Campbell CR6 Series, Operator'S Manual

Page: 449 / 690

Section 8. Operation
449
FIGURE 99: VSPECT Vibrating Wire Measurement
Wiring
8.1.4.1.2 Static VSPECT Measurement Theory
The CR6 uses an audio A-to-D
(p. 573)
converter to capture vibrating wire signals on
U terminals. The graph in the figure
Unconditioned Time Domain Data
(p. 450)
shows raw time series data recorded after the vibrating wire sensor was excited
with a swept frequency. Figure VSPECT Data
(p. 450)
shows the same data after an
FFT
(p. 583)
has been applied.
Each measurement acquires 4096 raw points. These data include all frequencies
vibrating the sensor wire. The FFT creates a frequency spectrum of these points.
The frequency spectrum shows each frequency with voltage amplitude. The
dominant frequency with the highest amplitude is the natural-resonant frequency.
Noise frequencies may be sourced from harmonics of the natural frequency,
electronic noise, or harmonics of the electronic noise. For example, 50 Hz or 60
Hz noise from ac mains grid power
(p. 590),
and associated harmonics, are common
noise sources. Noise frequencies may also originate from mechanical obstruction
of the taut wire, such as may be caused by a loose wire or when the wire vibration
is physically changed by sensor movement. VSPECT makes possible the
separation of the natural-resonant frequency from these other frequencies.
Note The FFT algorithm requires time for computation. Compile or
download errors will occur if the CRBasic program does not allow two
seconds for the measurement of each sensor.