An online trajectory modifier for the base link of biped robots to enhance locomotion stability

This paper proposes an online trajectory modification scheme for biped robots to cope with uncertainty of their environment. The trajectory of the base link in the vertical direction is modified depending on the magnitude of zero moment point deviation from its safety boundary such that appropriate angular moment is generated to maintain stable walking. The modified trajectory then gradually returns to the original trajectory using a 3rd- or 5th-order interpolation polynomial. We expand the gravity-compensated inverted pendulum mode to generate the base-link trajectory not only for single support phases but also double support phases. In the simulations to evaluate the proposed scheme, an impedance controller is used to control a 6-DOF biped robot and the environment of the biped robot is assumed to consist of nonlinear and linear compliant contact models. The simulation results show that the proposed online trajectory modification scheme effectively enhances the stability of locomotion