Simulation Study of the Workspace of the Parallel Machine Tool

A new three dimensional simulation method is introduced to study the workspace of a 6 PSS (P denotes a prismatic kinematic pair, S denotes a spherical kinematic pair) parallel machine tool. This algorithm adopts the method of numerical analysis to investigate the boundary points in a series of sections which form the surface of the workspace. That is, to study such points that have the largest polar radius on a certain section in a system of polar coordinates according to conditions of constraint. The constraint conditions considered in the article include the maximum and minimum displacements of each dieblock, the maximum and minimum angles of oscillation in each hinge. By converting the constraint inequalities into constraint equations, the largest polar radius corresponding to every constraint condition can be evaluated and the minimum one is used to decide the boundary point. This algorithm greatly simplifies the computational process and can be used to analyze any section of the workspace. It provides a theoretical basis for the structural design of such a machine tool.

STIFFNESS ANALYSIS OF THE MAIN MODULE FOR PARALLEL MACHINE TOOLS BY FINITE ELEMENT ANALYSIS

With the aid of commercial finite element analysis software package ANSYS,investigations are made on the contributions of main components to stiffness of the main module for parallel machine tools,and it is found that the frame is the main contributor.Then,influences of constraints,strut length and working ways of the main module have also been investigated.It can be concluded that when one of the main planes of the frame without linear drive unit is constrained,the largest whole stiffness can be acquired.And,the stiffness is much better when the main module is used in a vertical machine tool instead of a horizontal one.Finally,the principle of stiffness variation is summarized when the mobile platform reaches various positions within its working space and when various loads are applied.These achievements have provided critical instructions for the design of the main module for parallel machine tools.

3-PRS serial-parallel machine tool error calibration and parameter identification

3-PRS serial-parallel machine tool consists of a 3-degree-of-freedom (DOF) implementation platform and a 2-DOF X-Y platform. The error modeling and parameter identification methods were deduced based on 3-PRS serial-parallel machine tool. 3-PRS serial-parallel machine tool was researched, and the mechanism of error analysis, modeling, identification of error parameters and measurement equipment for the use of agency error of measurement were conducted. In order to achieve the geometric parameters calibration and error compensation of the serial-parallel machine tool, the nominal structural parameters of the controller was adjusted by identifying the structure of the machine tool. With the establishment of a vector space size chain, we can do the error analysis, error modeling, error measurement and error compensation can be done.

Dimensional tolerance allocation for new type of parallel machine tools

Presents an inverse error estimation method for deriving errors of links from cutter head errors to overcome the difficulty in obtaining analystic solutions for new type of machine tools due to the multi solution of direct kinematics so that the total dimension chain tolerance of machine tool links can be determined by Monte Carlo random simulation for a certain spindle dimension of the machine tool and the precision requirement specified, and the method of dimensional tolerance allocation developed by analysing the elements of machine tool links and the combination of machine tool precision with actual dimensional tolerance for justification of tolerance design for this kind of machine tools.

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.

Structural scheme and theory behind feedback compensation of new parallel machine tools

Analyses the errors of new parallel machine tools and presents a structural scheme for feedback compensation of machine tools to eliminate these errors, the theory behind and the relation between direction consine of normal vector of the workpiece surface and the rotation angles α,β,γ of the cutter head.

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.

INVERSE DYNAMIC FORMULATION OF A NOVEL HYBRID MACHINE TOOL

In recent years, hybrid devices have increasingly received more research.However, few of researchers studied the dynamic analysis. The inverse dynamic analysis of a novelhybrid machine tool designed in Tsinghua University is presented. The hybrid machine tool underconsideration consists of parallel and serial structures, which is based on a new 2-DOF parallelplatform and serial orientations. The kinematics and the dynamic equations are studied first for theparallel structure through Newton-Euler approach. And then, the dynamic analysis for serialstructures is conducted. Finally, a closed-form inverse dynamic formulation is derived by using someelimination techniques. Some simulation results are also given.

OPTIMAL ALGORITHM FOR NO TOOl-RETRACTIONS CONTOUR-PARALLEL OFFSET TOOL-PATH LINKING

A contour-parallel offset （CPO） tool-path linking algorithm is derived without toolretractions and with the largest practicability. The concept of ＂tool-path loop tree＂ （TPL-tree） providing the information on the parent/child relationships among the tool-path loops （TPLs） is presented. The direction, tool-path loop, leaf/branch, layer number, and the corresponding points of the TPL-tree are introduced. By defining TPL as a vector, and by traveling throughout the tree, a CPO tool-path without tool-retractions can be derived.

平行闸板阀阀座孔加工专用车削刀具设计及实验研究

平行闸板阀被广泛应用在石油钻采配套装备中。采用普通内孔车削刀具加工平行闸板阀阀座孔,需要调头加工,难以使用切削液,并且由于刀杆刚度和硬度较小引起“颤抖”现象,加工工件时不易保证阀座孔的加工质量和形位公差。基于此设计了一套专用车削刀具,增大刀杆的刚度和硬度,方便使用切削液,而且仅需一次装夹,避免掉头加工,以满足阀座孔的形位公差和加工质量要求。采用有限元法对所设计刀具进行了强度校核。通过实验验证,所设计的刀具可以提高加工效率,满足加工质量和形位公差精度要求。

基于累加式实时串并联变换算法的机械故障声学监测方法

针对基于物联网(IoT)的冲压机床故障监测问题,为了降低冲压机床故障监测的计算复杂度,并提高其低频识别的精度,提出了一种无需机器学习技术的实时性机械故障声学监测方法,即基于累加式实时串并联变换算法的机械故障声学监测方法。首先,研究了物联网场景中冲压机床声学低频分析的必要性,并给出了声学信号的表达式;然后,针对频率轴上多个周期信号重叠导致参数估计较为困难的问题,提出了一种累加式实时串并联变换算法,将输入的采样序列馈入多个具有不同输出端口的串并转换器,从累加的波形中检测出最大绝对值,并进行了比较;最后,通过样本时隙划分,将累加式实时串并联变换算法应用于机械故障监测;通过仿真和冲压机床实机测试,对累加式实时串并联变换算法和实时性机械故障声学监测方法的有效性进行了验证。研究结果表明:在无需大量信号样本的情况下,使用累加式实时串并联变换算法有利于提高低频带的识别精度;在直方图相关性方面,累加式实时串并联变换算法和Morlet小波变换具有相同的性能,且均明显优于短时傅立叶变换;同时,尽管累加式实时串并联变换算法需要的加法总数比Morlet小波变换多2.5倍,但是乘法总数减少了20447%,大幅减少了计算的复杂度。

基于神经网络的并联机床位姿误差分析与补偿研究

为了解决并联机床位姿误差补偿这一难题,以六自由度并联机床为研究对象,基于并联机床的运动学逆解建立了位姿误差模型,得到了并联机床驱动杆杆长误差和铰链点位置误差与并联机床动平台位姿误差的关系。铰链点位置误差可以通过调整杆长来弥补,由于驱动杆杆长误差难以测量,所以基于BP神经网络设计了一种改进的修正系统输入法对并联机床动平台的位姿误差进行补偿。仿真结果表明,经过补偿后的位姿误差明显小于补偿前的位姿误差,验证了并联机床位姿误差分析与补偿方法的可靠性。

基于视觉的并联机床位姿测量研究

为准确测量并联机床的位姿,提高并联机床的运动精度,提出一种基于视觉的位姿测量方法用于并联机构的末端位姿检测。采用Femto深度相机作为测量设备搭建测量平台,并用棋盘格标定板和MATLAB对相机进行标定;将标靶置于动平台进行位姿采集,利用点云库对采集后的数据进行过滤和提取;最后,将计算出的实际位姿与输入位姿进行实验比较。结果表明:X、Y、Z轴的最大误差分别为0.071、0.047、0.394 mm,绕X、Y、Z轴旋转的最大误差分别为0.09°、0.09°、0.14°,满足精度要求,证明了此方法的有效性。此方法操作高效便捷、成本低廉,在保证高检测精度的同时,可为位姿测量和后续标定实验提供新思路。

基于Virtools并联机床虚拟装配系统的研究

并联机床,机构简单而数学复杂,是一种知识密集型机构。它的出现积极响应了现代制造业对高速、高精度制造装配的需求,同时也带来了全新的机遇和挑战。为了弥补学校及企业在并联机床教学及培训方面的不足,提高学生及工人对并联机床工作机理及产品装配的掌握,设计了基于Virtools的并联机床的虚拟装配系统。通过UG建立并联机床的三维模型,通过3d Max渲染及制作运动仿真动画,在此基础上利用Virtools平台设计了交互式的并联机床的虚拟装配系统。该系统界面友好、交互性强,不仅节省开发时间和成本,而且大大减少对资源的利用,能够满足高校及企业的教学与培训需求。

6SPS型并联机床铣削参数优化

针对并联机床在不同位姿下因刚度、振动响应等因素造成的铣削过程不稳定问题,以6SPS型并联机床为研究对象,通过分析铣削参数,提出一种变进给速度优化方法。采用各推杆的总伸长量及标准偏差对位姿进行描述,将进给速度转化为进给倍率,建立进给倍率关于推杆总伸长量及标准偏差的数学模型,并采用不同的铣削路径、参数、刀具,分别进行两组实验方案下的恒进给与变进给速度的铣削实验。实验结果表明:实验方案一、二中恒进给铣削振动均方根变化范围比变进给铣削分别扩大179倍、8.5倍;实验方案一恒进给铣削表面质量差异明显,而变进铣削表面质量比较稳定。所提变进给速度优化方法有效提高了并联机床的铣削稳定性。

高精度数控机床的导轨平行度测试方法研究

基于平面度、直线度、平行度的测量及评价原理,对有效行程2 000 mm精密导轨平行度进行测量,并对导轨工作台单个滑块及组合滑块的角度偏差、直线度及导轨平行度进行理论分析和实验验证。结果表明:在相同的环境和装配工艺等条件下,水平面内滑块的角度偏转方向和趋势基本一致,滑块的最大角度偏差相近;组成导轨工作台的4个滑块在水平面内直线度曲线趋势相同均为中间凹,竖直平面内直线度受装配锁紧力和角度偏差的影响,角度偏差的转动方向相同,但趋势稍微有差异;组合滑块直线度的方向与单个滑块直线度的方向一致,因滑块组合后受力的平均效应作用,在竖直面和水平面内组合滑块直线度比单个滑块直线度好;以基准导轨与副导轨的直线度斜度代数运算的绝对值评价导轨间平行度的方法,可为精密导轨平行度的测量提供依据。

基于遗传算法整定参数的火力发电用电机伺服控制分析

同步发电机的控制精度对提高火力发电效率的影响较大,为进一步提高火力发电用电机伺服控制控制精度,选用遗传算法整定驱动轴参数。从正弦信号和直线信号两个方面开展进给测试,促进跟随性能的显著提升。研究结果表明:遗传算法优化可知,在当前人工经验条件进行整定得到的系统带宽接近64 Hz,响应性能较为理想。正弦信号下,采用遗传算法进行参数整定时,跟随误差更低,表现出更优的跟随性能。绝对误差极值降低27.3%,绝对误差积分降低42.4%。直线进给状态下,跟随误差更低,表现出更优的跟随性能。绝对误差极值降低28.6%,绝对误差积分降低39.1%。相对于人工经验整定方式,达到更优的误差波动控制状态,实现了较优的伺服稳定性。

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.

Dynamic Feedforward Control of a Novel 3-PSP 3-DOF Parallel Manipulator

Parallel manipulators with less than six degrees of freedom （DOF） have been increasingly used in high-speed hybrid machine tools. The structural features of parallel manipulators are dynamic, a characteristic that is particularly significant when these manipulators are used in high-speed machine tools. However, normal kinematic control method cannot satisfy the requirements of the control system. Many researchers use model-based dynamic control methods, such as the dynamic feedforward control method. However, these methods are rarely used in hybrid machine tools because of the complex dynamic model of the parallel manipulator. In order to study the dynamic control method of parallel manipulators, the dynamic feedforward control method is used in the dynamic control system of a 3-PSP （prismatic-spherical-prismatic） 3-DOF spatial parallel manipulator used as a spindle head in a high-speed hybrid machine tool. Using kinematic analysis as basis and the Newton-Euler method, we derive the dynamic model of the parallel manipulator. Furthermore, a model-based dynamic feedforward control system consisting of both kinematic control and dynamic control subsystems is established. The dynamic control subsystem consists of two modules. One is used to eliminate the influence of the dynamic characteristics of high-speed movement, and the other is used to eliminate the dynamic disturbances in the milling process. Finally, the simulation model of the dynamic feedforward control system of the 3-PSP parallel manipulator is constructed in Matlab/Simulink. The simulations of the control system eliminating the influence of the dynamic characteristics and dynamic disturbances are conducted. A comparative study between the simulations and the normal kinematic control method is also presented.The simulations prove that the dynamic feedforward control method effectively eliminates the influence of the dynamic disturbances and dynamic characteristics of the parallel manipulator on high-speed machine tools, and significantly improves the trajectory accuracy. This is the first attempt to introduce the dynamic feedfordward control method into the 3-PSP spatial parallel manipulator whose dynamic model is complex and provides a study basis for the real-time dynamic control of the high-speed hybrid machine tools.