Theoretical And Experimental Study On Adaptive Vibration Isolation

Passive vibration isolation can not meet the high performance requirements in comfort and acoustic stealth of underwater vehicles, which poses a new problem to the vibration and noise control for quiet underwater vehicles. To meet this challenge, studied in this thesis is the active vibration isolation, which works in combination with the passive vibration isolation and consequently has the advantages of active control at low frequencies and passive control at high frequencies. The disturbance induced by rotating machinery has distinct characteristics in the frequency domain, which can be distinguished by spaced lines in spectra. For this kind of disturbance, the adaptive control strateges are usually adopted to realize cancellation. This thesis attempts to present a feasible active control strategy based on an adaptive cancellation method and the classical control theory. The contents are organized in seven charpters.First of all, the active vibration isolation theory and applications are reviewed and the contents of this thesis are introduced in Chapter 1.In Chapter 2 discussed are the three feedback methods, namely velocity, acceleration and force feedback. The influence of the changes in dynamic parameters of the sytem on the closed loop performance is analyzed, and especially, the impact of time-delay on the closed loop stability is revealed by the gain-delay cureves– a theoretial reference to the design of active isolation.In Chapter 3, an adaptive control algorithm is presented to cancel harmonic disturbances with oscillating frequencies. The algorithm is embedded with online frequency estimators and tracking filters. Frequency estimation is realized on the subspace identification principal and the recursive computation of correlation series of reference signals. Center frequencies of the tracking filters are adjusted according to the estimated frequencies so that the filters can track signal components. Simulation results are given to demonstrate the superiority of the adaptive method with tracking filters over the method using filters with fixed center frequencies.The influence of saturation in controller output on the active isolation and the alleviation scheme are discussed in Chapter 4. Based on the analysis of the formation of saturation in controller output, an anti-saturation scheme with fast convergence is obtained by solving a constrained optimization problem, and a new adaptive controller is constructed subsequently in accordance with the anti-saturation algorithm. Simulation results have shown the negative effect of output saturation on the control performance and demonstrated the effectiveness of the saturation suppression scheme.Numerical simulation is conducted with the model of an experimental system and the results are given in Chapter 5. This section maily discusses two issues– coupling in multi- channel control and influence of controller output saturation. Firstly, the influence of coupling on control is analyzed. Secondly, the single channel control method with anti-saturation scheme is expanded and applied to multi-channel vibration control. Finally, by comparing the results under different control strategies with/without anti-saturation scheme, it is demonstrated that saturation alleviation is helpful to improve control performance, and a good control effect can be reached for multi-channel isolation by taking appropriate measures.In Chapter 6 presented are the experimental system and the verification of the anti- saturation performance, the cancellation effectiveness as well as the robustness to impact action of the multi-channel vibration control algorithm. The experimental system includes a cylindrical shell and four passive/active isolators installed inside. Accelerometers are placed on the structure and their responses are amplified by signal amplifiers and then transmitted to data-acquisition units in the controller. Control commands given by the adaptive algorithm, which is coded with Labview, are output to the data-acquisition units and then the power amplifiers, which impels the actuators to act on the structure. In order to assure the stability of actuators, velocity feedback is introduced to the actuators, and the stability of actuators is analyzed at the given sampling rate. The experiment not only puts theoretical methods into practice, but also considers the factors which are not included in theoretical analyses, which makes the control strategy more applicable. It is demonstrated that the proposed adaptive method has high adaptability and can be applied in real situations with abnormal disturbances.Finally in Charpter 7, the conclusions and contributions of this thesis are summarized and highlighted. Moreover, some problems for further study are suggested as well.