Adaptive and Dexterous Tendon-Driven Underactuated Finger Design with a Predefined Elastic Force Gradient

To address the current problem of decoupling between joints and the fixed synergy relationship of underactuated fingers, an adaptive and dexterous underactuated finger named the LMH finger was designed in this article. The LMH finger has two degrees of actuation (DoAs) and three degrees of freedom (DoFs) based on the predefined elastic force (PEF) gradient, which actively changes the joint coupling relationship and the synergistic control structure. With the changes in the driving forces and external forces, the LMH finger is used to establish three conditional synergistic control (CSC) laws for the mutual transformation of adaptivity and dexterity. With a double-tendon-pulleyed winding mechanism and a PEF gradient, the LMH finger provides three single-joint independent motion modes and four multijoint synergistic motion modes in a simple differential control of double driving forces. To improve the antagonistic control ability of bending and extension and the enveloping ability, the crank-slider and crank-rocker mechanisms of the LMH finger provide a force synergy mechanism and a continuous contact surface for replaceable modules. Depending on the control conditions of driving forces, external forces, and PEFs, the LMH finger can approximately achieve the dexterity of human fingers in the pregrasping stage and adaptivity in the grasping stage. Finally, the CSC method and the design paradigm are successfully applied to a two-DoA and six-DoF underactuated two-fingered hand for symmetric and asymmetric dexterous manipulations, which verifies the improvement in the grasping adaptability and manipulative dexterity.