• Journal of Theoretical and Applied Mechanics
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• 제3권1호
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• pp.66-80
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• 2002

The bilinear formulation proposed earlier by Peters and Izadpanah to develop finite elements in time to solve undamped linear systems, Is extended (and found to be readily amenable) to develop time finite elements to obtain transient responses of both linear and nonlinear, and damped and undamped systems. The formulation Is used in the h-, p- and hp-versions. The resulting linear and nonlinear algebraic equations are differentiated to obtain the first- and second-order sensitivities of the transient response with respect to various system parameters. The present developments were tested on a series of linear and nonlinear examples and were found to yield, when compared with results obtained using other methods, excellent results for both the transient response and Its sensitivity to system parameters. Mostly. the results were obtained using the Legendre polynomials as basis functions, though. in some cases other orthogonal polynomials namely. the Hermite. the Chebyshev, and integrated Legendre polynomials were also employed (but to no great advantage). A key advantage of the time finite element method, and the one often overlooked in its past applications, is the ease In which the sensitivity of the transient response with respect to various system parameters can be obtained. The results of sensitivity analysis can be used for approximate schemes for efficient solution of design optimization problems. Also. the results can be applied to gradient-based parameter identification schemes.

• Journal of Communications and Networks
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• 제16권1호
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• pp.45-55
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• 2014

The paper investigates the problem of designing the multiple-antenna relay in a two-way relay network by taking into account the imperfect channel state information (CSI). The objective is to design the multiple-antenna relay based upon the CSI estimates, where the estimation errors are included to attain the robust design under the worst-case philosophy. In particular, the worst-case transmit power at the multiple-antenna relay is minimized while guaranteeing the worst-case quality of service requirements that the received signal-to-noise ratio (SNR) at both sources are above a prescribed threshold value. Since the worst-case received SNR expression is too complex for subsequent derivation and processing, its lower bound is explored instead by minimizing the numerator and maximizing the denominator of the worst-case SNR. The aforementioned problem is mathematically formulated and shown to be nonconvex. This motivates the pursuit of semidefinite relaxation coupled with a randomization technique to obtain computationally efficient high-quality approximate solutions. This paper has shown that the original optimization problem can be reformulated and then relaxed to a convex problem that can be solved by utilizing suitable randomization loop. Numerical results compare the proposed multiple-antenna relay with the existing nonrobust method, and therefore validate its robustness against the channel uncertainty. Finally, the feasibility of the proposed design and the associated influencing factors are discussed by means of extensive Monte Carlo simulations.

• 산업경영시스템학회지
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• 제22권50호
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• pp.23-32
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• 1999

Reliability and maintainability allocation in the analysis of the system's design, with the objective of planning and installing the individual components in such a way that the system performance is achieved. This paper has been made to solve an important task in reliability management of manufacturing systems within the general objective being to increase productivity while maintaining costs low. Thus, the purpose of this paper is to provide an analytical approach to determine an optimal reliability and maintainability allocation, trading off among system performance and parts investment costs. Two important considerations will be addressed in this regard : (ⅰ) determine the reliability and maintainability allocation of parts which maximizes a given production index, having fixed the total cost of investments ; and (ⅱ) determine the reliability and maintainability allocation which minimizes the total cost of investments, having fixed a minimum acceptable level of productivity. The procedure proposed in this paper is able to provide to managers and designers useful indications on the reliability and maintainability characteristics of parts in series -parallel systems. And this heuristic model is a decision support tool for contractors who are involved in large scale design projects such as ship and aircraft design. Numerical examples prove that an approximate expression of the average throughput rate is sufficiently accurate to be used in a numerical optimization method.

• 한국기계가공학회지
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• 제17권6호
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• pp.75-82
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• 2018

This study suggests an optimal design process of an LMU, which is installed on the top side of offshore structures. The LMU is consist of EB(elastomeric bearing) and steel plate, and supports the vertical loads of offshore structures and assists its stable installation. The structural design requirement of the LMU is related to its stiffness. This study utilizes the finite element analysis to predict the stiffness. The stiffness of the EB depends on the size of the bearing. Thus, the design variables in this study are defined as the thickness, the width and the number of plates. Since the LMU has different loads for different locations, its stiffness should be designed differently. The multiobjective function is introduced to attain the target stiffness. In this process, the metamodel using the kriging interpolation method is adopted to replace the true stiffness.

• 한국산학기술학회논문지
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• 제19권4호
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• pp.683-688
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• 2018

최근 하이브리드 전기자동차의 보급 확대에 따라 전기에너지 수요가 증가하고 있다. 본 연구에서는 전기에너지 수요에 대응하기 위해 에너지 하베스팅 기술을 이용해 자가발전이 가능한 3가지 구조의 현가장치 적용 선형 발전 시스템을 ANSYS MAXWELL을 사용하여 전자기 시뮬레이션을 통해 각 구조의 발전 특성을 비교 분석 하였다. 다음으로 각 모델에 대해 상용 PIDO(Process Integration and Design Optimization)툴인 PIAnO(Process Integration, Automation and Optimization)을 사용하여 최적설계를 수행하였다. 3가지 설계변수를 선정하여 실험계획법 기법 중 직교 배열표(Orthogonal Array)를 이용해 도출한 18개의 실험 점에 대해 전자기 해석을 통해 완성한 실험계획법을 바탕으로 근사 모델을 생성하였으며 진화 알고리즘(Evolutionary Algorithm)을 이용한 최적 설계를 수행하였다. 마지막으로 초기 모델과 동일한 해석 조건을 사용해 최적 설계 결과 모델에 대한 전자기 시뮬레이션을 통해 최적설계 결과를 검증 하였다. 각 선형 발전기 모델에 대해 최적의 구조에 대한 발전 특성을 비교한 결과 8pole-8slot, 12pole-12slot, 16pole-16slot 구조에서 최대 발전량은 각각 366.5W, 466.7W, 579.7W로 slot, pole 조합 수가 많아질수록 발전량이 증가하는 결과를 확인하였다.

• 대한기계학회논문집A
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• 제37권11호
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• pp.1397-1405
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• 2013

본 연구에서 탑승자 머리 보호를 위한 센터 필라 트림의 리브 패턴 최적설계는 두 가지 방법에 의해 수행된다. 첫째는 실험계획법과 반응표면법을 이용한 근사최적화 기법으로써, 상대적으로 큰 비중을 차지하는 해석비용 저감을 위하여 근사모델 구성에 필요한 최소한의 해석만을 수행하고 실제 최적화 과정에는 구성된 모델을 이용함으로써 근사적으로 최적 점을 찾아가는 방법이다. 하지만 이러한 방법은 시행착오적인 반복과정을 거쳐야 하는 단점이 있다. 따라서 저자들의 선행연구에서 제안한 순차적 실험계획법과 인공신경망을 이용하여 인자의 상한 또는 하한에 걸리지 않는 근사최적 해를 체계적인 반복과정을 통해 도출하고자 하며, 이를 수학적인 예제와 구조물 문제에 적용함으로써 실용성을 확인하고자 한다.

• 대한기계학회논문집A
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• 제39권7호
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• pp.673-679
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• 2015

본 연구는 비지배 분류 유전알고리즘(NSGA-II)을 이용하여 흐트러진 쿼드콥터의 자세를 빠르게 회복 할 수 있는 최적화된 PID(Proportional-Integral-Derivative) 이득 값을 얻고자 하였다. PID 제어에 앞서 로터가 4 개로 이루어진 쿼드콥터의 간격을 전산유체해석을 통해 정의하였으며, 정의된 쿼드콥터 모델을 통하여 PID 제어 알고리즘을 생성하였다. 반응표면 모델을 생성하기 위해 실험계획법의 하나인 D-최적계획법 이용하여 실험점을 배치 시킨 후 반응표면모델을 생성하였다. Roll 과 Altitude 의 두 값을 동시에 만족할 수 있는 PID 의 이득 값을 NSGA-II 를 통해 쿼드콥터의 최단 시간의 자세제어를 할 수 있는 최적의 이득 값을 얻을 수 있었다.

• 한국전산응용수학회:학술대회논문집
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• 한국전산응용수학회 2003년도 KSCAM 학술발표회 프로그램 및 초록집
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• pp.1-1
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• 2003

In this talk, a reduced-order modeling methodology based on centroidal Voronoi tessellations (CVT's)is introduced. CVT's are special Voronoi tessellations for which the generators of the Voronoi diagram are also the centers of mass (means) of the corresponding Voronoi cells. The discrete data sets, CVT's are closely related to the h-means clustering techniques. Even with the use of good mesh generators, discretization schemes, and solution algorithms, the computational simulation of complex, turbulent, or chaotic systems still remains a formidable endeavor. For example, typical finite element codes may require many thousands of degrees of freedom for the accurate simulation of fluid flows. The situation is even worse for optimization problems for which multiple solutions of the complex state system are usually required or in feedback control problems for which real-time solutions of the complex state system are needed. There hava been many studies devoted to the development, testing, and use of reduced-order models for complex systems such as unsteady fluid flows. The types of reduced-ordered models that we study are those attempt to determine accurate approximate solutions of a complex system using very few degrees of freedom. To do so, such models have to use basis functions that are in some way intimately connected to the problem being approximated. Once a very low-dimensional reduced basis has been determined, one can employ it to solve the complex system by applying, e.g., a Galerkin method. In general, reduced bases are globally supported so that the discrete systems are dense; however, if the reduced basis is of very low dimension, one does not care about the lack of sparsity in the discrete system. A discussion of reduced-ordering modeling for complex systems such as fluid flows is given to provide a context for the application of reduced-order bases. Then, detailed descriptions of CVT-based reduced-order bases and how they can be constructed of complex systems are given. Subsequently, some concrete incompressible flow examples are used to illustrate the construction and use of CVT-based reduced-order bases. The CVT-based reduced-order modeling methodology is shown to be effective for these examples and is also shown to be inexpensive to apply compared to other reduced-order methods.

• 한국시뮬레이션학회논문지
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• 제15권3호
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• pp.103-114
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• 2006

최근의 자동물류센터는 자동창고(ASRS)와 자동주행대차(AGV)를 중심으로 각종 컨베이어시스템과 운반장치 등으로 구성되면서 매우 복잡한 시스템 형태를 갖게 되었다. 이러한 시스템의 복잡성에 기인하여 시스템 설계과정에서의 정확한 운영 수행도 파악을 위해서는 많은 종류의 설계변수들이 고려되어야 한다. 물류센터 설계에서 고려해야할 일반적인 설계 변수로는 보관설비 및 운반설비의 사양과 여러 가지 시스템 운영 규칙, 보관영역이나 보관물의 형태 등이 있다. 이 논문에서는 자동화물류센터에 대하여 시뮬레이션 실험과 반응표면모델을 이용하여 메타모델을 만들고 이를 통하여 설계변수들을 최적화하는 효율적인 방법에 대하여 소개 하고자 한다. 정확하게 정의된 시뮬레이션 기반의 메타모델은 시스템 함수의 근사적 표현으로서 수리적 계산을 통해 신속한 설계변수 최적화를 가능하게 한다. 이 논문에서 제시한 접근 방법은 자동물류센터와 같은 복잡한 물류 시스템의 설계 단계에서 시뮬레이션의 설계 참여도를 극대화시키고 최종 설계의 정확도를 향상시키는데 기여 할 것이다.

• 산업경영시스템학회지
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• 제38권4호
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• pp.98-108
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• 2015

This paper introduces the existence of purchase dependence that was identified during the analysis of inventory operations practice at a sales agency of dealing with spare parts for ship engines and generators. Purchase dependence is an important factor in designing an inventory replenishment policy. However, it has remained mostly unaddressed. Purchase dependence is different from demand dependence. Purchase dependence deals with the purchase behavior of customers, whereas demand dependence deals with the relationship between item-demands. In order to deal with purchase dependence in inventory operations practice, this paper proposes (Q, r) models with the consideration of purchase dependence. Through a computer simulation experiment, this paper compares performance of the proposed (Q, r) models to that of a (Q, r) model ignoring purchase dependence. The simulation experiment is conducted for two cases : a case of using a lost sale cost and a case of using a service level. For a case of using a lost sale cost, this paper calculates an order quantity, Q and a reorder point, r using the iterative procedure. However, for a case of using a service level, it is not an easy task to find Q and r. The complexity stems from the interactions among inventory replenishment policies for items. Thus, this paper considers the genetic algorithm (GA) as an optimization method. The simulation results demonstrates that the proposed (Q, r) models incur less inventory operations cost (satisfies better service levels) than a (Q, r) model ignoring purchase dependence. As a result, the simulation results supports that it is important to consider purchase dependence in the inventory operations practice.