Master Thesis - Fachbereich Informatik

4. Length Measurement Approach
While the previous chapter focused on the hardware setup, this chapter will present the
methodical part of the system. After a brief overview, the different steps including the
camera calibration and teach-in step as well as the tube localization, measuring point
detection, tube tracking and the good/bad classification are introduced. All assumptions
and the model knowledge used throughout these steps are presented before.
4.1. System Overview
The fundamental concept of the developed system is a so called multi-image measuring
strategy. This means, the goal is to measure each tube not only once, but in as many
images as possible while it is in the visual field of the camera. The advantage of this
approach is that the decision whether a particular tube meets the length tolerances can
be made based on a set of measurements. The total length is computed by averaging over
these single measurements leading to more robust results. Furthermore, the system is less
sensitive to detection errors. Depending on the conveyor velocity and the tube length one
can reach between 2 and 10 measurements per tube.
The system is designed to work without any external trigger that provokes the camera
to grab a frame depending on a certain event, e.g. a tube passing a light barrier. Instead,
the camera is operated in continuous mode, i.e. images are captured at a constant frame
rate using an internal trigger. The absence of an external trigger, however, requires fast
algorithms to evaluate whether a frame is useful, i.e. whether a measurement is possible.
In addition, the system must be able to track a tube while it is in the visual field of the
camera to assign measurements to this particular tube. Accurate length measurements of
tubes require the very accurate detection of the tube edges. A template based tube edge
localization method has been developed allowing for reliable, subpixel accurate detection
resultsevenunderthepresenceoftubeedgelikebackgroundclutter. Oncethereisevidence
that a tube has left the visual field of the camera, all corresponding measurements have
to be evaluated with respect to the given target length and tolerances. The resulting
good/bad decision must be delegated to the external controller handling the air pressure
based blow out mechanism. Model knowledge regarding the inspected tubes under the
constrained conditions is exploited if possible to optimize the processing.
Before any measurements can be performed, the system has to be calibrated and trained
to the particular target length. This includes camera positioning, radial distortion compensation
and an online teach-in step.
Figure 4.1 gives an overview on the different stages of the system. It can also be seen
as outline of this section. The underlying methods and concepts will be introduced in the
following in more detail.
Throughout this chapter all parameters will be handled abstract. Corresponding value
assignments used in the experiments are given in Section 5.1.1.
51