VTI Instruments EX1629, Руководство пользователя

Страница: 24 / 346

VTI Instruments Corp.
24 EX1629 Introduction
The advantage to specifying in this way is that it provides maximum values that correspond to real
world conditions. For in a production environment, it is impractical to allow, for example, only a
±1 ºC temperature swing. Similarly, since many strain tests can be very long in duration, it is
impractical to demand a daily self-calibration or excitation voltage measurement.
The disadvantage, however, is that these assumptions result in measurement uncertainties that are
overstated for test sequences that are significantly shorter in duration and subject to less
environmental movement. However, since the gain and offset temperature coefficients are
provided, the resultant performance improvements can be interpolated.
Wideband Outputs
The offset accuracy of the wideband outputs is provided referred to input (RTI). This is the
accuracy of the calculated input signal. When considered in its raw form at the connector, namely
referred to output (RTO), the listed accuracies must be multiplied by the gain of the range used.
The bandwidth specifications for each range were generated for an output sine wave signal of 2 V
peak-to-peak. Very large signals (as a percentage of range) will encounter slew rate limiting and
have a lower effective bandwidth.
Confidence Measurements
There are two specifications listed for excitation voltage measurement. The ±excitation voltage
accuracy refers to the uncertainty of each excitation source measurement as an independent
measurement. Conversely, the
total excitation voltage accuracy refers to the uncertainty of the
combined excitation source measurement, specifically the difference between the +excitation
voltage and the –excitation voltage. This quantity has a lower uncertainty, because, as a difference
measurement, it is subject to fewer error sources. While these performance characteristics are
provided, they should not be added to the listed quarter-bridge and full-bridge accuracy tables, as
these accuracy tables already contain the effects of the excitation voltage measurement.
The excitation current measurements are defined such that current flowing out of the source is
positive current and current flowing into the source is negative current. Consequently, the
+excitation current quantity is nominally a positive number, and the
–excitation current
quantity
is nominally a negative number. Moreover, the current measurements are defined to be the total
current of the source, not just that flowing in the external bridge. As shown in Figure 2-3, there is
a 20 kΩ resistance, represented by the back-half resistors, that is always connected between the
excitation sources. As a result, nonzero excitation source values will generate nonzero excitation
currents even if the input channel is open. For example, a total excitation voltage of 5 V on an
unpopulated channel will nominally display excitation currents of ±250 µA.
M
AXIMIZING
M
EASUREMENT
P
ERFORMANCE
This section discusses tips and procedures that can help maximize the actual performance realized
with the EX1629 and aid the user in avoiding some common pitfalls associated with strain gage
measurement.
Utilize self-calibration
Self-Calibration should be conducted as often as practical, especially if the ambient environment
has changed significantly since the previous calibration. However, fast ambient environmental
changes should ideally be followed by a period of thermal stabilization before conducting self-
calibration. The self-calibration process completes quickly and does not require removal of the
actual input connections, making it convenient to run often.