Getting Good Seam Tracking with Automated Welding Equipment

With manual and semiautomatic welding, we often take for granted the important function the welder has in placing the weld pool at the correct location in the joint. Sure, we have tonnes of fancy welding equipment available to us, but the skill, especially with seam welds, ultimately belongs to the welder.

Mechanized and automated welding equipment can easily improve the welder's productivity by appropriate increases in wire feed speed, travel speed and operating factor. However, the ability to visually track the arc in such systems may not be practical and in many cases an alternative means must be used to achieve correct bead placement.

In fact, inadequate seam tracking is one of the most significant sources of quality problems in mechanized and automated welding.

Aside from correct bead positioning, the welder also tries to maintain correct torch-to-work distances. In the case of consumable electrode processes such as SAW and GMAW, the contact tip-to-work distance is an important variable that determines the welding amperage for a given wire feed speed. Seam tracking systems are able to adjust both electrode positioning in the joint and contact tip-to-work distance and therefore can provide full three-dimensional positioning control.

When does a seam tracking system become necessary?

Mechanized welding systems

Mechanized welding can be defined as welding with equipment that requires manual adjustment of the equipment controls in response to visual observation of the welding operation. Typically, a major part of the manual adjustment is the seam tracking operation visually performed by the operator.

Seam tracking welding systems become necessary in mechanized welding when the operator is required to spend a disproportionate amount of time in the seam tracking operation. A rule of thumb is to consider a seam tracking welding equipment system when the operator spends more than 1/3 of his arcing time on seam tracking adjustments. Operators of multiple-arc mechanized systems will most often exceed this value, making seam tracking a necessary component of such equipment.

Automatic welding systems

Automatic welding can be defined as welding with equipment that requires only occasional or no observation of the welding and no manual adjustment of the equipment controls. A welding robot is a typical example.

With automated welding, the options are relatively clear. Either parts are accurately prepared and positioned relative to the path of the welding arc or a seam tracking system must be used. If existing material processing cannot produce consistent parts with consistent seam locations, it is unlikely that weld quality will be acceptable on a long term basis.

Requirements for joint location accuracy will depend on the weld size, welding position and, under certain conditions, the welding speed. For fillet welds in the range of 3 mm to 5 mm, a rule of thumb that the joint location be repeatable within one-half the wire diameter is sometimes specified. For the GMAW process using 1.2 mm wire, this would require that the joint be consistently located within 0.6 mm.

To meet the above requirements in an automated system, consideration needs to be given to part size and thickness; material processing tolerances (cutting, forming and fitting); tolerance build-up as individual parts are combined into assemblies; fixturing and tooling design; and control of contact tip wear, contact tip-to-work distance, wire cast and helix in robotic applications, the use of touch sensing to determine joint location.

When required tolerances cannot be met on a consistent basis, a seam tracking system needs to be considered.

Available Welding Equipment for Seam Tracking Systems

Four basic types of seam tracking systems are readily available for welding applications:

Mechanical systems: Mechanical seam tracking welding systems typically use an arrangement of guide wheels that roll in the joint or on adjacent plate surfaces to provide a seam tracking function. Mechanical systems are relatively simple, robust and low cost compared to other types of systems. However, they are limited in application to the mechanized welding of larger parts with relatively long joint lengths. This type of system is widely applied with portable tractors using the SAW process.

Tactile probe: Electromechanical seam tracking welding systems can also provide effective joint tracking. The probe runs in the joint and electrically feeds back to horizontal and vertical cross-slides to move the welding torch for accurate placement of the electrode.

Tactile probes are limited to joints that have an appropriate surface for the probe to contact. Furthermore, because tactile probes track at some distance in front of the arc, they may not always be able to provide the necessary positioning at the welding arc location.

Through-the-arc systems: Through-the-arc welding equipment systems make use of electrical welding arc signals to control electrode position for three-dimensional control. The basis of the system is the relationship between welding amperage and contact tip-to-work distance.

In the past, through-the-arc systems were limited in travel speeds, the number of passes and minimum thicknesses in lap joints. However, recent advances in hardware and software have improved capabilities in all of these areas. These systems are widely used in robotic applications. An obvious advantage of through-the-arc systems is that no external probes or other devices are needed.

Laser welding vision systems: Laser welding vision systems use a laser-based industrial sensor, image processing hardware and special software to provide three-dimensional control. Laser welding systems work in the following manner: First, the sensor projects a beam of laser light onto the surface of the weld joint. Then, a video signal of the laser light is captured by a Charged Coupled Device (CCD) camera within the sensor. Next, the camera is inclined at about a 25-degree angle to the light beam to provide a profile image of the joint.

Following that, a processing board conditions and digitizes the video signal, translates it into digital joint profile information and sends it to the software and profile monitor. And then, the joint position information is analyzed by the software which determines what action is necessary.

Finally, position corrections are made as necessary by feeding back to Cartesian cross-slides or directly to the robot controller as applicableLaser welding vision systems offer a very wide range of capabilities. Generally, these laser welding systems have relatively few limitations in terms of travel speed, joint configuration, number of passes and are applicable to both mechanized and robotic welding. The two most premier laser vision system manufacturers--Modular Vision Systems (MVS) and Servo Robot, are both located in Montreal.

Why Inadequate Seam Tracking Needs to be Stopped

Inadequate seam tracking is one of the most significant sources of quality problems in mechanized and automated welding. A variety of seam tracking tools are available to enhance weld quality. In order to assist in determining the correct seam tracking option, consideration must be given to a variety of factors including the type of welding system employed, part tolerances, tooling, joint type of length, and weld size.