Research On Several Problems Of Active Vibration Control

In recent years, active vibration control techniques have been widely applied to many fields, such as aerospace industry, ships, automobiles and so on. Theories of vibration and active vibration control have also been developed. However, in real engineering applications, there are still many problems to be deal with necessarily. Many kinds of space structures trend to become larger and more flexible. Large space flexible structures are characterized by lower and closely spaced natural frequencies, small structure damping, high degree of mode coupling, etc. Therefore their vibration control is difficult. On the other hand, techniques of active vibration absorber and active vibration isolation play more and more important role in reducing vibration and noise of ships, but the present techniques have low vibration reduction performance and need large control effort, which limits their applications. Therefore, research on new kinds of active vibration absorber and active vibration isolation system is valuable in theory and engineering application.In this thesis, some key issues on active vibration control of structures in aerospace industry, reduction of vibration and noise in ships are investigated through theoretical analysis. The research focuses on three aspects: active control of structures with closely spaced frequencies, active vibration absorber and active vibration isolation. Its contents are as follows.1. Active vibration control of structures with closely spaced frequenciesSo far, the control problems of structures with closed spaced frequencies using smaller number of actuators than the number of closely spaced frequencies have been limited to two closed spaced frequencies. In practice, large space flexible structures usually have a cluster of closely spaced natural frequencies which are continuously distributed in low frequency range. Therefore, active control of a typical structure with three continuously closely spaced natural frequencies by a single actuator with velocity feedback is discussed. The influence of frequency spacings and uncertainty parameters on control effect is analyzed. The results show that the frequency spacing is the key parameter with respect to control effects and a single input can only control one of the three modes of the closely spaced natural frequencies and the rest close modes will approach to be uncontrolled as the control gain increases. When the equivalent tuning parameters are small, the mode shapes are so sensitive to the uncertainty parameters that a little change of the parameters can make the mode shapes change greatly. The sum of modal damping is independent of uncertainty parameters of the system, but depends only on the control gain.For a structure with two closely spaced modes, different identification methods of structures with closely spaced frequency are discussed. The results show that the tuning parameter defined by the frequency spacing is a simple and effective index. Its calculation is easy and it can reflect the characteristics of structures with closely spaced frequency, that is, the mode shapes are sensitive to the perturbations of system parameters. Through the analysis of the influence of perturbation parameters on mode shapes, a physical interpretation is proposed for the difficulties in active control of structures with closely spaced frequency. The difficulties lie in that mode shapes are sensitive to the perturbations of structure parameters, and the sensitivity is caused by weak couplings in these structures. Based on the idea, enhanced-coupling control method is proposed. For the structure with two closely spaced modes, a single control input can control effectively the two closely spaced modes using the method.2. Active vibration absorberIn this research, the advantages and limitations of an adaptive-passive vibration absorber (APVA) are analyzed in detail. Based on the analysis, a novel kind of adaptive active resonator absorber (AARA) is proposed and a patent is applied for AARA based on magneto-rheological elastomer. It can be considered as the integration of APVA and active resonator absorber (ARA), so their advantages such as low cost, high performance and fail-safe are inherited.Dynamics modeling is carried out for experimental system used to evaluate the performance of dynamic vibration absorber. Then the performance of an adaptive tuned mechanical vibration absorber is investigated through theoretical analysis and the computational results are well coincident with experimental ones.Frequency-tuning methods of dynamic vibration absorber (DVA) under multiple harmonic excitation are studied. The performances of single DVA, dual DVAs and state-switched absorber (SSA) are compared. Furthermore, one-one method for frequency-tuning under multiple harmonic excitation is proposed and compared with the optimization method. It almost has the same performance as the optimization method and it does not need time-consuming optimization process. In addition, the method can be easily expanded to the case of the excitation including more frequency components.3. Active vibration isolation Typical problems in active vibration isolation systems are analyzed based on single-layer one and some improvements on their control methods and configurations are proposed. Typical active vibration isolation systems have the disadvantages of large feedback gains with less effective vibration isolation at low frequencies. A modified system is designed to reduce feedback gains. It can reduce the feedback gain but at the cost of increasing the actuator stroke. Moreover, both the control force and the actuator stroke are functions of the displacement. To overcome the limitations, a joint active vibration isolation system which includes an adaptive dynamic vibration absorber is then designed. The results show that the adaptive DVA significantly reduces the control force and the displacement and eliminates the need to compromise between the feedback gain and the actuator stroke. Therefore, the joint vibration reduction method combining an active vibration isolation system and a DVA is an effective method to enhance the performance of active vibration system.To improve the isolation performances of traditional floating raft systems, dynamic vibration absorber is introduced into floating raft. The vibration reduction effects of passive and semi-active DVAs under single frequency and multi-frequency excitation are investigated. The results show that DVAs can significantly improve the isolation performance of floating raft system and their placements have remarkably large influence on the performance and should be chose according to the condition of excitation.Power flow transmissibility is proposed as a performance index to evaluate the performance of isolation systems. It is defined as the ratio of the power flow input into the equipment and the power flow transmitted into the receiver. Based on a simple vibration isolation system, its relationships with other performance indices are given by theoretical and numerical analysis. The efficiency ratio can directly indicate the effectiveness of isolator, but it can not reflect the response characteristics of the whole isolation system. Power flow transmissibility justly avoids the limitation. In addition, power flow transmissibility can be estimated easily according to vibration acceleration level, so its measurement is more convenient than efficiency ratio. Furthermore, numerical simulations are performed to give the influences of several parameters such as the damping, loss factor and stiffness of isolator on power flow transmissibility. The results will be helpful for the design and evaluation of passive and active isolator.