| With the development of welding technology and industrial automation, the demand of welding quality is becoming higher and higher. But traditional analog welding machine have many problems in the application,for example aging and drifting of analog chips can lead to low control accuracy and analog circuit design has very low efficiency.Compared with analog welding machine, digital welding machine not only can greatly improve the welding efficiency but also can improve stability.So it becomes mainstream now. Compared with other welding methods Pulsed MIG welding has the following advantages:it has expanded the welding current adjustment range;it can effective control of droplet transfer and droplet size, it can conducive to all-position welding and it can effectively control the heat input. However, the process of digitalization encounters a considerable number of difficulties. Drive and protection of the digital PWM, Voltage and current control methods and so on.In this paper, we analyzes the existing problems of full digital pulsed MIG welding machine, then designs the high-speed digital inverter drive circuit based on FPGA, welding current control scheme, welding arc control scheme and human-machine interface.Through analyzing the control requirements of welding current, welding arc and melting droplet transfer of pulsed MIG, the paper designs a double feedback control loop based on welding current and welding arc voltage. Voltage loop is responsible for the stability of arc length.We design the arc length adjustment strategy. In order to keep the arc length stable, welding arc length control adjusts the base-value current time to make sure the average voltage of the arc output following a given average voltage input. Current loop is used for the constant current source. Variable PI controller is also analyzed and realized in FPGA in order to achieve better time response of pulse current control. The paper also designs digital PWM drive circuit. It uses the given signal, the feedback signal and the sawtooth wave together to generate the drive pulse. The digital PWM controller uses the secondary current as feedback signal to generate PWM pulse, and adopts primary current together to produce the over-current protection signal.Now in the digital welding machine,we often use DSP+MCU program.But in this paper, we select the FPGA chip as the hardware platform’s master unit, and a32-bit NiosⅡ processor is embedded into it.This paper also design16-bit microcontroller as the core display board and related hardware circuit. In the FPGA we seclected EP3C10E144Cyclonelll family,Altera Corporation.In the16-bit microcontroller we selected Freescale9S12series MC9S12DG128. The control of welding process is implemented in FPGA, including output current control, arc length control, arc-igniting process, arc collection process and short-circuits proecss. PWM drive is also realized in FPGA. PWM drive can be configured into different PWM freqencies and provides overcurrent protection of main circuit. Nios Ⅱ is used to process some communication tasks and welding timing control. Dual-port RAM is used to exchange data between softwares FPGA and Nios Ⅱ. Nios Ⅱ also control from the wire feeder, stop, close the valve and wire feed speed. As to display board, digital tube and LED are used to show the welding data, Rotary encoder and buttons are used to input the parameters, by which we can adjust the parameter quickly.It be responsible for the welding parameters of output; welding parameters and waveform data generated.The designed system is tested, including the digital PWM test, the control of arc test, the current loop test, welding process test and the whole welding test. It indicates that the welding process of the designed system is smooth and achieves the control requirement.Finally, the paper concludes the main work, points out the deficiencies of the system and makes improvement plans for the next step. |
PDF Full Text Request