A New Actuator With Adjustable Stiffness Based on a Variable Ratio Lever Mechanism

This paper presents the actuator with adjustable stiffness (AwAS-II), an enhanced version of the original realization AwAS. This new variable stiffness actuator significantly differs from its predecessor on the mechanism used for the stiffness regulation. While AwAS tunes the stiffness by regulating the position of the compliant elements along the lever arm, AwAS-II changes the position of the lever's pivot point. As a result of the new principle, AwAS-II can change the stiffness in a much broader range (from zero to infinity) even by using softer springs and shorter lever arm, compared to AwAS. This makes the setup of AwAS-II more compact and lighter and improves the stiffness regulation response. To evaluate the aptitude of the fast stiffness adjustment, experiments on reproducing the stiffness profile of the human ankle during the stance phase of a normal walking gait are conducted. Results indicate that AwAS-II is capable of reproducing an interpolated stiffness profile of the ankle while providing a net positive work and thus a sufficient amount of energy as required for the toe-off.

I. Introduction

In human–robot interaction area, compliance has an important role [1]–[4]. Physical passive compliance decouples the inertia of the actuator, which normally has a large value, from inertia of the link, and it decreases the stiffness of the joint [5]–[12]. Adaptable compliance indeed helps to improve the performance in different circumstances. For instance, energy efficiency can be improved by properly tuning the stiffness [13] [14]. In the literature, different design approaches for actuation systems that allow the stiffness regulation are proposed. These variable compliance actuation systems typically employ two actuator units in combination with passive elastic elements to control, independently, the compliance and the equilibrium position of the actuated joint. These systems can be implemented by both antagonistic and series configurations.