Micro machine tool oriented optimum design of 3-RPS parallel mechanism with large titling and uniform deflecting capacities

Parallel kinematic mechanism has shown its advantages as a high-precision motion stage in a machine tool. In this paper, a RPS-XY hybrid kinematic structure TRIPOD actuated by linear ultrasonic motors is proposed and designed for a five-axis ultra-precision micro machine tool. In the designing stage, both the titling capacity and the consistency of the deflecting abilities in all directions need to be carefully designed to overcome the platform's limitation in the freeform surface machining. This study proposes an optimizer that can automatically search for a set of design parameters of the 3-RPS parallel part to generate large titling and uniform deflecting capacities simultaneously. A specified orientation workspace is calculated to evaluate the ability of rotation. Additionally, a new index is introduced to justify the uniform deflecting capacity. By evaluating those factors as the objective functions, a modified NSGA-II algorithm is employed for the optimum design with the consideration of discretely selected linear guides and spherical joints from a predefined database, which contains the abstracted engineering models of commercial products. Optimal results verify the feasibility and effectiveness of the proposed optimizer in designing a physical platform. The technique is extensible to most of other parallel mechanisms as a standard procedure.