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 met...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.展开更多
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 compl...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.展开更多
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 spec...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.展开更多
The drag-free satellites are widely used in the field of fundamental science as they enable the high-precision measurement in pure gravity fields. This paper investigates the estimation of local orbital reference fram...The drag-free satellites are widely used in the field of fundamental science as they enable the high-precision measurement in pure gravity fields. This paper investigates the estimation of local orbital reference frame(LORF) for drag-free satellites. An approach, taking account of the combination of the minimum estimation error and power spectral density(PSD) constraint in frequency domain, is proposed. Firstly, the relationship between eigenvalues of estimator and transfer function is built to analyze the suppression and amplification effect on input signals and obtain the eigenvalue range. Secondly, an optimization model for state estimator design with minimum estimation error in time domain and PSD constraint in frequency domain is established. It is solved by the sequential quadratic programming(SQP) algorithm. Finally, the orbital reference frame estimation of low-earth-orbit satellite is taken as an example, and the estimator of minimum variance with PSD constraint is designed and analyzed using the method proposed in this paper.展开更多
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...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.展开更多
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...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.展开更多
A new high dynamic synchronization algorithm using cyclic spectral density was presented according to the theories of cyclic spectral density and its anti-interface and anti-noise properties.The closed forms of freque...A new high dynamic synchronization algorithm using cyclic spectral density was presented according to the theories of cyclic spectral density and its anti-interface and anti-noise properties.The closed forms of frequency error and phase error were obtained,and their performances were analyzed.The in-phase signal throw costas loop was normalized to obtain a cosine signal.Cyclic spectral density of the cosine signal of was computed to obtain the frequency error and the phase error and then results were put into NCO to synchronize.Finally,the performance of the presented algorithms was compared with the conventional algorithms by virtue of simulations,and the simulation results proved the correctness and the superiority of the new algorithms.展开更多
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 quant...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.展开更多
This paper studies spectral density estimation of a strictly stationary r-vector valued continuous time series including missing observations. The finite Fourier transform is constructed in L-joint segments of observa...This paper studies spectral density estimation of a strictly stationary r-vector valued continuous time series including missing observations. The finite Fourier transform is constructed in L-joint segments of observations. The modified periodogram is defined and smoothed to estimate the spectral density matrix. We explore the properties of the proposed estimator. Asymptotic distribution is discussed.展开更多
Direct current(DC)bus voltage stability is essential for the stable and reliable operation of a DC system.If an oscillation source can be quickly and accurately localized,the oscillation can be adequately eliminated.W...Direct current(DC)bus voltage stability is essential for the stable and reliable operation of a DC system.If an oscillation source can be quickly and accurately localized,the oscillation can be adequately eliminated.We propose a method based on the power spectral density for identifying the voltage oscillation source.Specifically,a DC distribution network model combined with the component connection method is developed,and the network is separated into multiple power modules.Compared with a conventional method,the proposed method does not require determining the model parameters of the entire power grid,which is typically challenging.Furthermore,combined with a novel judgment index,the oscillation source can be identified more intuitively and clearly to enhance the applicability to real power grids.The performance of the proposed method has been evaluated using the MATLAB/Simulink software and PLECS RT Box experimental platform.The simulation and experimental results verify that the proposed method can accurately identify oscillation sources in a DC distribution network.展开更多
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 ...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.展开更多
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...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.展开更多
The dynamics of two non-coupled qubits independently interacting with their reservoirs is solved by the time convolutionless projection operator method. We study two-qubit quantum correlation dynamics for two differen...The dynamics of two non-coupled qubits independently interacting with their reservoirs is solved by the time convolutionless projection operator method. We study two-qubit quantum correlation dynamics for two different types of spectral densities, which are a Lorentzian distribution and an Ohmic spectral density with a Lorentzian–Drude cutoff function. For two qubits initially prepared in the initial Bell state, quantum discord can keep longer time and reach larger values in nonMarkovian reservoirs for the first spectral distribution or by reducing the cutoff frequency for the second case. For the initial Bell-like state, the dynamic behaviors of quantum discord and entanglement are compared. The results show that a long time of quantum correlation can be obtained by adjusting some parameters in experiment and further confirm that the discord can capture quantum correlation in addition to entanglement.展开更多
Objective Repetitive transcranial magnetic stimulation(rTMS)has demonstrated efficacy in enhancing neurocognitive performance in Alzheimer’s disease(AD),but the neurobiological mechanisms linking synaptic pathology,n...Objective Repetitive transcranial magnetic stimulation(rTMS)has demonstrated efficacy in enhancing neurocognitive performance in Alzheimer’s disease(AD),but the neurobiological mechanisms linking synaptic pathology,neural oscillatory dynamics,and brain network reorganization remain unclear.This investigation seeks to systematically evaluate the therapeutic potential of rTMS as a non-invasive neuromodulatory intervention through a multimodal framework integrating clinical assessments,molecular profiling,and neurophysiological monitoring.Methods In this prospective double-blind trial,12 AD patients underwent a 14-day protocol of 20 Hz rTMS,with comprehensive multimodal assessments performed pre-and postintervention.Cognitive functioning was quantified using the mini-mental state examination(MMSE)and Montreal cognitive assessment(MOCA),while daily living capacities and neuropsychiatric profiles were respectively evaluated through the activities of daily living(ADL)scale and combined neuropsychiatric inventory(NPI)-Hamilton depression rating scale(HAMD).Peripheral blood biomarkers,specifically Aβ1-40 and phosphorylated tau(p-tau181),were analyzed to investigate the effects of rTMS on molecular metabolism.Spectral power analysis was employed to investigate rTMS-induced modulations of neural rhythms in AD patients,while brain network analyses incorporating topological properties were conducted to examine stimulus-driven network reorganization.Furthermore,systematic assessment of correlations between cognitive scale scores,blood biomarkers,and network characteristics was performed to elucidate cross-modal therapeutic associations.Results Clinically,MMSE and MOCA scores improved significantly(P<0.05).Biomarker showed that Aβ1-40 level increased(P<0.05),contrasting with p-tau181 reduction.Moreover,the levels of Aβ1-40 were positively correlated with MMSE and MOCA scores.Post-intervention analyses revealed significant modulations in oscillatory power,characterized by pronounced reductions in delta(P<0.05)and theta bands(P<0.05),while concurrent enhancements were observed in alpha,beta,and gamma band activities(all P<0.05).Network analysis revealed frequency-specific reorganization:clustering coefficients were significantly decreased in delta,theta,and alpha bands(P<0.05),while global efficiency improvement was exclusively detected in the delta band(P<0.05).The alpha band demonstrated concurrent increases in average nodal degree(P<0.05)and characteristic path length reduction(P<0.05).Further research findings indicate that the changes in the clinical scale HAMD scores before and after rTMS stimulation are negatively correlated with the changes in the blood biomarkers Aβ1-40 and p-tau181.Additionally,the changes in the clinical scales MMSE and MoCA scores were negatively correlated with the changes in the node degree of the alpha frequency band and negatively correlated with the clustering coefficient of the delta frequency band.However,the changes in MMSE scores are positively correlated with the changes in global efficiency of both the delta and alpha frequency bands.Conclusion 20 Hz rTMS targeting dorsolateral prefrontal cortex(DLPFC)significantly improves cognitive function and enhances the metabolic clearance ofβ-amyloid and tau proteins in AD patients.This neurotherapeutic effect is mechanistically associated with rTMS-mediated frequency-selective neuromodulation,which enhances the connectivity of oscillatory networks through improved neuronal synchronization and optimized topological organization of functional brain networks.These findings not only support the efficacy of rTMS as an adjunctive therapy for AD but also underscore the importance of employing multiple assessment methods—including clinical scales,blood biomarkers,and EEG——in understanding and monitoring the progression of AD.This research provides a significant theoretical foundation and empirical evidence for further exploration of rTMS applications in AD treatment.展开更多
Water hammer diagnostics is an important fracturing diagnosis technique to evaluate fracture locations and other downhole events in fracturing. The evaluation results are obtained by analyzing shut-in water hammer pre...Water hammer diagnostics is an important fracturing diagnosis technique to evaluate fracture locations and other downhole events in fracturing. The evaluation results are obtained by analyzing shut-in water hammer pressure signal. The field-sampled water hammer signal is often disturbed by noise interference. Noise interference exists in various pumping stages during water hammer diagnostics, with significantly different frequency range and energy distribution. Clarifying the differences in frequency range and energy distribution between effective water hammer signals and noise is the basis of setting specific filtering parameters, including filtering frequency range and energy thresholds. Filtering specifically could separate the effective signal and noise, which is the key to ensuring the accuracy of water hammer diagnosis. As an emerging technique, there is a lack of research on the frequency range and energy distribution of effective signals in water hammer diagnostics. In this paper, the frequency range and energy distribution characteristics of field-sampled water hammer signals were clarified quantitatively and qualitatively for the first time by a newly proposed comprehensive water hammer segmentation-energy analysis method. The water hammer signals were preprocessed and divided into three segments, including pre-shut-in, water hammer oscillation, and leak-off segment. Then, the three segments were analyzed by energy analysis and correlation analysis. The results indicated that, one aspect, the frequency range of water hammer oscillation spans from 0 to 0.65 Hz, considered as effective water hammer signal. The pre-shut-in and leak-off segment ranges from 0 to 0.35 Hz and 0-0.2 Hz respectively. Meanwhile, odd harmonics were manifested in water hammer oscillation segment, with the harmonic frequencies ranging approximately from 0.07 to 0.75 Hz. Whereas integer harmonics were observed in pre-shut-in segment, ranging from 6 to 40 Hz. The other aspect, the energy distribution of water hammer signals was analyzed in different frequency ranges. In 0-1 Hz, an exponential decay was observed in all three segments. In 1-100 Hz, a periodical energy distribution was observed in pre-shut-in segment, an exponential decay was observed in water hammer oscillation, and an even energy distribution was observed in leak-off segment. In 100-500 Hz, an even energy distribution was observed in those three segments, yet the highest magnitude was noted in leak-off segment. In this study, the effective frequency range and energy distribution characteristics of the field-sampled water hammer signals in different segments were sufficiently elucidated quantitatively and qualitatively for the first time, laying the groundwork for optimizing the filtering parameters of the field filtering models and advancing the accuracy of identifying downhole event locations.展开更多
In order to describe pavement roughness more intuitively and effectively, a method of pavement roughness simulation, i.e., the stochastic sinusoidal wave, is introduced. The method is based on the primary idea that pa...In order to describe pavement roughness more intuitively and effectively, a method of pavement roughness simulation, i.e., the stochastic sinusoidal wave, is introduced. The method is based on the primary idea that pavement roughness is denoted as the sum of numerous sines or cosines with stochastic phases, and uses the discrete spectrum to approach the target stochastic process. It is a discrete numerical method used to simulate pavement roughness. According to a given pavement power spectral density （PSD） coefficient, under the condition that the character of displacement frequency based on the time domain model is in accordance with the given pavement surface spectrum, the pavement roughness is optimized to stochastic equivalent vibrations by computer simulation, and the curves that describe pavement roughness under each grade are obtained. The results show that the stochastic sinusoidal wave is suitable for simulation of measured pavement surface spectra based on the time domain model. The method of the stochastic sinusoidal wave is important to the research on vehicle ride comfort due to its rigorous mathematical derivation, extensive application range and intuitive simulation curve. Finally, a roughness index defined as the nominal roughness index （NRI） is introduced, and it has correlation with the PSD coefficient.展开更多
A microphone and a seismic sensor always become a basic unit of UGS(unattended ground sensors) system. The mechanism of acoustic and seismic property of target and its propagation are described. The acoustic and seism...A microphone and a seismic sensor always become a basic unit of UGS(unattended ground sensors) system. The mechanism of acoustic and seismic property of target and its propagation are described. The acoustic and seismic signals of targets are analyzed with time frequency distribution according to its non stationary property. Narrow band energy function (NEF) and local power spectral density (LPSD) are proposed to extract features for target identification. Experiment results show that local power spectral density indicates corresponding target clearly.展开更多
The chip-scale integrated spectrometers are opening new avenues for a much wider range of applications than their conventional benchtop counterparts.While spectral reconstruction should be in command of both spectral ...The chip-scale integrated spectrometers are opening new avenues for a much wider range of applications than their conventional benchtop counterparts.While spectral reconstruction should be in command of both spectral resolution and bandwidth,a large number of spectral channels is among the key goals of the spectrometer design.However,the chip footprint eventually limits the spectral channel capacities of well-established spectral-to-spatial mapping structures like dispersive elements,filter arrays,random media,and so on.Here we suggest an alternative scheme by encoding the spectral information using on-chip diffractive metasurfaces.The in-plane metasurface is capable of producing intensity speckles to resolve the spectra.The spectral richness is greatly increased by scaling the architecture via three layers of cascaded metasurfaces.The readout of speckles is realized by two-dimensional imaging of the grating-diffracted pattern,enabling a large matrix for spectrum reconstruction.The spectrometer has a resolution of 70 pm over a bandwidth of 100 nm.Up to 1400 spectral channels were obtained within a compact chip area of only 150μm×950μm.The on-chip diffractive spectrometer has a benchmark channel density of up to 10021 ch/mm^(2),which compares favorably against other state-of-art waveguide structures.展开更多
The operational principle and the lumped parameters model of capacitive micro-accelerometer are introduced. The equivalent stiffness of different directions of the accelerometer is given. From the point of view of ene...The operational principle and the lumped parameters model of capacitive micro-accelerometer are introduced. The equivalent stiffness of different directions of the accelerometer is given. From the point of view of energy and mechanics, expressions of some key parameters, such as the damping, sensitivity, resolution of the accelerometer, are derived. The accelerometer noise behavior of mechanical-thermal noise in the open-loop system, along with the dynamic range of the open-loop system and closed-loop system is analyzed. The result is that the noise of the capacitive micro accelerometer is dominated by the magnitude of mechanical-thermal noise. At the same time, the magnitude of mechanical-thermal noise depends on the temperature and magnitude of mechanical damp. The result of the measurement from the implemented closed-loop microo-accelerometer system shows that the resolution is the level of rag, and the measurement range is from -50g to 50g.展开更多
文摘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.
基金This project is supported by National Natural Science Foundation of China(No.50675076).
文摘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.
基金The National Natural Science Foundation of China(No.51975117)。
文摘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.
基金co-supported by the Open Fund of Joint Key Laboratory of Microsatellite of CAS (No. KFKT15SYS1)the Innovation Foundation of CAS (No. CXJJ-14-Q52)
文摘The drag-free satellites are widely used in the field of fundamental science as they enable the high-precision measurement in pure gravity fields. This paper investigates the estimation of local orbital reference frame(LORF) for drag-free satellites. An approach, taking account of the combination of the minimum estimation error and power spectral density(PSD) constraint in frequency domain, is proposed. Firstly, the relationship between eigenvalues of estimator and transfer function is built to analyze the suppression and amplification effect on input signals and obtain the eigenvalue range. Secondly, an optimization model for state estimator design with minimum estimation error in time domain and PSD constraint in frequency domain is established. It is solved by the sequential quadratic programming(SQP) algorithm. Finally, the orbital reference frame estimation of low-earth-orbit satellite is taken as an example, and the estimator of minimum variance with PSD constraint is designed and analyzed using the method proposed in this paper.
文摘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.
文摘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.
基金Sponsored by the National Basic Research Program (Grant No. 2007CB310601)the High Technology Research and Development Program of China(Grant No. 2007AA12Z338)
文摘A new high dynamic synchronization algorithm using cyclic spectral density was presented according to the theories of cyclic spectral density and its anti-interface and anti-noise properties.The closed forms of frequency error and phase error were obtained,and their performances were analyzed.The in-phase signal throw costas loop was normalized to obtain a cosine signal.Cyclic spectral density of the cosine signal of was computed to obtain the frequency error and the phase error and then results were put into NCO to synchronize.Finally,the performance of the presented algorithms was compared with the conventional algorithms by virtue of simulations,and the simulation results proved the correctness and the superiority of the new algorithms.
基金National Natural Science Foundation of China(No.61372041,No.61001034)
文摘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.
文摘This paper studies spectral density estimation of a strictly stationary r-vector valued continuous time series including missing observations. The finite Fourier transform is constructed in L-joint segments of observations. The modified periodogram is defined and smoothed to estimate the spectral density matrix. We explore the properties of the proposed estimator. Asymptotic distribution is discussed.
基金supported in part by the National Natural Science Foundation of China(No.51807112)。
文摘Direct current(DC)bus voltage stability is essential for the stable and reliable operation of a DC system.If an oscillation source can be quickly and accurately localized,the oscillation can be adequately eliminated.We propose a method based on the power spectral density for identifying the voltage oscillation source.Specifically,a DC distribution network model combined with the component connection method is developed,and the network is separated into multiple power modules.Compared with a conventional method,the proposed method does not require determining the model parameters of the entire power grid,which is typically challenging.Furthermore,combined with a novel judgment index,the oscillation source can be identified more intuitively and clearly to enhance the applicability to real power grids.The performance of the proposed method has been evaluated using the MATLAB/Simulink software and PLECS RT Box experimental platform.The simulation and experimental results verify that the proposed method can accurately identify oscillation sources in a DC distribution network.
文摘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.
基金National Key Basic Research Program of China(973 Program)under Grant No.2013CB036300Ministry of Transport Application Foundation Research Project under Grant No.2013319822070+1 种基金the National Natural Science Foundation of China under Grant Nos.91215302,51222809 and 51178353Program for New Century Excellent Talents in University
文摘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.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11264011 and 11104113)the Natural Science Foundation of Hunan Province, China (Grant Nos. 13JJ6059 and 11JJ6007)the Natural Science Foundation of Education Department of Hunan Province, China (GrantNo. 11C1057)
文摘The dynamics of two non-coupled qubits independently interacting with their reservoirs is solved by the time convolutionless projection operator method. We study two-qubit quantum correlation dynamics for two different types of spectral densities, which are a Lorentzian distribution and an Ohmic spectral density with a Lorentzian–Drude cutoff function. For two qubits initially prepared in the initial Bell state, quantum discord can keep longer time and reach larger values in nonMarkovian reservoirs for the first spectral distribution or by reducing the cutoff frequency for the second case. For the initial Bell-like state, the dynamic behaviors of quantum discord and entanglement are compared. The results show that a long time of quantum correlation can be obtained by adjusting some parameters in experiment and further confirm that the discord can capture quantum correlation in addition to entanglement.
文摘Objective Repetitive transcranial magnetic stimulation(rTMS)has demonstrated efficacy in enhancing neurocognitive performance in Alzheimer’s disease(AD),but the neurobiological mechanisms linking synaptic pathology,neural oscillatory dynamics,and brain network reorganization remain unclear.This investigation seeks to systematically evaluate the therapeutic potential of rTMS as a non-invasive neuromodulatory intervention through a multimodal framework integrating clinical assessments,molecular profiling,and neurophysiological monitoring.Methods In this prospective double-blind trial,12 AD patients underwent a 14-day protocol of 20 Hz rTMS,with comprehensive multimodal assessments performed pre-and postintervention.Cognitive functioning was quantified using the mini-mental state examination(MMSE)and Montreal cognitive assessment(MOCA),while daily living capacities and neuropsychiatric profiles were respectively evaluated through the activities of daily living(ADL)scale and combined neuropsychiatric inventory(NPI)-Hamilton depression rating scale(HAMD).Peripheral blood biomarkers,specifically Aβ1-40 and phosphorylated tau(p-tau181),were analyzed to investigate the effects of rTMS on molecular metabolism.Spectral power analysis was employed to investigate rTMS-induced modulations of neural rhythms in AD patients,while brain network analyses incorporating topological properties were conducted to examine stimulus-driven network reorganization.Furthermore,systematic assessment of correlations between cognitive scale scores,blood biomarkers,and network characteristics was performed to elucidate cross-modal therapeutic associations.Results Clinically,MMSE and MOCA scores improved significantly(P<0.05).Biomarker showed that Aβ1-40 level increased(P<0.05),contrasting with p-tau181 reduction.Moreover,the levels of Aβ1-40 were positively correlated with MMSE and MOCA scores.Post-intervention analyses revealed significant modulations in oscillatory power,characterized by pronounced reductions in delta(P<0.05)and theta bands(P<0.05),while concurrent enhancements were observed in alpha,beta,and gamma band activities(all P<0.05).Network analysis revealed frequency-specific reorganization:clustering coefficients were significantly decreased in delta,theta,and alpha bands(P<0.05),while global efficiency improvement was exclusively detected in the delta band(P<0.05).The alpha band demonstrated concurrent increases in average nodal degree(P<0.05)and characteristic path length reduction(P<0.05).Further research findings indicate that the changes in the clinical scale HAMD scores before and after rTMS stimulation are negatively correlated with the changes in the blood biomarkers Aβ1-40 and p-tau181.Additionally,the changes in the clinical scales MMSE and MoCA scores were negatively correlated with the changes in the node degree of the alpha frequency band and negatively correlated with the clustering coefficient of the delta frequency band.However,the changes in MMSE scores are positively correlated with the changes in global efficiency of both the delta and alpha frequency bands.Conclusion 20 Hz rTMS targeting dorsolateral prefrontal cortex(DLPFC)significantly improves cognitive function and enhances the metabolic clearance ofβ-amyloid and tau proteins in AD patients.This neurotherapeutic effect is mechanistically associated with rTMS-mediated frequency-selective neuromodulation,which enhances the connectivity of oscillatory networks through improved neuronal synchronization and optimized topological organization of functional brain networks.These findings not only support the efficacy of rTMS as an adjunctive therapy for AD but also underscore the importance of employing multiple assessment methods—including clinical scales,blood biomarkers,and EEG——in understanding and monitoring the progression of AD.This research provides a significant theoretical foundation and empirical evidence for further exploration of rTMS applications in AD treatment.
基金support from the National Natural Science Foundation of China(No.52374019).
文摘Water hammer diagnostics is an important fracturing diagnosis technique to evaluate fracture locations and other downhole events in fracturing. The evaluation results are obtained by analyzing shut-in water hammer pressure signal. The field-sampled water hammer signal is often disturbed by noise interference. Noise interference exists in various pumping stages during water hammer diagnostics, with significantly different frequency range and energy distribution. Clarifying the differences in frequency range and energy distribution between effective water hammer signals and noise is the basis of setting specific filtering parameters, including filtering frequency range and energy thresholds. Filtering specifically could separate the effective signal and noise, which is the key to ensuring the accuracy of water hammer diagnosis. As an emerging technique, there is a lack of research on the frequency range and energy distribution of effective signals in water hammer diagnostics. In this paper, the frequency range and energy distribution characteristics of field-sampled water hammer signals were clarified quantitatively and qualitatively for the first time by a newly proposed comprehensive water hammer segmentation-energy analysis method. The water hammer signals were preprocessed and divided into three segments, including pre-shut-in, water hammer oscillation, and leak-off segment. Then, the three segments were analyzed by energy analysis and correlation analysis. The results indicated that, one aspect, the frequency range of water hammer oscillation spans from 0 to 0.65 Hz, considered as effective water hammer signal. The pre-shut-in and leak-off segment ranges from 0 to 0.35 Hz and 0-0.2 Hz respectively. Meanwhile, odd harmonics were manifested in water hammer oscillation segment, with the harmonic frequencies ranging approximately from 0.07 to 0.75 Hz. Whereas integer harmonics were observed in pre-shut-in segment, ranging from 6 to 40 Hz. The other aspect, the energy distribution of water hammer signals was analyzed in different frequency ranges. In 0-1 Hz, an exponential decay was observed in all three segments. In 1-100 Hz, a periodical energy distribution was observed in pre-shut-in segment, an exponential decay was observed in water hammer oscillation, and an even energy distribution was observed in leak-off segment. In 100-500 Hz, an even energy distribution was observed in those three segments, yet the highest magnitude was noted in leak-off segment. In this study, the effective frequency range and energy distribution characteristics of the field-sampled water hammer signals in different segments were sufficiently elucidated quantitatively and qualitatively for the first time, laying the groundwork for optimizing the filtering parameters of the field filtering models and advancing the accuracy of identifying downhole event locations.
文摘In order to describe pavement roughness more intuitively and effectively, a method of pavement roughness simulation, i.e., the stochastic sinusoidal wave, is introduced. The method is based on the primary idea that pavement roughness is denoted as the sum of numerous sines or cosines with stochastic phases, and uses the discrete spectrum to approach the target stochastic process. It is a discrete numerical method used to simulate pavement roughness. According to a given pavement power spectral density （PSD） coefficient, under the condition that the character of displacement frequency based on the time domain model is in accordance with the given pavement surface spectrum, the pavement roughness is optimized to stochastic equivalent vibrations by computer simulation, and the curves that describe pavement roughness under each grade are obtained. The results show that the stochastic sinusoidal wave is suitable for simulation of measured pavement surface spectra based on the time domain model. The method of the stochastic sinusoidal wave is important to the research on vehicle ride comfort due to its rigorous mathematical derivation, extensive application range and intuitive simulation curve. Finally, a roughness index defined as the nominal roughness index （NRI） is introduced, and it has correlation with the PSD coefficient.
文摘A microphone and a seismic sensor always become a basic unit of UGS(unattended ground sensors) system. The mechanism of acoustic and seismic property of target and its propagation are described. The acoustic and seismic signals of targets are analyzed with time frequency distribution according to its non stationary property. Narrow band energy function (NEF) and local power spectral density (LPSD) are proposed to extract features for target identification. Experiment results show that local power spectral density indicates corresponding target clearly.
基金funded by the National Natural Science Foundation of China(U21A20454)the Science,Technology,and Innovation Commission of Shenzhen Municipality(RCYX20210609103707009,JCYJ20220818102406013)the Natural Science Foundation of Guangdong Province for Distinguished Young Scholars(2022B1515020057).
文摘The chip-scale integrated spectrometers are opening new avenues for a much wider range of applications than their conventional benchtop counterparts.While spectral reconstruction should be in command of both spectral resolution and bandwidth,a large number of spectral channels is among the key goals of the spectrometer design.However,the chip footprint eventually limits the spectral channel capacities of well-established spectral-to-spatial mapping structures like dispersive elements,filter arrays,random media,and so on.Here we suggest an alternative scheme by encoding the spectral information using on-chip diffractive metasurfaces.The in-plane metasurface is capable of producing intensity speckles to resolve the spectra.The spectral richness is greatly increased by scaling the architecture via three layers of cascaded metasurfaces.The readout of speckles is realized by two-dimensional imaging of the grating-diffracted pattern,enabling a large matrix for spectrum reconstruction.The spectrometer has a resolution of 70 pm over a bandwidth of 100 nm.Up to 1400 spectral channels were obtained within a compact chip area of only 150μm×950μm.The on-chip diffractive spectrometer has a benchmark channel density of up to 10021 ch/mm^(2),which compares favorably against other state-of-art waveguide structures.
文摘The operational principle and the lumped parameters model of capacitive micro-accelerometer are introduced. The equivalent stiffness of different directions of the accelerometer is given. From the point of view of energy and mechanics, expressions of some key parameters, such as the damping, sensitivity, resolution of the accelerometer, are derived. The accelerometer noise behavior of mechanical-thermal noise in the open-loop system, along with the dynamic range of the open-loop system and closed-loop system is analyzed. The result is that the noise of the capacitive micro accelerometer is dominated by the magnitude of mechanical-thermal noise. At the same time, the magnitude of mechanical-thermal noise depends on the temperature and magnitude of mechanical damp. The result of the measurement from the implemented closed-loop microo-accelerometer system shows that the resolution is the level of rag, and the measurement range is from -50g to 50g.
