H-VSJ: A Hybrid Stiff-Compliant Variable Stiffness Joint With Decoupling Position and Stiffness Modulation

Abstract

The wide spectrum of robotic applications motivates the intrinsic stiffness modulation of the robot besides compliance control, especially for highly dynamic manipulation. Variable Stiffness Actuators (VSAs) are designed for improving impact resistance, stiffness adaptability, and dynamic performance, while many existing variable stiffness mechanisms still face coupling between position and stiffness modulation. This letter presents a Hybrid Stiff-Compliant Variable Stiffness Joint (H-VSJ) that employs a decoupling mechanism to achieve independent control of joint position and stiffness as well as hybrid stiff-compliant modes. By adjusting the force-transmission ratio between the elastic element and the output link along the joint axis, H-VSJ achieves the decoupled and accurate stiffness modulation. The decoupling mechanism also allows the joint to smoothly switch between the stiff mode and compliant mode. The stiffness model of H-VSJ is established, and the influence of key design parameters on stiffness is further analyzed. Finally, the effectiveness of the proposed H-VSJ is validated through experiments on buffering motion range, stiffness-position decoupling, stiffness analysis, dynamic stiffness modulation, and impact-buffering capability, which demonstrate H-VSJ has a maximum deflection of $30.20^\circ$ in the low-stiffness state, an average stiffness modulation error of $7.00$%, a stiffness range of 23.57–260.36 N m/rad in the compliant mode, and 8630.25 N m/rad in the stiff mode.