Civil Engineering Reference
In-Depth Information
camera sensor [
11
,
12
]. In order to provide a low uncertainty reference to these measurements, a laser interferometer
(resolution
m) has been placed at the bridge mid span (Fig.
9.1b
). The displacement measured by the interferometer
can be assumed equal to that of target 7 in Fig.
9.2
and it will be used to compute the measurement uncertainty associated to
the vision-based approach. By varying the portion of the bridge framed by the camera (i.e. the zoom level and the distance
between the camera and the structure), it has been possible to study the relationship between the scaling factor and the
obtained measurement uncertainty.
¼
2.56
μ
9.4 Experiments
In this section the results of the Erba bridge tests will be reported. The first series of tests aims at verifying the measurement
performance in frontal view, i.e. with the camera placed at a side of the bridge, with the optical axis normal to the train
direction and pointing several targets fixed onto the steel structure (Fig.
9.1c
). The second series of tests has been carried out
to verify the measurement performance in case a camera is being used to detect the bridge displacement without any target
fixed to the structure, relying on the structure natural texture. These tests cannot be considered a real and complete
innovation in case of controlled laboratory conditions: a smaller number of reported cases relates to field measurements.
It must be noted that these tests are being performed under uncontrolled environmental conditions and are applied to a civil
structure monitoring, where the image analysis must deal with the specific requirements of this practical application. In
comparison with other similar works about real structures [
8
,
9
], this one is focused on the uncertainty qualification of these
techniques applied under uncontrolled environmental conditions.
9.4.1 Frontal Measurements with Target Fixed to the Structure
These measurements have the purpose to qualify the results obtained by the image analysis when the camera is placed in
front of the bridge side. As explained in the previous section, several targets, one every 3.65 m, have been placed on the
structure (Fig.
9.3
). The tests specifications are summarized in Table
9.1
: for every test the value of the px/mm ratio is given
for the targets acquired by the camera. During test 1 the camera was near the structure and the zoom parameter has been set
to have only one single target enclosed in the field of view. Under these conditions the highest image resolution in terms of
px/mm ratio has been achieved, but for just one measurement point. All the further tests have been made by increasing the
number of acquired targets so that a wider description of the vertical displacement of the bridge deck could be obtained by a
single video. However, increasing the target number has the drawback of decreasing the image resolution in terms of the px/
mm ratio, which means to worsen the measurement accuracy. For this reason, the presented results will consider tests with
no more than five targets framed by the camera; further increase in the mm/px scaling leads to unacceptable measurements
uncertainty.
The aim of these tests was to evaluate how the accuracy of the estimated displacements is affected by the target number
and consequently by the image resolution. This evaluation has been carried out on a real big structure and with uncontrolled
environment conditions, which is the situation in which the whole process is expected to operate for structural monitoring
purposes. For every target the pattern matching technique is applied to the two blobs, whereas the edge detection is applied
to the tilt line and a mean value of its position at every frame is extracted (Fig.
9.3
). In this way it is possible to estimate the
target displacement as a function of time.
Figure
9.4a
shows the results obtained for test 1 (measurement point #7), where x axis is time and y axis gives the vertical
displacement at mid span. The bridge vertical displacement, due to the train passage, which is between 7 and 8 mm can be
Table 9.1
Frontal tests: framed targets and local scaling factor (px/mm)
Target
Test # (number of framed targets)
#3
#4
#5
#6
#7
1
1.561
2
0.354
0.337
3
0.195
0.191
0.187
4
0.132
0.133
0.133
0.133
5
0.107
0.109
0.110
0.110
0.108
