Kinetostatic analysis of a spatial cable-actuated variable stiffness joint

The demand for robots capable of performing collaborative tasks requiring interactions with the environment is on the rise. Safe interactions with the environment require attributes such as high dexterity and compliance around obstacles, while still maintaining the requisite stiffness levels for payload manipulation. Such attributes are inherent to biological musculoskeletal systems. Motivated by this realization, this paper proposes a cable-actuated spatial joint with variable stiffness, inspired by the tensegrity principles found in biological musculoskeletal systems. The paper provides a detailed analysis of the joint's mobility and mechanism kinematics. Based on the limits of the actuation forces, the paper also presents the wrench-feasible workspace of the joint. The paper also outlines the conditions that the cable actuation forces must satisfy to maintain the static equilibrium of the joint. The stiffness modelling presented in this work demonstrates the modulation of stiffness bounds as a function of cable actuation forces. Furthermore, the stiffness modulation as a function of the geometrical parameters is also presented.