LIBRARY ı6ıul 0) - Cranfield University

orientation can be modified to improve the welding process, but this was not
implemented
in this work.
Although the implemented software was also designed to generate the robot
program in ARLA language, this capability was only introduced to give the user the
logical sequence of events necessary for the accomplishment of the positioning and
external communication tasks. The actual implementation of the off-line generated
programs was not carried out due to the fact that the available welding robot did not
have any off-line programming facilities. However, it is well established that this can
be performed if suitable robot hardware and communications programs are available
[refs. 196,197]. The programs generated were tested by simulation in software using
Workspace4 versions 3.2 and 3.3. This was carried out manually "step-by-step",
taking the robot through the teach-points according to the order required in the
generated program. This proved the programs viable and the assumed torch
orientations adequate.
The logic sequence of inputs and outputs was successfully tested using an on-
line programmed robot and the sequence presented the expected performance. It
should be noted that the input/output sequence was the only means of communication
between the robot and the external equipment.
8.3 Process modelling
8.3.1 Models used in the welding parameters generator
The main objective of the modelling work in this project was to obtain models
to predict the bead geometry, the risk of defects (undercut, bum-through, lack of
fusion), the stability characteristics of the process and the levels of welding current
and voltage suitable for producing a stable process.
The modelling work by Ogunbiyi [ref. 51] was used as the starting point for
the development of the welding parameter generator. This was initially based on
models developed for the Migatronic BDH320 welding power source, using BOC
Argonshield 5 shielding gas and 1 mm mild steel wire (A18-8). However, in this
work, a different power source (Migatronic BDH550) was used. The specifications
for both power sources are shown in Appendix H.
The welding parameters obtained using the BDH320 models, which
have been
proven to produce stable process [ref. 51], did not produce stable welding conditions
with the BDH550. Analysis of the welding data acquired using the data acquisition
system (see Chapter 5) showed that BDH550 delivers a welding voltage that is
approximately 91% of the required power source set-up voltage (see Figure 6.1).
According to Ogunbiyi, the BDH320 produces a welding voltage which was not
significantly different from the power source set-up voltage. The power source
(BDH550) also delivers a higher current level for the same set-up wire feed speed and
stand-off compared to BDH320 (see Figures 6.2 to 6.3). This resulted in bad ignition,
spatter generation, unstable arc and poor bead appearance, mainly due to inadequate
voltage setting. To overcome these, the models were adapted (see section 6.1) and a
voltage controller was developed (see section 4.2). It should be noted that the models
4Robot Simulations Ltd, UK.
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