Robot Welding

The essential guide

Sensing equipment for welding robots
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Sensing technology

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.

Joint location sensor

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-components. Other less common methods of location the start position include a mechanical probes or a laser point sensor.

Seam following sensor

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 is illustrated below:
1) Middle of the joint
2) Weaving oscillation
3) Welding current / feedback signal

Laser tracking devices

These are bolt-on units that can both locate a joint and/or track the seam using a beam of light that is projected onto the surface of the material and reflected back into the sensing head. Joint location is very fast. In general these devices use a triangulation method to calculate the path and instigate off-sets to the robot program when tracking a joint. Since they are bolt on units, they can cause obstructions making it difficult for the robot get joint access. The investment cost is also fairly high. However, they can be used for light gauge welding applications including lap joints and may also have adaptive fill capability, which means that the welding process can be adjusted for varying gaps. It is highly unusual to see this kind of sensor in the average robotic welding application, but it can solve some pressing issues. An example could be the welding of door hinges on a car body where it is not practical to weld these manually.

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