Master Thesis - Hochschule Bonn-Rhein-Sieg

5. Algorithms Master Thesis Björn Ostermann page 93 of 126
From the intrusion map, shown in Figure 65a, the robot area is excluded.
In Figure 65b the algorithm calculates a line from the centre of the robot’s base position to the first
pixel on the border of the intrusion map. Along this line every pixel that is crossed by the line is
checked for intrusion. If a pixel containing an intrusion is found, as well as when the border-pixel is
reached, the algorithm stores that pixel as an unreachable pixel and the next border-pixel is tried to be
reached (Figure 65c and d – stopped at the blue pixels).
The algorithm checks each border-pixel once, until all border-pixels are checked, as shown in Figure
65e.
From the remembered pixels in Figure 65e, the border of the reachable space is generated (Figure 65f)
by drawing a closed line (borderline) through the remembered pixels. This borderline marks the first
pixels that can not be reached.
5.5.4 Selection of significant part
When the robot is moving from one point to another, not all of the reachable space is necessary to
compute the collision free space, but only the part that is between the current robot position and the
position of the goal.
Therefore, in this algorithm:
- The borderline is transformed from the camera’s coordinate system into the robot’s coordinate
system.
- The coordinates of the pixels of the borderline are transformed from Cartesian coordinate system
to Polar coordinate system, the system of the robot’s axes.
- The significant part of the borderline lying between the current robot’s angle and the goal
position’s angle is selected.
The first step in the selection algorithm, transforming the values of the borderline into the robot
coordinate system, is achieved by applying the transformation matrix determined in chapter 5.5.2.
There are problems that have to be solved before transforming the values:
- It was observed during this thesis that in the outer regions of the acquired distance images the
camera’s measurements sometimes wrongly show a very short distance.
- The safety distance has to be dependant on the tool centre point’s height, since the robot can
work directly below an intrusion, as long as the angle of the camera allows a clear separation (see
Figure 66) of the robot and the intruding object.
- The reverse of the second point is true as well, where a high position of the tool centre point casts
a blind spot on intruding objects that are farther away in the horizontal direction but on a lower
position.