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

2025-12-21

AI Summary

Post-stroke upper-limb motor impairments necessitate rehabilitation robots capable of safe, transparent physical human–robot interaction (pHRI); however, existing variable-stiffness actuators suffer from inherent coupling between stiffness and deflection angle, complicating modeling and control. This paper proposes a novel variable-stiffness actuator featuring complete decoupling of stiffness from output characteristics: adjustable-lever and hypocycloidal linear mechanisms enable continuous, stepless stiffness tuning (≈0 to ∞) with linear torque–angle response; integrated differential planetary gearing and cascaded PI control achieve simultaneous stiffness and pose regulation while suppressing disturbances. Prototype experiments demonstrate high-accuracy stiffness calibration, wide-range stiffness modulation, sub-newton-meter torque control precision, strict decoupling, and balanced dual-motor load distribution. The design establishes a new high-performance actuation paradigm for pHRI systems, particularly rehabilitation exoskeletons.

Technology Category

• Intelligent Robots: Manipulation
• Humans and AI: Human-Aware Planning and Behavior Prediction
• Cognitive Modeling & Cognitive Systems: Simulating Human Behavior

Application Category

• Responsible Web: Machine-in-the-loop, human agency and autonomy
• Search and Retrieval-Augmented AI: Assisted, interactive, and conversational search
• User Modeling, Personalization and Recommendation: User modeling and simulation for interactive and conversational systems

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.

Problem

• Decouples stiffness from deflection angle in actuators
• Enables linear torque-deflection and continuous stiffness modulation
• Facilitates safe physical human-robot interaction for rehabilitation

Innovation

• Decouples stiffness and output using variable-length lever
• Uses planetary gear differential for dual-motor load sharing
• Implements cascade PI control to suppress stiffness fluctuations