| Welding technology is one of the most important industrial technologies. With the development of modern industry, the technical requirements of the welding power source are becoming higher. Compared to traditional analog welding power source, digital arc welding inverter power source has many advantages, such as high control accuracy, good stability and flexible controllability and it is the future development trend of welding power source. Submerged arc welding (SAW) is a widely used welding process and it has the advantages of automated welding, high welding efficiency, and good labor conditions. Domestic SAW power source products are mainly analog. There is still a wide gap between domestic SAW power source and foreign SAW power source and the foreign companies still occupy the high-end SAW power source market. So the research and design on digital SAW welding inverter has important theoretical and applied significance.Two digitalization methods are analyzed and they are based on primary peak current feedback and secondary current feedback. Then a control scheme based on digitalized double close-loop peak current feedback is adopted. It controls the SAW welding inverter power source by the PWM generator. The scheme can restrain magnetic bias of transformer and has the advantages of fast system dynamic response and accurate current output. According to the control system requirements, the characters of SAW arc is analyzed and the arc length adjustment strategy is designed which is based on PWM generator controlled by the PID regulator. Welding current, welding voltage and carrier speed is the object of the closed-loop control strategy.In order to meet the hardware requirements of digitalized SAW inverter power source, the system hardware scheme is designed whose core is FPGA, which embed a32-bit Nios II processor. The hardware circuit of digitalized double close-loop peak current feedback is consists of D/A converter, slope compensation circuit, comparator and primary peak current collection circuit. According to the control requirements of welding voltage and carrier speed, the drive hardware circuit of wire feed motor and carrier motor is designed. The A/D converter circuit is designed which can convert the analog signal such as welding current, welding voltage and carrier motor armature voltage into digital ones. The communication circuit of main control system is designed and the welding parameters can read and write through man-machine interface.The software control system is divided into several modules, for example the welding process algorithms module, data processing module, digitalized double close-loop peak current control module, DPWM module and so on. These modules are designed by VHDL hardware description language. The embedded Nios II processor is based on C software language and it processes the data communications and welding timing. The DPRAM is designed to meet the requirements of the data interaction between FPGA and Nios II processor.The simulation model of digitalized double close-loop peak current feedback control system is designed to verify the stability and flexibility of the system and point out the direction for further research. All of the digital SAW inverter power source modules are tested and welding experiments are carried out. The experiment results show that the digital SAW inverter power source which is designed by this paper can stably control the welding arc and can meet the requirements of SAW process. |
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