Master Thesis - Fachbereich Informatik

5.3. EXPERIMENTAL RESULTS 109
Detection rate
1.05
1
0.95
0.9
0.85
0.8
black
transparent
0.75
0 10 20 30
Tube spacing [mm]
40 50 60
Figure 5.5: Detection rate of black and transparent tubes depending on the spacing between
consecutive tubes.
Thus, one can conclude the system is able to detect the synthetic tube edges very accurate
even under the presence of noise if there is a sufficient contrast between background
and foreground.
5.3.2. Minimum Tube Spacing
10 black and 10 transparent tubes are used to investigate the influence of the spacing
on the detection rate. The tubes have been placed on the conveyor at an approximately
constant spacing. Five gap sizes are tested: 60, 30, 20, 10, and 5mm respectively. Each
load of tubes passes the measuring area five times for each gap size at a conveyor velocity
of 30m/min. In this experiment the total detection rate Ωtotal is considered only, i.e. how
many tubes are detected by the system at least once. The results are averaged over the 5
iterations.
As can be seen in Figure 5.5 the detection of black tubes is uncritical indicated by
Ωtotal = 1 until the tube spacing is less than 10mm. This means no black tube can pass
the measuring area without being measured if the spacing is ≥ 10mm. The decrease at
5mm gaps to Ωtotal =0.98 (i.e. 1 tube out of 50 is not detected) may be due to the fact
that the manual tube placing can not guarantee an exact spacing of 5mm. It is likely
that the distance between two tubes has become even smaller leading to the failure. Since
the tests have been performed online it is not possible to locate the origin of the outlier.
Therefore, it has been investigated how small the gap between two black tubes must be
until the profile analysis fails to locate the tube. The results are shown in Figure 5.6. Even
a spacing of about 2mm as in (a) is large enough to reliably detect the background regions
between the tubes as can be also seen at the corresponding profile analysis results in (c).
A gap of about 1mm, however, is too small even for black tubes. Due to perspective the
points closer to the camera merge (see Figure 5.6(b) and (d)).
Thetransparenttubesshowadetectionrateof< 1 even for the largest tested gap
size of 60. This can be explained by the much lower contrast to the background. If the
system must able to overcome a strong non-uniform background brightness, one has to
make a larger compromise in terms of detection sensitivity. As it turns out there is no
parameter setting that can guarantee that all tubes are detected independent of the gap