User manual

Chapter 5 Transducer Conversion Functions
© National Instruments Corporation 5-5 NI-DAQ User Manual for PC Compatibles
The
Strain_Convert
routine has two remaining parameters—
strainVolts is the voltage that NI-DAQ read from the strain gauge,
and strainVal is the return strain value.
The
Strain_Buf_Convert
routine has three remaining
parameters—numPts is the number of voltage points to convert,
strainVoltBuf is the array that contains the voltages that NI-DAQ read
from the strain gauge, and strainValBuf is the return array that contains
the strain values.
Using This Function
The conversion formula used is based solely on the bridge configuration.
Figure 5-1 shows the seven bridge configurations supported and the
corresponding formulas. For all bridge configurations, NI-DAQ uses
the following formula to obtain V
r
:
V
r
=(strainVolts – V
init
)/Vex
In the circuit diagrams shown in Figure 5-1, V
out
is the voltage you measure
and pass to the
Strain_Convert
function as the strainVolts parameter.
In the quarter-bridge and half-bridge configurations, R
1
and R
2
are dummy
resistors that are not directly incorporated into the conversion formula.
The SCXI-1121 and SCXI-1122 modules provide R
1
and R
2
for a
bridge-completion network, if needed. Refer to your Getting Started
with SCXI manual for more information on bridge-completion networks
and voltage excitation.