GB2250704A - Arc ignition system for a welding gun and method of operation - Google Patents

Abstract

An arc ignition system for a tungsten/inert gas welding gun comprises a pair of circuits HF1, HF2, connected in parallel with each other and to a common DC power supply for the purpose of providing concurrent sinusoidal discharges across the gap 10 between the tungsten electrode 12 of the gun, and a workpiece 14, so as to provide an ionised and conductively enhanced path for a welding arc. <IMAGE>

Description

ARC IGNITION SYSTEM FOR A WELDING GUN AND METHOD OF OPERATION The present invention relates to an arc ignition system for a welding gun of the kind known as a tungsten/inert gas welding gun.

Tungsten/inert gas welding guns are well known, long existing devices. Despite this, the establishment of a full welding arc across the gap which in operation is defined by a tungsten electrode which is centrally disposed in the bore of the gun3and a workpiece, is still only achieved with difficulty, over a time period which is not predictable.

Many papers which address the problem have been published over the last twenty years, by such institutions as the "Welding Institute", the "International Institute of Welding" and the "British Journal of Applied Physics".

However, the difficulties remain.

The present invention seeks to provide an improved method of enabling a said full welding arc across the aforementioned gap, and to provide apparatus with which to effect the method.

According to one aspect of the present invention, a method of establishing an electrical welding arc across a gap defined between the electrode of a tungsten/inert gas welding gun and a workpiece, comprises the steps of applying first and second continuous, sinusoidal electrical discharges of differing frequencies from power supply means across the gap so as to first ionise a gaseous flow in said gap and then enhance the achieved ionisation such as to provide a conductive path across said gap which is suitable for the establishment of a welding arc.

According to a further aspect of the present invention there is provided an arc ignition system comprising a pair of circuits connectable in parallel with each other and to a common output from a DC power supply and to a tungsten/inert gas welding gun so that in operation, voltage and current can be applied across a gap defined by a tungsten electrode disposed centrally within the gun,and a workpiece, each said circuit comprising, in series, an oscillator for converting DC supply to continuous, sinusoidal form and amplifier means for manipulating the resultant frequency, voltage and current outputs to values which on application thereof across said gap, the output from one circuit ionises a gas flow in the gap, and the output from the other circuit enhances the resulting conductivity of the ionised flow such as to enable the establishment of a welding arc across said gap.

The invention will now be described, by way of example and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of apparatus in accordance with the present invention.

Figure 2 is a block diagram of each circuit in accordance with said one aspect of the present invention.

Figure 3 is a circuit diagram in accordance with said aspect of the present invention.

Figure 4 is a circuit diagram of a coupling device for coupling circuits of Figure 3 and a welding power source.

Figures 5 and 5a are- respectively voltage output waveforms of each of two circuits utilised by the present invention.

Figures 6 and 6a are respectively voltage and current wave forms for the discharge supplied by the circuits of 5 and 5a.

Figure 7 illustrates voltage and current wave forms developed during arc ignition from initiation, when using the apparatus of the present invention.

Referring to Figure 1. A gap 10 is defined by an arrowhead 12 and a horizontal line 14. The gap 10 represents the gap between an elongate electrode (the arrowhead 12) an end of which projects beyond a bore in the ceramic barrel of a known tungsten/inert gas welding gun (not shown) and the surface of a workpiece (the line 14).

A direct current welding power supply 16, which normally would be connected directly to the gun (not shown) is, for the purposes of the present invention, connected via a circuit board 18.

A tap 20 is taken from the output of the supply 16, and is then divided and the divided outputs are input, one each, to further circuit boards 22 and 24.

The purpose of each of the circuits on boards 22,24, is to convert the power received from the DC supply 16 into sinusoidal voltages and currents of given frequencies which are then imposed, one upon the other, across the gap 10 so as to achieve appropriate electrical conditions thereacross, as to ensure the successful superimposition thereon of a welding current from the DC supply 16.

The purpose of the circuit on the board 18, via which the outputs of the circuits 22,24 pass to the gap 10, is to protect the respective supplies 16,22 and 24 from each other as is described later in this specification.

Referring now to Figure 2. Each circuit 22 and 24 is made up by the appropriate arrangement together of an oscillator and buffer 26, a class A driver 28 and a class B push-pull power amplifier 30. A power-current feed back circuit 32 connects the amplifier 30 and oscillator and buffer 26.

The output 34 from each amplifier 30 is the input to the coupling board 18 (Figure 1).

Referring now to Figure 3 which is the common circuit for each of the power generators 22 and 24 (Figure 1). The circuitry is devised for use in connection with a tungsten/inert gas welding gun, which has an open circuit voltage value of 70V DC, and wherein a gap of about 3 mm exists between the projecting tip of the tungsten electrode, and the surface of the workpiece 14.

In the present example, the oscillator and buffer shown diagrammatically in Figure 2, consists of a wave form generator 36 which produces a sinusoidal signal of the required frequency, adjusted by a variable resistor 38 and a capacitor 40. Further resistors 42,44,46 and 48 obviate signal distortion, and variable resistor 50 sets the desired amplitude of the output.

Buffer amplifiers 52 and 54 are used to give a current boost before the signal is fed to the driver 56, which is a MOSFET. The gate bias voltage of the driver is set by a variable resistor 58 and a resistor 60 for class A operation, with a resistor 62 as the drain load resistance which is made small so as to lower the output impedance of the driver stage.

The drain of the driver 56 is coupled to the input of the power generator via a transformer 64. A capacitor 66 is placed across the drain and source connections of the driver 56 to prevent self oscillations which are reflected through the transformer. Capacitors 68,70 and 72 are blocking capacitors. The complete unit is pre-adjusted for best performance and then operated by use of a switch 74.

The power amplifier 30 has a pair of MOSFETs 76 and 78 which are connected in a common source, push-pull arrangement. The arrangement allows a higher efficiency and twice the output voltage for the same turns ratio of an output transformer 80 and input DC voltage. The even harmonics are also suppressed, and therefor, radio frequency interference is reduced, if not obviated, and better sinusoidal outputs are achieved.

The individual gate bias voltages are set for class B operation, and care should be taken to make the two MOSFETS 76 and 78 equally balanced. The 70v DC voltage is connected to the centre tap 90 of the output, push-pull transformer 80, and the current is fed to the drains of the MOSFETs 76 and 78 via high speed recovery diodes 94 and 96, which prevent reverse current flows through the MOSFETs 76 and 78.

Diodes 98,100,102 and 104 are high speed transient voltage suppressors which protect the MOSFETs against high voltage spikes which may be reflected from the arc gap 10 (Figure 1) and which would be sensed via the output transformer 80.

Ferrite beads 106,108,110 and 112, and resistors 114 and 116 prevent self oscillations.

The drain current (ID) of each MOSFET 76,78 is controlled by the impedance (R'L) that appears at its drain and which is determined by the turns ratio of the transformer 80 and the load impedance (RL).

It is important to design the transformer 80 so as to match the load which varies considerably over the operating range. The discharge controls the value of RL which changes from almost open circuit, to a low value when a 1A high frequency discharge is established.

It has been found that the required output of the circuit on board 24 is 300V at 1A peak. Therefore the load impedance is:

If the maximum drain current from each MOSFET 76 and 78 is 5A when operating in a linear mode, then:

but:

where N1 is half the primary turns and N2 is the number of secondary turns, of the transformer 80. Therefore:

Litz wire is used for the windings of the output transformer 80 on circuit board 24, so as to reduce copper losses caused by skin effect, at a frequency of 300KHz. Further, the primary inductance was made high (1.1 mH, twelve turns on each half) to reduce the magnetising current. A stack of three toroidal ferrite cores make up the transformer 80, which maintains leakage inductance at a minimum.

The characteristics of the output transformer for the circuit board 22 are determined by following the procedure described in connection with the transformer 80, except that a suitable television line transformer is used in push-pull mode, so as to produce the necessary high voltage at a frequency of 30KHz.

The overcurrent protection circuit 32 (Figures 2 & 3) with which each circuit board 22 and 24 is provided, is designed and connected so as to turn off the MOSFETs 76 and 78 if their peak drain currents exceed, in the present example, 5A. This is achieved by sinking the input to the buffer amplifiers 52,54 to ground, by applying a positive pulse to the gate of a transistor 118. Thus, no A.C signal is fed into the MOSFETs 76 and 78.

The drain currents are sensed by current transformers 120,122 respectively. A dual comparator 124 is used to give low output if the input voltage which represents the amperage, is of a value which indicates that 5A is being exceeded.

The comparator 124 has two outputs which go to a three input AND gate 126, and if any of the inputs is low, the output of the AND gate 126 which is input to a timer 128, will be low. The low input triggers the timer 128 which emits a positive pulse of a given duration, say one second, which is transmitted via line 130 to the transistor 118 which is turned on, to achieve switch off of the MOSFETs 76 and 78.

Referring now to Figure 4. The coupling circuit 18 operates as follows: A tuned circuit 132 resonates at the frequency of power generator circuit 24 (Figure 1) i.e. 300KHz, so that it appears as a large resistance to the output of generator 24.

Circuit 132 is needed, to eliminate the effect of a capacitor 134 at the output of circuit 24 at 300KHz, by placing circuit 24 in parallel with the capacitor 134.

Capacitor 134 itself is needed to suppress any high voltage spikes that would otherwise damage the output stage of circuit 24.

Tuned circuit 136 resonates at the frequency of power generator circuit 22 (Figure 1) i.e. 30KHz, hence, a high voltage drops across it from circuit 22. Thus, the 30KHz voltage appearing at the output of circuit 24 will be small.

Circuit 136 is capacitive to the current from circuit 24 (at 300KHz) and to increase the required output current, an inductor 138 is placed in series with the tuned circuit 136.

Tuned circuit 140 resonates at the frequency of circuit 24 (300KHz) and it blocks the current from that circuit.

Capacitors 142 and 144 block the DC current. An inductor 146 and a capacitor 148 provide protection for the DC power supply 16 and prevent loss of high frequency power which would otherwise occur, due to the diversion of current away from the arc gap 10.

Tests have shown the invention to be successful.

Figure 5a depicts typical waveforms for the circuit 22 (at 30KHz). The open circuit voltage across the arc gap 10 (Figure 1) was below the gap ionising voltage at about 2KV (which is almost purely sinusoidal) for an input voltage of 4V.

Figure 5b shows the waveforms for the circuit 24 (at 30KHz) which indicates an open circuit voltage of 300V for an input of about 6V. Under open circuit conditions, both circuits 22 and 24 operated without difficulty.

Figure 6a shows the high frequency discharge voltage and current waveforms after ionisation of the gap 10 (Figure 1) when supplied by circuit 22.

Figure 6b shows the waveform of the high frequency starting discharge of current of about 1A peak, sustained by circuit 24. Both discharges are stable and operation is achieved without difficulty.

The procedure to be followed, in order to achieve welding arc ignition in accordance with the present invention, is to switch on the DC source and then set the output voltage of circuit 22, to just below the ionising voltage value, and then switch on the circuit 24, which will cause an increase in voltage across the gap 10. Breakdown proper then occurs and a high frequency starting discharge is established.

Figure 7 shows voltage and current waveforms for a complete ignition process, from before the application of the high voltage (2KV) to the time of establishment of the DC arc, the latter being achieved when the DC arc current reaches about 1A (1 ampere). The waveforms shown in Figure 7 have proved to be typical in the context of operation described in this specification.

Claims (14)

Claims:

1. A method of establishing an electrical welding arc across a gap defined between the electrode of a tungsten/ inert gas welding gun, comprising the steps of applying first and second continuous, sinusoidal electrical discharges of differing frequencies from power supply means across the gap, so as to first ionise a gaseous flow in said gap and then enhance the achieved ionisation such as to provide a conductive path across said gap which is suitable for the establishment of a welding arc.

2. A method of establishing an electrical welding arc as claimed in claim 1 including the steps of applying the first continuous, sinusoidal discharges at a given high voltage, low current, high frequency and applying the second continuous, sinusoidal discharges at a higher frequency, lower voltage and higher current than said first discharge.

3. A method of establishing an electrical welding arc as claimed in claim 1 or claim 2 including the step of deriving said first and second discharges from a common power supply.

4. A method of establishing an electrical welding arc as claimed in claim 3 including the step of dividing the output from said common power supply to provide two parallel inputs one each to a respective high frequency generating, current and voltage modulation circuit, the outputs of which are those desired for enabling establishment of said welding arc.

5 A method of establishing an electrical welding arc as claimed in any previous claim including the step of using the gun welding power supply to supply power for the said continuous, sinusoidal discharges.

6. A method of establishing an electrical welding arc as claimed in any previous claim including the step of passing the gun welding supply and both continuous, sinusoidal discharges to the gun, via a circuit which is tuned so as to obviate self generated radio frequency interference between said supply and discharges.

7. A method of establishing an electrical welding arc across a gap defined between the electrode of a tungsten/ inert gas welding gun and a workpiece substantially as described in this specification with reference to the drawings.

8. An arc ignition system comprising a pair of circuits connectable in parallel with each other and to a common output from a DC power supply and to a tungsten/inert gas welding gun so that in operation, voltage and current can be applied across a gap defined by the electrode of the gun and a workpiece, each said circuit comprising, in series, an oscillator for converting DC supply to continuous, sinusoidal form and amplifier means for manipulating the resultant frequency, voltage and current outputs to values which on application thereof across said gap, the output from one circuit ionises a gas flow in the gap, and the output from the other circuit enhances the resulting conductivity of the ionised flow such as to enable the establishment of a welding arc across said gap.

9. An arc ignition as claimed in claim 8 wherein the amplifier means comprises a buffer amplifier for boosting oscillator current output, and a class B mode output power amplifier.

10. An arc ignition system as claimed in claim 8 wherein the class B output power amplifier includes a pair of MOSFETs which are connected in a common source push-pull arrangement.

11. An arc ignition system as claimed in any of claims 8 to 10 and including an overcurrent protection circuit for each circuit of said pair, which is arranged to sense excessive current and generate a signal in response thereto, with which to de-activate the or each of said pair of circuits.

12. An arc ignition system as claimed in any of claims 8 to 11 and including a coupling circuit connected in series with each of said pair of circuits and with the gun power supply, and tuned so as to substantially avoid the generation of radio frequency interference therebetween.

13. An arc ignition system as claimed in any of claims 8 to 12 wherein the DC power supply is common to the pair of circuits and the gun.

14. An arc ignition system substantially as described in this specification and with reference to the drawings.