Yu-Deok Seo

Optimal Design of the Flexure Mounts for Satellite Camera by Using Design of Experiments

Abstract The primary mirror system in a satellite camera is an opto-mechanically coupled system for a reason that optical and mechanical behaviors are intricately interactive. In order to enhance the opto-mechanical performance of the primary mirror system, opto-mechanical behaviors should be thoroughly investigated by using various analysis procedures such as elastic, thermo-elastic, optical and eigenvalue analysis. In this paper, optimal design of the bipod flexure mounts for high opto-mechanical performance is performed. Opto-mechanical performances considered in this paper are RMS wavefront error under the gravity and thermal loading conditions and 1st natural frequency of the mirror system. The procedures of the flexure mounts design based on design of experiments and statistics is as follows. The experiments for opto-mechanical analysis are constructed based on the tables of orthogonal arrays and analysis of each experiment is carried out. In order to deal with the multiple opto-mechanical properties, MADM (Multiple-attribute decision making) is employed. From the analysis results, the critical design variables of the flexure mounts which have dominant influences on opto-mechanical performance are determined through analysis of variance and F-test. The regression model in terms of the critical design variables is constructed based on the response surface analysis. Then the critical design variables are optimized from the regression model by using SQP algorithm. Opto-mechanical performance of the optimal bipod flexure mounts is verified through analysis.

T-spline Finite Element Method for CAD/CAE Integrated Approach

Abstract T-splines are recently proposed geometric modeling tools. A T-spline surface is a NURBS surface with T-junctions and is defined by a control grid called T-mesh. Local refinement can be performed very easily for T-splines while it is limited for B-splines or NURBS. Using T-splines, patches with unmatched boundaries can be combined easily without special technique. In this study, the analysis methodology using T-splines is proposed. In this methodology, T-splines are used both for description of geometries and for approximation of solution spaces. Two-dimensional linear elastic and dynamic problems will be solved by employing the proposed T-spline finite element method, and the effectiveness of the current analysis methodology will be verified. 1. 서 론 유한요소법은 1950년대에 도래하여CAE 분야의 보편적인 도구로서 자리매김하여 왔으며, 현재까지도 많은 연구 및 산업현장에서 광범위하게 사용되고 있다. 이에 반하여 CAD는 효율적인 기하학적 모델링을 목표로 하여 1970년대에 도래하였다. 기하학적 모델링이 유한요소법의 기본적 토대가 됨에도 불구하고 유한요소법에 비해 그 역사적 출현이 늦음으로 인한 아이러니는 현재까지도 기하학적 모델링과 이를 이용한 유한요소해석 및 최적화 과정에 지대한 영향을 끼치고 있다. 대부분의 유한요소 모델링에서 형상을 표현하기 위해 다항식이 사용되는 데에 반해 CAD 모델에서는 B-스플라인 또는 NURBS(non-uniform rational B-spline) 등이 사용됨으로 인해 CAD와 CAE에서의 형상표현이 서로 다른 수학적 표현식을 사용하게 되었다. 이러한 형상표현의 차이는 제품설계 과정을 훨씬 복잡하게 만드는 요인이다. 보편적 제품설계 과정을 살펴보면 미적인 관점과 기능, 생산성 등을 고려하여 먼저 CAD모델이 구성된 후 해석 및 최적설계를 통한 CAE과정이 이루어지게 된다. 이 과정에서 구성된 CAD모델은 CAE과정에서 바로 이용될 수 없다. 그 이유는 앞서 언급한 바와 같이 CAD와 CAE에서의 형상표현을 위한 수학적 도구가 서로 상이하기 때문이다. 따라서, 이 과정에서 CAE를 위한 해석모델의 추가적 구성이 필요하게 된다. 이러한 해석모델의 구성은 형상의 분할을 통한 격자구성을 필수적으로 수반하며, 이 과정은 잘 알려져 있는 바와 같이 CAE과정에서 가장 많은 시간을 요하는 지루한 작업이다. 일례로, 자동차, 선박, 비행기 등의 설계에 있어 해석

T-spline FEA for Trimmed NURBS Surface

In this present work, spline FEA for the trimmed NURBS surface of the 2D linear elasticity problem is presented. The main benefit of the proposed method is that no additional efforts for modeling of trimmed NURBS surfaces are needed and the information of the trimming curves and trimmed surfaces exported from the CAD system can be directly used for analysis. For this, trimmed elements are searched by using NURBS projection scheme. The integration of the trimmed elements is performed by using the NURBS-enhanced integration scheme. The formulation of constructing stiffness matrix of trimmed elements is presented. In this formulation, the information of the trimming curve is used for calculating the Jacobian as well as for obtaining integration points. The robustness and effectiveness of the proposed method are investigated through various numerical examples.

A Study on the Optimization of Ply Angles for Composite Tube using Design of Experiments

Composite has become one of the most frequently used material for a tube of satellite camera due to its attractive characteristics. However, laminated composites can be weakened by delamination which comes from interlaminar stress. Such failure mode cause structural instability of the camera as well as degradation of optical quality. Therefore composite tube should be robust in delamination. Also, composite tube should have high stiffness, sufficient high natural frequency and small coefficient of thermal expansion. The design procedures presented in this paper are based on design of experiments. The experiments for mechanical analysis are designed by the tables of orthogonal arrays. In order to manipulate the various mechanical properties systematically, multiple-attribute decision making(MADM) is employed. Through analysis of variance and F-test, the critical design variables which have dominant influences on mechanical performance are determined. Finally improved ply angles for composite tube are determined.

A Study on the Shear Characteristics of Adhesives in Primary Mirror Supports of Satellite Camera

The optical performance of the mirror fur satellite camera is highly dependent on the adhesive properties between the mirror and its support. Therefore, in order to design a mirror with high optical performance, the mechanical properties of adhesives should be well defined. In this research, the mechanical properties of three kinds of space adhesives are studied. In case of the materials which show nearly incompressible behavior such as space adhesives, it is important to measure shear modulus which governs deviatoric stress components. Also the experiment should be performed in circumstances similar to real manufacturing process of mirror, because extra factors such as size effects, the adhesion effects of primer and reactions between adhesive and primer affect the properties of adhesive regions. In this research shear moduli of the adhesives are determined by using a single lap adhesively bonded joint. For the shear tests, several temperatures have been selected from to which is operating temperature range of the adhesive. In the case of linear behavior materials, shear moduli are calculated through a linear curve fitting. Shear stress-strain relation is obtained by using an exponential curve fitting for material which shows non-linear behavior. The shear modulus of each adhesive is expressed as a function of temperature. Characteristics and adaptability of the adhesives are discussed regarding their temperature sensitivity.