Method and device for controlling a short circuiting type welding system

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METHOD AND DEVICE FOR CONTROLLING

A SHORT CIRCUITING TYPE WELDING SYSTEM DISCLOSURE

The invention relates to the art of welding with an electric arc and more particularly to an improved method and device for controlling a short circuiting type weld- ing system to drastically reduce spatter which normally ~5 accompanies this type of welding process. BACKGROUND OF INVENTION

In consumable electrode arc welding, one of the rec- ` ognized modes of opera~ion is the short circuiting mode, - wherein a power supply is connected across the consumable

LO electrode, or welding wire, and the workpiece onto which a ; weld bead is to be deposited. As an arc is created, the end of the electrode melts to form a globular mass of molten -~ metal hanging on the electrode and extending toward the work- piece. When this mass of molten material becomes large ~L5 enough, it bridges the g~p between the electrode and the work- piec~ ~o cau~e a short circuit. At that time, the voltage b~- tween the electrode and the workpiece drops drastically there- ;~ by causing the power supply to drastically increase the current through the short circuit. Such high current flow is sustained !O and i8 actually increased with time through the molten mass as the power supply inductance is overcome. Since this short cir- . cuit current continues to flow, an electric pinch necks down a portion of the molten mass adjacent the end of the welding wire, ~ The force causing the molten welding wire to neck down is pro- .>5 porti~nal to the square of the current flowing thrQugh the molten ~` metal at the end of the welding wire. Thi~ electric pinch effect is explained by the Northrup equation: , . .

G (dynes / cm2 ) = I2 ~R2 _ r2 100 7r R ` I is current density, r is the distance from the center of the welding wire and R i~ the diameter of the neck. During the

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short circuit, there is a need for a relatively high current flow, which flow naturally results when the short circuit occurs.

This high current ~low i6 desirable ~ cause the neck portion of the molten mass ~o form rapidly into a very small .~rea or neck which ultimately explodes like an electric Euse ~o separate the molten ball from the wire and allow it to be drawn into the weld pool by surface tension. This explosion of the neck causes spatter from the welding process. Spatter is deleterious to the overall efficiency o~ the welding opera- tion and requires a substantial amount of cleaning adjacent the weld bead after the welding operation is concluded. Since the current flow through the wire or rod to the workpiece when the neck or fuse explodes is ~ite high, there is a tre- mendous amount of energy released by the neck explosion add- ing to the propelled distance and amount of spatter. As can be seen, there is contradiction between the short circuit current which should be high to efficiently decrease the neck size by an electric pinch, but should be low to re- duce the energy of the fuse explosion and, correspondingly, reduce the spatter and distance over which the spatter particles will be propelled. A considerable amount of effort has been devoted to limit- ing spatter when the arc is rees~ablished by the explosion at the neck or fuse of the metal ball hanging from the welding wire and engaging the workpiece or weld pool. At first, it was suggested to reduce the diameter of the welding wire, i.e. use a 1/32 wire; however, this approach to reducing spatter caused all oE the inefficiencies normally associated with using small welding wire. For instance, it was diffi- cult to lay large amounts o~ weld bead and the wire sometimes stubbed or entered the weld pool without melLing. As the wire diameter increased to overcome these problems, spatter was subs~antially increased. Faced with this dilemma, it was suggested that a high frequency power supply be used as taught in United States Letters Patent 4,544,826, wherein a hi~h frequency inverter i`s ~u~ned

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off during a short circuiting condition or upon detection of a premonition of rearcing, i.e. blowing of the fuse.

To prevent circulating currents when a high frequency power supply is turned off just before a fuse explosion, this i United States Letters Patent illustrates a switch, SWD, which is opened to place a resistor in the output tank circult of the solid state inverter for rapid attenuation of the cir- culating currents. This system is not applicable for all power supplies and is predicated upon a complex logic control system which actually forms the shape of the current curve from the time a short is detected to the time when the arc is reestablished after explosion of the neck or fuse. Re- duction of current at the time of a short is by tuned at- tenuation, which phenomenon causes a time constant curve be- tween time tl and t2. At the detection of a neck or fuse which is about tP blow, this same attenuation concept is em- ployed. This feature is shown between the times t5 and t6 of this prior patent. The preselected wave shape, as shown in this patent, is heavily reliant upon the aforementioned o attenuation of the output tank circuit of a solid state in- verter which is a serious limitation especially in reducing ; the current flow through the neck itself at the moment of explosion. Such a preselected current shaping is applicable, if at all, to a high frequency solid state inverter power ` 5 supply which can be internally turned off without substantial output inductance. With a substantial inductive reactance in the output circuit attenuation by the resistor in parallel with switch SWD would be difficult and not always guaranteed.

Since direct current welding systel,ls have output in~uctance o this attenuation concept for lowering spatter has serious practical dr.lwb~cks.

Another patent showing a system for creatlng a repetition of a current cycle originally triggered by a short circuit detec~ion i~ Vnited States Letters Patent No. 4,546,234. A~ain, ~5 the current wave form is somewhat fixed. After a preselected

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time delay, current is applied across the shorted molten metal glo~ular or ball to facilitate metal trans~er. A constant current is maintained until necking is predicted, at which time ~he current drops rapidly to a low level and then im~ediately shifts up to a second high level. This system causes preselected current wave forms which are com- plex and generally usable, i~ at all, only wi~h a high frequency solid state inverter type power supply. As can be seen, there is a definite need Eor a rel- o atively simplified system for reducing weld spatter by exerting a limited amoun~ of actual control over weld current flow so that the current flow can assume n~tur~l operating char- acteristics over most of the cycle between the short and the fuse explosion. In addition, there i5 a substantial de- mand for a spatter reducing circuitry to be used with both transformer fed and solid state inverter type power supplies which do not depend upon output attenuation of low inductance circuits nor upon several distinct current level limitations. THE PRESENT INVENTION

The present invention overcomes the disadvantages of prior attempts to reduce spatter in a welding system of the type employing the short circuit, transfer mode which system requires a minimum of logic circuits and is applicable for a wide variety of power supplies with and without a substantial !5 amount of inductance in the outpu~ circuit. In accordance with the broadest aspect of the present invention, a main weld current is on whenever the arc voltage exceeds a preselected threshold level, such as 10 volts. When the arc voltage drops below this preselected value, the main welding current is turned off for a selected time and is then turned on again. During this period or cycle a low background current is maintained so the molten metal m~ss or ball hang- .~ ing from the welding wire and in contact with the workpiece either ` breakY away or ~eve:Lops into a metal transfer short under the in1uence of 3S only the low back~round current and not the main currenc. Consequently, ~ .

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L-7434 1~7340~3 any molten metal ball which does not actually ~ransf~r ~o the weld ~ead or weld pool on the workpiece will be subj~ct to only low background currcnt when it separates from the weld pool. Such low current does not tend to propel the ball or portions there- cf from the wire away from the weld pool. These bulbs or balls of molten metal may only momentarily engage the molten ~eld pool or weld bead thus causing a phenomenon referred to as an "incipient short". An incipient short is not a metal transfer short, but is the engagement of the ball with the weld pool succeeded by a bouncing away of the ball from the molten pool by electric pinch forces to again establish ~n ~rc lithout any metal transfer. . The momentary short would occur well within the selected time of low current.

In practice, this time is 1.0 ms. As incipient shorts are created, the main weld current is tu-ned off reducing pinch forces at the contact point betwe~n the molten metal ball and

the weld pool. By maintaining the welding action with a low

level current flow there is sufficient current to maintain

the melting action but generally insufficient current to create

high level Pinch forces tending to re-e!3tablish the arc and allow arc J~t

forces to propel the molten b~ rom the weld pool. Con-

sequently, the short converts into a metal transfer short and

progresses without forming an incipient short. The term

"workpiece" is used herein to indicate either the metal onto

which the weld bead is being laid or deposited, the bead it-

self or the weld pool. All of these are electrically grounded

to the power supply.

If the short circuit is only a momentary short associated with an incipient short condition, the main current will come on when the short is broken and the arc reestablished. If the incipient short condition is converted to a transfer short during the time of forced low current, which is the general result, a short condition rQmains after the initial preselected time of low current. The arc voltage remains at a low level, but the main current is turned on. When this occurs, the

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current through the mol~e~ th ~etween the weldil-g wire and the workpiece increases rapidly because of the continuing short circuit between the welding wire and the workpiece.

As main current continues to flow through the welding wire, the wire continues to heat and its resistivity increases.

The ball bridging the gap starts to neck down by the elec- tric pinch effect at a rate proporti.onal to the square of the weld current. As the resistivity increases and as the neck decreases in diameter, the voltage commences to increase. ) Since the necking action is general].y self-sustaining after it star~s, commencement of the neck signals an impending fuse blow preceded by an increased voltage and a change in sign o~ the current slope. A time derivative of the operat- ing voltage or o the weld current with the main current j applied across the shorted metal indicates when the metal is electrically pinched by the main current flow. A rapid ; rise in the voltage or change of slope of the weld current indicates an imminent blow of the Euse or neck. When an imminent fuse explosion is indlcated by the voltage or current ) (lerivative, the main current is again immediately turned off, ~just before the loss of metal contact. With the main current off, the low background current causes the fuse explosion at :he neck. This iB a low energy explosion without sacrificing efficient separation. During separation of the bridging metal, the plasma or arc is restored by the background current and the arc voltage increases. When the arc voltage exceeds the control value, the main current is again turned on awaiting ; the next short.

To prevent interruption of the main current immediately

O after a short is detected, the circuit measuring the time derivative of the voltage or current is deactivated for a short time immediately following the first time delay after a short circuit detection. This deactivation prevents a de- rivative mcasurement or detection when the main current is turned on af~er the time delay. If this derivative detection feature were to function immediately after the delay, the

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L-7434 ~27340~3

main current would again be turned off thereby preventing the formation of a high shorting current and development of a strong electric pinch. During this transition to the main current there are variations in the voltage which could be erroneously identified as a necking condition by the circuit measuring the time derivati~e of the arc voltage or time derivative of the weld current. The invention as defined above allows the main current to switch off so that a low level background current is maintained for a preselected maximum time immediately after a short circuit detection. ~pon detection of an imminent fuse breakage at the end of a metal transfer cycle the same low level background current is applied for a selected time delay. During these time delays, should the voltage increase above the selected value, thus indicating an arc condition, the main current i5 immediately applied and the welding process continues as if ~here were no control over the weld current. Act~al metal transfer in a short circuiting mode of operation follows a short circuit condition caused by the molten ball touching the weld pool. After the short circuit, the flow of current drastically increases through the shorted ball until the current ~low causes a necking of the molten nletal ball at the end of the weld wire. When that happens, the resistance through the molten metal ball increases causing a corresponding decrease in the applied main current. Im- mediately therea~ter, since the main current is still flowing, the neck size decreases until it explodes. By employing the present invention, the current actually flowing through the neck or fuse when it explodes is reduced to a level drastically below the normal current level experienced by using the main welding current during actual fuse explosion. A reduction in current flow at the time of rupture drastically reduces spatter by decreasin~ the energy of the fuse explosion.

By using a derivative of voltage or current, the neck can be accurately detected so the current can be reduced before the fuse blows.

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In accordance with the present invention, the first time delay is interrupted and the main current is immediately applied whenever an arc is reestablished after a short. This occurs during an incipient shor~. The main current then in- creases to the plasma level awaiting an actual transfer short.

Consequently, the metal ball at the end or the weldinK wire is not subjected to high propelling forces. The molten balls do not tend to grow by repeated momentary shorts separated by arc jets. In this manner, the present invention recognizes and overcomes the problems caused by incipient shorts by allowing actual metal transfer to ~he weld pool during normal transfer shorts, but also providing only a low current condition during the formation of incipient shorts. Thus, hi~h current flow does not cause drastic high activity breaking the molten ball rom the weld pool as occurring in a system with no reduction in ; current at the start of a short. However, the present inven- tion overrides the low or background current control feature whenever there is an arc, immediately following a short circuit condition, such as would occur with an incipient short, if one should appear. By using the invention, incipient shorts are generally avoided. Further, the weld current is not forced through a preselected combination of current levels based purely on time cycles, as sometimes employed in the prior art for reason other than control of incipient shorting. In summary, the concept of incipient shorting as a mechanical component of spatter and utilizing the present in- vention to eliminate nearly all incipient shorts is a sub- stantial improvement in spatter control. In accordance with ; the invention, every short is assumed to be an incipient short .md the welding current is reduced to a background level so the agitation of the weld pool and the forces of the arc are minimized. This greatly increases the probability that even an incipient cont:act between the molten ball and the mol~en weld pool will be converted into a normal, desirable transfer ~ 35 short by attract:Lon of the weld pool surface. Should the

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incpient contact or short be one which is abnormally violent and cannot, or does not, convert into a transfer short, the fact that the ball contacts and separates from the weld pool a~ the low background current level all but eliminates spatter normally generated by this nontransfer action referred to here- in as an incipient short. - By recognizing and correcting the disadvantages of the incipient shorts and by also correcting the problems of high energy fuse explosion with only a shift between main current ~10 and background current, the problem of weld spatter has been essentially eliminated. By utilizing the concept of the present invention, spatter can be reduced drastically over normal short circuiting type welding without the complexity and current wave shaping of other spatter reduction techniques. Use of the present invention reduces the violent oscillations of the molten weld pool which further reduces tendency of spatter. Further, a more quiescent weld pool allows the surface tension to better bridge the root gap between two pieces being welded. Also, the pool will better conform to the pieces being welded with the reduced `~ agitation caused by high energy fuse explosions and incipient shorts. The weld pool, when less agitated, allows use of larger electrodes in out of position welding. Short arc ` 25 lengths can be maintained without stubbing. Consequently, the invention allows use of larger electrodes, high deposi- tion rates, high currents and, also, less contamination by entrapment of shielding gases. The primary object of the present invention is the provision of a method and device for controlling current between a main welding current and a background welding current in a manner to drastically reduce spatter in a short circuiting type of welding system.

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Another obje~t o~ the present invention is the pro- vision of a method and device as defined above, which method and device does not control the welcling current in a series of preselected levels having imprecise correlation with the actual demand for reduced spatter.

Another object of the present invention is the pro- vision of a method and device, as defined above, which ; method and device prevents incipient shorts from causing large partlcle spraying from the welding operation. ~ 10 Yet another objec~ of the present invention is the pro- ; vision of a method and device, as defined above, which method and device allows use of a large range of wire ~iameters.

Consequently, larger welding wire can be used without dele- terious spatter. The terms "wire" and "electrode" are ; 15 used somewhat interchangeably to mean the elongated con- sumable metal element feed into the welding area to be transferred during the welding process. The invention may be used with manual stick electrodes. Another object of the present invention is the pro- vision of a method and device, as defined above, which methodand device can be used on power supplies of ~he inverter type, motor generator sets, and conventional transformer type.

In the past, spatter control systems were primarily limited to high frequency solid state inverters because these power supplies exhibited substantially reduced inductive reactance in the output circuit. The spatter control systems of the past required low output inductive reactance.

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~ Still a further object of the present invention is -~ the provision of a method and device, as defined above, which method and device employs a Darlington power trans- sistor. This type of transistor ha~; a rapid turn-off time and a high voltage and current rating. Still a further object of the present invention is the provision of a method and device which allows high current during the transfer short to initla~e formation of a neck between the rod and the ball of material being transferred and then, abrputly, reduces the current just before the neck acts as a fuse and explodes. This reduced ~i current at the time of "blow" or explosion produces a low energy during the actual restriking of the arc by rupture ~; of the neck. The high short circuit current at the initial 15 stage of forming the neck causes a substantial electric ~ pinch. This advantage is confirmed by the known electrical ; princlple that the electric pineh is A factor of the square of the currcnt flowing through the molten metal ball between the welding wire and the workpiece. Should current be turned off or reduced before the neck is well formed and nearing explosion, the desired necking phenomenon would~be ad- ` ~ versely affected. The neck is started with a high current and then the squeezing action at the neck continues with the low current applied directly a mere instant before the fuse ruptures.

Yet another object of the present invention is theprovision of a method and device for reducing spatter in a short circuiting type of welding operation which can

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function with a variety of shielding gases and with differ- ent electrode types and sizes. Yet another object of the present invention i8 the provis~on of a spatter reduction system which reduces both i neck exposing spatter at the end of a metal transfer pulse cmd also incipient short sp~tter at the start of the trans- ler pul~e. These and other objects and advantages will become apparent from the following description taken together ) with the accompanying drawings described below. BRIEF DESCRIPTION OF DRAWINGS

FIGURE 1 is ~ schematic diagram of a short circuiting type welding system employing the preferred embodiment of the present invention;

FIGURES 2A, 2B and 2C are schematic views illustrating ~ progression of the molten metal bath formed on the end of : the welding wire during the short circuit condition and at ~ the fuse break; : FIGURE 3 is a graph of the welding current during a

O somewhat standard metal transfer, as shown in FIGURES 2A, 2B and 2C; FIGURE 4 shows volta~e and current graphs ~etailing operating characterlstics of the presen~ invention; . FIGU~E 5 is a wiring diagram of an alternate system ~5 for predicing the occurrence of the fuse action or neck . explosion as illustrated in FIGURE 2C; :~ FIGURES 6A, 6B, 6C and 6D are schematic views illus- trating an "incipient short", as this term is employed in the present application and certain physical and electrical ~O characteristics of this surprising phenomenon; FIGURE 7 is a graph showing welding current exhibiting certain electrical characteristics associated with the incipient shorts phenomenon, shown schematically in FIGURES 6A-6D;

~ L-7434 ~L2~3~0~3

FIGURE 8 is a current curve showing two unwanted incipient shorts followed by a desired metal transfer short with tll~ time abscissa expanded and interrupted and the current ordinate somewhat exagerated; FIGURE 9 is a schematic diagram of the preferred embodiment of the present invention used in explainin~ a characteristic of the present invent:ion illustra~ed in

FIGURE 10; FIGURE 10 is a graph showing a welding current wave form using one feature of the inven~ion for reducing current at the neck;

FIGURE 11 is a wave shape graph showing a metal transfer pulse employing all aspects o the present invention; FIGURE 12 illustrates the preferred embodiment of the present invention with components for protecting the power ; Darlington transistor rom high voltage and high circulat- ing current; FIGURE 13 is a wiring diagram of the actual circuit employed in practicing the preferred embodiment of ~he present invention which diagram is divided into two sheets labeled FIGURES 13A and 13B; and,

FIGURE 14 is a schematic diagram of the welding current, ; arc voltage for each state of the welding operation and con- trasting a standard short circuit welding system with a system using thP new spatter control system. PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawing wherein the showings are for the purpose of illustrating the preferred embodiment and not for limiting same, FIGURE 1 illustrates a welding system A constructed in accordance with the present invention. System A contains a spatter reduction or control circuit SC which will be desired later. Since spatter control circuit SC is capable of addition to a standard short circuit type welding uni~, system A of FIGURE 1 illustrates components common to a welcling operation whether or not inventive

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circuit SC i6 used. FIGURE 1 will initially be employed to explain general background and concepts used in short circuit type welding. The commonly used components of system A include a conventional transformer type power j supply 10 having a constant voltage output wi~h inductive reactsnce internal to the power suplply that prevents rapid discontinuation of welding current using normal ~olid state switching procedures. Output leads 12, 14 are connected in series across gas no~zle 20, welding wire 22 and workpiece 0 30. These components are arranged in a series circuit schematically illustrated in FIGURE l; however, a mechanism for feeding the wire 22 toward the weld pool on workpiece 10 for the purpose of laying a bead along the workpiece would be an integral component of system A. Such normal wire feeding mechanism does not form a part of the present inven- tion. The present invention can be used in a stand~rt short circuiting mode of welding as so far explained with ~he aid of standard components illustra~ed in system A of FIGURE 1.

This system employs a conventional transformer type power '0 supply applying an arc voltaga across wire 22 and arc or plasma P. Such voltage causes a weld current to flow from wire 22 to workpiece 30. In accordance with general practice an appropriate shielding gas 40 is directed from nozzle 20 around wire 22 to r~duce oxidation and contamination of the metal being dep~sited in the weld pool on the workpiece 30.

Of course, either workpiece 30 or welding head lncluding nozzle 20 iq moved along a desired path ~o deposit a linear welding bead either on the surface of the workpiece or in a groove formed between two abutting workpieces. In a standard ~hort circuiting transfer mode of ~elding as so far discussed, the metal is deposited from the wire onto the workpiece by passage of a welding current through the wire and to the workpiece which alternates between a short circuit condition wherei.n the rod or wire touches the workpiece and an arc conditlon or plasma condition, wherein there is a gap

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between the wire and workpiece. This gap below the wir~ is spanned by an arc or plasma. The term "workpiece" refers to the weld pool already deposited on the surface being treated or the workpiece i~self, which definition is used herein for convenience. FIGURES 2A, 2B and 2C, schematically illustrate the short circuiting tran~fer phenomenon fc~r transferring metal melted at the end of wire 22 and deposited on the workpiece by the surface tension drawing the metal into the weld pool of molten metal containing the previously .0 deposited metal. This weld pool is dynamic in nature and retains its fluidity for a substantial time after nozzle 20 progressively moves along the workpiece. As is well known, the bottom of wire 22 is maintained spaced from the workpiece or molten weld pool previously deposited a distance b which

L5 iR substantially greater than the diame~er a of the wire. In practice, diameter 8 can vary substantially and still be used in a welding system using the present invention. In the past, small wire diameters, typically, 0.035 inches were used to re- duce spatter since such small wire was cradled in the weld pool and spatter, if any, was physically caught in the weld pool formed around the end of the wlre as it deposit-R metal lnto the weld pool. Since spacing b i8 greater than diameter a, a molten metal bulb or ball B, shown in FIGURE 2A, has a geometric spherical shape somewhat greater than a hemi- ~Z5 sphere. This ball shape facilitates separation and surface tension transfer from rod 22 to the molten weld pool or work piece 30. In summary, in the standard short circuit mode of welding, current passes ~hrough wire 22, and an arc is caused between the bottom of the wire and the weld pool or workpiece 30, whereby the heat of the arc or plasma causes the end of wire 22 to become molten. This molten material grows in size until it bridges the gap indicated as spacing _ by forming ball B and causes a short circuit. Since the circuit re- sistance drops clrastically when this short circuit occurs the weld current immedia~ely increases and the voltage

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immediately drops. The rapid current rise is shown a~ the left side of the metal transfer pulse MT, illustrated in

FIGURE 3 wherein the welding current over a normal short circuit me~al transfer is schematically illustrated. Weld- ing current IW has a normal arc or plasma current level Ip which is a minimum current level and is controlled primarily by the output resistance. Between arc or pLasma periods when plasma current Ip is flowing, there is a metal transfer short pulse MT where welding currenlt IW is increased and then ) decreased back to plasma current Ip. Pulse MT, which is stand- ard when a transfer short occurs, includes a leading, rapidly rising curved side 50, which increases to a maximum Erom the point S. A short condition is illu~trated in FIGURE 2A. As the current increases, ball B clings by surface tension to the molten weld pool and forms a dis~inct neck N in accordance with the electric pinch effect. The diameter of ~he neck N :is reduced by current induced forces in accordance with the

Northrup equation. This causes the resistance through ball

B to increase. Consequently, when the neck starts, the weld-

O ing current IW curve in FIGURE 3 reverses direction and starts downwàrd due to the increasing resistance as the diameter of neck N decreases rapidly. Since the necking force or elec~ric pinch effect varies as the square of the welding current, substantial forces are being exerted at the top of ball B to ~5 cause a rapid necking down. For that reason, as soon as the neck starts as sh~wn generally in FIGURE 2B, the neck diameter decreases rapidly at an accelerated rate. This results in a fuse action or neck explosion F, represented schematically in ~ FIGURE 2C. This explosion at point 54 in FIGURE 3 immediately restrikes a plasma or arc between wlre 22 and workpiece 30 so that the current IW first drops rapidly along portion 55 of ` pulse MT and then gradually along line 56 as the arc stabiliæes.

At top 52 of metal transfer short pulse MT the current derivatlve di/dt is reversed and becomes negative. As soon as the neck ~5 is formed, it immediately breaks as illustrated by steep line 57 between top 52 and explosion point 54. Time constant por- tion 56 of metal transfer pulse MT follo~s the initial rapid

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current drop along line 55 immediately after the explosion at point 54. As can be seen, the neck starts forming and is immediately blown away in short time TN in FIGURE 3, which is drawn to scale. In summary, ball B forms on the end of wire 22 and enlarges until it makes contact with the weld pool or work- piece 30 shown in FIGURE 2A. Surface tension then draws ball B from the end of rod 22 preparatory to the electric pinch effect causing ball B to neck down as shown a~ N in

FIGURE 2B. Immediately thereafter, the fuse breaks or blows, as shown in FIGURE 2C. As shown in FIGURE 3, at the fuse F, the current is nearly 300 amperes; therefore, tremendous energy is released when ball B separstes from wire 22. Thls causes spatter of molten metal, indica~ed as arrows SP in

FIGURE 2C. This spatter flies outwardly with a high momentum carrying spattered metal some distance away from the actual welding operation. To reduce spatter SP in the past, wire 22 was reduced in si~e 80 as to bury the arc in the weld pool, thus, permitting the weld pool to catch most of the spatter particles of molten metal and reduce the tendency to drive the spatter particles away from the weld pool. Also, complex circuits were sugge~ted to control the shape of metal transfer pulse MT during the metal transfer cycle. These prior arrangements tended to drive pulse MT in a preselected pulse shape. Of~en the pulses were repeated, irrespective of whether they were needed or not or whether the metal transfer actually occurred. The present invention overcomes these disadvantages by making only minor modifications in the conventional system so far discussed with respect to cer~ain components of system A in FIGVRE 1. The normal welding current curve, as shown in

FIGURE 3, is controlled by simple, easily accomplished struc- tural modifications of the normal short circuit system so ~ar described. Referring again to FIGURE 1 and to FIGURE 4, the components added in combination with the standard features of system A to aceompllsh the preferred embodiment of the invention are illus- trated in FIGURE 1 where spatter reduction circui~ SC includes '`

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resistor R connected in parallel wlth a Darling~on power transistor switch SW. This parallel circuit is connected in series with nozzle 20, wire 22 snd workpiece 30. To complete the preferred embodiment of the presen~ invention, spatter reduction circuit SC also imcludes solid state logic control system L for operating switch SW: The manner of operating the switch in relatlonship to parameter~ at the welding site substantially eliminates spatter SP experienced in standard short circuit welding. Control L commands ) Darlington switch SW ~o be either conductive or non-conductive. When non-conductive, resistor R is in series with the welding operation to create a low level welding current hereinafter referred to as the background current IB. In practice, re- sistor R is 1.0 Ohms which ~evelopes approximatey 300 volts across the resistor when switch SW is turned off while the ma~n current flow is near 300 amperes. Consequently, power supply 10 is never ac~ually disconnected or grounded to re- duce the current flow through the welding operation irrespectlve of the statu6 of switch SW. Merely switching between a dlrert ~0 connection through switch SW and curren~ flow through only resistor R causes welding current IW to either ~a) float, in accosdsnce with ~tandard pulse MT or (b) be drlven downward by opening switch SW with a logic control L. One aspect of the present invention is the use of a Darlington connested ~5 power transistor having a rating of several hundred amperes ` and several hundred volts. The Darlington connected transistor network SN is preferred because of capabilities of rapid switch- ing under heavy current conditions. This type switch is part of the present invention which must drop the welding current from a high level main current to a low level background current IB within less than aboutl20 microseconds when the switch is opened or in the non-con~ctive condition. An approprlate voltage sensos, schematically represented as device 60, and a current sensor 62 provide logic control L ;35 with ~nstantaneous weldin~ current IW and arc voltage E~RC :` ,::

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so that logic control L opens and closes switch SW to provide the weld current control as illustrated in FIGURE 4. The basic aspects of the present inventicn are set forth in graphic form in the upper graph of FIGURE 4, whi~h graph illustrates voltage fluc~uations. The lower graph iLlustrates welding current IW through the wire and workpiece as corre- lated on a time base with the upper voltage curve. FUSE SPATTER REDUCTION

To appreclate one aspec~ of the present invention to reduce the fuse energy to reduce spatt~r SP,attention is directed to the rlght hand end of the metal transfer pulse ~T (NEW) in FIGURE 4. This pulse has a front curved shape, side or line 100 correlating directly with portion 102 of the arc voltage which will be described in connection with ~; another aspect of the lnvention used at the left end of pulse MT. As current IW increases to the right, the slope de- creases eventually, which change in current at the right end of line 100 causes a slight increase in the vol~age slopè at portion 104. This increase in voltage corresponds with ~0 the upper, generally flat portion of transfer pulse MT (NEW). As the current approaches a zero slope and when the voltage experiences an increased slope, the neck N is starting to form, as shown in FIGURE 2B. At this instance, in accord- ance with this aspect of the present invention, either by '5 de~ecting di/dt or dv/dt, switch SW is opened. This im- mediately shifts the current to the low background current level IB schematically illustrated as about 50 amperes in

FIGURE 4. The welding current stays at this low background level only so long as the arc voltage EA~C does not exceed ~30 a preselected value, such as 10 volts illustrated in the upper voltage curve. Then the neck breaks, the short cir- cuit is removed and the voltage rises along vertical line 106. When arc voltage EARC exceeds the selected l~w level (i.e. in practice 10 volts) as it moves along line 106, switch SW is closed and the welding current IW moves along

- 19 -

~L~7340~ L-7434

a curve 110, which curve is a time constant curve raising to the plasma or arc welding current value Ip, as shown at portion 112. This plasma current is illustrated 8S be- ing substantially over 200 amperes. At the same time, the voltage assumes its steady state condition at portion 10~, illustrated as somewhat over 20 volts. Pulse MT (NEW) causes metal transfer from the welding rod to the workpiece, i.e. the molten weld pool, as did the conventional metal pulse MT. The time Tp in FIGURE 4 is the metal transfer portion of the pulse. The first aspect ~; of the present invention, as explained above, occurs at ~` the end of pulse MT (NEW) and involves recognizing when the neck starts to anticipate a fuse blow. Then welding current IW is shifted to a low level substantially be- low the plasma level Ip by opening switch SW and again allowing flow of only background current IB by inter- posing resistor R, as shown in FIGURE 1. A low current, low energy fusP explosion occur~. At the explo3ion ~ the metal transfer pulse is concluded, an arc is established and the volta~e rises along line 106. This clo6es switch

SW. The main current is permitted to flow and it increases along time constant portion llO wh~ch starts from ~he low level IB, illustrated to be less than 50 amperes in FIGU~E 4.

The rapid increase in welding current along line 110 toward the plasma level 112 is a time constant curve. By using switch SW, current restoration does not swing upwardly above the normal plas- ma level Ip. Such unwanted current swing would cause control difficulties in res~riking the arc ~ince the applied current would be substantially higher than needed for actual plasma 30 ignition and maintenance. In accordance with this first aspect of the present invention, the circuit SC is designated to drop the current to a low level upon recognizing an lmminent fuse

- 20 -

~ 73 4~8 L-7434

explosion. Further, ~he welding current IW is held to the low backgrount level IB or a time T3 which, in practice, ~s 1.0 ms. However, circuit SC has a control parameter so : ~hat ~henever voltage VARc exceeds a preselected level, 10 volts in this instance, switch SW is closed. Since the arc ~oltage rises when neck N breaks, the rise in voltage always occurs prior to the time T3. Consequently, the holding action of circuit SC for the! time delay T3 is only a fail-safe feature assuring that current I~ will ultimately 0 be released for movement to plasma ~evel Ip at point or posi- tion 112 after being shifted to the background level IB along generally vertical line 114 just before the neck blows. The prior art does not teach the concept of recognizing the necking, shifting to a low level background current value and then shiEting back to the plasma level itself without drastically exceeding the plasma level Iy upon reestablish- ing the arc or plasma. Switching to a low level value I~ is accompllahed along the vertical line 114, which i~ rapld enough to assure a reduction in current by a ratio of nearly `0 6:1. The prior art generally shifts the current along a con- trolled time constant curve and then releases the current at some ~elected time thereafter. Precise control of the weld- ing curren~ is accomplished by the present invention by using

Darlington switch SW which provides immediate cutoff of IW ~'5 current, wi~hout a current storage in output inductors or ~: power leads which would result in the return curve exceeding subs~antially the plasma level 112. ~eferring now to FIGURE 5, one circuit for detecting top 52 of a metal ~ranser pulse i3 illustrated as a dl/dt ~ detector, whereby as the current shifts from a positi~e slope ~0 8 zero slope, an appropriate output from line 120 is processed by logic control L and is used to open switch SW.

This circui~ takes the first derivative of the welding cur- rent IW by differentiator 122. The derivative in line 124 is amplified by amplifier 126 and dlrected to input 128 of

~ 7~ ~8 L-7434

compArator 130. The output pro~ides a logic signal indi- cating when K di/dt is at a preselected level. Other ar- ra~gements could be provided for detecting the top 52 of pul~e MT. The time between this top 52 and fuse break point i S4 is indicated by the spacing TN. As can be ~een, by the scale of the graph ln FIGURE 3, the time TN ~g quite small.

Consequently, a~ soon as a detect signal i8 generated in line 120, switch SW i~ lmmedlately opened. This plunges current IW down to the background current level IB along ) line 114. Current IB is substantially lower than the plasma current Ip, as indicated in FIGURE 4. INCIPIENT SHORT SPATTER REDUCTION

In accordance with another aspect of the present in- vention, spatter control SC i~ provided with a ~eature that reduces spatter caused by the phenomenon of incipient shorts.

In accordance with this feature, as shown graphically in

FIGURE 4 the arc or plasma current Ip i~ immedi~tely dropped to less than 50 amperes (i.e. the background level IB) when any type of short occurs. The ~oltage drops rapldly 3 along line 116, current I~ drops alon~ line 118 by logic control L opening switch SW. The welding current IW is shifte~ to the background current IB when the ~hort first occurs. Background current IB is controlled primarily by the value of resistor R and is held Eor a preselected time or cycle Tl at I~. Af~er this cycle or time Tl exp~res, switch SW ls closed and the voltage at low level 120 com- mences to rise ~o steady state level 102, as previously discussed. Thiq drop of current a3 soon as a short i8 de- tected and holdlng the low level for a selected time ellm- inates inc~pient s~atter. To appreciate how the spatter control reduces spatter ;Erom incipient shorts certaln technical features of the "incipient shor~" phenomenon are diagrammed in FIGURES 6A, 6B, 6C, 6D, 7 and 8. Referring now to FIGURE 6A, when ball

B i~ formed on the end of wire 22, the ball, in some instances,

. - 22 -

.

- ~73408 L^7434

does not immediately attach to the molten weld pool by surface ~ension. The ball may ~ust touch the weld pool as illustrated in FIGURE 2B. Thls is explalned by recognizing that the weld pool ~8 molten metal sub~ected to hlgh arc curren~s a~ well as gra~ity and magnetomotive forces. It resembles a wavy body of water. In many instances, the weld pool engages the ball B, as shown in FIGURE 6B, for a short time by a motten metal front striking ball B. The electrical and mechanical forces caused by wave fronts in the weld pool engaging ball B sometimes drives the ball away from the undulating weld pool.

This is shown in FIGURE 6C. Consequently, there is a short when ball B touches ths weld pool WP. This short can be immediately opened by mechanical forces, as shown in FIGURE 6C. As ~his process continues and ball B i5 not captured by the weld pool, the ball contlnues to grow by the continuing melting action at the end of wire 22. Ultimately, ball B can be driven away f~om the weld pool as a substan~ial mass of molten metal. Thi6 is a form of spatter. Due to the mass of ` the metal in the ball, the mechanical spat~er caused by these 0 :incipient shorts results in lumps of metal randomly deposited immediately adjacent the weld area. These spatter particles present substantial problems in cleaning and cause waste of weld material and welding energy. FIGURE 7 lllustrates the current and voltage fluctuation during an incipient short when ball ~ touches weld pool WP as shown in FIGURE 6B.

As the ball touches but does not transfer, it swings away as illustrated in FIGU~E 6C. The current through ball B increa~es as the ball touches the weld pool. This increased current cau~es forces as the arc is reestablished, in FIGURE 0 6C, tending to drive ball B away from the weld pool. An in- cipient short causes the arc voltage to shift downwardly as the current increase~ between the weld pool and the touching - ball. This is shown in the lower curve of FIGURE 7. As soon as there ls a break caused by mecha.lical forces between the ~S ball and the weld pool, the voltage moves vertically upwardly

- 23 - .

~73~8 L-7434

along line 130 to the plasma arc voltage level. Since there is no large area contact for surface tension to draw molten ball B into the weld pool in an incipient shor~ condition, the inciplent short c~Uses ball 8 to grow and causes high mechanical forces to exert momentum to the ball B tending to throw the ball away from the weld pool. The inciplent short phenomemon is schematically il].ustrated in FIGURE 8 wherein the weld current IW experiences a series of incipient shorts before an active transfer short. In many instances, the molten ball remains molten after one or more incipient shorts and then goes lnto a standard transfer without caus- ln~ l~rge ~altlclc ~I)a~Ler. Ilowever, often thi~ does not `~ O~`~UI'. 'IllC IllCll~L(!lll~ U1101 t.~ I,,'ll~lH~' bill] 1~ Lo y,row u~ be propelled outwardly by tlle high mechanical forces during the short circuit ~ondition. If the effect of incipient shorts, illustrated as small pulses 140, 142, in FIGURE 8, is ignored, ~ large particle spatter will occur. This type of weld spatter - presents more cleaning problems than spatter caused by break- age of fuse F in FIGURE 2C. By using the present invention to drop the welding current to the background level IB as soon as a short is detected by a decreased voltage level, lncipient shorts as shown schematlcally in FIGURES 6A, 6B, ` 6C, 6D, 7 and 8 do not occur. The low level current IB per- mits the metal contact area to increase and thu~ develop into a transfer short. There is no high current to force an inc~pien~ short. ~ he "incipient short" graph portion of F~GURE 4 illus- trates what happens should an incipient short occur a~ the result of some transient mechanical agitation, during time T when the low level current IB is applied. Referring again to FI~URE 4, the cycle or time Tl i9 1.0 ms, which time is substantially greater than incipient short duration de- termined by the width of pulses 140, 142 of the prior art as shown in FIGURE 8. Assuming an incipient short occurs in use of the present invention, as soon as the ~hort occurs, as shown in FIGURE 6B, the overriding control recognizes the reduction of the arc voltage to a value below the preselected.

- ;'4 -

~2734(~8 L-7434

trip value. Thus, the weld current is shifted downwardly by opening switch SW. In accordance with the present invention, the current flowing during any short is reduced thus reducing mechanical forces known to cause an incipient short condition.

The current IB is drastically below the main current level, thus facilitating large area contact and transfer of the ball to the weld pool. At point 150, shown in FIGURE 4l, should mechanical agita- tion move ball B away from the weld pool, as shown in FIGURE 6C, the arc will be reestablished at current level IB. This mini- mizes the energy evolved when this small fuse explodes to re- duce associated spatter at this small fuse action. The arc voltage rapidly shifts upward along curve 152. As soon as the arc voltage exceeded the preselected value, which is an override, trip condition valid at all times in control SC, switch SW would be closed. The weld current IW would then rise from the background current level IB to the plasma current level Ip. Power supply 10 is a constant voltage machine set at a plasma voltage of approximately 20-30 volta~e and it feeds a short in the general range of 1-6 volts. When using this type machine, applying a main current by closing switch SW allows the welding current to reach the normal unimpeded current level schemat~cally illustrated as the increasing line 100 in FIGURE ;~ 25 4. When switch SW is opened, resistor R controls the welding current at the low, background level IB. As a recapitulation in the present inventlon, the main current is the normal un- impeded current and the background current is the current con- ; trolled by the reslstor. GENERAL DESCRIPTION

Referring now to FIGURES 9 and 10, ~n advantage of em- ploying the present invention is illustrated. In accordance with the invention, during the meltal transfer, the current when switch SW is cloæed, rises, as shown by pulse MT (NEW), from a low level background current IB to a detected top por tion 52a, at which time switch SW is opened and ~he current plunges along portion 180. This action is affected by the time constant including inductive reactance XL of choke 190 which is usually inside power supply 10. This inductance

- 25 -

': ` ,' '.-.~ ,""' . . ' ' ~ ' ,

~-7434

J 273408

is also created by the length of corductors 12, 14 extend- ing from the power supply to nozzle 20 and workpiece 30.

The time to plunge ~ro~ top 52a to the background current

IB, represented in ~he disclosed formula as Io~ is relatlvely short. .Tku~, the current at the time of the fuse break will depend upon the lead time of switch turn off before the fuse break. Lead times have been selected to cause the fuse break at approximately 10% of the height at the top 52a when it is 200 amperes. This current reduction sfter reaching top 52a, combined with the initial drop from plasma current Ip to back- ground current IB, results in a stlbstantiaL reduction in the separa~ion energy when fu~e F explodes or blows in response to further current flow through welding wire 22. Reduction of energy is thus accomplished by two phenomena. One is starting the transfer pulse at a low background level, i.e. in practice less than 50 amperes. The second is usin~ a Darling~on transistor switch to immediately open ~he switch and drive current to a level controlled by resistor R. A power supply or welding system employing the present ) invention has three distinct sta~es determining the welding current. At first, the current is the plasma level during the arcing condition. Second~y, the current i~ depressed to a background level in response to a short circuit. Thirdly, ~ at the end of a short circuit formed by metal transfer, the current is again depressed. During the transfer of metal, the current is con~rolled by the electrical parameters o the circuit. A schematic illustration of these various welding current stages during a standard metal transfer pulse is found in FIGURE ll. This view corresponds generally to the pulse ~ MT (NEW) in the lower portlon of FIGURE 4. In spatter reduc- tion control circuit SC, whenever the arc voltage exceeds the preselected value, in thi~ case, 10 volts, switch SW is closed to direct main current through the switch to the welding opera- tion. The main switch is closed during the pulse MT (NEW); however,, background voltage VB in portion 120 of FIGURE 4

. 26 -

.: .

L-7434 ~ ~73~08

shifts upwardly to the generally horizontal portion 102. DETECT INHIBITOR AND TIME DELAY

FIGURE 11 illustra~es a time delay T2 which is, in practice, 100 microseconds and occur~ immediately after cycle Tl. When this delay cycle is employed in the pre- ferred embodimen~ of the present invention, it inhibits the circuit used to detect when the fuse is about to blow which , i8 at a posltion near top 52a of curve 100. Such a circuit is illustrated in FIGURE 5 to detect the change in sign of di/dt. FIGURE 13 disclo6es a circult which measures and de- tects a preselected slope or derivative o the arc voltage, i.e. dv/dt. These detector circuits are inhiblted until time delay T2 expires. The reason for this feature is appreciated when considering portlon 122 of the arc voltage ~j immediately upon closing switch SW as shown in FIGURE 4. The current causes a voltage rlse that, ln ~urn, produces a value for dv/dt which would erroneously trigger a dvtdt ~; circuit because of the increase in current. This would open switch SW erroneously. For that reason, a slight delay T2 is pro~ided after exp~ration of cycle Tl and closing of switch SW. At the time of the transfer fuse and the drop of current along line 114 before the fuse explodes, time ~3 is set. Time delay T3 follows the detection signal 200 caused by a detec^

S t-on of the point on curve 100 ~ust before fuse explosion or blow and has a maximum value. Time delay T3 is shown in

FIGURE 11; however, it ~s seldom, if ever used. When switch

SW is opened in response to a detect si~al 200, the fuse blows with low current flow. -An arc condition then forces the voltage along line 106 of FIGURE 4 to above the 10 volt limit, an action that closes switch SW and causes the current to rise along line 110 to portion 112, which is the plasma current level. Immediate increase in voltage by interru~ting the short reconditions the system of ~he present in~ention to seek the next metal transfer cycle and to repeat control at

- 27 -

~273408 L-7434

the forward and rear ends of the weld$ng current pulse. In the intermediate portion a floating condition exists along lines 100, 102, a~ explained in connection with FIGURE 4.

Should the arc not cause a voltage o-~erride to close the ' 5 switch, the switch will be closed after time T3. CURRENT CONTROI.

Control of the background curre!nt occurs when a control signal 210 opens switch SW as illust:rated schematically in

FIGURE 12. That event places resistor R ~1.0 Ohms) directly into the welding circuit and remove~ current flow through switch SW. For the purpose of explaining certain a~pects of the invention, it is assumed that the plasma current Ip, as shown in FIGURE 4, is approximately 300 amperes. When the switch is opened in response to the plunge in the arc 15 voltage by a short condition, the 300 ampere current flow- ing when the switch i9 closed, develops 300 volts across resistor R. This high voltage is applied directly acro~s

Darlington translstor switch SW. To reduce the voltage acro s switch SW during turn off, capacitor 192, having 8 value of , 20 30 mf, i8 connected in parallel with ~he resi~tor R. Con- sequently, resistor R immPdiately charge~ capa~itor 192 toward the 300 volts of the resistor. In this example, it would re- quire about 30 microseconds for capacitor 192 to charge to 300 volts. The actual fall time of the Darling~on collector current ~s le~s than 5 microseconds; therefore~ ch~rging of capacitor 192 protects Darlington transistor against exces~ive power switching dissapation. A snubber or diode 194 pre- vents di~charge of capacitor 192 through the swi~ch SW, should t.he switch close when capacitor 192 is fully charged. Re- sistor R has at least two distinct functions in accordancewith the present invention. The resistor establi hes the magnitude of the background current IB. If the voltage from power supply 10 i8 20 volts D.C. as ~hown in FIGURE 4, back- ground current is 20 amperes calculated by dividing 20 volts

- 28 -

,

. - . - . : , - ~ ,- ~, : ' ,

L-7434 ~734~)8

by 1.0 Ohm. As a second function, resistor R protects the

Darlington transistor switch SW against over voltage. A basic advantage o reslstor R is that with resistor R in parallel with the Darlington switch SW there is no need for a second source of current to produce the background aurrent.

Further, there is no need for complicated circuitry attempting to control current based on sensed conditions or parameter~.

Merely placing resistor R in parallel with Darlington switch

SW produces the main current, when the switch is closed, and l the backgr~und current IB when the switch is opened. This is the unique concept for providing current levels which are em- ployed in a unique ~ystem conveniently controlling spatter both of the elec~rical type caused by blowing of fuse F and the m~chanical type caused by the incipient short phenomenon. i With capacitor 192 connected in parallel with resi6tor R, stored energy from the inductor 190 is disslpated when the switch is first opened and non-conductive. This tissipation of energy occurs rapidly snd is accomplished before the swi~ch

SW i9 conductive or turned on as indieated by the dashed line ) 126 in FIGURE 4. Since the energy has been dissipated, curve ;~ 110 can be gradually merged into the plasma current level along a curve de~ermined by the time constant of inductance or choke 190. Wi~hout the resistor and its capacitor for dissipating stored energy in the inductance, there would be no background current and thus no ionization formed by low current fuse action and, ~hus, cause an unstable arc start up or restrike. PREFERRED CIRCUIT

FIGURES 13A and 13B, taken together, describe the pre- ferred circuit for pract~cing the invention, as best ~hown in FIGU~ES 4 and 11. Each of the components is labeled; therefore, the circuitry is somewhat self-explanatory and only a br~ef description of the operation of the clrcuit is sufficient to understand how the circuit complies with the parameter~ of the present invention. The arc voltage

- 29 -

L-7434 1~73408

passes through a nolse reduction circuit 300 having an output 302 labeled ARC VOLTAGE. A referen~e voltage in line 304 is combined with output 302 by compara.or 310 to produce a logic level in output 312. The logic in thi~ line controls a mono- stable multivibrator or ball pulAe generator 320 which is tr~ggered when the logic in line 312 ~hifts ~o zero lndicat- lng that the ARC VOLTAGE has decrea~ed below th~ reference set in line 304. When this occurs ~ a 1.0 ms negative pulse appears in the ~ outpu~ 322 for controlling NAND gate 330.

A logic 0 ln line 322 forces th~ output 332 of NAND gate 330 to be at a logic 1. Thi8 opens switch SW by an appropriate circuit 340. The control of llne 322 corresponds generally with control signal line 210 of FIGURE 12. In this manner, the switch SW i8 turned off for 1.0 ms corresponding to i cycle Tl. Should the voltage increase above the preselected value determined by reference line 304, reset genera~or 350 having an input 352 connected wi~h line 312 shifts outp~t line 354 to a logic 0, thus, clearing ball pulse generator 320 and terminating the Tl time pulse. A logic 1 is applied to ~he input line 32~ of gate 330. At the same time, the reset logic in line 354 resets fuse time generator 360, if ; it ~s not already reset or cleared. A lo~ic 1 is thus ~pplied, as a pulee, t~ line 362. Thi~ logic combines with logic l on input 322 to produce a logi~ 0 in line 332. This j turns the Darlington transistor switch SW on whenever arc voltage exceeds the preselected value se~, in ths preferred - embodiment, st 10 volts. Thls voltage is selected to be greater than thP voltage drop acro~s wire 22 ~o assure enough available voltage drop Eor proper detection. ) As previously discussed with respect to FIGURE 5, fuse F can be predicted when nerking occurs as detected by dv/dt.

In FIGURE l3, including both FIGURES 13A and 13B, a derivatlve of the arc voltage is compared to a constant K for th~ pur- pose of predicting the fuse immediately upon establishment of neck N. This is illustrated schema~ically in the upper

- 30 -

lZ 73L~08~ L-7434

portion of FIGURE 4 at portion 104. To accomplish this comparison of dv/dt with K, the preferred circuit in FIGURE 13 uses a sample and hold concept, A samplP oscillator 380 produce~ a series of sample pulses in output 382. This ls applied to the sample and hold circuit 390 to sample the voltage at precisely spaced lnstances as the voltage is received on line 302. The output of sample and hold circuit 390 is line 392 which compares a held sample voltage with an instantaneous voltage in line 302. These two time spaced voltages (i.e. VN and VN-l) are compared by comparator 400 to produce a differential in voltage as compared to time.

This differential signal is amplified by amplifier 410 to create a differential of voltage with respect to time in line 412. The constant or K is selected by pot 414 whlch directs 8 selected constant through line 416 at the input of flip-flow 420 through line 418. When the dv/dt exceeds slope K, flip-floF 420 is clocked. This applies a logic l in line 422 produc~ng a logic 0 in line 362 turning off circuit 340 by a logic 1 in line 332. As so far described, when the voltage is less than 10 volts, switch SW is open for a tlme Tl which may be overridden by a voltage increas- inp above lO volts, su~h as would occur in an incipient ; ~hort. This will cause interruption of eycle Tl, as indi- cated by line 152 in FIGURE 4. With ~he switch SW closed during a short circuit, flip-flop 420 i8 awaiting a signal :ln line 418 indicating that dv/dt has exceeded constant K. hen that occurs, a 1.O ms pulse occur~ in line 362 holding line 332 at a logic 1. Thls is the time or cycle T3, shown in FIGURE 4, which cycle i9 generally never reached and 1B only a feature providing back-up assurance that the system will shift to the plasma current after a short. Before that happens, arc voltage increases along line 106 to a level above a preselected value of 10 volts. This causes reset generator 350 to reset ball pulse generator 320 placing a loglc 1 ln line 322. At the same time, reset pulse in line 354 produces

. : - 31 -

~273408 L-7434

a logic 1 in line 362. These two logic 1 inputs to gate 330 produces a logic O in line 332, which turns on switch SW.

The ~nhibit feature indicated by time T2 as shown in

FIGURE 4 assures that the derivative fllp-flop 420 does not operate until 100 microseconds after Tl i8 concluded. This is accomplished by another monostable multivibrator 430 having an output 432 which is combined logicslly with the logic in ball pulse line 322 by NAND gate 440. The output 442 is labeled HOLD CLEAR. It remains at a logic 1 until cycle T expires as indicated by the logic in line 322. Thereafter the 100 microsecond negative pulse on line 432 expires.

This pulse is shown at the bottom of FIGURE 13B. As long 8S a logic 1 i8 maintained in line 442, fl~p-flop 420 can not toggle to place a logic 1 in line 422. Con- se~uently, time T2 at the end of cycle Tl maintains the differential circuit inactive for a short time allowing the voltage and current to stabilize and operate in a floating condition awa~ting an ultimate detection at or near the top 52a of metal transf~r pulse MT (NEW). ) FIGURE 14 Referring now to FIGURE 14, the top graph depicts stages of a standard short circuiting welding operation with arc jet forces and plssma created at the first stage

I. The ball ~tarts to grow in stages II and III. Then an incipient short occurs with ~pstter at stage IV. After the incipient short has been terminat~d and a transfer is started, the ball starts to neck at stage V. Then the fuse blows at stage VI. Current and voltage curves ~or the~e stages are illustrated for a system without a spatter re- duction device of the present invention and labeled "Standard

Process". The corresponding process curves using the present ~ invention are set forth at the lower portion of FIGURE 14. - As can be seen the present lnvention maintains a controlled plasma current as well as controlling the transfer pulse ~l5 to nearly eliminate weld spatter.

32

. . :.,

~7340a L-7~3~

In accordance with the present invention, there is a voltage responsive override whereby the main current is applied by closlng switch SW whenever the voltage exceeded a pre- selected value, in the illustrated embodiment 10 volts. This override voltage is generally selecl:ed as about half of the arc or plasma voltage.