Design Of Large Space Structures Research Articles

Traveling wave modes of flexible structures with distributed gyros

With the development of space structures, flexible structures are widely studied. To achieve attitude and vibration control of flexible structures, flywheels or control moment gyros can be introduced. However, the critical challenge in studying gyro flexible structures is establishing a systematic framework to manage its physical properties and calculate the control moment precisely. This study aims to construct a theoretical framework for a two-dimensional flexible structure with distributed gyros using classical methods and explore the contribution of the gyro moment on the dynamic mode shapes. Both eigenvalue and traveling wave mode shape analysis are applied to investigate the dynamical behaviors of the system. It is found that dynamic modes with nodal lines showing whirling phenomena have arisen instead of the traditional modes featured with static nodal lines triggered by the gyro moment, and the dynamic mode shapes can be displayed by the direction and frequency of the mode nodal lines motion. The discovery and research of gyro flexible structure traveling wave modes can provide a solid base for the design of large space structures, improve their stability, and avoid harmful vibrations.

Control-oriented modelization of a satellite with large flexible appendages and use of worst-case analysis to verify robustness to model uncertainties of attitude control

The design of large space structures (LSS) requires the use of design and analysis tools that include different disciplines. For such a kind of spacecrafts it is in fact mandatory that mechanical design and guidance navigation and control (GNC) design are developed within a common framework. One of the key-points in the development of LSS is related to the dynamic phenomena. These phenomena usually lead to two different interpretations. The former one is related to the overall motion of the spacecraft, i.e., the motion of the centre of gravity and motion around the centre of gravity. The latter one is related to the local motion of the elastic elements that leads to oscillations. These oscillations have in turn a disturbing effect on the motion of the spacecraft. From an engineering perspective, the structural model of flexible spacecrafts is generally obtained via FEM involving thousands of degrees of freedom (DOFs). Many of them are not significant from the attitude control point of view. One of the procedures to reduce the structural DOFs is tied to the modal decomposition technique. In the present paper a technique to develop a control-oriented structural model will be proposed. Starting from a detailed FE model of the spacecraft and using a special modal condensation approach, a continuous model is defined. With this transformation the number of DOFs necessary to study the coupled elastic/rigid dynamic is reduced. The final dynamic model will be suitable for the control design implementation. In order to properly design a satellite controller, it is important to recall that the characteristic parameters of the satellite are uncertain. The effect that uncertainties have on control performance must be investigated. A possible solution is that, after the attitude controller is designed on the nominal model, a Verification and Validation (V&V) process is performed to guarantee a correct functionality under a large number of scenarios. The V&V process can be very lengthy and expensive: difficulty and cost do increase because of the overall system dimension that depends on the number of uncertainties. Uncertain parameters have to be parametrically investigated to determine robust performance of the control laws via gridding approaches. In particular in this paper we propose to consider two methods: (i) a conventional Monte Carlo analysis, and (ii) a worst-case analysis, i.e., an optimization process to find an estimation of the true worst-case behaviour. Both techniques allow to verify that the design is robust enough to meet the system performance specification in case of uncertainties.

Instrumentation and system identification of a typical school building in Istanbul

This study presents the findings of the structural health monitoring and the real time system identification of one of the first large scale building instrumentations in Turkey for earthquake safety. Within this context, a thorough review of steps in the instrumentation, monitoring is presented and seismic performance evaluation of structures using both nonlinear pushover and nonlinear dynamic time history analysis is carried out. The sensor locations are determined using the optimal sensor placement techniques used in NASA for on orbit modal identification of large space structures. System identification is carried out via the stochastic subspace technique. The results of the study show that under ambient vibrations, stocky buildings can be substantially stiffer than what is predicted by the finite element models due to the presence of a large number of partitioning walls. However, in a severe earthquake, it will not be safe to rely on this resistance due to the fact that once the partitioning walls crack, the bare frame contributes to the lateral stiffness of the building alone. Consequently, the periods obtained from system identification will be closer to those obtained from the FE analysis. A technique to control the validity of the damping is employed that checks the presence of phase difference in displacements of different stories obtained from band pass filtered records and it is confirmed that the proportional damping assumption is valid for this structure. Two different techniques are implemented for identifying the influence of the soil structure interaction. The first technique uses the transfer function between the roof and the basement in both directions. The second technique uses a pre-whitening filter on the data obtained from both the basement and the roof. Subsequently the impulse response function is computed from the scaled cross correlation between the input and the output. The overall results showed that the structure will satisfy the life safety performance level in a future earthquake but some soil structure interaction effects should be expected in the North South direction.

Nonlinear spectral dynamic analysis of guyed towers. Part I: Theory

One of the important parameters in the analysis and design of large space structures is to account for the dynamic forces induced by wind and the subsequent progressive collapse. During the past two decades, problems of extreme loading conditions have been considered by some authors, and special attention has been devoted to the collapse modes. Nonlinear time history dynamic analysis, involving the post-buckling characteristics of the space structures, may be analyzed using the conventional finite element method. However, to guarantee stability and accuracy of the solution, the number of elements used to model the structure may be very large. In this respect, the nonlinear spectral element method (SEM) is developed for analyzing dynamic problems in large space structures. Through use of a computer program, the proposed method has been used to obtain the nonlinear dynamic response of guyed power transmission towers under severe wind loading condition. Key words: nonlinear dynamic analysis, space structures, guyed towers, spectral element method.

Design of Large Space Structures with Imperfection Using ANN-Based Simulator

Based on the advanced computational plasticity and an artificial neural network (ANN) simulator, a new design strategy has been presented for large space structures with imperfections. Nonlinear system identification approach has also been greatly spread among the researchers and engineers in the past few years. The neural network simulators as a non-parametric system identification approach present a robust and efficient way to simulate the nonlinear behaviour of engineering systems. In the paper herein an artificial neural network (ANN) simulator, a general back error propagating perceptron, is use to simulate random imperfection for nonlinear dynamic analysis of large space structures. It is also desirable to search for a procedure for wind pressure calculation with accuracy and reliability. In this respect, attention is paid to the advanced computational fluid dynamics (CFD). The use of the advanced CFD analysis can help engineers to estimate the wind pressure for the design of large space structures with complex geometries. The characteristics of the new design method have been shown graphically using a full documented numerical example, which highlights the efficiency of the new simulation method. The purpose of this paper is to present a new design method, which takes into account the effects of imperfection on the resulting dynamic responses of large space structures under gravity, temperature and wind loadings.

Simulation of Impact Effects on Large Space Structures Using Advanced SE Method

One of the important parameters in analysis and design of large space structures is to account for the possible forces induced by flying object impact and the subsequent progressive collapse. Conventional finite element methods have been used successfully in linear and nonlinear dynamic analyses of large space structures. During the past three decades, problems of contact mechanics have also been considered by some authors, and special attention has been devoted to high speed impact. Impact wave propagation in space structures may be analyzed using the conventional finite element method. However, in order to guarantee stability and accuracy of the solution, the number of elements used to model the structure may be very large indeed; more precisely, accurate results can be obtained only after a substantial computational effort. In this respect, the spectral element method (SEM) is developed for analyzing the wave propagation problems in large space structures. Using a computer program, the proposed formulation has been used to derive the impact global dynamic responses of a large dome structure.

On-orbit system identification experiments on Engineering Test Satellite-VI

On-orbit system identification experiments were performed using the Engineering Test Satellite-VI (ETS-VI), launched in 1994. The purpose of the experiments is to check the validity of a mathematical model for ETS-VI which was previously obtained by ground experiments. Two kinds of system identification schemes are utilized. One is a traditional scheme based on polynomial black-box models such as the prediction error method, and the other is a subspace method based on state-space models. The purpose of this paper is to compare these system identification schemes using experimental data. It will be shown that the subspace method proves to be promising for the system identification of large space structures typified by ETS-VI.

On-Orbit Identification Experiments by Engineering Test Satellite-VI

Abstract On-orbit system identification experiments have been performed using Engineering Test Satellite-VI (ETS-VΙ) launched in 1994. The purpose of the experiments is to check the validity of a mathematical model for ETS-VΙ which was previously obtained by ground experiments. Two kinds of system identification algorithms are utilized. One is a prediction error method and the other is a subspace-based method.The purpose of this paper is to compare these system identification algorithms using experimental data. It will be shown that the subspace method is promising for system identification of large space structures.

Partitioned model reduction for large space structural control problem

A model reduction method for control system design of large space structures is presented. The control model of the structure is partitioned into several subdomains, and a generalized Craig-Bampton representation is derived. In considering the placement of actuators/sensors that may affect the selection of the candidate modes, the representative degrees of freedom (DOF) of the actuators/sensors are retained as physical DOF in the final coordinate vector. A reduced model for the controller design is obtained based on this representation. As an example, a space truss subject to four actuator forces is analyzed. Comparison with a conventional reduction method based on a complete model of the truss has been made via eigenpairs and dynamic responses. Good agreement is seen to be achieved, and improved accuracy is obtained with the proposed method. The importance of the actuator/sensor related DOF and the effectiveness of the proposed approach in dealing with the very localized DOF are investigated under the extreme condition where only these four actuator/sensor related DOF are considered as the dimension for the model reduction.

Effects of Noise on Sensor Placement for On-Orbit Modal Identification of Large Space Structures

A method of sensor placement for the purpose on-orbit modal identification and test-analysis correlation is presented. The method is an extension of the Effective Independence method presented in past work to include the effects of a general representation of measurement noise. Sensor noise can be distributed nonuniformly throughout the structure as well as correlated between sensors. The only restriction is that the corresponding noise covariance intensity matrix is positive definite. The technique presented offers a fast and efficient approach for reducing a relatively large initial candidate sensor location set to a much smaller optimum set which retains the linear independence of the target modes and maintains the determinant of the Fisher Information Matrix resulting in improved modal response estimates. The noise covariance intensity matrix which has been introduced into the method can be thought of as a sensor weighting matrix which modifies the shape of the target modes. The mode shape coefficients are modified based upon the noise levels at the sensor locations. Inclusion of the noise model results in higher ranking of sensor locations with low noise levels and suppression of sensor locations with high noise levels. A criterion is also presented which can be used during the course of the sensor placement analysis to determine how many sensors are required to maintain a desired level of signal-to-noise ratio over all the target modes. Simple numerical examples are presented which clearly demonstrate the ideas and trends presented in the paper.

Effect of model error on sensor placement for on-orbit modal identification of large space structures

A theory is presented for the effect of errors in large space structure prelaunch finite element models on sensor placement for the on-orbit independent identification of a set of selected target modes. The idea of positive net information is introduced. If the net information matrix remains positive definite as sensor locations are deleted from an initial candidate set, the analytical model provides useful information for the identification of the real target modes. If the net information matrix becomes indefinite, the sensor placement analysis based on the prelaunch analytical model will actually detract from the independent identification of the target modes. A bound is presented that can be easily checked after each iteration during which a sensor was deleted to determine positive definiteness of the net information matrix. Numerical examples are used to illustrate the ideas that are presented.

Graph‐Theory Approach to Eigenvalue Problem of Large Space Structures

The dynamic analysis and control system design of large space structures involve the solution of the large‐dimensional generalized matrix eigenvalue problem. The computational effort involved is proportional to the third power of the dimension of the matrices involved. To minimize the computational time a graph‐theory approach to reduce a matrix to lower‐ordered submatrices is proposed. The matrix‐reduction algorithm uses the Boolean matrices corresponding to the original numerical matrices and, thus, the computational effort to reduce the original matrix is nominal. The computational savings directly depend upon the number of submatrices into which the original matrix is reduced. A free‐free square plate is considered as an example to illustrate the technique. In this example a matrix of 16th order is reduced to three scalars corresponding to three rigid‐body modes, and three matrices of order three and one matrix of order four.

Gravity effects on damping of a space structure with pinned joints

An area of concern for the design of large space structures is the amount of structural damping that will be present. The joints used to assemble a large, light-weight structure like the space station will provide some damping; however, an accepted methodology for predicting joint damping is yet to be established. This paper documents a research effort at Utah State University to better understand joint damping in large space structures. A miniature tetrahedral truss was constructed that uses pinned joints. A large tip mass was attached to the truss to lower the natural frequency to a range associated with the space station. Considerable ground testing has been conducted to characterize the damping of the truss in a l-g environment in different orientations and in a vacuum. It was determined that the contribution of air to structural damping was insignificant for this truss. These experiments show that gravity can dramatically influence the damping produced.

Optimal experiment design for identification of large space structures

This paper is concerned with optimal experiment design for on-orbit identification of modal frequency and damping parameters in large flexible space structures. The main result is a separation principle for D-optimal design which states that under certain conditions the sensor placement problem is decoupled from the input design problem. This decoupling effect significantly simplifies the overall optimal experiment design determination for large multi-input multi-output structural systems with many unknown modal parameters. The error from using the uncoupled design to be ℞( δ 2) where δ is a measure of the inherent damping of the structure. A numerical example is given, demonstrating the usefulness of the simplified criteria in determining optimal designs for on-orbit Space Station identification experiments.

Identification of large space structures - A factorization approach

Reliable techniques for on-orbit identification of the vibrational dynamics of large space structures are important chiefly for two reasons. First, the complexity of such vehicles makes accurate finite-element modeling difficult; second, extremely limited ground testing possible for these structures prevents correction of modeling errors before flight. This paper describes a new algorithm that uses in-flight vibration data to estimate directly a factored form for the positive definite symmetric mass matrix of the structure. Applications of matrix factorization methods to, for instance, Kalman filtering and least-squares problems suggest greatly improved accuracy over methods that estimate the mass matrix itself. The properties of the new identification algorithm are analyzed in detail here and illustrated by means of examples.