LIBRARY ı6ıul 0) - Cranfield University

4.1.3.1 Adjustment before welding
Before the start of each new weld, the robot should be moved to a wire
cutting station, where the electrode wire would be cut to a length of 11 mm. Such a
length was found to be adequate for preventing collision between the el ding torch
gas nozzle and the side plates of a 90 degree included angle fillet weld, during the
weld start point search. It would also allow the adjustment of the initial stand-off to a
value between 12 mm and 20 mm, the limits set in the off-line programming module.
In the search procedure, the robot moves the welding torch to the weld
approach point located at the top surface of the workpiece clearance box (ap3 or
ap4) (see Figures 3.22 to 3.24). It then stops and sends a signal to the table
controller, which in response moves the workpiece to the table zero position. This
zero position is used in the off-line programming module as the origin of the table co-
ordinates frame. In reaching this zero position, the table signals the robot to move
through the next approach points until it reaches apl; where the robot should stop
once again and signal the table to move the workpiece in the direction of the torch
approach vector by a programmable amount. ý This amount will depend on the
manufacturing tolerances of the workpiece, which will dictate the maximum difference
that is likely to occur between the required joint position and actual joint position.
When the table finishes the movement, it signals the robot to move the welding torch
to the programmed weld start point, On reaching this point, the robot stops, switches
the wire touch sensor on and signals the table controller to start the search routines.
When the table finishes searching and adjusting the workpiece position, it signals back
to the robot, which switches off the wire touch sensor and starts the welding
sequence.
All these movements and communication issues are supervised by the main
controller, which is implemented in a personal computer and connected to the table
controller via a serial communications channel. The main controller is also responsible
for downloading the off-line programming information to the table controller.
4.1.3 .2 On-line control
The on-line position control is based on the information provided by the
sensors. The position deviation is detected by the monitoring system and transferred
to the table controller, which calculates the necessary movements to be performed,
based on the directions of approach and tangent vectors at the point. If the approach
direction at the weld start point is different from the approach direction at the weld
end point, the table controller performs an - interpolation to estimate the current
approach orientation, based on the programmed welding travel speed and on the
welding time measured until the point of interest. It then calculates the movements to
be performed in each axis such that the detected position deviation is reduced.
The position control implemented in the present work is a particular case of
the strategy explained above, since it addresses only the stand-off control. The
implementation of the full three dimensional position control was not carried out due
to the unavailability of a suitable sensor for measuring the lateral deviation of the
torch relative to the joint longitudinal axis. A sensor based on the projection of an
annular shaped laser light around the welding torch was proposed but its development