D.H. Lammlein PhD Dissertation - Vanderbilt University

For full circumferential welds, the eccentricity of the experimental setup, shown
in Figure 104, is approximately 0.02”. This level of eccentricity results in significant and
unacceptable changes in the vertical position of the sphere surface relative to the tool, and
thus unacceptable variations in weld depth. An in-process, force feedback control
algorithm was therefore employed for the conical tool experiments as a means of
maintaining a constant penetration depth around the entire sphere. The vertical position
was adjusted based on torque signals received from a rotating cutting force dynamometer
oriented in series with the tool. For the conical type tools it was found that desired
position was better maintained if torque, rather than force, was used as the input in the
decision making algorithm. An auto-zeroing procedure is performed over the start of the
weld to determine the height of the material surface here relative to the tool. The tool is
then plunged to the assigned penetration depth and the weld traverse is started. After 1”
of weld traverse the measured torque from the dynamometer is set as the desired torque
and from this point forward this torque value is maintained by vertical adjustments of tool
position using the weld machine's vertical axis motor. Figure 123 in a later section shows
the weld torque increasing with penetration depth at the 1000rpm, 10ipm parameter
setting. The relationship between weld depth and process forces will be discussed later.
Figure 105 and Figure 106 below demonstrate the torque control process for two welds
runs(1000rpm, 10.4ipm, 0.095” weld depth). In the upper frame the axial force (blue)
and desired axial force (green) are shown. The desired axial force is not used in the
control algorithm and is shown for comparison with the measured axial force and the
torque panel below. Torque was found to be a more reliable indicator of depth. These
relationships are discussed later. The middle panels in Figure 105 and Figure 106 show
the measured torque (blue) and desired torque (green). It can be seen in the figures that
the desired torque, set after 1” of weld traverse, is followed closely. The bottom panel of
the two figures is the vertical position of the tool relative to the height of the material
surface at the weld plunge site (weld start). A linear encoder is used to track movements
relative to this initial calibration. In this manner the vertical adjustments made by the tool
to maintain the desired torque can be seen. As torque is highly sensitive to plunge depth
and a constant torque is closely maintained throughout the weld traverse, it can be
concluded that tool penetration depth is closely maintained throughout the weld traverse.
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