• The Proceedings of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.7 no.6
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• pp.27-33
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• 1993

In order to improve complicated construction and complex control which are didvantage of optimal PWM technique aimed at harmonic elimination method, this paper presented MRA(Mode1 Reference Adaptive) PWM technique that gating signal of inverter is generated by comparing the reference signal with the induced feedback signal at the reference model of load. Design of controller is composed of microprocessor and analog circuit. MRA PWM technique used in the paper is able to compensate the degradation of voltage efficiency to be generated by the ratio of the output voltage to the DC supply voltage being low for using conventional sinusoidal PWM technique. When the trapezoidal signal is employed as the reference signal. the low order harmonics of line current can be reduced and the switching pattern is made by on-line computation using comparatively simple numerical analysis.

• Journal of the Korean Society of Safety
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• v.31 no.5
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• pp.95-101
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• 2016

This study proposes a multi-scale dynamic system reliability analysis of control system as a method of quantitative evaluation of its performance in probabilistic terms. In this second paper, we discuss the control effect of the viscous damper on the seismic performance of the structure-level failure. Since the failure of one structural member does not necessarily cause the collapse of the structural system, we need to consider a set of failure scenarios of the structural system and compute the sum of the failure probabilities of the failure scenarios where the statistical dependence between the failure scenarios should be taken into account. Therefore, this computation requires additional system reliability analysis. As a result, the proposed approach takes a hierarchial framework where the failure probability of a structural member is computed using a lower-scale system reliability with the union set of time-sequential member failures and their statistical dependence, and the failure probability of the structural system is again computed using a higher-scale system reliability with the member failure probabilities obtained by the lower-scale system reliability and their statistical dependence. Numerical results demonstrate that the proposed approach can provide an accurate and stable reliability assessment of the control performance of the viscous damper system on the system failure. Also, the parametric study of damper capacity on the seismic performance has been performed to demonstrate the applicability of the proposed approach through the probabilistic assessment of the seismic performance improvement of the damper system.

• Journal of Korean Society for Quality Management
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• v.47 no.4
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• pp.739-754
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• 2019

Purpose: The purpose of this paper is to develop a reliability estimation procedure for the underground gas pipeline in the presence of corrosion defects. Methods: Corrosion is one of the major causes of the gas pipeline failure. Several failure forms caused by corrosion have been studied. Among them, small leak and burst are considered in this paper. The composite failure of the two is defined by limit state function, and it is expressed with pipe parameters. Given a modified corrosion dataset, in order to obtain reliability estimations, the method of first order and second moment is adopted because of its simplicity. The computation processes are conducted with MATLAB coding. Results: According to numerical results, the probability of composite failure is affected by both small leak and burst. In particular, when corrosion depth stays at low level, it is consistent with the probability of burst failure. On the contrary, it is more influenced by the small leak failure as corrosion depth is increasing. In such case, the probability of composite failure is fast approaching to the safety limit. Conclusion: By considering the composite failure, more practical predictions of remaining life can be obtained. The proposed method is useful for maintenance planning of the underground gas pipeline.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.11
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• pp.1390-1398
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• 1999

A numerical analysis for ROT sparger of PWR(Pressurized Water Reactor) is carried out. Computation is performed to investigate the flow characteristics as the change of design factor. As the result of this study, RDT sparger's flow resistance coefficient is K=3.53 at the present design condition if engineering mar&in is considered with 20%, and flow ratio into branch pipe is $Q_s/Q_i=0.41$. Velocity distribution at exit is not uniform because of separation in branch pipe. In the change of inlet flow rate and section area ratio of branch pipe for main pipe, flow resistance coefficient is increased as $Q_s/Q_i$ decreasing, but in the change of branch angle and outlet nozzle diameter of main pipe, flow resistance coefficient is decreased as $Q_s/Q_i$ decreasing. As the change rate of $Q_s/Q_i$ is the larger, the change rate of flow resistance coefficient is the larger. The change rate of pressure loss is the largest change as section area ratio changing. The optimal design condition of sparger is estimated as the outlet nozzle diameter ratio of main pipe is $D_s/D_i=0.333$, the section area ratio is $A_s/A_i=0.2$ and the branch angle is ${\alpha}=55^{\circ}$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.3
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• pp.283-288
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• 2014

The use of robots is no longer limited to the field of industrial robots and is now expanding into the fields of service and medical robots. In this light, a trajectory generation method that can respond instantaneously to the external environment is strongly required. Toward this end, this study proposes a method that enables a robot to change its trajectory in real-time using a convolution operation. The proposed method generates a trajectory in real time and satisfies the physical limits of the robot system such as acceleration and velocity limit. Moreover, a new way to improve the previous method (11), which generates inefficient trajectories in some cases owing to the characteristics of the trapezoidal shape of trajectories, is proposed by introducing a triangle shape. The validity and effectiveness of the proposed method is shown through a numerical simulation and a comparison with the previous convolution method.

• Journal of Korean Society of Steel Construction
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• v.17 no.2 s.75
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• pp.173-181
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• 2005

This study deals with a method to identify damages of free vibrating thin plate structures using the combined finite element method (FEM) and the advanced uniform micro-genetic algorithm.To solve the inverse problem using the combined method, this study uses several natural frequencies instead of mode shapes in a structure as the measured data. The technique described in this paper allows us not only to detect the damaged elements but also to find their numbers, locations, and the extent of damage.To demonstrate the feasibility of the proposed method, the algorithm is applied to a free vibrating steel thin plate structures with arbitrary damages. From the standpoint of computation efficiency, the proposed method in this study has advantages when compared with the existing simple genetic algorithms. The numerical examples demonstrate that the method using micro-genetic algorithms can possibly detect correctly the damages of thin plates from only several natural frequencies instead of their natural modes.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.1
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• pp.31-38
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• 2008

This paper deals with the nonlinear optimal control approach to helicopter maneuver problems using the indirect method. We apply a penalty function to the integral deviation from a prescribed trajectory to convert the system optimality to an unconstrained optimal control problem. The resultant two-point boundary value problem has been solved by using a multiple-shooting method. This paper focuses on the model selection strategies to resolve the problem of numerical instability and high wait time when a high fidelity model with rotor dynamics is applied. Four different types of helicopter models are identified, two of which are linear models with or without rotor models, as well as two models which include the nonlinear mathematical model for rotor in its formulation. The relative computation time and the number of function calls for each model are compared in order to provide a guideline for the selection of helicopter model.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.10
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• pp.918-926
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• 2007

Military aircrafts usually operate at the area with lots of threats such as radars and surface-to-air missiles. Aircraft also faces with the unexpected or pop-up threats. Under this environment, a safe flight path should be generated to lead a mission successful. In this paper, a new path planning algorithm is proposed to provide less dangerous flight path efficiently. Of many path planning algorithms, a potential method is considered, because it has advantages of computation efficiency and smooth path generation. Trajectory generation under the condition of maximum range is studied so that the aircraft may reach the target area without refueling. The algorithm to cope with an unexpected situation is also proposed by adopting the concept of initial direction vector, additional force, and a new mapping function. The performance of the proposed algorithms is demonstrated for SEAD (Suppression of Enemy Air Defences) mission by numerical simulation.

• International Journal of Vascular Biomedical Engineering
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• v.2 no.1
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• pp.17-24
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• 2004

Tumor angiogenesis was simulated using a two-dimensional computational model. The equation that governed angiogenesis comprised a tumor angiogenesis factor (TAF) conservation equation in time and space, which was solved numerically using the Galerkin finite element method. The time derivative in the equation was approximated by a forward Euler scheme. A stochastic process model was used to simulate vessel formation and vessel elongation towards a paracrine site, i.e., tumor-secreted basic fibroblast growth factor (bFGF). In this study, we assumed a two-dimensional model that represented a thin (1.0 mm) slice of the tumor. The growth of the tumor over time was modeled according to the dynamic value of bFGF secreted within the tumor. The data used for the model were based on a previously reported model of a brain tumor in which four distinct stages (namely multicellular spherical, first detectable lesion, diagnosis, and death of the virtual patient) were modeled. In our study, computation was not continued beyond the 'diagnosis' time point to avoid the computational complexity of analyzing numerous vascular branches. The numerical solutions revealed that no bFGF remained within the region in which vessels developed, owing to the uptake of bFGF by endothelial cells. Consequently, a sharp, declining gradient of bFGF existed near the surface of the tumor. The vascular architecture developed numerous branches close to the tumor surface (the brush-border effect). Asymmetrical tumor growth was associated with a greater degree of branching at the tumor surface.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.11
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• pp.74-79
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• 2005

Computational studies on the hydrodynamic behavior of the forward-sweep inducers for the rocket-engine turbopump are presented in comparison with the conventional backward- sweep inducers. In the present study, two kinds of forward-sweep inducers are designed and numerically investigated. Forward-sweep inducers have bigger tip solidity compared to backward-sweep inducers even with shorter axial length due to their forward-sweep leading edge profiles. It is shown that back flows at the inlet decreases dramatically for forward- sweep inducers. And the low pressure region at the back flow are also decreased, which is assumed to promote the suction performance of the inducers. It seems that the hub located upstream of the tip at the leading edge induces pre whirl at the inlet blade tip for the backward sweep inducer. And this pre whirl leads to the big back flow.