Digital Control Strategy Research And Implementation Of PMIG Arc Welding Power Source

Digital inverter welding power supply can improve the welding efficiency and quality, compared to the traditional analog welding power. So it has been the development direction in the welding field. However, there are many technical challenges for us to overcome, while it achieves the process from analog to digital control, such as digital PWM drive and protection, sampling and smoothing welding signals, the choices of control strategy, real-time processing of data. Embedded and EDA technology provide some new solutions for these difficulties.By analyzing characteristics of welding current and the format mechanism of noises, this paper points out that the high-frequency action of power switching device is the main source of interference signals. In order to improve the accuracy of digital signal, a high-speed FIR digital filter is designed to reject the high-frequency noise. Also, the paper analyzes the PWM control theories based on the model of current and voltage, combines with the characteristic of primary current feedback and secondary current feedback, and determines DPWM control strategy based on the current model. The DPWM controller uses the secondary current as feedback signal to generate pulse of PWM, and adopts primary current together to produce the over-current protection signal. The controller can output high-precision PWM pulse and protect the main circuit.The current loop control strategy based on piecewise PI adjustor is designed after analyzing the molten-drops transition form of PMIG welding and the influence of pulse waveform for welding quality. It can output accurate welding current waveform, and ensure the form of one droplet per pulse. According to the control requirements of arc welding, the arc length adjustment strategy is designed, which can limit the pulse frequency and realize real-time adjustment function by the synchronous pulse. In order to simplify the welding parameter setting and operation, a unified regulation scheme for parameters is determined according to the relationship between the welding parameters. You can just set the value of welding voltage, and the system will give other corresponding welding parameters automatically. The paper designs a welding parameter control and data storage solution based on the USB and Ethernet, combining the current development trend of welding technology. The user can use the USB device or Ethernet to set the welding parameters and storage the welding current or voltage data.In order to implement the strategy of welding signal extraction and control, the paper uses the FPGA chip as the hardware platform's master unit, and a 32-bit NiosII processor is embedded into it. A large number of data are exchanged between NiosII processor and each function module by DPRAM inside the FPGA. The function modules based on verilog hardware description language, complete all the algorithm and data processing of the welding, which include double closed loop of voltage and current control, data sampling and filter, DPWM drive and the control of welding process. This system can process signal fast and exchange data largely. For the 32-bit NiosII processor, it is developed by C language. The NiosII processor controls the data that exchanged between FPGA and other board, and manage the welding sequence. With the NiosII processor, the methods of communication and control for FPGA system are more flexible. As to display board, digital tube and LED are used to show the welding data, and they can display the data in a long distance. Rotary encoder and buttons are used to input the parameters, by which we can adjust the parameter quickly. And for the hardware platform of communication solution, we select MCF52259 chip, which integrates USB and Ethernet module. So the design of hardware circuit will be simple and reliable.Finally, the paper tests the design of digital pulse-MIG welding power control system, including the function of each module and related welding experiments. The results of experiments show that under the control of the digital pulse MIG welding power control system, welding arc burns steadily, droplets transfer consistently, and weld joint shape uniformly.