Adaptive gravity-balancing mechanism using adjustable spring for a 6-DoF serial cooperative manipulator

Abstract

Cumbersome high-power joint actuators are not suitable for lightweight cooperative manipulators, which require flexibility and high energy efficiency while maintaining moderate payload capacity. Thus, gravity compensation is proposed in this work to drive cooperative manipulators by small joint actuators. Most existing gravity compensation methods are focused on fixed load (i.e. dead-weight), while in this study, a variable-payload balanced 6-DoF (degrees of freedom) serial manipulator is realized by adaptive gravity-balancing mechanism. Since joint actuators can be small enough to withstand only joint friction and inertia, this manipulator is inherently safe for man-robot interaction. The adaptive gravity-balancing mechanism is composed of ZFL (zero-free length) springs with adjustable endpoints, which are actively adjusted by cable after payload changes. Three advantages of the proposed manipulator include: Firstly, balancing all joints affected by gravity by only one additional balancing motor; Secondly, the balancing mechanism can actively adapt to variable payloads; Finally, the adjustment of the balancing mechanism is independent of the manipulator’s pose. Through simulation, these three advantages have been verified, which provides a new scheme for the design of safe-interaction and energy-saving cooperative manipulators.