Microcomputer-Controlled TIG Welding Automatic Arc Starting System

Literature Overview

This 1995 paper by Zhou Haobin and Jia Changshen from Xi'an Jiaotong University describes a practical microcomputer-controlled automatic arc starting system for TIG welding. The system uses a microcontroller to mimic the manual contact-starting action, achieving high automation and a high first-attempt arc starting success rate. The system is simpler than auxiliary electrode methods and avoids the interference and damage that high-frequency arc starters can cause to control systems. The authors argue that this system enables full computer control of TIG welding equipment, contributing to higher automation levels.

Background and Motivation

Automatic arc starting is a critical function in automated TIG welding systems. In manual TIG welding, the welder initiates the arc by briefly touching the tungsten electrode to the workpiece and then lifting the electrode away. This contact-starting method is reliable and does not require additional equipment such as high-frequency oscillators or pilot arcs. However, replicating this action automatically is challenging because it requires precise control of the electrode movement, timing, and current.

Before this work, automated TIG welding systems typically used one of two methods for arc starting: high-frequency (HF) arc starters or auxiliary electrode (pilot arc) methods. HF starters generate a high-voltage, high-frequency signal that ionizes the gap between the electrode and workpiece. While effective, HF starters can cause electromagnetic interference with control electronics and can damage sensitive components. Auxiliary electrode methods use a separate electrode to strike the arc, which adds complexity and cost to the system.

System Architecture and Operation

The system described in the paper uses a microcontroller to control the arc starting sequence. The basic operation is as follows:

1. The microcontroller commands a solenoid or motor to move the tungsten electrode toward the workpiece.
2. When the electrode contacts the workpiece, a current sensor detects the contact.
3. The microcontroller immediately commands the electrode to retract to the welding position.
4. The welding current is established, and the arc is maintained.

The microcontroller provides fast data processing and precise timing control, which are essential for reliable arc starting. The contact detection is achieved using a current sensor that monitors the welding circuit for the characteristic current spike that occurs when the electrode contacts the workpiece.

Advantages of the Contact-Starting Method

The contact-starting method has several advantages over HF and auxiliary electrode methods. First, it does not require a high-frequency oscillator, which simplifies the system and reduces cost. Second, it does not generate electromagnetic interference, which is important for the reliability of control electronics. Third, it uses the same tungsten electrode as the welding process, eliminating the need for a separate auxiliary electrode and its associated consumables.

The microcontroller-based control provides additional benefits. The fast processing speed ensures that the electrode is retracted promptly after contact, minimizing the risk of electrode sticking or contamination. The precise timing control ensures that the gas flow is established before the arc is struck and that the arc is established before the electrode is retracted.

Reliability and Success Rate

The paper reports a high first-attempt arc starting success rate, which is critical for automated welding operations. A low success rate would require repeated attempts, leading to electrode contamination, workpiece damage, and reduced productivity. The reliability of the system depends on several factors:

• The condition of the tungsten electrode tip: A sharp, clean electrode tip provides a more reliable contact.
• The condition of the workpiece surface: Oxidation, paint, or other surface contaminants can impede contact.
• The force applied during contact: Too little force may not establish contact, while too much force can damage the electrode tip.
• The timing of the retraction: The electrode must be retracted quickly enough to prevent sticking but not so quickly that the arc is extinguished.

Engineering Practice Implications

This system is particularly relevant for automated TIG welding applications in pipe and fitting fabrication. In automated welding cells, the reliability of arc starting is critical because a failed arc start can disrupt the entire welding sequence. The contact-starting method, controlled by a microcontroller, provides a reliable and cost-effective solution.

In my experience with automated welding systems, the contact-starting method is particularly well-suited for welding materials that are difficult to arc-start using HF methods, such as aluminum and its alloys. The method is also well-suited for welding in environments where electromagnetic interference is a concern, such as near sensitive electronic equipment.

Study Insights and Reflections

This paper, though published in 1995, represents an important contribution to the automation of TIG welding. The use of microcontrollers for welding control has become ubiquitous in modern welding systems, and the principles described in this paper are still relevant today. The emphasis on simplicity and reliability is a valuable lesson for engineers designing automated welding systems.

One limitation of the system is that it relies on physical contact between the electrode and workpiece, which can lead to electrode wear and contamination over time. In high-production environments, this may require frequent electrode dressing or replacement. However, the simplicity and reliability of the method make it a practical choice for many applications.