The gap between where the program says the joint is and where it is on this particular part. Cutting, forming and tacking tolerances stack up, fixtures wear, and heat moves the work while welding. Tracking measures the real position continuously and offsets the path, which either raises quality at existing tolerances or allows looser, cheaper upstream tolerances.

A sensor mounted ahead of the torch projects a laser line across the joint and a camera reads its deformation, reconstructing the joint profile by triangulation. From that profile the controller extracts the seam centre, the gap and the area to fill, and offsets the robot path accordingly, typically many times per second while the weld runs.

Tracking that uses the arc itself as the sensor. As the torch weaves across the joint, welding current rises and falls with the distance from tip to work, and the controller infers the groove centre and height from that variation. It needs no hardware at the torch and is immune to spatter and smoke, but requires a weave and a joint with sensible geometry.

Adjusting deposition to a joint whose cross-section varies. When the sensor reports the area still to be filled, the controller changes wire feed speed, travel speed, weave width or the number of passes to suit. Without it, a program set for the nominal gap burns through where the gap is tight and leaves an under-filled weld where it opens up.

Laser tracking where gap varies, joints are complex or adaptive fill is wanted, and the budget supports it. Through-arc where joints are simple grooves and fillets, weaving is acceptable, and cost or sensor survivability matters. Touch sensing where the part's placement varies but the joint itself is consistent. Many cells combine touch sensing with one tracking method.

Aluminium and polished stainless are the difficult cases, because specular reflection can saturate or confuse the sensor. Sensors intended for them use specific wavelengths, filtering and exposure control, and mounting angle becomes more critical. It is worth trialling on the actual material and surface condition rather than relying on a general specification.

Ahead of the torch in the direction of travel, far enough to read undisturbed joint but close enough that the correction stays relevant, and angled to keep spatter and direct arc light out of the optics. Because the lead distance is direction-dependent, joints welded in several directions need the sensor position handled in the program.