The problem of reorienting a constellation of spacecraft, such that the fuel distributed across the constellation is both conserved and expended uniformly, is considered. Results are derived for constellations with an...
详细信息
The problem of reorienting a constellation of spacecraft, such that the fuel distributed across the constellation is both conserved and expended uniformly, is considered. Results are derived for constellations with an arbitrary number of spacecraft, assuming that the constellation is in free space, that the spacecraft mass is time invariant, and that the thrusters can produce thrust in any direction. an open-loop control algorithm is derived by minimizing a cost function that trades off total fuel minimization and fuel equalization. The associated optimization problem is shown to be amenable to standard algorithms, simulation results using a four-spacecraft constellation are given.
The use of response surface estimation in collaborative optimization, an architecture for large-scale multidisciplinary design is described. Collaborative optimization preserves the autonomy of individual disciplines ...
详细信息
The use of response surface estimation in collaborative optimization, an architecture for large-scale multidisciplinary design is described. Collaborative optimization preserves the autonomy of individual disciplines while providing a mechanism for coordinating the overall design problem and progressing toward improved designs. Collaborative optimization is a two-level optimization architecture, with discipline-specific optimizations free to specify local designs, and a global optimization that ensures that all of the discipline designs eventually agree on a single value for those variables that are shared in common. Results demonstrate how response surface models of subproblem optimization results improve the performance of collaborative optimization. The utility of response surface estimation in collaborative optimization depends on the generation of inexpensive accurate response surface models and the refinement of these models over several fitting cycles. Special properties of the subproblem optimization formulation are exploited to reduce the number of required subproblem optimizations to develop a quadratic model from O(n(2)) to O(n/2). Response surface refinement is performed using ideas from trust region methods. Results for the combined approaches are compared through the design optimization of a tailless unmanned air vehicle in 44 design variables.
An optimization algorithm based on simulated annealing is proposed, in which the domain of the search is successively reduced based on a probability concept until the stopping criteria are satisfied. By introducing th...
详细信息
An optimization algorithm based on simulated annealing is proposed, in which the domain of the search is successively reduced based on a probability concept until the stopping criteria are satisfied. By introducing the ideas of probability cumulative distribution function and stable energy, the selection of initial temperature and equilibrium criterion in the process of simulated annealing becomes easy and effective. Numerical studies using a set of standard test functions and an example or a 10-bar truss show that the approach is effective and robust in solving both functional and structural optimization problems.
Global optimization algorithms are required for optimal control of complex systems such as plant growth and crop yield. Processes for global optimization, selfprogramming and the determination of accurate fitness (eva...
详细信息
Global optimization algorithms are required for optimal control of complex systems such as plant growth and crop yield. Processes for global optimization, selfprogramming and the determination of accurate fitness (evaluation) functions are indispensable to modeling in the strong AI and strong A-Life projects. The modeling of complex processes like consciousness and evolution will be applied towards the development of global optimization algorithms. To date several models of the microscopic mechanisms involved in brain function have been proposed that are based on quantum dynamics. However biological systems comprise a hierarchy from the microscopic to the macroscopic level. Here we employ both quantum dynamics and a systems theoretical approach based on techniques derived from the modeling of consciousness and evolution to propose a global optimization algorithm that can avoid locally optimal states of non-linear functions.
A new method for extracting accurate stress information from reduced-order structural and aeroelastic models is presented. The method has second-order accuracy when approximate reduced-order direct and adjoint solutio...
详细信息
A new method for extracting accurate stress information from reduced-order structural and aeroelastic models is presented. The method has second-order accuracy when approximate reduced-order direct and adjoint solutions (based on different reduced-order bases) are used simultaneously to obtain approximate stresses, The method is applicable to both static and dynamic linear analysis. A review of four common methods for structural model order reduction [two variants of the mode displacement method (standard mode displacement and the fictitious mass method), the mode acceleration method, and the Ritz vector method] identifies sources of difficulty and causes of errors in stress behavior sensitivity calculations. Considerations used for selection of the reduced-order direct and adjoint bases are discussed. A series of static and dynamic test cases is used to assess accuracy of the new method in an analysis mode. Accuracy studies of sensitivity calculations follow. We hope to contribute to the held of design-oriented structural dynamics in terms of both insight and practice.
作者：
Maghami, PGNASA
Langley Res Ctr Guidance & Control Branch Hampton VA 23681 USA
A novel approach for the refinement of finite-element-based analytical models elf flexible structures is presented. The proposed approach models the possible refinements in the mass, damping, and stiffness matrices of...
详细信息
A novel approach for the refinement of finite-element-based analytical models elf flexible structures is presented. The proposed approach models the possible refinements in the mass, damping, and stiffness matrices of the finite element model in the form of a constant gain feedback with acceleration, velocity, and displacement measurements, respectively. Once the free elements of the structural matrices have been defined, the problem of model refinement reduces to obtaining position, velocity, and acceleration gain matrices with appropriate sparsity that reassign a desired subset of the eigenvalues of the model, along with partial mode shapes, from their baseline values to those obtained from system identification test data. A sequential procedure is used to assign one conjugate pair of eigenvalues at each step using symmetric output feedback gain matrices, and the eigenvectors are partially assigned, while ensuring that the eigenvalues assigned in the previous steps are not disturbed. The procedure can also impose that gain matrices be dissipative to guarantee the stability of the refined model. A numerical example, involving finite element model refinement for a structural testbed at NASA Langley Research Center (Controls-Structures-Interaction Evolutionary model) is presented to demonstrate the feasibility of the proposed approach.
A Cartesian grid generator has been coded to obtain a Cartesian grid around an arbitrary geometry in 2D. The grid generation is accomplished by a novel technique employing a Beetle algorithm. The significant parameter...
详细信息
A Cartesian grid generator has been coded to obtain a Cartesian grid around an arbitrary geometry in 2D. The grid generation is accomplished by a novel technique employing a Beetle algorithm. The significant parameters in defining the movement of the Beetle and its capture of intersection points are ascertained and implemented within the code. The sample grid generated around several shapes demonstrates the code's versatility. (CSA)
The overall tensile and bending behavior of unidirectional composite elements with monotone and nonmonotone, possibly multivalued, constitutive laws for each constituent are studied within a nonsmooth mechanics framew...
详细信息
The overall tensile and bending behavior of unidirectional composite elements with monotone and nonmonotone, possibly multivalued, constitutive laws for each constituent are studied within a nonsmooth mechanics framework. A nondifferentiable and possibly nonconvex, caused by degradation effects, potential energy is formulated for the whole mechanical system. For the structural analysis problem the potential energy minimization problem is considered. The arising variational or hemivariational (in the case of nonmonotone laws) inequality problems are solved by appropriate nonconvex-nonsmooth computational mechanics techniques. Parametric numerical investigations of typical composite elements are presented, which shed light into the complex behavior of the composite structural elements. On the other hand, the arising overall laws can he used as phenomenological laws for structural analysis of large-scale structures incorporating the studied composite elements.
Given measurement data, a nominal model, and a linear fractional transformation uncertainty structure with an allowance on unknown but bounded exogenous disturbances, easily computable tests for the existence of a mod...
详细信息
Given measurement data, a nominal model, and a linear fractional transformation uncertainty structure with an allowance on unknown but bounded exogenous disturbances, easily computable tests for the existence of a model validating uncertainty. set are given. Under mild conditions, these tests are necessary and sufficient far the case of complex. nonrepeated, block-diagonal structure. For the more general case that includes repeated and/or real scalar uncertainties, the tests are only necessary but become sufficient if a collinearity condition is also satisfied. With the satisfaction of these tests, it is shown that a parameterization of all model validating sets of plant models is possible. The nea parameterization is used as a basis for a systematic way to construct or perform uncertainty tradeoff with model validating uncertainty sets that hare specific linear fractional transformation structure for use in robust control design and analysis. An illustrative example is given that includes a comparison of candidate model validating sets.
The possibilities of computer-assisted and automated accelerator mass spectrometry (AMS) measurements were explored. The goal of these efforts is to develop fully automated procedures for "routine" measureme...
详细信息
The possibilities of computer-assisted and automated accelerator mass spectrometry (AMS) measurements were explored. The goal of these efforts is to develop fully automated procedures for "routine" measurements at the Vienna Environmental Research Accelerator (VERA), a dedicated 3-MV Pelletron tandem AMS facility. As a new tool for automatic tuning of the ion optics we developed a multi-dimensional optimization algorithm robust to noise, which was applied for C-14 and Be-10. The actual isotope ratio measurements are performed in a fully automated fashion and do not require the presence of an operator. Incoming data are evaluated online and the results can be accessed via Internet. The system was used for C-14,Be-10,Al-26 and I-129 measurements. (C) 2000 Elsevier Science B.V. All rights reserved.
暂无评论