Electric heating – Metal heating – By arc
Reexamination Certificate
2001-04-09
2002-12-24
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S1370PS, C219S137710
Reexamination Certificate
active
06498321
ABSTRACT:
The present invention relates to the art of electric arc welding and more particularly to an improved system and method for controlling the power supply during a welding process.
BACKGROUND OF INVENTION
In electric arc welding it is common to employ a high speed switching type power supply, such as an inverter or chopper, so the output current of the power supply can be adjusted by a high speed control loop involving current feedback. In such electric arc welders, there is an outer control loop for creating the current command in accordance with the comparison of the average arc voltage with a reference voltage. In some instances, the current command signal to the power supply is directly controlled by the outer control loop. Consequently, the inner high speed control is based upon current feedback and is adjusted to maintain an average voltage. These inner high speed control loops operate at somewhat close to the speed of the inverter or chopper. The outer voltage control loop operates at about one-tenth the speed of the high speed switching power supply. Furthermore, the outer loop operates without the knowledge of the actual current used in the high speed inner control loop. Consequently, at low current operations, the arc current can dip too low and may cause extinguishing of the arc. Welding of aluminum GMAW requires more accurate control to maintain high weld rates. Thus, controlling the output current of a high speed switching power supply often involves a current overshoot because the outer control loop has a relatively slow reaction time. In short circuit and pulse welding where the feedback is average voltage, a complete weld cycle or portion of a weld cycle must be processed causing a relatively slow response time in the outer control loop. The control loop must be slowed down to maintain an average voltage to adequately compensate for long durations near zero voltage in a short circuit arc welding process. The control loop is thus very slow and the arc length is allowed to vary drastically during the welding process. With such a slow control loop the current is forced to transition by a greater amount to drive the arc length to the desired control voltage set by the outer loop. This presents two problem areas. In short arc welding at low currents, the background current is already so low that swings in the current, as experienced in a constant voltage system, forces the current too low and allow the arc to extinguish. This is especially noticeable just after each short is cleared. As the current is reduced to the background level, the control voltage system may overshoot and force the actual current to be driven too low allowing the arc plasma to be extinguished. This phenomena may occur during each weld cycle of a welding process so that intermittent extinguishing of the arc can be experienced unless corrective, expensive procedures are in place. Thus, welding at low currents is normally quite difficult when operating in short circuiting mode. When welding aluminum, fast changes in stick out or arc length caused by slow control cause intermittent disruption of the welding process with the resultant lack of uniformity of the weld bead.
THE INVENTION
The problems experienced in using a slow outer control loop with or without a high speed inner current control loop, as explained above, have been overcome by the present invention which essentially uses only the inner high speed loop for controlling the welding process. This is made possible by utilizing both the sensed arc current and the sensed arc voltage to produce a feedback signal indicative of the current and voltage product, i.e. power. Essentially, the electric arc welding process, whether it is spray, pulse or AC, has the actual arc power controlled by the desired power. To accomplish this result, both the sensed current and the sensed voltage are used. In the past, the arc voltage controlled the outside loop with its slow response time. The inner high speed loop was merely a current responsive feedback system where a digital error amplifier adjusted the input to the pulse width modulator driving the high speed switching power supply. By utilizing the product of the actual output current and voltage feedback signals, the actual output watts or power is sensed. This signal P is compared to the desired power established by the wave shaper. This causes the output current to change in a manner that forces the actual power to equal the desired power. This is performed at a speed generally ten times as fast as an outer loop control. By using a feedback signal based upon a relationship between both the current and voltage, the arc length is automatically adjusted and the rapid response time prevents any current overshoot.
In accordance with the invention, the voltage and current product is multiplied by a factor k. This product kP is then introduced into the high speed controller for maintaining the arc power at a set amount correlated with the time position in a welding cycle. The slow outer loop control is no longer required. The welding current depends on both the voltage and current introduced into the high speed control loop of the welder. The preferred implementation involves a feedback signal comprising merely the product of arc voltage and arc current. This is the arc power P. This product may be modified by a multiplier k. This factor can be used to compensate for travel speed or wire feed speed. This feedback signal is kP, but k is normally 1.0.
In accordance with a further aspect of the invention, the power P of the arc is a feedback signal for comparison with a power signal or profile from a wave shaper. In this aspect, the wave shaper generates a power profile, especially during arcing conditions of the welding cycle. For instance, when a short circuit type welding is being performed, such as the STT welding process, the wave shaper provides a desired arc power to be created after a short has been broken and an arc is reestablished. When welding in a pulse weld process, the wave shaper outputs peak arc power and then background arc power for comparison with the power feedback signal kP in the high speed control loop. In an AC arc welding process, the desired arc power during the positive polarity is outputted by the wave shaper for comparison with the power feedback signal kP during the actual positive polarity. In a like manner, the desired power during the negative polarity is outputted by the wave shaper for comparison with the power feedback signal kP to control the arc power of the negative polarity. Of course, as used in aluminum welding, the power during the positive polarity can differ drastically from the power during the negative polarity. In summary, a power signal is provided and a power feedback signal kP is sensed. These two parameters are compared to control the current from the high speed switching power supply. Of course, a wave shaper is not necessary during spray or globular welding when a constant power signal P
set
controls the inner control loop and is used for comparison with the power feedback signal kP from the arc. The present invention results in substantial advantages, primarily at low current welding.
The feedback referred to as the power feedback function or signal kP is a relationship of sensed arc voltage and sensed arc current, which product may be multiplied by a factor k that is a constant or a variable. In the preferred embodiment, the multiplier is a constant 1.0 so that the feedback signal is merely the arc power. This power feedback function or signal is compared to the desired arc power for control of the output current of the power supply. The multiplier k may be a constant, as explained, or a linear equation, non-linear equation, or some other equation. The factor k may depend on the actual voltage or current and is employed to modify the feedback signal in a manner to compensate for various welding parameters, such as wire feed speed, electrode travel speed, shielding gas, wire diameter, wire material, etc. It has been found that multiplier k can be current p
Fulmer Bruce E.
Peters Steven R.
Fay Sharpe Fagan Minnich & McKee
Lincoln Global Inc.
Shaw Clifford C.
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