A Framework for Laser Scan Planning of Freeform Surfaces

This paper proposes a framework that can be used for automatic planning of the laser scanning operation. Highly complex and freeform shaped parts are used in industry more and more; however, it is difficult to capture the part accurately in a short time using a conventional measuring machine. The technology of the laser scanner has been developed greatly and this machine can now meet the requirements. There are few planning systems, however, that can support the scanning operation. The planning system proposed in this paper considers various system parameters, such as depth of field, view angle, and length of laser stripe, to develop an accurate measuring plan. The algorithms in the planning system consist of two steps, estimation of the initial scan plan and a feedback routine that checks for the scanning constraints and generates the final scan plan. The developed algorithms were verified by examining a part with Bezier surfaces.     

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.     

光学三角法全视场自扫描测头的设计与研究

本文基于光学三角法测量原理,设计了一种通过改变光平面与成像光轴夹角来实现对视场内被测零件表面扫描的视觉测头。通过紧凑的旋转镜片反射机构驱动光平面在物空间做扫描运动,将全视场自动扫描功能封装在在线结构光视觉测头内部,使扫描成为线结构光视觉测头的一项内置功能。光定位精度实验以及尺寸测量的重复性精度实验均表面这种设计方法的可行性。这种自扫描测头对中、小尺寸零件三维尺寸的快速检测有重大的实用价值。     

Analysis of the geometrical features detectability constraints for laser-scanner sensor planning

This paper presents a planning approach for automatic synthesis of the viewpoint locations of a laser scanner that satisfy the detectability constraints of the geometrical features of a given object. Dominant constraints taken into consideration were the laser scanner field of view, depth of view, and the maximum permissible view angle. The approach is task driven and sensor model based. Information is given about the scanned object and its selected features, as well as about the optical characteristics of the sensor. Planning is presented as an optimisation problem where the optimal laser-scanner viewpoint must satisfy all the constraints simultaneously. The approach is demonstrated by scanning a 2D feature, and through planning an experiment in which robotic assembly of a car door is assisted by a laser scanner.     

激光扫描追踪人体目标位姿的算法研究

针对人体目标位姿的追踪问题,对二维激光扫描仪的工作原理进行了归纳,对所测取数据的分析算法进行了研究,设计了一种基于激光图像扫描的新型算法,利用C语言与Matlab混合编程,通过对人体目标肩部截面进行椭圆拟合,实时测取了运动人体目标的距离、角度和方位信息,并通过Matlab GUI显示.二维激光扫描仪通过旋转的光学部件发射光脉冲,形成了二维的扫描面,以实现区域扫描及轮廓测量的功能.研究结果表明,利用激光扫描仪测取的人体目标的位置信息与真实的人体目标的位置信息相比基本吻合,距离范围为0m～4m,距离精度优于0.05 m,角度范围为240°,角度跟踪精度优于2°,方位范围为360°,方位精度最高1 °,该算法可用于需要对人体目标进行实时追踪的工程应用中,例如用于对人体目标实时跟随的移动平台或机器人中.     

A 2D MEMS mirror with sidewall electrodes applied for confocal MACROscope imaging

This paper presents microelectromechanical system micromirrors with sidewall electrodes applied for use as a Confocal MACROscope for biomedical imaging. The MACROscope is a fluorescence and brightfield confocal laser scanning microscope with a very large field of view. In this paper, a microelectromechanical system mirror with sidewall electrodes replaces the galvo-scanner and XYZ-stage to improve the confocal MACROscope design and obtain an image. Two micromirror-based optical configurations are developed and tested to optimize the optical design through scanning angle, field of view and numerical aperture improvement. Meanwhile, the scanning frequency and control waveform of the micromirror are tested. Analysing the scan frequency and waveform becomes a key factor to optimize the micromirror-based confocal MACROscope. When the micromirror is integrated into the MACROscope and works at 40 Hz, the micromirror with open-loop control possesses good repeatability, so that the synchronization among the scanner, XYZ-stage and image acquisition can be realized. A laser scanning microscope system based on the micromirror with 2 μm width torsion bars was built and a 2D image was obtained as well. This work forms the experimental basis for building a practical confocal MACROscope.     

A confocal laser microscope scanner for digital recording of optical serial sections

A confocal laser microscope scanner developed at our institute is described. Since an ordinary microscope is used, it is easy to view the specimen prior to scanning. Confocal imaging is obtained by laser spot illumination, and by focusing the reflected or fluorescent light from the specimen onto a pinhole aperture in front of the detector (a photomultiplier tube). Two rotating mirrors are used to scan the laser beam in a raster pattern. The scanner is controlled by a microprocessor which coordinates scanning, data display, and data transfer to a host computer equipped with an array processor. Digital images with up to 1024 × 1024 pixels and 256 grey levels can be recorded. The optical sectioning property of confocal scanning is used to record thin (～ 1 μm) sections of a specimen without the need for mechanical sectioning. By using computer-control to adjust the focus of the microscope, a stack of consecutive sections can be automatically recorded. A computer is then used to display the 3-D structure of the specimen. It is also possible to obtain quantitative information, both geometric and photometric. In addition to confocal laser scanning, it is easy to perform non-confocal laser scanning, or to use conventional microscopic illumination techniques for (non-confocal) scanning. The design has proved reliable and stable, requiring very few adjustments and realignments. Results obtained with this scanner are reported, and some limitations of the technique are discussed.     

Rapid and accurate reverse engineering of geometry based on a multi-sensor system

The reduction of the lead time in measurement and reverse engineering, and the increased requirements in terms of accuracy and flexibility, have resulted in a great deal of research effort aimed at developing and implementing multi-sensor systems. This paper describes an effective competitive approach for using a tactile probe to compensate the data from a laser line scanner to perform accurate reverse engineering of geometric features. With the data acquired using laser scanning, intelligent feature recognition and segmentation algorithms can be exploited to extract the global surface information of the object. The tactile probe is used to re-measure the geometric features with a small number of sampling points and the obtained information can be subsequently used to compensate the point data patches which are measured by laser scanning system. Then, the compensated point data can be exploited for accurate reverse engineering of a CAD model. The limitations of each measurement system are compensated by the other. Experimental results on three parts validate the rapidity and accuracy of this multi-sensor data fusion approach.     

Kernel PCA and approximate pre-images to extract the closest ultrasonic arc from the scanning of indoor specular environments

In this paper, the authors firstly develop a general detailed acoustic waves propagation model validated by the comparison with different types of ultrasonic sensors and then they deal with the problem of extracting an arc from the ultrasonic scan in order to differentiate direct reflections from other spurious readings. They use a mechatronic system to scan an indoor environment, locating their scanner in front of a regular target surface. The scanning device allows the rotation of ultrasonic sensors with a view for detecting strong echoes in each pointing direction. The first echo permits to estimate the time-of-flight (TOF) and subsequently to obtain the distance covered by the sonic pulse. The scanned space is approximated as being two-dimensional and the scanner is treated as a ray-tracer placing a dot along the sensor axis at the measured distance. In case the reflected-back energy is not able to exceed the detector threshold, the dot is plotted at the appropriate maximum range value. The process of drawing a set of points relating to the closest target surface was achieved by means of the Kernel Principal Component Analysis (KPCA) that maps the original data into a higher-dimensional feature space. The obtained data were then projected into a subspace of the feature space and the approximate pre-images were calculated to extract the looked-for arc. Simple matrix algebra makes the method an efficient alternative to other techniques based on distance threshold that generally require a priori information on the explored environment. The arc permits to estimate location and orientation of the target surface with respect to the scanning device. Several tests were carried out in order to appreciate the effectiveness of the described approach and the results were compared to the outcome of other existing techniques.     

激光主动探测系统二维均匀扫描设计

二维扫描振镜是激光主动探测系统实现一定视场范围内扫描探测的重要部件.针对振镜非均匀扫描的缺陷,讨论了二维扫描振镜的工作原理,分析了扫描激光光斑全覆盖条件,推导了等间距扫描量与驱动信号之间的关系式,结合单片机控制电路,给出了均匀扫描的软件实现方法,并对实现均匀扫描的电路进行了时序分析.该方法过程简单,激光光斑的均匀采样易...     

数字X线影像仪中激光扫描光学系统的设计

数字X线影像仪是计算机和激光技术快速发展的产物,而其中的激光扫描光学系统是其核心技术之一。针对数字X光影像仪的使用特点,对激光扫描系统中的Fθ镜头、光束扩展器、扫描器这几个关键部件进行了较详细的论述分析,提出并解决了其中一些关键问题,如光阑位置浮动对像质的影响、影响系统扫描光点大小的因素、扫描器的确定等。最后用ZEMAX光学设计软件对系统的光学性能进行了设计模拟,得到扫描光斑直径小于0.1mm、焦距和视场满足线性关系的设计结果。像质评价分析结果表明,所设计的镜头像质优良,轴上与轴外质量相当,像质达到衍射极限。     

Nanoscale characterization of gold nanoparticles created by in situ reduction at a polymeric surface

Transmission Electron Microscopy is used as a quantitative method to measure the shapes, sizes and volumes of gold nanoparticles created at a polymeric surface by three different in situ synthesis methods. The atomic number contrast (Z‐contrast) imaging technique reveals nanoparticles which are formed on the surface of the polymer. However, with certain reducing agents, the gold nanoparticles are additionally found up to 20 nm below the polymer surface. In addition, plan‐view high‐angle annular dark‐field scanning transmission electron microscopy images were statistically analyzed on one sample to measure the volume, height and effective diameter of the gold nanoparticles and their size distributions. Depth analysis from high‐angle annular dark‐field scanning transmission electron microscopy micrographs also gives information on the dominant shape of the nanoparticles.     

Error Compensation for Three-Dimensional Line Laser Scanning Data

In this paper, the problem of compensation of 3D line laser scanning data for improved inspection accuracy is addressed. This problem is important, as a 3D line laser scanner is about one order less accurate than a touch probe. The approach taken is to compensate through error characterisation. In other words, a software compensation is performed, instead of a hardware compensation, which is probably more expensive. To do so, the errors associated with a 3D line laser scanning system are first characterised. With error characterisation, an empirical formula is obtained relating the errors to the influencing factors including the projected angle and the scan depth. This empirical formula is used to compensate for the digitised surface data obtained by the corresponding laser scanning system. The results show that the proposed approach effectively improves inspection accuracy.     

新型压电光学扫描器的性能分析及控制

详细分析了压电扫描器的机械特性，及其对扫描特性的影响，通过调整输入波形的形式来修正压电扫描器的扫描畸变，从而达到计算机控制压电扫描器扫描的高精度和可控性。压电扫描器的扫描角度能达到３°以上，扫描频率为２０Ｈｚ以上，扫描线性度为 １％。     

特殊工件扫描数据处理

针对逆向工程中激光扫描仪在扫描大型异形工件时点云对齐质量差,点云数据精简效果不理想等问题,提出了先分部精简点云数据再对齐的方法。对坐便器进行扫描处理,实现了工件表面模型的建立。     

Large angle and high linearity two-dimensional laser scanner based on voice coil actuators

A large angle and high linearity two-dimensional laser scanner with an in-house ingenious deflection angle detecting system is developed based on voice coil actuators direct driving mechanism. The specially designed voice coil actuators make the steering mirror moving at a sufficiently large angle. Frequency sweep method based on virtual instruments is employed to achieve the natural frequency of the laser scanner. The response shows that the performance of the laser scanner is limited by the mechanical resonances. The closed-loop controller based on mathematical model is used to reduce the oscillation of the laser scanner at resonance frequency. To design a qualified controller, the model of the laser scanner is set up. The transfer function of the model is identified with MATLAB according to the tested data. After introducing of the controller, the nonlinearity decreases from 13.75% to 2.67% at 50 Hz. The laser scanner also has other advantages such as large deflection mirror, small mechanical structure, and high scanning speed.     

A small scanning tunnelling microscope with large scan range for biological studies

We have developed a scanning tunnelling microscope specially designed for biological applications presenting some new features: the scanner tube is mounted parallel to the surface of the sample which enables a high resolution optical microscope to be brought close to the sample when working in air or liquids. The maximum scan range is 5×20 μm with a vertical range of 20 μm and the total size of the system does not exceed 10×40 mm. The piezo-sensitivity of the scanner tube versus applied voltage was analysed by interferometry measurements and by using scanning tunnelling microscopes. We found a value for the piezoelectric constant d₁₃ of ?1·71 Å/V at low voltages (under a few volts) going up to ?2 Å/V for higher voltages. Large-scale images of a carbon grid showed a surprisingly good linearity of the scanner tube.     

Isoplanar-based adaptive sampling for model-unknown sculptured surface coordinate metrology using non-contact probe

Optimizing sampling size and point distribution is a difficult issue for sculptured surface measurement. Highly curved areas should be sampled densely and vice versa. To achieve this goal, an adaptive sampling using point laser sensor has been further researched based on isoplanar method for model-unknown surface coordinate metrology. Sampling control points must be previously predicted using the measured data. Therefore, a reference needs to be initially established, and then dynamically updated during sampling process. Isoplanar line method has been employed to plan sampling movement. Curvature-based adaptive step adjustment method and hybrid extrapolation mode have been developed and further employed to calculate the two control increments in both feeding direction and side direction. A group of planar curves, which are utilized as an initial reference for scanning, are primarily sampled in slow tracking stage. As a result, a guide path can be estimated to direct fast scanning for the next path. The surface measurement algorithm was designed and programmed on an open CNC platform. Finally, with the help of linear interpolation technique and macro variables, a practical application has been implemented to verify the proposed fast sampling approach.     

Novel Method for Spatial Angle Measurement Based on Rotating Planar Laser Beams

Spatial angle measurement,especially the measurement of horizontal and vertical angle,is a basic method used for industrial large-scale coordinate measurement.As main equipments in use,both theodolites and laser trackers can provide very high accuracy for spatial angle measurement.However,their industrial applications are limited by low level of automation and poor parallelism.For the purpose of improving measurement efficiency,a lot of studies have been conducted and several alternative methods have been proposed.Unfortunately,all these means are either low precision or too expensive.In this paper,a novel method of spatial angle measurement based on two rotating planar laser beams is proposed and demonstrated.Photoelectric receivers placed on measured points are used to receive the rotating planner laser signals transmitted by laser transmitters.The scanning time intervals of laser planes were measured,and then measured point's horizontal/vertical angles can be calculated.Laser plane's angle parameters are utilized to establish the abstract geometric model of transmitter.Calculating formulas of receiver's horizontal/vertical angles have been derived.Measurement equations' solvability conditions and judgment method of imaginary solutions are also presented after analyzing.Proposed method for spatial angle measurement is experimentally verified through a platform consisting of one laser transmitter and one optical receiver.The transmitters used in new method are only responsible for providing rotating light plane signals carrying angle information.Receivers automatically measure scanning time of laser planes and upload data to the workstation to calculate horizontal angle and vertical angle.Simultaneous measurement of multiple receivers can be realized since there is no human intervention in measurement process.Spatial angle measurement result indicates that the repeatable accuracy of new method is better than 10".Proposed method can improve measurement's automation degree and speed while ensuring measurement accuracy.     

A laser sensor with multiple detectors for freeform surface digitization

In this paper, an integrated laser scanning sensor for freeform surface digitization is presented. The sensor consists of a diode laser light source and four position-sensitive device (PSD) detectors. The stand-off distance of the sensor is 180 mm and the measurable range is 90 mm. The Lambert model is applied to calculate the displacement between the sensor and the measured point on the object surface along the optical axis, under the assumption of a diffusive surface. The inclination angle of the measured point from the vertical plane of the laser beam is calculated by mathematical inference. Those data are used in error compensation to improve the system precision. The new design of multiple detectors could increase the measurable angle and could solve the dead-space problem in the single-point laser of triangulation measurement. The computer simulation and actual measurements show that the displacement resolution is around 50 μm, and the system performs well in terms of stability and repeatability. The sensor system could be mounted on the NC machine or on the X–Y platform for freeform surface digitization.