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cause dynamic variation in the seam position and also in the gap size. These factors
may induce quality problems in the weld, which may lead to part rejection.
2.4.1 "Ideal world" versus "Real world"
"Ideal world" is adopted here to describe the idealised graphical models of
the robot and its environment, as used in the simulation and off-line programming.
"Real world" refer to the actual geometric shapes of the various components of a
welding cell, including robot links, as well as their relative positions. Graphical
modelling in the "ideal world" is based on the nominal values of dimensions and
positions of the several components contained in a robotic cell.
There are normally inevitable differences between the computer model and the
real world [refs. 73,90]. These differences can come from many sources. For
instance, the robot may not be built or perform exactly according to the
manufacturer's specifications, the torch mounting position is not exactly determined,
there will be tolerances associated with tooling and parts, the layout of the cell may
not be exactly as used in the simulation, some dimensional variation or even some
displacement
in the workpiece will occur during the manufacturing process as in the
case of welding. Whatever its source, the discrepancy normally induces the off-line
generated program to work improperly. There are several ways of reducing or
overcoming these problems. One solution is to edit the program on-line using the
robot's teach pendant [ref. 82]. Depending on the amount of editing involved, this
may or may not be a time and cost effective solution. Another method is to use the
real robot to locate some keypoints within the cell and to modify the world model on
the simulation system [refs. 88,91,92]. The simulation system will thus contain an
accurate model of the environment, as the robot "sees" it. This approach however
does not compensate for dynamic movement during welding process. A third
technique of compensation for these discrepancies is through the use of sensors on the
real robot [ref. 73,88]. These may take the form of vision systems, tactile sensors,
simple limit switches etc. and will depend on the particular application. The robot
program should be created off-line in such a way as to make use of the information
provided by these sensors to ensure that the positioning accuracy is maintained in the
real cell.
2.4.2 Robot calibration
The success of an off-line programmed robot operation will depend on how
close the "ideal world" matches the "real world". The robot plays an important role in
matching both "worlds" since it is responsible for carrying the tool (welding torch, in
the case of arc welding) through the programmed path. This is normally described in
terms of world co-ordinates which are defined relative to the robot's World Co-
ordinate Frame (see Figure 2.14). The robot controller uses a mathematical model of
the robot structure to calculate the positions of the axes necessary to be attained for
the tool to reach the command pose. If the model does not match the robot, the
attained pose will be different from the command pose, resulting in inaccuracy
problems.
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