ATP: Anatomical Torque with Passivity-based Control Framework for Safe Upper-Limb Exoskeleton Assistance

2026-08-06

AI Summary

This study addresses the challenge of achieving precise and safe anatomically aligned assistance in upper-limb exoskeletons during complex, non-periodic movements. For the first time, anatomical alignment is extended to non-periodic upper-limb tasks through a novel framework that integrates musculoskeletal simulation with reinforcement learning to generate generic anatomical reference torques. The approach features a unified muscle controller, an anomaly-aware online torque optimization mechanism, and a theoretically grounded energy tank-based passivity control architecture. Implemented on a compliant cable-driven exoskeleton, the method enables high-fidelity torque tracking and real-time generalization across diverse tasks. Electromyography (EMG) experiments demonstrate up to a 48% reduction in target muscle activity, significantly outperforming both gravity compensation and open-loop assistance strategies.

Abstract

Providing assistance across diverse movements is a central objective of exoskeletons, and anatomical knowledge can enable responsive support that generalizes across tasks. However, anatomical assistance has mainly been studied for lower-limb exoskeletons, where periodic, weight-bearing motions impose lower demands on torque precision. Extending such assistance to complex, nonperiodic upper-limb movements remains challenging. This paper proposes Anatomical Torque with Passivity-Based Control (ATP) for safe upper-limb exoskeleton assistance. First, a scalable musculoskeletal simulation framework trains a unified reinforcement-learning muscle controller that generalizes across upper-limb movements and generates anatomical reference torques without complex biomechanical computations. Second, an online torque-refinement scheme adapts the reference to diverse movements, suppresses tendon-induced spikes, and incorporates a learned anomaly score for safe and comfortable assistance. Third, an interaction torque controller delivers assistance through a cable-driven compliant exoskeleton without constraining motion to predefined trajectories, while an energy tank preserves passivity with theoretical guarantees on torque tracking and system passivity. Simulations and real-world experiments show accurate tracking of long-duration motion sequences and generalization to real-time human movements. The controller achieves accurate torque tracking while preserving passivity and resumes tracking after energy-tank replenishment. An EMG study with five participants further shows reduced target-muscle activity during static and dynamic tasks compared with gravity compensation and open-loop assistance, with reductions of up to 48% relative to movement without the exoskeleton in a dynamic multi-joint task.

Problem

• upper-limb exoskeleton
• anatomical assistance
• nonperiodic movements
• safe assistance
• torque precision

Innovation

• Anatomical Torque
• Passivity-based Control
• Reinforcement Learning
• Cable-driven Exoskeleton
• Energy Tank