Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
the master curve and deviations which exceed preset limits are reported,
used to initiate an alarm, or even used to initiate the printing of a ‘reject’
label. Welding current, voltage and time are also checked against preset
values to ensure that satisfactory tolerances are maintained.
The recorded welding parameters may be stored in the monitor’s memory
and measured data may be stored on a non-volatile memory device (e.g. a
floppy disc). This type of equipment may also be configured as a series of
local monitors mounted on individual welding machines which feed the
collected data back to a central computer for permanent storage or analysis.
Distributed computer-based dynamic resistance monitors of this type are
suitable for on-line surveillance of automation systems and robotic welding
stations. [222]
Arc welding deviation monitors. Arc welding processes may be monitored
using similar techniques to those described in Section 10.3; however, in this
case it is usually necessary to monitor a continuous weld rather than a discrete
spot. One computer-based device suitable for quality assurance of automated
GMAW welds measures arc current and arc voltage signals and analogue
circuitry is used to derive short-circuit resistance and a radio-frequency (RF)
component of the voltage waveform. [223] The data are analysed by comparing
the sample data with preset limits for voltage, current, RF component and dip
resistance. To allow continuous updating of the data collected the four input
channels are sampled at approximately 15 kHz and the data are analysed online.
In normal dip transfer GMAW welding operations, an assessment
equivalent to each 2 mm increment of weld length is stored, passed to an
external computer or used to initiate weld quality signals. Using real-time
analysis and a knowledge of the inter-relationships between the monitored
variables and potential weld failure modes, [224] the unit is able to predict
a large number of weld quality deviations from the limited input data. For
example, the dip resistance may be used to indicate variations in torch-toworkpiece
distance, whilst excessive voltage in conjunction with a satisfactory
dip resistance may indicate wire feeding problems. The RF component of the
voltage waveform is used to indicate process stability and, in particular, the
disturbance of gas shielding. Some of the common quality deviations which
can be monitored with a device of this type are indicated in Table 10.6.
When applied to an automated welding system a pass/fail assessment may
be made for each increment of weld and as a percentage of the individual
weld length; in addition the results of the assessment may be used to initiate
a controlled shutdown of the welding cell or operations such as torch cleaning.
The monitor is provided with a serial output port (e.g. RS232 or 422) to
enable data to be downloaded to a central computer from a number of welding
cells where they may be compared and production statistics and overall
performance may be assessed.
Monitoring systems which provide on-line indication of deviation from