GB2144595A - Control system for DC pulse modulated arc welding - Google Patents

Abstract

A control system for pulsed direct current arc welding operates automatically to adjust the width of the current pulses supplied at an arc gap (73) by a pulsed DC power supply (74), to control power flow to work pieces (70) at the arc gap. A suitable control system 80 comprises a variable resistance device connected in parallel with a normally closed time delay switch. This system is electrically connected in an impulsar control circuit (78, 79, 80, 81, 82) of the power supply. The control system automatically changes the resistance in the conventional impulsar control circuit to provide an adjustment to alter the impulsar output signal which controls the pulse width of the current pulses supplied by the power supply. <IMAGE>

Description

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SPECIFICATION

Control device for dc pulse modulated arc welding This invention relates to a control device for arc welding and, more particularly, relates to a control device for pulsed direct current (DC) arc welding.

There are many situations in which it is desirable to arc weld together two pieces of metal. For example, a heat exchanger for an air conditioning system may be made from sections of thin wall aluminum tubing which are joined to provide a continuous circuit for the flow of a refrigerant. The sections must be joined so that there are no leaks. One method for accomplishing this is by arc welding.

One problem encountered in arc welding is the presence of foreign materials on the surfaces of the work pieces which are being welded together. These foreign materials can degrade the quality of the weld if they are not removed. Metals such as aluminum, magne- sium, and beryllium copper, pose an especially difficult surface contaminant problem since oxides instantaneously form on the surfaces of these metals when they are exposed to air. Oxides may be removed by using a nonmetal chlorine or flourine base flux during the welding process but this flux is corrosive and is not compatible with the environment. Therefore, it is desirable to arc weld, especially to arc weld metals such as aluminum, magnesium, and beryllium copper, without using a flux.

Fluxless welding is possible by using certain alternating current (AC) arc welding techniques. United States Patents 3,894,210 to Smith, et al and 3,818,177 to Needham, et al disclose such AC arc welding techniques. These techniques are especially useful for welding certain materials, such as aluminum, magnesium, and beryllium copper, since a weld can be made even if oxides are present on the surfaces of the work pieces. However, there are many situations when it is desriable to use direct current (DC) arc welding. For example, it is difficult to weld thin wall sec- tions of aluminum tubing used in making heat 115 exchangers for air conditioning systems by using an AC arc welding technique. This is because AC arc welding requires a significant power flow to the work pieces to make a weld and dissipate oxides without using a flux. This power flow heats the work pieces to an undesirable temperature because the thin wall tubing does not provide a sufficient heat sink for conducting away heat energy. Thus significant sagging in the weld area can occur and there is a possibility that the work pieces will be burned through. This distortion of the weld area can be reduced if DC arc welding is used. Also, electrode life can be increased if DC arc welding is used rather than AC arc GB2144595A 1 welding. Furthermore, power flow to the work pieces may be more precisely controlled when using DC arc welding. These are just some of the advantages inherent in DC arc welding when welding certain materials such as the thin wall sections of aluminum tubing used in making heat exchangers for air conditioning systems. Therefore, it is preferable to weld these materials by using DC arc welding rather than by using other techniques such as fluxless AC arc welding.

One disadvantage of conventional DC arc welding is that this type of arc welding is not generally capable of fluxless welding of cer- tain materials, such as aluminum, magnesium, and beryllium copper, which form difficult to reduce oxides on their surfaces. However, there is a novel method of pulsed DC arc welding for welding these materials without using a flux. According to this novel method, special pulses of positive direct current are applied at an arc gap to arc weld work pieces at the arc gap. The special pulses have a form which is similar to conventional DC pulses except that the ratio of the magnitude of the peak current to the magnitude of the maintenance current at the leading edge of each current pulse is selected to have a special feature. Essentially, this ratio is maximized and the increase from the maintenance current level to the peak current value is adjusted to occur in a time interval whereby a thermal shock affect is created. A related kind of thermal shock effect is well known in the field of vacuum brazing as part of a multi-stage heat treatment process in which materials are joined together by brazing. Basically, this thermal shock effect results from rapidly heating work pieces having surface oxides with a coefficient of thermal expansion which is substantially less than the coefficient of thermal expansion of the underlying pure material. The rapid heating causes an uneven rate of expansion which fractures and splits apart the oxides on the surfaces of the work pieces.

The split apart oxides are pushed away from the weld area due to the melting and joining of the underlying pure materials during the novel arc welding process disclosed above. Other physical phenomena also may be responsible for the exemplary welds formed when using this novel arc welding method but the thermal shock effect is believed to be the primary mechanism by which the oxides are dissipated. Regardless of the exact physical phenomena underlying the oxide dissipation, the feature of maximizing the ratio of peak current to maintenance current at the leading edge of each current pulse is an essentially element of this novel method of DC arc welding. This feature is best explained when it is assumed that the thermal shock effect is the primary mechanism by which the oxides are dissipated.

The optimal values for the maintenance 2 GB2144595A 2 current, peak current and time duration in which the increase from the maintenance current level to peak current value occurs, when arc welding according to the novel arc welding method described above, are selected through a trial and error process. These optimal values depend on the kind of material being welded, the thickness of the work pieces being welded, and other such factors.

Also, power flow from the welding electrode to the work pieces is an important factor in determining weld quality. Good quality welds cannot always be made because of changes in this power flow as a function of time. It is especially difficult to continually make good quality welds on certain materials, such as thin wall aluminum tubing, when mass producing products, such as heat exchangers for air conditioning systems, because of this vari- ation in power flow. This problem is present even if the novel method of fluxless pulsed DC arc welding described above is used in the manufacturing process.

These changes in power flow usually are caused by variations in the resistance between 90 the welding electrode and the work pieces due to inhomogeneities in the ionized gas, variations in work piece dimensions resulting in a changing arc gap separation, naturally occurring fluctuations in power supply output voltage and other such phenomena. This variation in resistance between the welding electrode and the work pieces directly affects the amount of power which reaches the work pieces from the welding electrode. It is desirable to maintain this power flow at a constant optimal valve since it is this power flow which primarily determines weld quality.

Conventional arc welding systems of the pulsed DC type do not specifically address the problem of controlling power flow to the work pieces. Typically, these systems regulate current flow by adjusting the voltage applied across the arc gap in response to variations in arc gap resistance to maintain the current flow at constant preset levels. Therefore, the normal operation of a current regulated pulsed DC system results in variations in the power flow to the work pieces.

A method of controlling this power flow, when using a pulsed DC power supply, is by changing the pulse width of the current pulses supplied to the arc gap. If a periodic series of current pulses is being used this amounts to changing the duty cycle of the current pulses. Thus, this method can be called pulse width modulation or duty cycle modulation. This method of controlling power flow is especially useful when the form of the DC pulses must be maintained as required when arc welding according to the novel pulsed DC are welding method described above. Therefore, it is desirable to provide a control system for an arc welding system pulsed DC power supply flow to work pieces by modulatfing the pulse width of current pulses supplied by the power supply to the work pieces. Preferably, this pulse width modulation is done without other- wise altering the general form of the current pulses. Furthermore, it is desirable to have a control system for a pulsed DC are welding system power supply which automatically con trols the pulse width of current pulses sup plied by the power supply to the arc gap.

Preferably, this control should be capable of use with a conventional DC power supply for controlling the pulse width of any type of pulse generated by the power supply.

According to the present invention there is provided a control device for the power supply of a pulsed DC arc welding system used to weld work pieces at an arc gap comprising a welding electrode for supplying electrical power to the work pieces; power supply means for supplying a voltage to the welding electrode to create a periodic series of direct current pulses which are supplied across the arc gap to weld the work pieces, said control device characterized by an impulsar means for providing a control signal to the power supply means said control signal controlling the pulse width of the current pulses applied at the arc gap; and a programmable pulse width control means for automatically adjusting the impulsar control signal to provide a preselected adjustment to the pulse width of the current pulses without otherwise substantially affecting the form of the current pulses.

Advantageously, the impulsar means cornprises a comparator means having an input terminal and an output terminal, said output terminal providing the control signal to the power supply means for controlling the pulse width of the current pulses supplied cross the arc gap, and said input terminal receiving an electrical signal from the programmable pulse width control means which controls the signal generated at the output terminal of the com- parator.

The programmable pulse width control means may comprise a time delay normally closed switch means for providing a first electrical signal to the input terminal of the com- parator means which causes the impulsar means to generate a first pulse width control signal; and a variable resistance device electrically connected in parallel with the switch means for providing a different second electri- cal signal to the input terminal of the comparator means which causes the impulsar means to generate a different second pulse width control signal, said second signal effective only when the normally closed switch means opens to allow the variable resistance device to control the operation of the comparator means after a fixed time period during which the normally closed switch means is controlling the operation of the comparator means. 65 which is capable of precisely adjusting power 130 In a preferred embodiment of the present 3 GB2144595A 3 invention, a programmable pulse width control device, in the control circuitry for an arc welding system power supply, is used to adjust the pulse width of the current pulses supplied at the arc gap from the power supply. This adjustment compensates for changes in parameters effecting power flow to the work pieces at the arc gap. If the arc welding power supply is a conventional pulsed DC supply, the programmable pulse width control device may be simply a normally closed time delay switch connected in parallel with a variable resistance device. This programmable device operates to interpose the variable resis- tance device in a conventional impulsar control circuit for the arc welding system power supply when the normally closed switch is opened after a preselected time delay. The device is electrically connected between an impulsar pulse width adjustor and impulsar output signal generator to automatically alter the pulse width of the voltage control signal outputted by the impulsar. This causes the power supply to automatically alter the duty - cycle of the current pulses supplied at the arc gap since the impulsar directly controls the operation of the power supply of a conventional arc welding system. If a more complicated current program is desired then the insertion of more resistance devices with time delay switches can be used or another such circuit arrangement devised.

This invention will now be described by way of example, with reference to the accom- panying drawings, in which:- Figure 1 shows a block diagram of an arc welding system including a controller for automatically adjusting the pulse width of current pulses supplied by a power supply to an arc gap.

Figure 2 shows specific circuit components for the automatic controller shown in Fig. 1.

Figures 3 and 4 show how the impulsar output system shown in Fig. 2 operates to generate output voltage control signals of two 110 different duty cycles in response to input voltage control signals of two different magnitudes when the impulsar output system includes a a comparator.

Referring now to Fig. 1, a block diagram is shown for an arc welding system having a time delay programmable pulse width controller 80 used as part of the control system for the power supply of the arc welding system.

As shown in Fig. 1, work pieces 70 and an electrode 71 form an arc gap 73 across which a voltage is supplied by power supply 74. High frequency high voltage arc starter 75 is also connected across the arc gap 73 to provide an initial high voltage for ionizing the inert gas supplied at the arc gap 73 from the gas supply means 76 at the beginning of a welding cycle and for initiating current flow across the arc gap 73. After this initial ionization and after the initial current flow begins the arc starter 75 discontinues operation. The power supply 74 is a commercially available pulsed positive DC power supply. A conventional current regulator 77 controlled by a conventional impulsar is used to control the operation of the supply 74.

As shown in Fig. 1, the impulsar is depicted as divided into two parts. One part is designated an impulsar pulse width adjustor 79 and the other is designated an impulsar output system 78. The impulsar pulse width adjuster 79 is that part of the conventional impulsar which generates internal control signals for the impulsar, typically voltage signals, for controlling operation of electrical devices of the impulsar. Typically, the magnitude of an internal voltage control signal 101 supplied through an electrical lead 81 from the pulse width adjustor 79 the impulsar output system 79 determines what pulse width control signal 104 will be outputted from the impulsar, as depicted in Figs. 3 and 4. Other internal control signals may flow from the impulsar pulse width adjustor 79 to the impulsar output system 78 via electrical connector 82.

The impulsar output system 78 is that part of the conventional impulsar which generates an output control signal 104 for the current regulator 77 in response to the internal voltage control signal 101 from the impulsar pulse width adjustor 79. Typically, the impulsar output system 78 includes a comparator which compares a reference voltage signal 102, such as a voltage ramp function, to the internal voltage control signal 10 1 from the pulse width adjustor 79. Figs. 3 and 4 depict how the comparator operates to generate output voltage signals 103 of different pulse widths in response to internal voltage control signals 101 of different magnitudes. Basically, the comparator generates an output voltage signal 103 only when the reference voltage signal 102 equals or exceeds the internal voltage control signal 101. Thus, the internal voltage control signal 10 1 shown in Fig. 3 results in a smaller pulse width for the comparator output voltage signal 103 compared to when the reduced internal voltage control signal 101 shown in Fig. 4 is utilized. Other conventional circuit elements of the impulsar output system 78 respond to the comparator output voltage signal 103 to generate an impulsar output control signal 104 for the current regulator 77. This output control signal 104 is keyed to the off- times of the comparator output voltage signal 103 so that the duty cycle of the current pulses supplied by the power supply 74 to the arc gap 73 is keyed to the off-times of the comparator output voltage signal 103. Thus, an increase in the internal voltage control signal, 101, which causes a decrease in the pulse width of the comparator output voltage signal 103, results in a corresponding increase in the duty cycle 4 GB2144595A 4 of the current pulses supplied at the arc gap 73.

The impulsar pulse width adjustor 79 of the arG welding system directly controls the oper ation of the impulsar output system 78. How ever, according to the principles of the pre sent invention, a time delay programmable pulse width controller 80 is interposed be tween the conventional pulse width adjustor 79 of the impulsar and the impulsar output system 78. This programmable pulse width controller 80 operates to automatically adjust the impulsar output system 78 to control the current regulator 77, and thus power supply 74, to provide current pulses of varying pulse width according to a predetermined pro grammed sequence.

The programmable pulse width controller can be of a variety of constructions. The controller 80 can be most simply constructed by providing a variable resistance device 89 and a time delay switch 88 connected in parallel to each other and in series between the pulse width adjustor 79 and the impulsar output system 78. Such a controller 80 is shown in Fig. 2. It should be noted that the circuit shown in Fig. 2 is a simple example of a programmable pulse width controller 80.

Other circuits could be devised by one of ordinary skill in the art to provide other more complex current programs.

In operation, positive DC pulses are sup plied at the arc gap 73 by the power supply 74 as controlled by the current regulator 77 in response to the input from the impulsar output system 78. Initially a signal is supplied through the normally closed contacts of the time delay switch 88. However, time delay switch 88 operates after a preselected time delay to open the normally closed contacts.

This interposes the variable resistance device 89 between the impulsar pulse width adjustor 79 and the impulsar output system 78. This results in a different signal being provided to the current regulator 77 and in turn to the power supply 74. This different signal adjusts the duty cycle of the pulses supplied at the arc gap 73. Typically, the duty cycle of the current pulses is decreased after the time delay. Normally, a decrease is required since there is a heat build-up at the work pieces 70 during the start-up period of operation of the arc welding system. Thus, it is necessary to reduce the power flow to the work pieces 70 after a period of time to maintain the optimal power flow which will consistently achieve good quality welds. The particular time delay and amount of reduction in duty cycle to achieve optimal welding depends on the parti cular work pieces 70 being welded. These parameters are best selected through a trial and error process.

Finally, it should be noted that, although the pulse width modulation of DC pulses according to the principles of the present 130 invention is particularly suited for welding materials, such as aluminum, when using the special novel type of current pulse described previously, the present invention is not limited to use with this type of pulse. Pulse width modulation accordng to the principles of the present invention provides precise control of power flow to work pieces at an arc gap when arc welding practically any kind of material with DC pulses. For example, conventional DC pulses used in welding together stainless steel work pieces, especially thin wall pieces of stainless steel, can be modulated according to the principles of the present invention to pro- vide precise control of the power flow to the work pieces to make high quality welds. Therefore, while the present invention has been described in connection with particular embodiments, it is to be understood that various other embodiments and modifications may be made without departing from the scqpe of the invention heretofore described and claimed in the appended claims.

Claims (4)

1. A control device for the power supply of a pulsed DC arc welding system used to weld work pieces at an arc gap comprising a welding electrode for supplying electrical power to the work pieces; power supply means for supplying a voltage to the welding electrode to create a periodic series of direct current pulses which are supplied across the arc gap to weld the work pieces, said control device characterized by an impulsar means for providing a control signal to the power supply means said control signal controlling the pulse width of the current pulses applied at the arc gap; and a programmable pulse width control means for automatically adjusting the impulsar control signal to provide a preselected adjustment to the pulse width of the current pulses without otherwise substantially affecting the form of the current pulses.

2. A control device according to claim 1 wherein the impulsar means comprises a comparator means having an input terminal and an output terminal, said output terminal providing the control signal to the power supply means for controlling the pulse width of the current pulses supplied across the arc gap, and said input terminal receiving an electrical signal from the programmable pulse width control means which controls the signal gen- erated at the output terminal of the comparator.

3. A control device according to claim 2 wherein the programmable pulse width control means comprises a time delay normally closed switch means for providing a first electrical signal to the input terminal of the comparator means which causes the impulsar means to generate a first pulse width control signal; and a variable resistance device electrically connected in parallel with the switch GB2144595A 5 means for providing a different second electrical signal to the input terminal of the comparator means which causes the impulsar means to generate a different second pulse width control signal, said second signal effective only when the normally closed switch means opens to allow the variable resistance device to control the operation of the comparator means after a fixed time period during which the normally closed switch means is controlling the operation of the comparator means.

4. A control device for the power supply of a pulsed DC arc welding system substantially as described herein and with reference to the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

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