Digital Control System Design Of Pulsed MIG

With the development of industrial automation and welding process requirements, the digital control arc welding inverter power source has become the trend in welding field. Pulsed MIG welding is widely used for its characteristics of good direction transition, less material splash and nice welding appearance. Full-digital control arc welding inverter power source can greatly enhance the level of pulsed MIG welding for good flexibility, high control accuracy and quick system response.At first, this paper studies the types of droplet transition in pulsed MIG welding, then analyzes the functions of the pulsed current parameters, finally proposes a dual-loop control method for welding current and welding voltage. Parameter-varying PI algorithm is applied to the current loop to adjust the welding current. The system uses the average welding voltage to keep the arc stable.This paper has designed full-digital control system for pulsed MIG welding inverter power source by establishing hardware platform using ColdFire V2 core microcontroller MCF5213CAF80 and EP1C6T144C8. The main controller, MCF5213CAF80, is responsible for voltage-loop control and welding affairs management; and EP1C6T144C8, the coprocessor, realizes the current-loop control and digital PWM module. These two controllers communicate with each other through the parallel bus interface built in FPGA.Based on the digital PWM mechanism, this paper introduces a High-Speed Counter Method to design the digital PWM module in FPGA implemented by VHDL. Compared with analog PWM mode using dedicated PWM chips, this method simplifies the interface between control circuits and power conversion circuits, as well as enhancing the system's security and stability. Furthermore, the portability of real-time embedded operating systemμC/OS-Ⅱon MCF5213CAF80 makes it possible to design tasks independently and more convenient to manage welding affairs. Finally, this paper does some tests for the control system, including A/D conversion accuracy test, PWM output waveform test and control results test of welding process. Experiments prove that the system can control the current waveforms and average arc voltage precisely. The study helps to establish a solid foundation for subsequent complex research and expert system.