Structural Health Monitoring Using Smart Sensors - ideals ...

3
Imote2_2/Imote2_1
Imote2_1/Siglab_1
Siglab2/Siglab1
3
Imote2_2/Imote2_1
Imote2_1/Siglab_1
Siglab2/Siglab1
Magnitude of transfer function
2
1
Magnitude of transfer function
2
1
0
0 20 40 60 80 100 120 140
Frequency (Hz)
3
(a) longitudinal
Imote2_2/Imote2_1
Imote2_1/Siglab_1
Siglab2/Siglab1
0
0 20 40 60 80 100 120 140
Frequency (Hz)
(b) transverse
Magnitude of transfer function
2
1
0
0 20 40 60 80 100 120 140
Frequency (Hz)
(c) vertical
Figure 7.17. Magnitude of transfer function among Imote2s and reference sensors.
that signals from the same data acquisition system are close to each other, but signals from
the Imote2 and the Siglab spectrum analyzer have noticeable differences. The transfer
functions for the longitudinal acceleration fluctuate considerably, indicating that the four
signals have discrepancies. In terms of the transfer functions, the longitudinal acceleration
measurements seem to be noisy, though the problem is not only specific to Imote2s but
also for the PCB accelerometers.
Imote2 and PCB accelerometer signals are also compared in terms of coherence (see
Figure 7.18). While the transfer function magnitude can detect frequency-dependent
amplification factors in two signals, the coherence function indicates the degree of
linearity between two signals. Signals that are linearly related result in a coherence
function that is one over all frequencies. Small values in the coherence function, on the
other hand, suggest nonlinearity or noisy measurements in the corresponding frequency
range. Vertical and transverse accelerations show coherence functions close to one. The
gradual drop of the coherence function between an Imote2 and PCB accelerometer signals
may indicate imperfect time synchronization between the two signals. Coherence
functions for the longitudinal acceleration are small in some frequency ranges. The
116