Civil Engineering Reference
In-Depth Information
a
b
8
0.6
Edge
Pattern
Edge
Pattern
Laser
0.4
6
0.2
4
0
2
-0.2
0
10
15
20
10
15
20
25
time [s]
time [s]
Fig. 9.4
Displacements measured at point 7 (
a
) and discrepancies between camera and laser (
b
)
0.1
Edge
Pattern
Laser
0.08
0.06
0.04
0.02
0
2
4
6
8
10
12
freq [Hz]
Fig. 9.5
Moduli of the spectra of the bridge sag computed at point 7: test 1
appreciated. The green line identifies the laser displacement measurement which can the considered the reference for the
camera qualification. The red and blue lines represents the displacements estimated with the edge detection and pattern
matching techniques respectively. The figure shows a good correspondence among data obtained with the three techniques.
A direct comparison in the time domain between the displacement measured by the camera (the edge and the pattern
analysis) and the reference (the laser) is given in Fig.
9.4b
, where the maximum discrepancy is around the absolute value of
0.4 mm (this is the method uncertainty, as data are the same).
In Fig.
9.5
the moduli of the spectra (1-12 Hz) of the bridge sag computed starting from camera based measurements are
compared with the one of the reference interferometer. Both edge detection and pattern matching are able to provide a
reliable estimation of the dynamic displacement of the structure: the peak frequencies are correctly identified and their
amplitudes are properly estimated. An overall slight overestimation of the structure vibrations can be notice in all the
harmonic components characterized by low levels of vibration: this is a consequence of the lower signal to noise level of the
camera based measurements with respect to the reference interferometer.
The same evaluations have been made for the other tests, but a direct comparison with the reference signal is possible
only for target #7 (Fig.
9.2
) because no reference transducers are available for the other tested bridge sections. However, it
should be noted that the camera is mounted with the optical axis almost normal to the bridge, therefore, in each test, the
scaling factor is nearly the same for all the targets; thanks to this consideration we can assume that the validation for the
measurement at point #7 is valid also for the other targets. In order to give a complete description of what can be measured
with a single video, all the estimated displacements of test 5 are shown together in Fig.
9.6
. In this case data are obtained by
the pattern matching algorithm applied to all the five targets. The results give a description of the bridge sag at different
positions along the deck, and data are consistent with simple models of a beam supported at both ends, not being possible a
direct check with a reference measurements for all points.
