Towards standards for the evaluation of active back-support exoskeletons to assist lifting task

Back-support exoskeletons have been recently proposed to reduce the risk of injuries for workers performing repetitive lifting tasks. Appropriate standards for their evaluation do not exist, but their definition would promote large-scale adoption in workplaces. This paper presents relevant standards and evaluation metrics as applied to similar devices and discusses their applicability to back-support exoskeletons, with the final goal to propose a reference methodology

Towards design guidelines for physical interfaces on industrial exoskeletons: Overview on evaluation metrics

On exoskeletons, physical interfaces with the body are one of the key enabling component to promote user acceptance, comfort and force transmission efficiency. A structured design workflow is needed for any application-driven product, such as industrial exoskeletons. In this paper, we review objective and subjective evaluation metrics that can be applied to physical interfaces. These indexes can be evaluated to create an ordered list of requirements to guide their future design. Pressure magnitude, duration, distribution, direction and time to don and doff are relevant objective indexes related to interfaces. Pain, comfort and ease of operation are subjective indexes. We propose that collecting a suitable set of metrics will lay the foundation for effective design guideline for industrial exoskeletons

Actuation Requirements for Assistive Exoskeletons: Exploiting Knowledge of Task Dynamics

When selecting actuators for assistive exoskeletons, designers face contrasting requirements. Overdimensioned actuators have drawbacks that compromise their effectiveness in the target application (e.g. performance, weight, comfort). In some cases, the requirements on the powered actuator can be relaxed exploiting the contribution of an elastic element acting in mechanical parallel. This contribution considers one such case and describes an approach to fit the actuation requirements closely to the task dynamics, thereby mitigating the drawbacks of overdimensioned actuators

Effects of an inclination-controlled active spinal exoskeleton on spinal compression forces

Mechanical loading of the spine is a known risk factor for the development of low-back pain. The objective of this study was to assess the effect of an inclination-controlled exoskeleton on spinal compression forces during lifting with various techniques. Peak compression decreased on average by around 20%, and this was largely independent of lifting technique

Actuation Selection for Assistive Exoskeletons: Matching Capabilities to Task Requirements

Selecting actuators for assistive exoskeletons involves decisions in which designers usually face contrasting requirements. While certain choices may depend on the application context or design philosophy, it is generally desirable to avoid oversizing actuators in order to obtain more lightweight and transparent systems, ultimately promoting the adoption of a given device. In many cases, the torque and power requirements can be relaxed by exploiting the contribution of an elastic element acting in mechanical parallel. This contribution considers one such case and introduces a methodology for the evaluation of different actuator choices resulting from the combination of different motors, reduction gears, and parallel stiffness profiles, helping to match actuator capabilities to the task requirements. Such methodology is based on a graphical tool showing how different design choices affect the actuator as a whole. To illustrate the approach, a back-support exoskeleton for lifting tasks is considered as a case study

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

With mechanical loading as the main risk factor for LBP, exoskeletons (EXO) are designed to reduce the load on the back by taking over part of the moment normally generated by back muscles. The present study investigated the effect of an active exoskeleton, controlled using three different control modes (INCLINATION, EMG & HYBRID), on spinal compression forces during lifting with various techniques. Ten healthy male subjects lifted a 15 kg box, with three lifting techniques (free, squat & stoop), each of which was performed four times, once without EXO and once each with the three different control modes. Using inverse dynamics, we calculated L5/S1 joint moments. Subsequently, we estimated spine forces using an EMG-assisted trunk model. Peak compression forces substantially decreased by 17.8% when wearing the EXO compared to NO EXO. However, this reduction was partly, by about one third, attributable to a reduction of 25% in peak lifting speed when wearing the EXO. While subtle differences in back load patterns were seen between the three control modes, no differences in peak compression forces were found. In part, this may be related to limitations in the torque generating capacity of the EXO. Therefore, with the current limitations of the motors it was impossible to determine which of the control modes was best. Despite these limitations, the EXO still reduced both peak and cumulative compression forces by about 18%

Rationale, implementation and evaluation of assistive strategies for an active back-support exoskeleton

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