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In order to minimise the undesirable effects of contact tip wear, periodic
contact tip replacements must be made.
2.6 Monitoring and adaptive control for robotic welding
Considering all the aspects of the gas metal arc welding process, robotics
applied to arc welding and off-line programming highlighted so far, it is clear that the
final quality of a weld carried out by an off-line programmed robot depends on the
combination of a series of interrelated factors originated from different sources. In
order to have an effective control over the weld quality the extent to which these
factors affect the process must be known and controlled. The use of monitoring
techniques provides the necessary process information for control actions to be taken.
Norrish and Gray [ref. 88] suggest that, to ensure adequate performance of
off-line programmed robotic arc welding operations, some degree of adaptiveness is
necessary in the robot system. This could take the form of a seam tracking facility
allied with some on-line monitoring and control system. The authors also suggested
the implementation of statistical process control techniques for identification and
correction of adverse performance trends before they cause component rejection.
In order to perform process monitoring, sensing techniques and data
acquisition must be used. A brief discussion about these topics will follow.
2.6.1 Sensing
Sensing is the term used to define the measurement of qualitative or
quantitative information of a process and its environment by means of sensors. A
sensor for arc welding is defined as follows [ref. 114]:
"A detector capable of monitoring and controlling welding operation based on
its own capacity to detect external and internal situations affecting welding
results and to transmit a detected value as a detection signal"
The measured quantities are normally converted to process useful information
either directly, using the measured variable itself, or indirectly, via modelling.
2.6.1.1 Sensors for robotic welding
Sensing in robotic welding can be performed through either contact sensors or
non-contact sensors [ref. 51,115]. Contact sensors are those which need to have
physical contact with the measured medium in order to produce a useful output. The
use of thermocouples for measuring temperature, electromechanical probes for
measuring torch position relative to joint path and wire touch sensor for searching for
the weld joint are examples of contact sensing in welding [ref. 115].
Non-contact sensors, on the other hand, use the arc characteristics (voltage
and current), sound, electromagnetic devices and/or optics to extract information
about the process and torch positioning relative to the joint [ref. 115].
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