Design of vertebrae-inspired trunk mechanism for controlling walking behavior of semi-passive walker

A semi-passive walker that equips a small number of actuators attains locomotion through an interaction between its physical properties such as center of mass (CoM) and the ground in contact. Therefore, it is expected that the robot obtains a wide range of locomotion such as in its walking velocity by changing a trajectory of CoM. In this study, we focused on the trunk mechanism that largely influences the walking behavior by changing its CoM, and propose a vertebrae-inspired trunk mechanism in which tunable viscoelastic joints are embedded. By switching a mechanical property, that is, the viscoelasticity of the trunk joints, the cycle of the passive oscillation while walking is changed, and the robot obtains a wide range of the walking cycle. To verify this trunk mechanism, we developed a physical bipedal robot. The physical trunk mechanism does not require a supplemental actuator to oscillate the trunk actively, and does not require energy consumption to retain the viscoelasticity according to the devised mechanism. We also constructed simulation models equipping various types of trunk mechanisms for verifying the number and position of the trunk joints that influence the variation of the walking cycle. The simulation results suggest a criterion for the trunk mechanism design in which the walking cycle is influenced by the number and position of the joints. This paper concludes that the proposed trunk mechanism is a suitable and practical mechanical element that provides a wide range of the locomotion of semi-passive walker by utilizing its switchable mechanical property.