Acoustic source identification using a scanning laser Doppler vibrometer

This paper shows how a scanning laser Doppler vibrometer (LDV), an instrument designed to measure vibrations of structures or objects, can be used in a non-traditional fashion to identify acoustical sources. This is achieved by measuring the changes in the optical path induced by local fluctuation of the air refraction index to which the LDV is sensitive. The acoustical signal used is sinusoidal and may be recovered by scanning at a uniform rate over a subject area (continuous scan) parallel to the source axis and demodulating this signal. Due to the fact that the measured scan area is in fact a line integral over a measurement volume between the laser head and a rigid object needed to reflect the laser beam, multiple view planes around the axis of the acoustic source are usually measured. These are then passed through a tomographic algorithm, thereby reconstructing the full sound field. In this article however, only one view plane is measured, but the acoustic source is placed on a rotating surface with fixed rotational frequency, thereby imposing a modulation on the measured spectrum. Demodulation will allow reconstruction of the three-dimensional sound field.     

Two-axis-scanning laser Doppler vibrometer for precision engineering

Laser Doppler vibrometer (LVD) has been the most favorite instrument for precision dynamics measurement due to its non-contact, high accuracy and high resolution. However, LDV can only give the dynamic data of a particular location on the entire feature. In order to get the whole field data, a laser beam-scanning mechanism has to be implemented. Currently, motor-driven scanning mirror is used to move the measurement probe from one point to another. The mechanical vibrations of the scanning mirror will reduce the measurement accuracy. This paper introduces a novel scanning LDV optical system embodied in an acousto-optic deflector scanning mechanism. It can improve the measurement accuracy since there is no mechanical motion involved. One main advantage of this system is that it generates a laser scanning beam in parallel that is different from the beam scanning in the conventional scanning laser Doppler vibrometer (SLDV). The new system has a board scanning range. The measurement target size ranges from few tens of millimeters down to 10 μm. We have demonstrated the capability of the novel system on scanning measurements of features as big as ultra-precision cutting tool to features as tiny as AFM cantilever. We believe that the novel SLDV will find profound potential applications in the precision engineering field.     

Automatic vibration mode tracking using a scanning laser Doppler vibrometer

Optical measurement techniques are being used more and more for quality control in the production process. An important problem when trying to implement optical measurement systems, and in particular the scanning laser Doppler vibrometer (SLDV) which is considered in this paper, is that user interaction is always required. The need for user interaction implies an increase in testing time and cost and also brings an additional source of variability of the test results. In this article, a completely autonomous test procedure is developed to track the vibration behavior of a product during the production process (this includes automatic laser focussing, position calibration, object recognition, grid generation and mode estimation). The proposed procedure typically requires a few minutes only to automatically determine a high-resolution model of the vibration behavior. The method will be validated on measurements of an electronic circuit board.     

Reducing measurement time for a laser Doppler vibrometer using regressive techniques

In the last decade the laser Doppler vibrometer (LDV) has become a widely spread instrument for measuring vibrations. It often offers accurate measurements with a high spatial resolution. However, the measurement time of the LDV and especially for the scanning LDV is long. Therefore, reducing the measurement time is an attractive objective. A way to achieve this is to use a single sine excitation (on a resonance frequency). However, this technique has two major drawbacks: the inability to provide information on the damping and a operational deflection shape that can differ from the true mode shape. In this article two methods will be introduced to reduce measurement time without these defaults. In the first method introduced in this article a narrow band multisine is used as excitation signal and the measured vibration signal in the time domain is represented by a model using sines and cosines with these fixed narrow band frequencies. The coefficients of those sines and cosines are then estimated on a global scale by means of a least-squares estimator. An important advantage of this particular technique is that one does not have to measure a full period of the signal, reducing time. The second method accelerates the measurement time for scanning LDV measurements. Using the time domain sequence from each previous scan point and a limited number of time samples from the current scan point, the full time domain sequence of the current scan point can be estimated. Both these methods are a key benefit for in-line quality control, which can have upwards of 1000 spatial measurement locations. The proposed techniques will be validated on both simulations and experiments of varying complexity.     

全光纤激光多普勒测振仪空气耦合振动检测

采用外差探测方式, 建立了全光纤结构的激光多普勒测振仪,用来测量经空气传播耦合引起的散射体振动,以期感知振源振动信息。实验探索了散射体材料、固定方式和相对振源位置等对空气耦合振动的影响,得到了200~2700 Hz频率范围内6种散射体振动响应特性曲线。结果表明:通过空气耦合引起的散射体振动,在相同振源振幅下,散射体振动的振幅一般随频率升高而升高; 在材料、固定方式和相对振源位置等因素中,散射体材料对振动响应特性的影响是主要的,后两者只影响局部细节。     

Optical measurement of the dynamic strain field of a fan blade using a 3D scanning vibrometer

Understanding the origin of the stress and strain distribution is crucial to increase the durability of components under dynamic loading. Numerical simulations based on finite element (FE) models help with this understanding but must be validated by real measured data. Updating the FE model using the measured data is often the next step in the design process. In this paper the recently developed 3D-scanning laser doppler vibrometer (3D-SLDV) is used to measure the 3D-displacement of a fan blade, which is then used to calculate the dynamic strain distributions. The measurement principle and experimental setup are discussed thoroughly. The experimental results are validated by using a FE model on the one hand and strain gage measurements on the other. It is shown that this technique is capable of measuring normal strain far below 1 microstrain. This technique has the potential to fill in the gap of accurately measuring small (full-field) normal and shear strains at both low and high frequencies, where other optical techniques (and strain gages) would certainly fail.     

Damage detection in composite panels based on mode-converted Lamb waves sensed using 3D laser scanning vibrometer

This paper introduces damage identification approach based on guided ultrasonic waves and 3D laser Doppler vibrometry. The method is based on the fact that the symmetric and antisymmetric Lamb wave modes differ in amplitude of the in-plane and out-of-plane vibrations. Moreover, the modes differ also in group velocities and normally they are well separated in time. For a given time window both modes can occur simultaneously only close to the wave source or to a defect that leads to mode conversion. By making the comparison between the in-plane and out-of-plane wave vector components the detection of mode conversion is possible, allowing for superior and reliable damage detection. Experimental verification of the proposed damage identification procedure is performed on fuel tank elements of Reusable Launch Vehicles designed for space exploration. Lamb waves are excited using low-profile, surface-bonded piezoceramic transducers and 3D scanning laser Doppler vibrometer is used to characterize the Lamb wave propagation field. The paper presents theoretical background of the proposed damage identification technique as well as experimental arrangements and results.     

Global noise characteristics of a laser Doppler vibrometer-II. Experiments using beam dynamics

The use of a laser Doppler vibrometer to obtain velocity information from vibrating structures has gained wide acceptance in recent years. Although use of such an instrument can yield a spatially dense matrix of velocity information, several users have noted ‘noise’ at certain points in the spatial field. The technique by which the SLDV system operates results in occasional velocity ‘drop-outs’ which are unidirectional, always estimating the velocity response closer to zero than reality. These ‘drop-out’ areas occur more predominately at points of maximum velocity response with small rotational components. Alternatively, points exhibiting minimum velocity response with large rotational components are less susceptible to the ‘noise’. In this paper, an experiment to visualize the speckle pattern motions received by the photo-detectors during these vibration conditions is presented. Theories regarding the source(s) of the ‘noise’ are developed.     

基于编码标志点的数码相机三维测量与重构

介绍了利用数码相机作为图像传感器,在物体表面设置编码标志点,通过对编码标志点进行识别和检测,从而确定标志点的空间位置,实现了对物体表面某些特定点三维信息的无接触检测。介绍了其中的关键技术,如标志点的设计与检测、基础矩阵的求解、数码相机参数的标定以及内外参数的捆绑调整等,给出了检测实例并进行了精度验证。实验证明,该方法结构简单,成本低廉,数据采集快,移动方便,检测范围大,可满足三维物体特别是大面积物体空间检测的需求。     

Fusion of 3D laser scanner and depth images for obstacle recognition in mobile applications

The problem of obstacle detection and recognition or, generally, scene mapping is one of the most investigated problems in computer vision, especially in mobile applications. In this paper a fused optical system using depth information with color images gathered from the Microsoft Kinect sensor and 3D laser range scanner data is proposed for obstacle detection and ground estimation in real-time mobile systems. The algorithm consists of feature extraction in the laser range images, processing of the depth information from the Kinect sensor, fusion of the sensor information, and classification of the data into two separate categories: road and obstacle. Exemplary results are presented and it is shown that fusion of information gathered from different sources increases the effectiveness of the obstacle detection in different scenarios, and it can be used successfully for road surface mapping.     

Systematic error correction of a 3D laser scanning measurement device

Non-contact measurement techniques using laser scanning have the advantage of fast acquiring large numbers of points. However, compared to their contact-based counterparts, these techniques are known to be less accurate. The work presented in this paper aims at improving the accuracy of these techniques through an error correction procedure based on an experimental process that concerns mechanical parts. The influence of the three parameters, defining the relative position and the orientation between the sensor and the surface, is studied. The process used to build an experimental global model of error is presented and applied to a typical part composed of planes or skewed surface. The systematic errors have been reduced by half in comparison to the reference values, while the random errors have slightly increased. This phenomenon is due to the fact that the errors correction model does not take into account the local response of the laser sensor. A second model, taking into account the local defect, has been developed. Its application to an example of inspection of a mechanical part shows an improvement of the results of the correction.     

多视点立体显示系统建模及分析

建立了平行式摄像机阵列实拍立体显示系统的数学模型。根据光学成像原理和计算机视觉双目视差原理,通过一个4×4齐次矩阵描述了真实物体到虚拟物体的映射关系。该实拍模型可精确地调节拍摄和显示的参数,能有效地控制立体深度感,减小立体失真。利用该模型,定量分析了拍摄和显示参数对虚拟立体图像观看质量的影响。分析和计算结果表明,该模型具有很高的立体深度调节灵敏度,能通过调节拍摄和显示的参数,把形变系数控制在1左右,避免了纸板效应。通过理论和仿真证明了这一模型具有很好的实拍效果。     

利用激光三角测距法提高三维面型检测精度的方法

本文首先介绍了根据激光三角测距原理并采用片光照明、面阵ＣＣＤ接收的三维面形检测原理，其优点在于物面相对于测量头仅需一个方向移动即可完成二维扫描，使测量系统结构简单、便于控制。其次分析了影响其测量精度的几个主要因素，即散斑噪声、接收器件本身的分辨率、投影片光束的宽度与稳定性。散斑噪声造成了投影光斑的抖动，ＣＣＤ的分辨率限制了测量系统的分辨率，而片光束的质量影响着ＣＣＤ光敏面上像斑的准确定位。针对其各自的特点给出了相应的解决途径。     

基于三维空间域移动通信统计信道的多普勒效应

针对散射体均匀分布以及三维空间域(3 dimension,3D)移动通信环境,提出了3D空间统计信道模型.对在指向性天线覆盖下的室内微小区移动通信环境,模型能够估计多径衰落信道的重要空时信道参数,如波达信号在水平面以及垂直面的信号到达角度(angle of arrival,AOA)以及多普勒效应(Doppler spectra,DS)等.移动台(mobile station,MS)的移动特性会使接收信号产生多普勒效应,根据运动的相对性理论,基站(base station,BS)也会有相对运动,因此也会有多普勒效应产生.本文引入控制变量法导出三维空间域在MS以及BS端的多普勒效应,数值仿真结果与室外3D多径衰落信道对比表明,本模型的信道参数估计结果符合理论和经验,扩展了3D空间统计信道模型的研究和应用.     

机载激光3D探测成像系统的关键技术

介绍了机载激光3D探测成像系统的应用领域和国内外发展现状,概括了小型无人机载激光3D成像检测系统（JIGSAW）、机载激光测深系统（LADS）、直升机3D-LZ Imaging LADAR及机载对地高分辨详查系统的基本组成、技术指标以及各自的特点。阐述了激光3D成像系统的工作原理与扫描方式的分类,并重点对其单元核心技术即激光测距系统、激光光轴控制与指向测量系统、数据图像处理技术与显示系统进行了研究。最后,简单综述了机载激光3D探测成像系统的发展前景。     

A New 3D Digitizer by Cutting and Scanning Layer-by-layer

A crucial element in the reverse engineering process is to obtain 3D data of the part or its CAD model. A new method is presented in this paper, which combines layer-by-layer cutting and scanning on each cross-section of a part with a milling machine and an image scanner respectively. The method can capture the internal and external profile informations of a complex-shaped part at the same precision simultaneously. When data files of 2D edges are imported into the 3D CAD/CAM package, the 3D data or 3D CAD model is acquired. According to the customers' requirements, the system can reach the highest accuracy of 5.4 μm, and a resolution of 2.7 μm in obtaining the 2D edges. The product is completely made now, on which experiments are conducted, which demonstrate the higher accuracy and efficiency.     

机载激光3D探测成像系统的关键技术

介绍了机载激光3D探测成像系统的应用领域和国内外发展现状,概括了小型无人机载激光3D成像检测系统(JIGSAW)、机载激光测深系统(LADS)、直升机3D-LZ Imaging LADAR及机载对地高分辨详查系统的基本组成、技术指标以及各自的特点。阐述了激光3D成像系统的工作原理与扫描方式的分类,并重点对其单元核心技术即激光测距系统、激光光轴控制与指向测量系统、数据图像处理技术与显示系统进行了研究。最后,简单综述了机载激光3D探测成像系统的发展前景。     

复杂轮廓表面激光检测及三维重构技术的研究

基于零件实物样件的几何模型反求技术已成为CAD/CAM领域中的研究热点之一。以航空航天用MJ螺纹为对象 ,对逆向工程中的两大关键技术———表面数字化及三维重构技术进行了分析和研究。提出了用一种新型激光光纤传感器实现复杂轮廓表面的非接触数字化检测方法 ,描述了测量原理。通过二维截面轮廓图重构三维面型 ,初步的实验结果验证了此方法的有效性和实用性     

A New 3D Digitizer by Cutting and Scanning Layer by layer

１ＩｎｔｒｏｄｕｃｔｉｏｎＷｉｔｈｔｈｅｄｅｖｅｌｏｐｍｅｎｔｏｆｔｈｅｒａｐｉｄｐｒｏｔｏｔｙｐｉｎｇ,ｉｔｓｔｗｉｎｎｉｎｇｔｅｃｈｎｉｑｕｅ———ｔｈｅｒｅｖｅｒｓｅｅｎｇｉｎｅｅｒｉｎｇ［１］ｉｓｕｎｄｅｒｇｏｉｎｇａｒａｐｉｄｄｅｖｅｌｏｐ...