MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1980. / Includes bibliographical references. / by Jeffrey Hastings Lang. / Ph.D.

No Brasil é comum a utilização de barras tubulares com extremidades estampadas na confecção das treliças metálicas espaciais, uma vez que representam uma significativa redução de custos, quando comparado a sistemas mais elaborados, devido a sua simplicidade de fabricação e montagem, entretanto trata-se aparentemente de um sistema precário do ponto de vista estrutural, merecendo estudos que estabeleçam critérios para uma utilização segura. Neste trabalho são apresentadas análises teóricas e experimentais destas estruturas, onde foi analisada a situação construtiva usual de treliça com nós típicos (extremidade das barras estampadas, sobrepostas e unidas por um único parafuso), e com nós de aço (sistema de conexão formado por uma peça com aletas de aço soldadas). Os resultados experimentais foram obtidos em ensaios de quatro protótipos que simulam um trecho de treliça espacial, constituídos por uma malha quadrada sobre quadrada, composta de nove módulos piramidais de 2,5 x 2,5 x 1,5 (m), totalizando uma estrutura com dimensões iguais a 7,5 x 7,5 x 1,5 (m), apoiada por colunas nos vértices, permitindo comparar a rigidez, o modo de falha e a resistência entre estes protótipos. A análise teórica dos protótipos foi feita admitindo-se os casos de linearidade e também de não linearidade física e geométrica, levando-se em consideração a variação de inércia das barras junto aos nós. / In Brazil the use of tubular bars with stamping extremities is common in space steel structures, as it means a significant reduction of cost, when caompared with more elaborete system, due to its simplicity of manufacture and assembly. However it is apparently an uncertain system from a structural point of view, which requires studies establishing safe utilization criteria. This paper presents theorical and experimental analysis of this structures. The usual constructive situation of space structure with \'nós típicos\' (bars with flattened ends, superposed and connected by a unique bolt), and \'nós de aço\' (connection system formed by a piece with steel fin welded) was studied. The experimental results were obtained in tests of four prototypes which simulated a section of space structure composed of a mesh squareon- square, with nine 2.5 x 2.5 x 1.5 (m) pyramidal modules, totaliing a structure with 7.5 x 7.5 x 1.5 (m) dimensions, supported by columns in their vertices, permiting to compare the stiffness, the failure manner and the resistance of this prototypes. The theorical analysis of prototypes was carried out by assuming the cases of linearity and physical and geometric non-linearity, taking into consideration the variation of stiffness of the bars next to the nodes.

Made available in DSpace on 2016-04-16T00:15:22Z (GMT). No. of bitstreams: 7 Andre Luiz Ribeiro1.pdf: 2358539 bytes, checksum: 96b1600fbc289e03678f8980b8a90e1f (MD5) Andre Luiz Ribeiro2.pdf: 2257374 bytes, checksum: ec3d0b885fe685d218d41626070c7b77 (MD5) Andre Luiz Ribeiro3.pdf: 2676248 bytes, checksum: e640f67173bf7beed608261d65377cd8 (MD5) Andre Luiz Ribeiro4.pdf: 1669145 bytes, checksum: 0d233fd7c37ca8df5b6114d3752e4f2d (MD5) Andre Luiz Ribeiro5.pdf: 3180282 bytes, checksum: 3865a968b4f37d30ab444b820362e76d (MD5) Andre Luiz Ribeiro6.pdf: 1438158 bytes, checksum: f1c2cc81f218be303f39012ee96ef593 (MD5) Andre Luiz Ribeiro7.pdf: 2533571 bytes, checksum: 012d63a228d283086ec6eecc361dcce1 (MD5) Previous issue date: 2009-02-17 / Fundo Mackenzie de Pesquisa / The university campus is a space that has great importance in the development of the society. The formation of the knowledge in the university space is intrinsically related to all the set of elements in it: from the physical structure, to the knowledge and the people. The well understanding of what it means the physical structure of the campus will contribute strongly for this formation. The structures that compose the university campus and its own evolution passed through historical process analyses which are instruments of the process comprehension and relevance from its contribution in the society formation. This research analyzes campi from its characteristics applied in the monachal structures yet scholastic, the formation of the first universities and colleges in the Occidental Europe. Then, in the research s development, it slowly evaluates the campi of University of Virginia made by Jefferson, the Illinois Institute of Technology made by Mies van der Rohe and finally the Architecture s College of Porto in Porto University made by Alvaro Siza. The understanding of these structures will be certainty important in the project of new campi to be produced in XXI century. / O Campus universitário é um espaço que tem grande importância no desenvolvimento da sociedade. A formação do conhecimento no espaço universitário está intrinsecamente relacionada com todo o conjunto de elementos presentes nele: da estrutura física, ao conhecimento e as pessoas. O bom entendimento do que significa a estrutura física do campus contribuirá fortemente para esta formação. A análise do processo histórico pelo qual passam as estruturas espaciais que geram o campus universitário e a própria evolução do campus como tal são instrumentos de compreensão do processo e da relevância de sua contribuição na formação da sociedade. Este trabalho analisa os campi desde suas características presentes nas estruturas espaciais monacais ainda escolásticas, a formação das primeiras universidades e colleges na Europa Ocidental. Na seqüência avalia detidamente os campi da Universidade de Virginia de Jefferson, o Instituto Illinois de Tecnologia de Mies van der Rohe e finalmente a Faculdade de Arquitetura do Porto na Universidade do Porto de Álvaro Siza. A compreensão destas estruturas espaciais com certeza será importante no projeto dos novos campi a serem produzidos no século XXI.

The emergence of freeform architecture provides interesting geometric challenges with regards to the design and manufacturing of large-scale structures. To design these architectural structures, we have to consider two types of constraints. First, aesthetic constraints are important because the buildings have to be visually impressive. Sec- ond, functional constraints are important for the performance of a building and its e cient construction. This thesis contributes to the area of architectural geometry. Specifically, we are interested in the geometric rationalization of freeform architec- ture with the goal of combining aesthetic and functional constraints and construction requirements. Aesthetic requirements typically come from designers and architects. To obtain visually pleasing structures, they favor smoothness of the building shape, but also smoothness of the visible patterns on the surface. Functional requirements typically come from the engineers involved in the construction process. For exam- ple, covering freeform structures using planar panels is much cheaper than using non-planar ones. Further, constructed buildings have to be stable and should not collapse. In this thesis, we explore the geometric rationalization of freeform archi- tecture using four specific example problems inspired by real life applications. We achieve our results by developing optimization algorithms and a theoretical study of the underlying geometrical structure of the problems. The four example problems are the following: (1) The design of shading and lighting systems which are torsion-free structures with planar beams based on quad meshes. They satisfy the functionality requirements of preventing light from going inside a building as shad- ing systems or reflecting light into a building as lighting systems. (2) The Design of freeform honeycomb structures that are constructed based on hex-dominant meshes with a planar beam mounted along each edge. The beams intersect without torsion at each node and create identical angles between any two neighbors. (3) The design of polyhedral patterns on freeform surfaces, which are aesthetic designs created by planar panels. (4) The design of space frame structures that are statically-sound and material-e cient structures constructed by connected beams. Rationalization of cross sections of beams aims at minimizing production cost and ensuring force equilibrium as a functional constraint.

This dissertation is concerned with developing new time-optimal control techniques for higher-order linear and weakly nonlinear systems. As an application, we consider the simultaneous slewing and vibration suppression of a flexible beam, possibly with a tip mass. This application arises in the design of large space structures and flexible lightweight and accurate robotic arms. The solution of the soft-constrained time-optimal control problem is expressed in terms of the controllability Grammian. The properties of the open-loop solution are studied. A closed-loop control algorithm, which takes into account the mUltiplicity of extremal solutions, is then developed. The algorithm is based on the concept of continuation and reduces the computational complexity by as much as two orders of magnitude when compared to the brute-force approach. The amplitude of the soft-constrained time-optimal control is found to saturate as the state norm becomes large, thus suggesting a simpler but suboptimal feedback implementation. We develop and discuss the concept of saturating controls for linear systems, and we develop a design approach that generates a family of saturating control laws in which the speed of the response and amount of available control action can be explicitly traded off. The soft-constrained time-optimal cheap-control problem is formulated and solved using singular-perturbation theory. The solution procedures are illustrated with an example solved using the MACSYMA symbolic manipulation language. Regular-perturbation theory is then used to find the open-loop hard-constrained time-optimal control for a class of weakly nonlinear systems. The control is found by solving a nonlinear two-point boundary value problem (TPBVP) characterizing the control of the linearized system, and a second linear TPBVP. / Ph. D.

The digital transformation of engineering design processes is essential for the aerospace industry to remain competitive in the global market. Multidisciplinary design optimization (MDO) frameworks play a crucial role in this transformation by integrating various engineering disciplines and enabling the optimization of complex spacecraft structures. Since the design team consists of multiple entities from different domains working together to build the final product, the design and analysis tools must be readily available and compatible. An integrated approach is required to handle the problem's complexity efficiently. Additionally, most aerospace structures are made from composite panels. It is challenging to optimize such panels as they require the satisfaction of constraints where the design ply thicknesses and orientations can only take discrete values prescribed by the manufacturers. Heuristics such as particle swarm or genetic algorithms are inefficient because they provide sub-optimal solutions when the number of design variables is large. They also are computationally expensive in handling the combinatorial nature of the problem. To overcome these challenges, this work proposes a two-fold solution that integrates multiple disciplines and efficiently optimizes composite spacecraft structures by building a rapid design framework. The proposed model-based design framework for spacecraft structures integrates commercially available software from Siemens packages such as NX and HEEDS and open-source Python libraries. The framework can handle multiple objectives, constraint non-linearities, and discrete design variables efficiently using a combination of black-box global optimization algorithms and Mixed Integer Programming (MIP)-based optimization techniques developed in this work. Lamination parameters and MIP are adopted to optimize composite panels efficiently. The framework integrates structural, thermal and acoustic analysis to optimize the spacecraft's overall performance while satisfying multiple design constraints. Its capabilities are demonstrated in optimizing a small spacecraft structure for required structural performance under various static and dynamic loading conditions when the spacecraft is inside the launch vehicle or operating in orbit. / Doctor of Philosophy / The design of new spacecraft takes several years and requires significant resources. The primary design objective is to minimize spacecraft mass/cost while satisfying the mission requirements. This is done by altering the structure's geometric and material properties. Most spacecraft panels are made from composite materials where the orientations of fiber paths and the thickness of the panel determine its strength and stiffness. Finding the best values for these parameters cannot be done efficiently using existing optimization algorithms, as several combinations of orientations can give a similar performance which can be subpar. In this dissertation, mathematical programming is adopted for fast evaluation of optimum panel properties, thereby saving a significant amount of resources compared to conventional techniques. Moreover, the requirements that govern the design process are handled one at a time in an organization. This leads to discrepancies in the various teams' designs that satisfy all requirements. A framework is built to integrate all requirements to account for their conflicting nature and quickly give the best possible spacecraft structural design configuration.

This dissertation is concerned with some topics in the modeling and control of large flexible structures. In the finite element convergence toward the natural modes and frequencies of a structure, it is found that two mechanisms limiting the accuracy of higher modes are, first, a decrease in the number of active degrees of freedom for higher mode approximations due to orthogonality constraints, and, second, the fact that lower computed, rather than actual, eigenfunctions appear in the orthogonality constraints, so that inaccuracy in lower modes inhibits convergence to higher modes. Refining the elements using the hierarchical p-version proves to be far superior to refining the mesh, as demonstrated by numerical examples. In the third chapter, a method is presented for solving the algebraic eigenvalue problem for a structure, which combines attractive features of the subspace iteration method and the component-mode synthesis methods. Reduced substructure models are generated automatically and coupled exactly to form a reduced structure model, whose eigensolution is used to refine the substructure models. Convergence is much faster than in the subspace iteration method, as demonstrated by numerical examples. In the fourth chapter, the effectiveness of modal control (IMSC) and direct feedback control, in which the actuator force depends only on the local velocity and displacement, are investigated for suppressing traveling waves on a string and on a beam, both with slight material damping. Direct feedback proves superior for the string, as more modes must be controlled than can be handled by modal control with a limited number of actuators, but inferior for the beam, as effort is wasted suppressing motion in higher modes where damping is pervasive, while modal control focuses effort on those lower modes which need to be controlled. The optimal vibration control for a distributed system subjected to persistent excitation is not available, so a two-part control is proposed in chapter five for suppressing the motion of a distributed system with a moving support. The first part cancels the moving support's excitation to an optimal extent, and the second is a direct velocity feedback control. A numerical example demonstrates the effectiveness of this control method. / Ph. D. / incomplete_metadata

This study concerns the active damping of structural vibrations through the application of various forms of velocity feedback control. Active damping will be required for large space structures which are performance-sensitive to motion or inaccurate pointing. Several control forms, including modal-space active damping and direct rate feedback, are analyzed theoretically, and three laboratory models are described. A previous, unsuccessful attempt at control is reviewed and explained. The remaining control forms developed in the theoretical section were implemented successfully and the results compare favorably with theoretical predictions. Each control form is analyzed relative to its own merits and in comparison with other methods. An important point is the stability assured by a dual (colocated) configuration. of velocity sensors and control force actuators. Modal-space active damping is shown to be an effective control method with predictable performance in controlled modes and beneficial spillover into residual (non-controlled) modes. / Ph. D. / incomplete_metadata

Traditional large space structure construction incorporates the use of lightweight tubular metal alloys that have good strength to weight and stiffness to weight ratio. Recently, however, space structure construction has shifted focus on materials that are ultra lightweight, have high strength, have low package volume and possess excellent damping characteristics. Substantial damping is required in space since there is no surrounding medium to provide damping. Such a construction uses composites in a fabric form that displays viscoelastic behavior. The viscoelastic behavior is attributed to energy dissipation because of the shear stresses between the various fibrous strands that are kept in place by constraining viscoelastic layers. This type of vibration control falls under the rubric of passive damping of structures and has been found to have certain advantages over active damping such as less complexity as it does not require sensors, actuators and power supply that are needed for active damping. One such material consists of woven carbon strands constrained by layers of viscoelastic damping material. Dynamics and buckling behavior of a structure in the form of a tube made from this material with metallic end caps is modeled and analyzed using commercially available Finite Element Analysis code ABAQUS®. The current analysis deals with the non-pressurized tube since the structure can maintain the tubular configuration as well as support end caps on account of the stiffness provided by the composites. Since no simple analytical approaches are available to predict damping of these materials, experimental data was used to estimate the damping characteristics of the material. The mass of the end cap was also estimated from the experimental impulse response as exact mass of the end cap (that was rigidly fixed to the tube) was unknown. / Master of Science

The design of feedback control laws for discrete models of flexible structures is addressed. Two strategies are proposed. First, a parameter optimization method is used, in which the behavior of the controller is described by a performance measure and numerically optimized. A second method, based on continuation maps, allows several performance criterion to be met. These performance measures are quite general in nature; they may be functions of eigenvalues, eigenvectors, sensitivities, and other mission specific quantities. A model of a cantilevered flexible beam is developed and used to demonstrate capabilities and problems with the design methods. / M.S.