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Chapter 4
Method
At the beginning of this chapter some real world use-cases are presented. From the analysis of these
use-cases evolving requirements are formulated. Afterwards elements which allow the abstract description
of a component-based system are presented. Further a method to determine the quality of
a transformation chain is shown. Finally some examples using the different elements and the quality
measurement are presented, to show the generality of the proposed methods.
4.1 Use Cases
This section describes several use-cases where it is necessary to transform data between different
coordinate frames. The selected use-cases have relevance for service robotics where a great number
of actuators and sensors exist and where sensors are mounted on actuators. The chosen use-cases
increase in complexity, starting with a setup where no actuators are included. The next use-cases
mainly increase in the number of actuators involved in the transformation chain. Today humanoid
robots have the probably most complex tf tree. However, the challenges are the same as for the third
use-case, where a sensor mounted on an arm is considered. Thus these use-cases are a representative
subset for the use of transformations in a robotic system.
Each of the use-cases contains a description of the use-case and a summary of the most important
challenges occurring in this use-case. Based on these challenges the requirements for this work are
derived in Section 4.2.
4.1.1 Collision Avoidance in 2D
One task every robot has to fulfill is collision avoidance. Therefore most robots are equipped with
a planar laser scanner, like shown in Figure 4.2(a). This laser ranger is mounted at a fix position on
the robot. In a component-based system, there are typically three components involved (see Figure
4.1). The base component is responsible for controlling the robot base, the laser component which
publishes the newest laser scan and a collision avoidance component subscribing for laser scans. The
latter component calculates the next collision free steering command for the base component based
on the given sensor data.
To calculate the next steering command the current laser scan, which of course is captured in
the laser_frame, has to be transformed into the base_frame. This transformation is necessary
because the steering commands are always relative to the base_frame. As shown in Figure 4.2(b)
the tf tree for this case is a very simple one. The tree has only two nodes, namely the base_frame
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