Chapter 2
Using the Module
2-6
ni.com
Note
The ratio of the bridge output voltage and the excitation voltage is done internally
on the NI PXIe-4330/4331.
To convert module readings to strain use the following equation:
To compensate for lead resistance errors shunt calibration should be used.
Quarter-Bridge Type II
This section provides information for the quarter-bridge strain-gage configuration type II.
The quarter-bridge type II configuration measures either axial or bending strain. Figure 2-5
shows how to position a strain-gage resistor in an axial and bending configurations.
Figure 2-6 shows the quarter-bridge type II circuit wiring diagram.
Figure 2-5.
Quarter-Bridge Type II Measuring Axial and Bending Strain
A quarter-bridge type II has the following characteristics:
•
One active strain-gage element and one passive quarter-bridge element (dummy gage)
used for temperature compensation. The active element (R
4
) is mounted in the direction
of axial or bending strain. The dummy gage (R
3
) is mounted in close thermal contact with
the strain specimen but not bonded to the specimen, and is usually mounted
perpendicular to the principal axis of strain.
•
Completion resistors (R
1
and R
2
) provide half-bridge completion. These resistors are
provided by the NI PXIe-4330/4331 module.
•
Sensitivity ~ 0.5
μ
V/V per
με
, for GF = 2.0.
strain
ε
( )
4
V
r
–
GF
1 2
V
r
+
(
)
-------------------------------
=
(+ )
(+ )
Axial
Bending
R
4
R
4
R
3
R
3
