Fast replanning of a lower-limb exoskeleton trajectories for rehabilitation

TL;DR

This paper introduces a rapid numerical method for replanning lower-limb exoskeleton trajectories during rehabilitation, ensuring stability and responsiveness within milliseconds, thus enhancing patient-specific therapy adjustments.

Contribution

A novel, fast replanning algorithm based on optimal control and Pontryagin's principle for stable, real-time exoskeleton trajectory adjustments during rehabilitation.

Findings

Replanning resolution time below 1 ms

Effective handling of practical physiotherapy scenarios

Stable trajectory updates demonstrated in realistic simulations

Abstract

The paper addresses the rehabilitation of disabled patients using a lower-limb fully-actuated exoskeleton. We propose a novel numerical method to replan the current step without jeopardizing stability. Stability is evaluated in the light of a simple linear time-invariant surrogate model. The method's core is the analysis of an input-constrained optimal control problem with state specified at an unspecified terminal time. A detailed study of the extremals given by Pontryagin Maximum Principle is sufficient to characterize its feasibility. This allows a fast replanning strategy. The efficiency of the numerical algorithm (resolution time below 1 ms) yields responsiveness to the patient's request. Realistic simulations on a full-body model of the patient-exoskeleton system stress that cases of practical interest for physiotherapists are well-addressed.