Research On Low Heat Input Variable Polarity Short-circuit Transfer Gas Metal Arc Welding System

Wide application of sheet metal and ultra thin sheet metal welding technology in car, containerand other manufacturing leads to the requirements of low heat input during welding process.Short-circuit transfer control of variable polarity is a novel low heat input welding method, which hasbeen drawing more and more attention. However, this method is still in the initial stage ofdevelopment. Additionally, there is little foreign reference, and domestic research is still in its infancy.Therefore, studying control and energy distribution principle of short-circuit transfer in-depth,proposing ideal control schemes and developing a welding system platform are important topics in thedevelopment of welding technology.In order to realize the low heat input and short-circuit transfer control of variable polarity,dual-chip DSP controlled welding power supply system was introduced in this paper, which wascomposed of three parts, the main circuit, control circuit and wire-feeding system. Dual chips wererespectively responsible for the control circuit and the wire-feeding system, both communicating viaCAN bus. The two-stage inverter structure was adopted in main circuit. Full-Bridge Zero Voltage andZero Current (FB-ZVZC) soft-switching inverter was applied to the first inverter, which obtained therequired energy during welding process by PWM control. The half-bridge inverter structure withcoupled inductor was applied to the second inverter to realize current polarity transformation so that itcan achieve the purpose of low heat input. Control system based on DSP as the chip of the entiresystem timing control, used digital PID algorithm to digitized PWM control. Use numerical method offixed point sampling and full cycle control strategy to ensure the control precision and stability. Tosolve the problem that traditional soft-switching inverter cannot output low power below700W, thetwo-stage continuous control of PWM was creatively put forward to enable inverter switch betweenfull-bridge and half-bridge, thereby the problem caused by low power output of soft-switchinginverter can be solved and low heat input can be realized.The digital wire-feeding system is an important part of the digital welding power supply system,whose performance directly affect the accuracy and stability of the welding process. Therebyincreasing the steady-state accuracy and fast response of wire-feeding system is an inevitable issueduring the welding process. A limited unipolar reversible speed control circuit was designed in thispaper. Control chip adopted DSP and communicated with welding power control system throughCAN bus. According to the armature induction voltage sampling when current was zero, digital PImethod was used to adjust the PWM duty cycle to maintain the armature induced voltage constant, thus to ensure a constant wire-feeding motor speed. It used fuzzy PI control technology for onlinetuning of PI parameters to improve dynamic performance of wire-feeding system. Tests showed thatthis system can feed wire stably and its static and dynamic performance is better than that of thearmature voltage negative feedback control system. Variable speed wire-feeding system based on arcvoltage negative feedback was proposed to solve the problem of arc instability caused by sectionalconstant current control. This system used double-loop fuzzy PI control. The inner loop adoptedinduced voltage negative feedback fuzzy control to improve the stability of wire-feeding speed andthe ability of fast response. The outer loop adopted arc voltage negative feedback to adjustwire-feeding speed and ensure the stability of the arc length. Tests proved that this method canachieve constant arc length and equal penetration depth control.Basing on the force analysis of molten droplets during short circuit transfer process, a waveformcontrol method was adopted, that reducing current to eliminate the instantaneous splash in initial shortcircuit period and reducing current quickly before liquid bridge blasting off in the final period of shortcircuit. In arcing period the peak current and the background current was adopted to control arcenergy precisely and improve the weld formation. Basing on thermal effect analysis of the wire andthe work piece during DCEP and DCEN period, a variable polarity control method was adopted thatemploying DCEP to DCEP at the end of the short-circuit phase to ensure the stability of arc andmolten droplet transition, employing DCEN in the early of arcing for rapid melting of welding wire,less molten pool impact and high deposition efficiency, and employing DCEN to DCEP later in thearcing period to facilitate the droplet transition through shaping the drop. A current control methodwas proposed in order to avoid arc-quenching triggered by polarity reversal later in the arcing period.And large arc pulse was applied before the current through zero to ensure the smooth progress of thecurrent commutation. In addition, an arc method consisting of short-circuits current limiting and slowwire-feeding was adopted to improve the success rate of arc ignition.Basing on the analysis of the physical characteristics of short arc transition, a GMAW weldingpower-arc system dynamic simulation model was created by Matlab/Simulink. Power conversionunit used practical application circuits and devices as the prototype. This model was able to achieve afixed arm zero-current switching, zero-voltage switching of moving arm. Adjusted the duty of movingarm to verify the presence of the issue that soft-switching inverter circuit had minimum power output."Two-stage continuous PWM control method” could reduce the power output of the system. Digitalcontrol unit could realize real-time computing main circuit IGBT drive signal according to the arcvoltage and current signals to achieve sectional constant current control. So that the welding currentand the given wave control signal had a good consistency. Short circuit load cell took the droplet dynamic process into account during short period, which included the model of arc length changesand the liquid bridge circuit model during arcing period. Simulation waveforms were consistent withthe experimental results, which proved that the system simulation model was correct.After a lot of experimental researches carried out in a homemade variable polarity the GMAWwelding power platform, it was proved that this variable polarity short circuit welding method is a lowheat input welding method. The paper also elaborated on the experimental research relating to theeffect of various parameters on the energy allocation by using this method. The corresponding lawwas drawn in this paper as well.