A Robotic Assistance Personalization Control Approach of Hip Exoskeletons for Gait Symmetry Improvement

Healthy human locomotion functions with good gait symmetry depend on rhythmic coordination of the left and right legs, which can be deteriorated by neurological disorders like stroke and spinal cord injury. Powered exoskeletons are promising devices to improve impaired people's locomotion functions, like gait symmetry. However, given higher uncertainties and the time-varying nature of human-robot interaction, providing personalized robotic assistance from exoskeletons to achieve the best gait symmetry is challenging, especially for people with neurological disorders. In this paper, we propose a hierarchical control framework for a bilateral hip exoskeleton to provide the adaptive optimal hip joint assistance with a control objective of imposing the desired gait symmetry during walking. Three control levels are included in the hierarchical framework, including the high-level control to tune three control parameters based on a policy iteration reinforcement learning approach, the middle-level control to define the desired assistive torque profile based on a delayed output feedback control method, and the low-level control to achieve a good torque trajectory tracking performance. To evaluate the feasibility of the proposed control framework, five healthy young participants are recruited for treadmill walking experiments, where an artificial gait asymmetry is imitated as the hemiparesis post-stroke, and only the ‘paretic’ hip joint is controlled with the proposed framework. The pilot experimental studies demonstrate that the hierarchical control framework for the hip exoskeleton successfully (asymmetry index from 8.8% to − 0.5%) and efficiently (less than 4 minutes) achieved the desired gait symmetry by providing adaptive optimal assistance on the ‘paretic’ hip joint.

I. Introduction

Over the last two decades, there has been a flurry of research efforts in developing wearable lower-limb exoskeletons to augment functions for healthy individuals [1], [2] or provide rehabilitation/assistance for individuals with motor deficits [3], [4]. Focusing on the hip joint, various powered joint configurations, actuator designs, and control techniques have been reported to assist gait rehabilitation and human performance augmentation [5]–[9]. The rising research interest on hip exoskeletons lies in 1) the hip is important for powering upright locomotion and postural control [10], 2) the hip joint is capable of manipulating step length, step width, and associated gait symmetry during walking, 3) compared with the ankle joint, the hip joint needs higher metabolic cost for the generation of similar mechanical joint power owing to the differences in muscle characteristics [11], and 4) compared with ankle/knee exoskeletons, hip exoskeletons add less mass to the leg, altering the leg dynamics to a lesser degree. Typically, individuals with neurological disorders caused by diseases or injuries such as a stroke and spinal cord injury generally have muscle weakness, which could lead to insufficient force or torque at the hip joints during locomotion [12], which easily causes gait asymmetry, increase metabolic cost, and poor balance control [13] that deteriorate activities of daily living. Therefore, improving gait symmetry is significant for individuals with neurological impairments.