Passive joint compliance is a key feature of the human hand that plays an important role in realizing dexterous, precise and graceful movements. Our goal is to study the role of passive compliance in the human hand joints and develop ways for implementing human-like passive compliance in a robotic form. While a variety of factors influence joint stiffness in a tendon-driven system the focus of this paper is on investigating the effects of variation in moment arms, defined by the joint shape, on the joint stiffness characteristics. We present a method for analyzing the effects of variable moment arms on the joint stiffness variations and a mathematical technique to synthesize joint shapes based on the stiffness requirements. We study the role of variable passive stiffness by analyzing energy consumption and dynamic response with system models. We validate our theoretical results in shape synthesis with an experimental platform involving a single degree of freedom tendon driven joint. The analysis and results of our work have implications in the design of various robotic systems, including robotic hands, that seek to incorporate human-like joint stiffness characteristics.

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