This paper presents a novel impedance controller for THINGER (THumb INdividuating Grasp Exercise Robot), a 2-degree-of-freedom (DOF) spherical 5-bar exoskeleton designed to augment FINGER (Finger INdividuating Grasp Exercise Robot). Many rehabilitation and assessment tasks, for which THINGER is designed, are improved by rendering near-zero impedance during physical human-robot interaction (pHRI). To achieve this goal, the presented impedance controller includes several novel features. First, a reference trajectory is omitted, allowing free movements. Second, force-feedback gains are reduced near actuator limits and a saturation function limits the maximum commanded force; both allow more responsive (higher) force-feedback gains within the workspace and mitigate transient oscillations caused by external disturbances. Finally, manipulability-based directional force-feedback gains help improve rendered impedance isotropy. Validation experiments included free exploration of the workspace, following a prescribed circular thumb motion, and intentional exposure to external disturbances. The experimental results show that the presented impedance controller significantly reduces impedance to subject-initiated motion and accurately renders the desired isotropic low-impedance environment.
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