A Control Scheme That Uses Dynamic Postural Synergies to Coordinate a Hybrid Walking Neuroprosthesis: Theory and Experiments.

Naji A Alibeji,Vahidreza Molazadeh,Nitin Sharma,Brad E Dicianno

doi:10.3389/fnins.2018.00159

Abstract

A hybrid walking neuroprosthesis that combines functional electrical stimulation (FES) with a powered lower limb exoskeleton can be used to restore walking in persons with paraplegia. It provides therapeutic benefits of FES and torque reliability of the powered exoskeleton. Moreover, by harnessing metabolic power of muscles via FES, the hybrid combination has a potential to lower power consumption and reduce actuator size in the powered exoskeleton. Its control design, however, must overcome the challenges of actuator redundancy due to the combined use of FES and electric motor. Further, dynamic disturbances such as electromechanical delay (EMD) and muscle fatigue must be considered during the control design process. This ensures stability and control performance despite disparate dynamics of FES and electric motor. In this paper, a general framework to coordinate FES of multiple gait-governing muscles with electric motors is presented. A muscle synergy-inspired control framework is used to derive the controller and is motivated mainly to address the actuator redundancy issue. Dynamic postural synergies between FES of the muscles and the electric motors were artificially generated through optimizations and result in key dynamic postures when activated. These synergies were used in the feedforward path of the control system. A dynamic surface control technique, modified with a delay compensation term, is used as the feedback controller to address model uncertainty, the cascaded muscle activation dynamics, and EMD. To address muscle fatigue, the stimulation levels in the feedforward path were gradually increased based on a model-based fatigue estimate. A Lyapunov-based stability approach was used to derive the controller and guarantee its stability. The synergy-based controller was demonstrated experimentally on an able-bodied subject and person with an incomplete spinal cord injury.

Highlights

Paraplegia in persons with spinal cord injury (SCI) impairs walking function and lowers their quality of life
From the results it can be seen that did the synergy-based controller result in better tracking performance, but it did so while consuming less energy compared to the RISE controller
That when a leg takes the role of the stance leg, the synergy activation is zero which results in zero stimulation and zero desired feedforward motor activation

Summary

Introduction

Paraplegia in persons with spinal cord injury (SCI) impairs walking function and lowers their quality of life. Since FES systems that use either percutaneous or surface electrodes have been used to produce gait (Bajd et al, 1983; Marsolais and Kobetic, 1987; Kralj and Bajd, 1989; Granat et al, 1993; Kobetic et al, 1997; Hardin et al, 2007). Despite this progress, the issue of rapid onset of FES-induced muscle fatigue remains unresolved. The gait is still powered by FES during the swing movement and is affected by FES-induced muscle fatigue

Methods

Results

Discussion

Conclusion