Nonlinear Dynamics Analysis And Active Control Of Rotor Tilting Pad Journal Bearing System

With the trending to high speed, high precision and high load of the large scale rotating machinery, the increasingly nonlinear dynamic instability of rotor-bearing system have become the most serious problem. It is necessary to investigate the nonlinear dynamic behaviors, instability and the influences of operating parameters on dynamic stability of rotor-bearing systems. The fluid film force model is crucial to the nonlinear dynamics analysis of a rotor-bearing system. The formulation of a reasonable and manageable analytical fluid film force model is of great significance to the stability analysis and its engineering applications of rotor-bearing systems. On the other hand, the dynamic instability factors of the rotor-bearing system must be kept under control and the effective measures must be taken to improve the dynamic stability of the system. Once the abnormal phenomenon occurs, the effective corrective measures have to be put into action in case of the more serious consequences. The effectiveness of the active control to the rotor-bearing system mainly depends on the design of the control device and the regulation of the controller parameters.In this dissertation two challenging issues in the investigation of large-scale rotor-bearing system are investigated, i.e. the formulation of analytical fluid film force model of a tilting pad bearing and active control of the self-excited vibration problem. Firstly, the nonlinear fluid film force model of a tilting pad journal bearing is deduced based on the short bearing assumption. And then an active lubrication device is adopted to eliminate the self-induced vibration of the rotor-bearing system. Correspondingly, several active control strategies are designed to regulate the electro-hydraulic control system. The nonlinear dynamic responses of the active lubricated rotor-bearing system based on different controllers are worked out and the inhibitions of self-excited vibration of the systems with various controllers are compared. The main achievements are listed below:1. Based on the assumption of the short journal bearing,the fluid force model of the tilting pad journal bearing is formulated. Taking as a reference of the Capone model which is formulated based on the assumption of the short journal bearing,the fluid film pressure distribution is derived in each pad's own local reference frame. The relation of the pad rotation angle and motion of the rotor is figured out. Then the fluid film pressure distribution of the tilting pad bearing in global reference frame is obtained by coordinate transformations. The nonlinear fluid force model of the tilting pad is formulated by integrating the fluid film pressure distribution. A numerical model for an unsymmetrical flexible-rotor supported by two tilting pad bearings is given and the dynamic response is worked out. The fluid force model based on the finite difference model for the tilting pad journal bearing are deduced to confirm that the analytical model is valid to deal with the problem of nonlinear dynamics of the rotor-bearing system. The calculated results show that the analytical model proposed can be used to analyze the dynamical behavior of the rotor-bearing system qualitatively and to save the computing cost for solving the system.2. To eliminate the whirl and whip instability of the rotor bearing system, the active lubrication action is adopted to simultaneous reduction of wear and vibration between rotating and stationary machinery parts. The dynamic model of the electro-hydraulic active control system including the active lubricated tilting pad journal bearing and the electro-hydraulic system is formulated. In order to facilitate the realization of the control device, the design strategies of PI and PID controllers, applied to a tilting-pad journal bearing, are analyzed and discussed. Numerical results show that an effective vibration reduction of the unbalance response of an asymmetric rotor is performed for the system with PI or PID controllers. The feasibility of eliminating rotor-bearing instabilities (phenomena of whirl) by using active lubrication is also investigated, that illustrates clearly one of its most promising applications.3. To deal with the disadvantages of highly nonlinear, time-varying uncertainty and delay characteristics in the electro-hydraulic control system, the neural network (NN) technology is applied to improve the active lubricated control system. The most efficient neural network, radical basis function (RBF) and back propagation (BP) are designed to optimize the parameters of the PID controller to settle the uncertainty and inaccuracy of the hydraulic system. Then the dynamic performance requirements of hydraulic servo system are achieved. Then two kinds of NN-PID controllers are applied to an active lubricated tilting-pad bearing system, and the dynamic responses of the asymmetric rotor-bearing system are analyzed and calculated. Comparing with the results based on the PI and PID controllers, the NN-PID controller show better performance.4. The fuzzy PID control and fuzzy neural network PID control are designed to improve the active lubricated system after the successful application of neural network technology in optimizing PID parameters. Fuzzy controller does not rely on accurate models and possess flexible algorithms especially is suitable for the control of the system with nonlinear, large delay, time-varying characteristics. While the neural network has the advantage of strong robustness, memory capacity, non-linear mapping capabilities and a self-learning ability. So the fuzzy PID controller and fuzzy neural network PID controller are designed to regulate the active lubrication of the tilting pad bearing. Then the dynamic responses of the asymmetrical rotor-bearing system with fuzzy PID and fuzzy neural network PID controllers are worked out. Comparing with the results based on the other control laws, the fuzzy neural network PID controller shows better performance.5. The nonlinear dynamics analysis and active control of the overhang rotor system with double disks supported by two tilting pad journal bearings is investigated. The nonlinear dynamic model of the overhang rotor system is formulated based on the finite element theory. The analytical fluid film force model of the tilting pad journal bearing proposed in this dissertation are applied to analyze the nonlinear dynamic characteristics of the rotor system. Numerical method is used to solve the coupled dynamic equations and the result shows that whirl and whip phenomena occur when the rotational speed is increasing upto the critical case. Thus the active lubrication systems based on both fuzzy PID and fuzzy neural network PID controller are designed to suppress the vibration of the overhang rotor system. The results show that by active lubrication the whirl amplitude of the system is greatly suppressed and whip instability is totally eliminated. The stable operating speed range of the rotor system is widely broadened. The control performance of the fuzzy neural network PID controller is better than that of fuzzy PID controller.