Abstract:
A method and apparatus for controlling a gas supply to a plasma arc torch uses a proportional control solenoid valve positioned adjacent the torch to manipulate the gas flow to the torch, thereby extending electrode life during arc transfer and shutdown. Swirl ring design can be simplified and gas supply and distribution systems become less complicated. The invention also allows manipulation of shield gas flow to reduce divot formation when making interior cuts. The system can be controlled with a digital signal processor utilizing a feedback loop from a sensor.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to plasma arc cutting torches, and more particularly to control systems and methods for controlling gas supplies to such torches to enhance their operation.  
       BACKGROUND OF THE INVENTION  
       [0002]     Plasma arc cutting torches have a wide variety of uses, such as the cutting of aluminum sheet metal, thick plates of steel or stainless steel, or thin sheets of galvanized metal. As illustrated in  FIG. 1 , a plasma torch  10  includes a torch body  12 , an electrode  14  and a nozzle  16  mounted within the body. The nozzle has a central exit orifice  18  and can be surrounded by a shield  22 . An exit port  24  of the shield is generally aligned with the exit orifice  18  of the nozzle. A power supply (not shown) is used to create an arc between the electrode  14  and the nozzle  16  ionizing a plasma gas that is supplied from a plasma gas source  30 . The ionized plasma gas exits the torch  10  through the exit orifice  18  of the nozzle and the exit port  24  of the shield, and is used to cut a workpiece (not shown). Once the plasma arc has been initiated, the current flow can be transferred from the nozzle to the workpiece.  
         [0003]     The shield  22  is mounted to a retaining cap  26  on the torch body  12 . Shield gas from a shield gas source  40  can be introduced to a space between the nozzle  16  and the shield  22 . At least a portion of the shield gas exits the torch with the plasma arc (i.e., the ionized plasma gas) through the exit port  24  of the shield. The shield gas cools the shield and helps protect the shield from workpiece splatter during a cutting or piercing operation of the torch. The torch can include a swirl ring (not shown) in the flow path of the plasma gas and/or the shield gas to impart a swirling motion to the gas for improving torch performance.  
         [0004]     During operation of the torch, certain consumable parts become worn and have to be replaced. These consumables include torch electrodes, nozzles, and shields. Previous patents assigned to Hypertherm, Inc. of Hanover, N.H. teach techniques for prolonging the life of these consumables. For example, U.S. Pat. No. 5,070,227, the contents of which are incorporated herein by reference, teaches that the life of an electrode can be extended by controlled reduction of the plasma gas flow a short time before commencement of the torch current flow reduction, as the cut cycle is ended. U.S. Pat. No. 5,166,494, the contents of which are incorporated herein by reference, describes altering the flow of plasma gas in conjunction with the transfer of the current flow from the nozzle to the workpiece, and U.S. Pat. No. 5,170,033, the contents of which are incorporated herein by reference, explains that a chamber in the swirl ring can be created and sized to favorably affect the dynamic flow characteristics of the flowing gas when torch operating conditions are changed.  
         [0005]      FIG. 1  illustrates a known gas distribution feed system for a plasma arc torch. Gas from a gas supply system or a gas cylinder (e.g., plasma gas supply  30  or shield gas supply  40 ) is regulated to a desired operating pressure using a pressure regulator  31 ,  41 . The reduced-pressure gas passes through an on-off solenoid valve  33 ,  43  and optionally through a manually operated needle valve  35 ,  45 . On-off solenoid valves (e.g.,  33 ,  43 ) generally produce an exponentially decreasing pressure decay curve upon closure of the valve. After exiting the needle valve  35 ,  45 , the gas flows to a plasma torch  10 . The plasma gas is channeled to the plasma chamber, to a space located between the electrode and the nozzle. The shield gas flows to a space between the shield and the nozzle. A more complex gas distribution feed system is described in U.S. Pat. No. 5,396,043, assigned to Hypertherm, Inc., the contents of which are incorporated herein by reference. The &#39;043 patent describes a plurality of plasma and/or shield gas flow channels and valves of different sizes and configurations that can be used to provide a pre-flow, an operating flow, and a quick charge for use in different operating modes of the torch, such as during piercing operation of the torch, or cutting operation.  
         [0006]     Unfortunately, there are drawbacks associated with these different approaches. For example, although the gas flow reduction scheme of the &#39;227 patent extends the lifetime of the electrode, to fully optimize the technology it is necessary to customize the length of hose between the on-off solenoid valve  33  and the torch  10  to achieve a proper volume and resulting gas ramp-down characteristics for a particular torch and consumable set (e.g., electrode and nozzle). This hose volume customization needs to be matched, e.g., to the specific closing characteristics of the on-off solenoid valve  33 , such that a precise gas flow profile is achieved about the electrode  14  as the cut cycle is ended. More specifically, it was previously necessary to position on-off solenoid valves  33 ,  43  at a specified distance from the torch such as 12 inches, 4 feet, or 6 feet, depending upon the system being configured. Empirical determination of the pressure decay curve along with other mechanical adjustments and compensations were then performed to obtain a prolonged life of different consumable sets (e.g., electrodes and nozzles). Such tedious empirical determinations were performed for different torches, systems, consumable sets, and cutting conditions. In such systems, relocating the on-off solenoid valve by one foot, for example, from 4 feet away from the torch to 5 feet away from the torch without recalibrating the current ramp down rates resulted in a dramatic reduction in electrode life (on the order of 30%).  
         [0007]     These control difficulties are exacerbated by the rapid system dynamics, which can all take place within a time span of about 300 milliseconds or less. The determination of the proper hose length and current ramp down characteristics to achieve an acceptable termination gas flow profile is empirically acquired and is extremely time consuming. Similar developmental problems are encountered when customizing the gas flow characteristics required for optimal use of the &#39;494 patent, e.g., while transferring the current from the nozzle to the workpiece.  
         [0008]     The chambered swirl ring of the &#39;033 patent, while achieving an increase in electrode life, requires fabrication of a complex swirl ring design. Moreover, the inlet and outlet port diameters of such a swirl ring must be carefully fabricated to precise tolerances to achieve the desired gas flow characteristics. Although proper sizing of the swirl ring chamber volume and inlet and outlet port diameters can achieve the desired gas flow results, a given swirl ring is generally useful for only a certain torch type or consumable set necessitating the storage and availability of different swirl rings with varying design criteria. Performance of such swirl rings can also be adversely affected, e.g., by varying gas supply pressures and other gas flow parameters.  
         [0009]     Finally, considering the gas distribution feed system of the &#39;043 patent, the multiple flow channels for each gas stream are complex and require many component parts. What is needed is a less complicated, less expensive system to accomplish desired gas flow objectives.  
         [0010]     What is also needed is a method and apparatus that can reliably accomplish all of these objectives using fewer component parts and at a reduced manufacturing cost.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention achieves these objectives by positioning a programmable control valve in the gas line adjacent the torch and manipulating it to control the gas flow.  
         [0012]     One aspect of the invention features a method for extending the life of a torch consumable such as an electrode, nozzle, or shield that includes providing a first fluid line such as a plasma gas line for supplying a plasma gas to the torch, and positioning a programmable control valve in the first fluid line adjacent the torch to control a flow of the plasma gas. Manipulation of programmable control valve controls the flow of the plasma gas to the torch during operation of the torch and compensates for a volume in the first fluid line between the proportional solenoid control valve and the torch. Embodiments include locating the programmable control valve near the torch and adjusting an opening of the programmable control valve, such as a flow orifice, or a valve and seat assembly, to change or adjust the flow of the plasma gas a plurality of times during a ramp cycle of the plasma gas. Examples include plasma gas ramp up, such as during torch start up, and plasma gas ramp down near the end of a torch cutting cycle. A control output from a digital signal processor can be used to adjust the programmable control valve to perform at least one of the gas flow controlling or the volume compensating steps. In some embodiments the programmable control valve is a proportional solenoid control valve, such as a Burkert proportional solenoid control valve, although other types of control valves that have suitable control characteristics and response times can be used.  
         [0013]     The method for extending the life of the torch consumable can include a sensor disposed between the torch and the programmable control valve, such that the digital signal processor uses a signal from the sensor to adjust the control output to the programmable control valve. The sensor can be a pressure sensor, a flow sensor (such as a mass flow meter), an electrical sensor such as a current measurement, a temperature sensor such as an IR (infrared measurement), and more. The method can also include positioning the sensor in the first fluid line between the programmable control valve and the torch, sensing a parameter (such as one of those mentioned above) in the first fluid line, and using the sensed parameter to adjust the programmable control valve during the controlling step.  
         [0014]     The method can also include the step of extending the life of a consumable (such as an electrode or a nozzle) by controlling a ramp-up flow of the plasma gas during start-up of the torch using the programmable control valve, especially when the current flow is transferred from the nozzle to the workpiece. It can also include the step of controlling a ramp-down flow of the plasma gas during shut-down of the torch using the programmable control valve, which can also extend the life of the consumable. In some embodiment, both the ramp-up flow of the plasma gas and the ramp-down flow of the plasma gas are controlled using the programmable control valve. This can be used to reduce the cycle time of workpiece cuts by the torch, thereby increasing production line throughput and capacity. In some embodiments, the torch consumable either is or includes an electrode and operation of the torch includes controlling at least one of a ramp-up or a ramp-down of a flow of the plasma gas based on a type of the electrode installed in the torch, i.e., some electrodes perform better and/or last longer when start-up and/or shutdown is accompanied by a customized plasma gas flow curve.  
         [0015]     Another aspect of the invention features a method for control of a gas flow to a plasma arc torch that includes a plasma chamber disposed within a body of the plasma arc torch. The method includes providing a first fluid line for supplying a first gas to the torch, positioning a programmable control valve in the first fluid line adjacent the torch to control a flow of the first gas, and manipulating the programmable control valve (such as a proportional solenoid control valve). Embodiments include locating the programmable control valve near the torch and adjusting an opening of the programmable control valve, e.g., by using a valve with a rising stem-type plunger, to change or adjust the flow of the first gas a plurality of times during a ramp cycle of the first gas. Examples include first gas ramp up, such as during torch start up, and first gas ramp down near the end of a torch cutting cycle. Manipulation of the programmable control valve is used to control the flow of the first gas to the torch during operation of the torch and to compensate for a volume in the first fluid line between the proportional solenoid control valve and the torch. The programmable control valve can be a proportional solenoid control valve. In some embodiments the first gas is a plasma gas that supplies the plasma chamber. In others the first gas can be a shield gas that flows to space between a nozzle and a shield of the torch.  
         [0016]     This method can also include positioning a second programmable control valve in a second fluid line, which can be used to supply a second gas to the torch. The first gas can be a plasma gas and the second gas can be a shield gas, and the flow of the second gas can cool and protect from splatter a shield that surrounds a nozzle and is mounted on the torch body. The method can also include diverting at least a portion of the flow of the second gas (e.g., a shield gas) through a third fluid line to join with the flow of the plasma gas to the torch. A third programmable control valve can be positioned in the third fluid line adjacent the torch, to control the diverted shield gas flow. The third programmable control valve can be manipulated to control the flow of the diverted portion of the second gas and to compensate for a volume in the third fluid line between the third programmable control valve and the torch. In another embodiment, the method can include diverting at least a portion of the flow of the plasma gas through a third fluid line to join with the flow of the second gas (e.g., the shield gas) to the torch. The third programmable control valve can be positioned in the third fluid line adjacent the torch to control the diverted plasma gas flow. The third programmable control valve can be manipulated to control the flow of the diverted portion of the plasma gas and to compensate for a volume in the third fluid line between the third programmable control valve and the torch.  
         [0017]     In some embodiments the second and/or the third programmable control valve is a proportional solenoid control valve. A control output from a digital signal processor can be used to adjust any or all of the programmable control valves to perform at least one of their respective controlling and the compensating steps. A sensor, such as a pressure, flow, temperature, or mass flow sensor can be located between the torch and the programmable control valve, such that the digital signal processor uses a signal from the sensor to adjust the control output to the programmable control valve. Embodiments include the first gas comprising at least one of oxygen, nitrogen, hydrogen, methane, or argon. In some embodiments, the plasma chamber is defined by an electrode and a nozzle, and the first gas is a shield gas that cools and protects the shield from splatter, where the shield surrounds the nozzle and is mounted on the torch body, e.g., via a retaining cap.  
         [0018]     The method can also include the step of adjusting the flow of the first gas (e.g., a shield gas) to reduce formation of a divot in an interior cut of a workpiece, to control slag formation, and/or to improve the quality of a corner cut within a workpiece. Embodiments include controlling the flow of the shield gas to be a piercing flow during a piercing operation of the torch (e.g., when initially penetrating a workpiece), or to be a cutting flow during a cutting operation of the torch (such as during a prolonged cutting operation).  
         [0019]     Another aspect of the invention features a method for control of a shield gas flow to a shield surrounding a nozzle and mounted on a torch body of a plasma arc torch that includes providing a first fluid line for supplying a shield gas to the torch and positioning a programmable control valve in the first fluid line adjacent the torch, to control the flow of the shield gas. The programmable control valve is manipulated to control the flow of the shield gas to the torch during operation of the torch and to compensate for a volume in the first fluid line between the proportional solenoid control valve and the torch. The shield gas flow is adjusted to reduce formation of a divot in an interior cut of a workpiece. Embodiments include locating the programmable control valve near the torch and adjusting an opening of the programmable control valve to change or adjust the flow of the shield gas a plurality of times during a ramp cycle of the shield gas. Examples include shield gas ramp up, such as during torch start up, and shield gas ramp down near the end of a torch cutting cycle. In some embodiments the programmable control valve is a proportional solenoid control valve, although other types of valves with suitable flow characteristics and adequate response dynamics can also be used.  
         [0020]     Yet another aspect of the invention features a plasma arc torch for cutting a workpiece, wherein the plasma torch has a plasma gas source to supply a plasma chamber. An electrical current passing between an electrode and a nozzle of the torch produces a plasma arc that exits the torch through a nozzle exit orifice. The plasma torch includes a means for sensing a parameter in a first fluid line that supplies a plasma gas from the plasma gas source and a means for controlling a flow of the plasma gas to the plasma chamber based on the sensed parameter using a programmable control valve disposed in the first fluid line adjacent the plasma torch. In some embodiments the programmable control valve is a proportional solenoid control valve. The sensed parameter can be a pressure or a flow of the plasma gas. The torch can also include a means for controlling a flow of a shield gas from a shield gas source to the torch through a second fluid line, the means comprising a second programmable control valve disposed in the second fluid line adjacent the plasma torch. The second programmable control valve can be a proportional solenoid control valve.  
         [0021]     The torch can also include a swirl ring that imparts a swirling motion to at least one of the plasma gas or the shield gas, and a control output from a digital signal processor can be used to manipulate one or both of the programmable control valve and the second programmable control valves. The output of the digital signal processor can be adjusted based on the type or thickness of the workpiece to be cut, such that the plasma and/or shield gas flows are thereby adjusted to compensate for these variables. In some embodiments the plasma gas includes oxygen, nitrogen, hydrogen, methane, argon, or mixtures thereof. The nozzle of the plasma torch can be surrounded by a shield mounted to a retaining cap and having an exit port that aligns with the exit orifice of the nozzle.  
         [0022]     Another aspect of the invention features a plasma cutting system that includes a power supply and a plasma arc torch for cutting a workpiece. The plasma torch includes a plasma gas source to supply a plasma chamber such that an electrical current passing between an electrode and a nozzle produces a plasma arc that exits the torch through an exit orifice in the nozzle. The plasma torch includes a means for sensing a parameter in a first fluid line, such as pressure, temperature, or flow, which line supplies a plasma gas from the plasma gas source to the torch. The torch also includes a means for controlling a flow of the plasma gas to the plasma chamber based on the sensed parameter using a programmable control valve disposed in the first fluid line adjacent the plasma torch. The programmable control valve can be a proportional solenoid control valve. Other types of valves with suitable control parameters and response dynamics can also be used. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The foregoing discussion will be understood more readily from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which:  
         [0024]      FIG. 1  illustrates a known gas supply system for a plasma arc torch;  
         [0025]      FIG. 2  is a schematic representation of a plasma gas supply system to a plasma arc torch incorporating features of the invention;  
         [0026]      FIG. 3  is a chart depicting plasma gas flow control test data during startup of a plasma arc torch, in accordance with an embodiment of the invention  
         [0027]      FIG. 4  is a schematic representation of a shield gas supply system to a plasma arc torch incorporating features of the invention;  
         [0028]      FIG. 5  illustrates the control of divot formation achievable with an embodiment of the invention;  
         [0029]      FIG. 6  is a schematic representation of a combined plasma gas and shield gas supply system to a plasma arc torch; and  
         [0030]      FIG. 7  is a more detailed representation of a combined plasma gas and shield gas supply system to a plasma arc torch. 
     
    
     DETAILED DESCRIPTION  
       [0031]      FIG. 2  is a schematic representation of a plasma gas supply system to a plasma arc torch incorporating features of the invention. A plasma gas, e.g., including oxygen, nitrogen, and/or argon flows from a plasma gas supply  30  to a plasma gas supply line  201 . This first gas supply line channels plasma gas to the plasma torch  10  through a programmable control valve  205  such as a proportional solenoid control valve. The plasma gas flows to a plasma chamber  207  of the torch, and can pass through a sensor  210 , such as a flow sensor or a pressure sensor. In some embodiments a signal  212  from the plasma gas sensor  210  can pass to a digital signal processor (DSP)  215  (e.g., a microprocessor, computer, or PLC), which in turn can manipulate an output  218  to the programmable control valve  205  based on the signal  212 . However, the sensor  210  is optional and embodiments include a plasma gas supply line  201  with no sensor  210 , and manipulation of the output  218  of the DSP  215  can be based upon other, e.g., predetermined, operating parameters.  
         [0032]     Valves suitable for use as a programmable control valve  205  with the invention include actuated valves such as ball, plunger, needle, and varying orifice valves. Although valves that control flow using a flow orifice or opening of variable size are preferred, other valves having suitable flow characteristics and response times can also be used. Servo valves of the same valve styles can also be used. Another suitable programmable control valve is a proportional solenoid control valve such as a Type 6022 or Type 6023 proportional solenoid control valve available from Burkert Fluid Control Systems (http://www.burkert-usa.com) of Irvine, Calif. For example, Burkert valves with a valve coefficient (C v ) of 0.12 suitable for use with most aspects of the invention. The input to the valve (e.g., the output  218  from the DSP  215 ) can be of many different forms, such as an 800 Hz PWM (pulse width modulation) signal, a 0-10 volt DC signal, a 4-20 ma current loop signal (e.g., 24 volt DC), or others, such as are known to those of skill in the art. This listing of valves is not intended to be comprehensive and other valves and types of valves meeting the required performance characteristics can also be used to achieve the objectives of the invention.  
         [0033]     The programmable control valve  205  differs from previous valves used in gas supplies to plasma arc torches, such as those described above. Applicants have discovered that use of a programmable control valve  205  such as a proportional solenoid control valve having an analog-type control range, suitable flow characteristics, and fast response times, when positioned adjacent the torch allows for very precise dynamic (real-time) manipulation of the gas flow with a control precision that has a dramatic effect on torch operation and consumable life. An example of the precise gas flow control achieved by the invention is illustrated in  FIG. 3 . In order to achieve results such as these, it is necessary to position the programmable control valve  205  adjacent the torch, i.e., within a reasonable distance. Preliminary testing indicates the plasma gas programmable control valve  205  can be located up to 10 feet away from the torch if certain parameters, such as inside line size diameter are met. For example, for a plasma gas flowing to a plasma arc torch through a plasma gas line having an inside diameter of ⅛ of an inch at between 70 and 90 psig, adequate gas control characteristics can be achieved if the programmable control valve is located not more than 10 feet from torch. Embodiments also include locating the programmable control valve only 6 feet from the torch, and closer.  
         [0034]     As the proximity of the programmable control valve to the torch is increased, i.e., as the programmable control valve is located closer to the torch, the control dynamics of the system improve. Positioning the programmable control valve adjacent the torch permits an effective dynamic response time of the control system to be achieved, thereby reducing a time constant of the system response and allowing a precise and meaningful system response to be achieved. As can be seen from  FIG. 3 , the required speed of response for this example is significantly faster than 300 ms. The mechanical response of the programmable control valve  205  to system changes, e.g., as indicated by sensor  210 , occurs at least a plurality of times within a gas ramp (i.e., a ramp up or a ramp down) cycle (e.g., about 300 ms). This rapid mechanical valve response rate is advantageous for both increasing gas ramp cycles (ramp up) and for decreasing gas ramp cycles (ramp down). Moreover, such rapid valve response allows non-constant setpoints (e.g., flow or pressure) to be used, further enhancing the versatility and usefulness of the system. An optimized, predetermined setpoint curve can be used for a given set of cutting conditions, and the predetermined setpoint curve values can be controlled and maintained throughout the gas ramp cycle (e.g., over 300 ms) by the programmable control valve  205 . As explained above, achievement of these benefits requires that the programmable control valve  205  be positioned adjacent the torch to provide a suitably fast system response time to changes in valve position.  
         [0035]     Such positioning of the programmable control valve  205 ,  405  allows a single hose arrangement to be used for many different system configurations and consumable set combinations. This is especially advantageous for mechanized (i.e., robotic) systems and automated cutting tables. For example, the programmable control valve  205  can be manipulated to compensate for the limited volume in the hose between the programmable control valve  205  and the torch. However, due to the compressible nature of the flowing fluid, it is necessary that the programmable control valve  205  be positioned adjacent the torch  10  as described above.  
         [0036]      FIG. 3  is a chart depicting plasma gas flow control test data during startup of a plasma arc torch, in accordance with an embodiment of the invention. In this example, current flow (the curve labeled as “C”) is steadily increased from approximately 35 amps to approximately 130 amps from time zero to approximately 200 ms. Previous plasma gas flow results using on on-off solenoid valve  33  are labeled on this chart as curve P. Two different control schemes of the programmable control valve  205  are depicted. The curve labeled N shows a pressure controlled response to the torch using a PID controller and a pressure control setpoint of 85 psig. The curve labeled S illustrates a pressure controlled response using a similar PID controller, but including a step change in the pressure control setpoint. For this example, the initial pressure setpoint is 75 psig through about 350 ms, whereupon the pressure setpoint is changed to 85 psig. Other suitable control schemes will become apparent to the skilled artisan. For example, a pressure setpoint of 75 psig could be used until 250 ms followed by a step change in the setpoint value to 80 psig. At 500 ms a pressure control setpoint step change from 80 to 85 psig could be implemented. Many other setpoint control schemes are also possible, including many more step changes at varying times, or a continuously changing setpoint value over time that follows a smooth contiguous curve.  
         [0037]     As is apparent from  FIG. 3 , to maintain sufficiently precise control of the gas flow (or pressure) over the course of the gas ramp up cycle, a plurality of adjustments by the programmable control valve  205  to the gas flow are necessary. Preferably, these programmable control valve output adjustments are based on a feedback control loop using measurement signals from sensor  210 . For effective control, a plurality of programmable control valve output gas flow adjustments are made to cause the gas flow profile to match a desired, predetermined, or previously-used setpoint curve. Each programmable control valve  205  output adjustment results in a repositioning of the mechanical valve, e.g., of a valve plunger with respect to a valve seat within the valve trim section of the valve. When positioned adjacent the torch, not only can the programmable control valve  205  be used to match a setpoint curve of the form of curve P, but setpoint step changes and curves having a continuously changing setpoint value over time (such as those described above) can also be achieved. The programmable control valve is positioned adjacent the torch (as described above) to reduce the time constant of the system response, thereby enabling rapid system dynamics to achieve precise gas control. In this manner, a plurality of programmable control valve outputs to the gas flow, via a corresponding transmission of a plurality of adjustments via the mechanical valve mechanism, can be effectively achieved during a gas flow ramp cycle (e.g., within a few hundred milliseconds), resulting in the consumable life extensions and other benefits described herein. Embodiments also include only three to five programmable control valve output adjustments during a gas ramp cycle, while other embodiments include dozens or even continuous adjustments.  
         [0038]     In contrast, if the programmable control valve  205  is not located adjacent the torch, i.e., is too far from the torch, then the time constant of the system is too slow and rapid, fine valve trim adjustments of the programmable control valve  205  do not result in the fine, predictable, controllable flow required to accomplish the objectives of the invention.  
         [0039]     Moreover, the plurality of programmable control valve output adjustments described above can be used in conjunction with sensor  210 . A feedback control loop using a signal from the sensor  210 , e.g., via DSP  215 , can be used to efficiently control the gas flow. Using this technique in combination with programmable control valve  205  located adjacent the torch allows realization of unprecedented plasma torch control benefits to be achieved when the feedback loop cycles a plurality of times during a ramp cycle of the plasma (or shield) gas.  
         [0040]     In addition to the PID controller described above, other control schemes can also be used, such as a gap action controller, a hysteresis controller, and other control methods and techniques known to the skilled artisan. Moreover, sensor control parameters other than pressure can be used. For example, embodiments include using a flow sensor with a sufficiently rapid response time in place of a pressure sensor, a temperature signal from a strategic point within or about the torch, a mass flow sensor, or visual or electrical measurements (such as current flow).  
         [0041]     The invention can also be used to realize additional advantages. Now that precise flow or pressure control of the plasma gas is no longer linked to the specific hardware configuration employed, plasma gas ramp-up flow controls (e.g., during arc transfer from the nozzle to the workpiece) and/or plasma gas ramp-down flow controls (e.g., during torch shutdown) can be customized to more fully optimize the lifetime of torch electrodes, nozzles, and the like. Compensation for various nozzle orifice and shield exit port sizes, varying volumes into the plasma chamber and between the electrode and nozzle, and for trapped volumes between the torch  10  and the programmable control valve  205  can now be readily achieved. Moreover, consumable lifetime optimization can now be performed by precisely synchronizing and optimizing plasma gas flow with torch current increases and/or decreases. Such synchronization can take many forms. For example, the ramp rate of gas flow can be the same as or greater than the rate of change of the current during one portion of the cycle, and less than the rate of change of the current during another portion. Many combinations are now possible. Previous gas flow optimization results have been rudimentary in comparison with those achievable by the present invention, having been severely limited by the gas flow profiles previously obtainable. Achievable gas flow profiles have now been decoupled from the mechanical constraints of previously-used control hardware and system arrangements.  
         [0042]     The properly positioned plasma gas programmable control valve  205  can also be manipulated to achieve other objectives. For example, the function of the chambered swirl ring described in U.S. Pat. No. 5,170,033 (described above) can now be achieved using the programmable valve  205 . Whereas previously it was necessary to physically size the chamber inlet and/or outlet ports to achieve the desired swirl ring gas flow characteristics, the present invention allows similar results to be achieved using a swirl ring having no chamber. Moreover, the swirl ring ports can be oversized, and the requisite flow control can be achieved using the programmable control valve  205  and, optionally, sensor  210 . The resulting system is thus simpler and less costly to manufacture. The system can also be used to respond to plasma gas supply pressure fluctuations. This feature is particularly useful for shop operations having torches supplied from a header system that is prone to such fluctuations.  
         [0043]      FIG. 4  is a schematic representation of a shield gas supply system to a plasma arc torch incorporating features of the invention. A shield gas, e.g., including oxygen, nitrogen, hydrogen, methane, argon, and/or mixtures of these gases, flows from a shield gas supply  40  to a shield gas supply line  401 . This gas supply line channels shield gas to a space  407  between the nozzle  16  and the shield  22  through a programmable control valve  405  such as a proportional solenoid control valve. The types of valves described above in connection with the programmable control valve  205  for the plasma gas can also be used as a programmable control valve  405  for the shield gas. As described above, the programmable control valve  405  must be located adjacent the torch, e.g., within 2 feet.  
         [0044]     Preliminary testing indicates that for shield gas flow the programmable control valve  205  can be located up to 10 feet away from the torch if certain parameters, such as inside line size diameter are met. For example, for a shield gas flowing to a plasma arc torch through a gas line having an inside diameter of 3/16 of an inch at between 30 and 50 psig, adequate shield gas control characteristics can be achieved if the programmable control valve is located not more than 10 feet from torch. Embodiments also include locating the programmable control valve only 6 feet from the torch, and closer. As discussed above in connection with programmable control valve  205 , the proximity of the programmable control valve to the torch is increased, i.e., as the programmable control valve is located closer to the torch, the control dynamics of the system improve. The mechanical response of the programmable control valve  405  to system changes, e.g., as indicated by sensor  410 , must occur at least a plurality of times within a gas ramp cycle. Gas ramp cycle times for both plasma and shield gases can be about 300 ms, although longer ramp cycles, and considerably shorter ramp cycles, are sometimes used and are within the scope of the invention. As with plasma gas control, shield gas setpoints can also be variable over time having, e.g., the form of a curve, a step function, or a linear ramp through the gas ramp cycle. The programmable control valve  405  is located adjacent the torch to achieve benefits of this embodiment of the invention by providing a time constant of the shield gas response system that is sufficiently small to allow precise process control of the gas flow to be achieved. The operational requirements of the shield gas programmable control valve  405  are similar to those of the plasma gas programmable control valve  205  described above, in that a plurality of mechanical output adjustments by the programmable control valve  405  to the gas flow during the gas ramp cycle are required to achieve the objectives of the invention.  
         [0045]     After passing through the programmable control valve  405  the shield gas can be routed through a sensor  410 . An output  418  from a DSP  215  can be used to manipulate the programmable control valve  405  based on a signal  412  from the sensor  410 . Sensor types and control strategies described above for use with the plasma gas supply system can also be used with the shield gas system.  
         [0046]     Additional control objectives can be achieved using the shield gas control system. For example, torch performance is improved by appropriate shield gas control during piercing and cutting operations as taught in U.S. Pat. No. 5,396,043, described above. Such objectives can be readily obtained using the present invention, but using fewer components and gas lines than previously required. For example, the single shield gas supply line  401  of the present invention can be used to provide the pre-flow, operating flow, and quick charge flows described in the &#39;043 patent. Moreover, positioning the programmable control valve  405  near the torch  10  allows it to be used to manipulate the shield gas flow to provide both the piercing gas flow and the cutting gas flow at different times, as described in the &#39;043 patent. For example, during workpiece piercing operation (e.g., at the commencement of a cut) rapid and strategic increases in shield gas flow can be used to reduce slag formation and slag blowback to the torch shield  22 .  
         [0047]     Improved control of divot formation can also be obtained.  FIG. 5  illustrates divot formation results achievable with an embodiment of the invention. When creating an interior cut within a workpiece divots and dross are commonly formed, especially at the end of the cut. A divot is formed when the end of the cut perimeter is returned, e.g., to the starting point.  FIG. 5  illustrates the perimeter of an interior circular cut  505  within a workpiece  510 . In some situations an interior divot A can be formed when the start and end point of the cut is at A. In other situations an exterior divot B can be formed, when the start and end point of the cut is at B. Preferably, in this example, a circular interior cut  505  would be achieved resulting a circular hole without the formation of any divots (i.e., without A or B).  
         [0048]     Different techniques can be used to achieve a divot-free cut. For example, withdrawing the torch from the workpiece surface while still in full operating mode will reduce divot and dross formation, but results in premature damage to the torch electrode. Alternatively, the current flow can be slammed off (immediately truncated) at the end of the cut, but this also results in premature electrode failure. Another technique involves using a controlled ramp down of the shield gas flow, along with torch current ramp down flow management, to achieve a divot-free cut. The invention allows unprecedented benefits of this technique to be maximized. Embodiments of the invention also provide for cut optimization when performing interior corner cuts (not shown). Controlled reduction of shield gas and current flow as the torch makes an interior corner cut results in improved corner cut quality.  
         [0049]     Additional advantages can be realized utilizing the invention. For example, torch operation can be optimized to accommodate different end cut requirements, such as for small holes and other specialized workpiece cut designs and features. Moreover, the ability to precisely and independently control shield gas flow and torch current allows customization to be performed to better accommodate different workpiece materials and thicknesses.  
         [0050]      FIG. 6  is a schematic representation of a combined plasma gas and shield gas supply system to a plasma arc torch. A plasma gas programmable control valve  205 , such as a proportional solenoid control valve, is positioned adjacent the torch  10  in the plasma gas supply line  201 . A shield gas programmable control valve  405 , such as a proportional solenoid control valve, is positioned adjacent the torch  10  in the shield gas supply line  401 . Optionally, a sensor  210  is present in the plasma gas line  201  and provides a control signal  212  to a DSP  215 . Also optionally, a sensor  410  is present in the shield gas supply line  401  and provides a control signal  412  to DSP  215 . The sensors  210 ,  410  can measure different types of physical parameters, such as flow, pressure, and others, such as those described above.  
         [0051]     In addition to advantages described above, this embodiment of the invention allows additional advantages to be realized. For example, precise and dynamic real-time control of three independent variables is now possible-torch current, plasma gas flow, and shield gas flow. Manipulation of these variables allows unprecedented optimization of cut cycle times since faster ramp-up and ramp-down times can now be achieved without sacrificing consumable life. In many operations, and especially in automated mechanized operations, workpiece cut cycle times can be further optimized in view of, e.g., electrode and nozzle life. This embodiment of the invention allows processing time to be reduced using plasma gas ramp-up and ramp-down controls in conjunction with precise current controls, along with the time saving benefits achieved by faster and better shield gas piercing and cutting controls. A more precise optimization between cut cycle time and consumables life expectancy can now be achieved, resulting in greater productivity, more efficient utilization of manufacturing equipment and resources, and increased cost savings or throughput. Of course, the divot and dross minimization benefits described above can also be achieved.  
         [0052]      FIG. 7  is a more detailed representation of a combined plasma gas and shield gas supply system to a plasma arc torch that provides enhanced operating flexibility. This figure illustrates how seven different supply gases can be efficiently incorporated into a system providing two cut gases  701 ,  702 , each cut gas being available to provide plasma gas and/or shield gas.  
         [0053]     For example, different plasma gas compositions are preferred when cutting different workpiece materials or thicknesses.  FIG. 7  illustrates a system that can supply H5 (5% hydrogen content, 95% argon) via on-off solenoid valve  710 , H35 (35% hydrogen content, 65% argon) via on-off solenoid valve  711 , F5 (5% hydrogen content, 95% nitrogen) via on-off solenoid valve  712 , and/or methane (CH4) via on-off solenoid valve  713  as a first cut gas  701 , such as a plasma gas. Nitrogen (N2) as a first cut gas can also be supplied via on-off solenoid valve  714  and air via on-off solenoid valve  715 . A check valve  703  can be included in the first cut gas line. This first cut gas  701  can be channeled through a plasma gas programmable control valve  205  positioned adjacent the plasma torch, and through sensor  210  as described above.  
         [0054]     Nitrogen can also be supplied as a second cut gas  702  via on-off solenoid valve  720 , air via on-off solenoid valve  721 , and O2 (oxygen) via on-off solenoid valve  722 . A check valve  704  can be included in the second cut gas line. As illustrated, the second cut gas can be used as a shield gas, which can pass through a shield gas programmable control valve  405  positioned adjacent the plasma arc torch, and through sensor  410  as described above. Although not shown, a DSP  215  can be used to manipulate, e.g., the programmable control valves  205 ,  205 A,  405 , and  405 A, and/or some or all of the on-off solenoid valves  710 ,  711 ,  712 ,  713 ,  714 ,  715 ,  720 ,  721 ,  722 , and  730 .  
         [0055]     This embodiment also features crossover lines  730  and  732 . Crossover line  730  allows the first cut gas  701  to be used as a shield gas; crossover line  732  allows the second cut gas  702  to be used as a plasma gas, as illustrated. Each of the crossover lines includes a programmable control valve  205 A,  405 A, such as a proportional solenoid control valve, which is used to precisely control the amount of crossover gas flow. For embodiments incorporating this crossover flow functionality, it is important that any crossover programmable control valve  205 A,  405 A be positioned adjacent the plasma arc torch. Crossover gas flow can be used to augment or replace other gases already flowing in the system.  
         [0056]     Also illustrated in  FIG. 7  is an optional vent valve  730  that can be used to vent plasma gas to the atmosphere (ATM). Vent valve  730  can be an on-off solenoid valve. In some embodiments (not shown) vent valve  730  can be a programmable control valve. This embodiment is particularly useful when cutting conditions are encountered in which it is desirable to decrease the flow rate of plasma gas flowing to the plasma chamber  207  more rapidly than is possible with only the plasma gas programmable control valve(s)  205 ,  205 A.  
         [0057]     Of course, other combinations are possible. Gas supplies other than those illustrated can be used, and various techniques known to those of skill in the art can be used, e.g., to mix different gas sources. For example, a plasma gas mixture can be formed by mixing methane and H35 using techniques and instrumentation (not shown) known to the skilled artisan. In addition to the mixing techniques described above using crossover lines  730  and  732 , additional gas supplies can be added adjacent the torch and these additional gas supplies can be controlled using programmable control valves. An embodiment includes adding a reducing gas stream (such as methane) to the shield gas between programmable control valve  405  and sensor  410 , such that the flow of the reducing gas is controlled with a programmable control valve. More than one additional gas (such as the reducing gas stream) can be added in this manner. Similar techniques can be used for the plasma gas. Many other arrangements and combinations are also possible.  
         [0058]     While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.