Review of control strategies for lower-limb exoskeletons to assist gait

Abstract

Background Many lower-limb exoskeletons have been developed to assist gait, exhibiting a large range of control methods. The goal of this paper is to review and classify these control strategies, that determine how these devices interact with the user. Methods In addition to covering the recent publications on the control of lower-limb exoskeletons for gait assistance, an effort has been made to review the controllers independently of the hardware and implementation aspects. The common 3-level structure (high, middle, and low levels) is first used to separate the continuous behavior (mid-level) from the implementation of position/torque control (low-level) and the detection of the terrain or user’s intention (high-level). Within these levels, different approaches (functional units) have been identified and combined to describe each considered controller. Results 291 references have been considered and sorted by the proposed classification. The methods identified in the high-level are manual user input, brain interfaces, or automatic mode detection based on the terrain or user’s movements. In the mid-level, the synchronization is most often based on manual triggers by the user, discrete events (followed by state machines or time-based progression), or continuous estimations using state variables. The desired action is determined based on position/torque profiles, model-based calculations, or other custom functions of the sensory signals. In the low-level, position or torque controllers are used to carry out the desired actions. In addition to a more detailed description of these methods, the variants of implementation within each one are also compared and discussed in the paper. Conclusions By listing and comparing the features of the reviewed controllers, this work can help in understanding the numerous techniques found in the literature. The main identified trends are the use of pre-defined trajectories for full-mobilization and event-triggered (or adaptive-frequency-oscillator-synchronized) torque profiles for partial assistance. More recently, advanced methods to adapt the position/torque profiles online and automatically detect terrains or locomotion modes have become more common, but these are largely still limited to laboratory settings. An analysis of the possible underlying reasons of the identified trends is also carried out and opportunities for further studies are discussed.

A Survey on Design and Control of Lower Extremity Exoskeletons for Bipedal Walking

“…The existing reviews on bipedal walking exoskeletons have been performed for medical applications. A majority of the researchers in the last decade focused on modular rehabilitation of the ankle or knee joints [35][36][37][38] while some selected articles reviewed the leg composition, but with an additional supportive device [39][40][41][42][43][44]. There have been reviews, designs, and clinical gait evaluations in the last few decades, but no specific meta-analysis has been made combining several analyses, evaluations, design, and control methods to validate a model as a proof of concept for developing efficient, affordable, and safety-wise bipedal walking exoskeleton robots that require no additional crutches support.…”

Coordination Between Partial Robotic Exoskeletons and Human Gait: A Comprehensive Review on Control Strategies

“…Across the reviewed literature, the inconsistency found in the experimental validation of the reviewed control strategies is remarkable. As reported in other reviews ( Pinto-Fernandez et al, 2020 ; Baud et al, 2021 ), the lack of common validation protocols and metrics makes it extremely difficult to compare results from different devices and reliably assess the benefits of each control strategy. Regarding the reviewed works, we found two main issues that were divergent: 1) the subject population that tested the devices and 2) the evaluated outcomes.…”

“…In terms of control strategies, Tucker et al provided an overview of the different control strategies for lower-limb robotic prostheses and orthoses and introduced a three-level paradigm controller development ( Tucker et al, 2015 ). This same paradigm was recently updated and completed by Baud et al, who systematically analyzed the control strategies of lower-limb exoskeletons by dividing them into functional units ( Baud et al, 2021 ). Considering these three-level paradigms, Ma et al performed a deeper review of the high-level controllers responsible for the voluntary control of robotic devices ( Ma et al, 2019 ), while Miscon et al focused on middle-level controllers, in particular, joint trajectory generation for robotic exoskeletons ( Miskon and Yusof, 2014 ).…”

“…In contrast with complete lower-limb exoskeletons, which act over the hips and knees (even ankles) of both limbs, partial exoskeletons act partially over the locomotor system of the wearer by exclusively assisting one joint (single-joint exoskeletons) or one leg of the user (unilateral exoskeletons). Although they present several advantages [they are simpler and lighter than bilateral devices ( Baud et al, 2021 ) and can target the specific function of the assisted joint during gait ( Yan et al, 2015a )], they need to ensure proper interjoint coordination, especially with non-actuated joints. Unlike complete exoskeletons, which can impose the appropriate coordination between joints and legs, partial exoskeletons cannot actuate globally in the entire locomotor system; therefore, this interjoint coordination needs to be resolved by the controller of the device.…”

Control strategies used in lower limb exoskeletons for gait rehabilitation after brain injury: a systematic review and analysis of clinical effectiveness

“…The interest on lower-limb exoskeletons for gait re-69 habilitation has increased exponentially in the last 70 years, which is reflected in the considerable number 71 of reviews published within the last decade [38,. 72 However, the majority of these reviews focus on hard-73 ware, while only a few of them analyzed the control 74 strategies implemented on lower limb exoskeletons and 75 their effects on walking function in people with brain 76 injuries [38,41,42,[54][55][56][57][58][59][60]]. Yet, the control strategy -77 as ergonomics and robot actuation-might play a key 78 role on the effectiveness of the robotic treatment [61].…”