ANALYSIS FOR AIRCRAFT TAXIING AT VARIABLE VELOCITY ON UNEVENNESS RUNWAY BY THE POWER SPECTRAL DENSITY METHOD

In this paper a novel approach for the analysis of non stationary response of aircraft landing gear taxiing over an unevenness runway at variable velocity is explored, which is based on the power spectral density method. A concerned analytical landing gear model for simulating actual aircraft taxiing is formulated. The equivalent linearization results obtained by probabilistic method are inducted to treat landing gear non linear parameters such as shock absorber air spring force, hydraulic damping and Coulomb friction, tire stiffness and damping. The power spectral density for non stationary analysis is obtained via variable substitution and then Fourier transform. A representative response quantity, the overload of the aircraft gravity center, is analyzed. The frequency response function of the gravity overload is derived. The case study demonstrates that under the same reached velocity the root mean square of the gravity acceleration response from constant acceleration taxiing is smaller than that from constant velocity taxiing and the root mean square of the gravity acceleration response from lower acceleration taxiing is greater than that from higher acceleration.

基于PSD特征的FBCCA脑电信号识别方法

当前基于稳态视觉诱发电位(steady-state visual evoked potential,SSVEP)的脑机接口(brain-computer interfaces,BCIs)使用的都是单一识别算法,针对不同时间长度的识别准确率较低。提出了一种基于滤波器组的典型相关分析(filter bank canonical correlation analysis,FBCCA)与功率谱密度(power spectral density,PSD)分析相结合的SSVEP识别算法,可以提高SSVEP识别的普适性与准确率。该方法使用FBCCA寻找高相似度的参考频率信号,再通过多组PSD分析来锁定最终的响应频率,完成频率识别。该方法无需经过训练就能得到较高的识别准确率。实验结果表明:在刺激时长为1 s时,该方法能达到86.61%的准确率,比PSD分析方法提升了5.44%,比典型相关性分析方法(canonical correlation analysis,CCA)提升了10.38%的准确率,比FBCCA提升了8.86%的准确率。

ESTIMATION OF THE NUMBER OF CORRELATED SOURCES WITH COMMON FREQUENCIES BASED ON POWER SPECTRAL DENSITY

Blind source separation and estimation of the number of sources usually demand that the number of sensors should be greater than or equal to that of the sources, which, however, is very difficult to satisfy for the complex systems. A new estimating method based on power spectral density （PSD） is presented. When the relation between the number of sensors and that of sources is unknown, the PSD matrix is first obtained by the ratio of PSD of the observation signals, and then the bound of the number of correlated sources with common frequencies can be estimated by comparing every column vector of PSD matrix. The effectiveness of the proposed method is verified by theoretical analysis and experiments, and the influence of noise on the estimation of number of source is simulated.

Power spectral density analysis for nonlinear systems based on Volterra series

A consequence of nonlinearities is a multi-harmonic response via a monoharmonic excitation.A similar phenomenon also exists in random vibration.The power spectral density(PSD)analysis of random vibration for nonlinear systems is studied in this paper.The analytical formulation of output PSD subject to the zero-mean Gaussian random load is deduced by using the Volterra series expansion and the conception of generalized frequency response function(GFRF).For a class of nonlinear systems,the growing exponential method is used to determine the first 3 rd-order GFRFs.The proposed approach is used to achieve the nonlinear system’s output PSD under a narrow-band stationary random input.The relationship between the peak of PSD and the parameters of the nonlinear system is discussed.By using the proposed method,the nonlinear characteristics of multi-band output via single-band input can be well predicted.The results reveal that changing nonlinear system parameters gives a one-of-a-kind change of the system’s output PSD.This paper provides a method for the research of random vibration prediction and control in real-world nonlinear systems.

Four Years of Daily Photon Emissions That Have Predicted Major Earthquakes: Raw Data, Spectral Power Density Analyses and Implications for the Geosciences

The daily median ground surface flux densities per day from a continuously operating photomultiplier tube unit for four years in Sudbury, Ontario are presented for December 2010 to December 2014. Increases of about 2 to 4 PMT units (1 unit = 5 × 10-11 W·m-2) for median daily measures reliably occurred about two weeks before M ≥ 7.7 earthquakes anywhere on the planet. The PMT units, until June 2014, usually returned to baseline within a few days after the events. There has been a slow positive drift in flux power density since about 2012 and a conspicuous maintained increase after May, 2014. The equivalent energy per day if it were represented isotropically within the volume occupied by the earth is the same order of magnitude as the average daily global total seismic energy release. Spectral power densities revealed enhanced peaks (periodicities) between 100 to 150 days, 60 days, 30 days, and 25 days. Discriminable peaks in power were noted around 18 days, 14 days, and 4 to 6 days. These results suggest that continuous measurement of photonemissions within hyper-dark conditions may reveal geophysical processes that precede larger seismic events and could reflect the movement of the earth-solar system around the galactic center.

Roughness evaluation of three-dimensional asphalt pavement based on two-dimensional power spectral density

To solve the problem of the lack of comprehensive evaluation of three-dimensional(3D)asphalt pavement roughness,a method for evaluating the asphalt pavement roughness is proposed based on two-dimensional(2D)power spectral density(PSD).By calculating the 2D PSD of a 3D asphalt pavement and converting it into the longitudinal average asphalt pavement PSD,the relationship between the evaluation method of the 3D asphalt pavement roughness and the current evaluation standard of roughness is established.Combined with the road-fitting formula used in international standards,the elevation data of the A,B,C,and D grades of the 3D asphalt pavement are simulated by the harmonic superposition method.According to the proposed method,the longitudinal PSD of each level of simulated asphalt pavement is calculated and compared with the standard spectral line of each pavement level.This approach verifies the effectiveness of the proposed method in evaluating the roughness of the 3D asphalt pavement.Compared with the PSD of a certain horizontal profile elevation,it is verified that the fluctuation amplitude of the spectral line calculated by the proposed method is greatly improved.The results show that the proposed method can evaluate the roughness of asphalt pavements more comprehensively and accurately and has strong practicability.

EVALUATION OF POWER SPECTRAL DENSITY OF PASSIVE INTERMODULATION DISTORTION IN HIGH-POWER COMMUNICATION SATELLITE SYSTEMS

In order to analyze the deleterious effects of Passive InterModulation (PIM) on high power communication satellite systems, the basic concept of PIM is introduced, and an equation for the power spectral density of the n-th order PIM distortion insuch systems is derived by applying flat signal-power spectrum assumption and Fourier transform method. It is indicated that PIM level generally decreases with order and the lowest frequency receive channel in the receive band is the channel of most affected by PIM interference.

The Wavefront Power Spectral Density Measurement of Aspheric Lens with Long Focal-Length Using a Computer-Generated Hologram

To ensure the performance of the optical system, the machining accuracy of lens with long focal lengths is required to ensure the image quality. A new method for lens transmission wavefront power spectral density (PSD) in mid-frequency domain measurement using binary phase computer-generated hologram (CGH) is presented. This technique is widely applicable and is particularly useful for measuring large-size lenses with long focal lengths. A comparison experiment of the CGH measurement with results from a Fizeau sphere interferometry method is carried out to verify the accuracy and convenience of the measurement. Furthermore, measurement uncertainty due to CGH fabrication process is analysed. Analysis of the CGH test showed the overall accuracy of less than 1 nm RMS for a sphere lens with over 30 m focal length and Φ410 mm clear aperture. CGH can provide reference spheres with high precision, in the meantime greatly shorten air space, thus reducing the effect of vibration and air turbulence, therefore is of great importance for lens transmission wavefront PSD measurement. The realization of high precision, high efficiency and nondestructive testing of long focal-lens wavefront PSD ensure the ultra-precision and certainty level of machining, hence improving the comprehensive performance of the optical system.

The Changes of Heart Rate Power Spectral Density during Head-up Tilt Test

To study the role of autonomic nervous system in the period of developing syncope induced by head-up tilt test(HUT), we analysed the changes of heart rate power spectral density(HRPSD) in 50 patients with unexplained syncope, including 15 positive patients (Group 1) and 35 negative patients(Group 2), and 15 negative healthy persons(Group 3) in 5 minute periods before and after tilting and 5 minutes before the end of test. HRPSD and their changes in total(T), very low-frequence(VLF), low-frequence(LF), high-frequence(HF) and the ratio of low/high frequence(LF/HF) were similar (P>0.05) 5 minutes before and after tilting among three groups. Five minutes before the end of test, Group 1 had obvious increase of T, VLF, LF and LF/HF while Group 2 and 3 had not such significant changes. There was significant difference(P<0.01) compared Group 1 with Group 2, 3. The results showed that the abnormal regulatory function of autonomic nervous system played an important role in the mechanism of symcope induced by HUT, the positive group had abnormal increase of sympathetic tone and imbalance of sympathetic/parasympathetic neural tone before syncope appeared.

Porosity Evaluation and the Power Spectral Densities Analyses of Carbon-Nickel Composite Films Annealed at Different Temperatures

The densification and the fractal dimensions of carbon-nickel films annealed at different temperatures 300, 500, 800, and 1000℃ with emphasis on porosity evaluation are investigated. For this purpose, the refractive index of films is determined from transmittance spectra. Three different regimes are identified, T 〈 500℃, 500℃ 〈 T 〈 800℃ and T 〉 800℃. The Rutherford baekscattering spectra show that with increasing the annealing temperature, the concentration of nickel atoms into films decreases. It is shown that the effect of annealing temperatures for increasing films densification at T 〈 500℃ and T 〉 800℃ is greater than the effect of nickel concentrations. It is observed that the effect of decreasing nickel atoms into films at 500℃ 〈 T 〈 800℃ strongly causes improving porosity and decreasing densification. The fractal dimensions of carbon-nickel films annealed from 300 to 500℃ are increased, while from 500 to 1000℃ these characteristics are decreased. It can be seen that at 800℃, films have maximum values of porosity and roughness.

Nonlinear distortion analysis for multisine excitation constrained by power spectral density

Nonlinear distortion introduced by an amplifier when subjected to a multisine excitation decreases the measurement accuracy in many metrological and measurement areas. In this paper, we performed qualitative and quantitative analyses of the nonlinear distortion with the multisine excitations constrained by a constant power spectral density. We present the numerical results with respect to different tone spacings,nonlinear orders, harmonic phases and tone distributions. Moreover, three as-pects of contributions are made to further reveal the distortion mechanism. First, we find that the type Ⅱ components for Schr- oeder-phase multisines distribute uniformly but not all in anti-phase to the type I components for the second order nonlinearity. Second,we simulate the variance of the type I and Ⅱ components and their summation to explain the principle of reducing the type Ⅱ distortion by averaging the results obtained using multiple realizations of random-phase multisines. Third, we observe a special distortion distribution mechanism for the Schroeder-phase multisine excitation The results contribute to a better estima-tion and understanding of the nonlinear distortion.

Degradation of effective carrier-power-to-noise densityratio based on code trackingspectral sensitivitycoefficient for GNSS radio frequencycompatibilityin C band

The frequency band between 5 010 MHz and 5 030 MHz allocated as C band has been used as a candidate in the global navigation satellite systems （GNSS） along with more and more naviga- tion services in L band. The potential benefits and technical requirements of C band for satellite navi- gation have been analyzed before. However the degradation of effective carrier-power-to-noise densi- ty ratio（ A （C/No ）eu） based on code tracking spectral sensitivity coefficient（ CT_SSC ） as a compati- bility assessment methodology for potential GNSS radio frequency compatibility in C-Band has not been discussed clearly. So the compatibility of the signals in the C band between BeiDou （BD） B1 C and GPS L1C, L1C/A, Galileo E1Os as the interoperability or classical signals in L band is analyzed. Simulation results reveal the interference degree between BD III B1C and GPS L1C/A, L1C, Galileo E1OS. The results can also reveal that the multiplexed binary offset carrier （MBOC） and binary phase shift keying （BPSK） modulation is not appropriate for C band.

基于CEEMD-PSD算法的变电站控制电缆故障定位方法

频域反射法(frequency domain reflection,FDR)是电力电缆故障定位的有效办法,但尚未应用于控制电缆,且现有FDR法存在频域泄露严重、灵敏度低等问题,导致故障定位误差较大。该文提出了采用补充集合经验模态分解(complementary ensemble empirical mode decomposition,CEEMD)与功率谱密度(power spectral density,PSD)联合算法,实现了变电站控制电缆精准定位。首先通过CEEMD对首端反射系数谱实部进行分解,然后对模态分量进行加努塔尔-布莱克曼混合卷积窗(nutall-blackman window)处理;最后对加窗后的模态分量进行PSD分析,获得故障电缆的定位曲线。该文搭建了控制电缆模型进行仿真研究,并选取总长为100 m,在50 m存在故障的ZR-KVVP2-22 4×4型控缆进行实验验证。仿真及实验结果表明:与传统FDR定位方法相比,该文所提方法可以有效抑制频谱泄露,提升故障识别的灵敏度。

Power Spectral Analysis of Orthogonal Pulse-Based TH-UWB Signals

The paper analyzes power spectral density (PSD) of orthogonal pulse-based signals for time hopping ultra wideband (TH-UWB) systems. Our extensive studies show that the PSD of these signals not only depends on the time dithering code and the modulation schemes, but also on the energy spectral density (ESD) of orthogonal pulses. The different order orthogonal pulses provide different ESD which changes the shape of continuous spectral component with symbols. We show that orthogonal pulse-based signals reduce the dynamic range of amplitude of discrete spectral components. Further, we reduce the dynamic range by adopting longer TH code over orthogonal pulse-based signals. As a result, UWB system performance improves with average transmitted power. The theoretical analysis of PSD of orthogonal pulse-based TH-UWB signal is provided in details and verified through simulation results.

内蒙古地区强震动台站背景噪声与数据质量分析

计算了内蒙古地区193个强震动台站2022年连续观测数据的加速度功率谱密度,通过绘制不同时空、不同加速度计、不同频段的功率谱密度和功率谱概率密度函数图及背景噪声速度均方根值分布图,评估了该区强震动台站的背景噪声变化特征,对出现的功率谱密度异常进行了具体分析,并针对强震动台站数据异常检测及处理提出了相应的建议。研究结果显示:不同加速度计、不同时空强震动台站的背景噪声功率谱密度昼夜差异主要体现在1 Hz以上频段,且不同区域的昼夜差异变化较大;QA-2g型、JS-A2型和TDA-33M型三种加速度计在2 s频点的背景噪声功率谱密度随区域和三分向变化较小;从强震动台站2 s频点背景噪声的监控能得到可靠的观测数据异常,数据异常主要由加速度计零点漂移大、加速度计故障、脉冲干扰、系统参数有误造成,建议优化站点观测环境,进一步规范仪器安装、调试及JOPENS系统的参数配置和审核。

包装件随机振动时域响应和PSD快速获取方法研究

在公路运输过程中,包装件主要遭受的是随机振动。要获得包装件的随机振动响应,目前有两种方法,一是试验采集,二是仿真计算。本文基于Matlab软件的Simulink仿真模块建立路面不平度模型与车辆振动模型,生成车厢底板的振动响应,并将该响应作为包装件振动模型的激励,可以快速获取包装件的时域响应和其功率谱密度。上述方法适用于复杂非线性包装件模型,不仅可以为科研工作者提供参考,同样可以给运输包装课程学习提供帮助。

城市轨道交通轨面短波不平顺谱实测研究

针对城市轨道交通诱发的振动噪声问题,对轨面短波不平顺进行研究.首先,对上海市轨道交通线路轨面短波不平顺实测数据进行零均值化、剔除异常值、消除趋势项等预处理.其次,利用改进的协方差法计算得到轨面短波不平顺功率谱,分析轨道类型、曲线半径及钢轨接头对轨面短波不平顺功率谱分布规律的影响.然后,基于非线性最小二乘法对轨面短波不平顺谱进行拟合.最后,提出我国城市轨道交通轨面短波不平顺统计谱表达式.研究结果表明:直线段接头区轨面短波不平顺功率谱密度曲线在整个分析波长范围内具有较多特征峰值;直线段接头区上、下行短波不平顺功率谱密度值随波长的分布特征和数值相差不大,说明上、下行的轨面不平顺状态基本相同;钢轨的磨耗程度与曲线半径呈负相关,内、外轨面不平顺功率谱密度的差异随曲线半径的增大而减小,曲线半径是影响轨面不平顺状态的主要因素之一;线路类型对轨面短波不平顺功率谱的影响不大,不同曲线半径、焊接接头影响明显,在对轨道进行维护和养护时需关注小曲线半径区段以及焊接接头区段的不平顺状态.

自适应精简经验Ramanujan分解及其在复合故障诊断中的应用

Ramanujan傅里叶模态分解采用低频向高频扫描的方式获取分量信号,易出现过量分解和信息分散的现象,致使分解分量不具有单一完整的状态信息.为了解决上述问题,论文提出了一种自适应精简经验Ramanujan分解(Adaptive Concise Empirical Ramanujan Decomposition,ACERD)方法.在ACERD方法中,采用功率谱密度获取分割频带,旨在进行准确的频带划分.同时,利用Ramanujan傅里叶变换提取每个分割频带所对应的模式分量,提高周期分量的识别能力,并获得具有单一周期特征信息的模式分量.通过复合故障仿真信号和实测信号分析,结果表明:ACERD方法具有优异的频带分割和周期脉冲特征提取能力,适用于复合故障诊断.

随机振动下管径壁厚对航空液压管路的疲劳寿命的影响

根据目前存在由流量设计的不同管径液压管路的疲劳寿命达不到要求的情况,不同管径和壁厚的航空液压导管在随机振动工况下进行疲劳寿命的影响分析。将管路模型简化处理并进行模态分析和随机振动响应分析,最后结合疲劳损伤累积理论和材料的S-N曲线数据得出不同管径壁厚下的疲劳寿命时间。结果表明:当管径和壁厚越大时,对疲劳寿命时间的影响越大。

脑小血管病认知障碍患者脑电功率谱变化与中性粒细胞/淋巴细胞比值相关性及临床价值研究

目的:探索脑小血管病认知障碍(CSVCI)患者脑电功率谱变化与中性粒细胞/淋巴细胞比值(NLR)的相关性,并分析其临床价值。方法:收集脑小血管病(CSVD)患者61例,依据蒙特利尔认知评估量表(MoCA)评分,将其分为伴有认知障碍组(CSVCI组,29例)及无认知功能障碍组(CSVD组,32例),同期招募体检健康者20例为对照组(HC组)。对三组进行脑电图检查,比较全脑定量脑电图功率谱密度(PSD),并分析其与NLR的相关性。结果:CSVCI组年龄高于HC组,受教育年限低于CSVD组和HC组(均P<0.05)。CSVCI组和CSVD组高血压占比高于HC组(均P<0.05)。CSVCI组脑室旁及总脑白质Fazekas评分高于CSVD组和HC组,且CSVD组高于HC组(均P<0.05)。CSVCI组和CSVD组深部脑白质Fazekas评分高于HC组(均P<0.05)。CSVCI组δ、θ频段PSD值高于CSVD组,α频段PSD值低于CSVD组和HC组,CSVD组δ频段PSD值低于HC组(均P<0.05)。多因素Logistic回归表明α频段全脑PSD减低及受教育年限低为CSVCI发生的独立危险因素(均P<0.05)。CSVD患者α频段PSD值与MoCA评分呈正相关,δ频段PSD值与MoCA评分呈负相关,且脑室旁Fazekas评分、深部脑白质Fazekas评分及总脑白质Fazekas评分与MoCA评分呈负相关(均P<0.05)。NLR与θ频段全脑PSD呈正相关(P<0.05)。结论:CSVCI患者脑电快波活动减低而慢波活动增加。α频段全脑PSD值独立于Fazekas评分外影响CSVD患者认知障碍的发生,可能成为评估认知障碍严重程度、预测认知障碍发生的早期客观指标。NLR可能为导致脑电慢化的机制,有可能成为早期预测脑电慢化、认知障碍的客观指标。