VTI Instruments EX1629, Руководство пользователя

Страница: 25 / 346

www.vtiinstruments.com
EX1629 Introduction 25
Utilize excitation measurement
Measurement accuracy is notably improved by utilizing the EX1629’s ability to measure its
excitation source and use the measurement in the EU conversion. By doing so, the set point
accuracy of the excitation source ceases to be an error source. The strain accuracy tables are based
on the assumption that excitation measurement is performed.
While a tightly regulated supply, the excitation source does, nonetheless, have a temperature drift
characteristic. For this reason, it is best to conduct the excitation source measurement just prior to
the initiation of strain measurements, making the source measurement as current as possible.
Utilize proper strain gage wiring techniques
In addition to the accuracy of the measuring instrument, the total system accuracy of the strain
measurement is a function of the gage characteristics and the connection wiring of the gage to the
measuring instrument. Nonideal wiring techniques can create measurement inaccuracies far above
those of the measuring instrument.
For half- and full-bridge configurations, it is highly recommended that the remote excitation sense
lines be used on the excitation source, as shown in Figure 2-4 and Figure 2-5, respectively. These
lines should be connected at the same point that the ±Excitation lines are connected to the bridge.
Ultimately, the excitation source regulates based on the voltage present on its sense lines. Without
remote sense, this regulation point is at the EX1629 input connector. This is a nonideal connection
because the lead wire resistance between the EX1629 and the bridge will create a voltage drop,
lowering the effective excitation value at the bridge. Remote sense inherently compensates for the
lead wire resistance and delivers the correct excitation value.
For quarter-bridge configurations, it is highly recommended that the full 3-wire connection be
used, as shown in Figure 2-3. Specifically, it is important that the –Sense line be connected at the
gage, instead of locally at the EX1629 input connector. The 3-wire connection reduces the total
lead wire resistance seen by the gage by putting half of it in series with the completion resistor.
Not only does this reduce the static lead wire desensitization error, but it also provides an inherent
level of temperature compensation. Specifically, since the same lead wire resistance is in the
active leg as well as the completion leg, any variation of the resistance due to temperature
naturally occurs in both legs and cancels.
Compensate for lead wire desensitization error
Even if the 3-wire connection is employed in quarter-bridge configuration, there may still be
significant error in the measurement, as there is lead wire resistance that does not move with the
underlying strain, but is indistinguishable from the actual gage resistance. This results in the
measured strain being systematically in error. Depending on the length and gauge of the wire
employed, this error may be much larger than the underlying instrument accuracy. Precision
measurements consequently demand that lead wire compensation is conducted to eliminate this
error. The error arises from the fact that the measuring instrument cannot distinguish between the
resistance of the lead wires and the resistance of the strain gage. Specifically, when the resistance
of the gage changes under load, the measuring instrument reads a strain value lower than the true
value, because part of the total resistance it considers to be the gage is not changing. The extent of
the desensitization error is dependent on the resistance values of the gage and the lead wire,
related by this equation:
lead gage
lead
R R
R
error