Point Data Reduction Using 3D Grids

Reverse engineering refers to the process of obtaining a CAD model from an existing physical part. Advances in laser scanning technologies have facilitated this process by sampling part surface data with speed and accuracy. With the help of this technology, it is now possible to acquire the geometry of a part having complex and freeform surfaces. However, it creates the burden of large amounts of point data which must be manipulated, therefore, data from a laser scan must be significantly reduced to proceed with the computations and to lower the storage requirement. Many point data reduction methods for image processing have been developed in the past. However, there is little published work on laser-scanned data, and what exists focuses only on 2D point data. This paper presents a data reduction method that reduces the amount of 3D point data using part geometry information. The method reduces the point data, based on normal values of points using 3D grids. The method is applied to two sample models and the results are discussed.     

用于强反射表面形貌测量的投影栅相位法

为了实现强反射表面三维形貌的光学非接触测量,提出一种改进的投影栅相位法。分析了强反射表面的反射光特点及其对相位解算的影响,指出了反射光亮度范围与相机动态范围的不一致是导致传统投影栅相位法测量失效的主要原因;提出了亮暗条纹投射、多曝光时间采集图像和图像合成等技术,使相机亮度测量范围与强反射表面的反射光亮度范围相一致,并分析了此方法的可行性和适应范围。最后,给出了改进投影栅相位法的条纹投射与图像采集步骤。实验结果表明,改进的投影栅相位法克服了强反射表面引起的条纹图像饱和或过暗问题,能够成功测量出99.6%以上的三维点云,有效解决了测量点云缺失问题,能够实现强反射表面三维形貌光学非接触测量。     

基于激光共焦扫描显微镜方法的磨损表面三维数字化描述

表面形貌的精确描述在许多领域诸如材料、生物医学、摩擦学和机器状态监测等领域变得越来越重要。开发了一种基于激光共焦显微镜和图像处理技术的研究磨损表面及表面参数的新方法。首先用B io-rad Rad iance 2000激光共焦显微镜方法获得精确的三维表面形貌,然后用计算机辅助图像分析技术自动计算出表面特征参数。应用示例表明本文所研究的方法是可靠的,能对工程表面的表面粗糙度特征进行精确描述。     

A multichannel time-domain scanning fluorescence mammograph: performance assessment and first in vivo results

We present a scanning time-domain fluorescence mammograph capable to image the distribution of a fluorescent contrast agent within a female breast, slightly compressed between two parallel glass plates, with high sensitivity. Fluorescence of the contrast agent is excited using a near infrared picosecond diode laser module. Four additional picosecond diode lasers with emission wavelengths between 660 and 1066 nm allow to measure the intrinsic optical properties of the breast tissue. By synchronously moving a source fiber and seven detection fiber bundles across the breast, distributions of times of flight of photons are recorded simultaneously for selected source-detector combinations in transmission and reflection geometry either at the fluorescence wavelength or at the selected laser wavelengths. To evaluate the performance of the mammograph, we used breastlike rectangular phantoms comprising fluorescent and absorbing objects using the fluorescent dye Omocyanine as contrast agent excited at 735 nm. We compare two-dimensional imaging of the phantom based on transmission and reflection data. Furthermore, we developed an improved tomosynthesis algorithm which permits three-dimensional reconstruction of fluorescence and absorption properties of lesions with good spatial resolution. For illustration, we present fluorescence mammograms of one patient recorded 30 min after administration of the contrast agent indocyanine green showing the carcinoma at high contrast originating from fluorescence of the extravasated dye, excited at 780 nm.     

A Reverse Engineering Approach to Generating Interference-Free Tool Paths in Three-Axis Machining from Scanned Data of Physical Models

An NC tool path is usually generated by sweeping parametric surfaces of a CAD model. In modern design, freeform or sculptured surfaces are increasingly popular for representing complex geometry for aesthetic or functional purposes. Traditionally, a prototype is realised by machining the workpiece using the NC codes generated from a CAD model. The machined part can then be compared with the CAD model by measurement using a coordinate measuring machine. In this paper, a reverse engineering approach to generating interference-free tool paths in three-axis machining from the scanned data from physical models is presented. There are two steps in this procedure. First, a physical model is scanned by a 3D digitiser, and multiple data sets of the complex model are obtained. A surface registration algorithm is proposed to align and integrate those data to construct a complete 3D data set. A shortest-distance method is used to determine the connecting sequence of the neighbouring points between two adjacent scan lines, such that the scanned data are converted into triangular polygons. Tool paths are then generated from the tessellated surfaces. Using the Z-map method, interference-free cutter-location data are calculated, relative to the vertex, edges, and planes of the triangles. The algorithms for tool-path generation are usually different for cutters of different geometries. Some algorithms found in the literature require complex numerical calculations and are time consuming. In this paper, an efficient algorithm is developed to calculate interference-free cutter-location data by easy geometric reasoning without complex computation. This robust method is suitable for generally used cutters such as ball, flat, and filleted end mills, and the time taken to obtain full tool paths of compound surfaces is short. Some real applications are presented to validate the proposed approach.     

组合CCD图像和稀疏激光测距数据的建筑物三维信息提取

对组合高分辨率航空CCD图像和机载稀疏激光扫描测距数据自动提取城市建筑物的三维信息进行了研究。航空CCD图像能清晰地给出建筑物的几何形状和分布,因此采用了自适应的Canny边缘检测算法来提取CCD图像上的全部边缘信息,然后根据双向投影直方图和线段匹配方法来自动而准确地提取建筑物的平面轮廓信息,最后根据CCD提取的轮廓信息从机载激光扫描测距数据中提取建筑物的高度信息,从而实现了每栋建筑物的三维信息提取。通过实际数据的处理和提取,说明了组合CCD图像和机载激光扫描测距数据可以自动重建建筑物的三维信息。     

Compensation of Illumination Variation and Geometric Distortion in Scanning Bound Books

Illumination variation and geometric distortion within a scan of a bound book page are well known image quality problems. The degradation of the scanned image is caused by the book page not being in uniform intimate contact with the scanning surface of the scanner. A system that corrects for those artifacts within scanned book page will be presented. In the proposed system, illumination data is first acquired through a sampling window having a long and thin geometry. From the data, foreground and background illumination distributions are defined and a spatially varying illumination gain factor is generated to normalize the illumination across the book page. The extracted illumination variation is then used to estimate the distance between book page and the scanning surface through an illumination model. Once the distance at each pixel position is determined, the optical magnification ratio at the pixel position is calculated. Based on this magnification ratio, a dewarping factor can be determined and used to locally rescale and resample the scanned image to compensate for the geometric distortion. Compared to the original scanned image, the processed image of a book page possesses a more pleasing appearance, and is easier to read and further process for purposes such as OCR.     

Analysis of digitizing errors of a laser scanning system

The digitizing errors of a high-speed 3D laser scanning system are analyzed and characterized in this paper. As the laser scanner is an electro-optical device and based on the principle of optical triangulation, the measurement accuracy is affected by the measured part geometry and its position within the scanning window. Commercial laser scanners are often calibrated in the scanning plane to account for variation of the incident angle of the laser beam. The effects of the scan depth and the projected angle, characterizing the surface normal of the measured part external to the scanning plane, on the measurement accuracy are not considered in the standard calibration process and have been identified by experiments in the present work. Experimental results indicate that the random error of the scanned data is close to the nominal value provided by the manufacturer. The systematic error shows a bilinear relationship with the scan depth and the projected angle and has a maximum value of about 160 μm. The developed empirical model correctly predicts the systematic error with a maximum deviation of only 25 μm.     

Reconstruction of a scanned topographic image distorted by the creep effect of a Z scanner in atomic force microscopy

We analyzed the illusory slopes of scanned images caused by the creep of a Z scanner in an atomic force microscope (AFM) operated in constant-force mode. A method to reconstruct a real topographic image using two scanned images was also developed. In atomic force microscopy, scanned images are distorted by undesirable effects such as creep, hysteresis of the Z scanner, and sample tilt. In contrast to other undesirable effects, the illusory slope that appears in the slow scanning direction of an AFM scan is highly related to the creep effect of the Z scanner. In the controller for a Z scanner, a position-sensitive detector is utilized to maintain a user-defined set-point or force between a tip and a sample surface. This serves to eliminate undesirable effects. The position-sensitive detector that detects the deflection of the cantilever is used to precisely measure the topography of a sample. In the conventional constant-force mode of an atomic force microscope, the amplitude of a control signal is used to construct a scanned image. However, the control signal contains not only the topography data of the sample, but also undesirable effects. Consequently, the scanned image includes the illusory slope due to the creep effect of the Z scanner. In an automatic scanning process, which requires fast scanning and high repeatability, an atomic force microscope must scan the sample surface immediately after a fast approach operation has been completed. As such, the scanned image is badly distorted by a rapid change in the early stages of the creep effect. In this paper, a new method to obtain the tilt angle of a sample and the creep factor of the Z scanner using only two scanned images with no special tools is proposed. The two scanned images can be obtained by scanning the same area of a sample in two different slow scanning directions. We can then reconstruct a real topographic image based on the scanned image, in which both the creep effect of the Z scanner and the slope effect of the sample have been eliminated. The slope effect of the sample should be eliminated so as to avoid further distortion after removal of the creep effect. The creep effect can be removed from the scanned image using the proposed method, and a real topographic image can subsequently be efficiently reconstructed.     

Masked illumination scheme for a galvanometer scanning high-speed confocal fluorescence microscope

High-speed beam scanning and data acquisition in a laser scanning confocal microscope system are normally implemented with a resonant galvanometer scanner and a frame grabber. However, the nonlinear scanning speed of a resonant galvanometer can generate nonuniform photobleaching in a fluorescence sample as well as image distortion near the edges of a galvanometer scanned fluorescence image. Besides, incompatibility of signal format between a frame grabber and a point detector can lead to digitization error during data acquisition. In this article, we introduce a masked illumination scheme which can effectively decrease drawbacks in fluorescence images taken by a laser scanning confocal microscope with a resonant galvanometer and a frame grabber. We have demonstrated that the difference of photobleaching between the center and the edge of a fluorescence image can be reduced from 26 to 5% in our confocal laser scanning microscope with a square illumination mask. Another advantage of our masked illumination scheme is that the zero level or the lowest input level of an analog signal in a frame grabber can be accurately set by the dark area of a mask in our masked illumination scheme. We have experimentally demonstrated the advantages of our masked illumination method in detail.     

Multichannel Confocal Microscope Based on a Diffraction Focusing Multiplier with Automatic Synchronization of Scanning

Implementation of a laser scanning confocal microscope is described, where the specimen is scanned by an array of illuminating beams, which significantly increases the velocity of object image construction. The array formation is provided by using a diffractive optical element. Scanning by the array of laser beams over the specimen is performed by galvanometric scanners with moving refractive plane-parallel plates. Owing to application of such a scanning device, the beams in the illuminating channel and the signal beams in the receiving channel pass through one motionless array of confocal diaphragms; as a result, the scanning beams in the specimen plane and the signal beams in the plane of the photodetector matrix can be used without an additional synchronized pair of scanners. The proposed confocal microscope can be applied in problems that require a fast response.     

Machined surface generation using wavelet filtering

The surface geometry of a workpiece after machining is a major concern in the industry. Thus, it is important to generate its geometry, reconfiguring and modifying it by a wavelet-based filtering technique with confocal laser scanning microscopy (CLSM). This technique is well-suited to construct the three-dimensional surface of a machined workpiece. The problem of noise on the surface always occurs when treating the raw height data (HEI: Height Encoded Image) acquired by CLSM. The noise occurs in constructing the surface height image (HEI) due to an electronic and photographic image process and must be filtered. In this paper, a wavelet-based filtering algorithm is used to eliminate noise in the shape features of the surface on a micro scale so as to realize an accurate geometry for further topographic analysis. The filtered surface patterns enable us to predict the machined surface and analyze surface characteristics in varied shapes of different levels. Also, to consider the tilting that inevitably occurs in mounting a specimen on the table, the surface is compensated and flattened in the x and y-directions simultaneously. This newly developed technique combines CLSM and wavelet filtering generates various micro machined surfaces over a wide range of surface resolutions on a micro scale     

MECHANISM OF CHIP FORMATION IN HIGH-SPEED TURNING OF INCONEL 718

The mechanism of serrated chip formation during high-speed turning of Inconel 718 using PCBN cutting tools has been investigated with the aid of scanning electron microscopy and optical microscopy. A conceptual model of chip formation has been developed knowing the chip morphology. It is followed by the analysis of chip segmentation frequency and the chip forms. Further, the chip segment forms and geometry were quantitatively characterized as a function of machining parameters and the cutting edge geometry using statistical methods. The chip morphology has been correlated with the cutting forces, specific shearing energy and the resultant roughness of the machined surfaces.     

联合光度立体视觉和图像校正的磨粒三维形貌重建

分析铁谱技术在机械装备磨损状态检测中发挥了重要作用,但其仅能提供磨粒二维图像,导致磨粒形貌信息不足。为实现磨粒三维形貌的精确重建,联合光度立体视觉和图像校正,为精确重建磨粒的三维形貌,联合光度立体视觉和图像。该方法首先采用大津阈值法由全光源图像识别磨粒与背景区域;然后为消除LED发光强度差异对磨粒形貌重建的影响,结合平面形状和朗伯反射模型确定背景区域的理想成像亮度,并校正各光度图像序列的亮度;最后根据光度立体视觉方法,由校正后光度图像序列重建磨粒的三维形貌。以不同类型的磨粒为测试样本,将所提出的方法的重建结果与激光共聚焦显微镜的测量结果进行对比。结果表明：重建磨粒的形貌参数误差小于15%,表明提出的方法能够精确重建磨粒的三维形貌。     

Pulsed Nd:YAG laser treatment of monocrystalline silicon substrate

To minimize reflection and increase the opportunity for photovoltaic (PV) devices to absorb incident light, we produced rough-textured surfaces using a pulsed Nd:YAG laser. We investigated the effect of various operating parameters on surface features and optical performance. The parameters investigated were pulse repetition frequency, pulse energy, and textured surface exposure time. Following laser surface treatment, no chemical solution etching or anti-reflective coatings were necessary. Reflectance values were measured by spectrophotometer. Structural properties including surface morphology and surface roughness of the textured surfaces were analyzed by a three-dimensional confocal laser scanning microscope. The resulting surface reflection curves indicated that, of the different laser machining paths, the vertical and circular paths produced identical optical properties in the samples. Surface reflection values decrease as the pulse energy and exposure times increase. By contrast, surface roughness increases with exposure time. An increase in pulse frequency reduces surface roughness, although the extent of the reduction depends upon the pulse energy. Following surface texturing of the monocrystalline silicon samples, the total reflection was reduced from 40% to approximately 10% of incident light. Our experimental results indicate that the rougher surfaces attained by laser surface treatment provide better light-trapping properties for PV devices.     

3-D measurement and evaluation of surface texture produced by scraping process

The scraping process involves traditional manual work and is an important technique for producing flat bearing surfaces with lubricating grooves on a sliding surface. In order to meet the requirements of precision engineering, the scraped surface should have an equally distributed pattern with a required number of high points per unit area while retaining good flatness. In the machine tool manufacturer’s workshop, however, the quality inspection of scraped surfaces still depends on human eyes. In this study, a 2-D evaluation system is first developed using image processing so that the peak points per area of square inch (PPI) and the percentage of points (POP) can be quantified as parameters. A vision-assisted laser focus probe system is then developed to measure the 3-D form of the scraped profiles. The laser probe is made of a DVD pickup head based on the astigmatic principle. Driven by an XY stage, the entire scraped profile can rapidly be scanned. The quality of the scraped surface can thus be interpreted in a more scientific manner. Based on the measured 3-D data, new evaluation methods are proposed for five parameters, namely the PPI, POP, height of points (HOP) or depth of surroundings (DOS), flatness, and oil retention volume. Experiments show that the 3-D system is consistent with the 2-D system. It not only reveals more surface quality parameters but also uncovers more characteristic surface phenomena than the 2-D image system.     

Performance evaluation of outlier detection methods in GNSS vector networks using 1D and 3D component analysis

In most surveying studies, the computation of reference point coordinates in three-dimensional (3D) space has become inevitable with the extensive use of the Global Positioning System (GPS). Such computations are particularly required in the establishment and densification of 3D geodetic networks. In contrast to the classical measurements, GPS vector measurements, resulting from the processing of code and phase observations, are physically correlated. Therefore, in the analysis of observations components of GPS vectors can be considered either individually (i.e. 1D) or a whole (i.e. 3D). In this study, this consideration is applied for outlier detection process in a simulated GPS vector network and the results are compared with the outlier detection method considering each vector component individually. Results showed that using 3D components for outlier search was more effective when the gross error exists in all components of the baseline whereas the outlier search for 1D component was best conducted when an outlier exists in a component. Findings in this study clearly indicated that both methods should be applied to data sets to detect the outliers so as to ensure the elimination of outliers in the network.     

Sensor planning system for fringe projection scanning of sheet metal parts

Automatic sensor planning for 3D scanning with structured light scanners is a challenging task. That is true especially in the case of reflective surfaces that are typical for sheet metal parts. This paper describes a new sensor planning system for the automatic generation of scanning positions based on a computer model of the part for digitization of sheet metal parts. Compared to previous studies, visual properties of the scanned parts’ surface can be simulated precisely by application of a reflectance model proposed by Nayar and simplified by Ellenrieder. This model is suitable for materials that are characterized by the combination of diffuse and specular reflections. Results of the scanning that were obtained using an ATOS III Triple Scan fringe projection 3D scanner and a KUKA KR 60 HA industrial robot were compared to the simulation. Results obtained for two test parts show 16 and 26% differences in correspondence of acquired polygonal areas between simulation and scanning.     

Fabrication of porous copper surfaces by laser micromilling and their wetting properties

Porous copper surfaces show their great merits in the applications of chemical reaction, sound absorption and heat transfer. In this study, a laser micromilling method is proposed to fabricate porous surfaces with homogeneous micro-holes and cavities of the size about 1–15 μm on pure copper plates in a one-step process. The laser micromilling was performed by a pulsed fiber laser via the multiple–pass reciprocating scanning strategy. Based on the measurement of scanning electron microscope (SEM) and 3D laser scanning confocal microscope, the formation of surface structures was investigated together with the laser ablation mechanisms. The effects of laser processing parameters, i.e., laser fluence, scanning speed, number of scanning cycles and scanning interval, on the formation and surface morphology of porous surfaces were systematically assessed. Furthermore, the wettability of the porous copper surfaces was also evaluated by measuring the static contact angle of water. The results showed that the laser fluence played the most significant role on the formation of porous copper surfaces. The average depth and surface roughness of porous copper surfaces increased with increasing the laser fluence and number of scanning cycles while decreased with the increase in scanning interval. The scanning speed played little influence on the formation of porous copper surfaces. These results can be closely related to the variation of energy density and re-melting process during the laser micromilling process. Moreover, all the copper porous surfaces were found to be hydrophobic. The contact angle of porous copper surfaces was significantly dependent on laser fluence, but weakly affected by the scanning speed and number of scanning cycles.     

Construction of 3D solder paste surfaces using multi-projection images

This paper aims to develop the 3D surface construction applications for the solder pastes using multi-projection images and the neural network approach. The proposed solution uses the image features of multi-projection angles as the inputs and the laser surface scanning results as the outputs of the neural network model to perform precise 3D solder paste surface construction. In this manner, the proposed methodology can measure the 3D solder paste surfaces in a precise way like the laser scanning results. The advantages of this work is to use a low cost and high speed image solution to overcome the disadvantages of high cost and slow speed laser solution while the inspection accuracy is maintained. The multi-projection images are captured from the multi-channel light source and the coaxial light source to perform precise and efficient inspections, respectively. On the other hand, the back-propagation (BP) neural network approach is used to construct the 3D solder paste surface models for various solder pad geometries. Finally, the proposed system was experimentally verified. The experimental results showed that the multi-channel light source solution with pad based learning achieves 95% volumetric accuracy in average, and the coaxial light source with sub-area based learning just achieves 80% volumetric accuracy in average when compared to the actual laser surface scanning.