Method and apparatus for welding

Abstract

A short circuit arc welding system is disposed. The control scheme uses a current command signal to drive the output current. The command signal is comprised of a long-term current command that sets the long-term current command level and a real-time or short-by-short current command. Arc voltage feedback is used to determine if the desired arc length is present and to adjust the long-term command. The short-by-short current command is derived from real-time arc current feedback and is used to control the burn-off rate by an instantaneous, or short-by-short, adjustment of the current command. A function of the time derivative of arc power, less the time derivative of arc current, is used to detect, in real time, when the short is about to clear. A stop algorithm is employed that monitors the arc on a short-by-short basis. When the process is ending a very low current level is provided to avoid forming a ball. However, if a short is created, (indicated by a drop in arc voltage) after the low current level, a burst of energy is provided to clear of burn off the short. After the short is cleared, very low current is again provided to avoid forming a large ball. This is repeated until the wire stops and the process ends.

Description

FIELD OF THE INVENTION [0001] The present invention relates generally to the art of welding power supplies. More specifically, it relates to welding power supplies and the control thereof for short circuit welding. BACKGROUND OF THE INVENTION [0002] There are many types of welding power supplies and welding processes. One welding process is referred to as short circuit transfer welding. Short circuit transfer welding generally consists of alternating between an arc state and a short circuit, non-arc state. During the arc state the wire melts, and during the short circuit state the metal further melts and the molten metal is transferred from the end of the wire to the weld puddle. The metal transferred in one cycle is referred to herein as a drop, regardless of the size or shape of the portion of metal that is transferred. [0003] Short circuit transfer welding has many advantages, such as shorter arc length and less melting of the base plate. However, short circuit transfer welding h...

AI Technical Summary

Problems solved by technology

However, short circuit transfer welding has disadvantages, such as increased spatter.

Inappropriate burn-off rate will result in increased spatter.

Third, spatter is also caused by too much power when the short is cleared, i.e., the transition from a short circuit to an arc.

Some prior art patents do not teach control of the short circuit transfer welding process on a short circuit by short circuit basis.

While these may provide fast control, they may be relatively expensive or have insufficient peak current capacity.

Also, switching high current may increase reliability problems and switching losses.

Thus, prior art control schemes that use arc power (or arc energy) to control the burn-off rate are complex, and inaccurate.

However, the prior art attempts result in missed or false positive short clearing predictions.

One of the causes of instability in a short circuit transfer welding process relates to excessive pre-heating of the wire.

Variations in the wire / puddle interaction caused by operator movement and / or changing puddle geometry, can result in irregular pre-heating of the wire due to I2*R heat generation.

Too much pre-heating of the wire can cause the melting rate of the wire to increase to a point where the molten ball grows very quickly and may burn off following the transition from a short to an arc.

This quick melting, known as a flare-up, results in a rapid increase in arc length with a corresponding voltage increase.

If energy is not added quickly enough, the wire can eventually “stub” into the puddle.

The end result of such stubbing is either an explosive short clearing, or a sustained short-circuit with no arc (sometimes called noodle welding).

Unfortunately, most prior art controls adjust after a stub or flare-up has occurred.

Thus, the control actually exacerbates the problem.

While this may produce a uniform wire diameter at the start of the next weld, it wastes time, and the extra step would not be needed if the wire had a consistent diameter when each weld is stopped.

However, the BETA-MIG® did not provide a fast enough response, or an adequate control scheme, to produce the consistent ball size desired for short circuit transfer welding.

However, this method will not work for processes such as short circuit transfer welding, that do not tightly control the frequency of the output power.

Also this prior art does not desirably compensate for irregularities in the process, such as unintended shorts.

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