LIBRARY ı6ıul 0) - Cranfield University

errors', (i. e. joint positioning, joint fit-up, joint component tolerances) and that these
are normally restricted in size by the allowable manufacturing tolerances, the
adjustments required in workpiece positioning would be small, therefore involving
low speeds and consequently, low dynamic loads.
Another advantage of using a moving table is the freedom of the user in
setting the range of movement of the axes, thus allowing easy adaptation to different
requirements in torch-to-workpiece relative position adjustment. One disadvantage,
though, is the necessity to use a robot independent controller to control the
positioning table, which might be considered as an added complication. However, the
additional flexibility provided by this approach compensates for this supposedly
increased complexity in the system.
4.1.2 Positioning table
Considering the aspects discussed above, it is suggested to use a three or more
degrees-of-freedom positioning table, on which the workpiece would be mounted, for
joint position adjustment. During the welding process only three degrees of freedom
with orthogonal moving directions would be needed for adjusting the relative joint
position, the remaining degrees of freedom being used for improving the orientation
of the workpiece a pre-weld operation, such that flat and horizontal welding
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positions could be favoured.
To implement this type of system, however, it would be necessary to use a
robot independent controller to control the in-process moving axes. Such a controller
would move the workpiece based on information provided by sensors. The directions
of movement and the amount by which each axis should be moved could be easily
calculated by using the information provided by the sensors together with the torch
approach vector and the joint tangent vector. The torch approach vector provides the
direction of movement necessary for adjusting the stand-off and the cross product of
both vectors would give the direction of movement necessary for seam tracking.
These vectors are directly obtained from the teach points file generated by the off-line
programming system.
To synchronise the movements of the robot with the positioning table, digital
inputs and outputs can be used. These are normally available in robot controllers.
In the present work, a table with only one degree of freedom was implemented
in order to prove the effectiveness of the position control strategy. Figure 4.1 shows a
sketch of the positioning table implemented.
4.1.3 Proposed control strategy
The position control strategy was based on adjusting the initial position of the
weld joint in a pre-weld operation and on the in-process adjustments of position
deviation compared to a pre-specified reference value.
' Assuming that the robot and its cell are correctly calibrated and that the programming errors have
been eliminated
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