Strict Stealth Walking of Planar Point-footed Biped with Extra Control Torques

This paper discusses a body structure of biped robots advantageous in achieving strict stealth walking (SSW) on the frictionless road surface, and develop a design method to generate closed orbits for respective coordinates in the phase space taking the redundancy of the control torques into account. To strictly and preferentially control the motions of the stance and swing legs of a point-rooted biped, we consider to add a reaction wheel to each leg frame and increase the number of the control inputs. First, we introduce a model of a planar, point-rooted, 5-DOF biped robot with four control inputs, and derive an approximate analytical solution of the target initial state by using the linearized equation of controlled motion. Second, we propose numerical procedures to identify the target initial angular velocities of the upper body and reaction wheel of the nonlinear model, and investigate the validity of the proposed method through numerical simulations of the typical SSW gaits. Furthermore, we discuss the energy efficiency in terms of specific resistance and its improvement by using an elastic element.