Design of hybrid-kinematic mechanisms for machine tools

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.

Concept of Modular Kinematics to Design Ultra-high Precision Parallel Robots

Murielle Richard

Miniaturisation will be the key challenge for the next decade in numerous industrial fields, such as microelectronics, optics and biomedical engineering. Although most of their products already achieve footprints of some square millimeters, the trend towards the integration of a maximum number of elements in a minimal volume requires even more compact components. This tendency creates a growing need for industrial robots able to perform micromanipulation and microassembly tasks with a submicrometric precision. Nonetheless, the design of such machines is nowadays costly, both in time and money, mostly because of the twofold complexity of their development: first, from a kinematic standpoint, the use of a parallel structure consists in a particularly interesting approach to build ultra-high precision robots. However, the synthesis of such a kinematics proves especially challenging for machines presenting more than 3 degrees of freedom. Moreover, the resulting robots are scarcely flexible: if the industrial specifications are modified, which for example necessitates to add a degree of freedom or to change the position of a rotation centre, the design process has to be restarted, often from the very beginning. The second challenge consists in the mechanical design of flexure-based mechanisms: flexure hinges are joints which are based on the elasticity of the matter. They allow to perform motions which are without friction, backlash and wear; their use is thus mandatory to achieve the aimed submicrometric precision. Albeit the synthesis of planar and low-degree of freedom structures is now widely investigated, the development of a whole tridimensional flexure-based robot is still infrequent, especially in the industrial context. This thesis thus introduces a modular design methodology which significantly reduces the time-to-market of ultra-high precision robots. This procedure can be compared to a robotic Lego, where a finite number of conceptual building bricks allows to easily design and modify parallel robots. Furthermore, this work shows that the machines resulting from this approach present similar or even improved performances compared to robots developed more traditionally. The key aspect of this thesis consists in the concept of modular kinematics, which aims at facilitating the synthesis of parallel kinematics thanks to solution catalogues. At this step of the methodology, the conceptual building bricks and the kinematics are totally independent from any mechanical design: they can thus be used to synthesise a large variety of robots, from machine-tools to microscale robots. An exhaustive conceptual solution catalogue groups all kinematics generated by the combination of the building bricks. Then, a reduced solution catalogue for ultra-high precision is proposed: based on selection criteria linked with the design and machining of flexure-based mechanisms, it allows to reduce the total number of solutions and thus facilitates the practical use of the concept. The second part of this work details the mechanical design of the building bricks, whose main challenge consists in increasing the ratio between the working ranges of the mechanisms and their overall size. One or more flexure-based solutions have been developed for each motorised brick: a special emphasis is given to the original use of a Remote Centre of Motion, which allows to achieve high rotation angles while drastically reducing parasitic translations. The development of a standardised actuation sub-brick, common to all motorised bricks, introduces a new level of modularity, thus increasing even more the flexibility of the methodology. As for non-actuated bricks, original designs and uncommon uses of well-known mechanisms are proposed. A case study on a 5-degree of freedom robot, Legolas 5, finally illustrates the practical use of the methodology: first, the selection in the solution catalogue of a kinematics adapted to the specifications of the robot is detailed. Then, the development of the Legolas 5 prototype highlights the mechanical design of the necessary building bricks, as well as assembly subtleties, such as force alignment and gravity compensation, which allow to shrewdly design a high-performance robot. The measurements of this machine have shown motion resolution and repeatability of 50 nm in translation and 1.9 µrad in rotation (limited by the sensor resolution). This case study has generated the Legolas family, a new family of ultra-high precision parallel robots, which notably includes the orthogonal version of the Delta kinematics: using only 6 of the conceptual building bricks, one solution can be built for each of the 19 possible robot mobilities. The promising characterisation of the Legolas 5 tends to suggest that the robots from this family will be interesting candidates to fulfill the upcoming need for quickly designed and high-performance industrial ultra-high precision machines.

EPFL2012

Méthode d'étalonnage d'une machine-outil à cinématique parallèle à cinq axes à grands angles d'inclinaison

Hélène Frayssinet Mazerolle

A machine-tool is able to manufacture precisely only when it is calibrated. Calibration consists in correcting the model of the machine so that it represents the real kinematics. On no account calibrating a structure consists in changing some parts of the machine. The industrial machining prototype of the Hita-STT (Stiffness Tracking Technology) machine-tool has a four-axis parallel kinematics associated to a turning table as left hand. The spindle is fixed to the parallel kinematics; the workpiece is fixed to the turning table. This new machine-tool was developed by the Laboratoire de Systèmes Robotiques from EPFL and Willemin-Macodel S.A. For its industrialization, it needs an absolute accuracy of 10 microns after calibration considering a repeatability of 5 microns. These specifications should be obtained in the entire workspace between the spindle and the turning device. This doctoral thesis deals with the calibration of the Hita-STT machine-tool. Several methods can be developed. They differ in: the geometrical model that is used. For each kinematics, three geometrical models exist: the implicit model, the forward geometrical model and the inverse geometrical model; the way the corrections are applied. Pose errors are compensated correcting the commands sent to the controller; or the parameters of the structure are identified and updated on the controller. This last solution allows to use a model for the control of the machine that is closer to the real kinematics. These different methods are presented and compared. An optimal calibration method is proposed. Whatever calibration methodology is chosen, it is divided into four steps: modeling the kinematics. An unique distribution of the parameters of the Hita-STT parallel kinematics is proposed; measurement of the pose errors. The choice and the design of the measuring device are presented. A set-up and characterization procedure is explained. It is compatible with the industrial environment. The pose errors measurements are automatic thanks to special ISO functions added to the controller. Data processing is automatic; calculation of the corrections of the model. Several methods are compared in terms of accuracy that can be obtained. An "ideal" method is proposed for the parallel kinematics calibration. It combines the two families of calibration methods, it means an identification method followed by a direct calibration method; implementation. The "ideal" method is tested on the industrial machining prototype Hita-STT. The structure of the programs used to automate the calibration process are given. Tests used to verify the calibration quality are presented. They can also be used to make sure that the parallel kinematics is accurate enough to satisfy the required quality of the manufactured parts.

EPFL2007

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.

EPFL2004