PHD Thesis - Institute for Computer Graphics and Vision - Graz ...

1.6. Contribution of this thesis 7
environments. Global localization applies when a robot operates in a known environment, that
is the map has already been built. Global localization is then the computation of the pose of
the robot, consisting of position and heading, from the actual sensor reading, i.e. camera image
without using the previous pose information. Global localization is necessary quite frequently
when encountering the following situations:
Switching the robot on: After switching on a mobile robot the position is not known. However,
the robot may already possess a complete environment map e.g. from a previous run.
But before it can start useful operations (like navigation and path planning) its position
has to be determined first. Global localization allows to compute the pose of the mobile
robot from the actual camera image.
Kidnapped robot problem: The kidnapped robot problem has been stated by Engelson and
McDermott [27]. In the kidnapped robot problem a well-localized robot is teleported (or
simply moved with switched-off sensors) to some other location. The problem with this
scenario is, that the robot still believes to be at the location from where it has been
kidnapped. Path planning and navigation based on such an assumption will not work.
The environment predicted from the map and its assumed position will not match. The
kidnapped robot problem is basically a test for the ability of a robot to recover from a
catastrophic localization failure. Global localization is the solution to the kidnapped robot
problem. With the ability of global localization the robot can immediately determine that
it has been moved from outside.
Recover from failure: Recovery from a failure is also possible with global localization. Consider
the case when a mobile robot moves into an area without landmarks. Abruptly the
current sensor readings do not contain any landmarks. For vision based systems this can
occur easily when the robot is moving into an untextured area, e.g. a part of an room
containing only white walls. In such a case the robot would loose track. The robot would
then move around randomly to get back into an area where landmarks can be detected.
However, when he finally enters an area where landmarks re-occur his global position got
lost, global localization is necessary. It would be possible to rely on the robots wheel odometry
in the landmark-less area. However, keeping track with odometry only will introduce
too big deviations in pose estimation and global localization would still be necessary.
Loop closing: Loop closing is the ability of a mobile robot to recognize previously visited
areas. It is important in the stage of map building. During map building a mobile robot
traverses the environment and adds new structure and landmarks to the world map. If
the robot enters an already mapped area and does not recognize this, the same features
will be added twice, usually not on the same position because of small drift errors. Global
localization can be used to notice that the current location has already been visited, that
the loop has been closed. This gives the information, that the landmarks could already be
in the map and an update would be appropriate and not a simple insertion.
Homing: The last example for a situation where global localization would be required is homing.
Homing is the task that a mobile robot has to go back to some start position. The
start position may for instance mark an automatic charging device and the robot will go
there to recharge its batteries. Global localization can tell the robot that its target position
is reached.