An Investigation on structure-vibration isolator interactions for particular performance

Abstract

In engineering, particularly in certain application of vibration isolation, there is a need to achieve a special low or high supporting stiffness. There are two methods found to be effective in providing these particular performances mainly nonlinear vibration isolation and/or active vibration isolation. To assess the efficiency of a nonlinear isolator, interaction analysis is necessary, as for the structure control interactions, the dynamic characteristics of both structure and control system affect each other. There have been fewer publications that consider or discuss the interactions between a structure and a nonlinear vibration isolator, and
between a structure and an active isolator. This thesis presents the study of the interaction
between a beam and a nonlinear isolator and between a beam and an active isolator for low and high supporting stiffness. For the beam- nonlinear isolator , the system consists of an elastic beam- like structure and a geometrically nonlinear isolation system in which a horizontal degree provides a
physical approach for realising the required horizontal force. The generalised dynamic equations of the proposed interaction system are derived, from which three reduced models can be obtained by introducing the related conditions into the generalised model. The modal summation method is used to analyse the beam. The nonlinear dynamic behaviour on equilibria and stabilities of the system are investigated. The dynamic interaction mechanism between the nonlinear isolation system and the elastic structure is revealed. The beam nonlinear isolator design for low stiffness support and high stiffness support is discussed. Then
the interaction between the beam and the active isolator is found, theoretical design strategies
of the active vibration isolation system are developed, and two cases are investigated; an active
isolator with low suspension frequency and an active isolator with high suspension frequency.
It is found that the beam provides additional mass, stiffness and force to the nonlinear vibration isolator and to the active isolator. For the beam-nonlinear isolator and beam- active isolator with low stiffness support, the requirement to perform ground vibration test whereby
the rigid mode of the beam must be less than one third of the first elastic natural frequency of the free-free beam has been satisfied. The condition to achieve high stiffness support has also been satisfied. Nonlinear dynamical behaviour of the beam-nonlinear isolator indicates that period doubling bifurcation occurs when the excitation force is 1 and excitation frequency is 0.5Hz. Poincare maps reveals that the system form closed loops and no chaotic behaviour is
observed. Performance analysis in terms of force transmissibility of the nonlinear isolator shows that the nonlinear isolator performs better than a linear isolator and also performs better than a hardening HSLDS mount. For the conceptual design of the beam-active isolator,
the dynamic response of the system was done using Simulink Simscape block set and is found to be similar to the dynamic response found using Simulink alone. The investigated system in the thesis can provide extremely low or high supporting stiffness and frequencies to satisfy special engineering applications for high precision vibration isolation.

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