LIBRARY ı6ıul 0) - Cranfield University

1. Introduction
Welding is the third largest fabrication process used in the metal working
industry, after assembly and machining. It is a special process that requires skilled
operators to achieve good weld quality. However, the high cost of skilled welders and
the demand for higher productivity and consistent weld quality have led to an
increasing use of robots in welding operations.
Before a robot can execute a task it needs programming. In the case of robotic
arc welding, most of the operations are programmed on-line, via a teach pendant. This
results in a downtime, since the production line must be stopped during the
programming phase. For an established product design with high volume production,
this downtime might not be critical. However, the programming time could represent
a considerable amount of the total costs in cases such as small batch production and
short life products. To reduce this downtime, off-line programming can be used.
With off-line programming (OLP), the robot is programmed remotely without
interrupting the production machine, by using a computer work station or a personal
computer (PC), and suitable software. The robot movements can be programmed,
simulated (and corrected, if necessary) on the computer and finally downloaded to the
robot controller for prompt execution. Hence, off-line programming should solve one
of the outstanding robot application problems, which is the downtime cost due to on-
line programming. Another benefit of OLP is that the component design data available
in CAD drawings could be used to define the welds during the programming task.
However, OLP has not been widely adopted by the industry for demanding
applications such as resistance and arc welding, because the current systems require
lengthy calibration sessions after the programming phase and this may consume most
of the time saved by using the off-line technique. This is due to the inaccuracy of the
geometrical models used to represent the robot, the welding cell and the workpiece,
plus other factors such as robot absolute accuracy, calibration of the workcell,
fixturing and workpiece positioning, and dimensional tolerances.
It should be noted that the time for programming can be chosen to best fit in
with the manufacturing cycle. Also, in the event of positional errors due to errors in
tool offset (for example, due to accidental collision between the tool and the
workpiece), a previously calibrated robot need not be totally reprogrammed. These
make off-line programming an obvious choice for robot programming if the need for
post-programming calibration can be reduced or eliminated.
It is generally accepted that for quality welds to be produced, consistent and
precise positioning of the wire tip relative to the joint line and consistent joint fit-up
must be ensured. Inaccuracy in OLP could, therefore, influence weld quality by
causing joint-to-wire tip positioning errors, resulting in bead misplacement. This could
also cause variation in the contact tip-to-workpiece distance (stand-off) which could
result in arc instability and inadequate penetration. It should be noted that these
problems become more pronounced when welding thin sheets, since the weld sizes are
generally small, requiring tighter positioning tolerances. Therefore, for this work an
adaptive workpiece positioning system was proposed and a stand-off monitoring and
control strategy developed.
1