Advanced Welding Processes: Technologies and Process Control

Mains
input
Welding power source technology 39
It is not possible to select an ideal design from this list, but series regulator
designs are often only justified for very-high-precision and research
applications, whilst primary inverter-based designs are cost effective and
suitable for a wide range of production tasks. One important characteristic
that all of these systems share is the use of feedback control.
Feedback control
Power
regulator
Error
signal
Comparator
3.14 Principle of feedback control.
Feedback
signal
Reference
input
Feedback control is a useful technique which can be applied most effectively
with electronic power sources. The basis of the technique is illustrated in
Fig. 3.14; the output of the system is measured and compared with the
desired output parameters, any difference between the two values will cause
an ‘error’ signal to be generated and the feedback system then adjusts the
output to correct the imbalance. Although this type of control may be applied
to conventional power source designs, it is usually costly, complicated and
too slow. Hence, most conventional power sources have ‘open-loop’ control,
i.e. if the input varies, the output changes by a proportional amount. The
higher response rates and low signal levels available from electronic control
systems make ‘closed-loop’ or feedback control effective and economical
and give inbuilt stabilization of output.
3.5 Output level, sequence and function control
For most welding processes, a prescribed sequence must be followed when
initiating or terminating the process. In addition, it may be necessary to
control the rate of current rise or decay and modulate the output during
welding. The extent to which these features could be provided has in the past
been limited by conventional power source design technology and the use of
Arc