Welding is one of the most economical and efficient ways to join metals permanently. It is a common way of joining two or more pieces of metal to make them act as a single piece or monolithic structure. Welding is used to join all of the commercial metals and to join metals of different types and strengths. Welding is vital to the economy. It is often said that up to 50% of the gross national product of a country is related to welding in one way or another [1,2].
Welding is a very important technique used in manufacturing. It is also a very important technique used in construction, as almost everything made of metal is welded. The use of welding is still increasing. Arc welding equipment represents approximately half of total welding equipment, and arc welding is expected to grow at a rate of 6% annually.
1.2 What is Welding?
Welding is a joining process that produces coalescence of materials by heating them to the welding temperature, with or without the application of
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pressure or by the application of pressure alone, and with or without the use of filler metal. A weld is a localised coalescence of metals or non-metals produced through welding. Coalescence is the growing together or growth into one body of the materials being welded.
A weldment is an assembly of component parts, joined by welding. It can be made of many or a few metal parts whose compositions may differ.
The parts may be in the form of rolled shapes, sheet, plate, pipe, forgings, or castings. To produce a usable structure or weldment, weld joints are made between the various pieces of the weldment. There are five basic types of joints for bringing two members together.
• Butt joint: two parts in approximately the same plane.
• Corner joint: two parts located approximately at right angles to each other.
• T joint: parts at approximately right angles, in the form of a T shape.
• Lap joint: between overlapping parts in parallel planes.
• Edge joint: between the edges of two or more parallel parts.
The technology of welding is complex. Welding is continuing to grow, yet the industry is changing rapidly. Among these, gas metal arc welding (GMAW) and flux-cored arc welding (FCAW) have become the most popular methods of arc welding.
GMAW is an arc welding process that uses an arc between a continuous filler metal electrode and the weld pool. The process is used with shielding from an externally supplied gas and without the application of pressure. It was developed in the late 1940s for welding aluminium and has become very popular. This process is also called metal inert gas (MIG) welding.
There are many variations depending on the type of shielding gas, the type of metal transfer, the type of metal welded, and so on. These include, for example, MIG welding, C02 welding, fine wire welding, spray arc welding, pulse arc welding, dip transfer welding, and short-circuit arc welding.
FCAW is an arc welding process that uses an arc between a continuous filler metal electrode and the weld pool. The process is used with shielding gas from a flux contained within the tubular electrode with or without additional shielding from an externally supplied gas, and without the application of pressure. This is a variation of GMAW and is based on the configuration of the electrode.
1.3 Automated Welding
The demand for improved weld quality, reduced welding cost, and increased productivity continues, especially in view of the improved materials and fabricating methods. Automated robotic arc welding is no longer a metalworking curiosity but has become commonplace in the metalworking industry. It plays an important role in industries to speed up reliable fabrication work, as it produces high quality and consistent welds that pass the strictest radiographic checks. This has helped take the human welder away from the arc and fumes. It has helped to clean up the welding environment. Yet, the automation of welding has lagged behind the automation of other industrial production processes. This is because welding is more complex than many other industrial processes [2].
1.4 Need for Seam Tracking
As we know, the arc welding processes involve heat. High temperature heat is largely responsible for welding distortion, warpage, and stresses. When metal is heated it expands in all directions. When metal cools it contracts in all directions too. Distortion usually occurs in six main forms, such as, longitudinal shrinkage, transverse shrinkage, angular distortion, bowing and dishing, buckling, and twisting. Thus, the seam or the pre-programmed path for automated welding may be distorted once welding is in progress.
Hence, there is a need to detect the effect of these distortions and track the actual seam path rather than the pre-programmed path. This requires on- line seam detection and correction (Figure 1).
Figure 1 Weld seam.
The term "seam tracking" refers to a low-level control mechanism wherein the welding torch is precisely controlled during welding to position it on the weld seam irrespective of variations in the unstructured seam environment due to thermal distortion, fixture error, improperly prepared weld joints and other such causes.
There are quite a few ways to do seam tracking, such as using mechanical, electrical, sonic, magnetic, or optical sensors [3, 4]. However, two techniques that have received most attention are the Through-the-Arc sensing and optical sensing which will be discussed next.