Design Of Main Control System Of Full Digital Welding Power Supply Based On FPGA

Analog circuit controller of inverter welding machine has many disadvantages. Aging and drifting of analog chips can lead to low control accuracy. Analog circuit design has very low efficiency. Digital inverter welding power supply has the characteristics of good flexibility, high precision and fast response. It also can provide digital communication interfaces and friendly user interface. So it is the direction of developing welding power supply. However, the process of digitalization encounters a considerable number of difficulties. Digital PWM cannot suppress overcurrent of main circuit and magnetic bias of main transformer. Digital control is discrete and have calculation delays. The process becomes unreliable with the increase of calculation burden. All of this hampers the realization of digital control of inverter welding machine.By analyzing the whole structure of digital inverter welding power supply and the control methods of pulsed MIG, the paper determines the scheme of welding current control, the scheme of welding arc length control in pulsed MIG and the scheme of digitizing PWM drive. Welding process control is based on inner current control loop and ourter welding arc control loop. Current control loop uses variable parameter PID to achieve the desired output current according to given current waveform, which can guarantee the ideal form of melting droplet transfer. Arc length control adopts improved I-I mode control. The method depends on average arc voltage to change the pulse frequency, which will ensure the dynamic matching of wire melting speed and wirefeeding speed. And this dynamic matching ensures the stability of welding process.In the paper, the digital PWM drive is realized in Field Programmable Gate Array (FPGA). ADC samples the output current of the inverter welding power supply. And then PWM module uses the output current as the feedback to realize current-mode PWM. This ensures the reliable drive of IGBT brigde. At the same time, in order to protect the main circuit, the current of primary side of transformers will compare with a protection threshold. If over-current happens, the comparator will generate an over-current signal. This signal is sent to digital PWM module in FPGA. The digital PWM module will shutdown PWM output to protect the main circuit once it detects the over-current signal.This paper combines the current embedded technology and leading-edge EDA technology. And the paper selects " FPGA+embedded soft-core Niosâ…¡" as the control unit of the system. The control of welding process is implemented by VHDL in FPGA, including output current control, arc length control, arc-igniting process, arc collection process and short-circuit proecss. PWM drive is also realized in FPGA. PWM drive can be configured into different PWM freqencies and provides overcurrent protection of main circuit. Niosâ…¡is used to process some communication tasks and welding timing control. Dual-port RAM is used to exchange data between softwares relized by VHDL in FPGA and softwares realized by C programming language in Nios II.The paper debugs and tests the design of digital inverter welding power control system, including testing of digital driver board, the current inner loop test and dynamic load test. Experiments show that the welding power supply can output accurate current waveform and can ensure the stability of arc length. And it can handle the arc ignition, arc collection and short ciruit condition correctly.