• KEPCO Journal on Electric Power and Energy
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• v.4 no.2
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• pp.55-61
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• 2018

본 연구는 시각 동기 위상 측정 정보를 이용하여 전력계통에 나타나는 여러 가지 동요 현상을 검출하기 위한 기초 연구로써, 시계열 데이터 분석 분야로 분류된다. 제시한 방법은 비선형 동특성에 해석 기반으로 접근하여 전력계통에 나타날 수 있는 여러 동요 현상을 범용적으로 검출해 낼 수 있다. 비선형 동요 현상의 신호적 패턴을 수학적으로 기본 순시치 파형으로부터 피크치 샘플링을 통해 전개하여 계통 요소간 간섭으로 인한 원하지 않는 진동 모드를 검출하고자 한다. 계통의 변화로 진동 모드가 나타날 때, 2차원 평면에 실효치로 환산한 시계열 전압 데이터와 선형화된 플로퀘트 상수(Floquet multiplier)를 맵핑하여 도시하고, 정상상태 지점으로부터 거리를 계산하여 최대 리아프노프 지수 계산을 통해 계통이 불안정하게 되는 시간을 시계열 데이터 분석으로 추정하는 것이 본 방법의 핵심이다. 이러한 접근으로 제시한 비선형 동요 검출 알고리즘을 적용하여 디지털 필터 적용 또는 주파수 영역 해석과 같은 오프라인 Study와 달리 온라인으로 신속하게 계통의 현재 상태를 알 수 있게 된다.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.19 no.1 s.71
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• pp.85-92
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• 2006

This paper investigates critical loads of the tapered Beck's columns with clamped and spring supports, subjected to a subtangential follower force. The linearly tapered columns with the solid rectangular cross-section is adopted as the column taper. The differential equation governing free vibrations of such Beck's columns is derived using the Bemoulli-Euler beam theory. Both divergence and flutter critical loads are calculated from the load-frequency curves which are obtained by solving the differential equation. The critical loads are presented as functions of various non-dimensional system parameters: the taper type, the subtangential parameter and the spring stiffness.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.903-906
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• 2005

The purpose of this paper is to investigate free vibrations and critical loads of the uniform Beck's columns with a tip spring, carrying a tip mass. The ordinary differential equation governing free vibrations of such Beck's column subjected to a follower force is derived based on the Bernoulli-Euler beam theory. Both the divergence and flutter critical loads are calculated from the load-frequency curves that are obtained by solving the differential equation numerically. The critical loads are presented in the figures as functions of various non-dimensional system parameters such as the mass moment of inertia and spring parameter.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.11 s.104
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• pp.1287-1294
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• 2005

The purpose of this paper is to investigate free vibrations and critical loads of the Beck's columns with a tip spring, which carry a tip mass. The ordinary differential equation governing free vibrations of Beck's column subjected to a follower force is derived based on the Bernoulli-Euler beam theory Both the divergence and flutter critical loads are calculated from the load-frequency corves that are obtained by solving the differential equation numerically. The critical loads are presented in the figures as functions of various non-dimensional system parameters such as the subtangential parameter, mass ratio and spring parameter.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.1
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• pp.57-62
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• 1990

This paper deals with the flow caused by a two-dimensional pulsating source, which moves with a constant horizontal speed beneath the free surface. The analysis is based on lincar potential theory including surface tension effects. In the case of subcritical reduced frequencies $\tau<1/4(\tau=U_{\omega}/g$, U=constant speed, $\omega$=circular frequency, g=gravitational acceleration), six wave components arc found. Two of them are largely affected by surface tension, which propagate ahead of the source in the direction of and opposite to the steady translation, respectively. The rest are almost identical with those found by Haskind(1954), i.e. for which the surface tension effect is negligible. For low oscillation frequencies, the resonant frequency still exists at $\tau$ only slightly greater than 1/4. For oscillation frequencies greater than $\nu(={\omega}^2/g)>20$, the surface tension effect is so significant that it disperses generated waves and consequently the singular phenomenon is removed. However, in addition to the gravity breaking, capillary breakings occur when the translation speed coincides with the minimum capillary celerity.

• Journal of the Society of Naval Architects of Korea
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• v.45 no.4
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• pp.389-395
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• 2008

It is very well known that the natural frequency of an oscillating body on the free surface is determinable only after the added mass is given. However, it is hard to find analytical investigations in which actually the natural frequency is obtained. Difficulties arise from the fact that in order to determine the natural frequency we need to compute the added mass at least for a range of frequencies, and to solve an equation where the frequency is a variable. In this study, first, a formula is obtained for the added mass, and then an equation for finding the natural frequency is defined and solved by Newton's iteration. It is confirmed that the formula shows a good agreement with the results given by Ursell(1949), and the value of natural frequency is reduced by 21.5% compared to the pre-natural frequency, which is obtained without considering the effect of added mass.

• Journal of Advanced Navigation Technology
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• v.20 no.3
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• pp.175-181
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• 2016

This paper introduces a heave compensation system for offshore crane when it gets unexpected disturbances and external force. The dynamic model consists of crane assumed to be the rigid body, hydraulic driven winch, elastic rope and payload. To keep the payload from moving up and down, PD control algorithm is applied. By using the control, the oscillating amplitude of the payload is reduced. Also by using the estimated values involved with time-delay, the relative motion of payload in heave direction is dramatically shortened. This paper shows using the control algorithm with estimated value having time-delay 0.1 second is enough to heave compensation system.

• Journal of Advanced Marine Engineering and Technology
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• v.41 no.1
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• pp.105-110
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• 2017

This paper introduces a heave compensation system for offshore crane when it subjected to unexpected disturbances such as ocean waves, tidal currents or winds and their external force. The dynamic model consists of a crane which is considered to behave in the same manner as a rigid body, a hydraulic driven winch, an elastic rope and a payload. To keep the payload from moving upwards and downwards, PD(Proportional-Derivative) control was applied by using linearization. In order to achieve a better performance, the sliding mode control and the nonlinear generalized predictive control algorithm was applied according to the time-delay. As a result, the oscillating amplitude of the payload was reduced by the control algorithm. Considering the time-delay involved in the system to be one second, nonlinear generalized predictive controller with a robust controller was a suitable control algorithm for this heave compensation system because it made the position of te payload reach the desired position with the minimum error. This paper presented a control algorithm using the robust control and its simulation results.

• Journal of the Society of Naval Architects of Korea
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• v.50 no.2
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• pp.111-119
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• 2013

The concept of the natural frequency is useful for understanding the characters of oscillating systems. However, when a circular cylinder floating horizontally on the water surface is heaving, due to the hydrodynamic forces, the system is not governed by the equation like that of the harmonic one. In this paper, in order to shed some lights on the more correct use of the concept of the natural frequency, a problem of the heaving circular cylinder is analyzed in the frequency domain. Previously, it was thought that the theory of Ursell (1949) could not be used to get the added mass and wave-making damping for short waves, however, they were obtained by applying an accurate collocation method to the theory in this study. Using the so developed numerical method, we found the added mass and wave-making damping of the circular cylinder for the entire range of the frequency. Then, the MCFR(Modulus of Complex Frequency Response) was used to locate the frequency corresponding to the local maximum of MCFR and we define it as the natural frequency. Comparing our results with the previous investigation, we found that the pressure distribution on the cylinder gets close asymptotically to that of a cylinder in infinite fluid OR close to that of the cylinder, that the approximation of the natural frequency by Lee (2008) is different from our new value only by 0.64%, and that the approximation of the heaving system by an equivalent damped harmonic oscillation is not proper by the reason that is clearly shown from the comparison of the shape of the corresponding MCFRs.

• Journal of Navigation and Port Research
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• v.40 no.6
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• pp.361-368
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• 2016

A ship's rolling motion can make crew and passengers sick and/or apply forces to the structure that cause damage.. Therefore bilge keels are equipped in most ships for anti-rolling. In special cases, anti-rolling tanks (ARTs), fin stabilizers, or gyroscopes can be installed. However, ARTs require a large area to install, and fin stabilizers and gyroscopes are costly to install and expensive to operate. This paper suggests a Anti-rolling pendulum (ARP) to reduce roll motion. ARPs acts like ARTs. However, the ARP has a circular shaped guidance arc instead of the string or wire of a simple pendulum. The device suggested has about 1/ 8 the weight and 1/ 6 the volume of a ART and is more effective. This study derives the nonlinear and linear differential equations of system motion.