Page 124 - Welding Robots Technology, System Issues, and Applications

Robotic Welding: System Issues 111

In fact these laser cameras and related processing hardware and software, with
some customization to the selected application [20], are very useful for evaluating
most of the geometric parameters besides the already mentioned joint detection and
seam tracking features. Since they are available with powerful APIs for general
use, with standard interfaces for robot controllers and current computer hardware,
this type of sensors constitute a powerful tool for robotic welding.

Another very important and challenging parameter is the penetration. Basically, a
good weld has constant penetration along the weld path, and consequently the
welding system should be capable of keeping that goal despite possible variations
in the joint geometry. If full penetration is achieved then there are some methods to
observe the penetration on-line, but in cases of partial penetration there are no
means to monitor its evolution. Several methods were designed to measure the
penetration [21] when full penetration is achieved, but most of them require back-
face bead measurements requiring access to the back of the work-piece, which isn’t
always possible. Measuring front-face bead geometrical characteristics, along with
the weld bead temperature provides the means to estimate the penetration. This
means a good understanding of the welding process behavior, so that a precise
model, correlating two dimensional measurements of the weld pools with the three
dimensional shape of the same weld pool, can be written and used for on-line
penetration control. Using ultrasonic techniques is also possible [21] as an
alternative, since a full model description is difficult to obtain and, although
desirable, has not been achieved yet.

Metallurgical aspects are also important for the welding quality, since they
determine important mechanical characteristics like hardness, soundness, strength
and residual stresses. Those very important mechanical parameters are not easy to
measure on-line and are a consequence of several mechanisms. Nevertheless, they
all result from the heat generated in the welding process. And since the welding
process is basically based on heat, the following is needed to guarantee an
acceptable weld:

1. A certain peak temperature is needed to achieve a good metal fusion and
penetration.
2. A roughly uniform temperature distribution, centered in the weld joint, is
required to achieve a constant weld.
3. Acceptable cooling rates, compatible with the required metallurgical
characteristics of the final work-piece, are also needed.

All these requirements focus on the need to monitor all the thermal events of the
welding process, which adds to the other geometrical measurements requirements
that a successful robotic welding system must implement.