Stealth walking with reduced double-limb support phase and its extension to careful legged locomotion

Stealth walking is an underactuated walking movement completing in one step for a stable and cautious walk on irregular terrains. The generated gait generally consists of the single- and double-limb support phases; during the former, the leg angles are strictly controlled to follow the desired trajectories to make the forefoot land on the ground stealthily, whereas during the latter, the upper body is controlled to return to the initial state while keeping the vertical ground reaction forces acting on both feet positive. This causes, however, the increase of the step period and deterioration of the energy efficiency. To solve this problem, this paper discusses some methods for achieving high-speed stealth walking based on a reduction of the double-limb support phase. First, a model of an underactuated rimless wheel with an upper body is introduced for analysis. Second, a method for generating a stealth walking gait of the linearized model is proposed, and the instability inherent in the gait is mathematically investigated. Third, two methods for extending the obtained results to the nonlinear model are discussed. Fourth, the method is also extended to generate a careful walking gait on the frictionless road surface; the importance and significance of this study are discussed through investigation of a strict stealth walking.