Abstract:
Provided is a welding system and method for controlling a welding system with an improved pulsed waveform suitable for use with metal core welding wires. The waveform includes peak pulses (of voltage, current, power, energy or a combination thereof) that aid in transfer of molten metal from the wire to the weld pool. The pulse duration is sufficiently short to avoid damage to the metal core welding wire. The waveform enables welding of thin metals and reduces spatter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Non-Provisional Patent Application of U.S. Provisional Patent Application No. 61/066,138, entitled “Welding System and Method with Improved Waveform”, filed Apr. 30, 2007, which is herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates generally to the field of welding systems, and particularly to pulsed gas metal arc welding systems (GMAW-P), also known as pulsed metal inert gas (pulsed MIG) welding systems. 
         [0003]    Arc welding systems generally comprise a power supply that applies electrical current to an electrode so as to pass an arc between the electrode and a work piece, thereby heating the electrode and work piece to create a weld. In many systems, such as gas metal arc welding systems (GMAW), the electrode consists of a wire which is advanced through a welding torch. As the electrode is heated by the arc, the electrode melts and is joined to molten metal of the work piece to form the weld. By controlling the supply of voltage and current to the electrode, a GMAW system may control the manner in which the electrode is melted and deposited by the arc. For example, the voltage supply to the electrode may be held constant, while the current supply is varied so as to maintain a constant arc length independent of the distance between the contact tip and the work piece. A GMAW system may also supply voltage and current to an electrode in a periodic or pulsed manner, known as pulsed gas metal arc welding (GMAW-P) or pulsed metal inert gas (pulsed MIG) welding. 
         [0004]    In a GMAW system, the arc performs two critical functions. First, the arc melts the end of the electrode into a molten ball. Second, the arc then transfers the molten ball of electrode material onto the work piece and into the weld puddle. Of the two critical functions, transferring the molten ball of electrode material onto the work piece requires significantly higher power. Accordingly, a GMAW system operating at a single constant voltage must operate at a relatively high voltage. Maintaining a high constant voltage adds more heat to the work piece and consumes more power than necessary. The additional heat results in a weld puddle which may be too fluid for some applications, such as overhead or vertical welding. Excessive heat may also cause thinner work pieces to warp and distort. 
         [0005]    Rather than provide a single constant voltage- or constant current-controlled arc, GMAW-P welding systems supply voltage and current to the electrode according to a periodic pattern. For example, a GMAW-P welding system may supply a constant low voltage in a first phase (the background phase), and then supply a constant high voltage in a second phase (the peak phase). In such a way, an arc may provide only enough power to melt the electrode in the background phase, while providing sufficient power to transfer the molten electrode material to the weld puddle in the peak phase. A GMAW-P system may allow a variety of parameters to be programmed, such as constant voltage levels, fixed current beginning points, constant current ramp rates, minimum and maximum current limits, time allowed for each phase, and so forth. The increased control offered by GMAW-P may reduce overall current consumption and thus minimize excess heat and work piece distortion, lessen the fluidity of the weld puddle, allow for a smaller weld puddle, and offer greater control over weld penetration. 
         [0006]    Though GMAW-P represents an improvement over typical GMAW, some problems persist. Because the arc generally transfers the molten ball of electrode material onto the work piece during a high power peak phase, the transfer often occurs rapidly. The rapid transfer may cause the molten ball of electrode material or the weld puddle to spatter. The high power of the peak phase may also heat the molten ball of electrode material beyond its boiling point, resulting in “outgassing” and microspatter caused by electrode vaporization. 
         [0007]    Occasionally, the molten ball does not completely transfer. If part of the molten electrode material remains connected both to the electrode tip and the weld puddle, a short circuit occurs and the arc may be extinguished. Often, a restrike phase is then initiated, during which the current rises rapidly to force the transfer of molten electrode material and thus restrike the arc. While such a restrike phase usually succeeds, at the moment the arc restrikes the current and voltage may be very high. The high energy of the arc may cause the weld puddle to spatter and generate excessive heat. For at least these reasons, GMAW-P systems generally seek to avoid short-circuit conditions. While some software-controlled GMAW-P systems may take implement schemes to predict clearing of short-circuit conditions, such systems may be more expensive or unwieldy than more common GMAW-P systems. 
         [0008]    In addition to the foregoing issues with gas shielded welding applications, waveforms similar to those used in MIG welding are sometimes used with metal core welding wires. Such welding wires generally comprise a sheath or shell in which a metal powder core is disposed and compressed. The sheath may be the same as the core, or may be different from the core so as to create a composite metallurgy when the two are melted. The core may also provide materials for shielding the weld pool. Conventional pulsed waveforms have not been widely used in such applications, however, particularly due to the higher levels of heating that may adversely affect the core of the wires. 
       BRIEF DESCRIPTION 
       [0009]    The invention provides a system and method of welding using an improved waveform to address such needs. The waveform is particularly well suited for use with metal core welding wires and avoids damage to the wire core. In accordance with one aspect of the present invention, a welding method includes application of a pulsed waveform to a metal core welding wire. The waveform may include a peak pulse of voltage, current, power, energy or a combination thereof of a duration of less than approximately 0.2 ms, and in some embodiments of less than 1.8 or 1.0 ms. Moreover, a ratio of the duration of the peak pulse to the overall duration of the waveform (which is applied in repetition) is on the order of less than about 1%, and in some embodiments less than 0.6%. 
     
    
     
       DRAWINGS 
         [0010]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0011]      FIG. 1  illustrates an exemplary welding system in accordance with one embodiment of the present invention; 
           [0012]      FIG. 2  illustrates an exemplary voltage and current waveform in accordance with an embodiment of the present technique; 
           [0013]      FIG. 3  illustrates an exemplary voltage and current waveform in which a short circuit does not clear on its own and a restrike phase is employed to clear the short circuit, in accordance with an embodiment of the present technique; 
           [0014]      FIG. 4  illustrates an exemplary magnified view of a welding electrode during a first background phase in accordance with an embodiment of the present technique; 
           [0015]      FIG. 5  illustrates an exemplary magnified view of a welding electrode during a peak phase in accordance with an embodiment of the present technique; 
           [0016]      FIG. 6  illustrates an exemplary magnified view of a welding electrode during a second background phase in accordance with an embodiment of the present technique; 
           [0017]      FIG. 7  illustrates an exemplary magnified view of a welding electrode during a restrike delay phase in accordance with an embodiment of the present technique; 
           [0018]      FIG. 8  illustrates an exemplary magnified view of a welding electrode immediately following a restrike in accordance with an embodiment of the present technique; and 
           [0019]      FIG. 9  is a flowchart illustrating a method of controlling the voltage and current to the welding electrode in accordance with an embodiment of the present technique. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates an exemplary welding system  10  which is configured to utilize the present technique. Prior to continuing, however, it is worth noting that the following discussion merely relates to exemplary embodiments of the present technique. As such, the appended claims should not be viewed as limited to those embodiments described herein. 
         [0021]    The exemplary welding system  10  includes a base unit  12  operably coupled with a welding torch  14 . Placement of the welding torch  14  proximate to work piece  20  allows electrical current, supplied by power supply  24 , to form an arc  22  from electrode  16  to the work piece  20 . The arc  22  completes an electrical circuit from power supply  24  to electrode  16 , to the work piece  20 , then back to ground via ground clamp  18  and ground cable  40 , which is operably coupled to power supply  24  through control circuitry  30 . The heat produced by arc  22  causes the electrode  16  and/or work piece  20  to transition to a molten state, facilitating the weld. 
         [0022]    Base unit  12  supplies welding torch  14  with voltage and current from power supply  24 , electrode  16  from electrode supply  32  via wire feeder  26 , and shielding gas from gas supply  28  through conduit  38 . The electrode  16  may be of various types, including traditional wire electrode or gasless wire electrode. Shielding gas from gas supply  28  shields the weld area from contaminants during welding, to enhance arc performance, and to improve the resulting weld. 
         [0023]    To precisely control the deposition of molten material from electrode  16  onto work piece  20 , the control circuitry  30  varies the voltage and current supplied by power supply  24  to welding torch  14  according to a predetermined algorithm, as discussed in greater detail below. Control circuitry  30  monitors the supply voltage and current with voltage sensor  34  and current sensor  36 . By varying the voltage and current supplied by power supply  24  to welding torch  14 , the control circuitry  30  controls the intensity of the arc  22  and, accordingly, the manner in which the molten material from electrode  16  is deposited onto the work piece  20 . 
         [0024]    It should be noted that, as described below, pulsed waveforms provided by the present techniques may be applied to welding systems utilizing metal core welding wires as well. As will be appreciated by those skilled in the art, such wires may be used with welding power supplies similar or even identical to those used for solid wire welding applications. 
         [0025]      FIG. 2  illustrates an exemplary voltage waveform  42  with voltage axis  48  and current waveform  44  with current axis  50 , both across time axis  46 , as implemented by control circuitry  30  with power supply  24 . Voltage waveform  42  comprises segments of constant voltage, while current waveform  44  comprises segments which allow current to vary during corresponding constant voltage segments and segments which ramp current up or down at constant rates. The waveforms repeat at a predetermined frequency with a period  52 . 
         [0026]    Referring to voltage waveform  42 , first constant voltage segment  74  represents a first background phase, during which background voltage level  58  is held constant. On current waveform  44 , the corresponding current during the first background phase is allowed to vary so as to maintain arc  22 . In the exemplary voltage waveform  42 , background voltage level  58  is low enough such that short circuits will regularly occur during a later phase, as discussed in greater detail below, but high enough to preheat the tip of electrode  16  to form a molten ball of electrode material before the proximate voltage increase. Accordingly, in various embodiments the background voltage level  58  ranges from 17V to 20V, but depending on variables such as frequency, wire feed speed (WFS), peak voltage level  64 , choice of electrode  16 , etc., the background voltage may be higher or lower. 
         [0027]    Immediately following the first constant voltage segment  74 , the first background phase ends and a peak phase begins. Current increases at peak current ramp rate  62  to peak current level  66 . Once the current reaches peak current level  66 , voltage is commanded to reach peak voltage level  64  during segment  68 , rising at a rate  60  (not commanded, but resulting from the commanded ramp-up of the current waveform), and then may remain at level  66  until peak phase time  54  expires, ending the peak phase. During segment  68 , current is allowed to fluctuate while voltage remains constant until the peak phase time  54  expires. 
         [0028]    The peak voltage level  64 , peak phase time  54 , peak current ramp rate  62 , and initial peak current level  66  may be chosen so as to minimize overheating of the molten electrode material while substantially initializing the transfer of molten electrode material toward the weld puddle. Because the power flowing through arc  22  during the peak phase may be much higher than during a background phase, an embodiment of the present technique may employ a relatively short peak phase time  54 , constituting between approximately one-tenth to one-third the total waveform period. In an embodiment operating at 220 Hz and with background voltage level  58  at 17 V, a peak voltage level  64  may be 35 V and the peak phase time  54  may be 1.0 ms. For the same embodiment, the peak current ramp rate  62  may be 1000 A/ms and the initial peak current  66  may be 550 A. 
         [0029]    Continuing to refer to  FIG. 2 , the second background phase begins immediately after the peak phase time  54  expires. Amperage decreases at background current ramp rate  72  causing a reduction in voltage, as indicated by reference numeral  70 , until the a background current level  73  is reached. To maximize the precision of the voltage and current pulse, background current ramp rate  72  may be significantly faster than peak current ramp rate  62 . In one embodiment, background current ramp rate  72  is 2000 A/ms, double the peak current ramp rate  62  of 1000 A/ms. As during the first background phase, the voltage is then maintained at background voltage level  58  while the current varies for the duration of the second background phase. 
         [0030]    The molten electrode material may typically reach the weld puddle while still attached to the tip of electrode  16 , causing a short circuit and extinguishing the arc  22 . The short circuit may be detected at the point that voltage drop  76  crosses threshold voltage  78 , triggering the end of the second background phase and the beginning of the restrike delay phase. During the restrike delay phase, current may be temporarily held constant at a restrike delay current level to allow the short circuit to clear on its own. The restrike delay current level may be high enough to keep the molten electrode material substantially fluid while it transfers to the weld puddle, but low enough not to cause excessive spatter when the short circuit clears and the arc  22  restrikes. Accordingly, in one embodiment, the restrike delay current level is 80 A. 
         [0031]    The restrike delay phase ends when either the restrike delay phase time  56  expires or when the short circuit clears on its own, whichever occurs first. Note that restrike delay phase time  56  represents a fixed upper bound of time allowed for the short circuit to clear on its own and that the arc restrike  84  will not necessarily occur at the same time the restrike delay phase time  56  expires. If the short circuit does not clear by the time the restrike delay phase time  56  expires, the restrike phase will begin (not shown in  FIG. 2 ), discussed in greater detail below. It should be noted that as the molten electrode material detaches from the tip of electrode  16  and the short circuit begins to clear, a voltage increase  82  naturally occurs; the arc restrike  84  occurs approximately at the point the voltage crosses threshold voltage  78  and the system detects that the short circuit has cleared. 
         [0032]    If the short circuit clears before the restrike delay phase time  56  expires, the waveform immediately enters a restrike return phase. At the moment the voltage crosses threshold voltage  78  and enters the restrike return phase, current is rapidly ramped down at restrike return current ramp rate  86  and held constant at the restrike return current level for a desired time to allow voltage to stabilize around background voltage level  58 . By reducing the current when arc  22  restrikes and the voltage is rising, the total power of arc  22  is reduced. Undesired effects, such as excessive spatter and microspatter, may thus be minimized. For the same reason, the restrike return current ramp rate  86  should be chosen to very rapidly drop the current, and may reach greater than 1000 A/ms. After the desired time has expired for the restrike return phase, the cycle begins again at a first background phase. 
         [0033]    The waveform illustrated in  FIG. 2  exhibits a number of novel and advantageous features. For example, during programming, the duration  54  may be set, as may the ramp rate  62 . This effectively determines the duration that the voltage will stay at the “peak” level (i.e., the pulse width of the peak current pulse). In a presently contemplated embodiment, for example, the ramp rate  60  may be set to approximately 1000 A/ms, although other rates may be used. During the rise in voltage between the background level  74  and the peak level  68 , the system effectively controls current, while during the peak voltage segment  68 , voltage is controlled. Subsequently, during the ramp-down of voltage from the peak voltage segment, current is one again controlled. 
         [0034]    As discussed in greater detail below, the availability of the very short peak current pulse, and the ability to control the pulse offers the potential for application of the waveforms and control techniques both for pulsed MIG and for metal core wire welding tasks. The reduced input of power or energy enables the welding of thin materials that would otherwise burn through with more conventional pulsed and non-pulsed techniques. In the case of metal core wires, heating is reduced, avoiding potential damage to the wires, and particularly to their cores. 
         [0035]    A short circuit will ordinarily clear on its own during the restrike delay phase, but occasionally the short circuit does not clear before the restrike delay phase time  56  expires. Turning now to  FIG. 3 , the exemplary voltage waveform and current waveform represent alternative waveforms which result if, during a restrike delay phase, a short circuit instead fails to clear on its own before the restrike delay phase time  56  expires. Though the waveforms of  FIG. 3  remain substantially identical to the waveforms of  FIG. 2  from the first background phase through the restrike delay phase, the waveforms depicted in  FIG. 3  comprise a restrike phase after the restrike delay phase. The restrike phase is calculated to force a short circuit to clear when the short circuit fails to clear on its own during the restrike delay phase. 
         [0036]    As depicted in  FIG. 3 , if during the restrike delay phase the voltage does not cross threshold voltage  78  before restrike delay phase time  56  expires to signal the short circuit has cleared, a restrike phase may begin. During the restrike phase current ramps up rapidly at restrike current ramp rate  88  to the restrike current level, where the current is held high for a desired restrike phase time or until the short clears, whichever occurs first. To ensure the arc  22  restrikes during the restrike phase, the restrike current level may reach beyond one hundred amperes higher than the initial peak current level  66 , and the restrike phase time may span many times the ordinary period of the waveform. For example, when the initial peak current level  66  is set to 550 A, the restrike current level may be set to 700 A; when the ordinary waveform period  52  (depicted in  FIG. 2 ) is approximately 4.5 ms, the restrike phase time may endure 65.5 ms or longer. Because of the very high current through the molten electrode material during the restrike phase, the short circuit will almost certainly clear before the restrike phase time expires. 
         [0037]    Continuing to refer to  FIG. 3 , once the short circuit clears and is detected when the voltage crosses threshold voltage  78 , the waveform may enter a restrike return phase. The restrike return phase operates in substantially the same fashion as depicted in  FIG. 2 , ramping current down at restrike current ramp rate  86  and holding current constant at the restrike return current level for a desired time to allow voltage to stabilize around background voltage level  58 . When the desired restrike return time expires, the waveform returns to a first background phase, restarting the cycle. 
         [0038]      FIGS. 4-8  illustrate exemplary magnified views of the welding electrode  16  through the various phases discussed above in accordance with an embodiment of the present invention.  FIG. 4  portrays a molten droplet  90  forming at the tip of electrode  16  during a first background phase. Arc  22  initially stretches between the tip of electrode  16  and the work piece  20 , heating each. The heat from arc  22  melts the tip of electrode  16  to form molten droplet  90 , while maintaining weld puddle  92  fluid on the work piece  20 . When the molten droplet  90  forms on the tip of electrode  16 , arc  22  spans from molten droplet  90  to the weld puddle  92 . 
         [0039]      FIG. 5  represents the welding electrode  16  during a peak phase, when the molten droplet  90  substantially begins to transfer toward weld puddle  92 . To facilitate the movement of the molten droplet  90 , higher voltage and current through arc  22  cause the arc to emit considerably more heat. As the molten droplet  90  begins to move away from the tip of electrode  16 , a molten tail  94  begins to form between the tip of electrode  16  and the molten droplet  90 . 
         [0040]    In  FIG. 6 , which depicts a second background phase, the power of arc  22  has reduced as the voltage and current have returned from peak levels. Accordingly, the molten droplet  90  approaches the weld puddle  92  while the molten tail  94  elongates. 
         [0041]      FIG. 7  illustrates a restrike delay phase, when the molten droplet  90  reaches the weld puddle  92  before the molten tail  94  has fully detached from the tip of electrode  16 , extinguishing the arc and causing a short circuit. During the restrike delay phase, the current between electrode  16  and work piece  20  is maintained at a level high enough to keep the molten droplet  90  and molten tail  94  substantially fluid while transferring to the weld puddle  92 , but low enough not to cause excessive spatter or outgassing after the short circuit clears and the arc restrikes. 
         [0042]      FIG. 8  represents welding electrode  16  immediately after the molten tail  94  detaches from the tip of electrode  16 , causing the short circuit to clear and arc  22  to restrike. In some embodiments, the current may be reduced immediately when arc  22  restrikes to prevent spattering molten droplet  90  while transferring into weld puddle  92 . 
         [0043]    Turning to  FIG. 9 , flowchart  96  illustrates an exemplary method of controlling the voltage and current to welding electrode  16  in accordance with an embodiment of the present technique. When the flowchart begins at block group  98 , a molten droplet  90  may have already formed on the tip of the electrode  16  during a background phase  108 . Block group  98  represents a peak phase, during which the molten droplet  90  may begin to move toward the weld puddle  92 , assisted by step  110 , ramping voltage and current to peak levels, and step  112 , maintaining a constant peak voltage level  64  for a desired peak phase time  54 . 
         [0044]    The proximate block group represents a background phase  100 , wherein the molten droplet  90  may continue toward the weld puddle  92 , assisted by step  114 , ramping voltage and current down to background levels, and step  116 , maintaining a constant background voltage level  58 . As illustrated by decision block  118 , background phase  100  may end when a short circuit is detected and the process flow enters block group  102 . The system may detect a short circuit when voltage naturally drops below a threshold voltage level  78 , indicating that the molten droplet  90  has reached the weld puddle  92  without detaching from the tip of electrode  16  and may have extinguished the arc  22 . Decision block  140  illustrates that if a short circuit is not detected before a desired background time has elapsed, the process flow returns to the peak phase block group  98 . 
         [0045]    Block group  102  represents a restrike delay phase. Step  120 , ramping and holding the current to a constant restrike delay current level, is calculated to keep the molten droplet  90  fluid so the molten tail  94  may disconnect from the tip of the electrode  16  on its own. Decision blocks  122  and  132  test first whether the short circuit has cleared on its own, and second whether a restrike delay phase time  56  has elapsed. As long as neither is true, step  120  maintains the current at a constant level. If the system detects the voltage to cross above a threshold voltage level before time elapses, indicating the short circuit has cleared, the process flow may progress to a restrike return phase as represented by block group  106 . If time elapses before the short circuit is detected to have cleared, the process flow may transfer to a restrike phase, illustrated by block group  104 . When the system variables are properly calibrated, short circuits should most often clear before the restrike delay time  56  elapses. Accordingly, the process flow will first be discussed as if the short circuit has cleared on its own during the restrike delay phase, before discussing the alternative. 
         [0046]    Continuing to refer to  FIG. 9 , if the short circuit clears before the desired restrike delay time elapses, the process flow may enter a restrike return phase as represented by block group  106 . Comprising a single step  124 , ramping current down to a restrike return current level for a desired time, the restrike return phase aims to reduce the power of arc  22  upon restrike, so as to minimize spatter and other negative effects of the restrike on weld puddle  92 . 
         [0047]    After the desired restrike return phase time has passed, the process flow enters background phase  108 . To grow a molten droplet  90  at the tip of electrode  16 , background phase  108  comprises step  126 , ramping up voltage and current to background levels, and step  128 , maintaining voltage constant at a background voltage level. Decision block  130  ensures the arc  22  is maintained at a constant voltage until a desired background time has elapsed and the process returns to a peak phase, represented by block group  98 . To conform to a constant period, decision block  130  may measure the desired background time from the start of background phase  100 . In this way, when a short circuit clears during the restrike delay phase of block group  102 , the peak phase of block group  98  will consistently repeat at a period comprising the peak phase time added to the background time, without regard to time spent in a restrike delay phase. 
         [0048]    Referring again to block group  102 , if alternatively the restrike delay phase time  56  elapses before the short circuit clears, the process flow may transfer to a restrike phase represented by block group  104 . The restrike phase employs step  134 , ramping current up to restrike current levels, until either the short circuit clears (see decision block  136 ) or the restrike phase time elapses (see decision block  138 ). To balance clearing the short circuit relatively quickly with minimizing the current level at the moment the short circuit clears, the restrike current ramp rate  88  may be calculated such that the current reaches the restrike current level within approximately half of the ordinary waveform period. For example, for a period of 4.5 ms, a restrike current ramp rate may be 300 A/ms. The process may progress to a restrike return phase, as represented by block group  106 , when the short circuit is cleared or the restrike phase time elapses. From block group  106 , the process flow proceeds. 
         [0049]    As noted above, the ability to provide a very short peak voltage pulse, as well as the ability to control the pulse duration allows the waveforms and control techniques described above to be used with metal core welding wires. In particular, one or more settings may be provided for such metal core applications, with presently contemplated settings designed to accommodate 0.045 and 0.052 inch AWS E70C-6M wires. Other wire diameters, such as 0.062 inch wire. 
         [0050]    Similarly, due to the reduced energy input offered by the waveforms and control techniques, thin metals may be welded with reduced risks of burn-through. For example, the waveforms are believed to be particularly advantageous for welding steels, such as cold rolled steels, with thicknesses of 16 gauge to 0.375 inch, and more particularly in a range of from 14 gauge to 0.312 inch. 
         [0051]    Again, for metal core wires, a range of settings may be envisaged for the voltage peak pulse width to reduce ill effects on the wire core, and to reduced spatter. For example, for 0.045 inch wire, a pulse width on the order of 1.0 ms is contemplated for a wire feed speed of 200-500 inches per minute (IPM). For higher wire feed speeds, longer periods may be used, such as 1.2 ms at 600 IPM. Other durations might be used for other sizes of wire, such as 1.2-1.4 ms for 0.052 inch wire. In general, however, the ability to control the peak pulse width to below about 2.0 ms (and particularly to below about 1.8 ms) is believed to offer significant promise in terms of reducing weld spatter, burn-through and damage to metal core wires. 
         [0052]    Regarding the particular frequencies that may be used for the waveform (i.e., the rate of repetition of the overall waveform discussed above), a range of frequencies may be envisaged, which may depend upon the wire size and the feed speed. For example, it has been found that for 0.045 inch wire, a frequency of 190-340 Hz works well for feed speeds of between 200 and 600 IPM. 
         [0053]    Within these ranges, particular settings might include, for example, 190 Hz at 200 IPM, 220 Hz at 300 IPM, 250 Hz at 400 IPM, 280 Hz at 500 IPM, and 340 Hz at 600 IPM. Thus, depending upon the frequency of repetition of the waveform and the peak pulse width selected, it can be seen that the pulses are very short as compared to the length of the overall waveform. In particular, based upon the ranges discussed above, the ratio of the peak pulse duration to the waveform duration may be on the order of 40-67% for pulses of a duration of approximately 2 ms, and less for shorter duration peak pulses, such as on the order of 20-33%. 
         [0054]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.