Master Thesis - Fachbereich Informatik

5. Results and Evaluation
5.1. Experimental Design
There are several parameters influencing the measuring results both in the hardware setup
and in the vision algorithms. To yield meaningful results, it is important to vary not more
than one parameter within the same experiment. In the following the parameters that are
tested as well as the evaluation criteria and the strategies used are proposed.
5.1.1. Parameters
The different parameters of the system can be grouped into four main categories including
tube, conveyor, camera and software respectively. Table 5.1 summarizes the most
important representatives of each category.
Obviously, there are much more parameters which have been described in the previous
chapter that theoretically fall in the last category. However, most of these parameters
do not have to be changed (e.g. the number of profile scanlines or the local ROI width).
The corresponding value assignments have been determined empirically at representative
sequences and are summarized in Table 5.2.
αpeak =4.0 has been determined in a teach-in step as proposed in Section 4.7.2 and
yields best results for transparent tubes with the conveyor belt and the illumination used.
This assignment does also cover black tubes, although the threshold could be much larger
in that case. As long as the conveyor belt is not changed and the amount of dirt on
the conveyor does not change significantly, the detection sensitivity does not have to be
re-initialized each time.
A timeout period of ∆t = 5 frames for the tube tracking (see Section 4.6.3) has been
used throughout the experiments, which is a good compromise between the number of
expected per tube measurements and the distance to the light barrier.
Approximately 1/4 of all measurements (rounded to the next integer value) are not
considered for the total length computation with αoutlier =0.25 to eliminate outliers in
the single measurements as introduced in Section 4.6.4. The same value is used for the
outlier filter in the teach-in step (see Section 4.7.4)
The teach-in of the calibration factor fpix2mm (see Section 4.7.4) terminates if the root
mean square error does not change for more than ɛ =0.0001 between two iterations.
Since it is still very complex to test all permutations and assignments of the remaining
parameters, one has to make compromises in the experimental design. Therefore, some
of the parameters listed above have been adjusted before the experiments to meet the
assumptions made in Section 4.2. This includes the guide bar distance as well as the
illumination (fiber optical back light setup through the conveyor belt) and all camera
parameters, i.e. lens, working distance, exposure time and F-number. For all experiments
with 50mm tubes a 16mm focal length lens at a working distance of approximately 250mm
is used. The shutter time has been adjusted to 1.024ms which is a good compromise
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