There can be two issues that prevents the robot from welding successfully. Inaccurate joint location and poor joint fit up (i.e gaps). It is always preferable for the robot to weld quality parts, but sometimes this is not achievable, for instance in the case of thick plate welding where bending, or forming tolerances mean that a joint cannot be presented in the same location every time.
The location of a joint can easily be detected using a tactile sensor by the robot
itself using the end of the welding wire or welding shroud. These are charged with
a voltage. When the robot is put into search mode the wire or shroud will make contact
with the work piece. The robot stops immediately and registers the position by sending
the relevant information back to the robot controller. Searches can be carried out
in three directions, X, Y and Z as deemed necessary. Each directional search takes
about 1.5 seconds to perform, which is not of too much consequence for a heavy welding
application where welding cycle times are long, but for a small assembly it is not
cost effective since it increases cycle times too much to search for joint locations.
Customers should therefore ensure that the upstream equipment delivers quality sub-
For heavy welding applications it is not uncommon that due to the affect of heat input from the welding process distortion takes place. As a result the joint may deviate from the programmed path of the robot. This is also easy to adjust using a through the arc sensor. The robot is put into weaving motion and as the arc length shortens during the welding process, a predefined limit is reached which activates the robot to move in the opposite direction and so forth and thus following the seam. This kind of sensor is suitable for fillet welds of 6 mm length and above. It is not able to compensate for varying gap conditions.
The principle of the through the arc seam following sensor:
1) Middle of the join
2) Weaving oscillation
3) Welding current / feedback signal
These are bolt-