Automating Manufacturing Systems - Process Control and ...

continuous sensors - 23.15
integrated to find velocity and acceleration.
Currently accelerometers cost hundreds or thousands per channel. But, advances in
micromachining are already beginning to provide integrated circuit accelerometers at a
low cost. Their current use is for airbag deployment systems in automobiles.
23.2.4 Forces and Moments
23.2.4.1 - Strain Gages
Strain gages measure strain in materials using the change in resistance of a wire.
The wire is glued to the surface of a part, so that it undergoes the same strain as the part (at
the mount point). Figure 23.16 shows the basic properties of the undeformed wire. Basically,
the resistance of the wire is a function of the resistivity, length, and cross sectional
area.
w
L
-
t
I
+
V
R
V
= -- = ρ L I Ā - = ρ -----
L wt
where,
R = resistance of wire
VI , = voltage and current
L = length ofwire
wt , = width and thickness
A = cross sectional area of conductor
ρ = resistivity of material
Figure 23.16
The Electrical Properties of a Wire