VXI VT1422A, User Manual

Page: 92 / 529

90 Programming the VT1422A & VT1529A/B for Remote Strain Measurement Chapter 3
Verifying Correct Bridge Completion (Shunt Cal)
Verifying bridge configurations and connections is accomplished by
inserting a known resistance (shunt cal resistor) in parallel with one leg of
the bridge to imbalance it by a predictable amount. The VT1529A/B
provides a single, internal 50 k
Ω
shunt cal resistor that can be
programmatically connected to each of the 32 channels, one channel at a
time. The VT1529A/B also provides the same connection capability for an
optional external user supplied shunt cal resistor. The user’s shunt cal
resistor can be connected via the front panel "Shunt Cal Resistor" terminal
block. See Figure 3-19.
For the following discussion, refer to Figure 3-10 through Figure 3-12.
The OUTPut:SHUNt:SOURce INT | EXT,(@<
ch_list >) command selects
either the INTernal (built-in) or EXTernal (user supplied) shunt cal resistor.
Use the OUTput:SHUNt ON | OFF,(@<
ch_list >) command to actually
connect the shunt cal resistor to the bridge to be tested. For OUTP:SHUN,
<
ch_list > may specify only a single channel on any one VT1529A/B. This
is because a single resistor is used to shunt each of a VT1529A/B’s 32
channels. When the command is sent to connect another channel, the
previously closed channel is opened.
To perform shunt cal on multiple channels, the program will have to enter a
loop to connect the shunt cal resistor to sequential channels and read the
result from the shunted channel. Generally, only OUTP:SHUN OFF needs
to be sent to open the last channel closed on a particular VT1529A/B. For
quarter bridge completion, the shunt cal resistor is connected locally
(on-board the VT1529A/B). For both half and full bridge completion, the
shunt cal resistor is connected remotely via the -Real and +Real terminals.
The switches that route Real are automatically controlled by the bridge
configuration commands [SENSe:]FUNCtion:Q120,
[SENSe:]FUNCtion:Q350, [SENSe:]FUNCtion:USER,
[SENSe:]FUNCtion:HBEN, [SENSe:]FUNCtion:FBEN, and
[SENSe:]STRain:BRIDge[:TYPE].
See Figure 3-20 for a general shunt cal programming sequence. A C++
example source file (
shuntcal.cpp ) is available in the VXI plug&play help
file and on the CD supplied with the instrument. View the readme.txt file
provided with the VXI
plug&play driver for example program file location.
Figure 3-19. User Shunt Cal Resistor Connection
Shunt
Cal
Resistor