Civil Engineering Reference
In-Depth Information
Spot diameters for the high density processes vary typically from 0.2 mm to 1 mm, while the spot
diameters for arc welding processes vary from roughly 3 mm to 10 mm or more. Assuming 1
10 the spot
diameter for positioning accuracy, we conclude that a typical positioning accuracy requirement for the
high power density processes is on the order of 0.1 mm and for arc welding processes is on the order of
1 mm. The required control system response time should be on the order of the interaction time, and
hence, may vary from seconds to microseconds, depending on the process chosen. With these require-
ments it can be concluded that the required accuracy and response speed of control systems designed
for welding increases as the power density of the process increases. Furthermore, it is clear that the high
power density processes must be automated because of the human operator's inability to react quickly
and accurately enough.
7.4
Arc Welding as a Multivariable Process
In this section the Gas Tungsten Arc Welding (GTAW) process (see
Fig. 7.1
)
, sometimes referred to as
the Tungsten Inert Gas (TIG) process, is discussed as an example for artificial neural network applications.
In this process the electric arc is sustained between a nonconsumable tungsten electrode and the work-
piece. The electrode is usually at a negative voltage potential with respect to the workpiece and current
is, therefore, carried across the arc gap through ionized plasma. The power source providing the electrical
current is operated in a current-control mode. While the arc current is controlled by the power source,
the voltage across the arc is largely determined by the length of the arc gap. An inert gas, such as Argon or
Helium, is routed along the electrode down through the gas cup such that it encompasses the molten weld
pool as well as the arc. The gas serves to provide the medium for ionization and plasma formation, and
to shield the weld pool from the atmosphere and thus prevent oxidation of the molten metal. Filler wire
is usually fed into the pool from the front end of the moving torch, where it melts and combines with
the molten base metal of the workpiece.
Any arc welding process can be controlled by a number of parameters, and the ultimate objectives of
the process are specified in terms of numerous parameters as well (refer to
Fig. 7.2
).
As a result, any arc
welding process can generally be viewed as a multiple-input/multiple-output system. The lack of reliable,
general, and yet computationally fast, physical models of this multivariable system makes the design of
Torch Travel Speed
Wire Feed
Rate
Gas F
low
Reinforcement Height
Penetration
Cross Sectional
Area
Bead Width
FIGURE 7.1
GTA weld process parameters.
