Research On Hydrostatic Thrust Bearing Oil Film Control Of Heavy NC Lathe

The performance of hydrostatic thrust bearing will directly impact the machining quality and operational efficiency of the heavy NC machining equipments. How to improve the stiffness of hydrostatic thrust bearing is one of the most urgent missions now. The oil film stiffness is the main research content and engineering application is the final goal in this dissertation, which combine closely the theory, experiment and application to develop a kind of active control system on hydrostatic thrust bearing to enhance bearing stiffness.The effects of workpiece weight, height and cutting force upon oil film displacement are analyzed based on finite element analysis data for worktable of heavy NC lathe C5116E-NC. The research results show that variation of workpiece weight impact on overturning moment distribution for given cutting force, oil film displacement will increase gradually as workpiece weight decreases, especially when workpiece weight is less than half of the maximum workpiece weight, local oil film pressure will not be built; the overturning moment and oil film displacement will increase as workpiece height increases. The oil film displacement produced by workpiece weight and overturning moment is given quantitatively in this study. Based on Christensen model, the Reynolds equations on steady-state and perturbed-state are built. The effect of surface roughness on oil film characteristics is analyzed taking into account the effect of fluid inertia and compressibility. The research results show that bearing surface roughness provide a neglected effect upon the load-carrying and a significant effect upon the steady lubricant flow rate, dynamic stiffness and damping coefficients. This work provides precondition of improving the hydrostatic thrust bearing stiffness.Based on analysis of effect factors on oil film property, an active control scheme on flow group of controlled chamber is proposed. The electro-hydraulic position servo system is selected as flow adjusting mechanism, which can regulate and control the oil film thickness of four groups controlled chambers to realize desired pose of the worktable. Mathematical models of electro-hydraulic servo variable flow mechanism and flow models of controlled chambers are built. A kind of second-dimension mutually non-correlative exciting signals is designed, which can simplify four-channel system into four single-channel systems for parameter identification. This work provides theoretical basis for designing and realizing oil film active control system of hydrostatic thrust bearing.Oil film control model of hydrostatic thrust bearing is equivalent to worktable pose control model; kinematics and inverse kinematics of oil film control are modeled and solved. Based on requirements for control strategy of non-line time-varying oil film system, a self-adaptive fuzzy PID hybrid controller is designed, a fuzzy switching mode is employed to improve obviously the control performance, and the fuzzy control system stability is analyzed using the describing function method. Oil film control strategy simulation model is built and worktable pose simulation is analyzed. The simulation results show that the rapidity of dynamic response and more little overshoot and more highly steady-state precision are attained for fuzzy PID hybrid controller. This work provides basis for realizing oil film active control strategy of hydrostatic thrust bearing.Hardware-in-loop simulation experiment system is developed to realize oil film position close-loop control based on xPC rapid control prototype technique. The model of single channel oil film control system (servo test bench) is identified by close-loop identification method, and reasonability of model is proved by experiment result. Hardware-in-loop simulation experiment of PID controller, fuzzy controller and fuzzy hybrid controller are carried out, and results show that hybrid controller provides quickly rising time and strongly anti-disturbance ability and adaptability for parameters change of plant controlled. This work provides reliable basis for developing oil film active control system of hydrostatic thrust bearing.