Master Thesis - Hochschule Bonn-Rhein-Sieg

5. Algorithms Master Thesis Björn Ostermann page 86 of 126
transformation from pixel to real distance was programmed. This feature can be added in future work
(see chapter 7.2).
For the demonstration of this algorithm, impacting the robot’s current speed, the speed of the robot
was reduced stepwise from 100% at 15 to 0% at 5 pixels distance. These values have to be adapted in
future work, dependant on the respective safety analysis (see chapter 6.3).
5.5 Robot path planning
Building a collaborative workplace, a robot working together with humans, the basic solution is the
described speed control from proximity measurement (see chapter 5.4). This basic solution
implements the required safety, but it induces the problem that the robot often needs to wait for the
humans to clear the working path before it can continue with its task. As described in chapter 2.3.1,
planning a path for the robot around intruding objects in the workplace can reduce the time the robot is
waiting. This optimization in collaboration is the main goal of this thesis.
The path planning algorithm needs to control the robot’s complete process. A robot’s process consists
of moving towards a desired goal in a certain way (direct drive, point to point) – the control of the
trajectory – and in controlling the robot’s tools. Since the given robot was not equipped with any tools,
the control of any tool was neglected. Adding a tool to the robot’s arm would change the physical
boundaries of the robot. In this case, not only the tool handling would have to be implemented in the
controlling program, but the change of the robot’s physique would have to be considered, when
calculating the necessary safety distances during path planning (see chapter 6.3).
To plan a path for the robot around intruding objects, the following points have to be considered:
- The robot’s current position and its relation to the next goal position have to be known, in order
to allow path control (see chapter 5.5.1).
- The camera’s coordinate frame and the robot’s coordinate frame have to be determined and their
relation has to be calculated. This is necessary to compare the robot’s position and its destination,
which are both given in the robot’s coordinate frame, with those of the objects, which are given
in the camera’s coordinate frame (see chapter 5.5.2).
- The reachable space, the space the robot is allowed to reach, has to be determined (see chapter
5.5.3) in order to allow the calculation of an evasion path (see chapter 5.5.5), if intruding objects
are present in the path.
Contrary to the speed control in chapter 5.4, where the information about the object’s and the robot’s
position was solely acquired from the camera, the algorithms in this part rely on the robot’s internal
sensors, to acquire its current position. While the speed control requires the actual shape of the robot,
which is only delivered by the camera, the path planning can work on assumptions of the shape, which
are computed from the robot’s position. The position acquisition from the robot’s internal sensors,
apart from being more precise than the camera’s data, has the advantage of being considerably faster.