• Korean Journal of Mathematics
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• 제6권1호
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• pp.27-46
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• 1998

We are concerned with an optimal control problem governed by a Poisson equation in which body force acts like a control parameter. The cost functional to be optimized is taken to represent the error from the desired observation and the cost due to the control. We recast the problem into the mixed formulation to take advantage of the minimax principle for the duality method. The existence of a saddle point for the Lagrangian shall be shown and the optimality system will be derived therein. Finally, to attain an optimal control, we combine the optimality system with an operational technique. By achieving the gradient of the cost functional, a convergent algorithm based on the projected gradient method is established.

• 전자공학회논문지
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• 제50권7호
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• pp.231-235
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• 2013

Lens distortions have a significant impact on captured or projected image geometry. This paper proposes a lens distortion correction with gradient components for wide-angle lenses. In most cases, distortion coefficients are estimated using a distortion model by point correspondences. Corrected images using only point correspondences can be compensated excessively, therefore, producing bended lines into the opposite direction near the corners. To curtail these phenomena, we propose to adopt the gradient components in addition to positions to obtain the distortion coefficients. We verified the improved accuracy and the straightness of the proposed method through experimentation.

• 정보보호학회논문지
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• 제34권5호
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• pp.1047-1057
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• 2024

최근 심층 신경망을 이용한 강화학습 모델들이 자율주행, 스마트 팩토리, 홈 네트워크 등 다양한 첨단 산업 분야에 사용되고 있으나 적대적 공격(adversarial attacks)에 취약하다는 것이 밝혀졌다. 본 논문에서는 강화학습 기반의 딥러닝 모델인 DQN과 PPO를 자율주행 가상환경 HighwayEnv에 적용하여 FGSM(Fast Gradient Sign Method), BIM(Basic Iterative Method), PGD(Projected Gradient Descent) 그리고 CW(Carlini and Wagner)을 이용하여 적대적 공격을 수행하였다. 적대적 공격에 대응하기 위해 양방향 필터(bilateral filter) 알고리즘을 사용하여 적대적 이미지의 잡음을 제거함으로써 강화학습 기반의 딥러닝 모델들이 정상적으로 작동할 수 있는 방법을 제안하였다. 그리고 HighwayEnv 환경에서 에피소드 수행 길이(episode during)의 평균과 에이전트가 획득한 보상(episode reward)의 평균을 성능평가 지표로 사용하여 공격의 성능을 평가하였다. 실험 결과 양방향 필터를 통해 적대적 이미지의 잡음을 제거한 결과, 적대적 공격이 수행되기 이전의 성능을 유지할 수 있음을 보였다.

• 한국정보처리학회:학술대회논문집
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• 한국정보처리학회 2019년도 춘계학술발표대회
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• pp.404-407
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• 2019

최근 많은 영역에 딥러닝이 활용되고 있다. 특히 CNN과 같은 아키텍처는 얼굴인식과 같은 이미지 분류 분야에서 활용된다. 이러한 딥러닝 기술을 완전한 기술로서 활용할 수 있는지에 대한 연구가 이뤄져왔다. 관련 연구로 PGD(Projected Gradient Descent) 공격이 존재한다. 해당 공격을 이용하여 원본 이미지에 노이즈를 더해주게 되면, 수정된 이미지는 전혀 다른 클래스로 분류되게 된다. 본 연구에서 기존의 FGSM(Fast gradient sign method) 공격기법에 Triplet loss를 활용한 Adversarial 공격 모델을 제안 및 구현하였다. 제안된 공격 모델은 간단한 시나리오를 기반으로 검증하였고 해당 결과를 분석하였다.

• The Korean Journal of Ecology
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• 제23권4호
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• pp.293-298
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• 2000

To study the effect of global warming on the growth of plants and plant populations throughout their life cycle under a field-like condition, we constructed a Temperature Gradient Chamber (TGC) in Tsukuba, Japan. The chamber had slender shape : 30 m long. 3 m wide, and 2.5 m high. That satisfactory performance was confirmed by a test throughout all seasons in 1998: the projected global warming condition in the near future was simulated. That is, independent of a great daily or seasonal change in ambient meteorological conditions, air temperatures at the air outlet were warmed 5$^{\circ}C$ higher than those at the ambient (the annual mean was 14.3$^{\circ}C$) with precision of ${\pm}$0.2$^{\circ}C$ (the annual means were 19.2$^{\circ}C$) with a rising rate of approximately 1$^{\circ}C$ every 5 m. This chamber will enable us to study the effects of global warming on growth of plants and plant populations because their abilities to control air temperature are excellent. TGC is expected that it would be utilized for studying the effect of global warming on plant growth under natural weather conditions.

• 한국인터넷방송통신학회논문지
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• 제21권1호
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• pp.87-92
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• 2021

빅데이터 기반의 추천시스템 모델링에서 바이어스, 분산, 오류 및 학습은 성능에 중요한 요소이다. 이러한 시스템에서는 추천 모델이 설명도를 유지하면서 복잡도를 줄여야 한다. 또한 데이터의 희소성과 시스템의 예측은 서로 반비례의 속성을 가지기 마련이다. 따라서 희소성의 데이터를 인수분해 방법을 활용하여 상품간의 유사성을 학습을 통한 상품추천모델이 제안되어 왔다. 본 논문에서는 이 모델의 손실함수에 대한 최적화 방안으로 max-norm 규제를 적용하여 모델의 일반화 능력을 향상시키고자 한다. 해결방안은 기울기를 투영하는 확률적 투영 기울기 강하법을 적용하는 것이다. 많은 실험을 통하여 데이터가 희박해질수록 기존의 방법에 비해 제안된 규제 방법이 상대적으로 효과가 있다는 것을 확인하였다.

• 천문학회보
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• 제45권1호
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• pp.41.2-42
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• 2020

Understanding how the filamentary structure plays a role in the formation of the prestellar cores and stars is a key issue to challenge. We have observed two prestellar cores in surrounding filamentary environments in 13CO, C180 (3-2) and HCO+ (4-3) molecular lines with the Heterodyne Array Receiver Program (HARP) of the James Clerk Maxwell Telescope (JCMT), in order to search for the evidence related to the possible flow motions along the filament and/or the radial accretion (or infalling motions) of gas material toward the dense cores from their surrounding filamentary cloud. In L1544, the velocity gradient of 1.6 km s-1 pc-1 toward the core was measured in a small branch of filament lying on a radial direction of main filament while no velocity gradient along the main axis of filament in both 13CO and C18O lines. In L694-2, we found the velocity gradient of 0.6 km s-1 pc-1 along the filament in only 13CO lines. The projected accretion rate of ~6 M◉ Myr-1 was estimated in both cases. The infall (or radially contracting) velocity of gas material was measured ~0.16 km s-1 in both 13CO and HCO+ lines and in both L1544 and L694-2, which leads to estimate a mass infall rate of ~20 M◉ Myr-1. Our analysis suggests that our targets are at a stage where the gravitational contraction dominates the mass accretion through the surrounding filamentary cloud. This is consistent with the fact that our targets are highly evolved prestellar cores on a verge of star formation. More detailed results will be presented at the meeting.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2009년 추계학술대회논문집
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• pp.49-55
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• 2009

This paper evaluates performances of a recently developed divergence-free finite element method based on Hermite interpolated stream functions. Velocity bases are derived from Hermite interpolated stream functions to form divergence-free basis functions. These velocity basis functions constitute a solenoidal function space, and the simple gradient of the Hermite functions constitute an irrotational function space. The incompressible Navier-Stokes equation is orthogonally decomposed into a solenoidal and an irrotational parts, and the decoupled Navier-Stokes equations are projected onto their corresponding spaces to form proper variational formulations. To access accuracy and convergence of the present algorithm, three test problems are selected. They are lid-driven cavity flow, flow over a backward-facing step and buoyancy-driven flow within a square enclosure. Hermite interpolation functions from cubic to quintic are chosen to run the test problems. Numerical results are shown. In all cases it has shown that the present method has performed well in accuracies and convergences. Moreover, the present method does not require an upwinding or a stabilized term.

• 정보처리학회논문지B
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• 제8B권2호
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• pp.215-222
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• 2001

본 논문에서는 새로운 형태 특징값으로서 형태 correlogram을 제안하고 이를 기반으로 한 효과적인 내용기반 영삼검색(content-based image retrieval) 방법을 제시한다. 기존읜 색상 correlogram은 색상 정보에 공간적인 정보를 부여함으로써 영상검색 성능을 향상시켰다. 그러나 이 특징값은 형태 정보를 포함하고 있지 않아서 색상이 다르면서 비슷한 윤곽선 형태를 갖는 물체의 검색에는 좋은 효과를 보이지 못한다.이 문제를 해결하기 위해 예지(edge)들의 correlogram인 형태(shape) correlogram을 제안한다. 색상 correlogram이 색상들의 거리에 따른 상관관계를 나타내는데 반해 형태 correlogram은 에지 각도들의 상관관게를 나타낸다. 형태 correlogram은 gradient 축과 각도 축을 가지는 2차원 특징 벡터(feature vector)로 표현된다. 각 축은 24개 빈(bin)으로 나뉘어져서 총 576개의 원소를 가지게 된다. 또한 본 논문에서는 형태 correlogram의 데이터 크기를 줄이고, 회전에 대해 불변인 특성을 가지게 하기 위해 투영(projected) 형태 correlogram을 제안한다. 실험결과를 통하여 본 논문에서 제안한 형태 correlogram과 투영 형태 correlogram을 사용한 영상검색 방법이 기존의 방법보다 성능면에서 우수함을 입증한다.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1992년도 한국자동제어학술회의논문집(국제학술편); KOEX, Seoul; 19-21 Oct. 1992
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• pp.1-6
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• 1992

A principle of "orthogonalization" is proposed as an extended notion of hybrid (force and position) control for robot manipulators under geometric endpoint constraints. The principle realizes the hybrid control in a strict sense by letting position and velocity feedback signals be orthogonal in joint space to the contact force vector whose components are exerted at corresponding joints. This orthogonalization is executed via a projection matrix computed in real-time from a gradient of the equation of the surface in joint coordinates and hence both projected position and velocity feedback signals become perpendicular to the force vector that is normal to the surface at the contact point in joint space. To show the important role of the principle in control of robot manipulators, three basic problems are analyzed, the first is a hybrid trajectory tracking problem by means of a "modified hybrid computed torque method", the second is a model-based adaptive control problem for robot manipulators under geometric endpoint constraints, and the third is an iterative learning control problem. It is shown that the passivity of residual error dynamics of robots follows from the orthogonalization principle and it plays a crucial role in convergence properties of both positional and force error signals.force error signals.