LIBRARY ı6ıul 0) - Cranfield University

4. On-line Control Strategy
In this chapter, a detailed explanation about the implemented control strategy
will be given, along with a description of the control algorithms used and the
assumptions made.
4.1 Control of relative position between welding torch and workpiece
The control of torch-to-workpiece relative position involves two main aspects:
a) the control of the contact tip-to-workpiece distance (i. e. stand-off) and b) the
control of the torch lateral displacement relative to the joint longitudinal axis (i. e.
seam tracking).
Such control entails providing some means of adjusting the position of the
torch relative to the workpiece during the welding process. To accomplish this, two
approaches can be used: a) the adjustment of the robot wrist position relative to the
stationary workpiece or b) the movement of the workpiece relative to the torch. The
first approach is only possible if the dedicated systems designed by each robot
manufacturer are allowed for, which results in limited flexibility. The second approach
offers high flexibility, since the same system could be used for many different robots.
A comparison between both approaches will follow.
4.1.1 "Robot position adjustment" versus "workpiece position adjustment"
At a first look, it seems straightforward to use the robot to adjust the torch
position relative to the workpiece during the welding process, since it is already
moving the torch in a pre-programmed path with a pre-determined speed. However, it
is not so simple to modify this pre-programmed movement during program run time.
This would involve real time computationally intensive calculations, which are not
always available in the robot controller or accessible by the end user. Also, the
method each robot controller uses to generate reference signals for its joints in order
to move its end effector to certain positions in space within a specified time frame is
not standardised. Each robot manufacturer adopts different techniques and these are
generally not released to end users, for safety reasons. When the robot is supplied
with functions which allow modification of path during the program play-back, the
extent to which the end effector position can be changed is often very limited.
Therefore, unless dedicated systems are used in which adaptivity functions are
allowed for, the task of adjusting the torch-position in real time becomes very difficult
if not impossible.
On the other hand, if the workpiece can be moved to accommodate the
differences between robot programmed path and the actual weld joint location, the
task of maintaining their relative position constant as required by the process
constraints can be easily achieved. This solution however would imply that the
workpiece would have a limit in its dimensions. Although, considering that the
differences between robot path and weld joint would be attributed only to component
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