I. Introduction
In humanoid robotics, lightweight design plays a significant role due to limited energy storage capacity and collision safety in human interaction applications. However, existing lightweight robots often focus only on control, kinematics and actuator design rather than the optimization of the mechanical support structure. For example, Schäffer et al. [1] developed the DLR lightweight robot, in which low mass was realized via a lightweight actuation with harmonic drive gears and high density motors. Buchler et al. [2] placed the actuators of a robot arm in the base instead of the moving links in order to reduce the weight of the moving masses. Mori et al. [3] designed a humanoid robot arm, where the lightweight design was achieved by a cable-driven architecture with integrated low weight compressors in the arm links. Sureshbabu et al. [4] added structural rigidity in the humanoid forearm by adopting parallel mechanisms and entirely avoiding support structural topologies.