Design and development of a cybernetic exoskeleton for hand-wrist rehabilitation through integration of human passive properties

Abstract:

Robotic devices are excellent candidates for delivering repetitive and intensive practice that can restore functional use of the upper limbs, even years after a stroke. Rehabilitation of the wrist and hand in particular are critical for recovery of function, since hands are the primary interface with the world.
However, robotic devices that focus on hand rehabilitation are limited due to excessive cost, complexity, or limited functionality. A design and control strategy for such devices that bridges this gap is critical. The goals of the research effort are to analyze the properties and role of passive dynamics, defined by joint stiffness and damping, in the human hand and wrist during grasping and manipulation, and then mimic such properties in a wrist-hand exoskeleton for stroke rehabilitation. The project will culminate with device testing in collaboration with rehabilitation clinicians. A significant problem in robotic rehabilitation is how to provide assisted movement to the multiple degrees of freedom of the hand in order to restore motor coordination and function, with a system that is practical for deployment in a clinical environment.  Armed with a clearer understanding of the mechanisms underlying passive dynamics and control of systems exhibiting such behavior, this project informs the design of more effective wrist/hand rehabilitation devices that are feasible for clinical use. In addition, the project has created a unique interdisciplinary environment enabling education, training, and co-advising of graduate students, undergraduate research, and significant and targeted outreach activities to underrepresented groups in science and engineering. In the third year of the project, the PIs have made significant progress towards the goal of an integrated wrist-hand exoskeleton for robot-assisted stroke rehabilitation. Specifically, the team has developed the next version of index finger exoskeleton with the Bowden-cable-based series elastic actuation that allows for bidirectional torque control of the device with high backdrivability and low reflected inertia. We have explored methods for attachment of the exoskeleton to the patient’s body and developed a new design of the exoskeletal attachment to the metacarpal bone of the thumb constructed from galvanized steel wire.  We have carried out the characterization of the two versions of the wrist module (RiceWrist-S and RiceWrist), which included determining system parameters such as torque output, workspace, static and viscous friction, spatial resolution, and closed-loop position bandwidth. We have developed a novel Assist-as-needed (AAN) controller for the exoskeleton. Our controller introduces two fundamental and novel contributions: a feedback gain modification algorithm, in order to regulate the assistance level according to performance of the patient, and an online trajectory recalculation algorithm. Finally, we have also initiated a case study with a subject, a 45-year old male affected by chronic incomplete (AIS level C) SCI at the C3-5 level, to test the efficacy of our robot in delivering rehabilitation therapy. Concurrently, several outreach activities have also been undertaken, including laboratory tours for K-12 groups of underrepresented students at both the University of Texas and Rice University and course development.