A Trade-Off between Complexity and Interaction Quality for Upper Limb Exoskeleton Interfaces

Abstract

Exoskeletons are among the most promising devices dedicated to assisting human movement during reeducation protocols and preventing musculoskeletal disorders at work. However, their potential is currently limited, partially because of a fundamental contradiction impacting their design. Indeed, increasing the interaction quality often requires the inclusion of passive degrees of freedom in the design of human-exoskeleton interfaces, which increases the exoskeleton’s inertia and complexity. Thus, its control also becomes more complex, and unwanted interaction efforts can become important. In the present paper, we investigate the influence of two passive rotations in the forearm interface on sagittal plane reaching movements while keeping the arm interface unchanged (i.e., without passive degrees of freedom). Such a proposal represents a possible compromise between conflicting design constraints. The in-depth investigations carried out here in terms of interaction efforts, kinematics, electromyographic signals, and subjective feedback of participants all underscored the benefits of such a design. Therefore, the proposed compromise appears to be suitable for rehabilitation sessions, specific tasks at work, and future investigations into human movement using exoskeletons.

The Development of an Innovative Occupational Passive Upper Extremity Exoskeleton and an Investigation of Its Effects on Muscles

“…Repetitive muscle activity around the shoulder for over-the-head tasks is the main source of fatigue and resulting discomfort for workers. Exoskeletons are one of the most promising devices that prevent WMSDs [23]. Exoskeletons have also been introduced to reduce the physical strain on workers [24].…”

Human movement modifications induced by different levels of transparency of an active upper limb exoskeleton

“…Participants were connected to the exoskeleton at the arm and forearm levels by thermoformed orthoses. Two passive rotations and a passive translation were implemented at the level of the forearm to minimize unwanted interaction efforts and ensure a comfortable interaction ( Jarrasse and Morel, 2012 ; Verdel et al, 2022b ; 2023c ). The posture of the participant is described in Figures 2A, B .…”

“…These parameters have different consequences with regard to the potential acceptability of the controllers. Indeed, the absolute average effort is known to be tied to the long-term acceptability whereas the absolute maximum effort is known to be tied to the short-term acceptability and pain at the connection level ( Bessler et al, 2021 ; Verdel et al, 2023c ).…”

“…Transparency can be improved through several developments. First, the exoskeleton’s mechanical design, both in terms of the transmission ( Garrec, 2010 ; Gopura et al, 2016 ; Hutter et al, 2018 ) and of the human-exoskeleton physical interfaces ( Schiele and van der Helm, 2009 ; Jarrasse and Morel, 2012 ; Mallat et al, 2019 ; Verdel et al, 2022b ; 2023c ), is an important factor to minimize unwanted interaction efforts. The other necessary development to reach high levels of transparency is the control law definition.…”

“…Although these methods led to a decrease in velocity (compared to movements performed without exoskeleton), they still conserved numerous properties of natural human movement (Bastide et al, 2018;Verdel et al, 2021b). More advanced methods introduced a force feedback control based on force/torque (FT) measurements at the level of human-exoskeleton interaction points (Kim et al, 2012;Just et al, 2018;Verdel et al, 2021a;2022b;2023c), which could also be coupled to hierarchical optimization (Zimmermann et al, 2019), inertial measurement units (Zimmermann et al, 2020) or learning based predictions (Jarrasse et al, 2008a). Nevertheless, few of these studies compared the performances of different controllers with an extensive method, as done in Just et al (2018) between feedforward and disturbance observer controllers.…”