A Tentative Study on Dynamics of a New Parallel NC Machine Tool with Long Travel, Large Rotating Angle and High Moving Speed

In view of the structure of traditional five-coord in ate machine tool, the work-piece and machine tool often move along their respec tive guides simultaneously on the whole. In this kind of machine structure, the total mass of moving parts including work-pieces, fixtures, rotating table, wor king table and so on is often very large. Besides, the elastic reform of transmi ssion and the viscous friction force of the guide can not be ignored. As a resul t, the machine tool can not move with high velocity and acceleration, and can no t meet the needs of modern fast and efficient production. The emergence of virtual-axis machine tool has provided a new approach for the solution of the above problems. The kernel of the virtual-axis machine tool is the parallel mechanism. So far, research of parallel mechanism in the world has achieved many results and various applied equipments based on parallel mecha nism have been worked out, but the research generally focuses on the working spa ce and kinematics analysis, dynamics are rarely considered. To meet the requirements of the modern fast and efficient production, reduce the cost and promote the machine tool’s acceleration character, not only should we analyze the kinematics of machine tool, but also we should study its dynamics a nd optimize the structure on the basis of analysis. In this paper, the kinem atics and dynamics of a 5-DOF (degrees of freedom) machine tool with novel para llel mechanism that has three moving DOF and one rotating DOF are studied by Rob ot-Wittenberg method. The dynamics character of the parallel robotic machine is analyzed and used to guide the structure design of machine tool. At last, the c orrectness is verified through a numerical simulation of 5-DOF. Hence, the dyna mics model can generally solve the problems existing in the parallel and hybrid machine tools. The dynamics character of the parallel robotic machine is studied and analyzed in quantity. The dynamics equation of the system can be written as This is a set of differential equations of four DOF system. Theoretically, the c losed solution of the forward and inverse problems can be gained by solving the above equations. The system equations quite suit to program at the computer. Whe n the forces are given, the state variables’ numerical solution can be gain ed through integral; and when the dynamics parameters are given, the forces can also be solved. But the multiple valued phenomena can not be avoided. We have developed simulation software based on the dynamics model presented by t his paper. The different effects of the structure parameters can be given by numerical simulation.

The Kinematic Analyses of the 3-DOF Parallel Machine Tools

A 3-degree-of-freedom （3-DOF） parallel machine tool based on a tripod mechanism is developed and studied. The kinematics analysis is performed, the workspace is derived, and an analysis on the number of conditions of the Jacobian matrix and manipulability is carried out. A method for error analysis and manipulability is introduced. Hence, the manipulability analysis of the parallel machine tool is accomplished.

Calibration of a Parallel Kinematic Machine Tool

A calibration method is presented to enhance the static accuracy of a parallel kinematic machine tool by using a coordinate measuring machine and a laser tracker. According to the established calibration model and the calibration experiment, the factual 42 kinematic parameters of BKX-I parallel kinematic machine toot are obtained. By circular tests the comparison is made between the calibrated and the uncalihrated parameters and shows that there is 80 % improvement in accuracy of this machine tool.

Homing Strategy for a 4RRR Parallel Kinematic Machine

Returning home is the most important process of a parallel kinematic machine （PKM） with incremental encoders.Currently,most corresponding articles focus on the accuracy of homing process,and there lacks the investigation of the operation＇s safety.For a 4RRR PKM,all servoaxes would be independently driven to their zero positions at the same time based on the traditional homing mode,and that can bring serious interfere of the kinematic chains.This paper systemically investigates this 4RRR PKM＇s safety of homing process.A homing strategy usually contains three parts which are the home switches＇ locations,the platform＇s initial moving space,and each links＇ homing direction,and all of them can influence the safety of homing operation.For the purpose of evaluating and describing the safety of the homing strategy,some important parameters are introduced as follows：Safely homing ratio （SHR） is used to evaluate the probability of a machine＇s successfully returning home from an initial moving space;Synchronal rotational angle （SRA） is the four links＇ largest synchronal rotational angle with given directions from a given pose.Whether a machine can safely return home from a given pose can be judged by comparing the SRA with all four home switches＇ mounting angles.By meshing the initial moving space and checking the safeties of returning home from all the initial poses on the nodes,the SHR of this initial moving space can be calculate.For the sake of convenience,the platform＇s initial moving space should be as large as possible,and in this 4RRR PKM,a square zone in the center of the workspace with a giving initial rotation range is selected as the platform＇s initial moving space.The forward direction is selected as each link＇s homing direction according to custom,and the platform＇s initial rotational angle is selected as larger than 0° based on this 4RRR PKM＇s kinematic characteristics.The platform＇s initial moving space can be defined only by the side length of the initial moving square.By setting a probable searching step and calculating the SHR of the initial moving square,an optimal procedure of searching for the largest side length of the platform＇s initial moving square is proposed.The homing strategy proposed is based on a systemic research on the safety of homing process for PKM,and the two new indexes SHR and SRA can clearly describe the safety of homing operation.The homing operation based on this strategy is fast and safe,and the method can also be used in other PKMs with the situation of serious components＇ interference.

Step kinematic calibration of a 3-DOF planar parallel kinematic machine tool

This paper presents a novel step kinematic calibration method for a 3 degree-of-freedom(DOF) planar parallel kinematic machine tool,based on the minimal linear combinations(MLCs) of error parameters.The method using mapping of linear combinations of parameters in error transfer multi-parameters coupling system changes the modeling,identification and error compensation of geometric parameters in the general kinematic calibration into those of linear combinations of parameters.By using the four theorems of the MLCs,the sets of the MLCs that are respectively related to the relative precision and absolute precision are determined.All simple and feasible measurement methods in practice are given,and identification analysis of the set of the MLCs for each measurement is carried out.According to the identification analysis results,a step calibration including step measurement,step identification and step error compensation is determined by taking into account both measurement costs and observability.The experiment shows that the proposed method has the following merits:(1) the parameter errors that cannot influence precision are completely avoided;(2) it reflects the mapping of linear combinations of parameters more accurately and enhances the precision of identification;and(3) the method is robust,efficient and effective,so that the errors in position and orientation are kept at the same order of the measurement noise.Due to these merits,the present method is attractive for the 3-DOF planar parallel kinematic machine tool and can be also applied to other parallel kinematic machine tools with weakly nonlinear kinematics.

机器人化三腿磨削机床的研制

以钢坯修磨为实用目的研制的一台机器人化三腿机床，具有刚度大、负载能力强、机构简单、工作空间大、无运动耦合及奇异位形等优点，首次解决了钢坯局部修磨自动化问题，配以其它加工执行器还可实现多种工艺要求。建立了该机床的运动学逆、正解方程，对其工作空间和受力分析进行了介绍。

6-UPS型并联机构的刚体动力学模型

建立了6—UPS型并联机构的动力学模型。为了能使动力学模型包含所有构件的重力和惯性力,以6—UPS并联机构的支链为研究对象,运用D—H方法建立了各构件的坐标系,推导了支链的运动学反解的解析方程,并给出了各个构件的速度、加速度与动平台的速度、加速度的映射关系。然后用牛顿-欧拉方法推导了6—UPS并联机构的动力学模型,为支链中各个构件支反力的求解和整个机构的动力学分析奠定了基础。最后以并联机床为例,运用动力学模型进行了实际计算并绘制了连杆驱动力的变化曲线。

三杆五自由度并联机床运动学研究

分析了一种三杆五自由度并联机床机器人运动学问题 ,给出了其运动学正逆解位置方程 ,并对并联机床的速度及加速度求解进行了定性分析。研究表明该并联机床运动学逆解方程形式简单、计算量小 ,易于进行实时控制 ,在机械加工领域具有一定的应用价值。

并联机床——机床行业面临的机遇与挑战

并联机床是９０ 年代问世的新型数控加工装备，其研发使机床行业面临新的机遇与挑战。简要评述了并联机床的国内外发展现状和未来趋势，以及设计理论与关键技术的研究进展。结合样机设计与制造的实践经验，提出亟待解决的若干理论与技术问题，以及解决这些问题的可行途径。

并联机床不同位形下的运动精度评价指标

以6自由度6TPS型并联机床为模型,推导出机构输入和输出的误差传递方程,分析了单个支链和整个系统的误差传递情况。利用雅可比矩阵的奇异值分解,得到影响位形精度的三个误差敏感性指标,即误差综合敏感度、误差绝对敏感度和误差方向敏感度。定义了综合误差系数和综合误差度,并以此作为评价不同位形运动精度的指标。最后,利用一台实际的并联机床,对不同加工轨迹的运动精度进行了评价,确定最优的加工轨迹。

并联机床后置处理算法与系统

以6自由度6-TPS型并联机床为模型,阐述了生成控制数据的一般流程,指出后置处理必须解决的两个关键问题——工件坐标系定位和动平台位姿获取。针对不同位形,提出利用综合误差系数和综合误差度来评价终端运动精度,并据此将工件坐标系定位归结为一类兼顾作业空间和运动精度的有约束非线性规划问题;提出了一种利用5自由度刀位文件获得6自由度动平台位姿的方法;研究了笛卡儿空间的插补算法,并比较了两种不同方法所产生的不同刀具插补轨迹。最后介绍了基于以上算法所开发的后置处理系统的结构。

3-HSS并联机床运动学设计

提出以可实现三平动自由度并联机构作为主进给机构 ,辅以回转工作台实现多坐标数控加工的并联机床结构配置方案 ,并据此简要阐述了一种三自由度并联机床的总体设计方案和相应的运动学设计方法。

6-UPS交叉杆型并联机床的运动学正解算法

6-UPS交叉杆型并联机床是Stewart并联机构的改进形式,由于动平台分两层结构,不具有一般Stewart平台的对称性,因此很难有效地获得其运动学正解的解析解。采用罗德里格斯参数描述旋转矩阵,通过分析动平台位置和姿态变量之间的耦合关系,并采用消元方法消去运动基本方程中的位置矢量参数,最终得到了只关于罗德里格斯参数的三元四次方程组。在此基础上,利用牛顿迭代方法,构造出用于求解该方程组的迭代函数。通过对实际的并联机床分析,证明了该方法的正确性和有效性。

基于ADAMS的并联机器人运动学和动力学仿真

应用机械系统动力学仿真分析软件ADAMS,建立了Stewart型并联机构的虚拟样机模型,包括对并联机器人各部件的简化方法、在ADAMS中的模型描述及仿真过程控制,并利用该虚拟样机模型对并联机器人进行了运动学和动力学分析。为并联机器人系统的设计、制造和模拟运动作业提供了理论依据和主要参数。实现了在计算机上通过使用CAE仿真软件来对并联机器人的运动和动力性能进行分析,为并联机器人的设计提供了一套有效的分析方法。

基于全局动力学性能的并联机床结构参数优化

提出了一种以并联机床动力学模型为基础的结构参数的优化方法。以 6- UPS型并联平台机构支链为研究对象进行了运动学分析 ,给出了机构支链的运动学方程和速度加速度映射公式 ,以运动学为基础叙述了该机构动力学建模过程 ,建立基于全局指标结构参数优化模型 ,并对 6- UPS型并联机床进行了优化计算。为检验结果的正确性 ,采用仿真机构运动的方法对优化结果进行了验证。

新型三腿并联机床的速度及加速度分析

东北大学在前期研究的基础上最近又开发了一种新型三腿并联机床 ,该机床可用于立式加工 ,将工作头旋转一定角度后又可实现卧式加工·介绍了该机床的结构型式和特点 ,并对机床的运动学问题进行了研究 ,在分析平行约束机构的基础上 ,推出了相应的位置、速度、加速度等运动学方程 ,并得出了雅可比矩阵的计算公式·理论分析表明 ,该机床的运动方程为显式 ,形式简单 ,计算量小 ,易于进行实时控制·此种机床可望在木制品加工。

并联机床虚拟设计的理论方法

针对并联机床，从自由度数定理出发，确保了并联机床概念设计的可靠性；运用多体系统运动学理论，推导出并联机床运动学模型及加工空间与结构尺寸关系的模型，进而，通过并联机床相对运动约束方程，推导出并联机床刀具运动与驱动杆系参数的关系方程．这一整套研究思路，对指导并联机床的设计与控制，减小设计的盲目性提供了有力的理论依据．

并联杆系机床工作空间与精度分析

从工作空间和加工精度两个方面，对并联杆系机床的加工范围进行了分析。讨论了影响其加工范围的各主要因素，为并联杆系机床的设计提供了必要的依据，并做了实例计算。

并联机床的历史、现状及展望

本文介绍了并联机床的基本概念、分类方法及其基本特性 ,详细地阐述了并联机床在国内外的发展历史、研究状况以及关键基础零部件的发展状况 ,在此基础上 ,分析了并联机床结构上的优缺点 ,表明了并联机床走向实用化所面临的一些问题 。