Research On Digital Control Strategy Of VPPA Welding Power Supply

The rapid development of aerospace science and technology has put forward higher requirements for the welding quality of aerospace aluminium alloy materials,and high-performance digital variable polarity plasma arc welding power source is the key to improve the welding quality of aluminium alloy materials.At present,the main control circuit of welding power supply is relatively mature,and its performance largely depends on the design of digital controller,especially in the constant current closedloop control strategy.According to the time-varying and non-linear characteristics of arc welding inverters,it is necessary to design the control methods of arc welding inverters working in large and small signal range.Variable polarity plasma arc welding power supply is a dual operating system with repetitive characteristics.The first working condition is DCEN(workpiece is positive,tungsten is negative),and the second working condition is DCEP(tungsten is positive,workpiece is negative).In view of this characteristic,a data-driven control method for large-signal systems is proposed based on the principle of the effective iterative learning control algorithm in the field of robot control.Therefore,the closed-loop control of welding power supply is realized by combining the main feedback of the controlled object's post feedback and the local feedback of the controlled object's pre-feedback.The former feedback is obtained by sampling the historical data(driving duty cycle)of multiple control periods within the convergence error.It is used as the driving data to adjust in the large signal range.The physical significance of the historical data is explained by the large signal average model of the arc welding inverter power supply system.A large amount of historical data output by the controller during load test is obtained,and the basic characteristic of different current are analyzed and compared.The control system compares and analyses the real-time welding current and given current waveform by the microcontroller with Cortex-M4 core to divide the working range.The controller is regulated by incremental digital PI in the small signal working and by historical data drive in the large signal working range.A fully digital VPPA welding power supply with inverted frequency 20 kHz has been developed,which has the characteristic of stable welding current(small overshoot),good current response(fast rising time of current)and short adjusting time when welding process requirements of large specifications.To verify and guide the design method of variable structure controller,a closed loop Simulink simulation model based on S-Function was established.The simulation design of variable polarity arc welding inverter power supply is realized by combining the programming design of M language with the graphic design of circuit model.Through variable structure control strategy and current waveform modulation,the simulation model realizes DC,AC,pulse and other welding modes,and achieves good simulation results,and good simulation results of dynamic characteristics are obtained.Software design and integrated allocation of power control task module are carried out on independent MCU,and time selection and layout of tasks are arranged in time domain to ensure accurate and reliable output of inverted PWM waveform.Variable polarity AC and DC pulse welding tests were carried out on the load box for currents of different specifications.At high current 280 A,the current response speed was obviously improved,the overshot was small and the adjustment time was short.Finally 110 A variable polarity alternating current was used to test the welding process of aluminium alloy plate,and good weld morphology was obtained.The final test results verify the practicability and reliability of the power supply control technology.