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Title: Nonlinear vibration control : a frequency domain approach
Author: Ho, C.
ISNI:       0000 0004 5348 6083
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2014
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A vibration isolator, sometimes called an isolating mount, is the device situating between the vibration source and the sensitive system preventing the transmission of undesired disturbances. The performance is measured by the force or the displacement transmissibility, both functions of frequency. A good vibration isolation system has three main properties - a low resonant peak, a large isolation range and low transmissibility at non-resonant regions. Unfortunately, these characteristics cannot be achieved simultaneously by a simple linear vibration isolation system. The thesis addresses this problem for single-degree-of-freedom (sdof) vibration isolation systems by introducing nonlinear damping and stiffness devices into the system. First, theoretical studies were carried out to rigorously reveal the benefits of the proposed nonlinear vibration isolation systems over linear ones. Next, the performance of these nonlinear systems were analysed by simulations. Then, experimental studies were conducted to verify the theoretical and simulations results. Finally, a systematic approach was developed to design the parameters of the nonlinear damping and stiffness devices in order to satisfy specific vibration isolation requirements. Many vibration isolators can be modelled as a single-degree-of-freedom mass-spring-damper system. Many researchers have attempted to enhance the vibration isolation performance by designing springs with nonlinear stiffness. Others have focused on different types of damping nonlinearities. The new vibration isolation system proposed in the thesis combines both spring and damping nonlinearities in one system to exploit the advantages of both components while avoiding their undesirable effects. The theoretical properties of this proposed nonlinear vibration isolation system were analysed rigorously using the output frequency response function (OFRF) approach, a novel and unique method recently proposed at Sheeld. The stiffness nonlinearity is already a well researched area and can readily be realised in practice. Therefore, the implementation of the proposed nonlinear vibration suppression system focused on the realisation of the nonlinear damping component using commercially available magneto-rheological (MR) dampers which provide a damping force that is dependent on a control current. With feedback control, the force-velocity relationship of an MR damper can be shaped into a designed function. This implementation has been incorporated first in a vibration isolation system by simulation, then in a physical experimental rig which has a moving mass. The simulation and experimental data not only showed the successful realisation of a damping device with a particular nonlinear damping characteristic, but also confirmed the theoretical findings on the beneficial effects of nonlinear damping on a vibration isolation application. The final part of the thesis is devoted to the practical design of the proposed vibration isolation system. Given specific transmissibility requirements at certain critical frequencies, the values of the linear parameters are first designed, then the OFRF approach is applied to determine the nonlinear parameters. This pragmatic method simplifies the design of a complicated nonlinear system, which was traditionally difficult to work with, into a step-by-step guide and, therefore, has significant potential of industrial applications. The thesis has exploited the special effects of two nonlinear components on the performance of a passive sdof vibration isolation system. With the support of theoretical, simulation and experimental studies, the newly proposed configuration has shown substantial benefits to many vibration isolation problems. The simple yet effective design and implementation has significant implications for a wide range of engineering applications such as car suspension designs and building protection against earthquakes.
Supervisor: Lang, Z. Q. ; Billings, S. A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available