Master Thesis - Fachbereich Informatik

118 CHAPTER 5. RESULTS AND EVALUATION
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(a) black, 6mm diameter
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Figure 5.13: Ground truth distance in mm of all measured black tubes with a diameter of
6 and 12mm at 30m/min.
the experiments. Although it is less likely, it is also possible that a tube lies on the side
with the smaller contact area. This happens if the tube is leaned against a guide bar for
example. The result are measuring points above the measuring plane leading to a larger
length in the image.
Figure 5.12(b) shows a section of 21 black tubes with a diameter of 12mm measured at
30m/min. The larger distance to the ground truth data is clearly visible. However, the
system is again able to reliably detect all tubes correctly within the tolerances without
any false negatives (ΩFN = 0). As an example of how the deformation of a tube influences
the measuring results, images of the 7th and the 11th tube 2 of this sequence are shown
in Figure 5.14(b) and (c) respectively. The extension in vertical direction of tube No. 7
is definitely smaller than for the neighboring tubes. This is an indicator that the tube is
deformed and lies on the smaller side, thus, it is measured larger than it actually is. On
the other hand, tube No. 11 is larger in the vertical extension indicating it is lying on
the larger contact area. The result is a much shorter length measured by the vision-based
system which can be also seen in Figure 5.13(b). Like for 6mm tubes, the measurements are
mostly shorter compared to the ground truth, although the origin is different as introduced
above.
These results show the accuracy limits of the weak-perspective model. If higher accuracy
is needed, a telecentric lens could be used to overcome the perspective effects of different
depths, or the height of a tube in the image could be exploited to adapt the calibration
factor fpix2mm dynamically.
Transparent Tubes The experiments with different diameters have been repeated with
transparent tubes. Only 92% of all transparent tubes with a diameter of 6mm are detected
and measured by the system in this experiment. This is mainly due to the nonuniform
translucency of the conveyor belt. Especially the thin 6mm tubes are very sensitive to
2 Note: The tube number does not correspond to the (single) measurement number. The dashed lines
indicate which measurements belong to the same tube.