Campbell Scientific CDM-VW300 Series Dynamic Vibrating-Wire Analyzer System - Manual (Page 73)

Appendix A. Measurement Theory
A.1 Dynamic Vibrating-Wire Measurements
The key components of standard, single-coil vibrating-wire sensors are 1) a
taut wire suspended between two anchor points, and 2) an electromagnetic coil
positioned at the center of the wire. The coil serves two functions: first as an
actuator to put energy into the wire and, second, as a pickup to detect the
motion of the wire. Static vibrating-wire measurements are typically
performed in a two-step process of exciting the wire and then measuring the
wire response for a period of time to determine its resonant frequency. The
excitation waveform must be spectrally broad, such as a swept-frequency sine
wave, so as to provide sufficient energy at the unknown resonant frequency of
the wire. After the excitation, the resonant motion of the wire is sampled, the
frequency is determined, and then the resonant energy dissipates prior to
making the next measurement. The time required for these steps typically
limits this measurement method to rates slower than 1 Hz.
Dynamic measurements compress the measurement cycle and achieve much
higher sample rates by eliminating the broadband excitation and not allowing
the wire oscillation to decay. FIGURE A-1, Timing of dynamic vibrating-wire
measurements illustrates the timing of this process. If energy can be injected
into the oscillation at precisely the right frequency and with the correct phase,
then a very short excitation waveform with a small amplitude can maintain the
resonance of the wire. If the excitation is not phase-aligned to the wire motion,
then the resonance will be dampened rather than reinforced. In between these
excitation windows, the wire motion can be sampled and the resonant
frequency determined. Using the newly calculated frequency, the excitation is
adjusted slightly as needed to track the changes in the resonant frequency of
the wire. The excitation mechanism has very fine frequency resolution to
precisely track even very subtle changes in the resonant frequency. As can be
seen in FIGURE A-1, there are usually only a few oscillation cycles of the wire
available for determining the wire resonant frequency. This difficulty is
overcome by the spectral analysis algorithm, which can determine the
frequency of the wire precisely, even with this short data sample.
A-1