The human hand has extraordinary dexterity with more than 20 degrees of freedom (DOF) actuated by lightweight and efficient biological actuators (i.e., muscles). The average weight of human hand is only 400g [1]. Over the last few decades, research and commercialization effort has been dedicated to the development of novel robotic hands for humanoid or prosthetic application towards dexterous and biomimetic design [2]. However, due to the limitations of existing electric motors in terms of torque density and energy efficiency, the design of humanoid hands has to compromise between dexterity and weight. For example, commercial prosthetic terminal devices i-Limb [3] and Bebionic [4] prioritize the lightweight need (450g) and use 5-DOF motors to under-actuated 11 joints, which is only able to realize a few basic grasp postures. On the other hand, some humanoid robot hand devices like DLR-HIT I & II hands [5] prioritize the dexterity need (15 DOF), but weigh more than four times than their biological counterpart (2200g and 1500g, respectively).

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