Study On Challenge Technologies Of Multiplexed Force-Displacement Control In Pneumatic Servo System For Resistance Spot Welding

Resistance spot welding is the dominant sheet metal joining method in automobile and aerospace industry. For traditionally pneumatic gun used in spot welding, electrode force depends on pressures in the cylinder. Electrode force can not change with working conditions and stability of it can not be guaranteed. Also the impact between electrode and workpieces may influence the electrode service life and working environment. Pneumatic servo welding drive mechanism is an upgrade product of traditionally pneumatic welding drive mechanism. Using it can achieve high-precision positioning of the welding gun and flexible welding control. Today many users are very concerned about the development of it. Yet related research has not been carried out in China.This research is financially supported by “National Natural Science Foundation of China” and “State Key Laboratory program of Fluid Power and Mechatronic Systems of China”. A booster cylinder controlled by solenoid valves is considered. Based on nonlinear models of pneumatic servo system, pneumatic position servo system and force servo system are studied indepently. Multiplexed force-displacement control of pneumatic servo system is then studied. The main contents of the thesis are summarized as follows.(1) Experimental system of pneumatic servo driving mechanism for spot welding is designed, which includes hardware and software with virtual instrument technology. Prototype of pneumatic servo driving system for spot welding is designed.(2) Nonlinear models for a booster cylinder controlled by solenoid valves are developed. The dynamic characteristics of solenoid valves are firstly investigated and time delay is then considered in the next control process. The transfer function from duty ratio of the solenoid valve to pressure in the chamber of a cylinder is obtained considering mass flow discontinuity of the solenoid valve. Stribeck friction model of a booster cylinder is obtained. Parameters of it are recognized by experiment. This Stribeck friction model is further improved to be continuously differentiable.(3) To improve the robustness and the anti-disturbance of pneumatic servo system, regulate its stiffness, a scheme concluding linear active disturbance rejection controllers for pneumatic servo system is proposed. Two linear active disturbance rejection controllers are designed respectively to track the motion and pressure reference. Three order of linear active disturbance rejection motion controller is designed to track the position reference. One order of linear active disturbance rejection pressure controller is designed to regulate stiffness of pneumatic servo system. Simulation results show that the scheme can improve position tracking accuracy and enhance the anti-disturbance ability.(4) As to deal with the robust control problems against parameters uncertainty, noise and friction disturbance, a control method based on disturbance observer plus feed-forward control is employed. The control sheme is applied to pneumatic position and force servo system respectively. The simulation and experimental results verify the effectiveness of the proposed method.(5) According to the large impact and force errors of conventional impedance control, a hybrid controller based on non-contact impedance control is addressed for pneumatic servo system to contact with rigid electrode. Virtual force is considered to decrease large impact and a force controller is adopted to improve the tracking accuracy of force. Simulation results show that the designed hybrid controller is effective in the contact process between pneumatic servo system and rigid electrode.