A Guideline for Humanoid Leg Design with Oblique Axes for Bipedal Locomotion

The kinematics of humanoid robots are strongly inspired by the human archetype. A close analysis of the kinematics of the human musculoskeletal system reveals that the human joint axes are oriented within certain inclinations. This is in contrast to the most popular humanoid design with a configuration based on perpendicular joint axes. This paper reviews the oblique joint axes of the mainly involved joints for locomotion of the human musculoskeletal system. We elaborate on how the oblique axes affect the performance of walking and running. The mechanisms are put into perspective for the locomotion types of walking and running. In particular, walking robots can highly benefit from using oblique joint axes. For running, the primary goal is to align the axis of motion to the mainly active sagittal plane. The results of this analysis can serve as a guideline for the kinematic design of a humanoid robot and a prior for optimization-based approaches.

I. Introduction

The field of bipedal robotic locomotion is divided into the two main fields of walking and running. While there exist many walking robots with an e.g. inverted pendulum or rimless wheel locomotion template [1], [2], there are fewer humanoids which are capable of running. The widely used spring-mass or spring-loaded inverted pendulum (SLIP) templates of running require different kinematic solutions with a shift of thinking about bipedal robotic locomotion and especially about impacts, stability, and efficiency [3]–[6]. To achieve a feasible running performance, the kinematic is a valid starting point to optimize current design approaches.