Contact and non-contact approaches in load monitoring applications using surface response to excitation method

Surface response to excitation (SuRE) method is a low-cost alternative to electromechanical impedance based structural health monitoring (SHM) technique. The SuRE method uses one piezoelectric transducer to excite the surface of a structure with a sweep sine wave. Piezoelectric sensors or scanning laser vibrometer can be used to monitor the dynamic response of structure.In this study, the performance of the SuRE method was evaluated with the conventional piezoelectric elements and scanning laser vibrometer used as contact and non-contact sensors, respectively, for monitoring the presence of loads on the surface. In order to determine the accuracy and reliability of both monitoring approaches in detecting changes in level of applied load, three different experimental setups were studied. Response of a system in the presence of a single ​load applying and multiple loads applying and its performance in detecting tightness in a nut and bolt system were investigated. The spectrum of the dynamic response is collected at the optimal operating condition. Any significant change of the spectral characteristics may indicate defects, improper loading or loose fasteners. The performance of the SuRE method using contact and non-contact sensors indicated that both variations of the method could be successfully used in load monitoring applications.     

Case study: A comparison of error sources in high-speed milling

This paper describes a case study devised to quantify the relative contributions of geometric, thermal, contouring, and cutting force errors to machined part dimensional errors. Measurements were performed to independently evaluate the: (1) quasi-static geometric errors using the laser ball bar; (2) variations in geometric errors due to thermal effects; (3) spindle thermal growth errors using a capacitance gage nest; (4) two-dimensional contouring errors using a grid plate encoder; and (5) surface location error due to (stable) forced vibrations during cutting. The effects of the first three error components were related to part dimensions using a homogeneous transformation matrix approach integrated into a Monte Carlo simulation. A comparison of the individual influences of these error sources showed that the cutting force error was dominant for the high-speed machining center/tool-holder combination selected for this study.     

基于激光扫描的环形锻件径向尺寸测量方法

在环形锻件环轧成形过程中,径向截面尺寸数据在很大程度上影响着环形锻件的锻造精度。由于轧制环境的复杂性以及高强度的噪声干扰,扫描数据量很大,据此测量得到的径向尺寸精度较低,因此,提出了一种环形锻件径向截面尺寸测量新方法。对激光扫描数据建立信息空间集合,并以连续的同心圆划分信息空间;根据信息空间内各区域间的信息关系,引入一致连续性系数并结合梯度下降法来求解待定系数;根据扫描数据点密度设定精简比率,结合精简比率与待定系数来确定每一同心圆各部分需要保留的点数,删去其余点及两同心圆相交部分的重复点;重复上述方法遍历激光扫描数据,对环形锻件的径向截面激光扫描数据进行处理。将该测量方法应用在实验中,从处理后的激光扫描径向截面形线数据中提取径向截面各尺寸数据,将处理前后所测得的数据与真实值进行对比,发现处理后得到的数据精度更高,误差更小,证明了该方法的可行性。     

单点激光连续扫描测振的薄壁圆筒件模态测试

针对薄壁圆筒构件模态测试的需求,提出了基于单点激光连续扫描测振的薄壁圆筒构件模态测试方法。根据纯模态分析理论,在研究两端固定细弦自由振动的基础上,理论上建立了单点激光测振仪在两端固定的自由振动细弦上做连续扫描所得的振动信号和弦的模态振型之间的关系,研究了并提出了激光测振仪输出信号的处理方法。搭建了单点激光连续扫描测振的薄壁圆筒构件模态测试平台,对薄壁圆筒构件进行了模态实测,获得了薄壁圆筒构件前五阶固有频率和相应的模态振型,并对测试结果进行了有限元模态分析验证。结果表明,模态振型一致,固有频率误差在5%以内,证明了提出方法的正确性和有效性。     

声场的直接测量

为准确测量声场分布,研究了基于激光测振仪的声场直接测量技术。基于声音在空气中传播时会引起空气折射率周期性变化的原理,利用激光测振仪测量了激光通过声场时激光光程受空气折射率的调制发生周期性变化产生的振动速度。由于声场在激光方向上的投影即为激光测振仪测得的振动速度,测量了声场在不同投影方向上的振动速度,再由Radon逆变换重构声场复振幅分布,从而实现了对声场的直接测量。文中实验测量了2kHz声场引起的振动速度振幅分布和相位分布,进而重构得到声压振幅分布和瞬时声压分布。测量得到空间分辨率为2cm,声压振幅最大为0.026Pa,对应的声压级为62.3dB。实验结果表明,基于激光测振仪的声场分布直接测量方法是可行的,该方法解决了现有麦克风阵列接触式测量声场存在的问题。     

Continuous scanning laser Doppler vibrometry and speckle noise occurrence

Continous scanning laser Doppler vibrometry (CSLDV) is a type of “spatial field” non-contact technique for measuring structural vibrations by employing a laser Doppler vibrometer whose laser beam is moving continuously on the structure surface. When an LDV is scanned continuously along an arbitrarily line, the LDV output is an amplitude-modulated sine wave according to the structure operational deflection shape. Smooth mode shapes, which can be defined by polynomial functions across the scanned area, may be recovered as a set of polynomial coefficients derived from the LDV output analysed in the frequency domain, which spectrum comprises sidebands centred on the excitation frequency and spaced at multiples of the scan frequency(ies).This paper concentrates its attention to the influence of the speckle noise on the measured data quality, the speckle being an unavoidable phenomenon occurring when a coherent light beam is scattered back from an optically rough surface.     

Structural Damage Detection Method Based on Decomposition of the Operating Deflection Shapes

Full-field measurement techniques such as the scanning laser Doppler vibrometer (LDV) and the electronic speckle pattern interferometry systems can provide a dense and accurate vibration measurement on structural operating deflection shape (ODS) on a relatively short period of time.The possibility of structural damage detection and localization using the ODS looks likely more attractive than when using traditional measurement techniques which address only a small number of discrete points.This paper discusses the decomposition method of the structural ODSs in the time history using principal component analysis to provide a novel approach to the structural health monitoring and damage detection.The damage indicator is proposed through comparison of structural singular vectors of the ODS variation matrixes between the healthy and damaged stages.A plate piece with a fix-free configuration is used as an example to demonstrate the effectiveness of the damage detection and localization using the proposed method.The simulation results show that:(1) the dominated principal components and the corresponding singular vectors obtained from the decomposition of the structural ODSs maintain most of all vibration information of the plate,especially in the case of harmonic force excitations that the 1st principal component and its vectors mostly dominated in the system;(2) the damage indicator can apparently flag out the damage localization in the case of the different sinusoidal excitation frequencies that may not be close to any of structural natural frequencies.The successful simulation indicates that the proposed method for structural damage detection is novel and robust.It also indicates the potentially practical applications in industries.     

Note: Development of a high resolution six-degrees-of-freedom optical vibrometer for precision stage

A high resolution six degrees of freedom (6-DOF) optical vibrometer is proposed. 6-DOF vibrations can be simultaneously measured using the proposed optical vibrometer, which reduces measurement time and number. The performance of the proposed vibrometer is verified by experiments. The results show that the accuracy of the proposed optical vibrometer is ±30 nm∕200 nm and ±0.04 arcsec∕0.1 arcsec at 1000 Hz.     

Vibrational characteristics of sphere model installed in wind tunnel test section

The vibrational characteristics of a sphere model installed in a wind tunnel test section were quantitatively investigated using a noncontact laser vibrometer. The sphere was supported by two fine wires passing perpendicularly through the center of the model in an X-shape. Each end of the wire was fastened firmly to the frame of the test section with a secure turnbuckle. This fixation method was evaluated by laser vibrometry measurement of the vibration of the sphere and by spectral analysis. The maximum displacement of the sphere was found to be less than 30 nm with the characteristic frequency on the order of hundreds of kilohertz. The effects of the vibrational displacement and frequency of the model were negligible considering the model dimensions and the shedding frequency of the sphere wake.     

Benchmarking of deformation and vibration measurement techniques for nuclear fuel pins

Understanding the mechanical interactions between the coolant and the core structure in nuclear reactors helps to determine the lifetime, health or optimal design of the reactor core. The flow of the coolant produces vibrations in the reactor core containing the fuel assemblies that consists of a matrix of fuel pins. We report on an evaluation of the performance of different vibration measurement techniques considered for measuring the flow induced vibrations on a fuel pin mock-up. These techniques include a laser Doppler vibrometer (LDV), a grid method (GRID), fiber Bragg grating sensors (FBGs), electrical strain gages and two types of accelerometers. In this paper we first show the practical aspects of the validation experiments before proceeding with the influence of the techniques on the pin dynamics. Finally we compare the signal-to-noise ratio (SNR) and the level of determination of the response signal of the sensors for low amplitudes and low frequencies. We conclude that for our setup the optical techniques and MEMS-type accelerometer prove to offer superior performance. Considering the space constraints, we believe that the fiber Bragg gratings are the best candidate for vibration monitoring in nuclear reactor core mock-ups.     

Fault detection for quality control of household appliances by non-invasive laser Doppler technique and likelihood classifier

This paper addresses the problem of developing an on-line diagnostic system for mechanical quality control of household appliances. The selection of a suitable measurement technique for feature extraction is discussed; the choice of a laser Doppler vibrometer technique and a laboratory measurement station for washing machines is presented. Vibration velocity and displacement are measured over a grid of points on the machine surface and data are stored in a database suitable for processing, both with good appliances and with defect ones with known defects. Features from the vibration velocity spectrum are used as the input to a likelihood classifier, which is shown to achieve very good classification scores.     

A study on the effect of rectangular cut out on laser forming of AISI 304 plates

Laser bending of a rectangular AISI 304 plate with a rectangular cut out is investigated in the present work. In particular, effects of process parameters: laser power, scanning speed, and geometric parameters: dimensions of the cut out of sheet metal on temperature distribution and bending angle are explored with the help of numerical simulation. Analyses are carried out through a coupled thermo-mechanical formulation with finite element method using COMSOL MULTIPHYSICS. The temperature distribution and deformation of sheet metal have been obtained from numerical simulations. The bending angle is affected by process parameters, namely, laser power, scanning speed, and width of the cut out of the sheet metal. Position of the cut out vis-à-vis the scanning path and length of the cut out have very little effect on the bending angle.     

Detection of solder bump defects on a flip chip using vibration analysis

Flip chips are widely used in microelectronics packaging owing to the high demand of integration in IC fabrication. Solder bump defects on flip chips are difficult to detect, because the solder bumps are obscured by the chip and substrate. In this paper a nondestructive detection method combining ultrasonic excitation with vibration analysis is presented for detecting missing solder bumps, which is a typical defect in flip chip packaging. The flip chip analytical model is revised by considering the influence of spring mass on mechanical energy of the system. This revised model is then applied to estimate the flip chip resonance frequencies. We use an integrated signal generator and power amplifier together with an air-coupled ultrasonic transducer to excite the flip chips. The vibrations are measured by a laser scanning vibrometer to detect the resonance frequencies. A sensitivity coefficient is proposed to select the sensitive resonance frequency order for defect detection. Finite element simulation is also implemented for further investigation. The results of analytical computation, experiment, and simulation prove the efficacy of the revised flip chip analytical model and verify the effectiveness of this detection method. Therefore, it may provide a guide for the improvement and innovation of the flip chip on-line inspection systems.     

Quasi-optimal processing of laser Doppler vibrometer signals

A method of quasi-optimal processing of laser Doppler vibrometer (LDV) signals in the case of monoharmonic vibrations is considered. Under certain assumptions, the problem of quasi-optimal estimation of the information parameter of the LDV signal, related through a known constant multiplier to the relative amplitude of vibrations, is solved. A structure of the device for optimal estimation is proposed. Conditions of optimality of the method are given. Statistical characteristics of estimates of the relative vibration amplitude are investigated by a numerical modeling method. The error of these estimates is compared with the Rao-Kramer bounds.     

Analysis of Natural Vibrations of Round Flexing Membrane-Type Piezoelectric Transducers with an Arbitrary Dimension Ratio

The problem of natural vibrations of a round exing membrane-type piezoelectric transducer (piezoelectric plate) with an arbitrary dimension ratio is solved in the exact formulation using the finite element method for two variants of fastening the membrane (simply supported and rigidly fixed plate). The natural frequency spectra are analyzed in the resonance and antiresonance regimes. The displacement distributions over the plate surfaces are also analyzed and the dependences of the dynamic electromechanical coupling coefficient are investigated as functions of the geometric dimensions of the piezoelectric transducer, the plate material, and the piezoelectric ceramic composition. The optimum geometric dimensions of the transducer which ensure the maximum dynamic electromechanical coupling coefficient are determined and the value of the coefficient is estimated.     

Continuous serial thin sectioning for electron microscopy

The process of serial sectioning for electron microscopy has been refined such that loss of thin sections is kept below 0.1% and the series is continued at will. The method relies on microscopic control of all manipulative steps, Formvar casting on plate glass for coated slot grids, coating of the block with contact cement for reliable ribboning, pickup by a one-step method with grid support in the diamond knife trough, staining in LKB grid holders, gentle treatment of grids in the electron microscope, and a slight modification to the microscope for safe grid withdrawal. The results are particularly applicable to the reconstruction of neuronal microcircuits and larger volumes of neuropil.     

非接触压电微马达的仿真与实验研究

提出了一种基于四区和八区2种激振方式的新型非接触压电微马达,并进行了振动模态的有限元数值模拟。利用ANSYS有限元软件对定子进行模态分析,并用激光测振仪对四区和八区2种激振方式下的定子进行位移扫频和振型测试,得出:最优驱动频率分别为34.4 kHz和46.3 kHz,相应的模态分别为B₂₁和B₂₂,二者结果吻合较好。对微马达的输出性能进行了实验研究,结果表明:三叶片和六叶片转子在八区最优模态激振下转子的转速约是四区的2倍。说明了增加定子的分区数不仅改变了定子的最优驱动频率而且能够明显提高转子的转速。而且当A-B间相位差从90°调谐到270°时,行波的传播方向发生了改变,从而实现了转子的换向。实验还表明,转子的正反转的转速基本一致。     

TiC颗粒增强镍基合金激光熔覆工艺仿真

通过有限元分析方法对不同激光功率、扫描速率以及光斑直径下TiC/Inconel 718复合材料制造过程中的热-力学特征进行了研究。通过线性组分公式确定复合材料的热物理性能参数,选用半球高斯热源模拟激光温度载荷,利用生死单元技术实现金属粉末增材过程。采用间接法进行激光熔覆热-力耦合分析,基于温度分析结果转换单元类型进行热应力计算。研究表明个工艺参数与温度、温度变化率及残余应力的变化存在一定的规律,且激光加工功率在225～250 W之间、扫描速度在1.0～1.5 mm/s之间、光斑半径在2.5～3.0 mm之间达存在最佳加工参数,可以达到较好的熔覆效果。     

A new method for processing impact excited continuous-scan laser Doppler vibrometer measurements

A scanning laser Doppler vibrometer (LDV) can acquire non-contact vibration measurements from a structure with high spatial detail in an automated manner; one only need redirect the laser via computer-controlled mirrors to acquire measurements at additional points. However, since most LDV systems are only capable of measuring one point at a time, conventional scanning vibrometry cannot be effectively employed in some situations, for example when the time record is long at each measurement point or when the structure changes with time. Conventional scanning LDV systems are also difficult to employ with impact excitation because there is considerable variation in the impact location, angle and the character of the impacts, which leads to errors in the mode shapes that are extracted from the measurements. This paper presents a method by which one can determine the mode shapes, natural frequencies and damping ratios of a structure from as little as one response record by sweeping the laser continuously over the vibrating structure as the measurement is acquired. A novel resampling approach is presented that transforms the continuous-scan measurements into pseudo-frequency response functions, so they can be processed using standard identification routines to find the modal parameters of the structure. Specifically, this work employs a standard multi-input–multi-output identification routine and the complex mode indicator function to the continuous-scan laser Doppler vibrometry (CSLDV) measurements. The method makes no assumptions regarding the shape or properties of the surface and only requires that the laser scan periodically and that the structure vibrate freely. The method is demonstrated experimentally on a free–free beam, identifying the first nine mode shapes of the beam at hundreds of points from a few time histories. For this system, this represents a two-order of magnitude reduction in the time needed to acquire measurements with the LDV.     

Vision-Based Obstacle Avoidance in Sidewalk Environment Using Top-View Transform and Optical-Flow

This paper presents a vision-based obstacle avoidance method for blind pedestrians in an outdoor sidewalk environment.Unlike many existing travel-aid systems using stereo-vision based methods,the proposed method is able to get obstacle position as well as user motion information by just one monocular camera fixed at the belly of the user.To achieve this goal,a top-view transformation of the road image is used for obstacle detection and user motion estimation,based on which a grid map is generated for navigation.For detection part,the bottom points of erect obstacles are detected by extracting local-maxima and minima on the top-view image while user motion is estimated by analysing the optical flow vector field in the user surrounding area.For the obstacle avoidance part,a step score is calculated on the grid map for evaluating the safety of next moving step.Experiments with several sidewalk video-clips show that the proposed obstacle avoidance method is able to provide useful guidance instructions under certain sidewalk environments.