Structural Optimization of a Spherical Parallel Manipulator Using a Two-Level Approach

Abstract

Abstract The Agile Wrist, a spherical wrist with a parallel, isotropic architecture for highest orientational accuracy, is being designed as a module of an 11-degree-of-freedom (dof) robot. The wrist consists of two main elements, the base and the moving plates. The two plates are coupled by means of three identical legs, each of these composed of two links, proximal and distal, coupled to each other by a revolute joint. Each leg, in turn, is coupled to its proximal plate via revolute joints. Moreover, the three axes of the leg-revolute joints are concurrent at the center of the wrist, each axis making an angle of 90° with its neighbor. Direct-drive DC motors are used to rotate the wrist proximal links and electrical brakes and optical encoders are located on each of the motor shafts for control purposes. In this paper we introduce a two-level approach to the optimum design of the proximal link of the Agile Wrist. First, the shape of the midcurve producing minimum stress concentrations is obtained by means of the concept of curve synthesis using cubic splines. At the second level, the optimum cross-section along the midcurve producing a link of minimum weight is determined.