LIBRARY ı6ıul 0) - Cranfield University

Hence, the X-axis and the Y-axis of the torch co-ordinates frame will be contained in
the referred planes.
For approach orientations contained in planes between planes 3 and 4, the
torch co-ordinates frame will be rotated about its X-axis by an angle between 0 deg
and 90 deg, proportionally to the angle of rotation of the plane which contains the
approach direction, from 120 deg to 225 deg. Figure 3.19 and Figure 3.20 show the
resulting robot positions for approach orientations contained in planes between planes
3 and 4.
For approach orientations contained in planes between planes 1 and 2, the
torch co-ordinates frame will be rotated about its own X-axis by an angle between 0
deg and -90 deg, proportionally to the angle of rotation of the plane which contains
the approach direction, from 60 deg to -45 deg. Figure 3.21 shows the resulting robot
positions for approach orientations contained in planes between planes 1 and 2.
Approach orientations contained in planes between planes 2 and 4 cannot be
defined. These rules were devised in order to take advantage of the mounting position
of the welding torch relative to the robot's end joint.
Another important aspect of off-line programming is the definition of the
robot path for approaching and withdrawing from the weld joint. In order to generate
the points necessary to define such a path, it was assumed that all the welds were
located on the external surface of the workpiece. The path which the torch must
follow to achieve a weld start point is defined by the weld approach points and the
path the torch must follow to withdraw from a weld end point is defined by the weld
withdrawal points. Up to four weld approach points and up to four weld withdrawal
points are defined automatically by the program for each weld, depending on the
orientation of the approach vectors at the weld start and end points and on the size of
the workpiece. The aim of these approach and withdrawal points is to prevent
collisions between the welding torch and the workpiece. This is achieved by defining a
workpiece extension box, which is an imaginary box whose dimensions are such that
the workpiece is fully contained inside it (see Figure 3.22). This extension box is given
in terms of two points16, situated at two diagonally opposite corners of the imaginary
box: a) a point with the minimum co-ordinates of the box and b) a point with the
maximum co-ordinates of the box. With the minimum and maximum co-ordinates of
the box, the other corners are also obtained. These points are then transformed to the
robot world co-ordinates frame by using equation (3.16) and a second imaginary box
is defined, the workpiece clearance box (see Figure 3.22), whose corners are obtained
by subtracting a clearance value (e. g. 50 mm) from the minimum transformed co-
ordinates and adding the same clearance value to the maximum transformed co-
ordinates. The workpiece clearance box will define the limits outside which the
welding torch can be moved without colliding with the workpiece.
Figure 3.23 shows an example of how the weld approach points are obtained.
In the example of this figure, to achieve a weld start point, the robot must follow the
weld approach points from ap3 to apl. The robot is always restricted to linear
movements between points, thus ensuring a predictable path. Note that the torch
orientation in ap3 is such that the X-axis of the torch co-ordinates frame points
downwards (-Z world co-ordinate direction) and the Z-axis of the torch co-ordinates
16 Relative to the CAD world co-ordinates frame
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