Advanced Welding Processes: Technologies and Process Control

Monitoring and control of welding processes 205
The welding of joints to a predetermined procedure is effectively an openloop
system. Although every effort may be taken to ensure that the
predetermined parameters are maintained (e.g. by equipment calibration and
monitoring), any corrective action taken as a result of post-weld inspection
is indirect and occurs some time after the process is complete.
Closed-loop systems are also referred to as feedback systems, since a
signal derived from the output is fed back for comparison with a reference
parameter. The result of this comparison is an error signal which is fed to the
controller in such a way as to reduce the error. The error correction system
or controller may be a manual operator. In dip transfer GMAW operations,
for example, the welder often tunes the process performance by adjusting the
wire feed speed, voltage or inductance in response to the sound of the arc; in
this case, the sound is an indication of the short-circuit frequency. In more
advanced control systems, the human operator is replaced by an automated
control, which monitors the output and corrects the system continuously
using an error signal derived by comparing a suitable output parameter with
the preset reference. In automated closed-loop control systems, the rate at
which the error is corrected is critical, as slow response rates or over-correction
may both lead to instability and the design of suitable control strategies will
have a significant effect on system performance.
Several variations of closed loop control are used in welding applications.
These may be referred to as: direct control; indirect control; adaptive control;
and learning control.
Direct control implies the measurement and feedback of the parameter
which it is sought to regulate. For example in the case of a power source, the
current may be regulated by monitoring its output level as described in
Chapter 3. Unfortunately, in many welding situations, it is not possible to
use direct control and indirect systems based on the measurement of secondary
variables must be used. Several indirect control systems will be described
below, but typical examples are: weld bead temperature for penetration control
and dynamic resistance for resistance spot weld geometry.
The term ‘adaptive control’ is often used to describe recent advances in
welding process control, but strictly this only applies to systems which are
able to cope with dynamic changes in system performance. [237] Adaptive
systems should have the ability to self-adjust in accordance with unpredictable
changes in operating conditions. It has been pointed out [238] that, in welding
applications, the term adaptive control may not imply the conventional control
theory definition, but may be used in the more descriptive sense to explain
the need for the process to adapt to the changing welding conditions.
Learning control systems are systems which progressively improve their
performance by experience – the most common example is the human operator
– but computer-based learning systems have also been developed for welding
applications.