Any impairment of the electric contact between sensor scanning element and workpiece is, in the case of electric-contact sensors, problematic, e.g. welding spatters at the shielding gas nozzle, scale and rolling skin on the workpiece surface or through a wire electrode end which has molten spheroidally and has adherence of slag. When mechanical-contact sensors are used, the scanning elements must be adapted to the respective groove shapes. Butt welds with a square butt joint preparation must have a groove gap of more than 3 mm; in overlap joints the top plate must have a thickness of more than 3 mm. The sensor must be mounted separately from the welding torch. Thus, the groove scanning is mainly carried out in a leading position in front of the torch. If the welds are mainly straight, this adjustment is no problem. It is also possible to use scanning element arrays (e.g. fork callipers or separated scanning elements for height and lateral scanning which allow scanning in the torch level and thus weld scanning which is almost free from errors. Apart from the torch guidance along a weld groove, mechanical contact sensors can also be applied for detection of the weld start and end of the weld.
