Cam-Contour-Engineered Passive Shoulder Exoskeleton: Delivering Adjustable Assistance with Minimal Movement Interference

Abstract

Overhead tasks in industrial and maintenance sectors significantly increase the risk of work-related musculoskeletal disorders (WMSDs) due to prolonged non-neutral shoulder postures. This paper presents the design and evaluation of a novel passive upper-limb exoskeleton specifically optimized for overhead assistance. To address the limitations of traditional assistive devices, such as constant resistance during natural movement and non-adjustable assistance levels, two core mechanisms are proposed: the Cam-based Variable Torque Mechanism (CVTM) and the Ratchet-based Pretension Adjustment Mechanism (RPAM). The CVTM provides a piecewise assistive characteristic: it allows for unobstructed, low-resistance arm swing during natural movement (Phase 1) and delivers programmable, elevation-dependent torque during overhead tasks (Phase 2), with the transition controlled by a critical elevation angle. Simultaneously, the RPAM enables the wearer to discretely adjust the assistance level through simple, cyclic arm motions without external tools, utilizing a dual-pawl and ratchet system to modulate spring pretension. The integration of these mechanisms into a lightweight, carbon-fiber frame ensures both high load-bearing capacity and ergonomic comfort. Experimental results demonstrate that the system significantly reduces shoulder physical strain during overhead tasks.