Miniaturized Phase Measuring Deflectometry Setup for Machine Integrated Measurement of Specular Surfaces
Miniaturized Phase Measuring Deflectometry Setup for Machine Integrated Measurement of Specular Surfaces
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用于镜面表面机器集成测量的小型化相位测量偏转装置
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发表时间：
2011
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通讯作者：
Christian Röttinger
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作者：
E. Uhlmann;G. Häusler;M. Kurz;C. Faber;E. Olesch;Christian Röttinger
介绍了基于相位测量偏折术（PMD）的小型化测量装置的研究进展。它的开发和设计是为了在超精密机床有限的安装空间内光学测量小型工件的镜面。测量设置的规格和性能进行了讨论，并提出了机器集成测量的第一个结果。1动机超精密机床能够制造粗糙度Ra < 10 nm、形状精度优于P-V = 0. 5 μm的非球面光学表面[1]。为了测量这种镜面，通常使用触觉和光学测量设备。触觉系统是基于触针的轮廓仪或坐标测量机。针式测头也可作为机器集成系统提供，以便直接测量和校正加工表面，同时保持安装在超精密机器的真空吸盘上。不幸的是，这种解决方案可能会在测试表面上留下不利的划痕。因此，优选使用光学测量系统。然而，只有少数测量原理实际上能够测量具有相当陡峭角度的镜面自由形状。特别是对于超精密加工应用，除此之外，还需要强大的灵活性，以便能够测量各种不同的自由曲面。提供这种能力的一种测量技术是PMD，其用于测量e。G.渐进式眼镜[2]。但这些设备无法在机器集成版本中提供，以满足超精密加工的高精度要求。2011年5月，埃尔朗根-纽伦堡大学光学研究所和柏林工业大学机床与工厂管理研究所（IWF）共同开展了一项研究项目，以实现PMD的机器集成。用PMD覆盖直径达D = 100 mm的大型工件的完整视场涉及对机器内部的广泛修改[3]。但是有许多小工件是用超精密机床生产的，它们不需要大范围的测量。例如，在制造用于小透镜或测试样品的球面或非球面模具时，可以确定车床的当前刀具偏移。在这些情况下，减小的测量场允许小型化设置。本文提出的方法旨在简化测量设置，以便轻松检查直径达D = 20 mm的零件，在测量过程中保持夹紧在真空吸盘上。2相位测量偏转法相位测量偏转法是一种同时建立的测量技术，通过用CCD相机测量空间扩展的结构光源提供的探测光线如何被物体表面偏转来确定镜面物体的局部斜率[4]。空间和角度测量范围由系统的几何配置和光源的大小决定。图1显示了PMD的基本原理。图一：PMD 3 Mini-PMD的原理将测量范围限制为D = 20 mm，并将目标表面内的最大斜率限制为α = ±10°，这使得光源较小，并且CCD相机可以紧密排列在一起并靠近光源。这种最小化的设置准备好被安装到机床中。为了便于组装和调整铝型材用于这个原型。作为光源，使用屏幕对角线约为d = 180 mm的迷你TFT监视器。为了最大限度地减少相机的工作距离，成像系统选择了f = 16 mm的短焦距，从而获得了强大的摄影测量校准的额外好处。对于s = 120 mm的工作距离，系统的横向分辨率为r = 28.0 μm/像素。第一次测量表明，该设置的测量精度约为a = 0.5 μm。该装置足够小，即使在旋转工作台（B轴）顶部安装标准刀具，也可以放入5轴超精密加工系统摩尔Nanotech® 350 FG中。机器内的系统如图2（左）所示。下一个版本的设置将在没有标准铝型材的情况下实现，从而进一步减少。图2（右侧）示出了这种装置的第一概念设计。图二：超精密车床上的Mini-PMD设置，带B轴（左）和更紧凑的下一代设计的CAD图片（右）图3（左）显示了对直径为D = 20 mm的非旋转对称铜零件和最大振幅为P-V = 10 μm的正弦表面获得的首批测量结果之一。该部件使用Slow Slide Servo制造，并直接在机器上进行测量，无需在约5分钟内将其从真空吸盘上取下。相比之下，使用图3（右）所示的白光干涉仪ZygoLOT NewView 5010进行机外测量需要将部件从车床上取下并缝合。对于WLI，使用2.5倍物镜，测量场尺寸约为。20 mm x 20 mm是屏幕相机相机屏幕测量对象工具
This paper presents the progress of current research work on a miniaturized measurement setup based on Phase Measuring Deflectometry (PMD). It is developed and designed to optically measure specular surfaces of small workpieces within the limited installation space of ultra precision machines. The measurement setup’s specifications and performance are discussed and first results of machine integrated measurements are presented. 1 Motivation Ultra precision machine tools are able to manufacture aspheric optical surfaces with a roughness of Ra < 10 nm and a shape accuracy better than P-V = 0.5 μm [1]. To measure such specular surfaces, commonly tactile and optical measurement equipment is used. Tactile systems are stylus based profilometers or coordinate measurement machines. Stylus probes are also available as machine integrated systems in order to directly measure and correct machined surfaces while remaining mounted onto the vacuum chuck of the ultra precision machine. Unfortunately, this solution can leave unfavourable scratch marks on the surface under test. Therefore, the use of optical measurement systems is preferred. However, only a few measurement principles are actually capable of measuring specular freeforms with rather steep angles. Especially for applications in ultra precision machining, in addition to this a strong flexibility is required to be able to measure a wide range of different freeform surfaces. One measurement technique providing this capability is PMD which is used to measure e. g. progressive eye glasses [2]. But these devices are not available in a machine integrated version that meets the challenging accuracy demands of ultra precision machining. The Institute of Optics at the University of Proceedings of the 11 euspen International Conference – Como – May 2011 Erlangen-Nuremberg and the Institute for Machine Tools and Factory Management (IWF) at the Technische Universität Berlin are jointly working on a research project to realize the machine integration of PMD. Covering with PMD the complete field of view for large workpieces with diameters up to D = 100 mm involves extensive modification of the machine’s interior [3]. But there are many small workpieces produced with ultra precision machines which do not require setups with a large field of measurement. Examples can be found in the manufacturing of spherical or aspherical molds for small lenses or test specimen to determine the current tool offset of the turning machine. In these cases, the reduced field of measurement allows for a miniaturized setup. The approach presented in this paper aims to miniaturize the measurement setup for easy inspection of parts with diameters up to D = 20 mm, remaining clamped onto the vacuum chuck during the measurement. 2 Phase Measuring Deflectometry Phase Measuring Deflectometry is a meanwhile established measuring technique determining the local slope of a specular object by measuring with CCD cameras how probing rays provided by a spatially extended structured light source are deflected by the object’s surface [4]. The spatial and angular measurement range is determined by the geometrical configuration of the system and the size of the light source. Figure 1 shows the basic principle of PMD. Figure 1: Principle of PMD 3 Mini-PMD Constraining the field of measurement to D = 20 mm and limiting the maximum slope within the surface of interest to α = ±10° allows the light source to be small and Proceedings of the 11 euspen International Conference – Como – May 2011 the CCD cameras to be arranged closely together and near to the light source. Such a minimized setup is ready to be fit into a machine tool. For easy assembly and adjustment aluminium profiles are used for this prototype. As light source a mini TFT monitor with a screen diagonal of about d = 180 mm is used. In order to minimize the working distance of the cameras, a short focal length of f = 16 mm has been chosen for the imaging systems, granting the additional benefit of a robust photogrammetric calibration. The lateral resolution of the system is r = 28.0 μm per pixel, for a working distance of s = 120 mm. First measurements indicate a measurement accuracy with this setup of about a = 0.5 μm. The setup is small enough to be placed into a 5-axis ultra precision machining system Moore Nanotech® 350FG even with a standard tool mounted on top of the rotating table (B-axis). The system within the machine is shown in Figure 2 (left). The next version of setup will be realized without standard Al profiles and thus further minimized. Figure 2 (right side) shows the first conceptual design of such a device. Figure 2: Mini-PMD setup on an ultra precision lathe with B axis (left) and CAD picture of a more compact next generation design (right) Figure 3 (left) shows one of the first measurement results obtained of a non rotationsymmetrical copper part with a diameter of D = 20 mm and a sinusoidal surface with a maximum amplitude of P-V = 10 μm. The part was manufactured using Slow Slide Servo and measured directly on the machine without removing it from the vacuum chuck within about 5 min. In comparison, the off-machine measurement with a whitelight interferometer ZygoLOT NewView 5010 shown in Figure 3 (right) requires the removal of the part from the lathe and stitching. For the WLI a 2.5x objective was used, the measurement field dimensions of approx. 20 mm x 20 mm are Screen Cameras Cameras Screen Measurement Object Tool