Dynamics Of Large Space Structures Research Articles

Flight-test-based construction of structurally stable models for the dynamics of large space structures

A method is proposed for constructing dynamic models of large space structures (LSS) when their parameter values are uncertain and LSS state measurements in actual operation conditions are incomplete and subject to errors. The method can be used to construct structurally stable dynamic models of LSS when ground-based LSS tests are impossible.

On the use of the P–TFE method for panel flutter optimization

A numerical approach for structural analysis based on the concept of parameter transfer finite elements (P–TFE) was proposed by the authors in previous papers Barboni R, Castellani A, Mannini A. Un metodo analitico-numerico per l'analisi strutturale. Atti del Dipartimento Aerospaziale, Roma, Novembre, 1988; Barboni R, Gaudenzi P, Mannini A, Santini P. An application to the dynamics of large space structures of a new semi-analytic method for structural analysis. 17th Congress ICAS, Stockholm, September, 1990; Barboni R, Gaudenzi P, Mannini A. Parameter-transfer finite element method for structural analysis. AIAA Journal 1993; 31:923–9; Barboni R, Mannini A, Scarponi C. Structural optimization with parameter-transfer finite element. Meccania 1995; 30: 291–304. Such elements are able to take into account, like a transfer function, the whole behaviour of the structure under concern, subject to dynamic, aerodynamic or thermal actions, and to produce, in analogy with the finite element method, a numerical technique of discretization of a continuum. The purpose of this paper is to apply this methodology to optimization problems, in particular to the design of a minimum-weight monodimensional panel under critical flutter pressure constraint. The use of P–TFE in this aeroelastic optimization problem overcomes most difficulties of the usual techniques of solution, in particular, the evaluation of the sensitivity matrix, the terms of which can be calculated analytically in the frame of the proposed approach. Moreover, the critical conditions are directly imposed by the solution of a non-linear system of two equations in the eigenfrequency and the dynamic pressure unknowns. Several examples relevant to different boundary conditions are presented and the results compared with those obtained by other authors, using different methodologies.

Free-decay time-domain modal identification for large space structures

Concept definition studies for the Modal Identification Experiment (MIE), a proposed space flight experiment for the Space Station Freedom (SSF), have demonstrated advantages and compatibility of free-decay time-domain modal identification techniques with the on-orbit operational constraints of large space structures. Since practical experience with modal identification using actual free-decay responses of large space structures is very limited, several numerical and test data reduction studies were conducted. Major issues and solutions were addressed, including closely-spaced modes, wide frequency range of interest, data acquisition errors, sampling delay, excitation limitations, nonlinearities, and unknown disturbances during free-decay data acquisition. The data processing strategies developed in these studies were applied to numerical simulations of the MIE, test data from a deployable truss, and launch vehicle flight data. Results of these studies indicate free-decay time-domain modal identification methods can provide accurate modal parameters necessary to characterize the structural dynamics of large space structures.

Recent advances in structural dynamics of large space structures

Recent progress in the area of structural dynamics of large space structures is reviewed. Topics include system identification, large angle slewing of flexible structures, definition of scaling limitations in structural models, and recent results on a tension-stabilized antenna concept known as the hoop-column. Increasingly complex laboratory experiments guide most of the activities leading to realistic technological developments. Theoretical progress in system identification based on system realization theory resulting in unification of several methods is reviewed. Experimental results from implementation of a theoretical large-angle slewing control approach are shown. Status and results of the development of a research computer program for analysis of the transient dynamics of large angle motion of flexible structures are presented. Correlation of results from analysis and vibration tests of the hoop-column antenna concept is summarized.

The dynamics and control of large space structures after the onset of thermal shock

This paper considers the problem of predicting the open and closed loop dynamics of large space structures after the onset of thermal shock. The thermal gradients induced in space structures due to solar radiation heating result in thermal deformations. The temperature gradient time history, the related thermal deflection response, the moments acting on a thermally deflected beam, and the control effort required for different combinations of solar incidence angles and material properties are considered in this investigation. The analysis is performed for structures which are nominally inertially stabilized as well as gravity stabilized. For the case of basic orbiting structural elements, such as a free-free plate (modelled as a beam in-plane), depending on the orientation, and required pointing accuracy, it may be necessary to redesign control algorithms previously developed for models which exclude the thermal shock effects.

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.

On the accuracy of modelling the dynamics of large space structures

Recently new space missions which require large-scale, light weight space based structural systems have been proposed. Because of the inherent size of these structures, system dynamic performance must be accurately predicted prior to launch and assembly, usually by mathematical modelling and simulation. Santini has developed a mathematical formulation for predicting the motion of a general orbiting flexible body using a continuum approach, different from the finite element techniques which model structures as a system of lumped masses connected by a series of (restoring) springs. When finite element techniques are used to model a system, the coupling terms between the rigid and flexible modes and between different flexible modes are usually neglected. In this paper a computational algorithm is developed to evaluate the coefficients of the various coupling terms in the equations of motion as applied to the finite element model of the 122 m diameter Hoop/Column system. It is seen that when deflections are of the order of meters (still within small amplitude assumptions) the coupling between the flexible and rigid modes should be included in the rigid rotational modal equations of the mathematical model.

Experiments Using Lattice Filters to Identify the Dynamics of a Flexible Beam

An approach for identifying the dynamics of large space structures is applied to a free-free beam. In this approach the system’s order is determined on-line, along with mode shapes, using recursive lattice filters which provide a linear least square estimate of the measurement data. The mode shapes determined are orthonormal in the space of the measurements and, hence, are not the natural modes of the structure. To determine the natural modes of the structure, a method based on the fast Fourier transform is used on the outputs of the lattice filter. These natural modes are used to obtain the modal amplitude time series from the measurements. The modal time series provides the input data for an output error parameter identification scheme that identifies the autoregressive moving average (ARMA) parameters of the difference equation model of the modes. Results using this approach for experimentally identifying the dynamics of a flexible beam hardware are presented.

Identification of structural dynamics systems using least-square lattice filters

A new approach for identifying the dynamics of large space structures with noisy measurements is presented. In this approach the system's order is recursively determined on-line, along with mode shapes using recursive lattice filters that are widely used in adaptive signal processing. Order determination results are presented using both statistical hypothesis testing and threshold testing. The approach is illustrated for the problem of identifying the structural dynamic characteristics of a free-free beam. Simulation results are presented on the effectiveness and accuracy of the identification scheme for various noise levels, sampling rates, and data window sizes. Experimental results are presented for order and mode shape estimation. The mode shape estimation is compared with that obtained by an alternate nonlinear least-square algorithm.