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Philpott [ref. 21] developed an on-line quality monitoring system which was
able to detect quality problems caused by either insufficient shielding or arc instability
in dip transfer gas metal arc welding. The system was based on the fact that the
transient voltage and current signals and certain radio frequency (RF) components of
these signals present specific signatures, that correspond to the occurrence of
instability caused by inadequate voltage and by insufficient shielding. By considering
that a quality problem is very likely to occur when such instabilities happen, the
system was designed to label the length of weld corresponding to the block of data
that was analysed, as either conform or non-conform, depending on the level of
instability present. At the end of each weld, an estimate of the percentage of the total
weld length corresponding to non-conform quality was calculated and compared to a
specified limit, above which the weld was rejected. The indication of inadequate
shielding was used to command a gas nozzle cleaning operation, before starting a new
welding cycle. Although effective, this system could not prevent quality deviations
from happening.
It is well established that welding process stability is intimately related to the
resulting weld quality. By considering this, Norrish and Ogunbiyi [ref. 54] proposed a
strategy for monitoring and controlling GMA welding by means of off-line procedure
optimisation and on-line tuning of welding parameters. The off-line optimisation
would take place before the beginning of production and would select optimum
welding parameters that would produce a weld to the required specification. The
control strategy was anticipatory in nature and its main purpose was to detect any
quality deviation trend and correct it before it could cause weld rejection, by tuning
the welding parameters accordingly. The control strategy (shown in Figure 2.9) was
applied successfully in a prototype control system in a production environment [ref.
56].
2.2 Robotic arc welding
Robotic arc welding is a self-explanatory term that is normally used for
characterising the application of a robot for carrying an arc welding gun through a
three-dimensional pre-programmed path in order to perform an arc welding operation
without operator control [ref. 58]. A robotic welding system basically consists of a
welding robot, an integrated welding power source and auxiliary equipment such as
positioning table, gas nozzle cleaning station, wire cutter and torch head change
systems.
This section gives an overview on the basic components found in robotic arc
welding.
2.2.1 The arc welding robot
An arc welding robot is basically an industrial robot adapted to perform arc
welding. According to the British Robot Association
An industrial robot is a reprogrammable device designed to both
manipulate and transport parts, tools or specialised manufacturing
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