Advanced Welding Processes: Technologies and Process Control

Welding automation and robotics 231
in the ‘TRACE’ mode (without welding) for final checking, and the ‘PLAY’
mode, which performs the whole sequence in production, may be initiated
by a simple switch or push button connected to one of the i/o ports.
The procedure is very easy to learn and the programming operation is
extremely rapid, but, for curved or complex shapes, a large number of points
need to be recorded. An alternative system uses a continuous path recording
technique and a device mounted on the end of the torch to maintain the
correct torch to workpiece distance. One problem which is inherent in these
systems is mechanical backlash. The encoders which are recording the position
of the axes in the ‘TEACH’ mode do not take into account any distortion in
the arm caused by the method of leading the torch to the work. This may lead
to some inaccuracy in playback, although the rigidity of the small SCARA
systems tends to minimize this problem.
Point-to-point with interpolation
This system is the approach most commonly used on fusion welding robots.
A program identification number and the ‘TEACH’ mode are selected at the
controller. The robot is then driven through a path in space using the normal
actuators which are controlled from a manually operated pendant equipped
with push buttons or a joystick. At selected points, the position is recorded
by pressing a key on the pendant. The mode of travel between points, the
velocity and the choice of welding or non-welding operation are also possible
using appropriate keys on the pendant. The travel mode choice is usually
‘LINEAR, CIRCULAR or WEAVE’ and the computer will interpolate an
appropriate path based on the points that have been programmed. The taught
program is stored at the end of the sequence and again it is possible to edit
in further instructions, welding parameters or control sequences. This approach
gives improved accuracy and facilities such as software-generated weave
patterns, but the programming process takes significantly longer than the
teach-by-doing method and it requires more care to avoid accidental collisions.
11.5.5 Program storage
The taught programs are stored in non-volatile memory (e.g. battery-backed
RAM) in the controller. It is also possible to assemble a sequence of programs
together into a batch file to perform a particular job and to store regularly
used sets of welding parameters in a library file which may be called up
during the main welding program. 4 For additional security, or to release
4 A library file may, for example, contain instructions to start welding, set the current, set
the voltage and the travel speed or to decrease current, decrease voltage and turn off the
welding system.