VXI VT1422A - Manual (Page 170)

168 Advanced Programming with the VT1529B Chapter 5
Alternate Method of
Computing Strain
As stated above, to use the additional features of the VT1529B, the EU
conversions are normally done in the VXIplug&play driver, not by the
on-board DSP. For channels programmed using the new capability, all
measurement data in the FIFO, CVT, and algorithms is voltage, not strain.
For full- and half-bridge strain channels, there is an alternate method of
doing the EU conversions if having the strain engineering units in the
algorithms is needed while still using the most recent value of the excitation
voltage for more accurate data. This method involves making strain
measurements using the DSP while also scanning the excitation voltage.
Because the full- and half-bridge strain conversions are linear with respect
to the excitation voltage, an algorithm can be created that uses the ratio of
the initial and current excitation voltages to correct the DSP-computed value
to get a more accurate value.
Note
This method cannot be used for quarter-bridge strain channels because the
EU conversion for quarter bridge is non-linear with respect to the excitation
voltage. If a quarter-bridge strain measurements is required in an algorithm,
the full EU conversion must be done in the algorithm.
Here are the steps to use this method:
1. Configure the channels as one would for the VT1529A. The following
simplified command sequence assumes that the strain channels are 10000
through 10002 and that the common excitation voltage for these channels is
monitored on channel 10006. It also assumes that the excitation voltage
supply is short-term stable, so that the supply variation is negligible between
the time the first and last channels are measured.
MEAS:VOLT:EXC? (@10000:10002)
Measure excitation on strain
channels - the FIFO data needs
to read and saved for later
MEAS:VOLT:UNST? (@10000:10002)
Measure unstrained voltages
SENS:STR:GFAC 2.2,(@10000:10002)
Specify gage factor
SENS:FUNC:STR:FBEN (@10000:10002)
Setup for full-bridge strain on
channels (DSP will do EU)
2. To have only the corrected value appear in the FIFO and CVT, disable
sending the uncorrected values to the FIFO and CVT using the ROUT:DEF
command:
ROUT:DEF (@(0(10000:10002),3(10006)
Disable sending of uncorrected
strain data to FIFO and CVT;
enable sending to both for
excitation data
Alternatively, simply omit the channels for the uncorrected values from the
scan list – the VT1422A driver will automatically add them to the scan list
because they appear in the algorithm and won’t send them to the FIFO or
CVT.