Appendix A. VSPECT
measurement theory
In a vibrating wire sensor, the coil serves the following functions:
l
Actuator to put energy into the wire
l
Pickup to detect the motion of the wire
The resonant frequency of a vibrating wire cannot be measured at static rates faster than about 1
Hz. A static measurement excites the wire and measures the response. Excitation is a swept
frequency sine wave that supplies a broad spectrum of frequencies to the wire. If you listen
closely to most vibrating wire sensors, during a static measurement you can hear tones ascending
through the swept frequency excitation. Time is then required for the non-resonant frequencies
to die out, and then more time is needed to sample the resonant frequency. Additional time is
required for the resonant frequency to die out before the next excitation/measurement cycle
begins.
In contrast, the VWIRE 305 makes dynamic measurements at rates as high as 333.3 Hz. The key
difference in the dynamic method versus the static method is that the wire is maintained in a
continuously vibrating state. The dynamic process eliminates the swept frequency sine wave.
Instead, the VWIRE 305 injects energy into the oscillation at just the right frequency and phase,
so a very short excitation with small amplitude maintains wire resonance. Phase alignment of
excitation strengthens wire resonance and decreases measurement noise.
The system synchronizes all measurement channels to the data logger clock through the CRBasic
program and CPI bus. Synchronization has the added benefit of allowing for simultaneous
sampling of channels, so there is good measurement correlation across multiple sensors and
multiple modules. Synchronized excitation is a key difference between dynamic VSPECTTM
measurements and other auto-resonant excitation schemes.
To improve dynamic measurements, the VWIRE 305 measures a static frequency at 1 Hz and uses
it as a reference in noise rejection. Since the VWIRE 305 samples only a few cycles of the wire
each time it makes a dynamic measurement, the spectral resolution is coarser than when using all
captured data. As a result, occasional noise frequencies are gathered into the same FFT (Fast
Fourier Transform) bin as those of the signal, which causes measurement error. The VWIRE 305
measures the static frequency at a finer resolution to detect the noise and prompt you to take
corrective action.
VWIRE 305
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