Range of Motion Enhancement in a Passive Upper-Limb Exoskeleton: Design Improvements to a Prototype Based on Pneumatic Artificial Muscles

Industrial passive exoskeletons are used to assist workers in performing repetitive and physically demanding tasks, in fields such as manufacturing. These devices aim to reduce biomechanical stress on specific joint or anatomical regions, with the ultimate goal of reducing the risk of work-related musculoskeletal disorders. Among the various joints subjected to overwork, the shoulder joint presents a unique challenge due to its high degrees of freedom (DOF) and wide range of motion (ROM), which make it difficult to design support mechanisms that do not interfere with physiological movements. This thesis focuses on the biomechanical analysis and functional enhancement of a passive upper-limb exoskeleton prototype which employs two Pneumatic Artificial Muscles (PAMs). In its initial configuration, the exoskeleton provides assistive torque during arm elevation, but the current transmission architecture imposes limitations on the functional ROM, effectively constraining the shoulder flexion between 90° and 135°. This causes discomfort while using the engaged exoskeleton for elevation angles below 90°, due to the excessive torque at the shoulder joint provided by PAMs action. The main objective of this work is to extend the ROM of the passive exoskeleton, ensuring continuous or manually disengaged assistance, without compromising mechanical simplicity or user comfort. To achieve this, two distinct mechanical solutions have been developed and analyzed. The first solution involves the redesign of the bracelet, integrating a disengagement mechanism that allows the user to lower the arm with a simple gesture. This allows free movement of the arm down to a minimum angle of 26° with respect to the vertical axis. This adjustment does not alter the existing transmission architecture and preserves the advantages of the original prototype in terms of assistive torque. The second solution, based on a cam transmission system developed in the earlier stages of the prototypes design project, was analyzed in this thesis to evaluate its potential for torque modulation and integration into the exoskeleton because of its optimal ROM in terms of flexion. The cam profile has been designed to modulate the torque transfer from the PAMs as a function of the arm elevation angle. This solution has not been developed because of the complexity of the integration on the back structure and the poor supportive torque while holding an object. In order to evaluate the possibilities of the ROM expansion this solution has been optimized to be implemented in the current back-frame, evaluating the pros and cons of the solution. Both design strategies have been modeled using CAD and evaluated through static simulation. Performance indicators such as achievable ROM, integration with existing components, and theoretical torque profiles were compared. The cam-based mechanism was subjected to analysis regarding manufacturability and integration into the existing structure. The results demonstrate that both approaches effectively expand the exoskeleton’s functional ROM. The disengagement-based bracelet design allows the user to regain use of the lower workspace without adding complexity to the system and preserving the support torque while not disengaged, while the cam mechanism enables continuous support in the ROM but provides less support while having weight in the hands. A future work implementing passive adaptable springs in the current transmission design could be an aspect that preserves the pros of both solutions.