NI WSN-3214 User Guide and Specifications
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Figure 26.
Shunt Calibration Circuitry
Shunt calibration involves simulating the input of strain by changing the resistance of an arm in the
bridge by some known amount. This is accomplished by shunting, or connecting, a large resistor of
known value across one arm of the bridge, creating a known strain-induced change in resistance. You
can then measure the output of the bridge and compare it to the expected voltage value. You can use the
results to correct gain errors in the entire measurement path or to verify general operation to gain
confidence in the setup.
Note
The NI WSN-3214 does not support shunt calibration for the half-bridge and full-bridge
measurement types. The NI-WSN 3214 supports shunt calibration for quarter-bridge only.
Offset Nulling
For each new installed bridge-sensor, the bridge does not output exactly 0 mV/V when not under load.
Slight variations in resistance among the bridges legs generate some nonzero initial offset voltage. To
apply software compensation for the bridge, perform an offset nulling calibration.
The NI WSN-3214 measures the bridge while not under load, and then uses this measurement as the
initial bridge voltage when scaling readings from the bridge. This method is simple, fast, and requires
no manual adjustments. The disadvantage of the software compensation method (in contrast to hardware
compensation) is that software compensation does not physically remove the offset of the bridge. If the
offset is large enough, it limits the amplifier gain you can apply to the output voltage, thus limiting the
dynamic range of the measurement.
Noise Considerations
Using the NI WSN-3214 in noisy environments creates additional noise in the measurements.
One common scenario is to observe the 50 Hz or 60 Hz noise in the spectrum of your measurement.
This can be caused by a nearby device that is supplied from the mains. The WSN-3214 is capable of
filtering the 50 Hz and 60 Hz in this situation by selecting the filter acquisition modes in NI-WSN or in
NI LabVIEW WSN.
National Instruments recommends using shielded cabling to lessen the susceptibility to noise. For
full/half-bridge measurements, use a shielded, twisted pair cable. Pair EX +/– and RS +/– together.
When using full-bridge, pair IN +/–.
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