Master Thesis - Fachbereich Informatik

134 CHAPTER 6. CONCLUSION
or bending) have a certain influence on the measuring precision. However, the average
precision is still < 0.1mm for real tubes. In general, tubes of 8mm diameter have been
measured more precisely than 6mm or 12mm tubes.
The average accuracy (root mean square error) of the automated measurements, i.e.
the distance to some ground truth reference, is about 0.1mm for black tubes and about
0.2mm for transparent tubes at velocities of 30m/min. The ground truth has been acquired
manually under ideal laboratory conditions and has also a certain inter and intra human
deviation of about 0.1mm. While the velocity has only a minor influence on the accuracy of
black tubes, the accuracy of transparent tubes decreases significantly with higher velocities.
The main reason for this observation is the decreasing number of per tube measurements,
sinceaveragingoverthesinglemeasurementsgetsmoresensitivetooutliers.Inaddition,
the probability increases that a transparent tube is not detected at all if the background
contrast is poor. However, in general, the accuracy and precision has been good enough in
all experiments to reliably detected both black and transparent tubes of different length
and diameter with respect to the specified tolerances. Experiments with transparent tubes
of manipulated lengths have shown the system is able to separate the good ones from the
tubes that do not meet the tolerances successfully. The false negative rate, i.e. the number
of tubes that have been sorted out wrongly, is 2.6 .Lessthan4.3 of failures could
pass the measuring area. However, 80% of the false positives have not been detected at all
by the system. With the adaptation of the blow out strategy as suggested in Section 5.4
these tubes would have been blown out, too. Hence, the theoretically remaining false
positive rate is 0.87 for transparent tubes. Following the experimental results one can
assume that the false positive rate for black tubes will be less or equal.
The measuring results have a positive side effect, since it is possible to compute the
moving average over the last N measurements. An operator can compare the current
mean length to the given target length. This can be useful especially during the teachin
of the machine. At production, deviations can be corrected before the tolerances are
exceeded. In a more sophisticated solution the adjustment could be automated. If one can
assure the current mean length measured by the vision system equals the target length,
the blow out mechanism may never need to be activated and the probability for false
positives can be further decreased.
In addition, the system is able to store the inspection results in a file or database. Such
statistics can be also useful for the management or controlling since they include not only
the length distribution of the production, but also information about the total number of
tubesproduced,thetimeofproduction,aswellasthenumberofdefectives.
The good results of the prototype support the use of an optical inspection system for
length measurements of heat shrink tubes. Manual sample inspections as used currently at
production are influenced by many factors like concentration, speed, motivation, or tiredness
of the individual operator. In general, less precision can be assumed for measurements
at production compared to ideal laboratory measurements as used for evaluating the system.
The advantage of the automated vision-based system is the ability to inspect each
tube at laboratory precision without getting tired.