Hybrid FES-exoskeleton control: Using MPC to distribute actuation for elbow and wrist movements

Nathan Dunkelberger; Jeffrey Berning; Eric M Schearer; Marcia K O'Malley

doi:10.3389/fnbot.2023.1127783

Hybrid FES-exoskeleton control: Using MPC to distribute actuation for elbow and wrist movements

Front Neurorobot. 2023 Apr 6:17:1127783. doi: 10.3389/fnbot.2023.1127783. eCollection 2023.

Authors

Nathan Dunkelberger¹, Jeffrey Berning¹, Eric M Schearer², Marcia K O'Malley¹

Affiliations

¹ Department of Mechanical Engineering, Mechatronics and Haptics Interfaces Laboratory, Rice University, Houston, TX, United States.
² Center for Human Machine Systems, Department of Mechanical Engineering, Cleveland State University, Cleveland, OH, United States.

Abstract

Introduction: Individuals who have suffered a cervical spinal cord injury prioritize the recovery of upper limb function for completing activities of daily living. Hybrid FES-exoskeleton systems have the potential to assist this population by providing a portable, powered, and wearable device; however, realization of this combination of technologies has been challenging. In particular, it has been difficult to show generalizability across motions, and to define optimal distribution of actuation, given the complex nature of the combined dynamic system.

Methods: In this paper, we present a hybrid controller using a model predictive control (MPC) formulation that combines the actuation of both an exoskeleton and an FES system. The MPC cost function is designed to distribute actuation on a single degree of freedom to favor FES control effort, reducing exoskeleton power consumption, while ensuring smooth movements along different trajectories. Our controller was tested with nine able-bodied participants using FES surface stimulation paired with an upper limb powered exoskeleton. The hybrid controller was compared to an exoskeleton alone controller, and we measured trajectory error and torque while moving the participant through two elbow flexion/extension trajectories, and separately through two wrist flexion/extension trajectories.

Results: The MPC-based hybrid controller showed a reduction in sum of squared torques by an average of 48.7 and 57.9% on the elbow flexion/extension and wrist flexion/extension joints respectively, with only small differences in tracking accuracy compared to the exoskeleton alone.

Discussion: To realize practical implementation of hybrid FES-exoskeleton systems, the control strategy requires translation to multi-DOF movements, achieving more consistent improvement across participants, and balancing control to more fully leverage the muscles' capabilities.

Keywords: functional electrical stimulation (FES); hybrid control (HC); model predictive control (MPC); movement assistance; upper limb exoskeleton.

Grants and funding

This material is based upon work supported by the National Science Foundation under Grants NSF CBET 2025130 and 2025142.