自来水铸铁管道泄漏声信号频率特征研究

针对基于声发射技术的自来水管网泄漏检测定位方法，研究因泄漏而形成的声信号频率分布及不同泄漏量对频率分布的影响。实际泄漏检测时，通常采用互相关法估计泄漏信号到达不同传感器间的时间延迟实现漏点定位，因此，借助互相关分析法研究了管道的不同口径及泄漏信号传播距离对泄漏信号频率分布的影响。同时，泄漏声信号的传播不可避免要经过管道间的接口，因此分析了两种管道接口对信号频率成分的影响。进而为设计合理的管道泄漏检测过程提供依据，并为泄漏声信号形成及多种因素对泄漏声信号特征产生影响的机制研究奠定基础。     

Resonant characteristics of PZT-4 transducers for photoacoustic signal detection

Rectangular and circular types of PZT-4 transducers have been studied. Their resonant frequencies were determined in different media and related to the photoacoustic signal generation. Recommendations were made for maximum signal detection with these transducers.     

一种Duffing弱信号检测新方法及仿真研究

研究了利用混沌相变进行弱信号检测的理论及仿真试验.对基于Duffing振子初值敏感性检测弱信号的方法分析后指出,过渡过程会影响检测性能,提出一种改进的弱信号检测方法.对仿真输入噪声生成和仿真步长选择进行研究后建立了仿真模型,在典型噪声背景下检测弱正弦信号.实验结果表明：所提出的方法有较好检测性能;混沌临界态的Duffing系统对噪声敏感导致相变方法难以精确确定最小检测幅值.指出了这类方法的局限性. 关键词： 混沌 信号检测 周期信号 白噪声     

火控雷达动态目标探测跟踪建模仿真

火控雷达目标探测跟踪是现代与未来防空作战的首要任务,也是火控系统的关键环节。针对火控雷达连续、自动、快速跟踪目标的特点,首先分析了火控雷达系统闭环连续探测跟踪工作流程,然后利用信号级建模方法,在信号层面上进行了雷达回波信号生成、信号处理与跟踪控制的一体化建模,并给出了各模块模型,最后完成动态火控雷达闭环目标距离、方位角与高低角三维探测跟踪测量过程仿真与分析。仿真结果不仅验证了建模方法与模型逼真有效,也对以后火控雷达跟踪性能评估、数据处理等仿真研究有着一定的参考价值。     

A new strategy for fast radiofrequency CW EPR imaging: direct detection with rapid scan and rotating gradients

Rapid field scan on the order of T/s using high frequency sinusoidal or triangular sweep fields superimposed on the main Zeeman field, was used for direct detection of signals without low-frequency field modulation. Simultaneous application of space-encoding rotating field gradients have been employed to perform fast CW EPR imaging using direct detection that could, in principle, approach the speed of pulsed FT EPR imaging. The method takes advantage of the well-known rapid-scan strategy in CW NMR and EPR that allows arbitrarily fast field sweep and the simultaneous application of spinning gradients that allows fast spatial encoding. This leads to fast functional EPR imaging and, depending on the spin concentration, spectrometer sensitivity and detection band width, can provide improved temporal resolution that is important to interrogate dynamics of spin perfusion, pharmacokinetics, spectral spatial imaging, dynamic oxymetry, etc.     

A miniaturized prototype of resonant banana-shaped photoacoustic cell for gas sensing

A resonant photoacoustic cell intended for laser-spectroscopy gas sensing is represented. This cell is a miniature imitation of a macro-scale banana-shaped cell developed previously. The parameters, which specify the cavity shape, are chosen so as not only to provide optimal cell operation at a selected acoustic resonance but also to reduce substantially the cell sizes. A miniaturized prototype cell (the volume of acoustic cavity of ∼5 mm³) adapted to the narrow diffraction-limited beam of near-infrared laser is produced and examined experimentally. The noise-associated measurement error and laser-initiated signals are studied as functions of modulation frequency. The background signal and the useful response to light absorption by the gas are analyzed in measurements of absorption for ammonia in nitrogen flow with the help of a pigtailed DFB laser diode oscillated near a wavelength of 1.53 μm. The performance of prototype operation at the second longitudinal acoustic resonance (the resonance frequency of ∼32.9 kHz, Q-factor of ∼16.3) is estimated. The noise-limited minimal detectable absorption normalized to laser-beam power and detection bandwidth is ∼8.07 × 10⁻⁸ cm⁻¹ W Hz^−1/2. The amplitude of the background signal is equivalent to an absorption coefficient of ∼2.51 × 10⁻⁵ cm⁻¹. Advantages and drawbacks of the cell prototype are discussed. Despite low absorption-sensing performance, the produced miniaturized cell prototype shows a good capability of gas-leak detection.     

New techniques of local damage detection in machinery based on stochastic modelling using adaptive Schur filter

Vibration signal analysis is one of the most effective techniques of monitoring machinery and detecting local damage in their parts, e.g. bearings and gearboxes. However, such detection is sometimes difficult, especially in heavy industrial machines, because of a small proportion of damage-induced components in relation to the remaining components of registered signals. Therefore, more effective signal processing algorithms are being looked for. Moreover, local damage (cracking, pitting, spalling, breakage, etc.) in bearings and gearboxes generates broad-spectrum impulse signals, while the other type can be effectively modelled as a sum of narrowband signals. In this article, techniques based on Schur adaptive filter are proposed for local damage detection. In such an approach, the analysed signal is modelled by means of autoregressive process and the filter is described by so-called reflection coefficients. Schur algorithm is an effective algorithm with very good numerical properties and it is capable of tracking rapid changes in second order statistics of the analysed signal. Thus, the method is well-suited to analysing non-stationary signals and it is potentially interesting for use in bearing and gearbox monitoring.Reflection coefficients describing the signal model, defined with the use of Schur algorithm, may be applied in a variety of ways, giving a chance of employing different solutions in different conditions. In the first proposed solution, detection is based on the weighted sum of derivatives of reflection coefficients, while in the other one – on new signal obtained as power in frequency bands calculated from a parametric spectrogram, whose starting point are reflection coefficients. All these operations are aimed at enhancing changes that occur in the signal at the moments when damage-induced impulses appear. The article also presents guidelines for methods of determining parameter values in the employed analyses. The proposed solutions have been applied for analysing signals coming from a two-stage gearbox of a large machine driving a mining belt conveyor and the obtained results were analysed. They prove the effectiveness of the proposed techniques. It is worth emphasizing that these techniques can be easily adapted for monitoring machinery in varying operating conditions.     

稀疏驱动自适应线谱增强的水下目标谱熵检测

针对无人平台在水下复杂环境中的线谱弱目标自主检测问题,提出了一种采用稀疏驱动自适应线谱增强(ALE)为前处理的监督学习目标检测方法。该方法在ALE代价函数中引入稀疏性l_p范数,并将稀疏正则化推广到0<p<1;经过稀疏驱动ALE处理使目标声谱的熵特征差异更加明显,利用支持向量机(SVM)的小样本学习能力,对波束声谱的熵特性曲线进行分类,判别目标是否存在。仿真结果表明,输入信噪比为-20 dB情况下,l_1/2稀疏驱动ALE比常规ALE的处理增益高11.5 dB。利用水下无人平台海上拉距试验的数据对算法性能进行验证,在宽带强干扰影响下,该方法可有效检测远距离声源,虚警率为3.5%时,检测率达95.8%,有效提高了对线谱弱目标的检测概率,具有较强的环境适应性。     

A low-cost implementation of EPR detection in a dissolution DNP setup

The implementation of electron paramagnetic resonance (EPR) detection in a low-temperature dissolution dynamic nuclear polarization (DNP) setup is presented. Using a coil oriented parallel to the static magnetic field, the change of the longitudinal magnetization of free radicals is measured upon resonant irradiation of an amplitude or frequency modulated microwave (mw) field. The absorption EPR spectrum is measured if the amplitude of the mw field is modulated, whilst the first derivative of the spectrum is obtained with frequency modulation. Using a burst of pulses, it is also possible to perform pump-probe experiments such as saturation-recovery or electron-electron double resonance experiments. Furthermore, the magnetization could be monitored in a time-resolved manner during amplitude modulation, thus making it possible to record its transient as it is approaching an equilibrium value. Experimental examples are shown with frozen solutions of trityl radical and TEMPO, two commonly used radicals for dissolution DNP experiments.     

A new synthetic detection technique for trackside acoustic identification of railroad roller bearing defects

The detection of railroad roller bearing defects is of great importance for railway traffic. The acoustic defective bearing detector (ADBD) via a trackside acoustic detection system (TADS) is a popular and reliable tool for trackside identification of railroad bearing incipient defects. However, the defective trackside acoustic signal of bearing is confronted with three challenges: demodulation of acoustic signal, removal of Doppler effect and enhancement of defect frequency. This paper proposes a synthetic detection technique to overcome these difficulties successively. In the technique, the signal envelope is first extracted by a new variable-resolution ridge demodulation (VRRD) method. Then the Doppler effect is removed in the envelope via a dynamic signal resampling method. Finally, the rectified defect frequency is enhanced through the means of logarithmic transformation and piecewise linear detrending. The VRRD technique extracts the envelope from the signal time–frequency distribution (TFD) along the Doppler shift curve of resonance frequency traced by an improved ridge extraction algorithm. The resampling method is carried out on the envelope according to the Doppler shift curve of defect frequency obtained by the same ridge extraction algorithm. The sidebands around defect frequency can be suppressed by the logarithmic transformation and the low-frequency non-linear trend can be removed by the detrending method. The signal amplitude is unified at the same time. Two practical bearing signals with inner-race and outer-race defects separately verify the effectiveness of proposed technique.     

浅海小目标前向散射的非互易时反检测方法

将无源非互易时反(NRTR)用于浅海小目标前向散射探测海上实验研究,提出了一种基于频域加权松弛的信道估计算法(WRELAX)和时反方法相结合的小目标探测算法(WRELAX-NRTR)。该算法采用高精度的信道响应估计实现信道的空-时匹配,利用目标引起的时反声场结构的畸变检测前向散射信号对声场的扰动实现目标探测,并通过阈值法实现目标判决。在15~30 kHz的信号频率范围内进行海上实验,数据的处理结果表明,提出的WRELAX-NRTR探测算法与传统的NRTR算法相比,环境背景的扰动幅度从10 dB减小到4 dB,从而降低了虚警率,另外该算法对海洋环境噪声、目标聚焦深度、目标穿越距离和信道动态变化等均具有良好的鲁棒性。     

被动线谱检测的子带分解和分方位区间融合算法

目标辐射噪声中低频线谱丰富,而且谱级比高、强度稳定,相比调制谱检测具有优越性。基于子带分解处理的现有融合方法适用在信噪比相对较高情况下,而当干扰是相干的或强宽带信号时,对线谱目标有效检测仍没很好解决。本文从子带空间谱统计特性出发,利用线谱谱级高出连续谱10-25dB;线谱频带所在方位区间的输出方位波动小,而其他分区间的输出方位波动大特点,提出了一种在信噪比低,多目标并存情况下更有效的弱线谱提取融合方法。理论仿真和海试实验数据处理结果验证该方法具有创新性、较常规方位稳定算法适用范围更广、检测效果更好,适合工程应用。     

A novel method for crack detection in beam-like structures by measurements of natural frequencies

A novel method is proposed for calculating the natural frequencies of a multiple cracked beam and detecting unknown number of multiple cracks from the measured natural frequencies. First, an explicit expression of the natural frequencies through crack parameters is derived as a modification of the Rayleigh quotient for the multiple cracked beams that differ from the earlier ones by including nonlinear terms with respect to crack severity. This expression provides a simple tool for calculating the natural frequencies of the beam with arbitrary number of cracks instead of solving the complicated characteristic equation. The obtained nonlinear expression for natural frequencies in combination with the so-called crack scanning method proposed recently by the authors allowed the development of a novel procedure for consistent identification of unknown amount of cracks in the beam with a limited number of measured natural frequencies. The developed theory has been illustrated and validated by both numerical and experimental results.