LIBRARY ı6ıul 0) - Cranfield University

In section 4.2.5 it has been mentioned that two methods of communicating
with the welding power source could have been used: (a) via direct RS232 serial
communication and (b) by using a so called "Robot Interface" specially tailored to be
used with robots. The use of each alternative would imply in a different transport
delay, that is a time delay between the issuing of a command by the monitoring
computer and the actual implementation of this command by the welding power
source. In order to have an estimate of these time delays, voltage step input tests were
carried out and the corresponding welding data were acquired, such that the time
between the issuing of a command and the power source response could fit in one
window of data. This would ensure accuracy in the time measurement. Figure 6.31 to
Figure 6.34 show the results of the tests. It should be noted that for direct serial
communication, an approximate delay of 50 milliseconds is observed before the power
source responds to a voltage command, while a delay of approximately 200
milliseconds is observed in the case of the robot interface. This difference is expected,
since the robot interface case includes, further to the digital-to-analogue and
analogue-to-digital conversions, the delay of transferring data from the robot interface
to the power source main controller via serial communications. Also, in the case of
using the direct serial link, only the voltage command was issued, whereas in the case
of the robot interface, a continuous power source error check is also carried out,
which might increase the response time. Considering that the current implementation
of the serial communications protocol did not have any function for setting wire feed
speed, the robot interface option was chosen for implementing the proposed process
controller.
6.4 Control system tuning
In order to tune the welding process controller shown in chapter 4, a series of
welding trials were carried out and adjustments in the control algorithms were made
until a satisfactory performance was achieved at all the levels of wire feed speeds
studied. The sequence of adjustments and modifications in the algorithm were
described
in chapter 4.
The tuning of the stand-off controller has also been described on chapter 4. It
basically consisted of choosing the threshold values for the minimum and maximum
stand-off adjustments that would be allowed in each control cycle, based mainly on
previous process experience. Tests were also made to check which speed and
acceleration would be acceptable for moving the workpiece in one control cycle. A
speed of 8 nuns and an acceleration of 400 mm/s2 were found to produce good
performance without deteriorating the welding process stability.
6.4.1 Filtering of process estimates
Despite the improvement in accuracy provided by the dip resistance based
estimation model, if compared to the model based on the cumulative differences in
welding current the stand-off estimates obtained were still corrupted by random noise
(see Figure 6.15 to Figure 6.28). As already mentioned in chapter 4, a third order
moving average filter was used to filter the stand-off estimates. This filter was
implemented in such a way that it was reset after every stand-off control cycle and a
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