# Research of a Multi-Frequency Waveform Control Method on Double-Wire MIG Arc Welding
## 1 Introduction  
To improve heat efficiency and avoid excessive current which may deteriorate the quality of welding seam, double-wire submerged arc welding was proposed in the middle of the last century [1], and then was commonly employed in many relative occasions. However, because submerged arc welding employs granular materials as shield, it can only be applied in flat position of welding. To overcome this limitation, double-wire metal inert-gas (MIG) arc welding was then proposed, and increasingly employed in both of academic and practical industrial areas [2; 3]. During past decades, the introduction of automation and robotic operation in industry required real-time monitoring and control of the welding process [4]. To improve the double-wire MIG arc welding system and obtain products with more satisfactory quality, as well as more stable welding process, many scholars and engineers took much effort to explore how to improve the efficiency to achieve the goals during the process. [PERSON] et al. [5] studied the modeling and controlling of the consumable double-wire MIG arc welding process, and concluded that the process offered unique advantages but required appropriate control to materialize the advantages, and then they selected proper inputs and outputs for an interval model control system and obtained a desired welding process. [PERSON] et al. [6] evaluated the influence of parameters of adjacent arcs in double-wire MIG arc welding and single arc in gas tungsten arc welding (GTAW) on the occurrence of arc interruption, thenconfirmed that proper distances between the adjacent arcs and intervals between the current pulses can eliminate the arc interruptions. [PERSON] et al. [PERSON] et al. (2017) studied the effect of peak of pulse voltage on the metal transfer and welding seam formation by means of a self-designed experimental platform, and then concluded that the effect of peak of leading wire current on metal transfer was significant, high peak of pulse voltage could induce good welding seam, but excessive voltage might deteriorate the quality of welding seam. [PERSON] et al. [PERSON] et al. (2018) conveyed new observations on the arc stability in anti-phase synchronized synergic-pulsed two-wire MIG arc welding, and concluded that the separation of arcs in time by anti-synchronization and a synergic pulsed can be better utilized to control the arc instability. [PERSON] et al. [PERSON] et al. (2019, 2020) studied twin wire indirect arc by simulation and experiments with different arc lengths, and then obtained the characteristics of arc under welding currents with different values during the process. [PERSON] et al. [PERSON] et al. (2019) established a three-dimensional (3D) model to explore the effect of welding current on the arc plasma behavior, and then conducted actual experiments to compare the simulation results and obtained a good agreement. All of the these previous works showed that the application of double-wire MIG arc welding can play an important role in current practical industry and be focused by more and more relative researchers and agencies. As for the control action, majority of previous works focused on the current waveforms control because it was a main operation during the process. For the leading wire current and trailing wire current, there were some different forms, such as having the same frequencies but inverse phases (SFIP), or having the same frequencies together with the same phases (SFSP). Generally, previous works considered that, although the noise was small and the process steady, welding penetration was small when the SFIP was employed [PERSON] et al. (2019). In other work, [PERSON] et al. [PERSON] et al. (2019) used a method of delay the pulse off timing of trailing arc by 0.4-0.5 ms from that of leading arc, to prevent adverse effects caused by electromagnetic interactions between two adjacent arcs to prevent arc interruption. In this work, SFSP was used. In addition, [PERSON] et al. [PERSON] et al.