Task-defined Pulley Design for Nonlinearly Coupled Tendon-driven Actuation

Coupled actuation is a common strategy for reducing the number of actuators in robots with high degrees of freedom. Coupled actuation leads to an underactuation that lowers system complexity and weight. However, an approach for a controlled nonlinear coupling of multiple tendons does not exist for tendon-driven actuation. We propose a method to enable task-defined non-linearly coupled actuation by designing eccentric pulleys of variable radii. Given a target profile of tendon length changes, we perform piecewise fitting of the target curve to generate a composite pulley geometry. Theoretical verification of the method with 200 randomly generated tendon profiles shows stable results with an average of 2.19 % relative error. In addition, we built a practical demonstration by implementing non-linearly coupled control of a tendon-driven finger. The six tendons that control the robotic finger were all actuated using pulleys connected to a single motor, resulting in a 5.35 % average fingertip orientation tracking error. In conclusion, the pulley-shaping method proposed in this project enables the task-defined nonlinear coupling of multiple degrees of freedom to a single actuator of tendon-driven systems. We believe that the algorithm's generalizability will enable further practical use in tendon-driven dexterous hands, animatronics, manipulators, and legged robots, among various other possible applications.