Robotic assist devices typically consist of a manipulator attached to a robot arm. Often, the assist device must perform the functions of the human hand. Therefore, they should have sufficient dexterity to manipulate an object without putting it down. Four fingered manipulators are by their very nature more complex than then three fingered counterparts. But, with the additional finger, the four fingered hands can manipulate an object without setting it down. We describe a novel design that introduces an additional degree of freedom to one finger, and the design optimization process that insures that the six fundamental grasps (spherical, cylindrical, tip, hook, palmar and lateral) are possible. In addition, we discuss how this one additional degree of freedom increases the manipulability of a three fingered manipulator so that a limited array of object manipulation is possible.

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