Measurement Computing WaveBook rev.3.0 - Manual (Page 57)

WaveBook User’s Manual, ch03B 6-23-99 WBK Expansion Options, WBK16 3-25
Bridge Applications
WBK16 can accommodate many different strain-gage configurations. All strain-gage bridge configurations
consist of a 4-element network of resistors. The quarter, half or full designation of a strain gage refers to
how many elements in the bridge are strain-variable. A quarter-bridge has 1 strain-variable element; a half-
bridge has 2 strain-variable elements; and a full-bridge has 4 strain-variable elements.
Full-bridges generally have the highest output and best performance. Output signal polarity is determined
by whether the strain-variable resistance increases or decreases with load, where it is located in the bridge,
and how the amplifier inputs connect to it. Configuration polarity is not important in WBK16, due to an
internal software-selected inversion stage. This simplifies bridge configuration.
Each WBK16 channel has locations for five bridge-completion resistors. These BCR’s are for use with
quarter and half-bridge strain gages. The resistors make up the fixed values necessary to complete the
4-element bridge design.
A full-bridge gage requires no internal completion resistors, but they may still be installed for other
configurations in use. The additional resistors will be ignored when the software has selected full-bridge
mode. Both quarter- and half-bridge gages require an internal half-bridge consisting of header positions Rg
and Rh. The recommended minimum values are 0.1%, <5 PPM/
°C drift, 1 KΩ, and 0.25-watt resistors.
Lower values will dissipate more power and add heat. Values >1K Ω will increase the amount of drift and
noise. The same value half-bridge resistors can be used for any resistance strain gage. This internal half-
bridge will be automatically selected by the software when needed.
Internal 1 M Ω Ω Ω Ω shunt resistors are used to avoid open circuits.
These resistors are not suitable for high-accuracy/low-noise applications.
A quarter-bridge gage additionally requires a resistor of equal value to
itself. Up to 3 different values may be installed simultaneously in header
positions Ra, Rc, Re. All of these resistors are connected to the (-)
excitation terminal. An external jumper at the input connector determines
which resistor is utilized. Therefore, 3 different quarter-bridge values can
be supported without opening the enclosure. Each different value bridge
would simply have the jumper in a different location; when the gage is
plugged in, the proper resistor is then already selected. Configurations
with the completion resistor on the (+) excitation are redundant, due to
the internal inversion stage, and not used.
Kelvin-Type Excitation Leads
The bridge-configuration figures in the following text show various strain-gage configurations divided into
4 groups: Full-bridge, half-bridge, quarter-bridge, and high-gain voltmeter. Many of these configurations
can coexist but are shown individually for clarity.
Excitation Connection
Remote sense inputs are provided for the excitation regulators. The excitation voltage will be most accurate
at points where remote sense lines are connected—preferably at the bridge (this is often referred to as a 6-
wire connection). Long cables will reduce the voltage at the bridge, due to current flow and wire resistance,
if remote sense is not used. If the 6-wire approach is not used, the remote sense inputs must be jumpered to
the excitation outputs at the input connector. Internal 1 M
Ω resistors are also connected where the jumpers
would be located to prevent circuit discontinuities. These 1 M Ω resistors are not suitable for high-accuracy
excitation-voltage regulation. 3-wire quarter-bridge configurations do not benefit from external remote
sense connections—the lead resistance is actually a balanced part of the bridge. If the + remote sense input
is connected to the + input on a quarter-bridge, the voltage is regulated across the bridge completion
resistor. This results in a constant-current linearized quarter-bridge; otherwise, quarter-bridges are not
perfectly linear.