Specifications

Calculating Vr Residing in the instrument firmware are the equations used to calculate the
measured strain (Table 4-1 on page 52). One parameter common to each
equation is Vr. Vr is the difference in ratios of the Wheatstone Bridge
output voltage (Vout) to the bridge excitation voltage (Vs), under strained
and unstrained conditions. In equation form, Vr is represented as:
Vr = [(Vout/Vs)strained - (Vout/Vs)unstrained]
The strain measurement procedure involves measuring the voltages which
determine Vr. Once Vr is known, the firmware calculates the value of strain.
When
CALibration:STRain (@channel_list) is executed, the multimeter
measures the unstrained bridge output voltage (Vout) and the bridge
excitation voltage (Vs), and computes the unstrained reference (Vout/Vs)
for each channel.
When
MEASure:STRain:strain_function? (@channel list) is executed, the
multimeter measures the strained bridge output voltage (Vout) and the
bridge excitation voltage (Vs). The ratio of Vout/Vs(strained) is calculated
for each channel and is used with the channel’s unstrained reference to
determine Vr.
Note Vs unstrained and Vs strained is measured one time per channel list. If the
channel list includes more than one multiplexer, Vs is measured one time on
each multiplexer.
Downloaded
Unstrained
References
The strain gage multiplexers have the capability of measuring the unstrained
reference voltages, storing the references in a computer, and then
downloading the references at the time the strain measurements are made.
This feature is useful in applications where repeated strain measurements
are made once a single unstrained reference has been measured. The
commands to measure, store, and download the unstrained reference are:
CALibration:STRain (@channel_list)
(measures the unstrained reference)
STRain:UNSTrained? (@channel_list)
(queries the references and returns them to the output buffer from where
they are entered into the computer)
STRain:UNSTrained reference,(@channel_list)
(downloads the unstrained references)
An example of how unstrained references are downloaded is found on page 46.
Chapter 4 Understanding the Strain Gage Multiplexers 51
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