LIBRARY ı6ıul 0) - Cranfield University

7. Experimental Results
This chapter presents the results obtained with the control system developed
during the project. It starts by showing the ability of the welding process controller to
achieve and maintain process stability when intentionally starting the process in an
unstable region. A comparison of the welds obtained in flat position fillet joints with
and without stand-off control follows. The results obtained with the two controllers
combined together are then presented.
7.1 Tests of the voltage controller
In order to demonstrate the effectiveness of the welding process controller
developed, bead-on plate welding trials were carried out with fixed stand-off, wire
feed speed and welding speed. The initial power source set-up voltages were
purposely chosen such that unstable situations would occur. These tests concentrated
on the dip mode of metal transfer since this is more sensitive to variations of voltage
around the optimum value.
Figure 7.1 shows the bead appearance obtained using constant welding
parameters (WFS, Sw and SO) and inadequate initial power source set-up voltage for
both the non-controlled and controlled process. Figure 7.2 shows the corresponding
welding data and the evolution of both the control (power source set-up and mean
welding voltages) and the controlled (Power Ratio) variables.
Figure 7.3 shows the bead appearance obtained using constant welding
parameters (WFS, Sw and SO) and an excessive initial power source set-up voltage
for both non-controlled and controlled process. Figure 7.4 shows the evolution of the
welding variables along the weld. Note that in both inadequate and excessive initial
power source set-up voltage cases, the process as measured by the Power Ratio was
brought back to stability in less than three seconds.
If the variation in the control variable (power source set-up voltage) is
considered as the indication of when the controller achieved the final stability level, a
different stabilisation time was observed to occur for the inadequate (;:
ý- 4 s) and the
excessive (, t; 6 s) initial set-up voltage cases. This occurred possibly due to the fact
that the difference between the optimum power source voltage setting, as attained by
the controller, and the actual initial value chosen for both the inadequate and the
excessive voltage cases were different, that is, for the inadequate voltage case the
difference was approximately -2.8 volts and for the excessive voltage case, +7.3 volts.
Also, the instability in the latter case is characterised mainly by the level of spatter
generation, the mean voltage level being fairly stable. On the other hand, the
instability in the case of inadequate voltage is characterised by the wire stubbing in the
weld pool, causing large variations in the mean voltage level and therefore in the main
controlled variable, PR. However, despite this difference the resulting controlled weld
beads were still satisfactory (see Figure 7.1 and Figure 7.3).
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