Control of Smooth Biped Walking by Means of Heel-Off Motion

Abstract

We propose the control method of biped walking controlled by heel liftoff that eliminates impact when the foot contacts the walking surface that makes walking unstable. In presenting sagittal motion with heel liftoff, we use an approximate 1-mass model and derive dynamics based on this model. Sagittal motion involves two centers of rotation: the ankle joint (phase 1) and the toe (phase 2). In phase 2, heel leftoff lowers the impact when the foot contacts the walking surface. 1D autonomous dynamics (phase) around the contact point is derived by applying Passive Dynamic Autonomous Control, which we proposed previously [15, 16] to dynamics in both phases. Based on this dynamics, we propose simple stabilizing control by varying the Center of Gravity (COG) trajectory in heel-off phase is proposed, and the motion period of the sagittal motion that is employed in order to synchronize the lateral and sagittal motion is calculated. Numerical simulation confirmed that walking dynamics converges at a certain state, and that heel liftoff reduced impact when the foot contacted the walking surface. We studied the energy efficiency of heel-off walking and proposed the mechanics to improve energy efficiency.