tailieunhanh - Mobile Robots Navigation 2008 Part 8

Tham khảo tài liệu 'mobile robots navigation 2008 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 268 Mobile Robots Navigation Laser range sensor Fig. 4. Occlusion effect conceptual diagram. After this coordinate transformation the generated cloud of points can be represented through a graphical user interface GUI which can allow a human user to observe the information of the environment retrieved by the sensor. Depending on the range of the sensor large very detailed areas can be mapped with a single scanning procedure. An example of such cloud points representation can be seen in Fig. 3. A cloud of points is one of the multiple representations that can be produced from the raw sensor s data other representations include rectangular meshes quad-tree representations and triangular grids. Occlusion problem Exteroceptive sensors such as LIDAR-based devices and stereo cameras are capable to retrieve the distance that separates them from an object within its field of view. However these sensors are not able to look behind the objects they perceive. If for example a laser range sensor is placed in front of a wall and an object moves from one end of the wall to the other the sensor would register the object as it passes but it would also loose information about the area of the wall immediately behind the object. The effect that the passing object produces onto the laser range sensor while it registers the distance to the wall is known as occlusion. Consider the static environment presented in Fig. 4. The laser range sensor is placed facing a wall and an object is located between the sensor and the wall. Assuming a single plane sweeping action when the sensor s laser beam reaches the object a discontinuity from the wall reading spans until the occluding object has been completely registered. The occlusion caused by the object can be thought as the shadow the object projects onto the wall once a source of light is set upon the object. When a mobile robot constructs maps in a cluttered environment or in large irregular outdoor environments the occlusion effect must