Manual welding requires skilled workers, as small imperfections in the weld can lead to severe conse- quences。 Furthermore, welders are exposed to haz- ardous working conditions (fumes, problematic er- gonomic working positions, heat, and noise) so  that the use of robots has become beneficial in GMAW processes even for the smallest lot-sizes。 Commonly, the automatic arc-welding process is based on a con- sumable wire electrode and a shielding gas that is fed through a welding gun。 Modern welding robots are particularly suited through the following characteris- tics:

● Computer control allows efficient programming of task sequences, robot motions, external actuators, sensors, and communication with external devices such as welding sources。

Fig。54。13a–c Offline programming of a spot welding workcell。 (a) The robot workcell and the task execution are mod- eled on the basis of realistic robot models (geometry, kinematics, kinetics)。 (b) The shown laser tracker is a portable measurement system that relies on laser beams to accurately measure in a radial volume (accuracy of ˙10 ppm D 10 µm=m, up to 80 m in diameter, measuring rate up to 3000 points=s)。 If measured objects cannot be equipped with reflecting targets or reached by the tracker, handheld probes are tracked instead。 (c) A tracker in use for interactively measuring the geometry of the robot workcell (courtesy of Leica, now Hexagon MI)

Fig。54。14a–d GMAW welding of building trusses in lot-size one by robot。 Illustration (a) shows the CAD drawing of a steel truss with relevant information for welding process, (b) one-half of the welding workcell with two welding robots working simultaneously on the truss when the neighboring truss on the other half is loaded or unloaded, (c) the laser-based seam finding and tracking sensors and (d) the welding robot (courtesy of Servo Robot, Canada; Goldbeck, Germany)

● Free definition and parameterization of robot posi- tions or orientations, reference frames, and paths。

● High   repeatability   and   positioning  accuracy of

paths。 Typically repeatability is some ˙0:05 mm and positioning accuracy is better than ˙1:0 mm。 These values can be significantly improved through modern robot calibration methods [54。42]。

● High speeds of the end-effector of up to 8 m=s  for

quick approach and depart motions。

● Typically, articulated robots have six DOF so   that

commanded orientations and positions in their workspace can be reached, which in the welding case means there is one DOF free for rotation around a rotational-symmetric welding tool。 Ad- ditionally, workspace extensions by mounting the robot on a linear axis (seventh DOF) or even on mobile bases are common, especially for welding of large structures。

● Typical payloads range 6—150 kg。 Higher load ca-

pacities are required for spot-welding guns (typi- cally > 50 kg) and their cable package。

● Programmable  logic  controller  (PLC) capabilities

such that fast input/output control and synchroniz- ing actions within the robot workcell are accom- plished。

● Interfacing to high-level factory control through factory communication networks。

Electric current sources, torches, and peripheral devices for automatic cleaning and maintaining the GMAW torch (anti-splatter, wire-cutting, tool changer, etc。) are offered by specialized companies。 Often sen- sors are used to track welding gaps and measure weld seams either before or synchronously with the weld- ing process, thus adapting the robot’s trajectory in the presence of workpiece variation and distortion。 Also, collaborating robots have been introduced where one robot fixes and moves the workpiece in synchronization with another robot carrying a welding tool so that the weld can be performed with the pool of molten metal horizontal。