In this thesis, we examine the Linear DC Motor in a configuration such that it provides the forces necessary to damp vibrations in a large flexible structure. The design is broken down into three steps where in each step, a feedback loop is placed around the actuator and/or the structure. The first loop is a motor compensation loop which effectively decouples the motor model from the structure model by removing the effect of the velocity of the structure on the motor's performance. The second loop stabilizes the relative position response of the combined actuator/structure model. This loop also shapes the magnitude response of the system, thus determining the bandwidth of the actuator. Two designs are developed: a narrow bandwidth design and a wide bandwidth design. The third loop is the vibration suppression design loop and can be designed a number of ways. In this thesis, we develop two decentralized designs and a centralized design. The final system is simulated to check design results. The various nonlinearities of the proof mass actuator are considered and their effect on results noted. These nonlinearites, the stroke and current limits, determine the effectiveness of each vibration suppression design. The linear model is checked for robustness to parameter uncertainty. Results for the various designs are tabulated and discussed. / M.S.

The work that is reported herein deals with system identification methods for large flexible structures. Proposed space missions for the future include the deployment of large flexible structures, e.g., NASA's proposed space station. These structures must be controlled to maneuver the structure to desired locations and to suppress unwanted vibration. Before controlling any structure, it is necessary to have an accurate model which may include accurate estimates of the structure's natural frequencies and mode shapes. System identification is an important process that precludes system control. Precision structures such as those proposed for the Space Based Laser or the Aerospace Plane require high performance control systems which will require robust, computationally efficient system identification algorithms. This work attempts to experimentally verify, develop, and compare existing identification algorithms to determine their properties and improve their efficiency towards potential applicability in a space environment. To this end, we consider the Temporal Correlation Method and the Eigensystem Realization Algorithm. The algorithms are implemented on the Astronautics Laboratory Grid structure, and the results of the algorithms are compared in the presence of damping, noise, and residual modes. In addition, the Temporal Correlation Method is shown to be a constrained version of the Eigensystem Realization Algorithm for cases of light damping. / Master of Science

The purpose of this study is to model and analyze the nonplanar shear wall assemblies of shear wall-frame structures. Two three dimensional models, for open and closed section shear wall assemblies, are developed. These models are based on conventional wide column analogy, in which a planar shear wall is replaced by an idealized frame structure consisting of a column and rigid beams located at floor levels. The rigid diaphragm floor assumption, which is widely used in the analysis of multistorey building structures, is also taken into consideration. The connections of the rigid beams are released against torsion in the model proposed for open section shear walls. For modelling closed section shear walls, in addition to this the torsional stiffness of the wide columns are adjusted by using a series of equations. Several shear wall-frame systems having different shapes of nonplanar shear wall assemblies are analyzed by static lateral load, response spectrum and time history methods where the proposed methods are used. The results of these analyses are compared with the results obtained by using common shear wall modelling techniques.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 69-72). Also available via the Internet.

Neste trabalho são descritas análises estática e dinâmica em regime elástico de pórticos espaciais com incerteza no carregamento utilizando o método de elementos finitos (MEF). A abordagem do problema dinâmico estrutural de vibrações forçadas será feita dentro do regime elástico linear com amortecimento viscoso. O pórtico é abordado com teoria de viga fina onde são considerados os acoplamentos devido às forças normais e a torção sobre o modelo de flexão em viga fina de Bernoulli-Euler. A formulação forte do problema dinâmico da viga com carga axial é obtida a partir das equações de equilíbrio de Euler-Lagrange, decorrentes do princípio variacional de Hamilton e, a formulação forte do problema dinâmico do eixo longo sujeito a torção é determinada utilizando o modelo de torção de Saint-Venan. Para a determinação da formulação fraca para o elemento de viga tridimensional foi aplicado o método dos resíduos ponderados Galerkin. Também é descrita neste trabalho, a análise da confiabilidade estrutural considerando a incerteza no carregamento e, em algumas propriedades mecânicas dos materiais, utilizando o método de Monte Carlo (MMC). A série de Neumann foi utilizada como alternativa para reduzir o tempo de processamento do problema dinâmico. A análise simultânea das diversas variáveis foi abordada utilizando a estatística multivariada. Os resultados da analise estática e de vibrações livres dos exemplos numéricos são apresentados com o intuito de validar os métodos contidos neste trabalho comparando-os com resultados obtidos utilizando um software comercial de análise estrutural. / Submitted by Ana Guimarães Pereira (agpereir@ucs.br) on 2015-11-23T16:10:16Z No. of bitstreams: 1 Dissertacao Samuel Cornelli.pdf: 2680799 bytes, checksum: bcb4daa1a00ba8c03aafdea11fa42778 (MD5) / Made available in DSpace on 2015-11-23T16:10:16Z (GMT). No. of bitstreams: 1 Dissertacao Samuel Cornelli.pdf: 2680799 bytes, checksum: bcb4daa1a00ba8c03aafdea11fa42778 (MD5) / In this thesis, static and dynamic analysis of elastic space frames with regime uncertainty in loading using the finite element method (FEM) are described. A structural approach to the dynamic problem of forced vibrations will be made within the linear elastic regime with viscous damping. The structure is approached with thin beam theory which the couplings are considered normal forces due to twisting and bending about the model thin Bernoulli-Euler beam. The strong formulation of the dynamic problem of the beam with axial load is obtained from the equilibrium equations of Euler-Lagrange equations arising from the variational principle of Hamilton and the strong formulation of the dynamic problem of the long shaft under torsion is determined using the model of twist of Saint-Venan. To determine the weak formulation for three-dimensional beam element applies the Galerkin method of weighted residues. Is also described in this study, the analysis of structural reliability considering the uncertainty in loading and some mechanical properties of materials using the Monte Carlo method (MMC). Neumann series will be used as an alternative to reduce the processing time of the dynamic problem. The simultaneous analysis of several variables is addressed using multivariate statistics. Some results of numerical examples are presented in order to validate the methods contained in this work compared with the results obtained from structural analysis of commercial software.

Neste trabalho são descritas análises estática e dinâmica em regime elástico de pórticos espaciais com incerteza no carregamento utilizando o método de elementos finitos (MEF). A abordagem do problema dinâmico estrutural de vibrações forçadas será feita dentro do regime elástico linear com amortecimento viscoso. O pórtico é abordado com teoria de viga fina onde são considerados os acoplamentos devido às forças normais e a torção sobre o modelo de flexão em viga fina de Bernoulli-Euler. A formulação forte do problema dinâmico da viga com carga axial é obtida a partir das equações de equilíbrio de Euler-Lagrange, decorrentes do princípio variacional de Hamilton e, a formulação forte do problema dinâmico do eixo longo sujeito a torção é determinada utilizando o modelo de torção de Saint-Venan. Para a determinação da formulação fraca para o elemento de viga tridimensional foi aplicado o método dos resíduos ponderados Galerkin. Também é descrita neste trabalho, a análise da confiabilidade estrutural considerando a incerteza no carregamento e, em algumas propriedades mecânicas dos materiais, utilizando o método de Monte Carlo (MMC). A série de Neumann foi utilizada como alternativa para reduzir o tempo de processamento do problema dinâmico. A análise simultânea das diversas variáveis foi abordada utilizando a estatística multivariada. Os resultados da analise estática e de vibrações livres dos exemplos numéricos são apresentados com o intuito de validar os métodos contidos neste trabalho comparando-os com resultados obtidos utilizando um software comercial de análise estrutural. / In this thesis, static and dynamic analysis of elastic space frames with regime uncertainty in loading using the finite element method (FEM) are described. A structural approach to the dynamic problem of forced vibrations will be made within the linear elastic regime with viscous damping. The structure is approached with thin beam theory which the couplings are considered normal forces due to twisting and bending about the model thin Bernoulli-Euler beam. The strong formulation of the dynamic problem of the beam with axial load is obtained from the equilibrium equations of Euler-Lagrange equations arising from the variational principle of Hamilton and the strong formulation of the dynamic problem of the long shaft under torsion is determined using the model of twist of Saint-Venan. To determine the weak formulation for three-dimensional beam element applies the Galerkin method of weighted residues. Is also described in this study, the analysis of structural reliability considering the uncertainty in loading and some mechanical properties of materials using the Monte Carlo method (MMC). Neumann series will be used as an alternative to reduce the processing time of the dynamic problem. The simultaneous analysis of several variables is addressed using multivariate statistics. Some results of numerical examples are presented in order to validate the methods contained in this work compared with the results obtained from structural analysis of commercial software.

Surface errors on parabolic reflector antennas degrade the overall performance of the antenna. They cause amplitude and phase errors in the aperture field which lower the gain, raise the side lobes, and fill in the nulls. These are major problems in large ->space reflector antenna systems. F or example, future multiple beam antenna systems requiring spatial isolation to allow frequency reuse could be rendered useless if high side lobes are present. Space antenna structures are difficult to build. They must maintain a nearly perfect parabolic shape in a harsh environment while remaining lightweight. The restrictions on the structure become more severe as science and technology requirements demand electrically large antennas. Mechanically, there are technologies [4)r building antennas with adaptive surfaces that can compensate for many of the larger distortions caused by thermal and gravitational forces. However, as the frequency and size of the reflectors increase, the subtle surface errors become significant and degrade the overall radiation pattern. It is for this reason that another method must be used to further improve the radiation pattern. Electromagnetic compensation for surface errors in large apace reflector antennas has been the topic of several research studies. Most of these studies try to correct the focal plane fields of the reflector near the radiation pattern. The compensation is implemented by weighting the elements of an array feed. In most of the studies, a precise knowledge of the reflector surface is required. An alternative approach to electromagnetic compensation is presented in this study. The proposed technique uses pattern synthesis to compensate for the surface errors. It differs from previous methods in two major respects. The previous studies used global algorithms that try to correct the entire focal plane field near the focal point or the aperture plane field and, hence, modify the entire radiation pattern. The pattern synthesis approach uses a localized algorithm in which pattern corrections are directed specifically towards portions of the pattern requiring improvement. The second major difference is that the pattern synthesis technique does not require knowledge of the reflector surface, but instead uses radiation pattern data to perform the compensation. / Ph. D.

Recent demands in large space structure technology have suggested the use of active control actuators integral to a structures' construction. The concept of a 3-D (triangular cross-sectioned) active truss is presented. The linear equations of motion for one plane of the truss are derived. A model for a generic flexible beam is then appended to the planar truss model. A linear time-invariant optimal control law is found, followed by a presentation of an experimental planar truss built to test the concept. Physical parameters are then substituted into the dynamic model and several sets of control gains are found. The "Kalman'' gains are applied to the experimental structure. Experimental results are compared to expected theoretical results with good (30%) correlation. Conclusions are drawn and suggestions are made for further research. / Master of Science

In order to provide a realistic and challenging experimental test for active vibration control concepts applicable to large space structures, an experimental model is required that simulates the complicated dynamic characteristics of such structures. This study presents the design, theoretical analysis, and experimental testing of such a model - a large, flexible plane grillage, with an adjustable skew angle, free to rotate on knife edges. The plane grillage model was shown, by theory and experiment, to have high modal density at low frequencies (twelve modes below 11 Hz). It was also demonstrated, by analogy with published results for a cantilevered skew plate, that the model would have a pair of closely spaced modes, with distinct mode shapes, at a particular skew angle. By using an ana1ogy with a simple rigid bar model, the pendulum mode of the plane grillage was shown to have a frequency that could be driven towards zero, thereby simulating a rigid body mode. The theoretical analysis was conducted, for one skew angle, using MSC/NASTRAN, and included the effect of gravity. Experimental tests were conducted on the model, with the same skew angle, using frequency and transient response techniques. The theoretical and experimental results were compared, with good quantitative agreement for the natural frequencies (first ten modes within 10%), and reasonable qualitative agreement for the lower mode shapes. / M.S.

Thesis (MScEng (Civil Engineering))--Stellenbosch University, 2008. / Doubly symmetric I-section columns are often utilised in portal frame construction. The sheeting (or cladding) is carried by sheeting rails connected to the outer flange of these columns. Although it is common practice to include the sheeting rails in the longitudinal bracing system, by connecting the sheeting rail to the cross-bracing, designers must be wary because the connection between column and sheeting rail will not prevent twisting of the columns cross-section. It has been shown ([11], [12], [17]), that by including this eccentric restraint into the bracing of the column, that a torsional-flexural buckling mode of failure can occur when the column is subjected to axial load only. It was seen that this phenomenon is provided for in BS 5950 [18], but is not present in many other design codes of practice, in spite of this phenomenon being relatively well known. In some cases the compression resistance of a column can be significantly reduced when compared to that of a flexural buckled configuration. Previous work performed by Helwig and Yura [15] proposed specific column to sheeting rail connections which would allow for the sheeting rails to be used as elastic torsional braces and effectively rigid lateral braces. However, it is the objective of this investigation to determine if it is possible to include the eccentric sheeting rails into the bracing system, even when using a relatively simple cleat connection with only two bolts onto the sheeting rail. The objective of the research was investigated by conducting experimental tests coupled with a series of detailed finite element analyses. The purpose of the experimental set-up was to investigate the behaviour of a column laterally supported on one flange by a continuous sheeting rail and to compare it to the behaviour of a column laterally supported on both flanges by means of fly-braces (“kneebraces”). The behaviour of the columns, as determined by the experimental tests, was validated by the finite element analyses. The evident conclusion that can be drawn is that, for the case of a continuous sheeting rail, connected to column simply by two bolts and a cleat, that sufficient torsional restraint is provided to the column to prevent torsional-flexural buckling from being critical. This result is helpful, as it means that the buckling capacity of a column can be increased four-fold by enforcing the second flexural buckling mode instead of the first mode through utilising a continuous sheeting rail connected to a cross-bracing system as longitudinal bracing on the columns. This can be achieved without the need to provide any specific detailing to the column to sheeting rail connection. It is however, recommended that further experimental work be conducted on varying lengths of column in order to further validate the results of this work.