Master Thesis - Fachbereich Informatik

Abstract
Heat shrink tubing is widely used in electrical and mechanical applications for insulating
and protecting cable splices. Especially in the automotive supply industry accuracy
demands are very high and quality assurance is an important factor in establishing and
maintaining customer relationships. In production, the heat shrink tubes are cut into
lengths (between 20 and 100mm) from a continuous tube. During this process, however,
deviations from the target length can occur.
In this thesis, a prototype of a vision-based length measuring sensor for a range of heat
shrink tubes is presented. The measuring is performed on a conveyor belt in real-time at
velocities of up to 40m/min. The tubes can differ in color, diameter and length.
In a multi-measurement strategy, the total length of each tube is computed based on up
to 11 single measurements while the tube is in the visual field of the camera. Tubes that
do not meet the allowed tolerances between ±0.5mm and ±1mm depending on the target
length are sorted out by air-pressure. Both the engineering and the software development
are part of this thesis work.
About 70% of all manufactured tubes are transparent, i.e. they show a poor contrast
to the background. Thus, sophisticated but fast algorithms are developed which reliably
detect even low contrast tube edges under the presence of background clutter (e.g. belt
texture or dirt) with subpixel accuracy. For this purpose, special tube edge templates are
defined and combined with model knowledge about the inspected objects. In addition,
perspective and lens specific distortions have to be compensated.
An easy to operate calibration and teach-in step has been investigated which is importanttobeabletoproducedifferenttubetypesatthesameproductionlineinshort
intervals.
The prototype system has been tested in extensive experiments at varying velocities
and for different tube diameters and lengths. The measuring precision of non deformed
tubes can reach 0.03mm at a conveyor velocity of 30m/min. Even with elliptical deformations
of the cross-section or deflections it is still possible to achieve an average precision
of < 0.1mm. The results have been compared to manually acquired ground truth measurements,
which also show a standard deviation of about 0.1mm under ideal laboratory
conditions. Finally, a 100% control during production is possible with this system - reaching
the same accuracy and precision than humans without getting tired.
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