Mechatronic Design And Control Research Of The Closed-loop Electro-hydraulic Proportional Servo System For Precision Injection Molding

Along with the development of plastic industry, domestic enterprises devote great effort to developing and manufacturing precision injection molding machines, to reduce the injection technology gap with foreign machines. As one of the most critical technologies, electro-hydraulic control system constrains the injection performance improvement. The time-variant, nonlinearities of control parameters and uncertainties of thermoplastic during molding process make it hard to precisely control the system and product. Therefore, research on mechatronic design and control of electro-hydraulic systems for precision injection molding has significant utility.The research subject is a part of the National Science and Technology Support Project named 'Research on the Critical Technologies for Precision Plastic Injection Molding and Industrialization'. The content of this dissertation is arranged as follow:1) The development and technology characteristics of hydraulic control systems and automatic control strategies for injection molding are expounded in the paper. Then the control difficulties of electro-hydraulic injection process are presented.2) According to the requirements of precision injection molding, a new closed-loop electro hydraulic injection system is presented which controlled by a closed-loop proportional valve and an AC servo motor. On the basis of the operating principal of the proposed system and hybrid theory, an equivalent mathematic model of the main hydraulic loop is built to get the nonlinear differential equations, and then a simulation block diagram of the whole injection system is established in Matlab/Simulink. The mathematic model is linearized, so as to verify the stability, controllability and observability of the system.3) In order to compensating nonlinearities of electro-hydraulic injection control system and uncertainties of the thermoplastic, the overall control scheme is presented in this dissertation. That the injection velocity is controlled by Fuzzy_PID control algorithm and the packing pressure is controlled by Integral-Separated PID algorithm. To verify control performance of this system, a serial of simulations on injection velocity and packing pressure are made to compare with traditional control algorithm by Matlab/Simulink. The simulation results show the advantages of this system and control method in dynamic and static characteristics. 4) The application of closed-loop electro-hydraulic injection system is studied in HTH200 servo injection molding machine. Experiments on this machine are carried out to evaluate the effectiveness of injection velocity and pressure control. As experimental results show, the velocity and pressure tracking performance is very satisfactory with fast response, small overshoot and high steady precision. Furthermore, experiments about repeatability accuracy including the position of injection screw and the weight of plastic product are carried out to verify the proposed electro-hydraulic system and control method.