Chapter 4
Theory of Operation
©
National Instruments Corporation
4-3
Software Scaling and Equations
Once you have acquired the voltage signal
V
CH
, you can scale this voltage
to the appropriate strain units in software. This is done automatically for
you in NI-DAQmx using a strain task or strain channel. You can also scale
the voltages manually in your application using the voltage to strain
conversion equations provided in this document for each configuration
type.
Finally, voltage to strain conversion functions are included in LabVIEW
and NI-DAQmx. In LabVIEW, the conversion function, Convert Strain
Gauge Reading.vi, is in the
Data Acquisition»Signal Conditioning
subpalette. The prototypes for the NI-DAQ functions,
Strain_Convert
and
Strain_Buf_Convert
, are in the header file
convert.h
for C/C++,
and
convert.bas
for Visual Basic.
The names of the strain-gauge types in these sections directly correspond
to bridge selections in MAX and the LabVIEW Convert Strain Gauge
Reading VI.
Quarter-Bridge Type I
This section provides information about the quarter-bridge strain-gauge
configuration type I. The quarter-bridge type I measures either axial or
bending strain. Figure 4-2 shows how to position a strain-gauge resistor in
an axial and bending configurations. Figure 4-3 shows the quarter-bridge
type I circuit wiring diagram.
Figure 4-2.
Quarter-Bridge Type I Measuring Axial and Bending Strain
A quarter-bridge type I has the following characteristics:
•
A single active strain-gauge element is mounted in the principle
direction of axial or bending strain.
•
A passive quarter-bridge completion resistor (dummy resistor) is
required in addition to half-bridge completion.
R
4
(+ )
Axial
Bending
R
4
(+ )