LIBRARY ı6ıul 0) - Cranfield University

6.2 Development of stand-off estimation models
As already mentioned in section 4.2.3, an initial attempt was made to use the
stand-off estimation model proposed by Ogunbiyi [ref. 51] to estimate the stand-off
changes for control purposes. Again a new model was necessary to map the mean
welding current produced by the BDH550 as a result of the combination of the set-up
wire feed speed, the set-up voltage and the stand-off. The data shown in Appendix I
were used for this purpose. The model structure shown in equation 2.21 fitted well
the available data and was adopted. Equation (6.4) shows the resulting model.
I,,, ý, =28595+27.417"WFS+0.10391"SO"V -0.72938. SO. WFS
RZ = 0.9905
SE=7.425
where
RZ is the coefficient of determination of the regression model and
SE is the standard error of the model.
(6.4)
In order to check the quality of the estimates produced by the model, a series
of bead on plate welding trials were carried out with the stand-off starting at 15 mm
and changing linearly as shown in Figure 6.7. Two levels of final stand-off (SOf) were
tested: 20 mm and 10 mm. In order to pinpoint the exact moment when the robot
started and finished the slope path, a robot output was connected to one of the
analogue input channels of the monitoring system as shown in Figure 6.8. The
welding parameters used in the trials were generated by the welding parameter
generator (see section 3.3.2.1) and are shown in Table 6.6, together with the
measured value of the proportionality constant 4mß required in equation 2.22, and the
calculated value, 4. i obtained from equation 2.24 with the coefficients shown in
equation (6.4). This is repeated explicitly in equation (6.5) for easier reference.
4`°``
1
0.10391.6 t-0.72938. WFS
Was N
r-1
ASO,
where
I; mean welding current measured at monitoring cycle i
i=0 monitoring cycle immediately before the start of the stand-off slope
i=N monitoring cycle at the end of the stand-off slope
ASO,
v = SOf - 15.0
133
(6.5)
(6.6)