WO1993000195A1

WO1993000195A1 - Improved pulsed arc welding system

Info

Publication number: WO1993000195A1
Application number: PCT/AU1992/000306
Authority: WO
Inventors: Gary Lyn Cheesman; William Kenneth Brougham
Original assignee: Welding Industries Pty Ltd
Current assignee: Welding Industries Pty Ltd
Priority date: 1991-06-24
Filing date: 1992-06-24
Publication date: 1993-01-07
Anticipated expiration: 1993-12-24

Abstract

A pulsed arc welding system having a current pulse generating circuit including a primary welding current source (19) and a power transistor switching circuit (D) controlled by a transistor driver circuit (F) for applying current pulses to a load via an inductance (G) and a flywheel diode (H), a background current generating circuit including a background driver circuit (15) applying background current to the load via a resistor and an inductance, central controlling means (E) including arc voltage monitoring circuits and reference voltage circuits which maintain the average arc voltage at a substantially constant level, one of the reference voltages being less than a voltage sufficient to maintain the welding arc, and detection circuit means (Fig. 4) for detecting when the arc voltage is less than the latter reference voltage and for causing the transistor driver circuit (F) to switch the power transistors (D) on to apply a current pulse to the load, said transistor driver circuit (F) causing the transistors (D) to be turned off when the latter reference voltage is exceeded to thereby maintain the welding arc and avoid short circuits which cause the arc to be extinguished.

Description

IMPROVED PULSED ARC WELDING SYSTEM Field of the Invention

This invention relates to improvements in pulsed arc welding systems. Background of the Invention

Pulsed arc welding is an extension of the Metal Inert Gas (MIG) welding process in which the arc is supplied with current modulated between two specific levels, and where the metal transfer across the arc is in the form of individual molten droplets which form and transfer under the influence of the pulsed arc current.

High speed photography, illustrated schematically in Figure 1, indicates that during the period of elevated arc current (the current pulse), the end e of the electrode wire w progressively melts, forming an nominally spherical droplet d of diameter approximately equal to the diameter of the electrode wire. Under the influence of the magnetic field which is present around the wire due to the current flow through it, (the magnetic pinch effect), the droplet d is projected axially away from the end of the electrode wire, initially connected to it by a bridge b of molten metal. Ultimately the bridge b ruptures with the droplet d continuing its transfer towards the work piece. This cycle of events is repeated at a rate proportional to the rate of wire feed into the arc.

It has been observed that the last step of droplet separation may occur at a period just after the welding current pulse, i.e. in the early stage of the background current period. It is usually desirable for the length of the welding arc be kept to a practical minimum, and in this condition it is possible for the droplet to contact the workpiece while still connected to the electrode wire by the molten bridge. Should this occur during the period of pulse current, sufficient current flows in the bridge to aid rupture of the bridge. However, if the short circuit occurs during the low current background period, the bridge may not rupture and hence the welding arc is short circuited and extinguished. In pulsed arc welding, this is an undesirable condition. Prior art equipment which includes means to detect such a short circuit condition may respond by altering one or more of the following pulse parameters: pulse width, pulse height or pulse frequency. The alteration of these parameters may not cause an immediate change in the arc, and instability of the arc may result. In practice the operator may tend to increase the arc length control setting to avoid the above occurring, so restricting the pulsed arc process and reducing its benefits in some welding applications. ^*έ-- ^gjiimmar of Invention and Object

The object of this invention is to provide an improved pulsed arc welding system by means of which the above e fect may be reduced thereby enabling pulsed arc welding to be performed to maximum advantage.

The invention provides a pulsed arc welding system comprising a current pulse generating circuit and a background current generating circuit including a DC power supply circuit adapted for connection to a load via an electrode wire, switch means for causing current pulses generated by said current pulse generating circuit to be applied to the load, means for monitoring the voltage applied by said circuits across said load, means for comparing said monitored voltage with at least one predetermined reference voltage, characterized in that said at least one reference voltage is chosen to be less than the voltage sufficient to maintain a welding arc, means for detecting when said detected voltage is less than or equal to said at least one reference voltage, and means for causing said switch means to conduct until said at least one of said reference voltages is exceeded to thereby maintain the arc.

The activation of the switch means when the arc voltage is detected to be at or below the reference voltage causes a short current pulse to be applied to the arc causing any molten metal bridge between the electrode and the workpiece to be quickly broken, thereby reversing the short circuit in the system. Since the switch means should be rapidly rendered conductive, it is preferred that a high speed switch, such as a bipolar transistor arrangement, should be used. In a preferred form of the invention, other reference voltage(s) selected to contain a substantially constant average arc voltage across the load.

In most practical embodiments of the invention, the pulsed arc welding system will include : a three phase welding current transformer, a welding current rectifier and filter capacitor bank, a high current, high speed switch, control circuitry, and inductor in the welding current path.

The high current switch is controlled in a manner similar to that described in our United States Patent No. 758707, the contents of which are incorporated herein by cross-reference, to form suitable welding current pulses. A separate bank of lower rated switching devices may be employed to form the background current.

The invention also provides a method arc welding comprising the steps of generating current pulses and a background level of current, applying said current pulses and said background current to a load via an electrode wire, monitoring the voltage generated by the application of said current pulses and said background current to the load, comparing the voltage with at least one predetermined reference voltage (to maintain the average of said voltage at a substantially constant level,) characterized in that said at least one of said reference voltages is less than a voltage sufficient to contain a welding arc, detecting when said voltage is less than said at least one of said reference voltages and applying said current pulses to said load until said one of said reference voltages is exceeded to thereby maintain the welding arc.

Preferably, the voltage is also compared with one or more other reference voltages so as to maintain a substantially constant average arc voltage across the load.

As defined more generally above, the welding voltage is compared to a reference voltage, the level of which is chosen to be less than the voltage drop necessary to maintain a normal welding arc. When the welding voltage is detected to be below this reference voltage, a drive signal is generated to cause the high current switch to conduct. The switch continues to conduct until the welding voltage is detected to be above the reference level, thereby causing the arc to be re-established. A typical welding current wave form showing the additional molten droplet detaching pulse PSC is shown schematically in Figure 2 of the drawings.

It will be understood from the above that the improved system also acts beneficially at the time of initial establishment of the welding arc. In welding terms, the major effect of the improved system is to permit welding to be successfully conducted at an arc length considerably shorter than is possible using existing equipment, whilst maintaining stable operating conditions. Brief Description of the Drawings

One preferred embodiment of the invention will now be described with reference to the accompanying drawings in which

Figure 1 is a schematic representation of molten droplet detachment during pulsed arc welding;

Figure 2 is a schematic representation of a typical welding current wave form showing the generation of additional pulses which reverse the short circuits;

Figure 3 is a block circuit diagram of one preferred form of the pulsed arc welding system embodying the invention.

Figure 4 is a circuit diagram of a shunt hybrid circuit forming part of the micro processor control circuit. Description of preferred embodiment

Referring firstly to Figure 3 of the drawings, it will be noted that the pulsed arc welding system block circuit diagram has many similarities to the block circuit diagram of Figure 1 of United States Patent No. 4758707, and for this reason it is not necessary to fully describe the circuit of Figure 3. In the system described in the U.S. Patent, the current pulses are switched by means of an SCR 22 under the control of the SCR trigger circuit 9_> which is in turn controlled by the central processor 4 and associated circuitry. The SCR is turned off by the switch off driver circuit 23 in conjunction with the transistor circuit 24. In the commercial form of this system, an assembly of power MOSFETS are used to commutate the SCR off with current supplied from a rectifier circuit constituting the auxiliary current source 25-

In the embodiment of the present invention shown in Figure 3 of the drawings, in which circuit elements in common with those shown in Fig. 1 of the US Patent are given the same reference numbers, an array of bipolar transistors D are used to switch the pulsed current on and off, thereby performing the functions of the SCR 22, the assembly of power MOSFETS and the rectifier referred to above.

To implement the invention defined more broadly above, a processor control circuit E is used to control the switching action of the bipolar transistor circuit D. The microprocessor control circuit E includes a shunt hybrid circuit which is shown in greater detail in Figure 4 of the drawings. This sub-circuit is preferably realized as a thick film hybrid circuit and is configured to perform the following controls.

The arc voltage is monitored in the same general manner as is described in the above U.S_* Patent from a current signal derived from current shunts and this voltage signal is applied to pins 7 and 8 of the shunt hybrid circuit of Figure 4. This voltage signal is buffered by the operational amplifiers U3:A, U3:B and U3:C and associated circuitry. A voltage signal representing the actual arc voltage is output on pin 18 labelled VARC, which is referenced to pin 17 labelled OV. The same voltage is applied to pin 12 of operational amplifier U3:D, and is compared to a voltage reference level established by the ratio of resistors R42 and R43» AS mentioned above, this reference level is selected to be less than the voltage drop necessary to maintain a normal welding arc. When the arc voltage falls below this reference level, the output of operational amplifier U3:D goes low, and transistor Tl ceases to conduct. The collector of Tl is connected to pin 16 labelled VLO (voltage low) , and when transistor Tl ceases to conduct, the circuit node VLO is held high by a resistor (not shown) in the processor control circuit E. This voltage is applied to a NOR gate (not shown) in the processor control circuit E, and the other input of this NOR gate is connected to a circuit node (not shown) in the processor control circuit E. When the output of the NOR gate is low, the power transistors D are driven on by means of the transistor driver circuit F in Figure 3_» which also operates in a known manner to withdraw base current from the transistors D to turn the transistors D off. The driver circuit is powered by an auxiliary power supply A.

The rise and fall slope of the arc current is limited by the inductance G, with the flywheel current path via diode H, resulting in an arc current wave form as illustrated in Figure 2 of the drawings. The "chop mode" control of the pulsed current height is not central to this disclosure and is therefore not described further. Thus, the current excursion labelled "PSC" (positive short clearing) results from the arc voltage falling below the reference level, which is taken to indicate that a short circuit has occurred within the arc, and the corrective current pulse resulting from the power transistors D being rendered conductive until the reference voltage is exceeded whereupon the transistors D are turned off by the driver circuit F.

It should be noted that a diode (not shown) in the processor control circuit operates to hold^' the node VLO low thereby allowing the PSC facility to be enabled or disabled as required by the selected welding processor under the control of software associated with the microprocessor control circuit E. Background current is provided to the arc by the action of a background driver circuit B which operates in a known manner to control the current from the rain supply during the required background current periods, under the control of control circuit E, in a manner similar to that described in the above U.S. Patent.

Claims

CLAIMS:

1. A pulsed arc welding system comprising a current pulse generating circuit and a background current generating circuit including a DC power supply circuit adapted for connection to a load via an electrode wire, switch means for causing current pulses generated by said current pulse generating circuit to be applied to the load, means for monitoring the voltage applied by said circuits across said load, means for comparing said monitored voltage with at least one predetermined reference voltage, characterized in that said at least one reference voltage is chosen to be less than a voltage sufficient to maintain a welding arc, means for detecting when said detected voltage is lesss than or equal to said at least one reference voltage, and means for causing said switch means to conduct until said at least one reference voltage is exceeded to thereby maintain the arc.

2. The system of claim 1, further comprising means for comparing the voltage with other reference voltage(s) so as to maintain a substantially constant average arc voltage across the load. _*

3. The system of claim 1 or 2, wherein said means for monitoring said voltage includes a shunt hybrid circuit connected to a central control unit for controlling the welding parameters of the arc, said circuit including means for receiving a voltage signal representing the arc voltage, resistance means for establishing said at least one reference voltage, said means for detecting an arc voltage lower than and at least one reference voltage causing a transistor driver circuit to drive a transistor power switching circuit on to apply current pulse(s) to said load until said at least one reference voltage and exceeded whereupon the transistor driver circuit causes the switching circuit to be turned off.

4. A method of arc welding comprising the steps of generating current pulses and a background level of current, applying said current pulses and said background current to a load via an electrode wire, monitoring the voltage generated by the application of said current pulses and said background current to the load, comparing the voltage with at least one predetermined reference voltage, characterized in that said at least one reference voltage is less than a voltage sufficient to maintain a welding arc, detecting when said voltage is less than said at least one reference voltage and applying said current pulses to said load until said at least one reference voltage is exceeded to thereby maintain the welding arc.

5. The method of claim 4 wherein said voltage is also compared with one or more other reference voltages so as to maintain a substantially constant average arc voltage across the load.