• Smart Structures and Systems
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• 제13권2호
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• pp.281-303
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• 2014

In order to suppress the wind-induced vibrations of the Canton Tower, a pair of active mass driver (AMD) systems has been installed on the top of the main structure. The structural principal directions in which the bending modes of the structure are uncoupled are proposed and verified based on the orthogonal projection approach. For the vibration control design in the principal X direction, the simplified model of the structure is developed based on the finite element model and modified according to the field measurements under wind excitations. The AMD system driven by permanent magnet synchronous linear motors are adopted. The dynamical models of the AMD subsystems are determined according to the open-loop test results by using nonlinear least square fitting method. The continuous variable gain feedback (VGF) control strategy is adopted to make the AMD system adaptive to the variation in the intensity of wind excitations. Finally, the field tests of free vibration control are carried out. The field test results of AMD control show that the damping ratio of the first vibration mode increases up to 11 times of the original value without control.

• 천문학회보
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• 제44권1호
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• pp.38.4-38.4
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• 2019

Ultra-diffuse galaxies (UDGs) are intriguing in the sense that they are much larger than dwarf galaxies but have much lower surface brightness than normal galaxies. To date, UDGs have been found only in the local universe. Taking advantage of deep and high-resolution HST images, we search for UDGs in massive galaxy clusters in the distant universe. In this work, we present our search results of UDGs in three massive clusters of the Hubble Frontier Fields: Abell 2744 (z=0.308), Abell S1063 (z=0.348), and Abell 370 (z=0.375). These clusters are the most distant and massive among the host systems of known UDGs. The color-magnitude diagrams of these clusters show that UDGs are mainly located in the faint end of the red sequence. This means that most UDGs in these clusters consist of old stars. Interestingly, we found a few blue UDGs, which implies that they had recent star formation. The radial number densities of UDGs clearly decrease in the central region of the clusters in contrast to those of bright galaxies which keep rising. This implies that a large fraction of UDGs in the central region were tidally disrupted. These features are consistent with those of UDGs in nearby galaxy clusters. We estimate the total number of UDGs (N(UDG)) in each cluster. The abundance of UDGs shows a tight relation with the virial masses (M_200) of thier host systems: M_200 \propto N(UDG)^(1.01+/-0.05). This slope is found to be very close to one, indicating that efficiency of UDGs does not significantly depend on the host environments. Furthermore, estimation of dynamical masses of UDGs indicates that most UDGs have dwarf-like masses (M_200 < 10^11 M_Sun), but a few UDGs have $L{\ast}$-like masses (M_200 > 10^11 M_Sun). In summary, UDGs in distant massive clusters are found to be similar to those in the local universe.

• 한국산업정보학회논문지
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• 제15권5호
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• pp.19-27
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• 2010

본 논문에서는 IEEE 802.15.4 표준에 따른 저속 무선 개인영역 네트워크 환경에서 종래의 문제점을 해결하여 채널 에러 등의 발생 시에도 QoS를 제공할 수 있는 실시간 데이터 재전송 방안을 제공한다. IEEE 802.15.4 a에서는 데이터 전송률이 850Kbps까지 증가되기 때문에 스트리밍 성의 실시간 데이터를 보다 용이하게 전송이 가능해 진다. 본 연구에서는 IEEE 802.15.4 LR-WPAN 표준에서 비컨 활성화 모드에서, 실시간 데이터 서비스 중에 에러가 발생되는 패킷 재선송을 위한 특정한 슬롯이 다음 슈퍼프레임에 동적으로 배정되어, 시간적인 다이버시티를 얻을 수 있도록 하여 열악한 채널 에러 환경에서도 QoS가 보장되도록 하였다. 수치해석 결과에서 제안 방식은 저속 무선 개인영역 네트워크 환경에서 기존 병사보다 패킷 에러에 강인한 특성 및 우수한 전송률을 제공함을 알 수 있다.

• 한국농림기상학회지
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• 제20권1호
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• pp.101-116
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• 2018

지속가능성과학은 다양한 학문 배경과 관심을 가진 과학자, 전문직 종사자 및 이해당사자들 간의 소통과 협력뿐 아니라 결정론적 환원주의적 접근에서 오래전 기본으로의 패러다임 전환이 요구되는 떠오르는 초학문적 연구다. 생태-사회시스템은 많은 구성성분(또는 행위자)들로 이루어져 이들의 국지 규모의 무작위 상호작용이 자연스럽게 시스템 전체 규모의 질서를 만들어내는 공진화하는 복잡계다. 여기서, 시스템과 주변환경 간의 에너지와 물질과 정보의 흐름이 중요한 역할을 한다. 본 통신에서는 이렇게 계속 변화하는 역동적 시스템, 즉 '자기-조직화하는 계층구조의 열린 시스템(SOHOs)'의 개념적 틀을 소개한다. 먼저 SOHOs의 구조와 기능성을 이해하기 위해 물리학의 두 기본 법칙을 다시 논의한다. 두 법칙의 재해석을 통해 시스템의 운명과 지속가능성을 향한 보다 나은 경로, 또한 생태계의 온전함과 사회의 비전/가치 추구를 어떻게 조화시킬 것인가에 대한 이해를 돕고자한다. 그 다음에 소위 '비저니어링(V)'이라는 틀을 되먹임/전방급전(feedback/feedforward) 루프로 SOHOs 틀에 통합시켜서, '슬쩍 찌르는(nudged) 자기-조직화'가 시스템을 구성하는 행위자들이 합력하여 지속가능한 생태-사회 시스템을 이루어 가도록 유도한다. 마지막으로, SOHOs-V의 적용사례로서, 현재 탄자니아의 농촌마을에서 진행되고 있는 미래지구의 지속가능발전목표 연구실(SDG Lab)인 '농촌시스템 비저니어링(Rural Systems Visioneering)'을 예로 제시하였다.

• Smart Structures and Systems
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• 제9권3호
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• pp.231-251
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• 2012

The stochastic optimal control for a piezoelectric spherically symmetric shell subjected to stochastic boundary perturbations is constructed, analyzed and evaluated. The stochastic optimal control problem on the boundary stress output reduction of the piezoelectric shell subjected to stochastic boundary displacement perturbations is presented. The electric potential integral as a function of displacement is obtained to convert the differential equations for the piezoelectric shell with electrical and mechanical coupling into the equation only for displacement. The displacement transformation is constructed to convert the stochastic boundary conditions into homogeneous ones, and the transformed displacement is expanded in space to convert further the partial differential equation for displacement into ordinary differential equations by using the Galerkin method. Then the stochastic optimal control problem of the piezoelectric shell in partial differential equations is transformed into that of the multi-degree-of-freedom system. The optimal control law for electric potential is determined according to the stochastic dynamical programming principle. The frequency-response function matrix, power spectral density matrix and correlation function matrix of the controlled system response are derived based on the theory of random vibration. The expressions of mean-square stress, displacement and electric potential of the controlled piezoelectric shell are finally obtained to evaluate the control effectiveness. Numerical results are given to illustrate the high relative reduction in the root-mean-square boundary stress of the piezoelectric shell subjected to stochastic boundary displacement perturbations by the optimal electric potential control.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2003년도 ICCAS
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• pp.1714-1719
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• 2003

Antilock Brake System (ABS) is indispensable safety equipment for vehicles today. In order to develop new ABS ECU suitable for pneumatic brake system of a bus, a Hardware In-the-Loop Simulation (HILS) System was developed. In this HILS, the pneumatic brake system of a bus and antilock brake component were used as hardware. For the computer simulation, the 14-Degree of Freedom (DOF) bus dynamic model was constructed using the Matlab/Simulink software package. This model was compiled and downloaded in the simulation board, where the Power PC processor was used for real-time simulation. Additional commercial package, the ControlDesk was used to monitor the dynamic simulation results and physical signal values. This paper will focus on the procedure and results of evaluating the ECU in the HILS simulation. Two representative cases, wet basalt road and $split-{\mu}$ road, were used to simulate real road conditions. At each simulated road, the vehicle was driven and stopped under the help of the developed ECU. In each simulation, the dynamical behavior of the vehicle was monitored. After enough tests in the laboratory using HILS, the parameter-tuned ECU was equipped in a real bus, which was driven and stopped in the real test field in Korea. And finally, the experiment results of ABS equipped vehicle's dynamic behavior both in HILS test and in test fields were compared.

• 한국멀티미디어학회논문지
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• 제6권1호
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• pp.48-57
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• 2003

본 연구는 hidden Markov model(HMM)과 유전알고리 즘을 갖는 MLP(multilayer perceptron) 합성 분류기를 이용한 근전 신호의 인식에 관한 연구이다. 제안된 기법에서 유전알고리즘은 전역적인 탐색으로 신경회로망의 최적의 초기 연결강도를 선택하는데, 이로 인하여 학습속도 및 인식률이 향상되게 된다. 근전 신호의 동적 특성은 연속 운동 인식처럼 신호의 길이 및 시작점과 끝점이 일정치 않고 시변성이 큰 경우에 반드시 고려되어야 하나, 일반 신경회로망에서는 이의 적용이 용이하지 않다. 따라서, 본 연구에서는 신호의 동적 특성에 대한 적응성을 갖는 HMM과 MLP 신경회로망을 결합시킨 구조를 갖는 인식기를 제안한다. 이러한 구조는 인식기의 입장에서 볼 때 HMM의 신호의 동적 특성에 대한 적응성과, MLP의 정적인 신호에 대한 우수한 분류력이 결합되어 동적인 신호에도 높은 인식율을 갖는 특성을 갖는다.

• International Journal of Fluid Machinery and Systems
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• 제8권2호
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• pp.102-112
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• 2015

In the Nordic grid, a trend observed the recent years is the increase in grid frequency variations, which means the frequency is outside the normal range (49.9-50.1 Hz) more often. Variations in the grid frequency leads to changes in the speed of rotation of all the turbines connected to the grid, since the speed of rotation is closely related to the grid frequency for synchronous generators. When the speed of rotation changes, this implies that the net torque acting on the rotating masses are changed, and the material of the turbine runners must withstand these changes in torque. Frequency variations thus leads to torque oscillations in the turbine, which become dynamical loads that the runner must be able to withstand. Several new Francis runners have recently experienced cracks in the runner blades due to fatigue, obviously due to the runner design not taking into account the actual loads on the runner. In this paper, the torque oscillations and dynamic loads due to the variations in grid frequency are simulated in a 1D MATLAB program, and measured grid frequency is used as input to the simulation program. The maximum increase and decrease in the grid frequency over a 440 seconds interval have been investigated, in addition to an extreme event where the frequency decreased far below the normal range within a few seconds. The dynamic loading originating from grid frequency variations is qualitatively found by a constructed variable $T_{stress}$, and for the simulations presented here the variations in $T_{stress}$ are found to be around 3 % of the mean value, which is a relatively small dynamic load. The important thing to remember is that these dynamic loads come in addition to all other dynamic loads, like rotor-stator interaction and draft tube surges, and should be included in the design process, if not found to be negligible.

• 한국지구과학회지
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• 제34권1호
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• pp.51-58
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• 2013

대기 모델링 연구에서 시간 간격을 적절하게 결정하는 것은 중요한 문제이다. 본 연구에서는 비선형 대기 모형에서 수치 해의 시간 간격에 대한 민감도를 조사하였다. 이를 위해 간단한 무차원화된 역학 모형을 사용하여 시간 간격과 비선형성 인자를 바꾸어가며 수치 실험을 수행하였다. 실험 결과, 비선형성 인자가 영향을 줄 만큼 크지 않고 절단 오차를 무시할 수 있는 경우에는 수치 해가 시간 간격에 민감하지 않았다. 그러나 비선형성 인자가 큰 경우에는 수치 해가 시간 간격에 민감한 것으로 밝혀졌다. 이 경우, 시간 간격이 감소할수록 공간 필터의 강도가 증가하여 작은 규모의 현상이 약하게 모의되었다. 이는 일반적으로 시간 간격이 감소하면 절단 오차가 감소하여 더 정확한 수치 해가 도출된다는 사실과 상충한다. 이러한 충돌은 비선형 모형의 수치 해를 안정하게 하기 위해 공간 필터가 반드시 필요하기 때문에 피할 수 없다.

• 천문학회지
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• 제46권3호
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• pp.133-140
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• 2013

The empirical mass discrepancy-acceleration (MDA) relation of disk galaxies provides a key test for models of galactic dynamics. In terms of modified laws of gravity and/or inertia, the MDA relation quantifies the transition from Newtonian to modified dynamics at low centripetal accelerations $a_c{\lesssim}10^{-10}ms^{-2}$. As yet, neither dynamical models based on dark matter nor proposed modifications of the laws of gravity/inertia have predicted the functional form of the MDA relation. In this work, I revisit the MDA data and compare them to four different theoretical scaling laws. Three of these scaling laws are entirely empirical; the fourth one - the "simple ${\mu}$" function of Modified Newtonian Dynamics - derives from a toy model of gravity based on massive gravitons (the "graviton picture"). All theoretical MDA relations comprise one free parameter of the dimension of an acceleration, Milgrom's constant aM. I find that the "simple ${\mu}$" function provides a good fit to the data free of notable systematic residuals and provides the best fit among the four scaling laws tested. The best-fit value of Milgrom's constant is $a_M=(1.06{\pm}0.05){\times}10^{-10}ms^{-2}$. Given the successful prediction of the functional form of the MDA relation, plus an overall agreement with the observed kinematics of stellar systems spanning eight orders of magnitude in size and 14 orders of magnitude in mass, I conclude that the "graviton picture" is sufficient (albeit probably not a necessary nor unique approach) to describe galactic dynamics on all scales well beyond the scale of the solar system. This suggests that, at least on galactic scales, gravity behaves as if it was mediated by massive particles.