Conceptual Design of a Variable Stiffness Mechanism in a Humanoid Ankle Using Parallel Redundant Actuation

Abstract

Future robots will rely more than today on high precision, better energy efficiency and safe handling (e.g. human-machine interaction). An inevitable step in the development of new robots is therefore the improvement of existing mechanisms, since better sensors and algorithms do not satisfy the demands alone. During the last three decades, Parallel Redundant Mechanisms (PRM) came more into the focus of research, as they are advantageous in terms of singularity avoidance, fast movements and energy efficiency. Subsequently, yet another technology - the Variable Impedance Actuator (VIA) - emerged which proposes to change its inherent stiffness allowing an adaptation to its environment and to handle for example dynamic movements or shock absorptions. This work aims to create a new mechanism where a stiffness and position control for 2 degrees of freedom (DOF) is achieved by 3 actuators with flexible elements. It is thus a combination of the PRM and VIA, while taking advantage of both technologies but asking for a more sophisticated mathematical description. Practical implementation is intended for a humanoid ankle mechanism. Kinetostatic and stiffness models are derived and incorporated into the simulation of the mechanism. The simulations show that improvements in terms of singularity removal and dexterity are achieved. Furthermore, the adaptation of human like gait performances is presented.