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satisfy the required quality criteria (see section 3.3.2.1). The on-line voltage tuning
was used to ensure that an adequate voltage level would be attained and maintained
throughout the welding process (see Section 4.2). Good results were obtained with
the voltage controller (see Section 7.1).
The voltage controller presented in Section 4.2 is a combination of fuzzy
control and rule based incremental control. The use of this incremental control
architecture ensured that the controller would always achieve a stable state. The time
taken to reach the stable state is, however, dependent on how far the process is from
stability and also on the voltage increment size used in each control cycle and on the
control cycle duration. This includes the monitoring/processing time and the power
source response delay.
Although not implemented in this work, control of deposition rate by adjusting
wire feed speed and/or travel speed could be introduced if sensor(s) for measuring
gap and bead geometry were available [ref. 183]. In cases when the robot does not
allow on-line speed correction, the wire feed speed could be used to adjust the
deposition rate. However, a change in wire feed speed could lead the process to an
unstable condition. A synergic control scheme could be used for adjusting the welding
voltage to a level close to the new stability region and the automatic voltage tuning
algorithm could be used to fine tune the voltage, thus restoring the process stability.
Tests were performed in flat position fillet joints with no control, only stand-
off control (i. e. without voltage control) and both controls (stand-off and voltage)
acting together. All tests started using the off-line optimised welding parameters.
From the non controlled trials it can be seen that the process is very robust to changes
in stand-off. For example, changing the stand-off from 12 mm to 20 mm in dip
transfer and from 15 mm to 23 mm in spray transfer resulted in a small reduction in
the average penetration: from 0.78 mm to 0.46 mm in dip transfer and from 2.04 mm
to 1.47 mm in spray transfer (see Table 7.2). The minimum penetration values
obtained would still satisfy the minimum quality requirement for penetration.
The welding parameters generated for fillet welds using spray transfer resulted
in porosity, undercut and pronounced finger-like penetration profile (see Figures 7.9
to 7.13 and 7.19 to 7.21) due to excessive voltage. Analysis shows that the prediction
of these welding parameters is due to the fact that the threshold of the undercut model
was too high (see Section 3.3.2.1), thus allowing these inadequate conditions. If the
threshold value of Pr(und) was reduced to 0.16, these conditions would have not been
predicted. The results show that for the plate thickness used (3.2mm) Pr(und)= 0.293,
0.162 and 0.196 (runs M4, M5 and M6 in Table 7.1) correspond to an average
maximum' undercut depth of 0.41,0.36 and 0.20 mm respectively. The use of these
spray conditions, however, allowed the capabilities of the control system to be
demonstrated.
The results show that the control of stand-off and voltage reduces the depth of
undercut (see Table 7.5), eliminates porosity (see Figure 7.27) and produces a more
consistent weld. A surprising result in this work was the marked influence of the
voltage control on the finger like penetration profile observed in spray mode (see
Figures 7.29 and 7.31). The penetration profile changed from a pronounced finger like
8 Average of the maximum undercut depth of sections cut along the weld length
196