Recent Development of Unpowered Exoskeletons for Lower Extremity: A Survey

Abstract

Unpowered exoskeletons (UEs) have attracted extensive research attentions for their portability, handleability and simplicity. However, designing exoskeletons without any active actuators is a great challenge because essential criteria such as performance enhancement, low impedance, and comfort must be met. This paper reviews the UEs researches from three aspects including modeling for understanding gait pattern, mechanical designs, and validation methods, which are main considerations of designing UEs. This review is based on the reported works in the past two decades and underlines some practical opportunities and challenges. Statistical data analysis and critical reviews are stated to understand human gait patterns, designing of UEs, and ergonomic validation. This review is trying to inspire a common understanding about passive exoskeletons for lower extremity and promote discussion among researchers, developers, or robotic practitioners.

Cooperativity Model for Improving the Walking-Assistance Efficiency of the Exoskeleton

“…Through further calculations, the walking-assistance efficiencies of the exoskeleton under the five parameter combinations are shown in Figure 11 and the results were 17.95%, 16.65%, 14.36%, 16.97%, and 15.10%, in which the HESM with the optimal parameter combinations exhibited the highest walking-assistance efficiency [ 3 , 25 ]. In addition, the walking-assistance efficiencies obtained by the dynamic simulation were slightly lower than the target walking-assistance efficiency, which is 20%, and generally higher than the previous lower-limb-assistant exoskeleton [ 3 , 4 , 23 , 24 , 30 ], but far lower than the extreme walking-assistance efficiency, which is 52.36%.…”

“…Compared with the hip joint, the discrete assistance measure was implemented for the knee joint [ 25 , 28 ]. The evaluation for the KESM is based on the support period, and 100 Nmm/deg, 200 Nmm/deg, 400 Nmm/deg, 500 Nmm/deg, and 700 Nmm/deg were selected for the spring stiffness.…”

“…Passive exoskeletons have advantages such as convenience, stability, and low cost, but their walking-assistance efficiencies are generally lower than powered exoskeletons [ 25 , 26 ]. Considering the working mechanism of the passive exoskeleton with the walking-assistance function [ 3 , 4 , 20 , 21 , 23 , 24 ], an important way to improve its walking-assistance efficiency is to improve the energy reuse efficiency [ 3 , 27 ].…”

“…The powered exoskeleton outputs the matching torque to the human by monitoring the human physiological data [ 28 , 29 ], but this poses a challenge to the passive exoskeleton, which does not have real-time-control capabilities. Literature [ 25 ] analyzed the existing passive exoskeleton and concluded that the spring with the optimal stiffness is of great significance for the metabolic benefits of the exoskeleton. Literature [ 3 ] also found, in the test of a passive ankle exoskeleton, that there is an optimal spring stiffness that could produce the optimal metabolic benefits of the exoskeleton.…”

“…Literature [ 3 ] also found, in the test of a passive ankle exoskeleton, that there is an optimal spring stiffness that could produce the optimal metabolic benefits of the exoskeleton. The reasonable selection of the elastic coefficient of the exoskeleton system to enhance the metabolic benefits of the passive exoskeleton is the consensus of the existing research, but the selection mechanism of the optimal spring stiffness is not clear [ 3 , 23 , 25 ], especially the selection factors that affect the optimal stiffness, such as structural characteristics, walking-assistance principle, etc. To address this issue, in this paper, the cooperativity model was established to emphasize the research of the torque transmission law between humans and exoskeletons in torque cooperativity and the motion response of the exoskeletons in motion cooperativity, aiming to attach the working mechanism of the exoskeletons to the biomechanical law to more effectively realize the distribution of human energy.…”