Abstract:
A plasma arc torch having new electronic circuit concepts wherein main current regulated power means regulates the pilot current prior to main arc transfer. Further, the circuit may contain two inductors to which DC current initially flows but is interrupted upon main arc transfer such that one inductor maintains the pilot arc while the current in the second inductor forces the establishment of the transferred arc. Also, advantages are presented in pulsing the cutting arc as well as pulsing the pilot arc.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The subject matter of this application is related to and comprises a continuation-in-part of the patent application having Ser. No. 07/682,727, filed on Apr.  8, 1991 pending  now U.S. Pat. No. 5,189,277 , which application is owned by a common assignee. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is in the field of plasma torches and in particular is directed to a plasma torch having an improved pilot and main arc generating circuit. 
     Plasma torches, otherwise known as electric arc torches, are known in the art for performing operations, such as cutting, welding, etc , on workpieces, and operate by directing a plasma consisting of ionized gas particles towards a workpiece. An example of the conventional single gas plasma torch is illustrated in Hatch, U.S. Pat. No. 3,813,510, assigned to the assignee herein Other patents disclosing such torches are U.S. Pat. Nos. 4,225,769; 4,663,512; and 4,663,515. The disclosures of all of the above-mentioned patents are incorporated herein by reference thereto. As these patents illustrate, a gas to be ionized, such as nitrogen, is fed through channels in the torch mechanism in such a manner as to swirl in front of the end of a negatively charged electrode. The welding tip which is adjacent the end of the electrode has a sufficiently high voltage applied thereto to cause a spark gap to jump between the electrode and the welding tip, thereby heating the gas and causing it to ionize. A pilot DC voltage between the electrode and the welding tip maintains the pilot arc. The iononized gas in the gap appears as a flame and extends externally of the tip where it can be seen by the operator. The extension of the pilot arc and the flame, which for practical purposes, may be considered as being co-extensive depends upon the power in the gap—i.e., the arc current—as well as the pressure of the gas forced into the gap and out of the torch. The pilot arc provides a source of light which enables the operator to see the proper position for the torch before starting the welding or cutting operation. In actual practice, when the pilot arc is on, a loop-shaped arc extending out of the torch can be seen. As the torch head is brought down towards the workpiece, the pilot arc jumps from the electrode to the workpiece due to the fact that the impedance of the workpiece current path is lower than the impedance of the welding tip current path. 
     Conventional single gas plasma torches include pilot arc circuits which provide a 20-40 amp. pilot arc current at 100-200 volts across the electrode-tip gap, resulting in an extension of the arc about ¼-½ inch past the welding tip. As a consequence, the torch must be brought to within about ¼-½ inch of the workpiece before the transfer arc jumps to the workpiece. This creates difficulties in the starting of cutting or welding operations. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a plasma arc torch circuit which is more efficient than prior art circuits and which regulate the power source in response to sensing of the torch arch  arc transferring from between the pilot electrode and the torch electrode, to torch electrode and the work member.  
     It is another object of the invention to optimize the power regulator design with respect to semiconductor switch and diode stresses, transformer and inductor design and overall component count. 
     It is still a further object to provide a plasma arc torch with smoother on-plate transfer dynamics 
     Briefly stated, the present invention comprises new electronic circuit concepts for a plasma arc torch wherein main current regulated power means regulates the pilot current prior to main arch transfer Further, the circuit may contain two inductors to which DC current initially flows but is interrupted upon main arc transfer such that one inductor maintains the pilot arc while the current in the second inductor forces the establishment of the transferred arc. Also, advantages are presented in pulsing the cutting arc as well as pulsing the pilot arc. 
     These, as well as other objects and advantages will become more apparent upon a reading of a detailed description of the preferred embodiment in conjunction with the drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic wiring diagram of one prior art plasma arc torch operating circuit; 
     FIG. 2 is schematic wiring diagram of another prior art plasma arc torch operating circuit; 
     FIG. 2A is a schematic wiring diagram of a prior art pilot arc regulating circuit as used within FIG. 2; 
     FIG. 2B is a schematic wiring diagram of another prior art pilot arc regulating circuit as used within FIG. 2; 
     FIG. 3 is a schematic wiring diagram depicting a plasma arc torch operating circuit according to the principles of the present invention; 
     FIG. 4 is a schematic wiring diagram of a modified embodiment of the present invention as it would be configured within the circuit of FIG. 3; and 
     FIGS. 5A and B, in combination, are a more detailed schematic wiring diagram of the present invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings, and FIG. 1 in particular, there is shown a schematic wiring diagram for one prior art plasma arc torch, generally referred to by the reference numeral  10 . Torch  10  includes torch tip electrode  12 or torch electrode as is known in the art and annular torch pilot electrode  14 or tip as is known in the art spaced from tiptorch electrode  12 . An electronic pilot circuit P connected between tip  torch electrode  12  and pilot electrode  14  provides an electric potential between electrodes  12  and  14  to create a pilot arc which heats a supplied gas such as nitrogen causing it to ionize as is well known in the art. FIG. 1 shows prior art circuit C which uses a resistively regulated pilot arc having a current regulated power means  16  and a pilot regulator means  18  including a disconnect means  20  in series with a resistor  22 . A high frequenty pilot initiation means  24  is positioned in series with pilot regulator means  18  and may be inserted in the circuit adjacent either electrode  12  or electrode  14  as shown in FIG. 1 to initiate investigation  ionization of plasma gas to commence pilot operations. 
     A current sensing means  26  is connected in parallel with pilot regulator means  18  and connects with the metal to be worked at  series with work  28  in main circuit M. When tip  torch electrode  12  is placed sufficiently close to the metal work  28  the arc will transfer to the work  28  causing current to flow through main circuit M and current sensing means  26  will sense the current differential and act to disconnect pilot regulator means  18  by opening pilot regulator means  18  by opening its disconnect means  20 . 
     One problem associate with the prior art circuit  10  of FIG. 1 is that the circuit voltage of current regulated power means  16  must be large compared with the torch piloting voltage between tip  torch electrode  12  and pilot electrode  14  to allow the pilot regulator means  18  to perform the function of a current source during pilot opration. This causes circuit  10  to be inefficient, power being dissipated as heat in pilot regulator means  18 . 
     FIG. 2 shows another prior art circuit  10 ′ similar to that of FIG. 1 in that it also contains electronically controlled pilot regulator means  18 ′ in pilot circuit P′ and a parallel main circuit M′. The circuit  10 ′ of FIG. 2 also includes similar current operated power means  16 ′, tip  torch electrode  12 ′, pilot electrode  14 ′, current sensing means  26 ′, pilot initiation means  24 ′ (alternatively positioned as shown) and work piece or member  28 ′. The difference between circuit  10 ′ of FIG.  2  and circuit  10  of FIG. 1 is the provision of circuit connection on the opposite side of power means  16  from current means  26 ′ connected to pilot regulator means  18 ′ in order to provide a second current regulated control loop, one for pilot arc operation and one for transferred main arc cutting  that FIG. 2 uses an active loop with feedback, either linear (FIG.  2 a) or switching (FIG.  2 b) regulator, while FIG. 1 uses a passive current limiting means (resistor). 
     FIG. 2A shows one prior art pilot regulating circuit  18 ′a wherein the pilot is linearly regulated; that is, the pilot current is regulated against a set demand means  3 d by varying the conductance of a linear element  3 a. 
     FIG. 2B shows another prior art pilot regulating  18 ′b wherein the pilot is switch regulated. That is, the pilot is regulated against a set demand  3 d′ to vary the duty cycle of a switching element  3 g within a feedback loop. Either scheme can tightly regulate the pilot arc against AC line variations and against plasma gas in use, however, both add parts count and cost to the torch and are relatively inefficient. 
     FIG. 3 shows the preferred plasma torch circuit  100  in accordance with the principles of the present invention. Circuit  100  includes torch tip  electrode  112 , pilot electrode  114 , current regulated power means  116 , pilot regulator means  118 , alternatively positionable pilot initiation means  124 , current sensing means  126 , and metal work  128 . Pilot regulator means  118  comprises an electronic disconnect  120  in series with a current smoothing and energy storage inductor  130 , and a free-wheeling diode  132  connected in parallel with disconnect  120  and in series with inductor  130 . 
     According to the invention, current sensing means  126  not only controls disconnect  120  through line  134 , but also sends a current signal to comparator  136  through line  138  which controls the output of power means  116 . 
     During torch piloting, disconnect means  120  is “on” and is in its saturated state. The voltage seen between the metal work piece  128  and torch tip  electrode  112  is essentially the voltage at which the torch maintains the pilot arc determined by torch geometry and the plasma gas used. This voltage is considerably lower than the open circuit voltage used in prior art torch circuits. When the torch is brought sufficiently close to metal work piece  128 , ionization current is detected by current sensing means  126 . In response to sensing the working current, sensing means  126  acts through line  134  to force disconnect means  120  to its “off” or high impedance state. At the moment of arc trnasfer to work piece  128 , the pilot arc is maintained by current flowing through energy storage inductor means  130  and the free-wheeling diode means  132 . At the same instant, the current flowing in through the smoothing inductor means  140  of power regulator  116  is forced to flow between the workpiece  128  and torch tip  electrode  112 ,. At the moment of disconnect, disconnect  120  open circuits the inductor means  140  to generate a transient voltage between the torch electrode and the workpiece using the stored energy in the inductor means  140 , greatly in excess of the pilot voltage, that initiates and initially supports the transfer to the main arc, thereby maintaining the transferred plasma arc. When the energy is dissipated in the storage inductor means  130 , the pilot arc between torch tip  electrode  112  and pilot electrode  114  self extinguishes. When transfer is detected in current sensing means  126  the pilot demand means  1 e is changed and the power means  116  changes the power to that demanded for the torch operation on work piece  128 . 
     A further embodiment of the present invention resides in additionally pulsing the pilot current. 
     Instead of maintaining a constant pilot demand means ( 1 e), the demand may be pulsed between two (or more levels) at various frequencies and duty cycles. During this pulsing the pilot arc is maintained throughout and no high frequency arc initiation means  124  is required, as would be the case for a ‘blown-out’ pilot. 
     This pulsing feature offers several advantages. First, higher standoff instances between the work metal  128  and torch tip  electrode  112  at the moment of transfer. Second, a tip cleaning action is observed i.e. during plasma cutting molten metal is blown onto the tip face where it adheres in particulate form. At the same time, electrode material is removed from the torch electrode and adheres to the inside tip. Both forms of contamination can cause the tip orifice to become distorted. When the pilot arc is pulsed following each cut significantly more power is dissipated in the tip  torch electrode  112  for the pulse duration. This thermal modulation is believed to be responsible for dislodging metal particles from the inner and outer tip surfaces. 
     With reference to FIG. 4, there is shown an alternative circuit wherein a small resistor  142  is added in series with the pilot means  118 . This modification can further improve the obtainable standoff on some plasma torch designs. The pilot current (Ip) flows through resistor  142  to generate a voltage drop (Ip×R) which is in series with the pilot voltage measured between torch tip  electrode  112  and pilot electrode  114 . Thus the open circuit voltage between the metal work piece means  128  and the pilot electrode means  114  is increased, assisting the standoff at transfer. The power dissipated in this resistor is then a function of the pilot demand and pulse duration. 
     A further alternative circuit provides a pulsing cutting or main arc. From the invention pulsing the pilot arc before transfer it is clear that it is possible to pulse the means  1 e, after the arc has transferred and while the transferred plasma arc is cutting the work metal means  128 . This provision of pulsing the main arc offers several potential advantages. First, by selecting the appropriate pulse rate and duty cycle in relation to the cutting variables, it will offer a proportionally greater arc cutting capacity/penetration for a small increase in power consumption. Second, it allows the tip orifice size to be reduced in comparison to a conventional plasma cutting system operating in response to a DC demand level. This will, allow, a smaller focussed plasma column and result in smaller kerf widths. Plasma arc stability may also improve as a result of pulsing. 
     FIGS. 5A and B are a more specific electronic circuit schematic diagram embodying some of the concepts of the invention as enumerated above. Like reference numerals appearing in FIG. 5 refer to like circuit components or group of components as appear in FIGS. 3 and 4. Reference numberal  120 c depicts the control circuity for disconnecting means  120 . The power supply means is not shown in FIG.  5 . 
     It can therefore be seen that the novel circuity shown in FIGS. 3 through 5 fulfills the objects and provides the advantages set forth above. Inasmuch as numerous changes could be made to the circitry without departying from the spirit and scope of this invention, the scope of the invention is to be determined solely by the language of the following claims as interpreted by the patent laws and in particular the doctrine of equivalents.