Patent Description:
A wide range of welding systems and welding control regimes have been implemented for various purposes. In continuous welding operations, gas metal arc welding (GMAW), and more specifically, metal inert gas (MIG) techniques allow for formation of a continuing weld bead by feeding welding wire electrode shielded gas (typically an inert gas or gas containing inert agents) from a welding torch. Electrical power is applied to the welding wire and a circuit is completed through the workpiece to sustain an arc that melts the wire and the workpiece to form the desired weld.

Advanced forms of MIG welding are based upon generation of cyclic pulses that may cause controlled short circuits between the wire electrode and the advancing weld puddle comprising of melted metal of the workpieces and the wire electrode. That is, various pulsed regimes may be carried out in which current and/or voltage pulses are commanded by the power supply control circuitry to regulate the formation and deposition of metal droplets (and/or spray, and/or globules) from the welding wire, to sustain a desired heating and cooling profile of the weld puddle, to control short circuits between the wire and the weld puddle, and so forth.

While very effective in many applications, such pulsed and short circuit welding regimes may be subject to drawbacks. For example, depending upon the transfer mode, the processes may either limit travel speed, create excessive spatter (requiring timely cleanup of welded workpieces), provide less than optimal penetration, or any combination of these and other effects. Moreover, certain pulsed processes, such as ones operating in a spray mode of material transfer, may run excessively hot for particular applications. Others, such as short circuit processes, may run cooler, but may again produce spatter and other unwanted weld effects.

Moreover, in certain welding situations and with certain welding electrodes, pulsed welding processes that are trained to implement cyclic short circuits between the electrode and the workpiece may add excessive energy to the weld. For example, with cored wire electrodes, the electrode may be heated by excessive current added to the wire, particularly insomuch as the weld current tends to flow through the wire sheath, which can more easily melt than solid wires. As a result, the arc may flare (grow long). However, for spanning gaps, reducing burn-through, and increasing travel speeds, it may be desirable to maintain the arc length at a minimum. Unfortunately, this causes the wire to short to the progressing weld puddle and requires additional current to clear short circuits, again leading to heating of cored wire sheaths, and causing the arc to flare.

There is a need, therefore, for improved welding strategies that allow for welding in pulsed and short circuit waveform regimes while improving weld quality and flexibility.

The present invention provides a welding method according to claim <NUM> designed to respond to such needs. In accordance with an exemplary implementation, a welding method comprises generating a control waveform for welding power output, the waveform comprising a plurality of successive peak phases followed by a short circuit between a welding wire electrode and an advancing weld puddle; and regulating each then present peak phase, e.g. the voltage peak, based upon at least the immediately preceding short circuit to control the short circuit that will occur following the then present peak phase.

Turning now to the drawings, and referring first to <FIG>, an exemplary welding system is illustrated as including a power supply <NUM> and a wire feeder <NUM> coupled to one another via conductors or conduits <NUM>. In the illustrated embodiment the power supply <NUM> is separate from the wire feeder <NUM>, such that the wire feeder may be positioned at some distance from the power supply near a welding location. However, it should be understood that the wire feeder, in some implementations, may be integral with the power supply. In such cases, the conduits <NUM> would be internal to the system. In embodiments in which the wire feeder is separate from the power supply, terminals are typically provided on the power supply and on the wire feeder to allow the conductors or conduits to be coupled to the systems so as to allow for power and gas to be provided to the wire feeder from the power supply, and to allow data to be exchanged between the two devices.

The system is designed to provide wire, power and shielding gas to a welding torch <NUM>. As will be appreciated by those skilled in the art, the welding torch may be of many different types, and typically allows for the feed of a welding wire and gas to a location adjacent to a workpiece <NUM> where a weld is to be formed to join two or more pieces of metal. A second conductor is typically run to the welding workpiece so as to complete an electrical circuit between the power supply and the workpiece.

The system is designed to allow for data settings to be selected by the operator, particularly via an operator interface <NUM> provided on the power supply. The operator interface will typically be incorporated into a front faceplate of the power supply, and may allow for selection of settings such as the weld process, the type of wire to be used, voltage and current settings, and so forth. In particular, the system is designed to allow for MIG welding with various steels, aluminums, or other welding wire that is channeled through the torch. These weld settings are communicated to control circuitry <NUM> within the power supply. The system may be particularly adapted to implement welding regimes designed for certain electrode types.

The control circuitry, described in greater detail below, operates to control generation of welding power output that is applied to the welding wire for carrying out the desired welding operation. In certain presently contemplated embodiments, for example, the control circuitry may be adapted to regulate a pulsed MIG welding regime that promotes short circuit transfer of molten metal to a progressing weld puddle without adding excessive energy to the weld or electrode. In "short circuit" modes, droplets of molten material form on the welding wire under the influence of heating by the welding arc, and these are periodically transferred to the weld puddle by contact or short circuits between the wire and droplets and the weld puddle. "Pulsed welding" or "pulsed MIG welding" refers to techniques in which a pulsed power waveform is generated, such as to control deposition of droplets of metal into the progressing weld puddle. In a particular embodiment of the invention, a specialized pulsed welding regime may be implemented in which pulses are generated that have characteristics of both short circuit welding and spray welding, in a type of "hybrid" transfer mode as described in <CIT>.

As described more fully below, the present techniques allow for control successive voltage and/or current pulses based on previous pulses so as to control the promotion, occurrence, duration, and interruption of short circuits between the welding wire electrode and the advancing weld puddle. In particular, in certain presently contemplated embodiments, voltage peaks in waveforms are regulated based on one or more immediately preceding short circuit, or aspects of the short circuit, such as its duration. Aspects of the voltage peaks may include, for example, their duration, their magnitude, their rate of rise or decline, and so forth. Aspects of current waveforms may also be directly or indirectly regulated. Various bases may be employed for the analysis of the short circuits, heating, heat injection, and so forth, as discussed below.

The control circuitry is coupled to power conversion circuitry <NUM>. This power conversion circuitry is adapted to create the output power, such as pulsed waveforms that will ultimately be applied to the welding wire at the torch. Various power conversion circuits may be employed, including choppers, boost circuitry, buck circuitry, inverters, converters, and so forth. The configuration of such circuitry may be of types generally known in the art in and of itself. The power conversion circuitry <NUM> is coupled to a source of electrical power as indicated by arrow <NUM>. The power applied to the power conversion circuitry <NUM> may originate in the power grid, although other sources of power may also be used, such as power generated by an engine-driven generator, batteries, fuel cells or other alternative sources. Finally, the power supply illustrated in <FIG> includes interface circuitry <NUM> designed to allow the control circuitry <NUM> to exchange signals with the wire feeder <NUM>.

The wire feeder <NUM> includes complimentary interface circuitry <NUM> that is coupled to the interface circuitry <NUM>. In some embodiments, multi-pin interfaces may be provided on both components and a multi-conductor cable run between the interface circuitry to allow for such information as wire feed speeds, processes, selected currents, voltages or power levels, and so forth to be set on either the power supply <NUM>, the wire feeder <NUM>, or both.

The wire feeder <NUM> also includes control circuitry <NUM> coupled to the interface circuitry <NUM>. As described more fully below, the control circuitry <NUM> allows for wire feed speeds to be controlled in accordance with operator selections, and permits these settings to be fed back to the power supply via the interface circuitry. The control circuitry <NUM> is coupled to an operator interface <NUM> on the wire feeder that allows selection of one or more welding parameters, particularly wire feed speed. The operator interface may also allow for selection of such weld parameters as the process, the type of wire utilized, current, voltage or power settings, and so forth. The control circuitry <NUM> is also coupled to gas control valving <NUM> which regulates the flow of shielding gas to the torch. In general, such gas is provided at the time of welding, and may be turned on immediately preceding the weld and for a short time following the weld. The gas applied to the gas control valving <NUM> is typically provided in the form of pressurized bottles, as represented by reference numeral <NUM>.

The wire feeder <NUM> includes components for feeding wire to the welding torch and thereby to the welding application, under the control of control circuitry <NUM>. For example, one or more spools of welding wire <NUM> are housed in the wire feeder. Welding wire <NUM> is unspooled from the spools and is progressively fed to the torch. The spool may be associated with a clutch <NUM> that disengages the spool when wire is to be fed to the torch. The clutch may also be regulated to maintain a minimum friction level to avoid free spinning of the spool. A feed motor <NUM> is provided that engages with feed rollers <NUM> to push wire from the wire feeder towards the torch. In practice, one of the rollers <NUM> is mechanically coupled to the motor and is rotated by the motor to drive the wire from the wire feeder, while the mating roller is biased towards the wire to maintain good contact between the two rollers and the wire. Some systems may include multiple rollers of this type. Finally, a tachometer <NUM> may be provided for detecting the speed of the motor <NUM>, the rollers <NUM>, or any other associated component so as to provide an indication of the actual wire feed speed. Signals from the tachometer are fed back to the control circuitry <NUM>, such as for calibration as described below.

It should be noted that other system arrangements and input schemes may also be implemented. For example, the welding wire may be fed from a bulk storage container (e.g., a drum) or from one or more spools outside of the wire feeder. Similarly, the wire may be fed from a "spool gun" in which the spool is mounted on or near the welding torch. As noted herein, the wire feed speed settings may be input via the operator input <NUM> on the wire feeder or on the operator interface <NUM> of the power supply, or both. In systems having wire feed speed adjustments on the welding torch, this may be the input used for the setting.

Power from the power supply is applied to the wire electrode, typically by means of a welding cable <NUM> in a conventional manner. Similarly, shielding gas is fed through the wire feeder and the welding cable <NUM>. During welding operations, the wire is advanced through the welding cable jacket towards the torch <NUM>. Within the torch, an additional pull motor <NUM> may be provided with an associated drive roller, particularly for aluminum alloy welding wires. The motor <NUM> is regulated to provide the desired wire feed speed as described more fully below. A trigger switch <NUM> on the torch provides a signal that is fed back to the wire feeder and therefrom back to the power supply to enable the welding process to be started and stopped by the operator. That is, upon depression of the trigger switch, gas flow is begun, wire is advanced, power is applied to the welding cable <NUM> and through the torch to the advancing welding wire. These processes are also described in greater detail below. Finally, a workpiece cable and clamp <NUM> allow for closing an electrical circuit from the power supply through the welding torch, the electrode (wire), and the workpiece for maintaining the welding arc during operation.

It should be noted throughout the present discussion that while the wire feed speed may be "set" by the operator, the actual speed commanded by the control circuitry will typically vary during welding for many reasons. For example, automated algorithms for "run in" (initial feed of wire for arc initiation) may use speeds derived from the set speed. Similarly, various ramped increases and decreases in wire feed speed may be commanded during welding. Other welding processes may call for "cratering" phases in which wire feed speed is altered to fill depressions following a weld. Still further, in pulsed welding regimes, the wire feed speed may be altered periodically or cyclically.

<FIG> illustrates an exemplary embodiment for the control circuitry <NUM> designed to function in a system of the type illustrated in <FIG>. The overall circuitry, designated here by reference numeral <NUM>, includes the operator interface <NUM> discussed above and interface circuitry <NUM> for communication of parameters to and from downstream components such as a wirefeeder, a welding torch, and various sensors and/or actuators. The circuitry includes processing circuitry <NUM> which itself may comprise one or more application-specific or general purpose processors, designed to carry out welding regimes, make computations for waveforms implemented in welding regimes, and so forth. The processing circuitry is associated with driver circuitry <NUM> which converts control signals from the processing to drive signals that are applied to power electronic switches of the power conversion circuitry <NUM>. In general, the driver circuitry reacts to such control signals from the processing circuitry to allow the power conversion circuitry to generate controlled waveforms for pulsed welding regimes of the type described in the present disclosure. The processing circuitry <NUM> will also be associated with memory circuitry <NUM> which may consist of one or more types of permanent and temporary data storage, such as for providing the welding regimes implemented, storing welding parameters, storing weld settings, storing error logs, and so forth.

More complete descriptions of certain state machines for welding are provided, for example, in <CIT>; <CIT>; <CIT>; and <CIT>.

<FIG> generally illustrates logic for a welding regime in which waveforms are controlled to control short circuit characteristics. The logic, indicated generally by reference numeral <NUM> may be thought of as beginning with performing a pulsed/short circuit welding process, as indicated by block <NUM>. Such a process may be implemented by closed loop control of voltage applied to a welding electrode (with a circuit completed through the workpiece and advancing weld puddle), by closed loop control of current applied to the electrode, or by control of both voltage and current. In some embodiments, for example, control loops that regulate voltage in a closed loop manner may be alternated with phases of control in which current is controlled in a closed loop manner. In a presently contemplated embodiment, a short circuit is promoted or permitted in each cycle of a pulsed process, and parameters of the voltage and/or current waveforms of power applied to the welding electrode are controlled and regulated to control aspects of the short circuit, particularly its duration, but also its timing, heating, restrike energy and so forth.

At step <NUM>, then, the short circuit is monitored. Such monitoring will typically be done by detecting the voltage and current applied to the welding electrode, the welding torch, the weld cable, or some other point in the power stream. In currently contemplated embodiments, the voltage and current are sampled at a much higher rate than the frequency of the pulsed welding regime. For example, each pulse cycle may be on the order of several milliseconds (e.g., <NUM>), whereas the sampling may be performed on the order of microseconds (e.g., every <NUM>). As will be appreciated by those skilled in the art, the occurrence of short circuits will typically be indicated by drops in voltage. However, in most systems, the voltage and current sampling will occur throughout the welding process, and analysis of the voltage and/or current during the short circuit may be done from the data collected. The duration of the short circuit may be determined by examining how many sampling intervals occurred with the levels of voltage and/or the current were such as would indicate a short circuit (e.g., a threshold low voltage).

At step <NUM>, according to present invention, parameters of voltage peaks are computed or adjustments may be made as discussed below. In the present invention, the parameters of voltage peaks are computed, with the system controlling welding power in a voltage-closed loop manner during the peak phase of the pulsed waveform. That is, the voltage of the peak is controlled, including the magnitude of the peak, the duration of the peak, the rate of rise to the peak, and the rate of decline from the peak. Some embodiments may control fewer than these parameters. As discussed below, the peaks are regulated to appropriately control aspects of the cyclical short circuit, and particularly the timing and duration of the short circuit. As indicated by reference numerals <NUM>, <NUM> and <NUM>, one or more bases are used for regulating the peak, such as proportional control, integral control, and derivative control. In currently contemplated embodiments, for example, the magnitude and/or duration (and/or rates of increase and decrease) of the peak are regulated based upon the duration of the short circuit in a proportional manner. However, integral factors may allow for more stable, longer-duration (e.g., multi-pulse) control, to eliminate errors from the desired peak parameters. Derivative factors may also aid, such as to improve response to the short circuit duration, timing, and so forth. In practice, more than one of these factors may be utilized, and exact algorithms for the regulation of the peaks may be adapted for particular welding applications, such as particular materials, particular electrodes, particular positions, travel speeds, and so forth. Moreover, in practice, each factor utilized may be multiplied by a desired gain, and such gains may be used to "teach" or adapt the control regime to arrive at more predictability and controllable short circuits, reduce energy applied to the weld, improve travel speed, reduce spatter, and so forth. In the current embodiment, while the adjustments are made to the peak, the adjustments may be made to the any of the waveform segments to achieve similar affects. To regulate the duration of a short, the short response, short energy or max short current may be modified as well (e.g., restrike or added energy to clear the short).

The "dynamic" adjustment of the voltage peak by reference to the immediately preceding short circuit may be done in a variety of ways. For instance, the adjustment may be on the basis of short circuit duration alone. By way of example, if the short circuit is <NUM> long, it may be determined that this duration is appropriate, and no adjustment is made. If, then, the short duration of <NUM> is detected, it may be determined that the peak was too long and the short circuit was, consequently, too short (that is the molten ball was almost off the electrode before the short circuit occurred). In this case, the next peak duration may be made shorter, the magnitude of the peak may be changed, the ramp up or ramp down rates and durations may be changed, and so forth. Further, if the short does not occur at all (i.e., duration of <NUM>) or is very short, it may be determined that the peak was so large that the molten ball split from the electrode before any short occurred at all), and the peak may be more seriously altered to ensure a short circuit, and one that is consistent over time.

As for the particular calculations made, another possible basis for computing the short circuit parameter that is the basis for regulating the voltage peak is the resistive heating (i<NUM>r) of the electrode. For this calculation, a resistance may be estimated, and detected currents used to estimate the heating (power) applied to the electrode. Still further, because the resistance may not change materially, this calculation may be simplified to a summation of squared currents or currents during the short duration. Here again, the duration of the short circuit may be determined based upon the number of sampling intervals over which the voltage and/or the current indicated the electrode was shorted to the weld puddle.

At step <NUM>, the logic may determine whether another type of control may be useful to assist in short circuit duration and/or timing control. That is, if the short circuit is too long, a determination may be made that a "restrike" or "pinch" is needed. Such techniques may involve application of current (e.g., in a current-closed loop manner) or voltage to force re-establishment of the welding arc, and thereby to end the short circuit. If no such control is needed, the flow may return to step <NUM> to repeat the process for the next pulse. Or the control loop may take into account the duration and current of a desired short heating and use variations from the nominal much as the short duration control loop (discussed above).

In this process logic, it should be noted that the control of each voltage peak is regulated based upon the immediately preceding short circuit. Again, due to the higher sampling rate of voltage and current as compared to the frequency of the voltage and current pulses of the control regime, each short circuit may be analyzed prior to implementation of the immediately subsequent peak phase. That is, the then-voltage peak may be regulated based upon the immediately preceding short circuit characteristics, particularly the time of onset of the short circuit, the time of ending of the short circuit, short heating, short power and/or its duration. Moreover, in some embodiments, one or more characteristics of the short circuit (e.g., duration), one or more characteristics of the peaks, and/or one or more characteristics of adjustments made to the peaks may be low pass filtered by averaging over a number of pulse cycles. For example, the duration of the short circuit may be averaged to smooth response to the duration in making adjustments to the peak parameters. In general, however, at least the immediately preceding short circuit is used as a basis for altering or regulating the then-current peak. In the current embodiment, the peak is a regulated voltage peak controlled in a voltage-closed loop manner.

At step <NUM>, if it is determined that the short circuit is longer than desired, a restrike or pinch may be implemented. In general, these aspects allow for control of voltage in a voltage-closed loop manner to promote termination of the short circuit earlier than it has been detected to end. At step <NUM>, then, parameters of the voltage used to terminate the short circuit may be computed and/or altered. As with the voltage peak, these parameters are controlled based on proportional control, integral control, and/or derivative control, as indicated by reference numerals <NUM>, <NUM>, and <NUM>, typically again based on the duration of the short circuit. Moreover, here again, the restrike or pinch may be based on the immediately preceding short circuit. Also, more than one short circuit occurrence may be referred to for this control, and low pass filtering, such as by averaging a number of short circuit occurrences (e.g., their durations). In the present embodiment the voltage peak duration of the arc phase is modified by both the short heating and short duration, although the short response is envisioned as an alternative in some aspects of this control.

At step <NUM> the parameters and/or adjustments computed or implemented at steps <NUM> and <NUM> may be combined. For example, as will be recognized by those skilled in the art, adding energy by implementation of a restrike or pinch may affect the dynamics of the material transfer and heating, and this may need to be compensated for by altering the peak. Conversely, altering the peak may affect the short circuit (and typically will), so that more or less restrike or pinch is needed. In a presently contemplated embodiment, a control loop is implemented to compute a change in the peak parameter(s) based on short characteristics (e.g., duration), and another control loop is implemented to compute a change in the peak parameter(s) based on the restrike or pinch, and errors (adjustments or corrections) in the peak parameters resulting from these calculations are simply combined (e.g., added). Weighted sums may be generated in this approach by altering coefficients or gains of each correction from the two different loops.

<FIG> illustrates exemplary waveforms <NUM> for the improved process, these including a voltage waveform <NUM> and a current waveform <NUM>, displayed over time <NUM> for a number of cycles of the pulsed/short circuit process. Here again, the process may include alternating periods or phases of voltage-closed loop control and current-closed loop ramp control, in this case, the peaks being voltage-closed loop controlled, followed by switching to current-closed loop control. In this process, molten metal from the electrode is transferred to the advancing weld puddle in a pulsed process in which short circuits are promoted and controlled by regulation of power (e.g., by voltage control) applied to the electrode. In the present context, promotion and control of short circuits is intended to convey that relatively hard short circuits are avoided, and that brief and controlled short circuits may be established by monitoring and analyzing aspects of at least one immediately preceding short circuit. The metal is thus transferred without adding excessive energy that might be required if harder or longer duration short circuits were employed. In most cycles of the resulting welding process, no special short clearing sequence will be needed, although such sequences may be programmed and ready to be implemented in case a longer or more stubborn short circuit does occur, as discussed above. As described above, because the molten material (e.g., a ball of metal) is already detaching after a voltage and/or current peak, little or no current is added to clear the short circuit. The result is relatively lower heating and a stable arc while avoiding unnecessary heating.

As shown in <FIG>, voltage pulses are formed in the welding process that generally correspond in time to current pulses, the two being regulated, in this embodiment, by alternating between voltage-closed loop control and current-closed loop control. However, again, to control the duration and characteristics of the short circuits, voltage and current may be analyzed and used to provide a combined control that effectively regulates both the peaks and the short circuits (as well as other aspects of the pulses, when desired). The voltage peaks are indicated in <FIG> by reference numeral <NUM>. These peaks generally cause energy to be input into the arc to melt the welding electrode. Short circuits then occur, resulting in a significant drop in voltage, as indicated by reference numeral <NUM>. The duration of these short circuits, indicated by reference numeral <NUM> are controlled by regulating the energy (e.g., voltage) of the peaks, and it can be seen in the voltage trace that shorts tend to occur in every cycle of the process, and are of a quite uniform duration. After each short circuit is cleared, the system returns to a background voltage level to await the next peak, as indicated by reference numeral <NUM>. The current peaks occur generally in synchronicity with the voltage peaks, as indicated by reference numeral <NUM>. After the current peaks, current generally returns to a background level <NUM>.

<FIG> is a similar graphical illustration of a single cycle of this process. Here, the voltage waveform is indicated by reference numeral <NUM>, and the current waveform by reference numeral <NUM>. As can be seen, the voltage peak <NUM> is controlled and is generally aligned with the current peak <NUM>. At these peak levels, the voltage rises from a background level <NUM> to the peak level, the magnitude, ramp rate, and duration of which may be controlled. The current similarly rises from its background <NUM>. The peak phase of the cycle causes the welding electrode <NUM> to melt, forming a droplet of material <NUM> that will be deposited in weld. Following the peaks <NUM> and <NUM>, then, the voltage returns to a background or step level <NUM>, as does the current <NUM>. Again, the ramp rate, the shape of the ramps, and so forth, may be controlled during these declines in voltage and current. The decline in voltage <NUM> to <NUM> corresponds to the short circuit between the welding electrode <NUM> and the advancing weld, as the formed droplet <NUM> is deposited into the weld puddle <NUM>. The current may be dropped in response to this short as indicated by <NUM> to <NUM>, it may also be slightly increased or maintained. In some embodiments the current is dropped from <NUM> to <NUM> to encourage the short of the wire to the weld puddle to occur in this area of the waveform (preceding the actual short).

The short circuit is then broken by reduction of energy into the weld when the voltage and current levels are dropped even further, as indicated by reference numerals <NUM> and <NUM>. Here again, as discussed above, if it is determined that the peak, or any other period of energy input (or any other weld dynamics), has caused the short circuit to be longer than desired, a restrike or pinch can be implemented, in which interruption of the short circuit is promoted by increasing current (which, again, interplays with the voltage regulation, such as by combined control of the peak voltage characteristics). The short circuit is thus broken, and the voltage and current can return to their background levels, as indicated by reference numerals <NUM> and <NUM>, as the electrode material <NUM> becomes integrated into the advancing weld.

Claim 1:
A welding method comprising:
- generating a control waveform for welding power output, the waveform comprising a plurality of successive peaks (<NUM>, <NUM>) followed by a short circuit between an advancing welding wire electrode (<NUM>) and a weld puddle (<NUM>);
characterized by
- analyzing an immediately preceding short circuit prior to implementation of the immediately subsequent peak phase;
- computing parameters of the peaks,
- regulating each then present peak based upon the at least one of duration, energy, heating or maximum current of the immediately preceding short circuit to control the short circuit that will occur following the then present peak based on proportional control (<NUM>), integral control (<NUM>) and/or derivative control (<NUM>),
wherein the sampling rate of voltage and current is higher as compared to the frequency of the voltage and current pulses of the control regime; and wherein the peak is a regulated voltage peak controlled in a voltage-closed loop manner.