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20 CHAPTER 4. METHOD<br />

base<br />

steeringCommand<br />

collision<br />

avoidance<br />

laserScan<br />

laser<br />

Figure 4.1: Components needed for collision avoidance.<br />

and the laser_frame. These two frames are linked statically, because the laser is directly mounted<br />

on the base, with no actuator inbetween.<br />

(a) Static mounted planar laser scanner on base. [9] (b) Tf tree for<br />

a static mounted<br />

laser scanner.<br />

Figure 4.2: Hardware setup and tf tree of a statically mounted laser scanner.<br />

Main challenge in this use-case:<br />

• Publish transformation once: It is enough to publish the transformation only once to every<br />

interested component, because the transformation does not change over time. It is important<br />

that also components which are started later also receive the transformation once.<br />

4.1.2 Collision Avoidance in 3D<br />

The use-case described above only considers objects in the plane of the laser scanner. It is obvious that<br />

in this case not all collisions are avoided, thus collision avoidance should be performed in 3D. One<br />

way to do this is to mount a laser scanner on a tilt unit. Such a setup is for example used on the PR2<br />

robot (see Figure 4.3(a)). In this setup the laser scanner is tilted continuously up and down and thereby<br />

creates a 3D point cloud of the environment. In this point cloud the drivable area is determined. In the<br />

component-based system above only the collision avoidance component has to be changed, the two<br />

other components can stay the same.

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