Design and implementation of a synergy-based cable-driven humanoid arm with variable stiffness

Cable-driven arms have the advantages of light weight, large workspace, good compliance, high speed and acceleration. This paper proposes a cable-driven variable stiffness humanoid arm that can reproduce typical daily postures of the upper limb with a few actuators via kinematic synergy. A kinematic model of the arm is established to obtain the design parameters corresponding to different postures. The dimension reduction of the actuation is realized through a synergy analysis of the driving cables. A coupling actuation mechanism is designed to reduce the number of actuators required to generate specific postures of the arm via cables. Optimization of the geometric parameters of the joints is conducted to improve the posture reproduction accuracy. The stiffness of the arm could be regulated by adjusting the cable tension. Stiffness modeling of the joint is performed to evaluate the influence of cable tension. A prototype of the arm is designed. The workspace is analyzed under the actuation of the designed coupling mechanism. The transformation among the targeted postures is simulated to validate the feasibility of the actuation dimension reduction design of the arm. Robustness analysis is conducted which indicates the use of synergic actuation weakens the arm robustness. With the proposed dimension reduction method, the actuation dimensions are reduced from 9 to 4, which leads to the diminution of reachable workspace and manipulability. The reproduction accuracy of the targeted postures is 90.4%. The proposed method can be applied to the dimension reduction designs of other cable-driven robots.