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contact tip and the weld pool and consequently, the measured resistance. It was also
observed that this under estimation would depend on the deposition rate, that is, on
the ratio between the wire feed speed and the welding speed, WFS/S . For a fixed
wire feed speed, the smaller the travel speed is the smaller the estimation will result,
or in other words, a larger bead build-up will occur.
Figure 6.29 shows the comparison between the stand-off estimations obtained
for fillet welding trials carried out with the same setup welding voltage (20.3 V) , wire
feed speed (5.5 m/min) and stand-off (15 mm), and with two different levels of travel
speed: (a) 0.4 m/min and (b) 0.8 m/min. Both estimations were filtered with a third
order moving average filter. Note that the estimation obtained for the welding trial
carried out with 0.4 m/min travel speed is generally lower than the one obtained for
0.8 m/min travel speed. It should be noted that the difference between the actual
stand-off and the estimated values for both welding trials is almost constant.
Considering that, in normal welding conditions (constant gap or no gap at all),
the wire feed speed and the travel speed should be kept constant during the welding
process, it is acceptable to assume that the bead build-up below the welding arc would
also have a constant average size. Hence, the observed estimation error should also
have a constant average value and it would, therefore, behave as an off-set in the
stand-off estimation. Since in the implemented system the initial stand-off is always
known, such off-set can be estimated at the beginning of the weld and added to the
subsequent dip resistance based stand-off estimates. This strategy was successfully
implemented and the results are presented in Chapter 7.
6.3 System integration
The integration of the system involved determining and combining the
communication capabilities of each sub-system such that a master and slave control
scheme could be defined. Three different ways of communicating were available in the
sub-systems used: (a) digital inputs and outputs, (b) analogue inputs and outputs and
(c) RS232 serial communications ports.
Digital inputs and outputs were freely available in the robot controller and in
the table controller. Analogue inputs and outputs were available in the robot interface
installed in the welding power source and also in the monitoring personal computer
(PC). RS232 serial communication ports were available in the monitoring PC and in
the table controller. Having determined the communication capabilities of each sub-
system and the specific requirements of each type of communication channel, all the
sub-systems were interconnected in such a way that the monitoring PC could have a
supervisory function. Figure 6.30 shows the interconnections in a diagrammatic form.
Table 6.10 shows the interconnections between the digital inputs and outputs and
Table 6.11 shows the connections in the analogue inputs and outputs.
The communication between the robot controller and the monitoring PC was
made via the table controller through digital inputs and outputs. The interconnection
between the robot and the table digital inputs and outputs was provided by a specially
built interface box (see Appendix G), whose primary function was to convert the two-
terminal "switch-type" robot inputs and outputs to 24 V inputs' and outputs, as
required by the table controller. Such a box was also used to house the touch sensor,
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