An inverted pendulum based approach to biped trajectory generation with swing leg dynamics

Reference trajectory generation is one of the key problems in biped walking robot research. The linear inverted pendulum model (LIPM) is employed widely as a useful model which simplifies trajectory generation task. Many reference generation algorithms use the zero moment point (ZMP) criterion for the LIPM in order to achieve stable walking trajectories. However, LIMP ignores the dynamics of the swing leg. This can lead to tracking problems, especially when the legs are heavy. This paper uses a two-mass LIPM and proposes a fifth order state space description for the dynamics of the robot body and the swing leg in the swing phase. The body center of mass (CMB) reference trajectory is obtained for given foot placement references and the desired ZMP trajectory. An inverse kinematics based position controller is then employed for locomotion. The walking performances with the one-mass and one-mass-two-mass switching linear inverted pendulum models are finally compared via 3D full-dynamics simulations of a 12 degrees of freedom (DOF) biped robot. The results indicate that the proposed model switching between one-mass and two-mass models is useful in improving the stability of the walk.