| Parallel manipulators potentially possess some superior properties over their serial counterparts, e.g., high ratio of load to self weight, low inertia, high stiffness, etc. Differing greatly from that of a serial manipulator, however, the performance of a parallel manipulator highly depends on its dimensions. A dimensional synthesis is absolutely necessary to maintain advantages of a parallel manipulator and complying as close as possible with the performance needed for the task at hand.; This work intends to deal with the optimal kinematic synthesis problem of parallel manipulators. A unified framework is novelly proposed for optimal design of parallel manipulators. By observing that regular (e.g., hyperrectangular) workspaces are desirable for most machines, we propose the concept of effective regular workspace, which reflects both requirements on the workspace shape and quality. Dexterity index is utilized to characterize the effectiveness of the workspace. Other performance indices, such as manipulability, stiffness, and minimal natural frequency, can be readily included. The optimal design problem is then formulated to find a manipulator geometry that maximizes the effective regular workspace. Since the optimal design problem is a multimodal constrained nonlinear optimization problem without explicit analytical expression, traditional gradient based optimization techniques may have difficulty in searching the global optimum. The controlled random search (CRS) technique, which was reported as a robust and reliable direct search method, is applied to numerically solve the problem. Some typical parallel manipulators, a five-bar parallel linkage, a rotational Delta robot, and a Gough-Stewart platform are employed as examples to demonstrate the design procedure.; We prove rigorously that a fully parallel mechanism consisting of 3-P P*a sub-chains undergoes generically 3-DoF purely translational motion. Here, P*a represents a spatial parallelogram whose joints are all spherical joints. Using the novelly proposed design formulation, a thorough search of all parallel mechanisms having a 3-P P*a topology, is conducted. For mechanisms having 3-P P*a topology, the best architecture is the one whose all prismatic joint axes are perpendicular mutually and intersect at a common point. We name this architecture Orthopod. This mechanism is simple in topology and easy for assembly. A prototype machine is manufactured. Positioning control and velocity based trajectory tracking experiments are conducted. The prototype Orthopod shows good performance in positioning accuracy. |
