TIG Welding Seam Tracking Sensing System Using Image Method Without Auxiliary Light Source

Literature Overview

This paper by He Jing-Shan, Yang Chun-Li, Lin San-Bao, and Wang Qi-Long from the State Key Laboratory of Modern Welding Technology at Harbin Institute of Technology was published in the Welding Journal in 2000. The research presents an image-based seam tracking sensing system for TIG welding that operates without requiring an auxiliary light source, relying instead on the natural luminance of the welding arc and molten pool for feature extraction and tracking.

Core Technical Analysis

Seam tracking is a critical technology for automated welding of pressure vessels, where weld joints often have irregular geometries, fit-up variations, and complex configurations. Traditional seam tracking systems often require auxiliary light sources such as infrared lamps, ultraviolet emitters, or structured light projectors, which add complexity, cost, and potential interference with the welding process. This research demonstrates that the natural light emitted by the welding arc and molten pool contains sufficient information for reliable seam tracking.

System Architecture

The seam tracking system comprises several functional modules:

• Image acquisition: A high-speed camera captures images of the weld area at sufficient frame rate to track seam position changes.
• Image preprocessing: Noise reduction, contrast enhancement, and feature extraction algorithms process the raw images to identify seam edges.
• Feature recognition: The system identifies the weld groove geometry, including groove edges, centerline, and fit-up gap.
• Position calculation: Image coordinates are converted to real-world coordinates using camera calibration parameters.
• Control feedback: The computed seam position error is fed to the welding torch positioner for real-time correction.

Advantages of Auxiliary Light-Free Operation

The elimination of auxiliary light sources offers several advantages for pressure vessel welding applications:

• Reduced system complexity: Fewer components means lower maintenance requirements and higher system reliability.
• No spectral interference: The absence of auxiliary light eliminates potential interference with arc stability and plasma characteristics.
• Simplified shielding gas management: No additional light source means no additional obstacles to gas flow, maintaining optimal shielding conditions.
• Reduced cost: Eliminating auxiliary light sources and their associated electronics reduces system cost.

Technical Implementation Details

Image Processing Algorithm

The core algorithm extracts seam features from arc-lit images through the following steps:

1. Region of interest (ROI) selection: The area around the expected seam position is selected for analysis.
2. Intensity thresholding: The bright arc and molten pool regions are separated from the darker base metal.
3. Edge detection: Gradient-based algorithms identify the boundaries between base metal and groove.
4. Line fitting: Detected edges are fitted with mathematical lines to determine groove geometry.
5. Centerline calculation: The weld centerline is computed from the groove geometry.
6. Error computation: The difference between the actual torch position and the computed centerline is calculated.

Engineering Practice Applications

For pressure vessel fabrication, seam tracking systems are essential for:

• Circumferential welds: Tracking along the circumference of vessel shells, heads, and nozzles where fit-up variations are common.
• Nozzle-to-shell joints: Tracking complex geometric transitions where the groove geometry changes.
• Multi-layer welding: Maintaining weld position across multiple passes in thick-walled vessel components.
• Clad layer welding: Ensuring overlay layers are deposited precisely on the intended surface area.

Integration with Cladding Operations

In weld overlay cladding applications, seam tracking ensures that the overlay layer is deposited uniformly across the substrate surface. For pressure vessel cladding, this is particularly important when:

• Overlaying large surface areas with corrosion-resistant materials
• Maintaining consistent overlay thickness across curved vessel surfaces
• Achieving full coverage of the substrate surface without gaps or excessive overlap
• Controlling dilution by maintaining consistent torch-to-work distance and angle

Key Questions and Reflections

The auxiliary light-free approach raises important questions about robustness under varying conditions. The intensity and spectral characteristics of the welding arc can vary significantly with:

• Changes in welding current and voltage
• Variations in shielding gas composition and flow rate
• Different electrode materials and geometries
• Contamination of the weld area

These variations could potentially affect the reliability of image-based seam tracking. Additionally, the system must maintain tracking accuracy when the arc is occluded by spatter, smoke, or other visual obstructions common in production welding environments.

Study Insights and Outlook

This research demonstrates that the welding arc itself provides sufficient visual information for seam tracking, eliminating the need for auxiliary light sources. For the pressure vessel and cladding industry, this approach offers a cost-effective and reliable solution for automated welding of complex geometries. Future developments should focus on enhancing tracking robustness through advanced image processing techniques, expanding the system to handle multi-feature recognition (such as groove geometry, fit-up gap, and surface defects), and integrating with process control systems for real-time parameter adjustment based on tracked seam conditions.