Integration of posture and rhythmic motion controls in quadrupedal dynamic walking using phase modulations based on leg loading/unloading

In this paper, we intend to show the basis of a general legged locomotion controller with the ability to integrate both posture and rhythmic motion controls and shift continuously from one control method to the other according to the walking speed. The rhythmic motion of each leg in the sagittal plane is generated by a single leg controller which controls the swing-to-stance and stance-to-swing phase transitions using respectively leg loading and unloading information. Since rolling motion induced by inverted pendulum motion during the two-legged stance phases results in the transfer of the load between the contralateral legs, leg loading/unloading involves posture information in the frontal plane. As a result of the phase modulations based on leg loading/unloading, rhythmic motion of each leg is achieved and inter-leg coordination (resulting in a gait) emerges, even without explicit coordination amongst the leg controllers, allowing to realize dynamic walking in the low- to medium-speed range. We show that the proposed method has resistance ability against lateral perturbations to some extent, but that an additional ascending coordination mechanism between ipsilateral legs is necessary to withstand perturbations decreasing the rolling motion amplitude. Even without stepping reflex using vestibular information, our control system, relying on phase modulations based on leg loading/unloading and the ascending coordination mechanism between ipsilateral legs, enables low speed dynamic walking on uneven terrain with long cyclic period, which was not realized in our former studies. Details of trajectory generation, movies of simulations and movies of preliminary experiments using a real robot are available at: http://robotics.mech.kit.ac.jp/kotetsu/.