自由曲面光学元件气囊抛光进动运动控制技术

针对自由曲面光学元件的加工特点,研究气囊抛光自由曲面光学元件进动运动控制技术,用于求出气囊工具进动过程中两虚拟轴的转角,实现对气囊自转轴空间位置的控制.以气囊自转轴为研究对象,由于自由曲面光学元件上每个点的法线三维坐标都不相同且气囊进动抛光过程中气囊自转轴与工件加工点局部法线夹角不变,提出建立基坐标系和抛光点对应三维空间坐标系的方法,得到抛光过程中气囊自转轴的空间位置变化情况,而后利用旋转坐标变换得到气囊抛光进动运动控制模型;在所建立的自由曲面光学元件气囊抛光进动运动控制模型中加入控制算法,求出抛光自由曲面光学元件各点时气囊工具两个虚拟旋转轴的转角.利用Matlab对自由曲面光学元件不同方向截面进行仿真抛光试验,得到自由曲面各方向上气囊抛光进动运动曲线以及仿真进动角曲线,结果证明了自由曲面光学元件气囊抛光进动运动控制模型及控制算法的正确性.     

气囊连续进动抛光运动模型的研究

为了控制六自由度机器人实现气囊连续进动抛光功能,建立了相应的运动数学模型。首先基于气囊连续进动抛光原理,利用齐次坐标变换矩阵,建立了气囊定点进动抛光的数学模型;在此基础上,根据气囊连续进动要求,应用坐标系转换原理,推导了气囊连续进动抛光的数学模型;通过Matlab仿真绘制了气囊连续进动抛光的运动空间,并在六自由度机器人辅助气囊抛光系统中进行了直线连续进动的实验验证,实验验证结果和仿真结果完全吻合。研究结果表明,该气囊连续进动抛光数学模型是可行的。     

气囊抛光进动机构的运动建模与仿真

气囊抛光工具的进动由一类特殊的2自由度空间机构实现。针对此2自由度空间机构运动,运用机构学理论,推导了这一空间机构的基本坐标变换,建立了各参数之间的内在关系。在此基础上,根据进动要求建立了这一空间机构的进动运动方程,通过仿真研究了气囊进动过程中运动参数的变化规律。结果表明在进动过程中,合成进动运动的2自由度运动有一定的运动规律,同时所建立的模型为该空间机构的进一步分析和运动控制提供了依据。     

气囊抛光去除函数的数值仿真与试验研究

为提高光学元件的面形精度,提高加工效率,对超精密气囊抛光方法的去除函数进行了理论和试验研究.通过分析气囊抛光的原理,以Preston方程为基础,应用运动学原理推导了气囊抛光"进动"运动的材料去除函数,利用计算机仿真的方法,得到近似高斯分布的去除函数,通过仿真分析几个主要参数对"进动"抛光运动去除特性的影响,总结得到三点气囊抛光工艺过程中重要的结论.通过在一台超精密气囊式智能抛光机上的试验对比,两者吻合很好,并得到面形精度 RMS=0.012 6 μm的超精密的光滑表面,为开展气囊抛光的工艺研究提供了理论依据.     

气囊抛光数控机床后置处理算法的研究

利用坐标变换的方法,对机床的进动机构和进给机构进行运动学分析,建立了进动和进给运动方程,并根据方程和实际气囊抛光工艺提出了用于机床实际运动控制的数控自动编程后置处理算法,该算法在实验抛光加工中得到了应用.     

气囊式抛光机数控系统的研究与开发

气囊式抛光是一种新颖的非球面抛光工艺,是一种富于竞争性的制造技术。通过对气囊抛光进动原理和抛光试验机床结构的分析,并根据气囊抛光的工作机理和控制参数,提出采用以PC嵌入CNC控制卡方式的气囊抛光数控体系结构。其硬件系统以PC与PMAC控制器为核心,能够控制6个运动轴,数控系统的软件系统建立在Windows通用操作系统平台上,具有很强的开放性、可操作性以及功能扩展性。数控系统已应用在气囊抛光试验样机上。     

气囊抛光工艺中接触面积的计算与仿真分析

针对气囊抛光非球面工件时,气囊为了适应工件表面而使得自身形状发生改变,其接触边界复杂多变难以确定以及接触面积难以计算的问题,对气囊研抛头抛光大型椭圆非球面的研抛过程进行了分析及适当的假设,建立了研抛头与非球面接触区域的数学模型,提出了一种以几何方法建立的接触面边界求解参数模型,并借助有限元软件ABAQUS和ADAMS对接触边界进行了仿真计算.研究结果表明,采用该算法得到的参数模型计算结果和三维实体接触软件模拟计算结果接近,说明该算法可用于气囊抛光工艺中接触边界和面积的求解.     

非球面磁性复合流体抛光路径误差分析与仿真

针对光学系统中对非球面元件的精度要求,设计了直线光栅式的抛光轨迹,并用磁性复合流体以这一抛光轨迹抛光非球面。根据抛光轨迹和非球面方程计算出每个抛光点的坐标;根据抛光点坐标和抛光头的抛光姿态计算出对应的抛光头中心点的坐标;建立相邻两抛光点的弓高误差模型,仿真出弓高误差模型并分析弓高误差的变化规律;根据弓高误差变化规律,用等弓高误差变步长控制算法实现弓高误差的一致性,提高加工质量。     

连续进动气囊抛光行间距优化及实验研究

为提高和调控模具加工质量,针对行切法连续进动气囊抛光的行间距优化问题进行了仿真分析和实验研究。在取得优化进给速度和叠加次数的基础上,兼顾抛光效率与抛光效果,运用二分法原理,通过MATLAB仿真分析了行间距对气囊抛光材料去除特性的影响,获得了优化行间距D=8.2mm。基于建立的连续进动气囊抛光离线规划原型系统进行了实验对比验证,实验结果与仿真结果吻合。     

光学非球面零件机器人抛光工具的研究

以高硼玻璃零件为研究对象,自主研制设计了一种光学抛光加工的工具,可抛光非球面凹面光学零件;分析了机器人抛光工具的构成以及进动运动方式和抛光加工的原理。根据实际生产情况,抛光工具结合机器人对非球面零件凹面进行抛光加工,得到了较好的实验结果。表明这套抛光工具是一种实用的、经济的,可以用于光学非球面的高精度抛光,为应用机器人做光学非球面零件柔性抛光提供了一种有效的途径。     

Modeling of the static tool influence function of bonnet polishing based on FEA

The purpose of this paper is to investigate the model of the static tool influence function (sTIF) of bonnet polishing (BP). Three kinds of sTIF are mathematically modeled, which are static tool influence function of tilted polishing (sTIF_t), static tool influence function of discrete precession polishing (sTIF_d), and static tool influence function of continuous precession (sTIF_c), respectively. Pressure distribution in the contact area is confirmed based on finite element analysis (FEA) technology. A group of experiments to extract the polishing spots have been conducted to verify the accuracy of the sTIF model. Meanwhile, the difference between sTIF_d and sTIF_c is studied. It turns out that the removal depths of sTIF_d and sTIF_c are almost the same, and the continuous precession polishing can be replaced by discrete precession polishing to ease control in practical polishing process especially for the aspheric surfaces polishing.     

气囊抛光工具的研制及特性测试

气囊式抛光是一种新颖的非球面抛光方法。气囊是该方法的关键部件,为控制它的变形和柔性,采用层合复合材料的制作方法,对气囊结构进行改进,然后从机械特性和应用两个方面,对改进的气囊进行试验和分析。研究结果表明,改进后的气囊具有变形小、柔性好的特点,能满足实际应用要求。     

Effect analysis of the residual error evaluation method used in bonnet polishing process for aspheric lens

The purpose of this paper is to investigate the effect of the residual error evaluation method used in bonnet polishing (BP) process to the axisymmetric aspheric surface accuracy. In the past, few attentions have been paid to the normal direction error (NE) of the aspheric surface, not to mention its application in BP. In this paper, a new approach is proposed to calculate NE on the basis of the axis direction error (AE) of the aspheric surface. The comparison of them is presented through a group of experiments on four asxisymmetric aspheric lenses. The effect of using AE as the residual error in BP process to the final surface accuracy is tested. It is confirmed that it would result in the difference between the practical final surface form and the theoretical final surface form, and NE should be used as the residual error in BP process.     

回转对称非球面气囊抛光控制算法研究

介绍一种新型的非球面光学零件抛光方法———气囊抛光,这种方法可以得到高质量的光学表面。针对气囊抛光加工回转对称非球面的特点,进行抛光路径规划,对去除函数进行降维处理,提出抛光驻留时间的计算方法。最后,给出抛光算法的实现框图,并按照实现框图指导实际的玻璃非球面加工,取得满意结果,验证了算法的有效性。     

Experimental investigation and analytical modelling of the effects of process parameters on material removal rate for bonnet polishing of cobalt chrome alloy

Cobalt chrome alloys are the most extensively used material in the field of total hip and total knee implants, both of which need highly accurate form and low surface roughness for longevity in vivo. In order to achieve the desired form, it is extremely important to understand how process parameters of the final finishing process affect the material removal rate. This paper reports a modified Preston equation model combining process parameters to allow prediction of the material removal rate during bonnet polishing of a medical grade cobalt chrome alloy. The model created is based on experiments which were carried out on a bonnet polishing machine to investigate the effects of process parameters, including precess angle, head speed, tool offset and tool pressure, on material removal rate. The characteristic of material removal is termed influence function and assessed in terms of width, maximal depth and material removal rate. Experimental results show that the width of the influence function increases significantly with the increase of the precess angle and the tool offset; the depth of the influence function increases with the increase of the head speed, increases first and then decrease with the increase of the tool offset; the material removal rate increases with the increase of the precess angle non-linearly, with the increase of the head speed linearly, and increases first then decreases with the increase of the tool offset because of the bonnet distortion; the tool pressure has a slight effect on the influence function. The proposed model has been verified experimentally by using different Preston coefficients from literature. The close values of the experimental data and predicted data indicate that the model is viable when applied to the prediction of the material removal rate in bonnet polishing.     

Theoretical and experimental investigation of surface generation in swing precess bonnet polishing of complex three-dimensional structured surfaces

Three-dimensional structured surfaces (3D-structured surfaces) possessing specially designed functional textures are widely used in the development of advanced products. This paper presents a novel swing precess bonnet polishing (SPBP) method for generating complex 3D-structured surfaces which is accomplished by the combination of specific polishing tool orientation and tool path. The SPBP method is a sub-aperture finishing process in which the polishing spindle is swung around the normal direction of the target surface within the scope of swing angle while moving around the center of the bonnet. This is quite different from the ‘single precess’ and ‘continuous precessing’ polishing regime, in which the precess angle is constant. The technological merits of the SPBP were realized through a series of polishing experiments. The results show that the generation of complex 3D-structured surfaces is affected by many factors which include point spacing, track spacing, swing speed, swing angle, head speed, tool pressure, tool radius, feed rate, polishing depth, polishing cloth, polishing strategies, polishing slurry, etc. To better understand and determine the surface generation of complex 3D-structured surfaces by the SPBP method, a multi-scale material removal model and hence a surface generation model have been built for characterizing the tool influence function and predicting the 3D-structured surface generation in SPBP based on the study of contact mechanics, kinematics theory, abrasive wear mechanism, and the convolution of the tool influence function and dwell time map along the swing precess polishing tool path. The predicted results agree reasonably well with the experimental results.     

Material removal model considering influence of curvature radius in bonnet polishing convex surface

The bonnet tool polishing is a novel, advanced and ultra-precise polishing process, by which the freeform surface can be polished. However, during the past few years, not only the key technology of calculating the dwell time and controlling the surface form in the bonnet polishing has been little reported so far, but also little attention has been paid to research the material removal function of the convex surface based on the geometry model considering the influence of the curvature radius. Firstly in this paper, for realizing the control of the freeform surface automatically by the bonnet polishing, on the basis of the simplified geometric model of convex surface, the calculation expression of the polishing contact spot on the convex surface considering the influence of the curvature radius is deduced, and the calculation model of the pressure distribution considering the influence of the curvature radius on the convex surface is derived by the coordinate transformation. Then the velocity distribution model is built in the bonnet polishing the convex surface. On the basis of the above research and the semi-experimental modified Preston equation obtained from the combination method of experimental and theoretical derivation, the material removal model of the convex surface considering the influence of the curvature radius in the bonnet polishing is established. Finally, the validity of the model through the simulation method has been validated. This research presents an effective prediction model and the calculation method of material removal for convex surface in bonnet polishing and prepares for the bonnet polishing the free surface numerically and automatically.     

NC polishing of aspheric surfaces under control of constant pressure using a magnetorheological torque servo

In this paper, a method of maintaining a constant polishing pressure is proposed for a NC polishing system by controlling the polishing force during the polishing process. First, the NC polishing system is developed to resolve the force–position coupling problem encountered in common polishing processes. It mainly consists of a force control subsystem based on a magnetorheological torque servo to provide a controllable torque to polishing tool to generate the polishing force and a position control subsystem based on a general CNC lathe to control the position of the polishing tool. Second, a constant polishing pressure model is established by controlling the polishing force according to the variation of the curvature of the aspheric surfaces, and the polishing parameters for model are planned. Then, the control model of the polishing system is proposed, and a PID controller is designed for torque tracking with the actual torque feedback from a torque sensor. Finally, polishing experiments are conducted with constant force and constant pressure, respectively. Experimental results show that the surface roughness is greatly improved, the aspheric surfaces can be polished more uniformly with constant pressure than with constant force, and the PID controller can meet the requirements for the polishing force control.