• Tribology and Lubricants
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• 제8권2호
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• pp.14-19
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• 1992

rate of growth of "protective" films of oxides and compounds derived from the lubricant. The protection afforded by the films is strongly dependent on lubricant and steel composition, initial composite surface roughness and the load/speed sequence or history in the early stages of sliding. The protective films, formed by chemical and mechanical action between lubricant and mating surfaces, delayed the surface roughening and offered higher load capacity. Given the great number of variables involved it is not possible to follow more than a few of the chemical changes on surfaces at the end of experiments. Ellipsometry was therefore used to monitor the formation and loss of dual protective films in real time, and also to find the effect of protective films on scuffing and break-in depending on lubricant conditions.

• 한국정보통신학회논문지
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• 제9권3호
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• pp.547-554
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• 2005

최근 대부분의 전송기술이 이더넷으로 통일되는 경향에 힘입어, 예전에 비하여 이더넷은 대단한 주목을 받고 있다. 본 연구에서는 최적의 MAC 코어 설계에 대해 연구하였는데, 이는 상위 계층 인터페이스, 전송엔진, 플로우 컨트롤 블록, 수신엔진, 정합 부계층, 초기설정 블록, 상태전송 블록, XCMII 인터페이스 블록으로 구성된다. 하드웨어 설계를 위해 10Gbps 이더넷 Data Link 계층의 MAC 코어를 C언어로 성능평가를 실시하여 내부 GIFO와 파라미터 초기값을 도출하였다 내부 FIFO는 $95\%$의 트래픽이 발생시 512 크기로 사용 가능하고, $97\%$의 트래픽이 발생시에는 1024 크기가 적합하였다. 성능결과를 토대로, VHDL 언어로 설계하여 검증하였다. 설계된 MAC 코어는 64비트의 데이터를 처리하고 168.549MHz를 지원하여 전송효율이 최대 10.78Gbps까지 지원하므로, 10Gbps 이더넷의 스위칭 장비의 인터페이스 모듈로 응용이 가능하다.

• 한국통신학회논문지
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• 제37권5B호
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• pp.338-347
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• 2012

본 논문에서는 OFDMA 기반의 기업형 펨토셀 네트워크 환경에서 펨토셀의 성능 향상을 위한 협력 통신 기법(cooperative transmission scheme; CTS)을 제안한다. 제안된 알고리즘에서 UE(user equipment)는 현재 접속 중인 fBS(serving femtocell base station)에서 뿐만 아니라 주변 fBS에서 원하는 신호(desired signal)를 받게 된다. 따라서 UE는 동기화된 두 개의 신호에 의해 향상된 신호 대 간섭 잡음 비(SINR)를 얻게 된다. 알고리즘의 성능 평가를 위해 콜 레벨 QoS(quality of service)와 패킷 레벨 QoS를 성능 지표로 나타낸다. 먼저 다양한 offered load에서 하향 링크 자원에 대한 호 차단 확률과 자원 사용률을 측정한다. 그리고 측정된 값을 사용하여 시스템의 outage 확률과 시스템 처리량(effective throughput)을 시뮬레이션 한다. 시뮬레이션 결과를 통해 제안된 알고리즘이 기존 시스템에 비해 시스템 처리량에서 향상된 성능을 가짐을 보인다.

• 한국멀티미디어학회논문지
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• 제12권3호
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• pp.409-418
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• 2009

서비스 품질을 보장하기 위해 서비스 트래픽 흐름 경로상의 각 라우터는 자신에게 배분된 흐름의 지연규격을 준수해야 한다. 라우터에 가해지는 부하 량은 지연규격에 의해 결정되므로 라우터간 부하 불균형은 배분된 지연규격의 조정에 의해 해소될 수 있다. RSVP에 적용된 균등배분 방법은 간단하지만 자원 이용 효율이 낮다. 자원 이용 효율을 높이기 위해 라우터의 부하 상태에 따라 지연규격을 달리 배분하는 부하 밸런싱 방법이 연구되었으나 RSVP에 적용하기에 너무 복잡하다. 본 논문은 RSVP에 적용할 수 있으며 자원 이용 효율을 보다 개선할 수 있는 경로 부하 밸런성 방법을 제안한다. 제안된 방법은 흐름의 종단간 지연한계를 RSVP 절차에 의해 각 라우터에 배분한 후 경로의 병목상태와 링크의 대역폭 가용도에 따라 배분된 지연규격을 조정한다. 평가 네트워크를 대상으로 성능을 평가한 결과 제안방법은 기존방법에 비해 4 ${\sim}$ 17% 정도 높은 자원 이용 효율을 제공함을 보였다.

• Journal of Electrical Engineering and Technology
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• 제12권3호
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• pp.1027-1037
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• 2017

Penetration of renewable energy sources makes the modern interconnected power systems to have more intelligence and flexibility in the control. Hence, it is essential to maintain the system frequency and tie-line power exchange at nominal values using Load Frequency Control (LFC) for efficient, economic and reliable operation of power systems. In this paper, intelligent tuning of the Proportional Integral Derivative (PID) controller for LFC in an interconnected power system is considered as a main objective. The chosen problem is formulated as an optimization problem and the optimal gain parameters of PID controllers are computed with three innovative swarm intelligent algorithms named Particle Swarm Optimization (PSO), Bacterial Foraging Optimization Algorithm (BFOA) and hybrid Bacterial Foraging Particle Swarm Optimization (BFPSO) and a comparative study is made between them. A new objective function designed with necessary time domain specifications using weighted sum approach is also offered in this report and compared with conventional objective functions. All the simulation results clearly reveal that, the hybrid BFPSO tuned PID controller with proposed objective function has better control performances over other optimization methodologies.

• Steel and Composite Structures
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• 제6권6호
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• pp.459-477
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• 2006

A theoretical research project is undertaken to develop integrated analysis and design tools for long span composite beams in modern high-rise buildings, and it aims to develop non-linear finite element models for practical design of composite beams. As the first paper in the series, this paper presents the development study as well as the calibration exercise of the proposed finite element models for simply supported composite beams. Other practical issues such as continuous composite beams, the provision of web openings for passage of building services, the partial continuity offered by the connections to columns as well as the behaviour of both unprotected and protected composite beams under fires will be reported separately. In this paper, details of the finite elements and the material models for both steel and reinforced concrete are first described, and finite element studies of composite beams with full details of test data are then presented. It should be noted that in the proposed finite element models, both steel beams and concrete slabs are modelled with two dimensional plane stress elements whose widths are assigned to be equal to the widths of concrete flanges, and the flange widths and the web thicknesses of steel beams as appropriate. Moreover, each shear connector is modelled with one horizontal spring and one vertical spring to simulate its longitudinal shear and pull-out actions based on measured load-slippage curves of push-out tests of shear connectors. The numerical results are then carefully analyzed and compared with the corresponding test results in terms of load mid-span deflection curves as well as load end-slippage curves. Other deformation characteristics of the composite beams such as stress and strain distributions across the composite cross-sections as well as distributions of shear forces and slippages in shear connectors along the beam spans are also examined in details. It is shown that the numerical results of the composite beams compare well with the test data in terms of various load-deformation characteristics along the entire deformation ranges. Hence, the proposed analysis and design tools are considered to be simple and yet effective for composite beams with practical geometrical dimensions and arrangements. Structural engineers are strongly encouraged to employ the models in their practical work to exploit the full advantages offered by composite construction.

• 한국위성정보통신학회논문지
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• 제5권1호
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• pp.63-68
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• 2010

인공위성 구조체는 발사환경과 궤도환경하에서 탑재체 및 여러 구동기 등을 안전하게 지지할 수 있도록 설계되어야 한다. 위성체의 형상설계가 이루어지면 상세설계를 위하여 발사체에서 공급하는 규격에 의한 준정적하중을 사용하여 구조해석을 수행한다. 이 때 준정적하중을 이용하여 설계된 위성체의 구조 건정성을 확인하기 위하여 발사체 업체는 위성체와 발사체를 연성한 후 연성하중해석을 수행한다. 현재 개발중인 위성체의 경우, 연성하중해석을 수행하기 위하여 위성체 모델을 Craig-Bampton 모델로 축약한 후, 발사체 제작업체로 전달하였다. 발사체 제작업체에서는 위성체 모델과 발사체 모델을 이용하여 연성하중해석을 수행하였으며, 가속도 결과와 변위결과를 계산하여 이를 전달하였다. 전달받은 가속도 결과와 변위결과로부터 위성체는 안전하게 설계되었으며, 위성체 내/외부에서 간섭이나 충돌의 위험성이 없다는 것을 확인하였다.

• 한국지반공학회논문집
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• 제27권12호
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• pp.97-106
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• 2011

본 연구에서는 강관합성 현장타설말뚝의 수평재하시험을 수행하고, 공내재하시험과 표준관입시험 결과에 의한 지반반력계수를 평가하여, 말뚝 머리의 수평거동에 대한 다양한 해석법과 비교 분석하였다. 분석 결과, 표준관입시험의 N값에 의한 지반반력계수를 이용하는 방법은 말뚝의 허용 변위량 내에서 수평거동을 평가하기에 적합하지 않은 것으로 나타났다. 반면 공내재하시험에 의한 지반반력계수를 이용하는 방법은 말뚝의 초기 수평거동을 비교적 잘 모사할 수 있었으며, 프레셔미터 곡선을 직접 말뚝머리의 p-y 곡선으로 이용하는 방법에서, 말뚝 재료에 대한 보정계수를 도입하여 말뚝머리의 수평거동을 타당하게 평가할 수 있었다.

• Smart Structures and Systems
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• 제21권1호
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• pp.53-64
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• 2018

Nondestructive testing methods are required to assess the condition of civil structures and formulate their maintenance programs. Axial force identification is required for several structural members of truss bridges, pipe racks, and space roof trusses. An accurate evaluation of in situ axial forces supports the safety assessment of the entire truss. A considerable redistribution of internal forces may indicate structural damage. In this paper, a novel compressive force identification method for prismatic members implemented using static deflections is applied to steel beams. The procedure uses the Euler-Bernoulli beam model and estimates the compressive load by using the measured displacement along the beam's length. Knowledge of flexural rigidity of the member under investigation is required. In this study, the deflected shape of a compressed steel beam is subjected to an additional vertical load that was short-term measured in several laboratory tests by using fiber Bragg grating-differential settlement measurement (FBG-DSM) sensors at specific cross sections along the beam's length. The accuracy of midspan deflections offered by the FBG-DSM sensors provided excellent force estimations. Compressive load detection accuracy can be improved if substantial second-order effects are induced in the tests. In conclusion, the proposed method can be successfully applied to steel beams with low slenderness under real conditions.

• Structural Engineering and Mechanics
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• 제32권3호
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• pp.399-406
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• 2009

A three-dimensional panel system, which was offered as a new method for construction in Jordan using relatively high strength modular panels for walls and ceilings, is investigated in this paper. The panel consists of two steel meshes on both sides of an expanded polystyrene core and connected together with a truss wire to provide a 3D system. The top face of the ceiling panel was pored with regular concrete mix, while the bottom face and both faces of the wall panels were cast by shotcreting (dry process). To investigate the structural performance of this system, an extensive experimental testing program for ceiling and wall panels subjected to static and dynamic loadings was conducted. The load-deflection curves were obtained for beam and shear wall elements and wall elements under transverse and axial loads, respectively. Static and dynamic analyses were conducted, and the performance of the proposed structural system was evaluated and compared with a typical three dimensional reinforced concrete frame system for buildings of the same floor areas and number of floors. Compressive strength capacity of a ceiling panel is determined for gravity loads, while flexural capacity is determined under the effect of wind and seismic loading. It was found that, the strength and serviceability requirements could be easily satisfied for buildings constructed using the three-dimensional panel system. The 3D panel system is superior to that of conventional frame system in its dynamic performance, due to its high stiffness to mass ratio.