Master Thesis - Fachbereich Informatik

6. Conclusion
In this thesis a functioning prototype for a vision-based heat shrink tube measuring system
has been presented allowing for an 100% online inspection in real-time. Extensive experiments
have shown the accuracy and precision of the developed system which is reaching
the quality of accurate human measurements under ideal laboratory conditions. The advantage
of the developed system is that this accuracy can be achieved even at conveyor
velocities of up to 40m/min.
A multi-measurement approach has been investigated in which each decision whether
a tube has to be sorted out is based on 2-11 single measurements depending on the tube
type and conveyor velocity. This requires video frame rates of ≥ 50fps to be processed
in real-time. Fast algorithms, heuristics and model knowledge are used to improve the
performance in this constrained application. Tube edge specific templates have been defined
that are able to locate a tube edge with subpixel accuracy even in low contrast
images under the presence of background clutter. In the prototype setup, the tube edge
detection has been complicated by the strong vertical structure of the conveyor belt and
an inhomogeneous translucency leading to non uniform bright background regions. The
consequences for transparent tubes have been discussed including the possibility of tubes
that can pass the visual field of the camera without being detected.
Since black tubes are not translucent, they yield an optimal contrast to the background
with a back lighting setup. On the other hand, transparent tubes are much more sensitive
to the structure of the background and the local tube edge contrast. All parameters
adjusted for transparent tubes turned out to have no disadvantage for black ones. Thus,
the parameters for transparent tubes are used in general, leading to a more uniform
solution in the system design.
Beside the algorithmic part of the work the engineering of the whole system including
the proper selection of a camera, optical system, and illumination has been solved. The
integration of the micro controller and the air blow nozzle completes the prototype, allowing
for concrete demonstrations of how tubes that do not meet the tolerances are blown
out.
A simple and intuitive initialization of the system has been developed. Most parameters
can be trained interactively and automated without complicated user interactions. Even
an unskilled worker should be able to perform the teach-in step after a few instructions.
The only critical part of the teach-in is the camera positioning. To exclude as many sources
of error the camera should be mounted as stable as possible at fix orientation (which has
to be calibrated only once). The required height adjustments to cover the range of tube
lengths should be automated if possible.
The maximum measuring precision of 0.03mm was reached for a metallic tube model
simulating an ideal tube (at a conveyor velocity of 30m/min). During the experiments
it has been observed that deformations of real heat shrink tubes (elliptical cross-section
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