基于改进四阶矩估计法的六轴机器人运动可靠性分析

针对六轴机器人运动可靠性问题,提出一种基于改进四阶矩估计法的机器人运动可靠性评估方法。首先根据等效极值原则推导六轴机器人的极限状态函数,然后采用改进四阶矩方法对六轴机器人的可靠度进行求解。在理论分析的基础上,引入实例进行了可靠度求解,通过与蒙特卡洛法和四阶矩法的对比,验证了上述改进四阶矩方法的正确性。该研究为后续进行优化设计奠定了理论依据。     

PCBN刀具车削镍基高温合金GH4169再生颤振仿真与试验研究

针对PCBN刀具车削镍基高温合金GH4169中颤振对加工效率和质量的影响,建立一种根据切削参数有效判定切削状态的方法。首先建立了二自由度外圆车削再生颤振模型,然后利用锤击法对数控车床的刀具系统进行模态试验,测定了1阶频率、模态阻尼、模态刚度、模态质量等模态参数;设计了二自由度外圆车削再生颤振MATLAB/Simulink仿真模型,根据仿真结果预测了稳定极限切削深度和颤振的主振方向。最后利用PCBN刀具进行变切深试验。试验结果表明,仿真模型正确。     

车削加工颤振稳定性可靠度蒙特卡罗法仿真

在工程实际中,车削系统刚度、阻尼及切削力等参数的随机性严重影响车削加工的稳定性。针对此问题,提出了一种车削加工再生型颤振稳定性可靠度计算方法。考虑随机因素的影响,采用蒙特卡罗数值模拟方法,研究车削加工再生型颤振稳定性的统计分布规律。建立车削加工再生型颤振动力学模型,采用拉氏变换获取机床车削的极限切削宽度及所对应的主轴转速。根据数控车床切削系统动力学参数的分布信息抽取样本,代入再生型颤振模型进行计算,获取极限切削宽度的样本,并统计其概率特性,以实际切宽是否小于极限切宽为判别条件提出一种基于蒙特卡罗模拟的车削加工再生型颤振稳定性可靠度预测方法。     

二元机翼颤振可靠性研究

机翼气动弹性模型参数一般具有不确定性,为了定量分析参数不确定性对二元机翼颤振可靠度的影响,采用响应面法结合确定性颤振分析对其进行了研究。通过v-g法求解气动弹性运动方程的特征值,获得确定性颤振临界速度,建立描述机翼颤振可靠度的隐式极限状态函数,利用响应面法进行函数拟合。采用验算点法计算颤振可靠度指标,获得设计验算点以及失效概率,分析颤振可靠性灵敏度,从而确定参数变化对机翼颤振的影响程度。最后通过算例分析,得出考虑参数随机性影响的可靠度比确定性分析的可靠度提高了4%,证明本方法切实可行,为机翼结构的颤振分析与优化设计提供理论依据。     

基于最大熵法的车辆零部件可靠性分析

采用最大熵法、可靠性设计理论和工程概率设计技术研究了随机参数服从任意分布的可靠性设计问题.在对随机变量统计的基础上,运用信息熵的概念和最大熵原理,建立了随机变量的概率密度函数和结构失效概率模型,给出了零部件可靠性状态函数的前四阶矩值,采用最大熵法得出最少带有人为偏倚因素的概率密度函数的估计函数,进而获得车辆零部件的可靠度,并与蒙特卡罗方法和四阶矩方法的计算结果进行比较,获得了一致的设计结果.为车辆零部件的可靠性设计提供了理论参考.     

非正态分布的连续体结构可靠性拓扑优化设计

由于具有不确定性参数的连续体结构普遍存在,因此研究不确定性参数连续体结构的可靠性拓扑优化问题具有十分重要的意义。在基本随机变量前四阶矩已知的情况下,利用四阶矩技术以及有限元方法求解连续体结构的可靠度,对隐式的可靠度约束进行显式化处理。将连续体的拓扑优化设计视为一种对单元的模式识别,将模式识别领域的K邻近方法引入到连续体结构拓扑优化设计领域,以结构的单元应力作为识别的变量,利用应力的欧拉距离作为判别的标准,对连续体结构进行可靠性拓扑优化设计。通过数值算例与确定性设计结果进行对比,结果表明,考虑了可靠度影响的拓扑优化结果要优于确定性参数的优化结果,同时表明该计算方法是可行的。     

基于过程阻尼的钛合金丝材表面车削精整加工稳定性研究

针对航空紧固件原材料钛合金丝材盘圆细长结构特性和加工难度大的问题,采用无心车床进行钛合金丝材盘圆的表面车削精整加工。考虑车削后高精度表面质量的要求,为提高无心车床的加工稳定性,建立考虑过程阻尼效应的多自由度耦合颤振动力学模型。通过对模型求解,绘制无心车床刀盘切削系统稳定性叶瓣图进行稳定性分析,得到稳定可加工参数组。进一步分析刀具结构参数的影响,刀尖圆弧半径为0.1～0.3 mm、刀具前角为1°～2°和刀具后角为2°～5°时,可以获得设计安全性最高的稳定可加工参数组。通过搭建试验平台验证理论分析结论,所得到的刀具结构参数选择域和稳定可加工参数组可为钛合金丝材盘圆的表面精整加工提供参考。     

基于MATLAB/Simulink再生车削颤振仿真研究

首先建立了再生型外圆车削颤振模型,对该模型进行了理论分析,阐明了含重合度不为1的条件下颤振瓣图的绘制过程。随后通过锤击法对某型号数控机床的刀具系统进行模态测试,获得其刀具系统在进给及吃刀方向的传递函数,并获得其模态参数作为仿真参数,借助与MATLAB/Simulink对颤振模型进行数值仿真。仿真结果表明,随着进给量的降低,重合度随之增大,再生效应即增大,导致稳定极限切深减小。同时,颤振仿真是在机床主轴转速较高的条件下进行的,忽略了过程阻尼的影响。仿真结果对于颤振的预测、监测研究具有一定参考意义。     

基于切触点高斯过程的直纹面侧铣可靠度分析

针对不确定因素导致直纹面侧铣精度预测困难及计算效率低的问题,提出了基于刀具切触点GP模型的直纹面侧铣可靠度分析方法。首先在高斯随机过程描述中引入驱动误差和参数不确定性的刀具切触点坐标误差,然后基于某型机床的误差传递模型,利用多体理论构建直纹面侧铣加工的精度分析模型;通过带有随机常数Zi的确定函数的线性组合方式,简化模型的输入变量,并根据重要性指标和精度指标选择有用样本和更新模型;最后基于更新模型采用蒙特卡洛法获得加工精度可靠度。实验结果表明,所提方法准确预测了加工精度变化趋势,具有较高的计算效率和预测精度,为直纹面侧铣精度预测提供了有效的方法。     

时间参数为非正态分布的CAN总线数据传输可靠性分析

数据传输可靠性对于CAN总线系统的设计影响重大,关系到整个总线系统的正常运行。以CAN总线可调度理论为依据针对总线系统中报文传输进行研究分析,利用可靠性四阶矩概率分析方法通过推导公式,以一个算例对CAN总线系统报文传输可靠性进行分析并计算;编程利用MONTE-CARLO方法进行模拟对比,从而得到了总线报文的传输可靠度。利用四阶矩可靠度计算方法对报文传输的可靠性灵敏度进行分析计算,得到的结果与定性分析一致,验证了四阶矩方法可靠性灵敏度计算的正确;通过以上的计算方法可以为调度理论的进一步研究提供理论依据,具有十分重要的实际意义。     

Chatter reliability prediction of side milling aero-engine blisk

Reliability analysis of a dynamic structural system is applied to predict chatter of side milling system for machining blisk. Chatter reliability is defined as the probability of stability for processing. A reliability model of chatter was developed to forecast chatter vibration of side milling, where structure parameters and spindle speed are regarded as random variables and chatter frequency is considered as intermediate variable. The first-order second-moment method was used to work out the side milling system reliability model. Reliability lobe diagram (RLD) was applied to distinguish reliable regions of chatter instead of stability lobe diagram (SLD). One example is used to validate the effectiveness of the proposed method and compare with the Monte Carlo method. The results of the two approaches were consistent. Chatter reliability and RLD could be used to determine the probability of stability of side milling.

     

Theoretical and experimental study of the chatter vibration in wet and MQL machining conditions in turning process

Turning is one of the most commonly used cutting processes for manufacturing components in production engineering. The turning process, in some cases, is accompanied by intense relative movements between tool and workpiece, which is called chatter vibrations. Chatter has been identified as a detrimental problem that adversely impacts surface finish, tool life, process productivity, and dimensional accuracy of the machined part. Cooling/Lubrication in the turning process is normally done for some reasons, including friction and force reduction, temperature decrement, and surface finish improvement. Wet cooling is a traditional cooling/lubrication process that has been used in machining since the past. Besides, a variety of new cooling and lubricating approaches have been developed in recent years, such as the minimum quantity lubrication (MQL), cryogenic cooling, nanolubrication, etc., due to ecological issues. Despite the importance of cooling/lubrication in machining, there is a lack of research on chatter stability in the presence of cutting fluid in cutting processes. In this study, the chatter vibration in turning process for two cooling/lubrication conditions of conventional wet and MQL is investigated. An integrated theoretical model is used to predict both the metal cutting force and the chatter stability lobe diagram (SLD) in turning process. This model involves deriving a math equation for predicting metal cutting force for both wet and MQL conditions using experimental training force data and a Genetic Expression Programming (GEP)-based regression model. Also, the traditional single degree of freedom chatter model is used here for predicting the SLDs. The chatter model is discussed and verified with experimental tests. Then, the experimental results of the tool's acceleration signal, work surface texture, surface roughness, chip shape, and tool wear are presented and compared for wet and MQL conditions. The results of this study show that the cooling/lubrication systems such as wet or MQL have a considerable effect on the SLDs. Also, the predicted results of metal cutting force and SLD for both wet and MQL techniques are in good agreement with the experimental data. Therefore, it is recommended that for each lubrication condition including wet, or MQL, the SLD be determined to achieve higher machinability.     

Chatter and dynamic cutting force prediction in high-speed ball end milling

Machine tool chatter is a serious problem which deteriorates surface quality of machined parts and increases tool wear, noise, and even causes tool failure. In the present paper, machine tool chatter has been studied and a stability lobe diagram (SLD) has been developed for a two degrees of freedom system to identify stable and unstable zones using zeroth order approximation method. A dynamic cutting force model has been modeled in tangential and radial directions using regenerative uncut chip thickness. Uncut chip thickness has been modeled using trochoidal path traced by the cutting edge of the tool. Dynamic cutting force coefficients have been determined based on the average force method. Several experiments have been performed at different feed rates and axial depths of cut to determine the dynamic cutting force coefficients and have been used for predicting SLD. Several other experiments have been performed to validate the feasibility and effectiveness of the developed SLD. It is found that the proposed method is quite efficient in predicting the SLD. The cutting forces in stable and unstable cutting zone are in well agreement with the experimental cutting forces.     

Optimization of the Tuned Mass Damper for Chatter Suppression in Turning

The tuned mass damper(TMD) has been successfully applied to the vibration control in machining,while the most widely adopted tuning is equal peaks,which splits the magnitude of the frequency response function(FRF) into equal peaks.However,chatter is a special self-excited problem and a chatter-free machining is determined by FRF at the cutting zone.A TMD tuning aiming at achieving the maximum chatter stability is studied,and it is formulated as an optimization problem of maximizing the minimum negative real part of FRF.By employing the steepest descend method,the optimum frequency and damping ratio of TMD are obtained,and they are compared against the equal peaks tuning.The advantage of the proposed tuning is demonstrated numerically by comparing the minimum point of the negative real part,and is further verified by damping a flexible mode from the fixture of a turning machine.A TMD is designed and placed on the fixture along the vibration of the target mode after performing modal analysis and mode shape visualization.Both of the above two tunings are applied to modify the tool point FRF by tuning TMD respectively.Chatter stability chart of the turning shows that the proposed tuning can increase the critical depth of cut 37% more than the equal peaks.Cutting tests with an increasing depth of cut are conducted on the turning machine in order to distinguish the stability limit.The tool vibrations during the machining are compared to validate the simulation results.The proposed damping design and optimization routine are able to further increase the chatter suppression effect.     

基于有限元分析的滚齿SLD构建

针对稳定性耳垂线图(SLD)应用于滚齿颤振预测时存在动力学参数获取困难、切削深度难以确定等问题,建立了考虑滚刀刀杆柔性的三维滚齿系统动力学模型,通过滚刀极限切屑厚度与轴向进给量的关系计算滚齿系统动力学参数,并在此基础上绘制出滚齿SLD。设计了滚齿颤振实验方案,通过采集的实验数据分析平稳切削与颤振切削振动的特征量,确定颤振频率及颤振主体,证实了所建立的动力学模型及SLD的有效性。所提方法为滚齿工艺颤振预测、切削参数选取提供了一种新手段。     

A new process damping model for chatter vibration

This paper presents a new analytical process damping model (PDM) and calculation of Process Damping Ratios (PDR) for chatter vibration for low cutting speeds in turning operations. In this study a two degree of freedom complex dynamic model of turning with orthogonal cutting system is considered. The complex dynamic system consists of dynamic cutting system force model which is based on the shear angle (φ) oscillations and the penetration forces which are caused by the tool flank contact with the wavy surface. Depending on PDR, the dynamic equations of the cutting system are described by a new mathematical model. Variation and quantity of PDR are predicted by reverse running analytical calculation procedure of traditional Stability Lobe Diagrams (SLD). Developed mathematical model is performed theoretically for turning operations in this study and simulation results are verified experimentally by cutting tests.     

Identification of bearing dynamics under operational conditions for chatter stability prediction in high speed machining operations

Chatter is a major problem causing poor surface finish, low material removal rate, machine tool failure, increased tool wear, excessive noise and thus increased cost for machining applications. Chatter vibrations can be avoided using stability diagrams for which tool point frequency response function (FRF) must be determined accurately. During cutting operations, due to gyroscopic moments, centrifugal forces and thermal expansions bearing dynamics change resulting in tool point FRF variations. In addition, gyroscopic moments on spindle–holder–tool assembly cause separation of modes in tool point FRF into backward and forward modes which will lead to variations in tool point FRF. Therefore, for accurate stability predictions of machining operations, effects of operational conditions on machine tool dynamics should be considered in calculations. In this study, spindle bearing dynamics are identified for various spindle rotational speeds and cutting forces. Then, for a real machining center, tool point FRFs under operating conditions are determined using the identified speed dependent bearing dynamics and the mathematical model proposed. Moreover, effects of gyroscopic moments and bearing dynamics variations on tool point FRF are examined separately. Finally, computationally determined tool point FRFs using revised bearing parameters are verified through chatter tests.     

基于Lab VIEW的数控轧辊车床车刀检测系统

讨论了车刀材料、几何参数和切削用量对车削过程切削力的影响;开发了基于Lab VIEW的数控轧辊车床车刀实时检测系统;提出了利用该系统检测数据改进刀具设计与切削工艺的实例。     

基于实验模态分析的集中参数法建模

利用集中参数模型的建立,不仅拓展了实验模态分析(EMA)的使用范围,还能大量缩减原有模型的自由度并保持结构的动态特性。在对一立铣床进行模态测试以及振型可视化的基础上,采用集中质量、弹簧阻尼单元建立了该铣床的7自由度集中参数模型,并在该模型基础上合成刀尖频响函数及预测颤振稳定域,揭示出机床颤振与振型的联系。与原有测试频响函数的比较表明,该模型准确度较高。