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In a lower extremity musculoskeletal leg, the actuation kinematics define the interaction of the actuators with each other and the environment. Design of such a kinematic chain is challenging due to the existence of the redundant biarticular actuators which simultaneously act on two joints, generating a parallel mechanism. Actuator kinematics is mainly dependent on the moment arm profile of the actuation system. It is a common practice to select a constant moment arm value for robotic actuation system; nevertheless, biological muscles feature a distinctive nonlinear moment arm profile that has been ignored in the design of the musculoskeletal robots. In this paper, we propose a design paradigm for compliant robotic leg Carl based on the direct replication of the human leg anatomy. The resulting mechanical system should (a) demonstrate a similar moment arm profile as in leg musculature, (b) exhibit expected physiological behavior of the muscles, (c) provide insight into the interaction of the actuators and possible improvement in the efficiency of the movements. We provide a comprehensive analysis of the moment arm profile of the leg musculature. The actuator kinematics of the designed leg is validated by comparing the contraction velocities of the muscles and actuators. The biological characteristics of the actuators are analyzed using the jump experiment data conducted on the previous version of the leg. The major physiological characteristics of the biarticular muscles, ligamentous action, and distal power transfer, is successfully demonstrated by the robotic leg. Our analysis demonstrates that the proposed structural design of the actuation system can improve the mechanical efficiency of this particular jump experiment up to 16% compared to the leg without actuator redundancy. Compared to the previous version of the leg, by only modifying the moment arm profiles, we can achieve an efficiency improvement of approximately 5%.

Bio-inspired and compliant control approaches have been studied by roboticists for decades to achieve more natural robot motion. Independent of this, medical and biological researchers have discovered a wide variety of muscular properties and higher-level motion characteristics. Although both disciplines strive to better understand natural motion and muscle coordination, they have yet to meet. This work introduces a novel robotic control strategy that bridges the gap between these distinct areas. By applying biological characteristics to electrical series elastic actuators, we developed a simple yet efficient distributed damping control strategy. The presented control covers the entire robotic drive train, from abstract whole-body commands to the applied current. The functionality of this control is biologically motivated, theoretically discussed, and finally evaluated through experiments on the bipedal robot Carl. Together, these results demonstrate that the proposed strategy fulfills all requirements that are necessary to continue developing more complex robotic tasks based on this novel muscular control philosophy.