System Design, Integration, and Preliminary Evaluation of a Compensation-Controlled Active Wrist for a Supernumerary Robotic Limb

Abstract

This paper presents the system-level integration and first experimental evaluation of a 1-DoF active pronation–supination wrist into the SoftHand X, a supernumerary robotic limb developed for upper-limb assistance. Building on an established compensation-control framework, the system combines device-specific kinematic adaptation, a dedicated mechanical interface, an embedded inertial measurement unit (IMU), actuator electronics, and real-time control. The robotic hand retains three alternative control inputs—electromyography (EMG), a finger-mounted flex sensor, and force—while the wrist is independently driven by IMU-based estimates of its deviation from a user-specific relaxed reference orientation. A within-subject study with 12 non-disabled participants compared fixed-wrist and Compensated Wrist Control (CWC) conditions during pouring and cylinder-reorientation tasks. Functional feasibility was assessed through task success and completion time, and perceived usability and workload through the System Usability Scale and NASA Raw Task Load Index. Success remained high across conditions (91.7–100%), with no significant differences in success or completion time. CWC increased usability for the flex and force inputs and reduced workload for the EMG and force inputs in both tasks. Selected IMU trajectories provided a proof-of-concept illustration consistent with the intended compensation behavior. These results support the system-level feasibility of integrating compensation-based active-wrist control into a physical supernumerary limb, with benefits primarily in user experience rather than short-term task performance.