains that would cause the gauges to plastically deform). As a result, Wheatstone
bridges are used to amplify the small change. In this circuit the variable resistor R2
would be tuned until Vo = 0V. Then the resistance of the strain gage can be calculated
using the given equation.
V+
R
R 2
R 1
strain
= ------------- when Vo = 0V
R 3
R2
R1
R4
-
+
Vo
Rstrain
R3
R5
Figure 23.18
Measuring Strain with a Wheatstone Bridge
A strain gage must be small for accurate readings, so the wire is actually wound in
a uniaxial or rosette pattern, as shown in Figure 23.19. When using uniaxial gages the
direction is important, it must be placed in the direction of the normal stress. (Note: the
gages cannot read shear stress.) Rosette gages are less sensitive to direction, and if a shear
force is present the gage will measure the resulting normal force at 45 degrees. These
gauges are sold on thin films that are glued to the surface of a part. The process of mounting
strain gages involves surface cleaning. application of adhesives, and soldering leads to
the strain gages.