CAD와의 연동을 고려한 T-스플라인 유한요소해석

엄태경(Tae-Kyoung Uhm),윤성기(Sung-Kie Youn) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5

Recently, the new finite element method which uses NURBS as shape functions was proposed. It is very promising because it can directly use CAD data to describe geometry and discretize problem domain. In this case, CAE models are not approximated but represent exact geometry. So, it can contribute to more accurate results. In addition, it can greatly reduce CAE costs in that simulation models don’t have to be made up independently. But in spite of these advantages, the method using NURBS have also some disadvantages. NURBS surface cannot be refined locally. T-splines are recently developed surface modeling technique. A Tspline surface is a NURBS surface with T-junctions and is defined by a control grid called T-mesh. The Tjunctions enable T-spline surfaces to be refined locally. That is, it is possible to add a single control point to a T-spline control grid without propagating an entire row or column of control points and without altering the surface. In this research, the finite element analysis using T-splines is studied. In this analysis, CAD data are used directly for engineering analysis. Some problems with complex geometry are solved. And the results will be compared with ones of conventional FEM.

국부세분화가 가능한 T-스플라인 기반 유한요소법의 동적해석

엄태경(Tae-Kyoung Uhm),윤성기(Sung-Kie Youn) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.10

T-splines are recently proposed geometric modeling tools. A T-spline surface is a NURBS surface with Tjunctions and is defined by a control grid called T-mesh. Local refinement can be performed very easily for Tsplines 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 research, an analysis framework using T-splines is proposed. In this framework, T-splines are used for both description of geometries and approximation of solution spaces. This is a CAD/CAE integrated approach. In this approach, CAD models are directly used without additional finite element modeling. Because the geometries are represented exactly, errors of solutions by geometric approximation can be avoided. Some dynamic problems will be solved using this analysis framework, and the effectiveness of the current analysis framework will be verified.

CAD/CAE 통합 접근을 위한 T-스플라인 유한요소법

엄태경(Tae-Kyoung Uhm),김기승(Ki-Seung Kim),서유덕(Yu-Deok Seo),윤성기(Sung-Kie Youn) 대한기계학회 2009 大韓機械學會論文集A Vol.33 No.2

T-splines are recently proposed geometric modeling tools. A T-spline surface is a NURBS surface with Tjunctions and is defined by a control grid called T-mesh. Local refinement can be performed very easily for Tsplines 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.

T-스플라인 기반 유한요소법을 이용한 쉘 구조물의 해석

엄태경(Tae-Kyoung Uhm),윤성기(Sung-Kie Youn) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5

Shell formulation based on the T-spline finite element method is presented. Although the spline-based finite element method has many advantages, it cannot be directly employed for the analysis of shell structures. Shell formulation based on NURBS or T-splines has fundamental limitations because rotational d.o.f., which are necessary in the shell formulation, cannot be directly defined on control points. Various approaches have been attempted to apply spline-based finite element method to the analysis of shell structures. But, they need some special techniques and reduce the merits of the spline-based finite element method. In our research, the idea for interpolating normal vectors and their rotation using T-spline shape functions is proposed. Using this concept, rotational d.o.f. can be granted to control points and T-spline finite element method can be easily extended into shell analysis. The proposed shell formulation will be verified through various benchmarking problems.

Shack-Hartmann 센서를 이용한 파면측정의 정확도 향상에 관한 연구

노경완,엄태경,김지연,박상훈,윤성기,이준호,Roh, Kyung-Wan,Uhm, Tae-Kyoung,Kim, Ji-Yeon,Park, Sang-Hoon,Youn, Sung-Kie,Lee, Jun-Ho 한국광학회 2006 한국광학회지 Vol.17 No.5

Shack-Hartmann 파면측정 센서는 적응광학 분야에서 파면측정용으로 가장 널리 사용되고 있다. Shack-Hartmann 센서에서 파면측정 알고리즘은 크게 점영상의 중심점 탐색 알고리즘과 파면복원 알고리즘으로 나눌 수 있다. 이 중 점영상의 중심점 탐색은 파면측정 결과에 크게 영향을 미치나 많은 연구결과에도 불구하고 최적의 중심점 측정 알고리즘 및 내부 변수 설정에 대한 일반적인 해결책은 아직 제시되지 못하고 있다. 현재, 중심점 탐색 알고리즘으로는 무게중심법(center of gravity)이 가장 널리 사용되고 있다. 본 연구에서는 중심점 탐색 알고리즘에 의한 측정 정확도 분석을 위해 무게중심법과 상관관계법(correlation)을 함께 고려하였다. 이를 위해 Shack-Hartmann 센서의 파면측정 과정을 전산모사하였고, 이로부터 중심점 탐색 알고리즘과 관련하여 무게중심법 및 상관관계법에서의 노이즈 특성을 비교, 분석하였다. 또한, 무게중심법에서 문턱값(threshold value)의 설정에 따른 파면측정의 정확도에 대한 분석을 통하여 노이즈 제거를 위한 최적의 문턱값을 제안하였다. 이와 더불어 Shack-Hartmann 센서 시스템을 구성하여 디포커스(defocus)를 발생시키고 이를 측정하는 실험을 수행함으로써 파면측정 알고리즘 및 전산모사 결과를 검증하였다. The SharkHartmann wavefront sensors are the most popular devices to measure wavefront in the field of adaptive optics. The Shack-Hartmann sensors measure the centroids of spot irradiance distribution formed by each corresponding micro-lens. The centroids are linearly proportional to the local mean slopes of the wavefront defined within the corresponding sub-aperture. The wavefront is then reconstructed from the evaluated local mean slopes. The uncertainty of the Shack-Hartmann sensor is caused by various factors including the detector noise, the limited size of the detector, the magnitude and profile of spot irradiance distribution, etc. This paper investigates the noise propagation in two major centroid evaluation algorithms through computer simulation; 1st order moments of the irradiance algorithms i.e. center of gravity algorithm, and correlation algorithm. First, the center of gravity algorithm is shown to have relatively large dependence on the magnitudes of noises and the shape & size of irradiance sidelobes, whose effects are also shown to be minimized by optimal thresholding. Second, the correlation algorithm is shown to be robust over those effects, while its measurement accuracy is vulnerable to the size variation of the reference spot. The investigation is finally confirmed by experimental measurements of defocus wavefront aberrations using a Shack-Hartmann sensor using those two algorithms.

대구경 경량 주반사경과 마운트 설계에 관한 연구

김지연(Ji-Yeon Kim),서유덕(Yu-Deok Seo),박상훈(Sang-Hoon Park),엄태경(Tae-Kyoung Uhm),김현중(Hyun-Jung Kim),윤성기(Sung-Kie Youn),이상률(Sang-Ryool Lee),이승훈(Seung-Hoon Lee),이덕규(Deog-Gyu Lee),이응식(Eung-Shik Lee),장홍술(Hong-Su 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6

In this paper, a study on the design of the primary mirror for satellite camera and its mounts is presented. In the design of the mirror, reducing its weight as well as improving optical performance is crucial design requirements. For high optical performance with light-weight, partially open back type and hexagonal cell pattern are adopted. Height of mirror, cell diameter and wall thickness are chosen as design variables. In order to obtain optimal design variables, a parametric study is carried out. Also, Optical performance of primary mirror is highly dependent on the mountdesign and severely deteriorated due to over-constraining by the mounts. Therefore, design of the mounts is carried out for high optical performance. Finally, the performance of the mirror system is investigated by numerical analysis of the system. In the analysis, the mechanical properties of the adhesives which are used to attach the mirror to the mounts are considered.

우주용 접착제의 중첩 전단 강도 시험

서유덕(Yu-Deok Seo),김현중(Hyun-Jung Kim),박상훈(Sang-Hoon Park),김지연(Ji-Yeon Kim),엄태경(Tae-Kyoung Uhm),윤성기(Sung-Kie Youn),이상률(Sang-Ryool Lee),이승훈(Seung-Hoon Lee),이덕규(Deog-Gyu Lee),이응식(Eung-Shik Lee),장홍술(Hong-Su 한국항공우주학회 2006 韓國航空宇宙學會誌 Vol.34 No.10

반사경 지지부의 접착 특성은 인공위성카메라 반사경의 광학적 성능에 지대한 영향을 미치므로 이에 대한 연구가 필수적이다. 이를 위해 본 연구에서는 세 종류의 우주용 접착제의 접착 특성에 관한 연구를 수행하였다. 반사경의 재료인 제로더(Zerodur)와 금속이 접착된 시편을 사용하여 중첩 전단(lap shear) 시험을 수행하였고 금속과 금속이 접착된 시편에 대해서도 시험을 수행하였다. 또한 다양한 온도 환경을 겪더라도 안정적인 접착력이 보장되는지의 여부를 확인하였으며 세 접착제의 접착력을 비교하였다. Optical performance of the mirror for satellite camera is highly dependent on the adhesive properties of its support. Therefore, in this paper, the adhesive properties of three kinds of space adhesives are investigated. For this purpose, the lap shear test is performed with Zerodur-to-metal lap shear specimen as well as metal-to-metal one. And it is also investigated whether reliable adhesive strength can be secured after the specific thermal cycle. Finally, the properties of three adhesives are compared with each other.

우주용 접착제의 기계적 특성에 관한 연구

서유덕(Yu-Deok Seo),김현중(Hyun-Jung Kim),박상훈(Sang-Hoon Park),김지연(Ji-Yeon Kim),엄태경(Tae-Kyoung Uhm),윤성기(Sung-Kie Youn),이상률(Sang-Ryool Lee),이승훈(Seung-Hoon Lee),이덕규(Deog-Gyu Lee),이응식(Eung-Shik Lee),장홍술(Hong-Su 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6

Optical performance of mirror for satellite camera is highly dependent on the adhesive properties of its support. Therefore, in order to design the mirror with high optical performance, the mechanical properties of adhesives should be well defined. In this paper, the mechanical properties of three kinds of space adhesives are studied. First, the elastic moduli of the adhesives are investigated. Tension tests of the adhesives are performed at some chosen temperatures within the prescribed temperature range. From the tension test results, the elastic moduli of the adhesives are expressed as a function of temperature and the sensitivity with respect to temperature are discussed. Secondly, the adhesive strengths of the adhesives are investigated by the lap shear test. The lap shear tests are performed with Zerodur-to-metal lap shear specimen as well as metal-to-metal one. And it is also investigated whether reliable adhesive strength can be secured after the specific thermal cycle.

우주용 접착제의 탄성계수 측정 및 특성 규명에 관한 연구

김현중(Hyun-Jung Kim),서유덕(Yu-Deok Seo),박상훈(Sang-Hoon Park),김지연(Ji-Yeon Kim),엄태경(Tae-Kyoung Uhm),윤성기(Sung-Kie Youn),이상률(Sang-Ryool Lee),이승훈(Seung-Hoon Lee),이덕규(Deog-Gyu Lee),이응식(Eung-Shik Lee),장홍술(Hong-Su 한국항공우주학회 2007 韓國航空宇宙學會誌 Vol.35 No.2

인공위성 카메라 주반사경의 광학적 성능은 반사경과 반사경 지지부의 접착특성에 크게 영향을 받는다. 따라서 고성능의 주반사경을 설계하기 위해서는 접착제의 기계적 특성규명이 필수적이다. 본 연구에서는 3종의 접착제를 선별하여 이에 대한 기계적 특성을 규명한다. 접착제의 탄성계수는 인장 시험을 통해 얻을 수 있다. 비선형 거동을 보이는 접착제의 경우 지수함수 형태로 곡선맞춤을 수행하여 응력-변형률 관계를 얻고, 선형거동을 보이는 접착제의 경우 선형 맞춤을 통해 탄성계수를 얻는다. 접착제의 작동 온도 범위내 8개 온도점을 선택하여 시험을 수행한다. 시험을 통해 탄성계수를 온도에 따른 함수로 표현하고, 이를 바탕으로 접착제의 특성에 대해 논의 한다. The optical performance of the mirror for 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 paper, the mechanical properties of three kinds of space adhesives are studied. Elastic moduli of the adhesives are determined by tensile tests. Stress-strain relation is obtained by using exponential curve fitting for the adhesive which shows non-linear behavior. In case of the linear behavior material, elastic modulus is calculated through linear curve fitting. For the tensile tests, several points have been selected in the operating temperature range of the adhesive. The elastic modulus of each adhesive is expressed as a function of temperature. Characteristics of the adhesives are discussed regarding their temperature sensitivity.