The Chevron Bridge
When measuring strain with
1
⁄
4
and
1
⁄
2
bridge arrangements, the strain gage
accessories use a variation of the Wheatstone Bridge known as the Chevron
Bridge (Figure 4-6). The
1
⁄
4
bridge and
1
⁄
2
bridge arrangements still
complete the Wheatstone Bridge; however, the Chevron Bridge enables the
bridge excitation voltage (VS) and the precision internal half bridge
resistors (R1/R2) of the bridge completion circuitry to be shared among all
channels.
Because the multiplexers use an unbalanced bridge for strain measurements
(see “ Unbalanced Bridge Measurement Technique” on
page 50
), the
accuracy of the measurement depends on the resistance of the bridge arms
remaining constant. The resistance must remain constant for the unstrained
measurement and the strain measurement. The only resistance which
changes is the resistance of the active gage(s).
The Chevron Bridge ensures that the bridge arm resistance remains constant
by switching the outputs of the bridge completion channels (HI (high), LO
(low), G (guard)) to the input of the multimeter. By switching the outputs
rather than switching the bridge arms, the gages are "permanently" wired to
the bridge completion channels via the +E, -E1, and -E2 terminals. This
eliminates any resistance changes due to self heating and variations in
switch or relay contact resistance.
Note
G (guard) is not switched on the FET multiplexers.
MULTIMETER
(VIA ANALOG BUS)
Figure 4-6. The Chevron Bridge
56 Understanding the Strain Gage Multiplexers
Chapter 4
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