Measurement Computing StrainBook/616 - Manual (Page 25)

StrainBook/616 967794 Operation Reference 2-11
(4) Know an unbalanced bridge when you configure one.
Theoretically, a strain gage bridge is balanced with zero output until strain is applied producing an output
voltage linearly proportional to the strain. In the real world the bridge is slightly unbalanced due to
component tolerances. There are two approaches that allow accurate strain measurements with the slightly
unbalanced bridge: (1) balance the bridge, or (2) compensate for the error with correction factors. If you
do neither, the bridge will provide erroneous results.
(5) Take it easy on the excitation regulators.
The excitation outputs of the StrainBook and WBK16 expansion module each will deliver up to 85 mA
without any degradation in output voltage. If this level of current is exceeded the voltage is reduced to
protect the regulator.
It is important to consider the current drawn by the internal reference node resistors. These resistors are
never switched off. They continue to load the excitation regulator no matter what bridge configuration is
chosen. If these resistors are 120 ohm resistors [which they never really need to be] they will be drawing
41.7 milliamps at 10 volts. An external full bridge of 120 ohm resistors will require an additional
83.3 milliamps. This will overload the regulator, resulting in both a reduced excitation level and an
incorrect signal level.
The best choice for the reference node resistors is 1000 ohms. There are two reasons: (1) The parts will
draw less excitation current, thus helping the regulators to stay cooler. (2) The lower degree of self-heating
will result in less drift.
(6) Provide adequate input power to each StrainBook and to each WBK16 expansion
module in the system.
Providing the proper level of input supply voltage is very important. Insufficient input voltage can cause
the StrainBook or WBK16 expansion module to exhibit channel-to-channel excitation interaction. All
individual channels can be set properly and then begin to lose voltage as additional channels are connected.
It is imperative that the unit not be “starved” for input voltage. This can easily happen if more than one
WBK16 is powered from a TR-40U power adapter. It could also happen if an inadequate power source
was being used for the StrainBook or WBK16.
A StrainBook or WBK16, each can require as much as 25 watts of input power if configured for eight
channels of 120 ohm bridges at 10 volts of excitation. The 15V, 3.33A (50 watts) output of the TR-40U is
not sufficient for two WBK16 modules. A variation of this problem can occur if a group of WBK16
modules is daisy-chained together with an insufficient wire size feeding the group. Voltage drop in the
wiring can also starve the modules to a greater degree as distance from the source increases. It is strongly
recommended that individual WBK16 units each operate from the TR-40U provided or from an individual
power lead from an adequately sized source such as a large battery or power supply.
A StrainBook and a WBK16 [together] may be powered from a single TR-40U [15 V, 3.33A] by:
(a) connecting the TR-40U to the WBK16 DIN5 POWER IN connector and then, (b) connecting a CA-115
cable from the WBK16 DIN5 POWER OUT connector to the StrainBook DIN5’s POWER IN connector.
This is advantageous in that it reduces incursion of common mode noise [resulting from power supplies].
(7) Handle channel configuration headers carefully.
The 16-position, machined-pin IC sockets [into which the bridge completion headers are inserted] have
demonstrated a tendency to become unreliable if the headers are rocked sideways to remove them, or if
resistors with larger leads than those we supply are plugged directly into them. The unreliability manifests
itself with widely fluctuating readings, especially if touched, or if the unit is subjected to shock or
vibration. Cold solder joints on the headers will exhibit similar symptoms.