You may want to go back to your robot company and see what they have to offer... I've worked with ABB robots, and through the years they have used optical type, laser type, and through the arc tracking. Some of the characteristics of each goes like this... heavy smoke and spatter may become a problem with optical lenses and somewhat with laser lenses,,, shiny metals or heavy scratches on alum. can be difficult for laser,,, also clamps or fixturing to close to seam may become the weld seam with optics and laser.. Don't know your application but I would say through the arc tracking is what you want.(If your MIG welding)????? ABB's latest tracking equip. is called A.W.C " Advanced Weld Control" , it compares the Impeadence of the welding arc on the left and right side of the seam, if the seam moves - robot adjusts. Also can sense a width change and will speed up or slow down the robot for proper fill. Need more info drop me a line.

LARRY