Méthode systématique de conception de cinématiques parallèles

Abstract

Parallel kinematics structures currently find increasing industrial applications, particularly in packaging and assembly. With very few exceptions, machine-tools with parallel kinematics are limited to mechanisms with three or six degrees of freedom. Moreover, the spindle tilt is limited to very small angles. The advantages of parallel kinematics structures are high precision, lower fabrication cost, good dynamics and high stiffness. This dissertation proposes a systematic configuration design method for the development of new parallel kinematics structures. The parallel structures obtained with this method combine large spindle tilt angles with the advantages of parallel kinematics structures. First, components and characteristic elements of parallel structures are defined and the description of a proposed systematic method for configuration design, the Design Cube Method, follows. The method enables the design of a family of parallel structures with a specified number and arrangement of mobilities. Methods such as Gruebler's Formula and the Method of Intersecting Trajectories serve to quickly verify the desired number of degrees of freedom of the structure and the optimal arrangement of the kinematic chains. Several mechanisms are described, that once integrated in a parallel structure, significantly increase the range of tilt of the end-effector, while keeping the stiffness as constant and as high as possible. A new parallel kinematics structure for a five-axis machine-tool has been developed using the new design methods described above. The performance specifications are listed, the desired arrangement of the axis is described and the design cube method is used to generate a new family of parallel kinematics structures. The various structures are evaluated in terms of how suited they are for machine-tool applications, and of the ease of integration of a mechanism to improve the tilt of the end-effector. A laboratory mock-up, demonstrating the various principles, and an industrial prototype of the new machine, the Hita-PDR, have been developed and fabricated. The bar lengths of the structure were selected so as to avoid singularities of the structure in the desired workspace. The specified high stiffness of the industrial prototype required the development of a new type of joint, the simple and double spherical joints with line contact. The high performances of the industrial prototype confirm the machine concept and the relevant choice of the parallel kinematics structure. This dissertation gives a global vision, of the various stages in the design of parallel kinematics structures with large rotations of the end-effector and high stiffness, starting with initial specifications, all the way up to the detailed design and manufacturing of the machine.