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Appendix J), it was observed that the proportionality constant 0 used in equation 2.22
and calculated using equation 6.5 differs from the measured value (see Figure 6.9).
The value of the difference observed was found to be dependent on the wire feed
speed and the mode of metal transfer.
This observed inaccuracy in the estimation of 0 was possibly caused by the
fact that the model does not fully describe the non-linearities present in the process.
Also, the model (see equation 2.21) is not a theoretical model but an empirical one
and the model inversion carried out could be a possible source of error.
While adequate for indicating changes in stand-off, Ogunbiyi's model does not
provide the accuracy and robustness necessary for control purposes, especially when
using low wire feed speed dip transfer (see Figure 6.9).
Previous work by Philpott [ref. 131] has shown that the dip-resistance can be
used for stand-off monitoring. Based on this, a method for monitoring the dip
resistance was developed and the dip resistance measurements were correlated to
stand-off variation. A stand-off estimation model based on the dip resistance was
developed and used in conjunction with the windowing technique developed by
Chawla [ref. 161] (see section 2.6.3).
The dip resistance based stand-off estimation model was found to be less
sensitive to process instabilities in dip transfer because it uses the resistance measured
during the short circuiting phase only. Hence, voltage and current spikes do not affect
the prediction as was the case in the Ogunbiyi's model (see Figures 6.15 to 6.24).
It should be noted that the model proposed by Ogunbiyi used an integral
approach' to reduce the effect of random variation commonly observed in the welding
current. However, the use of the integral approach makes the model very sensitive to
the stability of the process at the start of the welding. Also, Ogunbiyi's model only
outputs estimated changes in the stand-off while the dip resistance based model
produces an absolute stand-off estimation.
The dip resistance model was also extended to the spray transfer mode, the
only difference being that, in this mode, the measured resistance also included a
component due to the welding arc (see section 6.2 and Figures 6.13,6.14 and 6.25 to
6.27).
The stand-off models for dip and spray transfer were validated successfully for
bead-on-plate welding trials. However, when transferred to fillet joints in the flat
position, it was observed that the models would produce a prediction smaller than the
actual stand-off by a constant average value. This was hypothesised to be caused by
the weld pool build-up under the welding arc which occurs due to the restricted flow
of the molten metal resulting from the geometry of the fillet joint. This was confirmed
by changing the deposition rate and comparing the resulting estimation errors (see
Figure 6.29). In order to compensate for the weld pool constriction effect found in
fillet joints, the offset in the stand-off prediction (illustrated in Figure 6.29) was
estimated at the start of the weld and its value was added to the subsequent
estimations (see section 6.2). It should be noted, however, that this approach can only
be applied if the initial stand-off is known, as was the case in this work. Good results
were obtained for the fillet weld trials (see Figures 7.6 to 7.14).
7 Characterised by the use of a cumulative summation of the differences between successive welding
current values.
193