Tham khảo tài liệu 'mobile robots perception & navigation part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | A Pursuit-Rendezvous Approach for Robotic Tracking 471 The time-to-go The time-to-go is the time it takes the robot to reach the moving goal. The time-to-go is very important for any comparison between control strategies. The time-to-go can be estimated by the following equation - r ho-go 30 In general it is difficult to estimate the time-to-go since it depends on many factors that are time-varying. The most important factors are the velocity ratio and the target manoeuvers. The time-to-go may be used to determine the appropriate value of b to adjust c t . The only case where it is possible to find the time-to-go analytically is when the goal moves in a straight line c const vR and vG are constant and the robot is applying a pure rendezvous approach. In this case the time-to-go is given by to go vG cos ffG n Vr cos cấ 31 It is obvious that the time-to-go is proportional to the initial range. 5. In the presence of obstacles It is clear that the problems of navigation and reaching a moving object in the presence of obstacles are among the most difficult problems in robotic navigation. They combine local path planning for collision avoidance with global path planning for reaching the goal. In our formulation the robot moves in two modes the navigation mode and the obstacle avoidance mode. Clearly the obstacle avoidance mode has the priority. The collision avoidance is accomplished by building a polar histogram of the environment. The polar histogram is based on the angular information obtained from the sensory system. Only obstacles that appear within a given region called the active region are considered. The polar histogram allows determining free directions and directions corresponding to the obstacles. A snapshot of the local environment from a given position of the robot is a characterization of the visible obstacles and the angles they make with the robot. The first stage in the polar histogram is to represent the robot s surrounding environment using .