A Variable Stiffness Actuator Module With Favorable Mass Distribution for a Bio-inspired Biped Robot.

David Rodriguez-Cianca,Maarten Weckx,M.Carmen Sanchez-Villamañan,Diego Torricelli,Jose Gonzalez-Vargas,Rene Jimenez-Fabian,Bram Vanderborght,Dirk Lefeber,Massimo Sartori,J Luis Pons,Karsten Berns

doi:10.3389/fnbot.2019.00020

Abstract

Achieving human-like locomotion with humanoid platforms often requires the use of variable stiffness actuators (VSAs) in multi-degree-of-freedom robotic joints. VSAs possess 2 motors for the control of both stiffness and equilibrium position. Hence, they add mass and mechanical complexity to the design of humanoids. Mass distribution of the legs is an important design parameter, because it can have detrimental effects on the cost of transport. This work presents a novel VSA module, designed to be implemented in a bio-inspired humanoid robot, Binocchio, that houses all components on the same side of the actuated joint. This feature allowed to place the actuator's mass to more proximal locations with respect to the actuated joint instead of concentrating it at the joint level, creating a more favorable mass distribution in the humanoid. Besides, it also facilitated it's usage in joints with centralized multi-degree of freedom (DoF) joints instead of cascading single DoF modules. The design of the VSA module is presented, including it's integration in the multi-DoFs joints of Binocchio. Experiments validated the static characteristics of the VSA module to accurately estimate the output torque and stiffness. The dynamic responses of the driving and stiffening mechanisms are shown. Finally, experiments show the ability of the actuation system to replicate the envisioned human-like kinematic, torque and stiffness profiles for Binocchio.

Highlights

Creating bipedal robots that can walk stably and efficiently as humans has been an open challenge since long time in robotics research (Vukobratovicand Borovac, 2004)
The understanding of human locomotion has led to usage of variable stiffness actuators (VSAs) in humanoids due to their inherent advantages
Striving toward a more accurate biologically inspired robotic counterpart of humans, these VSAs have to be implemented in mechanical multi-degree of freedom (DoF) joints to replicate the rich variety of movements human present

Summary

Introduction

Creating bipedal robots that can walk stably and efficiently as humans has been an open challenge since long time in robotics research (Vukobratovicand Borovac, 2004). Traditional approaches focus on multiple degree-of-freedom (DoF) platforms controlled by classic control paradigms that ensure quasi-static stability, e.g., the Zero-Moment Point (ZMP) (Vukobratovic, 1975; Vukobratovicand Borovac, 2004). Despite their good performance on flat terrain, most of these robots show important limitations, such as high energetic costs, slow walking motion, poor robustness on uneven terrains, and unnatural kinematic patterns (Torricelli et al, 2016). Different from this approach, the “dynamic walking” principle emerged to improve the human-like properties of bipeds, realizing natural, and efficient motion with little or no actuation. These mechanisms are central for adapting to a large variety of terrains (Ferris et al, 1998) and for naturally adjusting to biomechanical and energetic demands (Farley and Gonzalez, 1996)

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