Parallel wrists for enhancing grasping performance

Good grasping and effective manipulation heavily depend on the performance of robotic wrists such as, e.g., the number of degrees of freedom, the kind of motion that is generated, the dexterity of the operations, the stiffness, and the size of the mechanical structure; such characteristics heavily affect kinematic and dynamic performance of the manipulation and can lead to a successful grasp or to an unexpected failure, if not taken into consideration since the early design steps. This chapter, after an introduction recalling the wrist structure of the industrial manipulators, focuses on parallel kinematics wrists, a rather new kind of mechanical architecture that has not found so far relevant industrial applications but shows very promising features, such as mechanical stiffness, high accuracy, lightweight construction, and so on. After presenting a powerful kinematical tool for the synthesis of parallel kinematics machines (SPM), which is based on Lie algebra, the design of a novel spherical wrist is discussed in details. A prototype machine, actuated by three brushless linear motors, has been built with the aim of obtaining good static and dynamic performance.