基于声压时间反转镜的水下目标被动定位技术

文章围绕基于时间反转镜技术的水下运动目标被动定位问题,讨论了被动声压反转镜（Pressure Time Reversal Mirror,PTRM）定位的基本原理,在此基础上,对PTRM进行了定位仿真分析,并对湖上试验数据进行了处理,试验结果与理论结果基本一致。结果表明,利用PTRM技术可以有效地对水下运动目标进行被动定位。     

被动迭代时间反转镜研究

抗除多途信道的干扰,优化被动检测性能是声纳信号处理关注的问题之一。通过仿真揭示了多途信道中单水听器时间反转镜的聚焦效应,研究了被动迭代时间反转镜技术,对其多目标定位选择性、多目标选择聚焦性能作了研究。结果表明利用了海洋信道的相干多途特性,被动时间反转镜可实现多个目标的空间匹配滤波：利用迭代算法,被动迭代时间反转镜可在抑制环境噪声干扰的同时,在信道输出总能量强的目标处实现选择性聚焦。     

基于MSS-PDS和单阵元PTRM信道均衡的深海远程水声通信方案

为了提高水声通信的性能,提出了一种联合运用M元扩频（MSS）通信和Pattern时延差编码（PDS）水声通信体制,并采用单阵元被动式时间反转镜（PTRM）来实现声信道均衡的深海远程水声通信方案。该方案的M元扩频-Pattern时延差编码通信技术既能胜任远程水声通信,又能提高通信速率;单阵元被动式时间反转镜信道均衡技术既可抑制多途扩展产生的码间干扰,又能提高信噪比。根据深海声道特性分析,该方案将通信固定节点置于声道轴以获取会聚增益,提高通信距离。计算机仿真结果证明,所提出的深海远程水声通信方案具有很好的鲁棒性和可行性。     

Deflectometric Measurement of Specular Surfaces

Even small variations of the curvature of a surface may cause malfunction of a part or be perceived as visually unpleasant. However, in the case of specular and painted surfaces, curvature defects can hardly be detected with traditional measurement methods like triangulation or stereoscopy. We describe new strategies to measure the curvature of such surfaces based on deflectometry. This technique mimics the behavior of a human observer by using the object under examination as a mirror. By analyzing its imaging properties, a partial inference on the surface shape is possible. With this approach, much better results can be achieved as compared with triangulation methods. Furthermore, a reliable measurement of defects showing depths of only a few micrometers is enabled.     

Ultrasonic beam focusing through tissue inhomogeneities with a timereversal mirror: application to transskull therapy

Time reversal of ultrasonic fields allows a very efficient approach to focus pulsed ultrasonic waves through lossless inhomogeneous media. Time reversal mirrors (TRMs) are made of large transducer arrays, allowing the incident field to be sampled, time reversed, and reemitted. Time reversal method corrects for phase, amplitude, and even shape aberration and thus, is more efficient than time shift compensation techniques. However, this technique needs the knowledge of the Green's function of a dominant scatterer available in the medium. Aberration correction for ultrasonic hyperthermia could be achieved by combining TRM with an artificial acoustic source or sensor implanted inside the treatment volume. In this paper, time reversal method has been experimentally applied to the focusing through the skull bone. It is shown that the skull induces severe attenuation of ultrasound and reduces the efficiency of the time reversal approach. Then, an amplitude correction method is proposed to focus through an attenuating layer located close to the array. This method consists in inversing the amplitude modulation and then time reversing these signals. Finally, this method is combined with numerical backpropagation to compensate for an attenuating layer located some distance away from the transducer array     

基于纵向超声导波管道非通透缺陷检测研究

管道经过长期服役后,由于磨损、腐蚀和意外损伤等原因,对管道正常运行产生危害。在实际工况中,小的腐蚀缺陷容易发展成腐蚀穿孔,浅层的小裂纹容易发展成穿透型裂纹,因此探究如何有效检测管道小宽度非通透缺陷是很有必要的。利用ANSYS有限元仿真软件模拟纵向模态导波对管道的周向和斜向非通透缺陷的检测,得到了周向非通透缺陷的反射系数曲线,并证实时间反转法可以显著提高导波对非通透斜裂纹的检测能力。还针对空管和充水管非通透斜裂纹的定位问题进行了研究,结果表明利用时间反转法可以得到缺陷较为精确的周向与轴向位置及大致形状。     

Independent Component Analysis–Based Feature Extraction Technique for Defect Classification Applied for Pulsed Eddy Current NDE

With the development of nondestructive detection, the emerging testing techniques provide new challenges to signal analysis and interpretation approach applied to the inspection evaluation. Some researchers have developed the methods that focus on feature analysis of detected signals. This article presents a new feature analysis by the Independent Component Analysis (ICA) approach to evaluate the defects tested by the pulsed eddy current (PEC) technique. ICA is a high-order statistics technique used to separate multi-unknown sources, which has been successfully applied to facial image identification and separation of the components of 1D signal. In this article, the ICA approach is utilized to project the response signals of various defects into the independent components (ICs) feature subspace by signal representation model. Dependent on the selected ICs, each defect is represented by different projected coefficients, which are proposed to discriminate and classify the defects that belong to three categories. The improved ICA model is proposed to improve the classification of two similar categories of single defects: metal loss and subsurface defects. The evaluation using the series of experimental data has validated the classification of single defects and the defects with lift-off effect by our ICA approach. The comparison with Principal component analysis (PCA)–based approach further verified the better performance of the ICA-based model.     

Thermal Reversal of Magnetic Grains Under Time Varying Pulse Field

Media thermal magnetization switching under an arbitrary time varying field, pulse shape, and duration is an important issue in many recording applications. This paper shows a scale dilemma in using the time dependent energy barrier approach to describe long time thermal reversal of magnetic grains under a time varying pulse field. A new model approach based on an optimal reversal path is proposed to characterize the thermal reversal behavior of magnetic grains under an arbitrary time varying pulse field. Using this new model approach, the effects of pulse field duration, width, and amplitude on magnetic grain thermal switching behavior are studied.     

Method to obtain a clear fringe pattern with a zone-plate interferometer

When a zone-plate interferometer is used, a bright spot appears at the center of the image plane. The spot makes it difficult to analyze the interference fringes. A simple technique that is based on the principle of fringe-intensity reversal is proposed to analyze the fringes efficiently. A zone plate with a phase fraction of π/2 or 3π/2 is used in this technique to diminish the bright spot. Unlike the masking technique, no part of the data on the fringes is lost. The fringes can, therefore, be analyzed completely. The technique is described in detail, and the results of an experiment in which the shape error of a concave mirror was measured with the proposed zone plate is presented. The experimental results agree well with the results obtained with the Fizeau interferometer.     

基于时间反转镜的宽带匹配场处理

声场的互易性和线性波动方程的时反不变性能用时间反转来代替频域的相位共轭。因此,时间反转技术可以应用于宽带的匹配场处理中。时间反转匹配场处理时,仍然需要知道海洋环境的先验知识,例如声场的声速分布,海洋边界条件等等。文章对时间反转应用于匹配场处理进行了仿真并分析海上试验数据。仿真和试验数据分析结果表明:该套时间的反转处理方法可以实现匹配场定位,定位精度较高,而且多元阵信号处理的增益也十分明显。     

Optimality properties and performance analysis of co-operative time-reversal communication in wireless sensor networks

The properties and performance of a technique, called time reversal, for co-operative communication on power-constrained wireless sensor networks are studied. A brief discussion of the optimality properties of this approach is presented, and performance is studied experimentally via a simulated indoor environment containing multiple wireless sensors. Using numerical simulation, the behaviour of the peak power received at a target sensor as a function of the number of co-operating transmitting sensors as well as the level of transmitted signal distortion and timing synchronisation errors, is studied. The simulation results demonstrate that, subject to some rather stringent synchronisation requirements, time reversal is an effective generalisation of beamforming that provides an efficient basis for co-operative communication on broadband multipath channels     

小波滤波在Shack-Hartmann传感器上的应用

在使用Shack-Hartmann传感器进行大口径非球面镜面检测中,外部环境的各种振动影响以及气流、温差的干扰都会使检测精度下降。针对这个问题,提出了一种新的时域小波滤波技术。这项技术可以对传感器的信号干扰在时间域上进行不同层次的小波分析,提取干扰信号的先验特征,对测量数据进行有效的滤波,减小波前的扰动起伏,以更准确地探测质心。实验结果表明,采用这种技术后,Shack-Hartmann波前传感器对光学镜面检测的静态测量精度提高了50%以上,离散性减少到原来的20%-30%。     

Contrast agent response to chirp reversal: simulations, optical observations, and acoustical verification

Active response of a microbubble is characterized by its resonance behavior where the microbubble might oscillate after the excitation waveform has been turned off. We investigate in this paper an excitation approach based on this resonance phenomenon using chirps. The technique, called chirp reversal, consists in transmitting a first excitation signal, the up-sweep chirp (UPF) of increasing frequency with time, and a second excitation signal, the down-sweep (DNF) that is a replica of the first signal, but time reversed with a sweep of decreasing frequency with time. Simulations using a modified Rayleigh-Plesset equation were carried out to determine bubble response to chirp reversal. In addition, optical observations and acoustical measurements were carried out to corroborate the theoretical findings. Results of simulations show differences between bubblesiquest oscillations in response to up-sweep and down-sweep chirps mainly for transmitted center frequencies above the bubbles resonance frequency. Bubbles that are at resonance or far away from resonance engender identical responses. From the optical data, the larger bubbles showed different dynamics when up-sweep or down-sweep chirps were transmitted. Smaller bubbles (< 2 mum diameter) appear to be less sensitive to frequency sweep at 1.7 MHz center frequency. However, driven at a higher center frequency, smaller bubbles tend to be more sensitive. These results were confirmed through the acoustical measurements. We concluded that simulations and experimental data show that significant differences might be observed between bubblesiquest responses to UPF and DNF chirps. We demonstrate in this study that, for an optimal use of chirp reversal, the transmit frequency should be higher than the resonance frequency of the contrast microbubbles.     

Demonstration of a linear optical true-time delay device by use of a microelectromechanical mirror array

We present the design and proof-of-concept demonstration of an optical device capable of producing true-time delay(s) (TTD)(s) for phased array antennas. This TTD device uses a free-space approach consisting of a single microelectromechanical systems (MEMS) mirror array in a multiple reflection spherical mirror configuration based on the White cell. Divergence is avoided by periodic refocusing by the mirrors. By using the MEMS mirror to switch between paths of different lengths, time delays are generated. Six different delays in 1-ns increments were demonstrated by using the Texas Instruments Digital Micromirror Device as the switching element. Losses of 1.6 to 5.2 dB per bounce and crosstalk of -27 dB were also measured, both resulting primarily from diffraction from holes in each pixel and the inter-pixel gaps of the MEMS.     

A non-linear damage accumulation fatigue model for predicting strain life at variable amplitude loadings based on constant amplitude fatigue data

A new phenomenological technique for using constant amplitude loading data to predict fatigue life from a variable amplitude strain history is presented. A critical feature of this reversal-by-reversal model is that the damage accumulation is inherently non-linear. The damage for a reversal in the variable amplitude loading history is predicted by approximating that the accumulated damage comes from a constant amplitude loading that has the strain range of the particular variable amplitude reversal. A key feature of this approach is that overloads at the beginning of the strain history have a more substantial impact on the total lifetime than overloads applied toward the end of the cycle life. This technique effectively incorporates the strain history in the damage prediction and has the advantage over other methods in that there are no fitting parameters that require substantial experimental data. The model presented here is validated using experimental variable amplitude fatigue data for three different metals.     

Single Mode Tuning Effects on Lamb Wave Time Reversal with Piezoelectric Wafer Active Sensors for Structural Health Monitoring

Lamb wave time reversal method is a new and tempting baseline-free damage detection technique for structural health monitoring. With this method, certain types of damage can be detected without baseline data. However, the application of this method using piezoelectric wafer active sensors (PWAS) is complicated by the existence of at least two Lamb wave modes at any given frequency, and by the dispersion nature of the Lamb wave modes existing in thin-wall structures. The theory of PWAS-related Lamb wave time reversal has not yet been fully studied. This paper addresses this problem by developing a theoretical model for the analysis of PWAS-related Lamb wave time reversal based on the exact solutions of the Rayleigh-Lamb wave equation. The theoretical model is first used to predict the existence of single-mode Lamb waves. Then the time reversal behavior of single-mode and two-mode Lamb waves is studied numerically. The advantages of single-mode tuning in the application of time reversal damage detection are highlighted. The validity of the proposed theoretical model is verified through experimental studies. In addition, a similarity metric for judging time invariance of Lamb wave time reversal is presented. It is shown that, under certain condition, the use of PWAS-tuned single-mode Lamb waves can greatly improve the effectiveness of the time-reversal damage detection procedure.     

改进的被动时间反转阵处理水下通信

时间反转技术由于不受环境制约,能自适应实现信道多途压缩和对指定点的空间聚焦,降低多用户通信过程中的码间干扰和同道干扰,广泛应用在水声通信领域。利用垂直阵加权处理的方法,在被动时间反转的基础上增加了一抵消项,抑制其中一个用户在接收阵的接收幅度以达到对另一用户指定通信的目的。模拟仿真显示该方法比单独只用被动时间反转处理,误码率能降低一半。海上试验数据处理结果与模拟仿真基本一致。     

Reflective grating interferometer: a folded reversal wave-front interferometer

We present a new, folded reversal interferometer. It is based on the reflective grating interferometer and can be applied for optical isolation and testing of coma aberration. The interferometer has several advantages in respect to other existing optical reversal configurations. A carrier can be easily added for phase retrieval in interferometric fringe patterns for mapping coma aberration. Furthermore, in an asymmetric optical configuration a lateral shear can also be added, transforming it in a reversal shearing interferometer. The principle of operation of the interferometer is described, and the application for measuring the coma aberration of a parabolic mirror used off axis is demonstrated.     

Two-deformable-mirror concept for correcting scintillation effects in laser beam projection through the turbulent atmosphere

A two-deformable-mirror concept for correcting scintillation effects in laser beam projection through the turbulent atmosphere is presented. This system uses a deformable mirror and a Fourier-transforming mirror to adjust the amplitude of the wave front in the telescope pupil, similar to kinoforms used in laser beam shaping. A second deformable mirror is used to correct the phase of the wave front before it leaves the aperture. The phase applied to the deformable mirror used for controlling the beam amplitude is obtained with a technique based on the Fienup phase-retrieval algorithm. Simulations of propagation through a single turbulent layer sufficiently distant from the beacon observation and laser beam transmission aperture to cause scintillation shows that, for an ideal deformable-mirror system, this field-conjugation approach improves the on-axis field amplitude by a factor of approximately 1.4 to 1.5 compared with a conventional phase-only correction system.     

模态声发射在结构材料缺陷定位中的研究

针对在役结构材料中间时存在静态和动态缺陷时难于进行缺陷平面定位的问题，利用声发射技术实时记录声发射波形时域信号的特点，结合薄板中Lamb波的频散特性和Gabor小波时频分析方法，研究并提出了一种新的实用定位方法。该方法通过先后采用三角形和椭圆平面定位方法可分别反演动态缺陷、静态缺陷的位置，并利用低频时频散不明显的对称模态波速和同一时域信号的相对时差来避免波速变化与各传感器间灵敏度差异和门槛值不同对椭圆定位精度的影响。经过实验研究证明，应用这种方法反演得到的缺陷位置和实际缺陷位置一致。