Master Thesis - Fachbereich Informatik

132 CHAPTER 5. RESULTS AND EVALUATION
The blow out mechanism was tested successfully in the prototype setup. The advantage
of this mechanism is that it works almost independent of the conveyor velocity and the
position of the light barrier relative to the measuring area. One has to assure only that
no tube passes the light barrier before the good/bad decision of the measuring system
reaches the blow out controller.
One drawback of the current strategy is the sensitivity to ghosts. If the system detects
a tube where actually no tube is, the resulting classification of the ghost is send to the
controller anyhow and stored in the FIFO memory. Since a ghost is never detected by
the light barrier, the good/bad decision of the ghost is still in the memory when the next
tube passes the light barrier. Instead of considering the decision belonging to this tube
(appended to the FIFO memory) the decision of the ghost is evaluated. This leads to a
loss of synchronization, i.e. a tube T is related to the decision of tube T − 1. Over time
this effect can increase and the reliability of the system is obviously violated.
A potential solution of this problem can be achieved by replacing the FIFO memory by
a single register that is able to store only the latest decision. Without loss of generality
a0inthisregistermightcorrespondtoblowingoutthenexttubewhilea1indicatesthe
next tube can pass. The register is set to 0 by default. Each time the inspection system
measures a tube to be within the allowed tolerances a signal is send to the controller that
sets the bit in the register to 1. As soon as the tube has passed the light barrier, the
register is reset to 0. This has to be done before the next tube is measured. Therefore
the light barrier has to be placed quite close to the measuring area. The advantage of this
approach is that the memory contains always the current decision belonging to the tube
that passes the light barrier next. A timer can be used to reset the register if no tube
intersects the light barrier within the expected time. Thus, ghosts become uncritical.
Furthermore, since the register is reset each time, this helps also to prevent the problemsofnondetectedtubes,i.e.
tubesthathavepassedthevisualfieldofthecamera
without being measured. In the outlier experiment (see Section 5.3.6) the false positive
rate increased drastically if tubes could not be detected. In this case the system does not
send a good/bad decision for the missed tube to the controller. The light barrier, however,
detects every tube independent of being measured or not. With the single register strategy
these tubes are blown out by default. Thus, only tubes that have been measured by the
system and meet the allowed tolerances are able to pass the blow out nozzle.
If tubes are not detected at all or measurements do not result in a meaningful length
value (e.g. the standard deviation of the single measurements is too large), the corresponding
tubes define another group U including all unsure measurements that can not
definitely be assigned to G ′ 0 , G′ −,orG ′ +. All tubes of this class should be blown out by
default to ensure no outlier can pass the quality control. These tubes do not have to
be considered as rejections, but could be measured by hand afterward or recirculated to
be inspected again by the vision-based measuring system depending on the frequency of
occurrence.
The experiments have shown that more than 80% of the total processing time is needed
for the template based edge localization. In the current implementation the left and
right ROI are processed sequential. One possible optimization could be to parallelize this
problem. This means, the computation within the left and right ROI could be performed
in separate threads to exploit the power of curret dual core architectures. This is possible,
since the processing in the two ROIs is independent of each other.