Advanced Welding Processes: Technologies and Process Control

CCD camera
Travel
direction
Torch
Monitoring and control of welding processes 211
Weld pool
Window 1
Window 2
10.16 Concentric video monitoring system for GTAW.
Chemical composition sensor. A novel approach to seam tracking is the
use of a component of the arc spectrum to detect the presence of particular
chemical species. [250] The arc emission is collected by a fibre optic and
passed to an on-line analysis device, which monitors the level of the signal
produced by specific element. In a dissimilar metal joint, this signal indicates
the lateral position of the arc and may be compared with a reference level to
generate an error signal for correction of the torch position. In joints of
uniform composition, tracer elements may be introduced in the form of
fusible inserts.
Arc length control
Control of arc length is important to ensure consistent heat input, constant
melting rate, stable process performance and adequate shielding. Arc length
may be maintained by good joint preparation, careful torch positioning or
tactile sensors of the type described in this section. Several closed-loop
systems have also been used for arc length control.
Voltage measurement. In the GTAW process constant-current power sources
are normally used and the arc voltage gives a clear indication of arc length
at a given current. These systems directly measure arc voltage, compare the
actual value with a reference and control the torch height to correct the
error. 10 The arc voltage depends on current, shielding gas, workpiece
composition, polarity and electrode vertex angle and the system reference
10 Arc length control by means of voltage is often referred to as AVC or arc voltage
control.