Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
If the full-scale reading of the instrument is 256 V, an eight-bit ADC
would give a resolution of 1 V whilst a 16-bit device would provide 0.004 V
resolution. For many welding applications, eight-bit accuracy is adequate,
particularly if the signal is conditioned to limit the full-scale value. The
advantage of digital measuring systems is that they provide a direct numeric
output and this may be stored or recorded electronically as described below.
Digital systems are generally more robust than analogue meters, but they are
sometimes susceptible to electrical interference. The disadvantage of the
digital approach is that it is difficult to interpret the digital display if the
parameter being measured is fluctuating rapidly and the ‘mean’ values may
be arrived at in several different ways (e.g. by electronic processing of the
incoming signal or by calculation). This may lead to slight discrepancies
between the values measured with different digital meters and may be
responsible for large variations when compared with traditional analogue
meters.
Computer-based data loggers
Computer-based data loggers as used in general process control and biomedical
applications were originally only used in welding research applications, but
purpose-built monitors for calibration and control of welding processes are
now available. [203]
The principle of computer-based instrumentation is illustrated in Fig.
10.4. The analogue signal to be measured is amplified or attenuated by a
signal-conditioning circuit, which consists of standard electronic components.
The output of this stage may be electrically isolated from the remainder of
the instrument by isolation amplifiers, and hardware filters may be incorporated
to reduce electrical noise. Isolation is particularly important when high voltage
welding signals are being measured, where common-mode problems occur 5
and to avoid spurious signals when low currents or voltages are being monitored
(e.g. from thermocouples).
The analogue signal is digitized by an ADC similar to that used in digital
voltmeters. The use of eight-bit ADC converters is again adequate for many
welding applications providing the input is scaled to an appropriate level. In
order to provide facilities for monitoring several welding parameters, the
conditioned analogue signal from a number of inputs may be scanned by a
multiplexer before being passed to the ADC.
5 Common mode problems: when measuring welding current and voltage simultaneously,
it is possible to connect the instrumentation in such a way as to short circuit the output
of the power supply. A high-current path may accidentally occur either in the interconnecting
leads or, more seriously and less obviously, through the ground or earth connection of the
instrument. Such faults can result in serious damage to welding instrumentation.