• 대한전기학회:학술대회논문집
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• 대한전기학회 2004년도 하계학술대회 논문집 D
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• pp.2149-2151
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• 2004

The time-frequency domain reflectometry(TFDR) is well known to detect and locate a fault in a coaxial cable[3]. Traditional reflectometry methods have been achieved in either the time domain or frequency domain only. However, the time-frequency domain reflectometry utilizes time and frequency information of a reflected signal passed through a cable to detect and locate the fault. The purpose of this paper is to find appropriate time-frequency distribution function suitable for a TFDR system. Choosing the appropriate time-frequency distribution function implies one can detect the fault and estimate the location accurately. We consider and compare adequate time-frequency distribution function on the basis of experimental results.

• Structural Engineering and Mechanics
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• 제49권3호
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• pp.411-425
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• 2014

This paper presents the processing of nonlinear features associated with a damage event by quadratic time-frequency distributions for damage identification in a frame structure. A time-frequency distribution is a function which distributes the total energy of a signal at a particular time and frequency point. As the occurrence of damage often gives rise to non-stationary, nonlinear structural behavior, simultaneous representation of the dynamic response in the time-frequency plane offers valuable insight for damage detection. The applicability of the bilinear time-frequency distributions of the Cohen class is examined for the damage assessment of a frame structure from the simulated acceleration data. It is shown that the changes in instantaneous energy of the dynamic response could be a good damage indicator. Presence and location of damage can be identified using Choi-Williams distribution when damping is ignored. However, in the presence of damping the Page distribution is more effective and offers better readability for structural damage detection.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2003년도 춘계학술대회논문집
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• pp.827-832
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• 2003

There has been a number methods for the presentation of time-frequency analysis of non-stationary signal. In this paper, STFT(short time Fourier transform), wavelet transform, Wigner distribution, and higher order Wigner distribution are discussed in details with simulation signals. They are also applied to the analysis of the wave propagation of a semi finite beam. Wigner distribution and higher order Wigner distribution have good time-frewuency resolutions. Wavelet transform is required for impact analysis but should be applied carefully. STFT suffers from time-frequency resolutions. Each method is has its advantage and disadvantage depending on each application signals.

• 전기학회논문지
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• 제57권2호
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• pp.268-275
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• 2008

In this paper, we introduce a new method for detecting and estimating faults on a power line using the time-frequency domain reflectometry system. The system rests upon time-frequency signal analysis and uses a chirp signal which is multiplied by Gaussian envelope. The chirp signal is used as a reference signal, and we can get the reflected signal from a fault on a wire. To detect and estimate faults, we analyze the reflected signal by Wigner time-frequency distribution function and normalized time-frequency cross correlation function. In this paper we design an optimal reference signal for power line and implement a system for estimating fault distance on a power line with the TFDR implemented by PXI equipments. This approach is verified by some experiments with HIV 2.25mm power lines.

• 한국통신학회논문지
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• 제30권11C호
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• pp.1060-1067
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• 2005

시간 영역과 주파수 영역을 사이의 공간을 뜻하는 부분 푸리에 영역으로 (fractional Fourier domains) 선형 시간-주파수 분포의 옮김 불변 특성을 일반화한다. 다른 선형 사주파수 분포와 달리 짧은 시간 푸리에 변환은(short time Fourier transform: STFT) 부분 푸리에 영역에서 크기 (magnitude-wise) 옮김 불변을 지니는데, 이 짧은 시간 푸리에 변환을 쓰면 분포를 좀더 쉽게 해석할 수 있다. 특히, 부분 푸리에 영역에서 크기 옮김 불변인 선형 분포는 짧은 시간 푸리에 변환뿐이라는 것을 밝힌다.

• Wind and Structures
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• 제20권5호
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• pp.661-682
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• 2015

This study investigates the use of time-frequency coherence analysis for detecting and evaluating coherent "structures" of surface pressures and wind turbulence components, simultaneously on the time-frequency plane. The continuous wavelet transform-based coherence is employed in this time-frequency examination since it enables multi-resolution analysis of non-stationary signals. The wavelet coherence quantity is used to identify highly coherent "events" and the "coherent structure" of both wind turbulence components and surface pressures on rectangular prisms, which are measured experimentally. The study also examines, by proposing a "modified" complex Morlet wavelet function, the influence of the time-frequency resolution and wavelet parameters (i.e., central frequency and bandwidth) on the wavelet coherence of the surface pressures. It is found that the time-frequency resolution may significantly affect the accuracy of the time-frequency coherence; the selection of the central frequency in the modified complex Morlet wavelet is the key parameter for the time-frequency resolution analysis. Furthermore, the concepts of time-averaged wavelet coherence and wavelet coherence ridge are used to better investigate the time-frequency coherence, the coherently dominant events and the time-varying coherence distribution. Experimental data derived from physical measurements of turbulent flow and surface pressures on rectangular prisms with slenderness ratios B/D=1:1 and B/D=5:1, are analyzed.

• 소음진동
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• 제6권3호
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• pp.341-347
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• 1996

진동해석을 이용한 기계계통의 진단에는 시간영역(time domain)에서의 해석 과 FFT(Fast Fourier Transform)를 이용한 주파수영역(frequency domain)에서의 해석 을 생각할 수 있다. 이중 FFT 방법은 고속연산기의 출현과 물리적인 이해의 편이성 등오로 인하여 널리 사용되고 있으나, 시간 함수인 비정상 상태신호(nonstationary signal)의 경우는 주파수영역 해석만으로는 물리적 이해를 구하는데는 한계가 있다. 그래서, 최근 신호처리기법 분야에서는 주파수영역 해석과 시간영역 해석을 보완적 으로 표현할 수 있는 기간-주파수영역 해석기법에 많은 연구가 진행되고 있다. 그중 대표적인 신호처리 기법은 Wigner-Ville Distribution이며, 특히 본 Wigner-Ville Distribution은 많은 물리적 의미를 갖고 있어 주요 연구 대상이며, 많은 응용분야 를 갖고 있다. 그러나, 기계계통중 회전체의 진동신호을 분석하여 고장 진단 및 감시를 용이하기 위해서는 새로운 형태의 시간.주파수영역 해석기법이 필요하다. 본 논문에서는 회전체의 진동신호 분석이 용이하도록 물리적인 의미와 응용상에 중점을 둔 방향서 Wigner-Ville 분포라는 시간-주파수 분석기법을 제안하였고, 회전체를 이용한 실험을 실시하였다. 그 결과 제안된 방향성 Wigner-Ville Distribution은 회전체 진동신호를 시간-주파수 영역에서 잘 표현하고 있으며, 특히 회전체의 수직 및 수평방향 진동신호로 부터 얻어지는 방향성 Wigner-Ville Distribution은 각 주파수 성분의 방향성 정보를 갖고 있어 이를 회전체의 고장 진단 및 감시에 이용 하였다.

• 한국군사과학기술학회지
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• 제8권2호
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• pp.38-48
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• 2005

Instantaneous frequency of doppler signals is used to get the information of the relative velocity and the miss distance between a missile and a corresponding target. In this paper, we have performed time-frequency analysis and instantaneous frequency estimation with Short Time Fourier Transform(STFT), Wigner Ville Distribution(WVD) and Continuous Wavelet Transform(CWT) about the doppler signals generated by moving targets. Performance evaluation was performed using simulated doppler signals generated by a single moving target and two moving targets. From the results of the time-frequency analysis, we found that WVD method was the most efficient instantaneous frequency estimator among the three methods. But in case of two moving targets, WVD method got cross talks and CWT method got oscillation when two doppler frequencies were close to each other.

• 전기학회논문지
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• 제57권4호
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• pp.729-734
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• 2008

Vibroarthrographic(VAG) signals, emitted by human knee joints, are non-stationary and multi-component in nature and time-frequency distributions(TFD) provide powerful means to analyze such signals. The objective of this paper is to classify VAG signals, generated during joint movement, into two groups(normal and patient group) using the characteristic parameters extracted by time-frequency transform, and to evaluate the classification accuracy. Noise within TFD was reduced by singular value decomposition and back-propagation neural network(BPNN) was used for classifying VAG signals. The characteristic parameters consist of the energy parameter, energy spread parameter, frequency parameter, frequency spread parameter by Wigner-Ville distribution and the amplitude of frequency distribution, the mean and the median frequency by fast Fourier transform. Totally 1408 segments(normal 1031, patient 377) were used for training and evaluating BPNN. As a result, the average value of the classification accuracy was 92.3(standard deviation ${\pm}0.9$)%. The proposed method was independent of clinical information, and showed good potential for non-invasive diagnosis and monitoring of joint disorders such as osteoarthritis and chondromalacia patella.

• 비파괴검사학회지
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• 제19권4호
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• pp.270-276
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• 1999

지금까지 퓨리에 변환이 신호처리법으로 가장 널리 사용되고 있다. 그러나, 이 방법은 신호의 주파수성분이 시간에 대하여 어떻게 변화하는지를 표현하지 못한다. 따라서, 최근 이와 같이 비정상신호를 표현하지 못하는 퓨리에 변환의 단점을 보완하여, 신호의 시간과 주파수에 대한 정보를 동시에 표현할 수 있는 시간-주파수 해석법들이 개발되기 시작하였다. 본 연구에서는 인공열화된 SUS 316강의 초음파신호를 시간-주파수 해석법으로 분석하였다. 특히, 단시간 퓨리에 변화법과 위그너 빌 분포법을 이용하여 초음파 신호의 주파수 성분과 특성을 분석하였다.