Rationale, Implementation and Evaluation of Assistive Strategies for an Active Back-Support Exoskeleton

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.

The effect of control strategies for an active back-support exoskeleton on spine loading and kinematics during lifting

“…In total, the EXO has a mass of 11 kg. Details of the device (EXO) can be found in Toxiri et al (2018). In short, the EXO spans the trunk and upper legs, with a waist/abdomen fixation.…”

Spine-Inspired Continuum Soft Exoskeleton for Stoop Lifting Assistance

“…Rigid exoskeletons rely on transmission mechanisms made of rigid components [19], which typically limit the natural movement of wearers. Toxiri et al [20] developed a powered back-support exoskeleton that reduced 30% muscular activity at the lumbar spine. Naf et al [21] proposed a passive back exoskeleton with a 25% increase of the range of motion of the trunk in the sagittal plane compared with the rigid powered design [20].…”

Characterizing Human Box-Lifting Behavior Using Wearable Inertial Motion Sensors

“…If a real-time algorithm was developed, the lift characterization data could be used to control assistive devices. For instance, an exoskeleton could be provided with lift information in order to properly support the user during lifting tasks, thereby increasing the effectiveness of such a device [3]. Such intent-detecting controllers have been designed for assistive devices before, such Parri et al's whole-body awareness controller for an active transfemoral prosthesis [13].…”

“…Thus, a system for automatically monitoring lifting behavior over time could prove useful. Furthermore, the information provided by such a real-time system could be used as control feedback for assistive devices such as trunk exoskeletons that support the user during lifting tasks, thereby preventing musculoskeletal disorders or reducing the consequences of such disorders [3][4][5].…”