DSO-VSA: a Variable Stiffness Actuator with Decoupled Stiffness and Output Characteristics for Rehabilitation Robotics

TL;DR

The paper introduces the DSO-VSA, a variable stiffness actuator with decoupled stiffness and output characteristics designed for rehabilitation robotics. It integrates a Variable Stiffness Mechanism based on a variable-length lever and a hypocycloidal straight-line path with a Differential Transmission Mechanism that aggregates power from two motors via a planetary gear system. A cascade PI controller maps stiffness and torque commands to coordinated motor velocities, enabling stable torque output across a wide stiffness range. Experimental validation demonstrates decoupled stiffness–torque behavior, effective dual-motor load sharing, and robust stiffness regulation, highlighting potential for safer, more capable rehabilitation exoskeletons. While offering high stiffness range and improved control, the design acknowledges weight and speed limitations and points to modular optimization as a path forward.

Abstract

Stroke-induced motor impairment often results in substantial loss of upper-limb function, creating a strong demand for rehabilitation robots that enable safe and transparent physical human-robot interaction (pHRI). Variable stiffness actuators are well suited for such applications. However, in most existing designs, stiffness is coupled with the deflection angle, complicating both modeling and control. To address this limitation, this paper presents a variable stiffness actuator featuring decoupled stiffness and output behavior for rehabilitation robotics. The system integrates a variable stiffness mechanism that combines a variable-length lever with a hypocycloidal straight-line mechanism to achieve a linear torque-deflection relationship and continuous stiffness modulation from near zero to theoretically infinite. It also incorporates a differential transmission mechanism based on a planetary gear system that enables dual-motor load sharing. A cascade PI controller is further developed on the basis of the differential configuration, in which the position-loop term jointly regulates stiffness and deflection angle, effectively suppressing stiffness fluctuations and output disturbances. The performance of prototype was experimentally validated through stiffness calibration, stiffness regulation, torque control, decoupled characteristics, and dual-motor load sharing, indicating the potential for rehabilitation exoskeletons and other pHRI systems.

Figures (13)

• Figure 1: CAD model of the DSO-VSA.
• Figure 2: The principle of the VSM. (a) The analysis of the decoupled stiffness and output characteristics. (b) Zero stiffness with $l_s=0$. (c) Infinite stiffness with $l_s=l_t$. (d) The analysis of hypocycloidal straight-line mechanism.
• Figure 3: (a) The stiffness $\delta$ versus $l_s$. (b) The maximum deflection angle $\theta_{\tau,max}$ versus $l_s$, when $l_s<26$, $\theta_{\tau,max}$ is limited by mechanical interference, when $60>l_s>26$, $\theta_{\tau,max}$ is constrained by the maximum deflection of the spring.
• Figure 4: The mechanical composition of the VSM.
• Figure 5: Basic concept of the DTM. (a) CAD model. (b) The kinematical diagram. (c) The schematic.