Page 105 - Welding Robots Technology, System Issues, and Applications

Sensors for Welding Robots
91

preventing the electrode from getting stuck during short circuitings by
disconnecting the current control when the peak voltage decreases the certain
preset value. The current can thereby increase until the short-circuit is broken.

To select the optimal welding parameters (W f, I p, I b, T p and T b) by trial and error is
a time-consuming procedure. Most manufacturers of welding sources thus use
synergic control to simplify the parameter setting. With this method the operator
simply chooses one parameter, e.g. the wire feed rate. The remaining parameters
are adjusted automatically to their optimal values according to certain optimization
criteria. The criteria to be met are [25]:

1. Arc stability: background current must exceed a minimum limit for stable
arcing.
2. Metal transfer: spray type metal transfer must be produced.
3. Arc length: a constant arc length must be maintained.

The arc stability is maintained if a minimum background current is kept. It is also
important to meet other conditions like clean metal surfaces and proper shielding
gas flow during welding.

The metal transfer to obtain spray mode can be arranged by modulating the current
in different ways and still keep the mean current at a certain value. The pulse width
can be constant while varying the pulse repetition rate or the pulse repetition can be
kept constant while varying the pulse width. The amplitude of the current pulse can
also be varied to maintain a constant pulse size.

The arc length (l a) can be estimated by measuring the weld voltage. The voltage
over the electrode stick-out is small compared to the arc voltage and can in general
be omitted for most monitoring and control purposes. The arc voltage can be
expressed as

E 3
U E 1 ˜ I E 2 E 4 l ˜ a 3.6
a
I

where ȕ 1, ȕ 2, ȕ 3 and ȕ 4 are empirical constants. If we can assume that the time
constant of the process is much less than the time constant of the pulse duration
and fixed current, the arc voltage can be expressed as

U K E 4 l ˜ a 3.7
5
a

where K 5 is a function of the current. A change in arc length will cause a change in
arc voltage and also a change in weld voltage which makes it possible to estimate
the arc length by measuring the weld voltage. As the arc length can be estimated, it
is possible to build a control system that keeps the arc length constant.