Robotic Knee Tracking Control to Mimic the Intact Human Knee Profile Based on Actor-critic Reinforcement Learning

TL;DR

This paper presents an actor-critic reinforcement learning approach for robotic knee prosthesis control, enabling the prosthesis to mimic the natural knee profile during various walking conditions, with theoretical guarantees and simulation validation.

Contribution

It introduces a novel RL-based tracking control algorithm for robotic knees, with analytical stability guarantees and practical simulation results demonstrating its effectiveness.

Findings

Effective knee profile tracking demonstrated in simulations

The control algorithm ensures stability and convergence

Enables adaptive walking on different terrains and speeds

Abstract

We address a state-of-the-art reinforcement learning (RL) control approach to automatically configure robotic prosthesis impedance parameters to enable end-to-end, continuous locomotion intended for transfemoral amputee subjects. Specifically, our actor-critic based RL provides tracking control of a robotic knee prosthesis to mimic the intact knee profile. This is a significant advance from our previous RL based automatic tuning of prosthesis control parameters which have centered on regulation control with a designer prescribed robotic knee profile as the target. In addition to presenting the complete tracking control algorithm based on direct heuristic dynamic programming (dHDP), we provide an analytical framework for the tracking controller with constrained inputs. We show that our proposed tracking control possesses several important properties, such as weight convergence of the learning networks, Bellman (sub)optimality of the cost-to-go value function and control input, and practical stability of the human-robot system under input constraint. We further provide a systematic simulation of the proposed tracking control using a realistic human-robot system simulator, the OpenSim, to emulate how the dHDP enables level ground walking, walking on different terrains and at different paces. These results show that our proposed dHDP based tracking control is not only theoretically suitable, but also practically useful.