Acquisition of Obstacle Avoidance Actions with Free-Gait for Quadruped Robots

Although legged mobile robots are inferior to wheeled or crawler types in mobility efficiency on the flat ground, they demonstrate high motion performance and adaptation capability to the ground by utilizing their high degrees of freedom (DOF). Since such robots choose stable leg-placement, stable movements can be performed on irregular terrains ( afak & Adams, 2002). Moreover, they demonstrate some unique functionalities, e.g., the scaffold of a stable activity at the time of rest can be formulated from taking the posture of legs which are hard to topple (Hirose & Yoneda, 1993). However, it is very difficult to decide robot gait due to its high DOF. When the legs of the robot are simply controlled by a fixed command, adaptation capability to the terrain is remarkably restricted and sometimes it is impossible to maintain a stable walk. Moreover, when a leg is unable to be placed properly an optimum leg placement must be efficiently found from among other candidates. Therefore, it needs for a legged mobile robot to sequentially decide the progression of legs. For that purpose, the robot predictively perceives and recognizes geographical features of the terrain, and it consequently gets over any obstacle by using an adaptive ability acquired in advance. From this fact, legged robots are not necessary to avoid all the obstacles by altering their path, unlike wheeled or crawler types, because they can avoid an obstacle by crawling-over or striding, according to the obstacle’s nature and the current state of the robot. Thus, it can be found that the mobility efficiency to reach a destination is improved by such action. Moreover, when robots have many legs like 4-legged or 6-legged types, the movement range is affected by the order of swing leg. We studied path to a destination and obstacle avoidance of a quadruped robot considering free-gait. In general, quadruped robots can realize two types of walk: one is the static walk which retains the stability statically, keeping the center of gravity (COG) in the polygon constructed by the support legs, and the other is the dynamic walk which retains dynamic stability, though it is statically unstable. In the static walk, one feature is that an irregular-terrain walk is easily realized, whereas excelling in walking-speed or consumption energy is a feature of the dynamic walk (Kimura et al., 1990, Kimura, 1993). Thus, static walk is suitable for action acquisition of quadruped robots in geographical environments where obstacles, such as a level difference, exist (Chen et al., 2002, Chen et al., 2002).