The exoskeleton-human body system: a multibody model for fitting design analyses

Occupational exoskeletons have been proven to provide a substantial support to workers; nonetheless, their efficiency is tightly related to proper fitting to the human body, and the acceptability of these devices might be hampered by discomfort produced by high pressures or reduced stability of the harnesses, that can result in scrubbing at the belts. This work aims to establish a numerical model that is computationally efficient and able to virtually assess the fitting of the exoskeleton and its comfort. A multibody model has been set up for this aim, where a discretised approximation was implemented for the flexible belts, paying special care to the proper setup of contact parameters between these belts and the human body. The results provided reliable estimations of contact pressures, being the numerical results within the confidence interval of experimental peak shoulder pressures found in literature. The model was then validated using experimental data collected immediately after the exoskeleton was worn and during squat and stoop movements. The model accurately estimated the internal strip force required for the exoskeleton stability under gravity (30.6 N) without generating skin injuries, in accordance with experimental results (35.6 ± 14.1 N). Relative movements between the exoskeleton and the human body were investigated for the stoop and squat actions; results showed that the exoskeleton moved of 10.2 cm during the stoop and 7.1 cm during the squat, so replicating the actual system behaviour (9.34 ± 2.20 cm and 9.04 ± 1.81 cm in experimental tests). The numerical model was proved to be able to accurately replicate the interaction of the exoskeleton-human body system, so representing a useful tool for the design of these devices.