The 3D Laser Scanner.

As there is no commercial 3D laser range finder available that could be used for mobile robots, it is common practice to assemble 3D sensors out of a standard 2D scanner and an additional servo drive [6,12]. The scanner that is used for this experiment is based on a SICK LMS 291 in combination with the RTS/ScanDrive developed at the University of Hannover. Different orientations of the 2D scanner in combination with different turning axes result in a number of possible scanning patterns. The scanning pattern that is most suitable for this rescue application is the yawing scan with a vertical 2D raw scan and rotation around the upright axis (see Fig. ). The yawing scan pattern results in the maximal possible field of view (360$^\circ$ horizontal and 180$^\circ$ vertical) and an uniform distribution of scan points.

As 3D laser scanner for autonomous search and rescue applications needs fast and accurate data acquisition in combination with low power consumption, the RTS/ScanDrive incorporates a number of improvements. One mechanical improvement is the ability to turn continuously, which is implemented by using slip rings for power and data connection to the 2D scanner. This leads to a homogeneous distribution of scan points and saves the energy and time that is needed for acceleration and deceleration of panning scanners. Another improvement that becomes more important with short scanning times of a few seconds is the compensation of systematic measurement errors. In this case the compensation is done by sensor analysis and hard real-time synchronization, using a Linux/RTAI operation system. These optimizations lead to scan times as short as 3.2s for a yawing scan with 1.5$^\circ$ horizontal and 1$^\circ$ vertical resolution (240x181 points). For details on the RTS/ScanDrive see [17].