In biped robots, the number of actuators is less than the number of degrees of freedom. Hence, these robots are inherently underactuation and unstable systems. In this research work, stability analysis of walking gaits and robustness analysis are developed for a five-link and four-actuator biped robot. In order to have a cyclic motion with faster convergence, we propose a stable gait planning for a biped without feet. Stability conditions are derived by studying unactuated dynamics and using the Poincaré map associated with periodic walking gaits. A stable gait is designed by an optimization process satisfying physical constraints and stability conditions. Also, considering underactuation problem, a time-invariant control law is designed to track the stable motion of biped. Validation of proposed approach is achieved by numerical simulations. Moreover, the effect of torso on the energy efficiency of biped and the stability of motion on the uneven surface are investigated.
