Monica Pasca

Nonlinear Oscillations of a Nonresonant Cable under In-Plane Excitation with a Longitudinal Control

The nonlinear oscillations of a controlled suspended elastic cable under in-plane excitation are considered. Active control realized by longitudinal displacement of one support is introduced in order to reduce the transverse in-plane and out-of-plane vibrations. Linear and quadratic enhanced velocity feedback control laws are chosen and their effects on the cable motion are investigated using a two degree-of-freedom model. Perturbation analysis is performed to determine the in-plane steady-state solutions and their stability under an out-of-plane disturbance. The analysis is extended to the bifurcated two-mode steady-state oscillations in the region of parametric excitation. The dependence of the control effectiveness on the system parameters is investigated in the case of the first symmetric mode and the range of oscillation amplitudes in which the proposed control ensures a dissipation of energy is determined. Although control based only on in-plane response quantities is effective in reducing oscillations with a prevailing in-plane component, addition of out-of-plane measures has to be considered when the motion is characterized by two comparable components.

Three-dimensional vibrations of tethered satellite systems

The three-dimensional free vibrations of an orbiter-towed tethered satellite system with nominally vertical tether during stationkeeping phase are studied analytically. The tether is modeled as an elastic continuum with mass and the orbiter and the subsatellite as two concentrated masses. The linearized equations of motion with variable coefficients are solved by means of a perturbation technique. An analysis of the order of magnitude of the different terms involved permits evaluation of the coupling between longitudinal and transversal in-plane vibrations coming to an approximate closed-form solution of the eigenvalue problem.

Stability and control of transversal oscillations of a tethered satellite system

The tethered satellite system is characterized by weak nonlinearities but it practically works in conditions of internal resonance which produces unstable oscillations. The effect of a longitudinal control force is investigated. Since the displacement component in the orbit plane is always present in the motion due to the nonlinear coupling, the control force is assumed depending only on this component and also when a prevailing out-of-plane oscillation is considered. The harmonic balance method and numerical solutions of amplitude modulated equations are used to obtain stationary and nonstationary oscillations, respectively; the Floquet theory is followed in the stability analysis. The assumed control force is shown to be effective in reducing the primary and secondary instability regions of oscillations perturbed by internally resonant disturbance components.

Nonlinear control of tethered satellite system oscillations

Abstract The control of in-plane transversal vibrations of a tethered satellite system by means of a longitudinal force exerted by subsatellite thrusts is analyzed, A one-mode model is derived for describing the in-plane dynamics of such a system. On the basis of this model, two different nonlinear (and simple to be implemented) control laws are proposed for the stabilization of the tethered satellite system. The Liapunov stability theory is used to show the global asymptotic stability of the station-keeping phase of the tethered satellite system, subjected to either control laws. The effectiveness of the proposed control laws is shown by means of both analytical arguments and simulation runs.

Two Analytical Models for the Analysis of a Tethered Satellite System in Atmosphere

A tethered satellite system to be flown in the relatively dense atmosphere is considered. The relevant dynamic problem is characterised by strong non-linearities due mainly to aerodynamic effects. Two mechanical models, with different degrees of fidelity, are developed for analysing the static equilibrium of the system; the former assumes a straight tether while the latter treats the tether as a perfectly flexible continuum. Both of them model the tether as an elastic continuum with mass and aerodynamic forces distributed along the system. The results of the first model, which are an approximation of the system behaviour, are used as a starting point for the numerical procedure adopted for computing more accurately the tether shape with the flexible model.

Finite-discrete element modelling of masonry infill walls subjected to out-of-plane loads

In this paper, the out-of-plane response of infill walls is investigated by means of non-linear monotonic (push-over) analyses through a combined finite and discrete modelling approach. The model accounts for material deformability, crack formation, sliding, separation and formation of new contacts. Masonry units are modelled as finite elements, and different material models are assumed for the masonry. Contact between masonry units, and between masonry and frame elements is modelled by means of interfaces, which permit tangential motion with frictional sliding. Frame elements are modelled by means of a linearelastic material. The results of the numerical analyses are compared with those of experimental tests available in the literature. The advantages and disadvantages of the adopted modelling strategy are investigated.

On the Control of a Building Model Subject to Seismic Excitation

Abstract The control problem of a building model, subject to seismic excitation, is considered in this paper. First, a feed-forward compensator is designed to compensate the ground acceleration; secondly, an LQG optimal feedback controller is added to the control scheme to improve the control performances. In order to reduce the complexity of the resulting controller, the computations design procedure is carried out with respect to an approximate model of the building, obtained by eliminating some of the state variables from a balanced state space realization of the original model. Simulation runs confirm the effectiveness of the proposed control structures.