An On-Line Trajectory Modifier for the Base Link of Biped Robots to Enhance Locomotion Stability

This paper proposes an on-line trajectory modification scheme for biped robots to cope with uncertainty o] their environment. For stable locomotion, biped robots should have robust properties against various disturbances such as ground irregularity and external pushing or pulling .forces. Since such uneertainty is not known a priori, biped robots should have the ability to adapt to them on-line. In the proposed scheme, the trajectory of the base link in the vertical direction is modified depending on the magnitude of ZMP deviation from its sa#ty boundary such that appropriate angular moment is generated to maintain stable walking. The modified trajectory then gradually returns to the original trajectory using a 3rdor 5th-order interpolation polynomial. And this paper expands the gravity-compensated inverted pendulum mode (GCIPM) 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 on-line trajectory modification scheme effectively enhances the stability of locomotion.