• 한국소음진동공학회논문집
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• 제13권7호
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• pp.524-531
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• 2003

This paper presents an active vibration control of a flexible beam structure using a hybrid mount which consists of elastic rubber and Piezoelectric material. After identifying stiffness and damping properties of the rubber and piezoelectric elements, a mechanical model of the hybrid mount is established. The mount model is then Incorporated into the beam structure, and the governing equation of motion is obtained in a state space. A sliding mode controller is designed in order to actively attenuate the vibration of the beam structure subjected to high frequency and small magnitude excitations. The controller is experimentally realized and control responses such as acceleration of the beam structure and force transmission through the hybrid mount are evaluated. In addition. a comparative work is done between the passive and hybrid mount systems.

• 항공우주시스템공학회지
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• 제7권4호
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• pp.18-26
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• 2013

The technique of sliding mode control has been introduced and designed for jet engine controller. For designing the controller for controlled element, the state space model of the turbojet engine is derived in advance from the perturbation of non-linear engine dynamic equation at operation point. Based upon the jet engine model, the robust sliding mode controller is proposed associated with the optimum sliding mode function. The numerical simulation demonstrates that the designed sliding mode controller proves its effectiveness for the jet engine by showing superior control performances over the conventional PI controller with fast responses and robustness to disturbance.

• 전기의세계
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• 제31권3호
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• pp.204-208
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• 1982

The objective of this paper is to present a digital method for improving the lateral resolution of the B-scan images in the medical applications of ultrasound. The method is based upon a mathematical model of the lateral blurring caused by the finite beam width of the transducers. This model provides a simple method of applying a recursive scheme for image restoration with fast computation time. The point spread function (P.S.F.) can be measured by the reflective signals after scanning the small pins located along the depth of interest. From the measured P.S.F., one can compute the coefficient matrices of the inverse discrete-time dynamic state variable equation of the blurring process. Then, a recursive scheme for deblurring is applied to the recorded B-scan to improve the lateral resolution. One major advantage of the present recursive scheme over the transform method is in its applicability for the space-variant imaging, such as in the case of the rotational movement of transducer.

• 천문학회보
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• 제44권2호
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• pp.59.2-59.2
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• 2019

In this talk, I will explain the implications of a rapid appearance of dark energy between the redshifts ($z$) of one and two on the expansion rate and growth of perturbations. Using both Gaussian process regression and a parametric model, I show that this is the preferred solution to the current set of low-redshift ($z<3$) distance measurements if $H_0=73~\rm km\,s^{-1}\,Mpc^{-1}$ to within 1\% and the high-redshift expansion history is unchanged from the $\Lambda$CDM inference by the Planck satellite. Dark energy was effectively non-existent around $z=2$, but its density is close to the $\Lambda$CDM model value today, with an equation of state greater than $-1$ at $z<0.5$. If sources of clustering other than matter are negligible, we show that this expansion history leads to slower growth of perturbations at $z<1$, compared to $\Lambda$CDM, that is measurable by upcoming surveys and can alleviate the $\sigma_8$ tension between the Planck CMB temperature and low-redshift probes of the large-scale structure.

• 항공우주시스템공학회지
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• 제14권5호
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• pp.1-8
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• 2020

구조적 특성 및 공력의 상호작용과 조종면 작동기의 제어 시스템의 영향을 고려한 서보공력탄성 해석을 수행하였다. 공탄성 안정성 해석을 위해서 선행되어야 하는 구조모델의 자유진동 해석은 유한요소 해석 프로그램 MSC Nastran을 사용하였다. 비정상 공기력 계산에 ZAERO를 사용하였다. 비정상 공기력 검증에 Doublet Hybrid Method를 사용하였다. Karpel의 최소상태근사법을 이용하여 주파수 영역의 공기력을 라플라스 영역으로 근사하였다. 공탄성 상태방정식을 구동기의 동역학 상태방정식과 결합하여 서보 공탄성 모델의 상태공간방정식을 구성하였다. 고세장비 모델의 승강타 입력에 따른 안정성 해석을 수행하였다. 근궤적법과 시간적분법을 사용하여 주파수영역과 시간영역에서의 서보공탄성 안정성 해석을 수행하였다.

• 한국항공우주학회지
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• 제39권7호
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• pp.596-604
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• 2011

원심 압축기의 원판 마찰 손실은 동력 손실의 한 종류로써, 원심 압축기의 전체 효율 향상을 위해 원판 마찰 손실을 줄여야 한다. 본 연구에서는 원심 압축기의 임펠러 디스크 면과 케이싱 사이의 축 간격 및 표면 조도 변화에 따른 원판 마찰 손실을 분석하였고, 원판마찰손실 저감을 위한 새로운 이론식을 제안하고자 한다. 원심 압축기 임펠러의 정상상태 해석을 위해서 상용 전산해석 코드인 FLUENT의 회전 좌표계와 2-equation k-${\omega}$ SST 모델을 사용하였다. CFD 해석 결과, 원심압축기 임펠러의 원판 마찰 손실은 축 간격의 변화보다는 표면조도의 변화에 더 큰 영향을 받는 것으로 분석되었다. 원심압축기 임펠러의 원판 마찰 손실을 최소화하기 위해서 축 간격은 이론적인 경계층 두께와 동일하도록 설정하고 표면조도는 최소화해야 한다.

• International Journal of Reliability and Applications
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• 제4권1호
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• pp.1-12
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• 2003

Many software reliability growth models (SRGM's) based on a nonhomogeneous Poisson process (NHPP) have been proposed by many researchers. Most of the SRGM's which have been proposed up to the present treat the event of software fault-detection in the testing and operational phases as a counting process. However, if the size of the software system is large, the number of software faults detected during the testing phase becomes large, and the change of the number of faults which are detected and removed through debugging activities becomes sufficiently small compared with the initial fault content at the beginning of the testing phase. Therefore, in such a situation, we can model the software fault-detection process as a stochastic process with a continuous state space. In this paper, we propose a new software reliability growth model describing the fault-detection process by applying a mathematical technique of stochastic differential equations of an Ito type. We also compare our model with the existing SRGM's in terms of goodness-of-fit for actual data sets.

• 항공우주시스템공학회지
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• 제17권4호
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• pp.43-57
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• 2023

우주 물체 및 우주 구조물의 초고속 충돌 시뮬레이션을 LS-DYNA를 사용하여 수행하였다. 구형, 원뿔형, 및 속이 빈 원통형의 다양한 형상의 우주 물체는 SPH(Smoothed Particle Hydrodynamics)를 사용하여 모델링하였다. 다양한 두께의 우주 구조물은 직접 충돌 영역과 간접 충돌 영역으로 나누어, 각각 SPH 및 유한 요소를 사용하여 나타내었다. 초고속 충돌에서 금속 재료의 비선형 거동을 나타내기 위하여 Johnson-cook 재료 모델과 Mie-Grüneisen 상태 방정식을 사용하였다. 우주 물체의 형상, 우주 구조물의 두께, 충돌 각도, 및 충돌 속도의 다양한 충돌 조건을 고려하였다. 파편운은 우주 물체와 우주 구조의 초고속 충돌로 인해 발생되며, 발생된 파편운의 형상을 정량적으로 분석하였다. 본 연구의 모든 충돌 조건에서, 원뿔 형상의 우주 물체로 인한 파편운이 가장 위험한 형상임을 확인하였다.

• 전력전자학회논문지
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• 제19권5호
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• pp.422-430
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• 2014

This study proposes a small-signal model and control design for a two-stage DC-DC-AC converter to investigate its dynamic characteristics in relation to battery energy storage system. When the circuit analysis of the two-stage DC-DC-AC converter is attempted simultaneously, the mathematical procedure of deriving the dynamic equation is complex and difficult. The main idea of modeling the two-stage DC-DC-AC converter states that this topology is separated into a bidirectional DC-DC converter and a single-phase inverter with an equivalent current source corresponding to that of the inverter or converter. The dynamic equations for the separated converter and inverter are then derived using the state-space averaging technique. The procedures of building the small-signal model of the two-stage DC-DC-AC converter are described in detail. Based on the derived small-signal model, the individual controllers are designed through a frequency-domain analysis. The simulation and experimental results verify the validity of the proposed modeling approach and controller design.

• 제어로봇시스템학회논문지
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• 제15권12호
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• pp.1175-1183
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• 2009

In photovoltaic(PV) generator systems, DC-DC converters are significantly considered for control system performance in power quality point of view. This paper presents a novel adaptive control method for DC-DC converters applied in PV generator systems. First, we derive a state-space average model of the converter system and then propose a reset control methodology to enhance transient response performance for time-varying PV systems. For estimating parameters of a reset control, a gradient descent optimization is utilized and an adjustment rule of them are derived respectively. An objective of the optimization is that characteristic equation of an augmented system model which is formed with an converter system model and an reset control is to trace a predefined polynomial given as a reference characteristic model. Next, we accomplish stability analysis by means of a well-known Lyapunov theory for nonlinear converter systems including time-varying voltage excitation from a PV generator. Numerical simulation demonstrates reliability of our control methodology and its superiority by comparison to a traditional control strategy.