Study On Dgital Control System Of Variable Polarity PMIG Welding Equipment

Variable Polarity Pulsed MIG (VP PMIG)is a consumable electrode AC pulse weldingprocess, it can used to weld aluminum alloy, mild steel and stainless steel. The advantages such aswelding arc energy, wire melting rate and penetration of weld can be controlled by regulate theratio of EN time to the period of varied polarity pulse current. VP PMIG is appropriate to weldthin plate of aluminum alloy and efficiency is great.A digital control system of VP PMIG arc welding power source is developed based on DSP,the topology and the output current waveform designed can meet the requirement of thin platewelding. Waveform of Varied polarity current is realized by using IGBT as switches of secondaryinverter, constant welding current is realized by inside loop of feedback of current, regulate of arclength can be realized by outside loop of feedback of arc voltage. A full bridge high voltage pulsedarc stabilizing circuit which was powered by single power source is designed to avoid ofextinguishing of cross zero welding arc. This circuit can maintain the stable of arc di-directionallywhen small background current cross zero, ensure the stable burning of welding arc when weldingcurrent cross zero.DSP (TMS320F2812) is the control kernel of VP PMIG power source. Hardware circuit issimplified by using software to control complicated welding process.As to control of weldingprocess,a way of digitital PI arithmetic is used for closed loop of constant-current.CPLD(EPM7128STC100) is the PWM kernel of digital control system.The digital control ofPWM is developed,without the fault of former ananlog control of PWM,the new control of PWMis much more convenient,dependable and cheaper.With the ability of logic programed as toCPLD,the protect circuit is designed to control the signal of PWM.The digital controlling system is used in welding power source .we program constant-currentcontrol software and research respectively the current and voltage step response of power source.The experiment proved that the welding inverter can control the constant-current waveformsprecisely and meet the design requirements.