Development of AC Pulsed MIG Inverter Welding Power Source Based on Microcontroller Control

Literature Overview

This paper by Song Zheng, Hang Zhengxiang, Hua Xueming, and Wu Yixiong, published in Mining Research and Development in 2006, describes the design and implementation of an AC pulsed MIG inverter welding power source controlled by an Intel 80C196KC microcontroller. The work was conducted at the Welding Engineering Research Institute of Shanghai Jiao Tong University and the School of Materials Science and Engineering at Shenyang University of Technology. The paper details both the hardware architecture and software design of the power source, demonstrating stable and reliable operation that meets welding process performance requirements.

Technical Architecture

The AC pulsed MIG welding power source described in this paper represents a significant advancement in welding power supply technology. The system architecture comprises several key subsystems:

Hardware Design

The 80C196KC microcontroller was selected for its integrated PWM generation capabilities, sufficient processing speed for welding control algorithms, and proven reliability in industrial applications. The microcontroller manages the following control functions:

1. DC bus voltage regulation: Maintaining a stable intermediate DC voltage through PWM control of the front-end rectifier.
2. Inverter frequency control: Setting the switching frequency of the IGBT inverter stage, typically in the range of 10–30 kHz.
3. Pulse waveform generation: Producing the characteristic pulsed current waveform with defined peak current, background current, pulse frequency, and on-time.
4. Arc voltage regulation: Closed-loop control of welding voltage through comparison with a setpoint.
5. Wire feed speed coordination: Synchronizing wire feed motor speed with welding current to maintain arc length stability.

Software Design

The software architecture follows a modular design with interrupt-driven control for real-time response requirements:

• Main loop: Handles user interface updates, parameter storage, and non-critical monitoring functions.
• Timer interrupt (PWM generation): Generates the high-frequency inverter PWM signals at microsecond-level precision.
• ADC interrupt (sampling): Acquires welding current and voltage signals for feedback control.
• Timer interrupt (pulse waveform): Generates the low-frequency pulse waveform modulation by switching between peak and background current states.

The pulse waveform control algorithm implements the following logic:

1. At the start of each pulse cycle, the current ramps up to the peak current value.
2. The peak current is maintained for a defined on-time, during which the droplet is accelerated toward the workpiece.
3. The current drops to the background current level, allowing the molten pool to solidify partially and reducing heat input.
4. The cycle repeats at the set pulse frequency.

Process Performance

The AC pulsed MIG power source described in this paper demonstrates several advantages over conventional DC pulsed MIG welding:

• Reduced spatter: The alternating current component disrupts the magnetic pinch effect that causes excessive spatter in DC welding, resulting in cleaner welds with less post-weld cleanup.
• Improved penetration in thick sections: The AC waveform provides deeper penetration during one half-cycle and shallower penetration during the other, enabling better control of weld geometry in thick material.
• Reduced porosity in certain alloys: The AC component helps break up oxide films on aluminum and its alloys, improving wetting and reducing porosity formation.

Engineering Practice Considerations

In steel pipe manufacturing, particularly for the welding of thick-walled pipes and pipe fittings, AC pulsed MIG welding offers advantages that are worth considering. For example, in the manufacture of large-diameter spiral welded pipe (SSAW), the multi-pass welding of thick pipe walls benefits from the deep penetration capability of AC pulsed welding. Similarly, in the repair welding of pipe defects, the reduced spatter and improved weld appearance reduce the need for post-weld machining.

However, several practical considerations must be addressed:

• Electrode stickiness: AC welding can cause electrode stickiness at zero-current crossings, which must be mitigated through proper parameter selection.
• Arc stability: The alternating current component can destabilize the arc, particularly at low current levels. The microcontroller-based control system must implement robust arc stability algorithms.
• Equipment compatibility: The power source must be compatible with the wire feed system and gas shielding arrangement of the existing welding equipment.

Key Questions and Reflections

The use of the 80C196KC microcontroller in this 2006 paper reflects the state of the art at that time. Modern welding power sources typically employ more powerful DSPs or FPGA-based controllers that offer higher processing speed, richer peripheral integration, and more sophisticated control algorithms. However, the fundamental control principles described in this paper — PWM generation, closed-loop current and voltage control, and pulse waveform modulation — remain relevant and are implemented in modern systems with enhanced capabilities.

One area that deserves further investigation is the integration of adaptive control algorithms into the microcontroller-based system. In production welding environments, variations in material properties, joint fit-up, and environmental conditions can affect weld quality. Adaptive control algorithms that adjust welding parameters in real-time based on measured welding signals (current, voltage, acoustic emission) can compensate for these variations and maintain consistent weld quality.

Conclusion

This paper documents a well-designed AC pulsed MIG welding power source that demonstrates the capabilities of microcontroller-based digital control in welding applications. The systematic approach to hardware and software design, combined with the demonstrated process performance, provides a valuable reference for engineers developing or evaluating welding power sources. The transition from analog to digital control in welding power supplies represents a fundamental shift in the industry, enabling more precise process control, better repeatability, and greater flexibility in accommodating different welding applications.