Upper Limb Deweighting Using Underactuated End-Effector-Based Backdrivable Manipulanda

Abstract

In rehabilitation for neurological injuries, gravity compensation or deweighting of the upper limb is often performed, as it allows patients with limited muscle activities to realize movements. Deweighting cancels the gravitational effect of the human arm allowing the available muscle forces to produce acceleration, and thus movement of the arm. In robotic devices designed for rehabilitation, deweighting is performed by applying forces to the arm, either joint-by-joint (e.g., with exoskeletons) or at a single point (e.g., with end-effector-based manipulanda). This paper formulates the gravity compensation strategy for spatial (three-dimensional) manipulanda, considers the effect of the force applied by the robotic device on the generalized dynamics of the upper limb and critically evaluates the advantages and limitations of this approach. Additionally, the proposed strategy is validated on the EMU robot, designed with the emphasis on its dynamically transparent mechanical transmission to allow an impedance control approach. The configuration of the EMU robot used in this letter only has 3 degrees of actuation, making it underactuated with respect to the task of regulating a human arm up to the wrist modeled with 4 degrees of freedom. The underactuation resulted in an uncompensated gravity induced moment about the swivel angle axis, which is a line connecting the shoulder and wrist points of the human arm. The experimental validation demonstrates the effectiveness of the proposed gravity compensation technique in cancelling the effect of gravity on the user's arm.