They are used for very long welding or difficult access applications, as in big diameter tanks. They have a tactile sensor with a little stick which touches the joint. This is connected to a control which at the same time is joined together to motorized adjustments in two axes “X” and “Y”. The sensor is placed few inches in front of the welding torch and every time the joint moves out of the center, the control compensates with the adjustments.
They are quoted according to the size of the adjustments, from 3×3 up to 20×20 inches of free movement (although they can be of any size). It is possible to program start and finish points and tolerance ranges.