Conceptual Bases of Robot Navigation Modeling, Control and Applications
Silas Franco dos Reis Alves, João Martins Pizauro, Humberto Ferasoli, Liz Rincon, Rosana Yamasaki · InTech eBooks · 2011
Conceptual Bases of Robot Navigation Modeling, Control and Applications 5 Robot Navigation systems and methodsNavigation is the science or art of guiding of a mobile robot in the sense of how travel through the environment (McKerrow, 1991).The problems related to the navigation can be briefly defined in three questions: "Where am I", "Where am I going" and "How do I get there?"(Leonard & Durrant-White, 1991).The first two questions may be answered by an adequate sensorial system, while the third question needs an effective planning system.The navigation systems are directly related to the sensors available on the robot and the environment structure.The definition of a navigation system, just like any aspect of the r o b o t w e h a v e s e e m s o f a r , i s i n f l u e n c e d b y t h e r e s t r i c t i o n s i m p o s e d b y b o t h t h e environment and the robot very purpose.The navigation may be obtained by three systems: a coordinates based system, a behavior based system and a hybrid system.The coordinates based system, like the naval navigation, uses the knowledge of one's position inside a global coordinate system of the environment.It is based on models (or maps) of the environment to generate paths to guide the robot.Some techniques are Mapping (Latombe, 1991), Occupancy Grid Navigation (Elfes, 1987), and Potential Fields (Arkin et al., 1987).The behavior based system requires the robot to recognize environment features through its sensors and use the gathered information to search for its goals.For example, the robot must be able to recognize doors and corridors, and know the rules that will lead it to the desired location.In this case, the coordinate system is local (Graefe & Wershofen, 1991).Information about the statistical features of the environment is important to both cited systems.The modeling of the environment refers to the representation of objects and the data structure used to store the information (the maps).Two approaches for map building are the geometric and phenomenological representation.Geometric representation has the advantage of having a clear and intuitive relation to the real world.However, the geometric representation has no satisfactory representation of uncertain geometries, as well as is not clear if knowing the world shape is really useful (Borenstein et al., 1995).The phenomenological representation is an attempt to overcome this problem.It uses a topological representation of the map with relative positioning which is based on local reference frames to avoid the accumulation of relative errors.Whenever the uncertainty grows too high, the robot sets a new reference frame; on the other hand, if the uncertainty decreases, the robot may merge frames.This policy keeps the uncertainty bound locally (Borenstein et al., 1995, as cited in Engelson & McDermott, 1992).Mobile robots can navigate using relative or absolute position measures (Everett, 1995).Relative positioning uses odometry or inertial navigation.Odometry is a simple and inexpensive navigation system; however it suffers from cumulative errors.The inertial navigation (Barshan & Durrant-White, 1995) uses rotation and acceleration measures for extracting positioning information.Barshan and Durrant-White (1995) presented an inertial navigation system and discusses the challenges related to mobile robot movement based on non-absolute sensors.The most concerning issue is the accumulation of error found in relative sensors.The absolute positioning system can use different kinds of sensors which are divided in four groups of techniques: magnetic compass, active beacons, landmark recognition and model matching.Magnetic compasses are a common kind of sensor which uses Earth's natural electromagnetic field and does not require any change on the environment to be able to navigate through the world.Nevertheless, magnetic compasses readings are affected by power lines, metal structures, and even the robot movement, which introduces error to the system (Ojeda & Borenstein, 2000).