Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-6
ni.com
unchanged. The voltage drop across the lead resistance on a quarter-bridge
type I configuration is uncompensated in hardware. It is important to
accurately determine the gauge lead resistance and enter it in MAX or in
the application software equation so the software can compensate for the
voltage drop.
You can neglect lead resistance (
R
L
) of the wiring if shunt calibration is
performed or if lead length is very short (
∼
<10 ft), depending on the wire
gauge. For example, 10 ft of 24-AWG copper wire has a lead resistance
of 0.25
Ω
.
Quarter-Bridge Type II
This section provides information about the quarter-bridge strain-gauge
configuration type II. The quarter-bridge type II measures either axial or
bending strain. Figure 4-4 shows how to position a strain-gauge resistor in
an axial and bending configurations. Figure 4-5 shows the quarter-bridge
type II circuit wiring diagram.
Figure 4-4.
Quarter-Bridge Type II Measuring Axial and Bending Strain
A quarter-bridge type II has the following characteristics:
•
One active strain-gauge element and one passive, temperature-sensing
quarter-bridge element (dummy gauge). The active element is
mounted in the direction of axial or bending strain. The dummy gauge
is mounted in close thermal contact with the strain specimen, but not
bonded to the specimen, and is usually mounted transverse
(perpendicular) to the principle axis of strain.
•
Often confused with the half-bridge type I configuration. There is a
key difference between the quarter-bridge type II and half-bridge
type I configurations. In the half-bridge type I configuration, the R
3
element is active and bonded to the strain specimen to measure the
effect of Poisson’s ratio. In the quarter-bridge type II configuration, the
R
3
element is not necessarily bonded to the surface, but is in close
thermal contact with the specimen or with another piece of the same
material at the same temperature.
R
4
(+ )
R
3
(+ )
Axial
Bending
R
3
R
4
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