• 대한전자공학회논문지SD
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• 제39권7호
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• pp.37-45
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• 2002

본 논문에서는 3차원 FDTD를 이용하여 진행파형 흡수 CPW(Coplanar waveguide) 광 변조기의 마이크로파 특성을 분석하여 최적화 설계하였다. 진행파형 구조에 있어서 마이크로파의 특성은 진성 영역의 두께와 폭에 영향을 받게 되고 신호전극과 접지전극의 위치와 신호전극의 폭에도 영향을 받게 된다. 진성 영역이 InAsP/InGaP (1.3Q) 의 양자우물로 구성되었을 때, 도파관의 폭이 $2{\mu}m$, 진성영역의 두께가 $0.9{\mu}m$ 신호전극과 접지전극 사이의 간격이 $3{\mu}m$일 때 마이크로파 손실을 최소화 하며 광파와의 속도정합을 이루었으며 이때의 임피던스는 약 50${\Omega}$으로 광 변조기의 최적화를 이룰 수 있었다. FDTD를 이용하여 다양한 구조의 변화가 마이크로파에 미치는 영향을 분석 하였으며 이를 이용한 보다 정확한 소자 설계가 가능함을 보였다.

• Smart Structures and Systems
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• 제13권4호
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• pp.587-614
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• 2014

In recent years the interest in online monitoring of lightweight structures with ultrasonic guided waves is steadily growing. Especially the aircraft industry is a driving force in the development of structural health monitoring (SHM) systems. In order to optimally design SHM systems powerful and efficient numerical simulation tools to predict the behaviour of ultrasonic elastic waves in thin-walled structures are required. It has been shown that in real industrial applications, such as airplane wings or fuselages, conventional linear and quadratic pure displacement finite elements commonly used to model ultrasonic elastic waves quickly reach their limits. The required mesh density, to obtain good quality solutions, results in enormous computational costs when solving the wave propagation problem in the time domain. To resolve this problem different possibilities are available. Analytical methods and higher order finite element method approaches (HO-FEM), like p-FEM, spectral elements, spectral analysis and isogeometric analysis, are among them. Although analytical approaches offer fast and accurate results, they are limited to rather simple geometries. On the other hand, the application of higher order finite element schemes is a computationally demanding task. The drawbacks of both methods can be circumvented if regions of complex geometry are modelled using a HO-FEM approach while the response of the remaining structure is computed utilizing an analytical approach. The objective of the paper is to present an efficient method to couple different HO-FEM schemes with an analytical description of an undisturbed region. Using this hybrid formulation the numerical effort can be drastically reduced. The functionality of the proposed scheme is demonstrated by studying the propagation of ultrasonic guided waves in plates, excited by a piezoelectric patch actuator. The actuator is modelled utilizing higher order coupled field finite elements, whereas the homogenous, isotropic plate is described analytically. The results of this "semi-analytical" approach highlight the opportunities to reduce the numerical effort if closed-form solutions are partially available.

• 한국멀티미디어학회논문지
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• 제21권4호
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• pp.450-463
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• 2018

Sound based machine fault diagnosis is the process consisting of detecting automatically the damages that affect the machines by analyzing the sounds they produce during their operating time. The collected sounds being inevitably corrupted by random disturbance, the most important part of the diagnosis consists of discovering the hidden elements inside the data that can reveal the faulty patterns. This paper presents a novel feature extraction methodology that combines various digital signal processing and pattern recognition methods for the analysis of the sounds produced by the drills. Using the Fourier analysis, the magnitude spectrum of the sounds are extracted, converted into two-dimensional vectors and uniformly normalized in such a way that they can be represented as 8-bit grayscale images. Histogram equalization is then performed over the obtained images in order to adjust their very poor contrast. The obtained contrast enhanced images will be used as the features of our diagnosis system. Finally, principal component analysis is performed over the image features for reducing their dimensions and a nonlinear classifier is adopted to produce the final response. Unlike the conventional features, the results demonstrate that the proposed feature extraction method manages to capture the hidden health patterns of the sound.

• 드라이브 ㆍ 컨트롤
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• 제15권1호
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• pp.28-37
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• 2018

When the wind speed rises above the rated wind speed, the produced power of the wind turbines exceeds the rated power. Even more, the excessive power results in the undesirable mechanical load and fatigue. A solution to this problem is pitch control of the wind turbines. This paper presents a systematic design method of a collective pitch controller for the wind turbines using a discrete fuzzy Proportional-Integral (PI) controller. Unlike conventional PI controllers, the fuzzy PI controller has variable gains according to its input variables. Generally, tuning the parameters of fuzzy PI controller is complex due to the presence of too many parameters strongly coupled. In this paper, a systematic method for the fuzzy PI controller is presented. First, we show the fact that the fuzzy PI controller is a superset of the PI controller in the discrete-time domain and the initial parameters of the fuzzy PI controller is selected by using this relationship. Second, for simplicity of the design, we use only four rules to construct nonlinear fuzzy control surface. The tuning parameters of the proposed fuzzy PI controller are also obtained by the aforementioned relationship between the PI controller and the fuzzy PI controller. As a result, unlike the PI controller, the proposed fuzzy PI controller has variable gains which allow the pitch control system to operate in broader operating regions. The effectiveness of the proposed controller is verified with computer simulations using FAST, a NREL's primary computer-aided engineering tool for horizontal axis wind turbines.

• International Journal of Fluid Machinery and Systems
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• 제2권4호
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• pp.392-399
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• 2009

The paper concerns the description of the step by step development process of the new fixed blade runner called "Mixer" suitable for the uprating of the Francis turbines units installed at the older low head hydropower plants. In the paper the details of hydraulic and mechanical design are presented. Since the rotational speed of the new runner is significantly higher then the rotational speed of the original Francis one, the direct coupling of the turbine to the generator can be applied. The maximum efficiency at prescribed operational point was reached by the geometry optimization of two most important components. In the first step the optimization of the draft tube geometry was carried out. The condition for the draft tube geometry optimization was to design the new geometry of the draft tube within the original bad draft tube shape without any extensive civil works. The runner blade geometry optimization was carried out on the runner coupled with the draft tube domain. The blade geometry of the runner was optimized using automatic direct search optimization procedure. The method used for the objective function minimum search is a kind of the Nelder-Mead simplex method. The objective function concerns efficiency, required net head and cavitation features. After successful hydraulic design the modal and stress analysis was carried out on the prototype scale runner. The static pressure distribution from flow simulation was used as a load condition. The modal analysis in air and in water was carried out and the results were compared. The final runner was manufactured in model scale and it is going to be tested in hydraulic laboratory. Since the turbine with the fixed blade runner does not allow double regulation like in case of full Kaplan turbine, it can be profitably used mainly at power plants with smaller changes of operational conditions or in case with more units installed. The advantages are simple manufacturing, installation and therefore lower expenses and short delivery time for turbine uprating.

• Journal of Biosystems Engineering
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• 제36권1호
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• pp.23-32
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• 2011

In this work, the finite element method was used to investigate the shifts of resonance frequencies and quality factor of whispering-gallery-mode (WGM) for an opto-fluidic ring resonator (OFRR) biosensor. To describe the near-field radiation transfer, the time-domain Maxwell's equations were employed and solved by using the in-plane TE wave application mode of the COMSOL Multiphysics with RF module. The OFRR biosensor model under current study includes a glass capillary with a diameter of 100 mm and wall thickness of 3.0 mm. The resonance energy spectrum curves in the wavelength range from 1545 nm to 1560 nm were examined under different biosensing conditions. We mainly studied the sensitivity of resonance shifts affected by changes in the effective thickness of the sensor resonator ring with a 3.0 mm thick wall, as well as changes in the refractive index (RI) of the medium inside ring resonators with both 2.5 mm and 3.0 mm thick walls. In the bulk RI detection, a sensitivity of 23.1 nm/refractive index units (RIU) is achieved for a 2.5 mm thick ring. In small molecule detection, a sensitivity of 26.4 pm/nm is achieved with a maximum Q-factor of $6.3{\times}10^3$. These results compare favorably with those obtained by other researchers.

• 경영정보학연구
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• 제13권1호
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• pp.61-74
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• 2011

소프트웨어 산업은 아직도 뿌리깊은 문제에 시달리고 있다. 어떻게 보면, 제조업과 같은 보다 성숙한 산업의 모범사례로부터 뭔가를 배워야 할 지 모른다. 제조업에서의 '린' 원칙을 소프트웨어 개발에 적용하는 것도 한 방안일 수 있다. 소프트웨어공학 문헌에서 이런 '린' 원칙은 시도해 볼만 한 것으로 언급되었다. 그러나 이들 원칙을 실제 어떻게 적용할 수 있을 가에 대한 구체적 방안을 제시한 연구는 별로 없었다 본 연구는 '린' 원칙을 소프트웨어 개발에 적용하는 방안을 제시하고자 한다. 이 방안의 핵심은 낭비제거라는 린의 관점을 구체화하기 위하여 프로젝트에서의 리드 타임과 결함을 누적 흐름도(Cumulative Flow Diagram)을 통해 관리하는 방법이다. 또한 이 방안을 실제 프로젝트 사례에 적용함으로써 타당성을 검증하고 적용 방법에 대한 이해를 돕고자 하였다.

• 한국산업정보학회논문지
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• 제13권5호
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• pp.55-65
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• 2008

기업의 전략적 의사결정자는 불확실성과 복잡성에 직면해왔다. 그리고 그들은 이러한 환경 하에서 의사결정을 내려야 함에도 불구하고 그들의 의사결정에 필요한 충분한 시간, 인력, 예산, 그리고 지식이 충분히 주어지지 않는다. 그래서 그들은 그 분야에 대한 암묵적 지식을 지닌 전문가들의 지원을 받는 수 밖에 없다. 그러나 의사결정자들이 어떤 문제에 직면할 때 마다 의사결정에 필요한 새로운 지식을 창조하고 변형하고 결합하고 응용하는 다른 절차나 방법을 발견하는 것은 매우 어렵다. 본 논문은 노나카에 의해 제안된 지식변환과정을 이용함으로써 전략적 의사결정을 지원할 새로운 방법을 제안한다. 본 연구는 컨설팅 산업분야에서 전략적 의사결정의 응용사례를 예시한다. 이 논문은 제안된 방법에 기초한 의사결정지원기법으로서 인지지도를 사용한다.

• 대한기계학회논문집A
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• 제41권6호
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• pp.483-489
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• 2017

본 연구에서는 초음파 기법을 이용하여 메가헤르츠 대역에서 작동하는 마이크로 캔틸레버의 공진 주파수를 측정하였다. 고출력 초음파 펄스 발생기와 탐촉자를 이용하여 실리콘 재질의 마이크로 캔틸레버를 가진하였으며, 532 nm 연속파장(continuous wave) 레이저를 광원으로 하는 마이켈슨 간섭계를 사용하여 자유진동하는 마이크로 캔틸레버의 시간영역 파형을 획득하였다. 고속 푸리에 변환(fast Fourier transform)을 통한 주파수 응답 특성으로부터 마이크로 캔틸레버의 고유진동수를 평가할 수 있었으며, 유한요소법을 이용한 수치해석을 통하여 그 타당성을 검증하였다. 본 연구는 기존 원자현미경 기반의 측정기술과 대비하여 민감도를 향상시킬 수 있는 특성평가 기법을 제시한다.

• 한국항공우주학회지
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• 제33권2호
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• pp.54-59
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• 2005

구동기의 백래쉬와 동강성을 고려한 미사일 조종날개의 비선형 공탄성 해석이 수행되었다. 아음속 비정상 공기력 계산을 위해 DHM을 사용하였고 최소상태접근법을 사용하여 근사하였다. 비선형 플러터 해석을 위해 백래쉬는 유격으로 모델하고 기술 함수법을 사용하여 선형화하였다. 또한, 동강성은 주파수의 함수로 모터의 운동방정식으로부터 계산하였다. 선형 및 비선형 플러터 해석 결과들은 공력탄성학적 특성들이 백래쉬와 동강성에 중요한 영향을 받는다는 것을 보여준다. 비선형 플러터 해석에서 다양한 제한 주기 운동이 선형플러터 속도 이하에서 관측되었다. 또한 플러터 특성과 응답을 시간영역에서도 조사하였다.