Spider DSA User’s Manual
210
Gages measure changes on the micro strain level (micro inch/inch). Hence a very
sensitive resistance measurement tool is needed. That tool is the Wheatstone
Bridge.
Figure xx5: A basic Wheatstone Bridge circuit
The output voltage of the Wheatstone Bridge circuit is given by:
For the condition R
1
R
43
= R
2
R
4
, the output, V
Out
, is zero and the bridge is said to be
nulled, zeroed or balanced. If we replace Resistor R
1
with a strain gage and select
R
2
to be equal to the unstrained (or starting condition) resistance of the gage, the
bridge is nulled for any R
3
= R
4
value. Starting from this condition, the voltage
output of the bridge tracks any change in strain in accordance with:
Hence the bridge’s output is proportional to the driving voltage, V
in
, and
approximately proportional to the strain, ε. Since the gage factor, GF, is nominally
about 2 and ε is of the order 10
-3
(1000 microstrain) or less, the non-linear term is
ignorable.
Spider-80SG Wheatstone Bridge Hardware
The actual hardware in each Spider-80SG is somewhat more complex. Firstly, all
of the very high precision bridge-completion resistors may be switched into and
out of the circuit as needed.
Secondly, upon software command, a 100 kΩ precision resistor may be switched
into the circuit as a calibration shunt. This resistor is applied to a balanced bridge
to simulate a known strain change in the gage. When 350 Ω gages are used, the
100 kΩ shunt reduces the arm’s resistance by 1.22 Ω, simulating a strain of -
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