Mechanical Structure Design and Actuation Characteristics Analysis of the Parallel Driven Variable Stiffness Actuator With Unrestricted Rotation Range of the Output Shaft

Abstract

Among many types of flexible robot actuators, the variable stiffness actuator (VSA) is an actuator that can adjust the compliance of the actuator output shaft by adjusting the equivalent stiffness of its internal elastic element. Compared with the traditional rigid actuators, the VSAs have inherent flexibility and adjustable stiffness because they contain elastic elements and stiffness adjusting mechanisms. They have advantages in human-robot physical interaction security and task adaptability. The independent controllability of position and stiffness and the ability of stiffness adjustment make the VSAs have research value and application prospects in the fields of rehabilitation training equipment, prosthetics, wearable devices and so on. The mechanical scheme design of the VSA is an open research field. At present, there is no perfect mechanical structure design of any type of VSA. Optimizing and improving the mechanical structure design of the VSA is helpful to improve its actuation characteristics and application value. Due to the limitation of the mechanical structure design, most of the existing VSAs have limited rotation angle range of the output shaft, which limits their application. The purpose of this paper is to design the parallel driven VSA whose rotation angle range of output shaft is not limited. Moreover, inspired by the mechanical structure design ideas of some stiffness adjustment mechanisms with good implementation schemes and elastic elements with good force transmission characteristics, this paper makes some improvement, synthesis and innovation in the mechanical structure design of the VSA, so as to improve the structural compactness design, assembly modular design, mechanical transmission reliability design and actuation characteristics. In this paper, six types of parallel driven VSAs with different mechanical structure design schemes are proposed. Their common point is that the rotation angle range of the output shaft is not limited, and the differences are that they have different implementation schemes of variable stiffness mechanisms and elastic elements. The transmission structure diagrams of the designed VSAs show the working principle and stiffness adjustment principle. The actuation characteristics of the designed VSAs are analyzed and their differences are compared. The detailed mechanical structure design of the VSA is shown. In the designed VSAs, the model of the VSA based on the symmetrical Archimedes spiral cam groove mechanism is printed and assembled to show the feasibility of the implementation of the designed mechanical structure scheme. Finally, a manual adjustment experiment verifies the stiffness adjustment ability of the assembled VSA model, and shows that the rotation angle range of the output shaft of the assembled VSA model is unlimited.INDEX TERMS variable stiffness actuator, variable stiffness mechanism, output shaft with unlimited rotation angle range, parallel driven structure, mechanical structure scheme design, actuation characteristics I. INTRODUCTION

Toward Inherently Safer Human-Robot Interaction Using Compliant Actuators With High Torque-to-Inertia Ratios and Low Torque-to-Stiffness Ratios

“…However, a lower stiffness reduces the output position control bandwidth and affects the robot's repeatability. To solve both the problems of control bandwidth and safety, variable-stiffness actuators have been developed [21][22][23][24]. However, additional actuators are required to adjust the output stiffness, which makes variable-stiffness actuators much more complicated than SEAs.…”