| ABSTRACT:Electric-hydraulic servo valve is widely used in national defense industry areas such as aerospace and ships as of its characteristics of big transfer power, fast response, and high accuracy. As the key assembly unit of electric hydraulic servo system, precise electric hydraulic servo valve has a complex structure and high precision. Its stability, reliability and rapidity affect the performance of electric hydraulic servo system and the success of spacecraft fight mission to a great extent. When performing on-off control actions, the internal flow oscillation of electric hydraulic servo system often produces self-excitation phenomenon. This phenomenon makes the torque motor and piston generate larger vibration amplitude and seriously affects their normal operating characteristics, and even their working lives. This dissertation takes the precise flapper-nozzle electric hydraulic servo valve used for some aerospace types for carrier and deeply study the mechanism and characteristics of fluid-solid coupling vibration as well as the influence rules of related factors. This study is meaningful for adopting proper control measures to reduce coupling vibration and keeping good dynamic performance of the system.This dissertation firstly introduces the background of this study. By concluding the research results of both abroad and domestic scholars in these aspects of fluid-solid coupling vibration performances, electric hydraulic servo system vibration, vibration control and so on, the key questions of fluid-solid coupling vibration analysis and control are put forward. Then clearly define the direction, goal and meaning of this study. At last the technology roadmap is given.According to the operating principle and process of electric hydraulic servo system, this dissertation analyzes the static and dynamic performances of the system and sets up a nonlinear mathematical model of twin flapper-nozzle servo valve. By proper nonlinear state feedback transform based on differential geometric control theory of nonlinear system, the system input-output linearization is realized and the zero-dynamic stability is investigated. The model is closer to the reality compared to the incremental linear method based on operating points. This provides the theory foundations for analyzing electric hydraulic servo valve fluid-solid coupling vibration phenomenon and vibration control.Servo valve self-excitation is caused by non-constant electric current of the system and belongs to the dynamic transition process when flow parameters change in steps. The flapper-nozzle, as a prestage driver structure, has a high speed of internal jet flow field, many cavitations and strong fluidity of shear flow. The coupling vibration, which caused by the flapper-nozzle, often makes servo valve generate a high-frequency self-excitation. This dissertation combines hydromechanics fundamentals with the nonlinear model of electric hydraulic servo valve and sets up mathematical models of jet flow field between flapper-nozzles. Moreover, study the flow performances of servo valve jet flow field by simulations and experiments. Give out the internal relations of jet pressure pulsation, the prestage parameter structure and servo valve self-excited oscillation.Slide valve vice-module, as the second level of servo valve amplification structure, forms a free shear layer with a great velocity gradient when fluid enters a relatively slow valve chamber flow field with a high speed by valve ports. Effected by the instability of the shear layer, the small perturbation of the shear separation point enlarges in the propagation and causes new disturbances by the reflection between upstream and downstream. This is how the self-excited oscillation comes out. Applying the fluid vibration theory and the numerical method of fluid behavior equation, this dissertation takes the flow through the pipelines as nonlinear parameter system and calculates all the valves or actuators as the boundary conditions of fluid motion. Then complete the calculation of steady boundary and give out the solution of equations. At last put forward a semi-analytic fluid-solid coupling model according to the analysis of structural features and operating performances.To fully understand how the micro-compression of the fluid can change its momentum in the transient process, an improved fluid dynamic model of pipeline is proposed. The correctness of the simulation is approved by the comparison of calculation data with experiment data. The analytical model is carried out based on the one-dimensional fluid transient theory. Adopt the method of characteristics and finite difference method and have a numerical calculation of the transient flow of pipeline oil-hammer in hydraulic system. Meanwhile conduct experiments on the dynamic performances of oil pipelines. The results show that these models give more reasonable descriptions for the transient performances of hydraulic system and the phenomena of pipeline oil-hammer and pipeline flow oscillation.Lastly, develop a test bench to research on the electric hydraulic servo self-excitation performances and perform experiments on the static and dynamic performances as well as fluid-solid coupling vibration performances. The results prove that the mathematical models based on the input-output linearization theory can recognize the stability of the system effectively as well as study the relations between input signals and output displacements. The fluid-solid coupling vibration models can be used to effectively analyze the principle of how the servo valve self-excitation comes out. The self-excitation frequency of servo valve and the trend of vibration amplitude are nearly the same in experiments and calculations and this in turn proves the validity of fluid-solid coupling vibration models. Moreover, the high-frequency electromagnetic vibration exciters are applied in experiments to reduce electric hydraulic servo valve self-excitation amplitude and the results also verify the effectiveness of control measures. |
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