Long-term working in fixed postures often leads to muscle fatigue and musculoskeletal disorders because muscles are not well-adapted to continuous, long-lasting contractions, even at low-level exertion. Under the influence of gravity, the shoulder and elbow joints experience maximum torque when the upper and lower arms are extended forward in a horizontal posture while performing a repetitive task such as drilling.
Despite the increase of automation in industry, work-related musculoskeletal disorders are often seen in overhead and ahead working, resulting in a global occupational health issue. Exoskeletons designed to offset exertion have been introduced as a way to reduce work-related musculoskeletal disorders. An exoskeleton is a wearable robotic device that works with a user to enhance or replace their physical capabilities. In industry, they are often used to assist workers with heavy lifting or repetitive tasks.
Two types of exoskeletons have been designed for this use: passive exoskeletons and active exoskeletons. Existing upper-limb passive exoskeletons have helped to alleviate muscular fatigue and mitigate injury risks by compensating for the gravitational forces acting on the arm, the weight of the handheld object. These lightweight and compact structures use elastic elements such as springs to generate support torques. However, assistance is only provided during arm elevation. During arm lowering, resistance is encountered, putting strain on muscles and joints.
Active exoskeletons offer adjustable torque, position, and speed outputs but face limitations due to their weight and size due to complex structures and high-capacity batteries, limiting comfort and portability.
To address these challenges, this study introduces a backpack-style exoskeleton designed to enhance wearing comfort and reduce resistance to user's normal motion. The design features three pneumatic-driven variable-stiffness units: a bending actuator, a tensile actuator, and a teeth-engagement clutch.
(a) Composition of the variable-stiffness bending actuator. (b) Working principle of the bending actuator, whose bending stiffness can be enhanced in vacuum pressure. A sectional view of its right half is displayed. (c) Prototype of the variable-stiffness bending actuator.
Based on these designs, a portable semi-active upper-limb exoskeleton was developed for sustained overhead and ahead postures. The exoskeleton is controlled by a portable pneumatic system (a vacuum pump) and the total mass is 3.35 kg. In flexible state, the exoskeleton imposes minimal resistance to user's motion. In rigid state, it restricts user's joint motion and provides support for posture maintenance.
Overview of the proposed upper-limb exoskeleton. (a) Components of the exoskeletal system. Note that air tubes are eliminated for more concise displaying. (b) Wearing of the exoskeleton. (c) Main DOFs of the exoskeleton. (d) Schematic diagram illustrating the principle of passive alignment of the center of rotation. (e) Working principle of stiffness adjustment for rotational joints. Details of Rotation Joint I and II are displayed with enlarged drawing.
The feasibility of applying the proposed variable-stiffness units in exoskeleton design was verified by promising results of performance evaluation in 12 subjects, who maintained four common assembly postures with and without the device. Compared to the condition without wearing the exoskeleton, using the exoskeleton demonstrated an average reduction in muscular activation of 42.3 ± 2.4% across all postures, confirming the exoskeleton's potential for ergonomic support. In addition, the bending variable-stiffness actuator achieved 50.98-Nm/rad bending stiffness and 87.9-fold stiffness variation ratio at a relatively low vacuum pressure of 40 kPa.
The authors believe this indicates the promise of applying the proposed exoskeleton for long-term working assistance in fixed overhead and ahead postures.
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