| In order to reduce the percentage of defective parts and improve productivity,a stamping machine can be equipped with a die cushion system,for the die cushion plays an important part in the stamping process,which helps stretch parts to a greater extent and improve material fluidity.At present,die cushions made in China have problems such as low control precision and low productivity,which are mainly caused by the low position control precision of the die cushion system.Therefore,a hydraulic die cushion with high-precision position control can greatly improve the above shortcomings.In this paper,the hydraulic control system of the die cushion of the 500 k N stamping machine was designed and researched according to the actual requirements of production.Based on a complete review of Chinese research on the die cushion system,the main content of this research issue was determined.By comparing the advantages and disadvantages of different designs,valve-controlled cylinders and cost-effective servo proportional valves were finally determined as a hydraulic power unit and hydraulic control unit respectively and an accumulator was used as the auxiliary power source to improve the energy utilization rate of the system.According to the designed hydraulic scheme,the parameters and models of major components of the hydraulic system were calculated and selected,and AMESim was used to match the load characteristics of the valve-controlled cylinder system,which proved the correctness of parameter calculation and model selection of servo proportional valve and servo hydraulic cylinder.A mathematical model of the closed-loop position control system was built to derive the block diagram of the system transfer function.The Bode diagram was used to judge the stability of this model.A simulation model of the closed-loop position control system was built through AMESim to deeply study the dynamic characteristics of the hydraulic control system.In addition,considering the requirements of blank holder force control when the workpiece is stretched,a global model of the hydraulic control system was built.According to the simulation results of the above two models,the slow dynamic response of position control and the control precision of 0.1mm cannot meet the technical requirements,so the system needs to be further corrected;the stable pressure control and the blank holder force fluctuation range 250.12-250.32 k N meet the technical requirements.In view of the position control precision below the technical index,an in-depth study on the control strategy was carried out.Conventional PID correction was first conducted,and system performance got improved,but it still could not meet the technical index.On this basis,the compound control strategy “feedforward and acceleration feedback-PID correction” was employed to further correct the system,which reduced lag errors and improved position control precision.As a result,both dynamic response and static control met the technical requirements.In addition,the effect of factors such as target signal curve(control signal),hydraulic valve resolution and hysteresis,and oil supply pressure on system performance was studied in detail,and appropriate control parameters were selected.A hardware-in-the-loop simulation(HILS)platform was built to validate the correctness of the control strategy.The platform consisted of a hardware environment and a software environment.The hardware environment was composed of PLC,communication card and computer,and the software environment was comprised of AMESim and Simulink.After the platform was built,a HILS model of the closed-loop position control system and a global HILS model of the hydraulic control system were built in the software environment for simulation calculation.According to the final simulation results,the dynamic and static characteristics of the hydraulic control system met the technical requirements.This proved the correctness of the control strategy and meanwhile provided strong support for the feasibility of the design. |
PDF Full Text Request