Page 107 - Welding Robots Technology, System Issues, and Applications

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Sensors for Welding Robots

wire to the work-piece should be as regular as possible. Experiments indicate that
in the short-circuiting mode, optimal process stability occurs when the short-circuit
frequency is equal to the oscillation frequency of the weld pool [26],[27]. The
experiments also show that the weld pool oscillation frequency is mainly
determined by the width of the weld.

When the short-circuit frequency and oscillation frequency of the weld pool are
unsynchronized, the oscillating weld pool surface will fail to contact the growing
droplet at the electrode tip at regular intervals. Hence, larger variation in arc time,
short-circuiting time and transferred droplet mass will occur, and as a result there
will be a lower process stability. When the short-circuit rate is synchronized with
the weld pool oscillations, optimal process stability is obtained.

From the physics of the short-circuit GMAW process it follows that a natural
choice of feature to monitor is the mean and standard deviation of the short-circuit
frequency. Other features have been suggested in the available literature and
include for example the use of an arc stability index which is based on the standard
deviation of short-circuit time, the arcing time, the average short-circuit current
and the average arc current. The specific features used may be dependent on the
specific algorithm and the welding condition. However, studies made have shown
that the mean and standard deviation of the arc time, the short-circuit time, the
peak current, the short-circuit rate and the standard deviation of the short-circuit
rate etc. are less effective features than the variance of the weld voltage in order to
detect a defect weld [1].

3.6 Spray GMAW

In spray GMAW the metal drops are smaller or of the same size as the diameter of
the electrode wire. This was the original type of metal transfer used when GMAW
was initially developed. Due to the large heat input to the work-piece when
welding in spray transfer mode, the weld joint produced has good penetration,
which often is desirable. On the other hand, the considerable heat input also creates
a large weld pool which may be difficult to control. Thus normal spray transfer is
limited to flat and horizontal positions and cannot be used to weld thin materials.

The standard type of power source for GMAW has voltage amplitude
characteristics which are not constant but which drop slightly with increasing
current. The advantage of such power sources is that they have a tendency to
maintain a constant arc length. This has been termed “self-adjusting arc” [23].
When the contact tube to work-piece distance is increased the arc length is also
increased. The disturbance in the form of arc length enlargement results in a
change in the working point along the power source slope. Arc voltage increases in
this way by a change in voltage, at the same time as the mean current decreases. It
should be noted here that a small variation in arc voltage results in substantial
current swing. Reduction of the current leads to a decrease in the wire burn-off rate