Page 53 - Welding Robots Technology, System Issues, and Applications

Welding Robots
38
2.2.2 Welding Equipment
Basic equipment for conventional GMAW is consists of the power source, the
electrode feed unit, the welding torch and the shielding gas regulator, as
represented schematically in Figure 2.9.

2.2.2.1 Power Source

Most common GMAW power sources are of the inverter type with an architecture
similar to that represented in Figure 2.3, but providing a constant-voltage output. A
constant-voltage power source used in conjunction with a constant speed wire
feeder can provide self-adjustment and stabilization of the arc length, in order to
compensate for the variations in the torch to work-piece distance that occur mainly
during manual welding operations. In a power source with approximately constant-
voltage characteristics any change in the arc length is compensated by the
modification of the weld current and consequently of the burn-off behavior of the
electrode. Figure 2.10 illustrates the effect of increasing the arc length from L 1 to
L2, which corresponds to an increase of the torch to work-piece distance. This
increase of arc length produces an increase of the arc voltage and consequently a
decrease of the weld current from I 1 to I 2 and of the burn-off rate from B 1 to B 2. As
the wire feed speed is constant and burn-off decreases the arc tends to assume the
initial length.

In addition these machines provide slope control of the power source
characteristics and of the inductance in order to control spatter in short-circuiting
transfer [3]. Inductances introduced in the output circuit reduce the rate of rise of
current during the short-circuiting, reducing in this way the risk of explosion of
metal droplets. In the case of thicker electrodes, which show a small variation of
burn-off rate with current, or for materials having high conductivity, such as
aluminum, process control is achieved by using a variable-speed wire feed unit that
reacts to the arc length changes by adjusting the electrode feed speed.

GMAW inverters are also used to generate pulsed current with pulsed repetition
rates (PRR) (number of pulses per second) typically between 100 and 200 PRRs
[15]. Pulsed parameters are defined by algorithms in the controller. New synergic
pulsed GMAW inverters can control melting rate through the modulation of the
pulse shape and of the pulse frequency, being the process managed by a
microprocessor [3].