Abstract
Active exoskeletons are potentially more effective and versatile than passive ones, but designing them poses a number of additional challenges. An important open challenge in the field is associated to the assistive strategy, by which the actuation forces are modulated to the user’s needs during the physical activity. This paper addresses this challenge on an active exoskeleton prototype aimed at reducing compressive low-back loads, associated to risk of musculoskeletal injury during manual material handling (i.e., repeatedly lifting objects). An analysis of the biomechanics of the physical task reveals two key factors that determine low-back loads. For each factor, a suitable control strategy for the exoskeleton is implemented. The first strategy is based on user posture and modulates the assistance to support the wearer’s own upper body. The second one adapts to the mass of the lifted object and is a practical implementation of electromyographic control. A third strategy is devised as a generalized combination of the first two. With these strategies, the proposed exoskeleton can quickly adjust to different task conditions (which makes it versatile compared to using multiple, task-specific, devices) as well as to individual preference (which promotes user acceptance). Additionally, the presented implementation is potentially applicable to more powerful exoskeletons, capable of generating larger forces. The different strategies are implemented on the exoskeleton and tested on 11 participants in an experiment reproducing the lifting task. The resulting data highlights that the strategies modulate the assistance as intended by design, i.e., they effectively adjust the commanded assistive torque during operation based on user posture and external mass. The experiment also provides evidence of significant reduction in muscular activity at the lumbar spine (around 30%) associated to using the exoskeleton. The reduction is well in line with previous literature and may be associated to lower risk of injury.
Highlights
Exoskeletons are wearable devices generally aimed at supporting physical tasks by generating appropriate forces on one or multiple human joints
A similar approach described in Hara and Sankai (2010) was based on a sEMG measurements on the muscles at the lower back, which is considered relatively invasive for an industrial application as well as prone to artifact due to the contact with the exoskeleton structures
The authors expect the state of the art on strategies for active back-support exoskeletons to advance substantially in the near future, as more research and development focuses on industrial applications
Summary
Exoskeletons are wearable devices generally aimed at supporting physical tasks by generating appropriate forces on one or multiple human joints. During manual material handling large compressive forces over 5000N on the lumbar spine are generated, leading to a high risk of physical injury Norman et al (1998); Coenen et al (2014); OHSA (European Agency for Safety and Health at Work) (2000). Guidelines for workplace safety and ergonomics often result in very strict limitations on the weights that can be handled depending on operating conditions such as frequency and posture Konz (1982); Waters et al (1993); Garg (1995). These strict limitations give rise to opportunities for novel technical solutions, among which wearable exoskeletons have attracted great interest. A number of devices aimed at supporting the lower back have been designed as prototypes for research studies or developed as commercial products
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