Zhuojin Pipe Fitting Co., LtdMicrocomputer Control System for Welding Current in TIG Pipe Welding Machine
Literature Overview
The paper by Lei Yi from the Department of Mechanical Engineering, China University of Petroleum, published in Petroleum Engineering Construction in 2000 (Vol. 26, Issue 3, pp. 29–33), describes a microcomputer-based control system for the welding current of a TIG pipe welding machine. This work is particularly relevant to the oil and gas industry, where long-distance pipeline construction requires high-quality, consistent welds over thousands of kilometers of pipe. The paper introduces a control system that uses a single-chip microcomputer (MCU) to implement a parabolic fitting algorithm for smooth welding current regulation, replacing the traditional segmented current control approach with a continuous, programmable current profile.
Technical Background and Problem Statement
In TIG pipe welding, the welding current profile is critical for achieving consistent weld quality, particularly for all-position welding of large-diameter pipes. The traditional approach uses a segmented current control, where the welding current is set to discrete values for different phases of the welding process (e.g., start-up, steady-state, and termination). This approach results in abrupt changes in current, which can cause arc instability, spatter, and weld defects such as undercut, porosity, and incomplete fusion.
The problem is exacerbated in all-position welding, where the gravity effects on the molten pool vary with the welding position. In the overhead and vertical positions, the molten pool tends to sag, requiring higher current to maintain penetration, while in the flat position, lower current may be needed to prevent burn-through. The segmented current control cannot smoothly transition between these different current levels, leading to inconsistent weld quality.
Control System Architecture and Algorithm
The proposed control system uses a single-chip microcomputer (MCU) as the central processing unit, with a parabolic fitting algorithm to generate smooth current profile curves. The parabolic fitting algorithm approximates the desired current profile using a series of parabolic segments, each defined by three points (start, peak, and end). The MCU computes the current value at each time step and outputs a control signal to the power supply to adjust the welding current accordingly.
The system architecture includes the following components:
| Component | Function |
|---|---|
| Single-chip microcomputer (MCU) | Central processing unit; executes the parabolic fitting algorithm |
| Current sensor | Measures the actual welding current and provides feedback |
| Power supply | Generates the welding current based on the MCU control signal |
| Operator interface | Allows setting of welding parameters and monitoring of the process |
| Anti-interference circuitry | Protects the MCU from electromagnetic interference (EMI) from the welding arc |
The parabolic fitting algorithm is a significant improvement over the segmented approach because it provides a continuous, smooth transition between current levels. The algorithm is implemented in the MCU firmware and can be adjusted by the operator to match the specific welding requirements of the pipe joint.
Anti-Interference Measures
One of the key challenges in implementing a microcomputer-based control system for welding is the electromagnetic interference (EMI) generated by the welding arc. The welding arc produces high-frequency noise and voltage spikes that can disrupt the MCU operation and cause erroneous control signals. The paper describes several anti-interference measures, including:
- Hardware filtering: Using RC filters and ferrite beads to attenuate high-frequency noise on the power supply and signal lines.
- Software filtering: Implementing digital filtering algorithms in the MCU firmware to reject noise spikes and smooth the control signal.
- Grounding and shielding: Proper grounding of the MCU and signal lines to minimize common-mode noise.
- Isolation: Using optocouplers or transformers to isolate the MCU from the high-voltage welding circuit.
These measures are essential for ensuring the reliability and accuracy of the control system in the harsh electromagnetic environment of a welding operation.
Engineering Practice and Application
The microcomputer-based control system described in this paper is particularly suited for automated TIG welding of pipelines, where consistency and reproducibility are critical. The oil and gas industry requires welds to meet stringent standards such as API 1104, ASME B31.3, and SY/T 4109, which specify requirements for weld quality, non-destructive testing, and mechanical properties. The smooth current profile provided by the parabolic fitting algorithm helps achieve these requirements by reducing arc instability and weld defects.
The system can be integrated into a larger automated welding cell, where the MCU coordinates the welding current, travel speed, torch position, and filler wire feed rate. This level of integration is essential for achieving high productivity and consistent weld quality in large-scale pipeline construction projects.
The paper also highlights the importance of the anti-interference measures, which are often overlooked in the design of welding control systems. In practice, EMI can cause intermittent failures, erratic current control, and even damage to the MCU. The described measures provide a practical framework for designing robust control systems that can operate reliably in the welding environment.
Reflections and Study Insights
This paper represents an early application of microcomputer technology to welding process control, a field that has since seen significant advancement in digital control, programmable logic controllers (PLCs), and industrial robots. The parabolic fitting algorithm is a simple but effective approach to generating smooth current profiles, and the concept can be extended to more complex profiles using higher-order polynomial fitting or spline interpolation.
The paper is also notable for its emphasis on anti-interference measures, which are critical for the reliability of any electronic control system in a welding environment. The described measures—hardware filtering, software filtering, grounding, and isolation—are standard practices in industrial electronics, but their specific application to welding control systems is valuable and practical.
One limitation of the study is the lack of experimental data comparing the performance of the parabolic fitting algorithm with the traditional segmented approach. The paper describes the system architecture and algorithm but does not provide quantitative data on weld quality, arc stability, or defect rates. Future work should include comparative experiments to demonstrate the advantages of the smooth current profile over the segmented approach.
Additionally, the paper does not address the adaptability of the control system to different welding conditions, such as variations in pipe diameter, wall thickness, or material grade. In practice, the welding parameters must be adjusted for each specific joint, and the control system should provide a flexible interface for the operator to input these parameters.
In conclusion, this paper describes a practical and effective microcomputer-based control system for TIG pipe welding, using a parabolic fitting algorithm to generate smooth welding current profiles. The system addresses the limitations of traditional segmented current control and provides a robust solution for automated pipeline welding. The emphasis on anti-interference measures is particularly valuable for engineers designing control systems for welding applications, and the paper serves as a useful reference for the implementation of digital control in welding processes.