Méthode systématique de conception de cinématiques parallèles

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.

Design of hybrid-kinematic mechanisms for machine tools

Patric Pham

The machine tool industry is a well established, old and extremely important branch of today's manufacturing industry. With the ongoing globalization and the resulting increase of competition in this industry, the manufacturers have to push their technology to the limits in order to stay competitive. The architecture (kinematics) of most machine tools is based on a serial arrangement of joints and segments, like a human arm. The requirements regarding dynamics, stiffness and precision of these machines brought the scientists and industries to evaluate parallel kinematics for this type of application. Parallel kinematics possess a much higher potential to fulfill these demands, and they would therefore allow the access to a next level of machine performance. Whereas the success of parallel kinematics in domains like packaging is incontestable, it proved to be less evident in machine tools. The low rotation amplitudes and the complexity of the mechanism, the main weak points of parallel kinematics, slow down the development and integration of this kind of machines. In the last few years however, we could observe an increase in development, and more important, in the sales (1)(37)(54) of hybrid kinematic machines. Hybrid kinematics can, by appropriate combination of parallel and serial axes, present a well performing compromise, especially in the machine tool domain where 5 axes/mobilities and high rotation amplitudes are common. The present document is concerned with the mechanical, industrialized design of hybrid-kinematic machine tools and their mechanical elements, and will show that "Hybrid-kinematic mechanisms can outperform fully-parallel mechanisms considering all attributes for a successful and industrialized machine design." The work will point out the limits of fully-parallel mechanisms and justify the use of hybrid solutions. The most important elements of the mechanisms, thereof particularly the spherical and universal joints, will be treated in a detailed manner. Industrialization aspects will be analyzed, the difficulty for their integration will be shown, and solutions provided in order to increase the accessibility of hybrid and parallel mechanisms. A design methodology will be synthesized from all these elements and applied to three case studies. The methodology will point out important and often neglected steps and provide elements and tools to support the designer in the whole process of creation. Furthermore, by providing a broad catalogue of both new and existing hybrid and parallel kinematics, this work is intended to stimulate and inspire the creativity of the designer. The three final cases studies, each differing in their application domain and representing each an unpublished concept, will illustrate and validate the methodology. The work took place around multiple industrial projects and therefore always keeps in mind the practical feasibility, with respect to an industrial environment, and the economic aspects and risks.

EPFL2009

Conception systématique de structures cinématiques orthogonales pour la microrobotique

Patrick Helmer

The rapid evolution in microsystems and optoelectronics creates the demand for robotic production means with increased performance. The required precision for specific tasks in machining, positioning and assembly is a factor 10 to 100 better than 1 micrometer, while keeping high dynamic properties and motion ranges of up to 1 centimeter. Every application has its own constraints, for the layout of the desired degrees of freedom of the robot as well as for the available volume, and induces a considerable development duration. This thesis aims at making the first phase of manipulator design process more efficient, from the specifications gathering down to the kinematics structure design. The solutions space of all possible kinematics in 3D being incredibly broad and rich, several simplifications have been introduced, in order to respond to the designer's need for structure and pragmatism. On the one hand, only structures having forces aligned with the main orthogonal axes in space are considered, which allows a systematic classification of the kinematics chains and allows us to define the generic concept of "functional" joint with well defined kinematics properties. On the other hand, the proposed methodology is limited to qualitative aspects of motion, for small motion ranges, with a systematic use of the "parasitic" motion concept. The first phase of the design problem is solved in two steps: first, the "elementary" kinematics function of the robot is identified, and second, alternative structures are generated in a creative process, following a set of principles and using a variety of partial and global solutions. The generation of structures with parallel or hybrid kinematics is thereby greatly enhanced and possible problems of mechanical overconstraint are systematically identified. Elastic solid state joints are incorporated in the generated structures on the one hand through a set of design recommendations and on the other hand through specific substitutions. A 6 degrees of freedom micromanipulator with high dynamics and nanometric position resolution was designed during this thesis and a patent application was filed. The prototype "Sigma 6" was built and successfully used to perform an active fiber alignment procedure. A new family of rotational structures with 1 or 3 degrees of freedom, a remote rotation center and reduced parasitic rotation center shift was designed. A new decoupled arm with increased mechanical load capability made of 18 elastic joints was designed and a prototype built, as an alternative to traditional arms ended by a universal and a spherical joint.

EPFL2006

Haptic interface design and control with application to surgery simulation

Ulrich Spälter

This thesis proposes a framework for haptic devices interacting with virtual environments. As application of the theoretical results a haptic device for the surgical training of hysteroscopy, a gynecologic intervention, is presented. Surgery simulators, which can be described as 'flight simulators for surgeons', comprise a virtual reality for modeling and visualization of organs, and the haptic device: While the surgeon interactively follows the scene on the computer screen, he perceives the contact forces at the tool handle of the surgical instrument. The haptic device tracks the position of the surgery tool and transforms the virtual contact forces to real forces, which can be actually felt by the surgeon. This thesis treats haptic devices with application to surgery simulators for minimally-invasive surgery. Haptic interfaces are complex mechatronic systems and show inherent trade-offs between haptic realism and system stability. Human factors play an important role, as the human operator can both stabilize or destabilize the system. The haptic perception, which can vary between individuals, as a criterion for haptic rendering quality is still not formalized. The optimization of haptic interfaces, within focus of research since 15 years ago, has remained a complex task. However, with look at the emerging applications for haptics, it has become an even more important topic. For this reason it is necessary to integrate mechanical design, control design, mechatronic elements and human factors in a unifying approach. This thesis highlights the principal trade-offs of haptic interfaces and works out design guidelines for new developments and quantitative information on where and how to optimize haptic systems. As application example a haptic interface for the simulation of hysteroscopy, a gynecologic intervention on the female uterus, has been realized as prototype. The device takes the characteristics of hysteroscopy into account and introduces features like insertion and complete removal of the surgery tool from the haptic interface. A design methodology is proposed, which combines aspects of theoretical kinematics based on Lie-Groups with integrated product development – a systematic approach for engineering tasks. The result, a novel kinematic design, has been realized as haptic interface prototype. The well-known affine parameterization for all stabilizing controllers is shown to be a well-suited tool for haptic control synthesis. It provides insights into the control trade-offs and allows to integrate human factors at controller design stage. The resulting controller, demonstrated by experimental results, can reduce parasitic effects (friction, device dynamics) close to and below the human perception threshhold, thus making the device transparent within the specified bandwidth and suitable for real-time implementation. For numerical controller representation the δ-operator is discussed. It unifies continuous and discrete time domain and has beneficial numerical properties. As there is still no standardized procedure for technical performance evaluation of haptic interfaces methods proposed in literature are summarized. In the near future a large market for haptic technology can be expected with applications from surgery-assist devices and drive-by-wire interfaces in automobiles to entertainment, which highlights the relevance of the treated topic.

EPFL2006

Haptic interface design and control with application to surgery simulation

Ulrich Spälter

EPFL2006

Conception systématique de structures cinématiques orthogonales pour la microrobotique

Patrick Helmer

EPFL2006

Design of hybrid-kinematic mechanisms for machine tools

Patric Pham

EPFL2009