Neuromechanical model-based adaptive control of bi-lateral ankle exoskeletons: biological joint torque and electromyogram reduction across walking conditions

TL;DR

This paper introduces a neuromechanical model-based control system for bilateral ankle exoskeletons that adaptively supports various walking conditions, reducing biological torque and muscle activity without predefined profiles or rules.

Contribution

It presents a novel adaptive control approach using person-specific neuromechanical models that operate across unseen walking conditions, enabling more natural and flexible exoskeleton assistance.

Findings

Reduced biological ankle torque by 22-24% across conditions

Decreased ankle muscle EMG activity by 12-14%

Enabled control during complex tasks like moonwalking

Abstract

To enable the broad adoption of wearable robotic exoskeletons in medical and industrial settings, it is crucial they can adaptively support large repertoires of movements. We propose a new human-machine interface to simultaneously drive bilateral ankle exoskeletons during a range of 'unseen' walking conditions and transitions that were not used for establishing the control interface. The proposed approach used person-specific neuromechanical models to estimate biological ankle joint torques in real-time from measured electromyograms (EMGS) and joint angles. A low-level controller based on a disturbance observer translated biological torque estimates into exoskeleton commands. We call this 'neuromechanical model-based control' (NMBC). NMBC enabled six individuals to voluntarily control a bilateral ankle exoskeleton across six walking conditions, including all intermediate transitions, i.e., two walking speeds, each performed at three ground elevations, with no need for predefined torque profiles, nor a priori chosen neuromuscular reflex rules, or state machines as common in literature. A single subject case-study was carried out on a dexterous locomotion tasks involving moonwalking. NMBC always enabled reducing biological ankle torques, as well as eight ankle muscle EMGs both within (22% torque; 12% EMG) and between walking conditions (24% torque; 14% EMG) when compared to non-assisted conditions. Torque and EMG reductions in novel walking conditions indicated that the exoskeleton operated symbiotically, as exomuscles controlled by the operator's neuromuscular system. This opens new avenues for the systematic adoption of wearable robots as part of out-of-the-lab medical and occupational settings.