VXI VT1422A - Manual (Page 81)

Programming the VT1422A & VT1529A/B for Remote Strain Measurement 79 Chapter 3
Measure Strain Using
Built-in Strain EU
Conversion
This method lets the VT1422A convert the strain bridge readings to units of
strain (
∈) before they are stored in the CVT and/or FIFO or accessed by
algorithms. There is no speed penalty and there is significant convenience in
allowing the VT1422A to make the Engineering Unit conversion to strain.
In fact, this is considered the "normal" VT1422A measurement method.
When the command SENSe:FUNC:STRain:<bridge_type> is sent, the specified
bridge type is configured by switches in each VT1529A/B, the channel
inputs are connected to the bridge outputs (see Figure 3-10 through
Figure 3-12 starting on page 70) and when the INIT command is sent, bridge
voltage readings are automatically converted to strain before being stored
into the FIFO buffer and/or CVT (current value table).
Before the VT1422A can convert a channel’s bridge output voltage reading
to strain, the gage factor, the excitation voltage and the unstrained reference
voltage for that channel must be known.
The user provides the above information to the VT1422A and below are the
methods/commands to do so:
1. The gage factor default is 2.00 for each channel. To change any
channel’s gage factor value, use the SENSe:STRain:GFACtor
command.
2. The unstrained reference voltage default value is 0.0 on each channel.
There are two ways to change any channel’s value.
a. Use the MEAS:VOLTage:UNSTrained? query (recommended)
which will take an average of 32 voltage readings on each
specified channel and save the values internally for use later by the
strain EU conversion process. When using this method, loaded
algorithms are not executed to avoid putting extraneous readings
into the FIFO buffer. The voltage readings are also sent to the
FIFO buffer for review.
b. Measure the voltage directly using the following series of
commands:
ROUTe:SEQ:DEFine (input the list of channels to measure)
SENSe:FUNC:VOLT (set measurement to voltage)
INIT (take the measurement)
SENS:DATA:FIFO? (read the data)
Next, the unstrained voltage values read above must be sent back
to the VT1422A’s EU conversion routine by using the command:
SENS:STRain:UNST <voltage value>,channel list