• Structural Engineering and Mechanics
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• 제23권1호
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• pp.75-95
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• 2006

Two-step identification approaches for effective bridge health monitoring are proposed to alleviate the issues associated with many unknown parameters faced in real structures and to improve the accuracy in the estimate results. It is suitable for on-line monitoring scheme, since the damage assessment is not always needed to be carried out whereas the alarming for damages is to be continuously monitored. In the first step for screening potentially damaged members, a damage indicator method based on modal strain energy, probabilistic neural networks and the conventional neural networks using grouping technique are utilized and then the conventional neural networks technique is utilized for damage assessment on the screened members in the second step. The effectiveness of the proposed methods is investigated through a field test on the northern-most span of the old Hannam Grand Bridge over the Han River in Seoul, Korea.

• 천문학회지
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• 제45권6호
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• pp.147-156
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• 2012

We present a test of the emission statistics of active galactic nuclei (AGN), probing the connection between the red-noise temporal power spectra and multi-modal flux distributions known from observations. We simulate AGN lightcurves under the assumption of uniform stochastic emission processes for different power-law indices of their respective power spectra. For sufficiently shallow slopes (power-law indices (${\beta}{\leq}1$), the flux distributions (histograms) of the resulting lightcurves are approximately Gaussian. For indices corresponding to steeper slopes (${\beta}{\geq}1$), the flux distributions become multi-modal. This finding disagrees systematically with results of recent mm/radio observations. Accordingly, we conclude that the emission from AGN does not necessarily originate from uniform stochastic processes even if their power spectra suggest otherwise. Possible mechanisms include transitions between different activity states and/or the presence of multiple, spatially disconnected, emission regions.

• 한국반도체및디스플레이장비학회:학술대회논문집
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• 한국반도체및디스플레이장비학회 2004년도 춘계학술대회 발표 논문집
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• pp.163-168
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• 2004

Linear motor can directly apply to the system needed linear mot ions without rotary mot ions. To control a high-speed and high-resolution, the development of the linear motors is recently required in the high-integrated and speed process industry. This paper presents vibration analyses as well as measurement standards of the newly developed linear motors through analyzing the vibration characteristics and thermal behaviors of the advanced products. Vibration experiments are conducted for identifying the hysteresis and vibration level during operation. They are also included in the modal test to analyze the vibration. Analytic data using Finite Element Method (FEM) are compared with the results of the modal. Loss of temperature generated the linear motor leads to a serious deformation within its parts. The thermal behaviors are very important factor in linear motor. The FEM and experiments make it possible to understand these characteristics.

• 한국산학기술학회논문지
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• 제7권2호
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• pp.107-112
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• 2006

본 연구에서는 설계단계에서 소형 직류 모터의 동특성을 파악할 수 있는 유한요소 해석모델을 개발하였다. 유한요소해석 결과의 검증을 위하여 충격실험을 실시하였으며 모터의 고유진동수와 모드형상을 추출하여 비교하였다. 개발된 유한요소 해석모델을 이용하여 모터의 형상변화가 동특성에 미치는 영향을 분석할 수 있었으며 이는 향후 고품위 저진동 직류모터를 개발하는 데 효과적으로 이용될 수 있다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2004년도 추계학술대회논문집
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• pp.637-640
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• 2004

Structural damping enhancement of composite flexures and aeroelastic stability of a hingeless rotor system are investigated. Constrained layer damping (CLD) treatments are applied in order to increase structural damping of flexures. Material damping property of viscoelastic layer is modelled as complex modulus. Modal analysis of composite flexures with attached viscoelastic layers and constraining layers are performed using MSC/NASTRAN and the effects of CLD treatments are verified with the modal test results. The composite flexures with CLD are applied to a 4-bladed, 2-meter diameter, Froude-scaled, soft-in-plane hingeless rotor system. The aeroelastic stability is tested at hovering condition and the effects of CLD are investigated. It is shown that the CLD treatment effectively enhance the aeroelastic stability at hover.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 추계학술대회논문집
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• pp.88-93
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• 2000

In this paper, described is a research on the modal analysis of small dc motor by finite element method for the vibration reduction. An impact test is performed to obtain the natural frequencies and modal shapes of dc motor, which valuate the usefulness of the finite element analysis. From the study, we show that this finite element model can be applicable for designing a new motor with improvement in vibration characteristics. As an example, a new motor shape is suggested and its vibration characteristics is discussed in comparison with those of original shape.

• Structural Engineering and Mechanics
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• 제70권6호
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• pp.649-659
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• 2019

Vibration-based structural damage detection through optimization algorithms and minimization of objective function has recently become an interesting research topic. Application of various objective functions as well as optimization algorithms may affect damage diagnosis quality. This paper proposes a new damage identification method using Moth-Flame Optimization (MFO). MFO is a nature-inspired algorithm based on moth's ability to navigate in dark. Objective function consists of a term with modal assurance criterion flexibility and natural frequency. To show the performance of the said method, two numerical examples including truss and shear frame have been studied. Furthermore, Los Alamos National Laboratory test structure was used for validation purposes. Finite element model for both experimental and numerical examples was created by MATLAB software to extract modal properties of the structure. Mode shapes and natural frequencies were contaminated with noise in above mentioned numerical examples. In the meantime, one of the classical optimization algorithms called particle swarm optimization was compared with MFO. In short, results obtained from numerical and experimental examples showed that the presented method is efficient in damage identification.

• 한국소음진동공학회논문집
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• 제13권5호
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• pp.327-333
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• 2003

Several tests are performed to evaluate road booming noise. Baseline test delivers the information of road noise characteristics. Coupling effect between structure and acoustics is obtained from the mode shapes and the natural frequencies by the modal test. Equivalent stiffness at joint areas between chassis and car-body system can be determined by the input point inertance test. Noise sensitivity of body mounting point of a chassis part can be obtained from the noise transfer function test with input point inertance test. Operational deflection shape makes us analyze the actual vibration modes of the chassis system under actual leading and find noise sources very easily. Finally, the transfer path analysis is used to Identify noise Paths through the chassis system. The objectives and the procedures of the tests are described in this Paper Also, the guideline for efficient road noise evaluation test can be found.

• 소음진동
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• 제8권1호
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• pp.57-74
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• 1998

구조체의 연결부위에서 미지의 결합강성으로 인하여, 실험과 유한 요소법으로 구한 모달 매개변수들은 종종 일치하지 않는다. 본 논문은 실험으로 추출된 동특성 데이타에 근거하여, 해석적인 방법을 통해 새로운 모델링 방법을 제시하고 있다. 대표적인 연결방법으로 리벳을 이용한 Lap이음보에서 동강성을 측정하기 위한 비선형 진동실험이 제안 되었으며, 이를 Lap 이음판에 적용하기 위하여 동강성에 해당하는 설계변수로서 빔요소를 도입하였다. 이 유한요소 모델링은 범용 패키지인 PATRAN과 ABAQUS를 사용하였으며, 빔요소의 직경을 실험치의 고유진동수와 일치하도록 조절함으로써 직경의 최적치 경향을 관찰하였다. 제시된 모델링 기법과 기존의 모델링 기법으로 얻은 결과치들을 실험치와 비교하였다.

• 한국지진공학회논문집
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• 제12권4호
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• pp.19-33
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• 2008

구조물에 고유한 진동수,모드형태 및 감쇠비 등과 같은 동적성능치를 추출하기 위하여 25층 및 42층 건물에 대하여 자연진동에 의한 동적계측실험을 수행하였다. 고려된 건물은 주요 횡하중 지지기구로서 코아벽체 혹은 전단벽체가 추가된 철근콘크리트건물이며, 입면 혹은 평면상으로 골조가 혼합된 구조형식을 나타낸다. 특히, 25층 건물은 측면에 위치한 코아벽체 이외에 상부로부터 내려오는 전단벽 구조가 4층 바닥이하에서 골조형식으로 전환되는 복잡한 구조이다. 이와 같은 이유 및 건물 주방향의 유사한 강성배치로 매우 근접하고 혼합된 모드형태가 예상되어 시스템판별 시 어려움이 예상된다. 현재까지 개발된 다양한 시스템판별법을 대상건물의 자연진동 실측기록에 적용하여 모달계수를 유도하였으며, 그 결과를 비교 분석하였다. 3개의 주파수영역 및 4개의 시간영역에 근거한 응답의존 시스템판별법이 고려되었다. 서로 다른 시스템판별법에 의하여 추출된 고유진동수 및 감쇠비는 대체로 상당한 일치를 보였으나, 모드형태는 사용된 방법에 따라 정도가 다르게 불일치를 나타냈다. 실험으로부터 추출한 성능치와 초기 유한요소해석 값을 비교해 본 결과 대상건물 모두 적어도 저차 3개의 고유진동수에서 2배 정도의 차이를 나타냈다. 실험과 해석결과의 일치를 위하여 몇몇 수동모델향상이 시도되었으며, 허용할 만한 결과를 획득하였다. 사용된 시스템판별법에 대하여 각자의 장, 단점에 대하여 기술하였으며, 본 연구와 같은 실제 대형구조물에 대하여 자동모델향상기법을 적용할 시 예상되는 문제점에 대하여 토의하였다.