Qualification of parallel kinematics machines in high-speed milling on free form surfaces

Serial milling machines may one day find their limits in high-speed milling due to their limited dynamic characteristics. Indeed, the major drawback of a serial structure is that it consists of a pile of actuated joints; hence the mass on board for the axis underneath can be huge. This is mainly the reason why parallel structures are of interest in milling: in order to go faster.Such structures have been developed since 1980 for robotic tasks, while the first parallel kinematics machine tool appeared only 14 years later. Since then, very few papers have been published that deal with the potential of these structures in milling. The objective of this study is to show the potentialities of parallel structures in milling and especially in high-speed milling of free form surfaces, in comparison to serial structures. To do so, experiments have been realised on four serial kinematics machines (SKMs) and four parallel kinematics machines (PKMs). These experiments were of two kinds: either a real piece has been milled, or the output axis encoder data have been saved (this method of measurement has been previously developed in our team).So far, a comparison between the two structures (serial and parallel) has been possible, which permitted us to show that PKMs can provide interesting results in terms of time and precision. Hence, the goal of this study has been reached, as the PKMs potentialities in milling free form surfaces have been highlighted. With more experiments, a generalisation, by taking into account all the parameters (shape and material of the part, structure of the machine, etc.), will be attempted. In parallel, the development of our simulator for PKMs is necessary in order to be able to predict milling on these machines.     

新型6-SPS三维平台并联机床工作空间研究

工作空间是并联机床的重要性能指标。研究了一种以 6 -SPS并联三维平台机构为原形的新型 6自由度并联机床的工作空间 ,基于位置反解方程并考虑其结构约束条件 ,求出其定位姿工作空间任意截面的轮廓图 ,分析了机床结构参数对工作空间大小的影响 ,这对该并联机床的合理设计和使用很有意义。     

DSX5—70型三杆五自由度并联虚轴机床的运动仿真

基于开发研制的三杆五自由度并联虚轴机床，概述了实现虚轴机床三维实体模型数控加工的运动仿真方法，利用开发的虚轴模块，解决了在虚轴机床上实现CAD／CAM集成的难题，实现了三杆虚拟轴机床CAD／CAM的一体化、软件的模块化设计，使仿真系统具有良好的扩展性和通用性，操作方便。     

新型并联机床的力／运动传递各向同性性能及其分布

以并联机器人机构为原型的并联机床,其力/运动传递各向同性性能是随其运动平台的位姿变化而变化的。本文应用并联机器人机构学理论,提出基于6-SPS并联三维平台机构的新型6自由度并联机床的力/运动传递各向同性性能评价指标,并给出这些指标在定位姿工作空间内的分布情况,这对该结构形式的并联机床的设计及其任务规划和控制有重要意义。     

Kineto-static Optimisation of PKMs

One or the main drawbacks of PKMs Is the non uniformity of force and velocity transmission and stiffness characteristics In the workspace. This non uniformity can be dramatically reduced and in some cases, but the case of 6 dof PKMs, even eliminated using efficient kineto-static optimization criteria in the design phase. In this paper new performance Indexes for the optimisation of 6 dof fully parallel PKMs, developed by the authors, are presented. These indexes overcome the fad that the Jacoblan matrix J is not consistent in units and allow an independent study of the linear and the angular behaviour of the machine. The well known octahedron proposed by Duffy is analysed using the new indexes. The first Italian PKM for 5 axis milling operations - Celerius - developed by ITIA-CNR using an optimisation criteria based on the presented Indexes is introduced. This machine exhibits an Isotropic behaviour in the middle of its workspace and is almost Isotropic in all the other points of the useful workspace.     

提高大型高精立式机床精度保持性的新型转台技术研究

针对国内生产的高速、高精大型立式机床以及车铣复合机床等高档数控机床迫切需要提高精度保持性的重大需求,开发一种能提高大型高精立式机床精度保持性的数控回转工作台。该回转工作台采用一套精密推力向心组合圆锥滚子轴承,取消了传统结构中的主轴及其复杂的轴承支承系统。推力向心组合圆锥滚子轴承的底圈和上圈都安装在同一个轴台上,实现了上圈、底圈与轴台无间隙的紧配合,消除了轴承所有套圈和它们配合件之间的间隙,并能实现过盈配合,很大程度上提高了机床的精度保持性。     

基于3-PRS-PP的并联机床工作空间研究

工作空间是机床操作器的工作区域,它是衡量并联机床性能及加工能力的重要指标,是机床工作能力的直接反映.本文研究了一种基于3-PRS-PP并联机床工作空间,通过考虑其结构约束条件,采用极坐标搜索法确定了并联机构的工作空间边界点,并在此基础上绘出了工作空间边界曲面的图形,得到了机床工作空间的三维仿真模型,并分析了机床结构参数对工作空间的影响.     

Workspace Enlargement for Parallel Kinematic Machines

This paper presents an approach that allows parallel kinematic machines to move through singular poses typically connected with them. Using this the usable workspace of parallel kinematic machines can be significantly increased. This compensates the main disadvantage of parallel kinematic machines, i.e. the sometimes poor ratio of workspace to machine size. Their main advantage, the higher structural stiffness, is maintained. To efficiently apply this to the kinematic design of production machines, different types of workspaces are defined. Finally a robot is presented that passes through singular poses and shows the possibilities in increasing machine performance by applying the presented method.     

Assessment of machine tool trunnion axis motion error, using magnetic double ball bar

A method is proposed in this paper to assess the axis motion errors of a trunnion-type A-axis using the magnetic double ball bar (DBB) as the measuring instrument. The proposed method consists of five DBB tests with a single setup for all of the tests and the exclusive motion of the trunnion axis during data acquisition. The single setup helps to reduce non-productive time by limiting the intervention of the operator within the machine workspace whereas the exclusive trunnion axis motion prevents the data from being contaminated by other axes motion errors within each test. Simulations show that setup errors cause eccentricities and radius changes of the ball bar data when viewed on a polar plot. Finally, the proposed method is applied to a VL30 Mitsui-Seiki vertical machine tool to identify its trunnion axis motion errors. The results show the effectiveness of the proposed method as well as its ease of use and the short time required.     

Investigation of accuracy of aerostatic guideways

Aerostatic guideways are often used in machines requiring very high motion accuracy such as coordinate measuring machines. Currently, positioning error analysis for such machines focuses on the relationship between volumetric errors on one hand and axes’ motion errors and axes’ relative location errors on the other. The internal mechanisms causing motion errors are rarely considered. In order to gain a deeper understanding of aerostatic guideways, this paper investigates the relationship between the motion errors of the axis’ carriage and the guideways’ geometric errors both mathematically and experimentally. The analytical model uses bearings location and stiffness, guideway geometry and static equilibrium to produce a model in matrix form. Validation experiments are conducted on a machine axis moving on aerostatic guideways with and without preload.     

七轴并联机床加工汽轮机叶片技术及其工作空间干涉检查研究

并联机构自从1980年就应用于机器人作业,但是第一台并联机床却出现在14年以后.目前,研究并联机床在铣削加工能力方面的文献很少,文章着力于研究并联机床在铣削加工尤其是加工自由曲面的能力.文中描述了并联机床在铣削加工汽轮机叶片的实际应用. 研究了基于UG CAD/CAM铣削汽轮机叶片技术,并考虑了并联机床粗加工和半精加工的效率和精加工的精度控制.UG CAM产生的刀具轨迹文件,不能直接被并联机床数控系统识别,所以必须进行后置处理,文中提出了面向七轴并联机床的刀具轨迹后置处理技术,以生成并联机床CNC系统可读的加工代码.最后为了避免机床在加工中的干涉,针对七轴并联机床的特性,提出了快速工作空间检查算法.     

一种新型运动冗余并联机构的运动学研究

针对传统3-RPR型非冗余平面并联机构运动过程中存在奇点,且需要较大驱动力矩无法实现平滑轨迹跟踪问题,提出了一种新型3-PRPR运动冗余并联机构。建立了新型并联机构的位置、速度和加速度模型,基于运动学模型进行了工作空间仿真分析,分别对不同末端执行器位置和大小、不同基圆半径对工作空间的影响进行了对比分析,并对工作空间运动轨迹进行了分析。仿真结果表明:末端执行器位置只影响定向工作空间,对可达工作空间无影响;定向工作空间和可达工作空间均与基圆半径成正比;末端执行器大小与定向工作空间成反比,与可达工作空间成正比。相同条件下,与传统3-RPR并联机构相比,所提出的新型并联机构工作空间明显增大,证明了该新型并联机构的有效性。     

五轴铣床运动链构形方案分析

目前五轴CNC机床已经得到广泛的应用。大多数机床的运动学都是基于直角坐标系统。论文根据理论上自由度的可能性组合，将五轴铣床概念设计及其配置方案进行分类，并分析了各类设计的优缺点。在此理论分析的基础上，文中基于机床坐标系统的定义和机床构形码，进一步得出常用五轴铣床运动链设计限制条件及其可行构形数。该研究结果有益于工程师进行五轴铣床的方案设计以及评价。     

Analysis and dimensional design of a novel hybrid machine tool

This paper presents a four degrees of freedom (DOF) hybrid machine tool based on a novel planar 3-DOFs parallel manipulator and a long movement of the worktable. Closed-form solutions are developed for both the inverse and direct kinematics about the parallel manipulator. Three kinds of singularities are presented. Based on the formulation of condition number and the definition of the position workspace and orientation workspace, the effects of the design parameters on the dexterity and workspace were discussed. The change ratios of condition number were defined and used as the evaluated indicator to analyze the effects of the design parameters on the performances. A dimensional design approach that could satisfy the requirement of motion platform realizing orientation capability, dexterity and workspace was proposed. A novel 4-DOFs hybrid machine tool was manufactured. The methodology presented in this paper can be of great help in the design, trajectory planning, control and application of such devices.     

Parallel kinematic concept for stationary high performance cutting in wood machining centers

In recent years parallel kinematic machines for wood machining have come into use more frequently. Despite first promising prototypes, these machines are single solutions for specific applications. To meet the requirements of shorter product life cycles and higher product diversity, high flexibility is demanded of the machining system. This paper presents a new wood machining center obtaining both, the reduction of the primary and secondary processing times. The machine concept, based on a parallel kinematic structure, allows high operating speeds and accelerations not only for workpiece machining but also for handling. Thus, the machine can be used without any external handling devices. The kinematic structure originates from a plane closed five-bar chain with two linear drives and additional drive axes for stroke and rotation. In order to increase the useable workspace a continuous motion between different assembly modes is realized. To guarantee a high feed rate and to minimize set-up times, an optimized dust exhaustion is included.     

球头碗球面加工组合机床设计

根据球面形成原理,利用铣刀和工件的两个圆运动,合成球头碗的不完整内球面。采用组合机床,结构简单,调整、维护方便,主轴弹性卡头便于工件装卸。对零件尺寸、形状和位置误差进行了分析。通过对铣刀和主轴电机的变频调速,找出最佳切削参数,实现了内球面的高效率高精度加工。     

A systematic optimization approach for the calibration of parallel kinematics machine tools by a laser tracker

Parallel kinematics machine has attracted attention as machine tools because of the outstanding features of high dynamics and high stiffness. Although various calibration methods for parallel kinematics machine have been studied, the influence of inaccurate motion of joints is rarely considered in these studies. This paper presents a high-accuracy and high-effective approach for calibration of parallel kinematics machine. In the approach, a differential error model, an optimized model and a statistical method are combined, and the errors of parallel kinematics machine due to inaccurate motion of joints can be reduced by this approach. Specifically, the workspace is symmetrically divided into four subspaces, and a measurement method is suggested by a laser tracker to require the actual pose of the platform in these subspaces. An optimized model is proposed to solve the kinematic parameters in symmetrical subspaces, and then arithmetical mean method is proposed to calculate the final kinematic parameter. In order to achieve the global optimum quickly and precisely, the initial value of the optimal parameter is directly solved based on the differential error model. The proposed approach has been realized on the developed 5-DOF hexapod machine tool, and the experiment result proves that the presented method is very effective and accurate for the calibration of the hexapod machine tool.     

燃汽轮机缸体倾斜孔数控铣削加工参数化编程

在机械加工中,常会遇到与机床主轴不垂直的空间大斜孔系的铣削加工。在三轴联动的数控镗铣床上,采用万向角度铣头改变机床主轴方向,使孔轴线与铣刀轴线一致,运用空间几何建立各加工变量之间的数学模型,实现空间斜孔铣削加工数控程序参数化。结果表明:只需一次装卡工件,赋予相应变量值,就可完成孔系的加工。该方法的加工精度高,省工省力,可推广到类似空间斜面、倾斜槽的加工。     

8-SPU并联机器人工作空间分析及参数优化

并联机器人工作空间是评价并联机构工作能力的重要指标,而结构参数是影响工作空间的基本因素。通过圆弧相交法求定姿态的工作空间,得到其截面形状,利用梯形积分法计算出工作空间体积。以工作空间体积为目标函数,通过遗传算法对机构的结构参数进行优化,获得了性能优良的结构参数。利用MATLAB软件编程得到了该并联机构的工作空间图,可以更直观观察工作空间的变化,为该并联机构性能的进一步研究和推广应用奠定了基础。     

Design and Kinematics of a Parallel Manipulator for Manufacturing

This paper presents a new model of parallel manipulator for manufacturing, whose mechanism comes from the general Stewart platform which has the advantages of high rigidity and high load capacity. A scheme of the link mechanism is also proposed to make the link structure very simple. The mechanism parameters are optimized by workspace analysis of the parallel mechanism. With the view of kinematics equivalence between parallel mechanism and serial mechanism, by way of hypothesis serial mechanism and branch hypothesis serial mechanism, an efficient kinematics algorithm has been developed. The problem, derived motion produced by the link structure, has also been solved conveniently.