Touch Sensing - http://www.fanucrobotics.com/products/Robots/welding-robots.aspx
Touch Sensing is a low cost solution for locating weld seams when the part geometry or tooling.
Touch Sensing is used to teach the FANUC robot where a part is in space. In Touch Sensing, two searches are performed on the X-axis and the Y-axis to determine the location of the part in relation to the robot.
The locations are then applied to the weld path to assure the path is aligned with the part. The search is repeated on a part that is misplaced and a new weld path is created based on the position of the part.