Weld Seam Tracking Control Using Externally Applied Magnetic Field Swaying TIG Arc

Literature Overview

This paper, published in the Journal of Tianjin Polytechnic University (2025, Vol. 44, No. 5, pp. 93-99) by Yue Jianfeng and colleagues from Tianjin Polytechnic University and Beijing University of Chemical Technology, presents an innovative approach to weld seam tracking control in TIG welding. The method employs an externally applied transverse alternating magnetic field to induce flexible arc swaying, enabling extraction of the welding torch center deviation signal. This work addresses a practical challenge in automated TIG welding: maintaining weld accuracy during swing welding operations where the short electrode-to-workpiece distance creates a risk of tungsten contamination and damage.

Core Technical Findings

Problem Statement

In conventional TIG welding with mechanical swing, the short distance between the tungsten electrode and the workpiece creates a risk of:

• Tungsten electrode contacting the workpiece during swing motion
• Tungsten contamination from contact, leading to arc instability
• Electrode damage requiring frequent replacement
• Interrupted welding and reduced productivity

The proposed solution uses magnetic field-induced arc swaying instead of mechanical swing, eliminating the physical movement of the torch and thereby avoiding electrode-workpiece interference.

Magnetic Field Arc Swaying Principle

The transverse alternating magnetic field interacts with the electric current flowing through the TIG arc, producing a Lorentz force that deflects the arc laterally. The key characteristics are:

The magnetic field causes the arc root to oscillate laterally, creating variations in the arc voltage signal that encode information about the weld seam position relative to the torch centerline.

Signal Processing and Control

The signal processing chain comprises:

1. Extended Kalman Filter (EKF): Applied to the arc voltage signal to remove noise and extract the position information content.
2. Integration difference method: The filtered arc voltage signal is integrated over the left and right intervals of the arc swing, and the difference is calculated to determine the torch position deviation.
3. Mathematical model: A model relating the integration difference to the physical deviation is established.
4. Fuzzy self-tuning PID control: A fuzzy logic algorithm adjusts the PID controller parameters in real time to achieve stable correction and disturbance rejection.

Performance Results

The method achieves:

• Maximum tracking error controlled within 0.25 mm
• Significant improvement in TIG welding efficiency
• Enhanced weld seam tracking accuracy
• Stable torch correction and disturbance rejection capability

A maximum error of 0.25 mm is highly competitive with state-of-the-art weld tracking systems, which typically achieve errors in the range of 0.3-1.0 mm depending on the sensing method and control algorithm.

Engineering Practice Implications

Advantages Over Conventional Methods

The elimination of mechanical torch movement during swing welding is a significant advantage, as it removes a major source of electrode damage and arc instability. The use of the arc voltage signal itself as the tracking signal eliminates the need for additional sensors, reducing system complexity and cost.

Application Scenarios

This technology is particularly suitable for:

• Longitudinal seam welding of pipes and pressure vessels
• Circumferential seam welding of large-diameter pipes
• Welding of thick-section components requiring multi-pass swing welding
• Automated welding cells where torch manipulation is complex
• Applications where electrode contamination must be avoided

Integration with Existing Systems

The magnetic field swaying system can be integrated with existing automated welding setups by:

1. Adding transverse magnetic field coils around the welding area
2. Implementing the EKF signal processing in the welding controller
3. Adding the fuzzy self-tuning PID control loop
4. Calibrating the system for the specific welding parameters and joint geometry

Key Questions and Reflections

Several technical questions arise from this work:

1. Magnetic field strength optimization: What is the optimal magnetic field amplitude and frequency for different welding currents and joint geometries?
2. Interaction with other processes: How does the transverse magnetic field affect weld quality, penetration profile, and microstructure?
3. Multi-pass welding: Can the tracking system maintain accuracy across multiple passes with varying joint geometry?
4. Disturbance rejection: How does the system perform under conditions of varying workpiece geometry, misalignment, or vibration?

The use of the arc voltage signal as the tracking signal is elegant in its simplicity, but it also raises questions about signal quality and robustness. Arc voltage signals are inherently noisy and can be affected by many factors unrelated to torch position, including:

• Arc length variations
• Shielding gas flow rate changes
• Workpiece surface condition variations
• Electrode wear

The Extended Kalman Filter is well-suited to handle these disturbances, but the performance in real-world conditions with multiple simultaneous disturbances may differ from laboratory results.

Study Insights and Outlook

This research demonstrates a novel and practical approach to weld seam tracking in TIG welding that addresses a genuine engineering challenge. The use of magnetic field-induced arc swaying eliminates electrode-workpiece interference while providing a built-in sensing mechanism through arc voltage variations. The combination of Extended Kalman Filtering and fuzzy self-tuning PID control provides robust signal processing and control. The achieved tracking accuracy of 0.25 mm is competitive with existing systems, and the elimination of mechanical torch movement offers significant advantages in terms of electrode life and arc stability. Future work should focus on validating the method under production conditions, optimizing magnetic field parameters for different welding scenarios, and investigating the impact of the transverse magnetic field on weld quality and microstructure. This technology has the potential to improve the productivity and quality of automated TIG welding in pipe fabrication and pressure vessel manufacturing.