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was considered to be outside the scope of this work. The proposed concept is
described in Appendix D.
The stand-off measurement was provided by applying estimation models to the
welding current and voltage waveforms. The aspects involved in this estimation are
discussed
in section 4.2.3.
4.2 Welding process control
The welding process control in this work was aimed at ensuring that the most
stable process was always attained. The control strategy is based on the objective
assessment of the process state provided by the monitoring indices developed by
Ogunbiyi [ref. 51] and on the adjustment of the welding voltage as a means to
achieving process stability. It was assumed that the wire feed speed and the travel
speed predicted by the off-line welding parameters generator are adequate to produce
the required weld quality. Although the correction of the travel speed and/or wire feed
speed might also be necessary to compensate for the excess weld volume caused by
the presence of an unexpected gap, this was not implemented in the present work
mainly due to the unavailability of a robust sensor for accurately measuring the joint
gap. The sensor arrangement proposed in Appendix D could also be used for this
purpose.
It should be noted that there is already a growing trend in equipping welding
power sources with on-line automatic voltage control to ensure process stability [ref,
200]. This is achieved by monitoring and objectively assessing the welding process
stability in real time and then controlling the welding voltage based on the result of the
assessment.
The development of the control rules necessary to accomplish the control
objectives in this work and the implementation of these rules in a digital control
system involved the consideration of several influential factors, which are discussed in
the subsections below.
4.2.1 Process stability and control algorithm
It is generally accepted that the gas metal arc welding process stability
depends on the mode of metal transfer. If the deposition rate is adequate and the
stand-off is kept constant, then the only controllable welding parameter influencing
the process stability is the welding volta ge.
In order to implement the proposed control strategy, it was necessary to
provide a means of objectively assessing and quantifying the process stability. The
monitoring indices developed by Ogunbiyi [ref. 51] were found to produce a good
indication of the process stability state. Based on these monitoring indices and on the
threshold limits shown in Figure 2.8, a set of rules were developed in order to classify
the process stability into different states and to generate the welding voltage
correction signals. The rules were used to develop a rule-based incremental controller
[ref. 194], which initially used only the value of the Power Ratio as the stability
assessment factor and the controlled variable. The controller objective was to keep
the measured value of the power ratio within the defined stable process limits for dip
and spray modes of metal transfer (see Figure 2.8). The globular mode was
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