GB2136169A - Regulating Pulses for Arc Welding - Google Patents

Abstract

An electrical current pulse generating circuit for pulses arc GMA welding, has a main power supply (P1) for delivering current to an arc between an electrode (E) and a workpiece (W), a switch (1) controlling the formation of current pulses in the arc, and controllers (2, 3) for turning the switch (1) on and off. The arc voltage is monitored (6), sampled, and averaged, and then compared with a preset arc voltage. The pulse frequency is altered within preset limits so as to maintain the average arc voltage at the preset level, in order to ensure single droplet transfer for each current pulse. The peak and background current levels are monitored and controlled by high frequency switching. <IMAGE>

Description

SPECIFICATION Improved Current Pulse Generator This invention relates to improvements in electrical current pulse generating circuits of the type suitable for pulsed arc GMA welding.

Pulsed arc welding was first proposed in the early 1 960's and since that time has become one of the most useful developments in arc welding.

However, pulsed arc welding has never reached its full potential due mainly to the number and complexity of the controls which an operator must manipulate in order to achieve satisfactory welding conditions, in particular, the most desirable welding condition whereby one welding metal droplet is detached with each welding current pulse irrespective of the wire feed rate.

The variables which exist in a pulsed arc welding system include: (a) pulse height-the amplitude of the current pulse, (b) pulse width-the duration of the current pulse, (c) wire feed rate-the lineal speed at which the electrode is fed to the welding arc, (d) pulse frequency-the repetition rate of the current pulses, (e) background current-the relatively low value of dc current which flows in the arc in the periods between current pulses, (f) the average arc current, and (g) the average arc voltage.

The pulse height in combination with the pulse width define the energy content of the pulse and this must always be sufficient to form and detach the weld metal droplet. The pulse height must be above "the threshold" current level in a particular wire size/type/shielding gas combination necessary to achieve spray transfer of the weld metal. It is desirable for a practical welding system to include a feedback mechanism of some type to take account of changes in the welding parameters, such as variations in mains voltage and wire feed rate due to changes in the motor speed or slippage in the wire feed system.

Although various forms of feedback system have been proposed, none have had the desired effect of maintaining welding parameters which ensure the detachment of a single droplet for each current pulse generated by the system. This is principally due to the fact that the circuitry does not adequately adapt to the changes in welding conditions caused by changes in the wire feed rate and changes in the position of the electrode relative to the workpiece (arc length). Most attempts to compensate for such changes have concentrated on controlling the pulse width to maintain a relatively constant electrode burn-off rate.However, such an approach ignores the importance of the correct relationship between the pulse frequency and the wire feed rate resulting in the production of more than one metal droplet per current pulse, or in the need for more than one pulse for the detachment of the metal droplet, which in turn results in spatter of the weld metal and other undesirable effects.

It is one object of the present invention to provide an improved electric pulse generating circuit which ensures, as far as is practicable that only a single droplet of weld metal of the desired size is formed and detached during each current pulse.

In the light of the above object, the present invention therefore provides a current pulse generating circuit comprising a dc power supply circuit adapted for connection to a load, switch means for causing current pulses to be applied to said load, means for detecting the voltage applied by said power supply to said load, means for averaging said detected voltage and means for controlling said switch means to control the pulse repetition rate to maintain a substantially constant predetermined average voltage at said load.

Where the current pulse generating circuit is used in an electric arc metal transfer process in which an electric arc is struck from a wire which is fed towards a workpiece at a predetermined wire feed rate, a reference voltage proportional to said wire feed rate is used to determine said predetermined average arc voltage and the difference between said predetermined average arc voltage and the detected average arc voltage is used to control the switch means to in turn control the pulse repetition rate of said circuit.

It will be appreciated from the above that any change in the instantaneous feed rate of the electrode wire at the weld zone because of wire feeding problems or changes in the position of the welding gun relative to the workpiece are immediatley met in accordance with the present invention with a corresponding change in the pulse repetition rate or frequency to maintain a constant average arc voltage resulting in the formation and detachment of a single droplet for each current pulse.

For a given wire size/type shielding gas combination, the relationship between wire feed rate and average arc voltage has been found to be substantially linear, the wire feed rate varying in practical applications from about 2.5 meters per minute for a large diameter wire to about 1 6 metres per minute for a small diameter wire, the average arc voltage varying between about 22 volts and 32 volts.

In a preferred form of the invention, the switch means includes an SCR which is triggered to its 'on' stage by a pulse generated by a pulse width circuit and is driven to its 'off' state by means of a transistor circuit triggered by another pulse from the width circuit.

The height of each current pulse is also preferably controlled. One means of achieving this is to use the switch in such a way that when the pulse current exceeds a given current level, the switch is opened to allow the current to fall, and when the pulse current falls below a second lower given current level, the switch is again closed. The repetition of this cycle during the pulse period causes the pulse current to be confined between two given current levels and hence to be controlled in height.

The preferred circuit is preferably programmable so that the operator may select a predetermined number of different welding parameters. However, once a particular program has been selected, the only control required to be adjusted by the operator, apart from standard fine adjustments, is the desired current level. Thus, the circuit may be regarded as a "one knob" controlled welding circuit.

Preferred embodiments of the invention will now be described with greater detail with reference to the accompanying drawings in which: Figure 1 is a simplified schematic block diagram of a welding system embodying the present invention, Figure 1 A is a schematic representation of the current pulse profile produced by the system of Figure 1, Figure 2 is a more detailed welding circuit diagram, and Figures 3, 4 and 5 are detailed circuit diagrams of the principal elements of the control circuitry included in the circuit of Figure 2.

Referring firstly to Figures 1 and 2 of the drawings, the pulsed-arc welding machine embodying the invention will be seen to include a three phase main power supply P1 and an auxiliary power supply P2 of standard configuration as shown in Figure 2. Since the power supplies are quite standard and since further description of a similar power supply is included in the specification of PCT/AU80/0008 (Publication No. WO81/01323) further description of the power supplies will not be provided.

The power supplies P 1, P2 are connected to the arc electrode E supported by an arc gun (not shown) to which arc electrode wire E is fed by a wire feed motor M under the control of a gun switch G to create a welding arc between the electrode E and a workpiece W. The arc current is controlled by a pulse switching circuit incorporating a main current switch SCR1 controlled by a triggering circuit 2, and a main drive off circuit 3. The triggering circuit 2 is shown in greater detail in Figure 3 of the drawings and will not be described in any further detail. The main drive off circuit 3 is comprised of a bank of transistors T which are configured to rapidly divert current from SCR1 when the drive off circuit 3 is suitably triggered. One suitable transistor bank configuration is described in GE SCR Manual, 4th Edition, page 99. "Class E-External" pulse source for commutation.While SCR1 is off, background current is supplied to the electrode E from the power supply P 1 under the control of a background drive circuit 4.

The welding current is supplied to the workpiece W by way of a welding inductance Lw.

The welding current is detected as the voltage drop across a shunt 5 while the arc voltage is monitored in the manner shown. An arc voltage and current processing and isolation circuit 6 is connected to the current and voltage signal leads and to the common lead shown. The processing circuit 6 (see also Figure 3) amplifies the voltage drop across the shunt 5 by a factor of 160, divides the signal by 10, buffers the signal by means of IC8/2 and IC8/3 and then multiplies the signal by 10. This process electrically isolates the current signal which has low noise and remains accurate even at the low background current levels which the circuit must monitor. The arc voltage is similarly divided by 10, buffered and multiplied by 2 so that the resultant signal is 1/5th of the detected voltage signal.

A current and voltage sensing circuit 7 is used to detect the existence of valid welding conditions for a reason to be discussed further below. The circuit 7 will be seen to comprise IC3/4 and IC3/3 which are configured as comparators detecting the presence of arc voltage and current respectively. When the required levels of voltage and current are detected, both outputs are high and transistor T5 goes on and an internal signal G goes high to indicate that valid welding conditions exist.

The G signal is passed to a ramp-up circuit 8 which operates at the beginning of welding once the G signal indicates that valid welding conditions have been achieved. Referring to Figure 3, the ramp-up circuit will be seen to comprise T3, R14, R18, D1 and D2 configured as a constant current generator. C4 is held discharged by T4 until G goes high and C4 is then linearly charged by the constant current generator to the voltage level on pin 3 of IC2/1 which corresponds to the setting of the current selection potentiometer 9 (main Figure 3) which is in turn supplied by one of two constant current generators 10 and 1 1 (Figure 3). The other constant current generator 11 also supplies an arc length selection potentiometer 12.The use of constant current generators 10, 1 1 minimizes the picking up of spurious signals on cables running from the current and arc length controls which are remote from the welding machine.

A series of program selection switches S1 to S6 are provided to enable selection of predetermined sets of the welding parameters, wire feed speed, voltage, current pulse frequency, background current level, current pulse width and current pulse height, the upper and lower limits of which are set by potentiometers RV1 to RV10. As will be seen in Figure 4, the program selection switches are CMOS switches which are connected via buffers to the limit potentiometers RV1 to RV10. In the present embodiment, six programs may be selected and the upper and lower limits of each parameter are set for each program in the factory.

Four of the six parameters: wire feed rate, voltage, frequency and background current all vary in direct proportion to the setting of the current selection potentiometer 9, as modified by the ramp-up circuit 8 at the beginning of a weld.

Each of these parameters is the result of the addition of two voltages: a program constant voltage which sets the level of each variable at the lowest setting of the current potentiometer 9 and a voltage which is scaled from the voltage as set by the current potentiometer 9. These are referred to as the low and high end sets respectively and potentiometers RV1, RV3, RV5 and RV7 are the low end potentiometers while RV2, RV4, RV6 and RV8 are the high end potentiometers. Potentiometers RV9 and RV10 set the fixed parameters of pulse height and pulse width. Summation is performed by networks R1, R2 and buffers lC5/4 and so on as shown in Figure 4.The six resultant program voltages are gated onto an analogue bus by bilateral switches contained in IC1 and in part of IC2. As mentioned above, the number of programs has been selected as six but more or less could be accommodated as required. The detected arc voltage from the processing circuit 6 is fed to a sample and hold circuit 13 followed by an averaging circuit 14. The sample and hold circuit 13 is in the form of a bilateral switch IC1 in which segment A is gated by the G signal so that any short circuits which take place in the arc are not seen by the averaging capacitor Cl.

The programmed arc voltage signal, the lower and upper limits of which are set by RV3 and RV4.

is applied along with the arc length signal set by potentiometer 12 to a summing amplifier 15 (Figure 5) comprising lC2/l, R7 and R8 which permits the programmed arc voltage to be modified in response to the setting of the arc length potentiometer 12 (fine Figure 5). A comparator 16, in the form of error amplifier IC2/2, compares the average arc voltage from the averaging circuit 14 and within the limits set by diodes D1 to D4, which maintain system stability, the output is used to modify the programmed pulse frequency signal at the summing point R17, R18 leading to operational amplifier 17 (lC2/3). In this way, any change in the average arc voltage caused by changes in the wire feed speed or the position of the electrode E will cause a corresponding change in the pulse frequency signal.

Frequency generator 18 has a characteristic frequency proportional to the voltage at pin 10 of lC2/3. IC2/3 is an adjustable current sink by which the charge rate of C4 is varied. IC2/4 acts as a comparator so that when the voltage at pin 13 falls lower than the voltage at pin 12, the output goes high, T2 goes on and triggers the timing circuit IC3. The output from timer IC3 is a pulse frequency marker signal which also discharges C4 via T3 to begin the next cycle.

The width of each current pulse is determined by pulse width timer 19. As shown in Figure 5, R37, R38, R39, D9 and T1 1 form a constant current source which charges C9 at a factory set rate of 1.5 V/mS. To enable inter-changeability. A linear ramp is therefore developed across C9 and this is reset to zero by the pulse frequency marker signal It4/4 is a comparator which compares the ramp to a buffered program pulse width signal to determine the pulse time. The output at pin 14 triggers IC5 on the leading edge to provide an ontrigger to SCR1 and on the trailing edge triggers IC6 to provide an off signal to the transistor bank T to actuate the main drive off circuit 3 to turn SCR1 off.

Pulse height is controlled by circuitry including a constant current source 20 and pulse height comparators 21, 22 connected respectively to the SCR trigger on circuit 2 and to the main drive off circuit 3. As shown in Figure 5, the programmed pulse height voltage is buffered by IC7/2 and is present on pin 10 of IC7/3. A constant current source R51, R53, R54, T12 and Dl 7 passes fixed current through R52 to make the voltage at pin 13 of IC7/4 always at a fixed level above the voltage at pin 10 to create a pulse height window.

The arc current signal is present on pin 12 of IC7/4 and on poin 9 of IC7/3, both of which act as comparators. When the arc current signal pin 12 exceeds the upper window pin 13, output pin 14 goes high triggering IC8 to generate a pulse off signal. When the arc current signal pin 9 falls below the voltage level at pin 10, IC9 is triggered to generate a pulse on signal to SCR1.

The background current is controlled by a further constant current source 23, connected to comparators 24, 25 having their outputs connected to the set and reset terminals of a flip flop 26. As shown in Figure 5, the constant current source 23 comprises R73, R75, T13, D27 and D28 which develop a constant voltage across R74. It10/2 is a sink only buffer and the voltage on pin 6 is the programmed background level while the voltages at pins 9 and 12 are the lower background window level and the upper background window level respectively. IC10/1 detects the trailing edge of a welding current pulse and lifts the voltage across C23, within the limits confined by D25, and D26 which prevents the arc being extinguished at the point at which the background current must take over from the pulse current.When the voltage at pin 10 of IC10/3 falls below the voltage at pin 9, output pin 8 goes high triggering IC1 1 on. Pin 3 of IC11 then goes high providing gate drive to the background current transistors. When the voltage at pin 13 goes higher than the voltage at pin 12, output pin 14 goes high, resetting IC1 1 (flip flop 26) causing pin 3 of IC1 1 to go low thereby removing drive from the background transistors.

In use the operator chooses correct electrode wire and shielding gas combination to suit the job in hand. The correct program must then be chosen by use of the program selection switch S1--S6.

To begin welding the operator closes the gun switch G which is usually integral to welding handping. This action causes 1) Gas valve to operate to supply shielding gas to welding zone, 2) Energies the welding power source main contactor which connects 3 phase power to the main welding transformer therefor to the 'main' and 'aux' rectifiers P 1 and P2, 3) Enables 'motor drive circuit' causing electrode feed motor M to commence rotation, hence electrode wire is fed into the arc zone.

In the period prior to the electrode wire touching the workpiece the 'and' gate denoted sense V and I' will detect only V. At the time at which the electrode wire first touches the workpiece, an electrical short circuit is formed (but no arc) and the 'and' gate will sense only I.

After a period of resistive heating due to the high current flow, the electrode wire will rupture and an arc will be established. The 'and' gate will now sense V and I resulting in the signal G going high.

While G is low all programmed variables except frequency are kept at their respective low end setting. Once G goes high, denoting the establishment of valid welding conditions, the "ramp-up- circuit is initiated allowing all programmed variables to run up to their respective levels as set by the 'current' potentiometer 9.

A second function of G, when it is low, is to override the programmed frequency during the arc starting phase to provide an elevated pulse frequency as an aid to arc initiation.

During welding the following closed loop is in operation. Arc voltage is processed through sample and hold circuit 13 and averaged at 14.

This average arc voltage is compared to programmed arc voltage (as modified + or - by the 'arc length' control). The result of the comparison is in turn used to modify pulse frequency which is nominally set by the program.

The significance of average arc voltage is that it is representative of average arc length. The overall effect as follows: Assume initial stable welding conditions. If the wire feed rate relative to the workpiece is reduced by either a change in torch position relative to workpiece or slowing of the actual wire feed rate ,due to some mechanical cause, arc length will tend to increase as the burn-off rate is (initially) unchanged, but the apparent feed rate has been lowered. The increase in average arc length causes average arc voltage to rise arithe power source responds by lowering the pulse frequency, hence lowering the burn-off rate to match the prevailing conditions. Because it is the pulse frequency which is varied to achieve a reduction in burn-off rate, the volumetric relationship necessary for 'one drop per pulse' is retained.

When gun switch is released machine returns to the standby condition.

Claims (6)

1. A current pulse generating circuit comprising a dc power supply circuit adapted for connection to a load, switch means for causing current pulses to be applied to said load, means for detecting the voltage applied by said power supply to said load, means for averaging said detected voltage and means for controlling said switch means to control the pulse repetition rate to maintain a substantially constant predetermined average voltage at said load.

2. The circuit of claim 1 further comprising means for limiting the pulse repetition rate between predetermined upper and lower limits within which circuit operation is substantially stable, said means for controlling said pulse repetition rate operating to maintain said predetermined average voltage whilst remaining within said pulse repetition rate limits.

3. The circuit of claim 1 or 2, comprising means for detecting the voltage at said load, means for averaging said voltage, means for comparing said average voltage with a predetermined set voltage and for adjusting the pulse repetition rate to maintain said average voltage at said predetermined set voltage.

4. The circuit of claim 1,2 or 3, further comprising means for detecting the current at said load and for controlling said switch means to maintain the current pulse within the limits of a current pulse window so that each pulse is composed of a discrete number of on periods of said switch means.

5. The circuit of claim 1, 2, 3 or 4, further comprising means for supplying background current to said load, including means for controlling the background current to remain within the limits of a background current window.

6. The circuit of any one of claim 1 to 5, wherein said pulse repetition rate is controlled by the frequency generator which generates a pulse frequency marker signal which in turn controls the operation of a pulse width controller which causes the generation of on and off signals to control said switch means.