A method for positioning a welding head or welding torch of a robot welding system over a workpiece sends microwaves as a measuring signal from a transmitter arranged on the welding head to the workpiece. The microwaves reflected on the workpiece are received by at least one receiver arranged on the welding head, and the received microwaves are evaluated by an evaluation module for determining the position of a workpiece edge. The microwaves are sent from at least one transmitter in different positions on the welding head, and the reflected microwaves are received, with a change of polarization, by the at least one receiver, having a polarization plane arranged at an angle to the polarization plane of the transmitter. The position of the edge is determined by the evaluation module at least on the basis of a phase change of the respective microwaves reflected on the different positions.