LIBRARY ı6ıul 0) - Cranfield University

It should be noted, however, that this is not a complete solution to the problem, since
further degrees of freedom would be needed to provide seam tracking, if a proper
sensor was available.
In this work, a touch sensor was developed to provide pre-weld joint search
capabilities to the system. The touch of the welding wire on the workpiece was
detected by measuring the parallel combination of the power source internal resistance
and the wire-to-workpiece resistance. The parallel combination would have either the
value of the power source internal resistance or zero, when the welding wire touched
the workpiece. This parallel combination was connected in series with a fixed resistor
and the resulting combination was made an arm of a Wheatstone bridge circuit (see
Section 5.5 and Figure 5.1). This would become unbalanced in the event of a touch
between the welding wire and the workpiece, thus generating a detection signal.
Although this design was effective in the laboratory controlled trials, it cannot be used
in a non-controlled environment without modification to the circuit, since the low
voltage used (7.5 V dc) might not be enough to overcome the insulating effect caused
by grease, oxidation and/or dirt normally present in the industrial environment.
The position control was based on the stand-off estimation provided by the dip
resistance based models. Depending on the mode of metal transfer (dip or spray), a
suitable model was chosen by the control software (see Section 6.2). A third order
moving average filter was used to reduce the noise present in the estimation. Also, a
"forgetting" property was introduced in this filter in order to prevent the controller
from responding to signals that have been dealt with before. This was accomplished
by resetting the filter every time a filtered stand-off estimate was sent to the table
controller and only sending a new filtered value after obtaining three new non-filtered
estimates (see section 4.2.4). Threshold values were introduced in the position
controller so that it would only respond to errors greater than 0.5 mm and the
maximum correction per cycle was limited to 1 mm. This resulted in good
performance without oscillation (see Figures 7.15 to 7.32). However, the
performance was found to be affected by the travel speed. This was due to the limit
imposed on the maximum correction per cycle, which restricted the table movement
to a maximum average speed of 1.3 mm/s. This speed was dependent on the time
taken to acquire each `window of data" and on the speed of the processing hardware.
A faster speed could have been obtained if a higher sampling frequency (in spray
transfer) was used and if a faster hardware was available.
The use of an independent controller for the moving table in this work can be
viewed as an added complication to a welding cell. However, considering the fact that
on-line robot position adjustment is only possible when allowed by the robot
manufacturers and usually only with their own specific sensing hardware, this
perceived added complexity then becomes very important by giving added flexibility
to the system.
8.4.2 Welding process control
The welding process control strategy in this work was implemented in two
stages, namely: (a) the off-line optimisation of the welding parameters and (b) the on-
line tuning of the welding voltage and control of stand-off. The off-line optimisation
was used to predict the welding parameters which would produce a weld that would
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