1 Introduction
It is common for robot manipulators to operate in workspaces where the manipulator mobility is limited due to the presence of obstacles. In general, an obstacle can be any object which is present in the robot workspace and must be properly avoided for achieving the correct behaviour of the robot. Other objects may be present in the robot workspace but, depending on the application, they may not be considered as obstacles (e.g., the target object of a grasping device). An application example where obstacles play an important role is the autonomous motion of a robot in a space shuttle, where obstacles are represented by people, tools, instruments and other manipulators [16].