Zhuojin Pipe Fitting Co., LtdAC Pulsed MIG Arc Welding Power Source and Arc Length Control System
Literature Overview
The paper by Hang Zhengxiang, Yin Shuyan, and Huang Pengfei, published in Transactions of the China Welding Institution (2003, Vol. 24, No. 2, pp. 83-85), presents the design and control methodology for an AC pulsed MIG (ACP-MIG) arc welding power source with arc length control. The research was conducted at Beijing University of Technology, a leading institution in welding research in China.
Process Principles
The ACP-MIG process operates on a "one pulse per cycle, one droplet per cycle" control mode. The pulse current is designed to occur during the electrode positive (EP) polarity period of the AC waveform, while the remaining time is occupied by the base current. This arrangement exploits the deep penetration characteristics of DCEN welding (electrode positive) while maintaining the AC benefits of self-cleaning action on oxide films and reduced arc blow.
| Parameter | Description |
|---|---|
| Control mode | One pulse per AC cycle, one droplet per cycle |
| Pulse timing | During EP polarity period |
| Base current | Occupies remaining time of AC cycle |
| Primary inverter | Controls current fast dynamic response |
| Secondary inverter | Controls arc polarity |
Control System Architecture
The control system employs a dual closed-loop structure:
- Inner loop: Analog control of arc current, providing fast response to current transients
- Outer loop: Microcontroller-based control of arc voltage and arc polarity, using an 80C196KC single-chip microcomputer
This hierarchical control architecture separates the fast current regulation from the slower voltage and polarity control, allowing each control function to operate at its optimal bandwidth.
Four Control Rules
Based on the arc voltage deviation signal and arc state, the system implements four control rules with corresponding parameter calculation methods:
- Normal welding state: Variable frequency control of AC pulse frequency to maintain stable arc length
- Arc length too long: Increase pulse frequency or adjust current parameters to increase arc energy and pull the arc closer
- Arc length too short: Decrease pulse frequency or adjust current parameters to reduce arc energy and allow the arc to extend
- Arc instability: Adjust pulse parameters to stabilize droplet transition and arc characteristics
Engineering Significance
The ACP-MIG process addresses several challenges in AC welding of aluminum and aluminum alloys:
- The AC waveform provides cathodic cleaning action that removes oxide films from the workpiece surface, which is critical for aluminum welding
- The pulsed current control enables short-circuit-free or reduced short-circuit droplet transfer, minimizing spatter and improving weld quality
- The arc length control system ensures consistent weld geometry and penetration depth throughout the welding process
For aluminum alloy welding applications, the ACP-MIG process offers advantages over conventional AC MIG welding:
| Feature | Conventional AC MIG | ACP-MIG |
|---|---|---|
| Droplet transfer | Spray or short-circuit | Pulsed, controlled |
| Spatter level | Moderate to high | Low |
| Arc stability | Variable | High |
| Penetration control | Limited | Precise |
| Heat input control | Limited | Precise |
Study Insights
The paper represents a significant contribution to AC welding power source technology, particularly for aluminum alloy welding applications. The dual closed-loop control architecture with microcontroller-based outer loop represents a practical implementation of modern digital control in welding power sources.
The "one pulse per cycle, one droplet per cycle" control philosophy is elegant in its simplicity and effectiveness. By synchronizing the pulse with the AC cycle and the droplet transition, the process achieves a highly stable welding arc with consistent energy input. This level of process control is essential for automated welding applications where repeatability and consistency are paramount.
The remaining challenge is the practical implementation of the four control rules in real-time. The 80C196KC microcontroller, while adequate for the time, represents a relatively modest computational platform. Modern implementations would use more powerful processors and advanced signal processing algorithms to achieve even better arc control. Nevertheless, the fundamental control philosophy presented in this paper remains valid and has been adopted in subsequent ACP-MIG power source designs.
The work by Hang et al. exemplifies the integration of power electronics, control theory, and welding physics in the development of advanced welding processes. For practitioners involved in aluminum alloy welding, the ACP-MIG process offers a viable alternative to conventional AC MIG welding, particularly for applications requiring high quality and consistency in automated or semi-automated welding operations.