Campbell Scientific CDM-VW300 Series Dynamic Vibrating-Wire Analyzer System - Manual (Page 16)

CDM-VW300 Series Dynamic Vibrating-Wire Analyzers
Vibrating-Wire
Sensors
CDM-
VW300
PC
LoggerNet
CR3000
Datalogger
SC-CPI
FIGURE 4-6. Field-mode data-acquisition system diagram
4.6 Data Uses
Users should consult authoritative sources concerning the use of vibrating-wire
data in structural analysis applications. Following is a short introduction to the
use of data made available by CDM-VW300 series analyzers.
4.6.1 Static Measurements
Each sensor is measured for a static frequency once per second. The static
frequency is used to obtain a result with finer spectral bin resolution than that
which can be achieved at the dynamic measurement rates. This measurement
can be helpful in detecting the rare case, for example, in which a noise
frequency very near the resonant frequency affects dynamic sampling
measurements.
4.6.2 Dynamic Measurements
Each analyzer can be configured in a 20, 50, 100, 200, or 333.3 Hz sampling
mode. By capturing multiple readings each second from a vibrating-wire
sensor, rapid changes or events can be captured and evaluated. Due to the
Nyquist sampling theorem, only responses equal to or less than half of the
sampling frequency can be detected and characterized. Event-based data
capture techniques programmed into the controlling datalogger can be helpful
to ensure useful data are captured under the right event conditions. In this way,
storage and retrieval of less-useful data are avoided.
4.6.3 Thermistor Measurements
Each vibrating-wire sensor input channel has inputs for the thermistor that is
usually built into a vibrating-wire sensor. The temperature measurement is
normally used to correct the frequency measurement for temperature changes.
A 24-bit measurement circuit ensures high-accuracy resistance measurements,
which are converted to temperature. Guidance about the impact of temperature
on the vibrating wire is usually provided by the manufacturer in documentation
provided with each individual sensor. Contact the sensor manufacturer if more
information is required.
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