Research on ARM and CPLD-Based Pulsed TIG Welding Power Supply

Literature Overview

This research paper by Liu Qiang and Song Yonglun from Beijing University of Technology, published in Electric Welder in 2012, presents the development of a multifunctional TIG welding system based on dual ARM microcontrollers and Complex Programmable Logic Device (CPLD) technology. The system employs a combination of a pulse power supply and a base current power supply to achieve flexible current output for various welding applications. The paper details the system architecture, control algorithms, and experimental validation of the power supply performance.

Core Technical Findings

The system architecture features two independent power supply units: a pulse power supply and a base current power supply. The pulse power supply uses a dual-loop PI (Proportional-Integral) control method to achieve precise pulse current output, while the base current power supply uses a single-loop control method to maintain constant base current output. This dual-power-supply architecture allows the system to generate complex current waveforms that combine pulsed and DC components, enabling advanced welding processes such as pulsed TIG, spray transfer, and short-circuit transfer.

The dual-inverter architecture is a key innovation of the system. Each power supply unit contains an independent inverter stage that converts the DC bus voltage to the required AC output frequency. The two inverter stages operate independently but are synchronized by the ARM microcontrollers to produce the desired combined current waveform. This architecture provides greater flexibility than a single-inverter system, as the pulse and base current components can be controlled independently.

The experimental results demonstrate the system's ability to accurately reproduce programmed current waveforms across a range of pulse frequencies and pulse widths. The current response characteristics show fast rise and fall times with minimal overshoot, indicating effective control loop design. The system also incorporates special handling for overcurrent and overvoltage conditions, with automatic shutdown and alarm functions that enhance system reliability and safety.

The PI control algorithm is implemented with digital filters to improve noise rejection and dynamic response. The A/D sampling system includes anti-aliasing filters to prevent frequency folding, and the control period is synchronized with the inverter switching frequency to minimize control delay. The system achieves a current control accuracy of within ±2% of the setpoint, which is sufficient for most TIG welding applications.

Process and Standards Analysis

Pulsed TIG welding is widely used in pipeline welding for root pass applications, as specified in standards such as ASME B31.3, API 1104, and EN ISO 15614. The pulsed current waveform allows precise control of heat input, penetration, and bead geometry, which is critical for achieving consistent root pass quality across all positions and pipe sizes.

The pulse parameters (pulse frequency, peak current, background current, and duty cycle) directly influence the weld bead characteristics:

• Pulse frequency: Controls the number of pulses per second, affecting bead width and cooling rate.
• Peak current: Determines the penetration depth per pulse.
• Background current: Maintains the arc and provides baseline heat input.
• Duty cycle: Ratio of peak current time to total pulse period, controlling total heat input.

For nuclear and critical service applications, the welding power supply must comply with specific requirements for traceability, reliability, and quality assurance. The system described in this paper incorporates features that support compliance with these requirements, including parameter recording, fault detection, and safety interlocks.

The CPLD technology used in the system provides advantages over traditional microcontroller-based control in terms of response time and parallel processing capability. The CPLD can handle fast-switching logic and timing functions that would be difficult to implement in software alone, enabling precise control of the inverter switching sequence and current waveform generation.

Integration with Engineering Practice

In pipeline welding practice, pulsed TIG welding is the preferred method for root pass welding of girth welds on large-diameter pipes (typically DN200 and above). The pulse parameters are optimized for each pipe size, wall thickness, and material grade to achieve full penetration with minimal distortion and controlled HAZ width. The power supply described in this paper offers the flexibility to implement these optimized parameters with high accuracy and repeatability.

A practical application scenario involves the welding of X70 carbon steel pipe with a wall thickness of 12.7 mm for a long-distance transmission pipeline. The root pass is performed using pulsed TIG welding with the following parameters: pulse frequency of 10 Hz, peak current of 200 A, background current of 80 A, and duty cycle of 40%. The power supply system must accurately reproduce these parameters throughout the entire weld length, with automatic adjustment for changes in pipe diameter and wall thickness due to manufacturing tolerances.

For stainless steel pipe welding, the pulsed TIG process is used to control heat input and minimize intergranular corrosion susceptibility. The pulse parameters are selected to maintain the interpass temperature below 150°C and to achieve a cooling rate that promotes a fine-grained microstructure. The power supply system must provide precise control of the background current to maintain the arc without excessive heat input during the off-pulse period.

Key Questions and Reflections

The paper raises several important questions for the practical application of the developed power supply system. First, the dual-inverter architecture increases system complexity and cost compared to a single-inverter system. The cost-benefit analysis must demonstrate that the improved current waveform flexibility justifies the additional hardware and development costs. For high-volume pipeline welding applications, the improved weld quality and reduced rework rates may justify the higher system cost.

Second, the system's reliability and maintainability must be evaluated for field deployment. Pipeline welding is often performed in remote locations with limited access to spare parts and technical support. The system must be designed for robust operation in harsh environments, with protection against moisture, dust, vibration, and temperature extremes. The use of modular design and standardized components can improve maintainability and reduce downtime.

Third, the integration of the power supply with the overall welding system (including torch, wire feeder, positioner, and control system) must be carefully planned. The communication protocol between the power supply and the welding controller must be reliable and support real-time parameter adjustment. The system should also support remote monitoring and data logging for quality assurance purposes.

Study Insights and Implications

This research demonstrates the effectiveness of ARM and CPLD-based digital control for pulsed TIG welding power supplies. The dual-power-supply architecture provides the flexibility to generate complex current waveforms that are essential for advanced welding processes. The dual-loop PI control method achieves precise current regulation with fast dynamic response, which is critical for maintaining consistent weld quality. For pipeline and pressure vessel manufacturing, this technology offers a pathway to improved welding quality, productivity, and process control, contributing to the development of more reliable and efficient welding systems for critical infrastructure applications.