Master Thesis - Fachbereich Informatik

126 CHAPTER 5. RESULTS AND EVALUATION
Larger tubes can be covered by the 16mm focal length lens like 50mm tubes, but the
camera has to be placed further away from the conveyor to yield a larger field of view. The
resulting pixel representation, i.e. the length a pixel represents in the measuring plane,
increases as mentioned before. Hence, the precision decreases.
In each experiment a charge of 50 tubes (transparent and black) of 30mm and 70mm
length and 8mm diameter is used as test data. Each charge has been measured by hand and
is evaluated with respect to mean and standard deviation. Each tube passes the measuring
area once in this experiment and is measured as often as possible (single measurements)
while it is in the visual field of the camera. The mean over the computed total lengths
as well as the standard deviation are determined and compared to the ground truth data.
The results are summarized in Table 5.8 and visualized in Figure 5.21 in terms of Gaussian
distributions.
The number of per tube measurements ΩPTM of 30mm tubes is slightly smaller compared
to experiments with 50mm tubes at the same velocity. This is due to the smaller
field of view of the camera. Obviously the tubes leave the measuring area faster. However,
there are still more than 3 single measurements of each tube both for black and transparent
tubes on average. The larger 70mm tubes have been measured even more often
than 50mm tubes with 6.12 single measurements for black and 4.85 for transparent tubes
respectively. This can be explained by a larger field of view.
The mean value over a sequence of tubes µseq equals the expectation µGT in almost
all experiments. Only the 30mm transparent tubes differ from the ground truth of about
0.01mm which is acceptable small. This indicates the calibration factor between pixels
and mm has been trained perfectly in all experiments.
The standard deviation is much smaller for 30mm tubes both in the manual and automated
measurements compared to 70mm tubes. In general black tubes are measured with
higher precision than transparent tubes by the system according to the observations in
previous experiments. The higher precision for 30mm tubes is important with respect to
the specified tolerances (see Table 1.2). In all experiments beside the 70mm black tubes
the manual precision is only slightly better than the precision of the visual inspection
system. However, the results of the system have been always precise enough to allow for
reliable measurements in terms of the allowed tolerances. At 70mm black tubes the system
performed even better than humans with a standard deviation of 0.14 compared to 0.16
measured by hand.
It is important to state that the precision in these experiments depends both on the
measuring variance of the system and the real variance of the tubes. Accordingly one
can not compare the results directly with those in Section 5.3.5 where only one tube was
measured several times in one experiment.
One can conclude the visual inspection system is able to measure also tubes of different
lengths as accurate as humans on average.
5.3.8. Performance
Finally, the performance of the system is evaluated on an Athlon64 FX-55 (2.6GHz, 2GB
RAM) platform.
The total processing time can be divided into five main groups including profile analysis,
compensation for radial distortion, edge detection and template matching, as well as length