LIBRARY ı6ıul 0) - Cranfield University

of a robot in its zero position12 with a welding torch attached to the end of the robot
arm. The torch position relative to the robot arm in its zero position is assumed to
follow the configuration shown in Figure 3.14. The torch co-ordinates frame is
defined such that its origin lies at the end of the electrode wire" with its X-axis
parallel to the contact-tip longitudinal axis, pointing to the same direction "'the
movement of the wire when it is being fed. The Z-axis and the Y-axis are expected to
follow the directions shown in Figure 3.14. In order for the robot controller to
recognise the torch co-ordinates frame used for off-line programming, a Tool Centre
Point14 (TCP) must be defined accordingly. The procedures involved in defining such
a TCP vary with the robot type and are normally specified in the robot's operation
manual.
Considering that the approach vector (X-axis) is provided for a point in the
weld, it is only necessary to define the orientation of one more axis to fix the
orientation of the torch in that point. The rules used to determine the torch orientation
were devised from experience in on-line programming an IRB2000 industrial robot
with a curved welding torch attached to the robot end joint (see Figure 3.14). It is
assumed that the robot is installed with its base fixed on the floor and the positioning
table is placed in front of it, as in Figure 3.5.
In order to explain the torch orientation rules, consider the sphere shown in
Figure 3.15. It is used to illustrate how the robot should behave in order to achieve
different approach orientations, which are represented here as normal vectors to the
sphere surface. The co-ordinates frame with origin in the centre of the sphere is
parallel to the robot world co-ordinates frame, i. e. they have the same orientation.
Consider that meridians are traced on the sphere surface as in Figure 3.15: each
meridian together with the X-axis of the sphere co-ordinates frame will define a semi-
plane that is rotated by a certain angle around the X-axis, from the XY-plane of the
sphere co-ordinates frame. Now consider the semi-planes shown in Figure 3.15 and
Figure 3.16. These planes are used to define regions in which different orientation
rules are used. No matter what the torch approach direction is, it will always be
perpendicular to the sphere surface, thus defining a plane rotated about the sphere's
X-axis by a certain angle. For angles between planes I and 3 (rotation angle between
60 deg" and 120 deg from the XY-plane) the Z-axis of the torch co-ordinates frame
is fixed such that it is tangent to a meridian. This implies that both X-axis and Z-axis
of the torch co-ordinates frame will be contained in the plane formed by the meridian
and the X-axis of the sphere co-ordinates frame. Figure 3.17 and Figure 3.18 show
the resulting robot positions for approach orientations contained in planes between
planes 1 and 3.
For approach orientations contained in planes 2 and 4 (rotation angles of -45
deg and 225 deg from the XY-plane, respectively) the Y-axis of the torch co-
ordinates frame is fixed such that it is tangential to the corresponding meridian.
12 The robot zero position is defined as the robot configuration in which all the robot joints are in
their respective zero positions.
13 Considered to have 15 mm stick-out and to be aligned with the contact tip longitudinal axis.
14 The Tool Centre Point defines a transformation matrix between the co-ordinates frame attached to
the end of the tool and the co-ordinates frame attached to the end of the robot arm. This
transformation matrix vary depending on the tool configuration.
13
"deg" in this context stands for angular degrees. 1 deg - 1/360 of a complete revolution.
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