Abstract:
Methods and systems for transferring a plasma arc from between an electrode and a tip to between an electrode and a workpiece and back as dictated by the conditions at the cutting arc are provided. The present disclosure allows for arc transfer detection without use of a current sensor at the workpiece or knowledge of a precise pilot circuit limit value through a novel plasma arc control circuit. In one embodiment, the plasma arc control circuit provides a programmable current source and a current sink configured to limit current in a pilot arc control circuit. The pilot arc circuit may be configured to signal its limiting status to a controller, which may switch the pilot arc control circuit in or out of the current path. Certain embodiments may include a pulse width modulation control in the pilot arc control circuit for controlling current flow through the pilot arc circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Non-Provisional Application of U.S. Provisional Patent Application No. 61/036,530, entitled “Method for Detecting Current Transfer in a Plasma Arc”, filed Mar. 14, 2008, which is herein incorporated by reference. 
     BACKGROUND 
     The invention relates generally to metal cutting systems, and more particularly, to systems and methods for forming a first plasma arc between an electrode and a tip of a plasma cutter then transferring that arc such that it forms a second plasma arc between the electrode and the work lead. 
     A plasma cutting system harnesses the energy in plasma (e.g., high temperature ionized gas) to cut metal or other electrically conductive material. Prior to cutting, the first plasma arc, the pilot arc, is struck between the negatively charged electrode and the tip of the plasma cutter. The arc must then be transferred to the work piece to initiate cutting. The tip to work potential determines the favorability of the plasma shift from the tip to the workpiece and thus the transfer height (i.e., the height at which the pilot arc will transfer and become the cutting arc) of the system. Since a large transfer height is desirable, multiple methods, such as the placement of resistors in series with the pilot switch, are currently employed to increase the tip to work potential. However, these methods fail to maximize transfer height and often lead to lossy circuits. 
     After a pilot arc has been established, it is necessary to detect that current will readily flow to the work piece so that cutting current can be applied and the pilot circuit can be disabled. Since the arc transfer is a critical step in the initiation of plasma cutting, this requires a precise and accurate measurement technique. Traditionally, a work current sensor, such as a Hall-based current sensor, is connected to the work lead to measure the current in the work lead prior to transfer. However, it is now recognized that these sensors are costly and comprise a large portion of the overall machine cost. Accordingly, it is now recognized that there exists a need for plasma cutting systems equipped to maximize transfer heights and tip to work potential while minimizing cost. 
     BRIEF DESCRIPTION 
     The present disclosure is directed to systems and methods relating to a plasma arc control circuit. One embodiment of the present disclosure relates to arc transfer detection without use of a current sensor in the work lead or knowledge of a precise pilot circuit limit value. In particular, the present disclosure provides methods and systems for transferring a plasma arc from between an electrode and a tip to between an electrode and a workpiece and back as dictated by the conditions at the cutting arc. In one embodiment, the plasma arc control circuit provides a programmable current source and a current sink configured to limit current in a pilot arc control circuit. The pilot arc circuit may be configured to signal its limiting status to a controller, which may switch the pilot arc control circuit in or out of the current path. Certain embodiments may include a pulse width modulation control in the pilot arc control circuit for controlling current flow through the pilot arc circuit. 
    
    
     
       DRAWINGS 
       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: 
         FIG. 1  is a perspective view of an exemplary plasma cutting power supply unit in accordance with aspects of the present disclosure; 
         FIG. 2  is a circuit diagram illustrating an exemplary embodiment of the power supply circuitry in accordance with aspects of the present disclosure; 
         FIG. 3  is a circuit diagram illustrating one embodiment of the power supply circuitry in accordance with aspects of the present disclosure; 
         FIG. 4  is a block diagram illustrating exemplary processing logic that may be used to control the current source output and the pilot control circuitry in accordance with aspects of the present disclosure; 
         FIG. 5  is a block diagram illustrating exemplary logic that may be used to establish the pilot arc and the cutting arc in accordance with aspects of the present disclosure; 
         FIG. 6  is a graphical representation of exemplary current waveforms through the tip, the electrode, and the work piece during cutting arc initiation in accordance with aspects of the present disclosure; 
         FIG. 7  is a graphical representation of exemplary voltage potential waveforms during cutting arc initiation in accordance with aspects of the present disclosure; 
         FIG. 8  is a block diagram illustrating exemplary logic that may be used to transfer the cutting arc back to the pilot arc in accordance with aspects of the present disclosure; 
         FIG. 9  is a graphical representation of exemplary current waveforms through the tip, the electrode, and the work piece during transfer back to the pilot arc from the cutting arc in accordance with aspects of the present disclosure; and 
         FIG. 10  is a graphical representation of exemplary voltage potential waveforms during transfer back to the pilot arc from the cutting arc in accordance with aspects of the present disclosure; 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary plasma cutting power supply  10 , which powers, controls, and provides consumables to a cutting operation in accordance with aspects of the present disclosure. A torch  14  and a work lead clamp  16  are communicatively coupled to the power supply unit  10  and may be utilized to perform cutting operations. The front side of the power supply unit  10  in the illustrated embodiment contains a control panel  12 , through which a user may control the supply of materials, such as power, gas flow, and so forth, to the cutting torch  14 . The work lead clamp  16  typically connects to a workpiece to close the circuit between the torch  14 , the work piece, and the supply unit  10 , and to ensure proper current flow. The portability of the unit  10  depends on a handle  18 , which enables the user to move the power supply unit  10  to the location of the workpiece. 
     Internal components of the power supply unit  10  receive power from a wall outlet, a generator, a battery, or the like and then supply power to circuitry that enables the formation of plasma arcs necessary for the plasma cutting operation.  FIG. 2  illustrates an arc control circuit  20  that controls the formation of the plasma arcs between a workpiece  22 , a torch tip  24 , and an electrode  26  by controlling current flow through circuit components. A first current path through a pilot arc circuit may be established when a programmable current source  28  outputs a first current  30  that flows through a node  32  into a current regulator  34 , which is capable of programmable switching. The current regulator  34  outputs a second current  36  (e.g., pulse width modulated current). In one embodiment, the pulse width modulation of the second current  36  is hysteretic, wherein hysteretic means maintaining current between a lower limit and an upper limit. The second current  36  flows through the tip  24  and the electrode  26 , establishing a pilot arc  38  between the tip  24  and the electrode  26 . The second current  36  then returns to the current source  28 , completing the first current path through the components that define the pilot arc circuit. A second current path through a cutting arc circuit may be established when the programmable current source  28  outputs the first current  30  that flows through a node  32  into the workpiece  22  and the electrode  26 , establishing a cutting arc  40  between the workpiece  22  and the electrode  26 . The first current  30  then returns to the current source  28 , completing the second current path through the components that define the cutting arc circuit. The amount and path of the current flow through the components of the arc control circuit  20  define a voltage potential  42  between the electrode  26  and the tip  24 , a voltage potential  44  between the tip  24  and the workpiece  22 , and a voltage potential  46  between the electrode and the workpiece  46 . 
     In one embodiment, the arc control circuit  20  achieves a current limit through the pilot arc circuit by employing a chopper switch in the current regulator  34 . In one embodiment, the current source  28  is configured to programmably provide a range of output currents limited only by its rated output voltage. The current regulator  34  may comprise a fixed current limiter and may be switched in or out of the active circuit to achieve the current limit through the pilot arc circuit. The chopper switch in the current regulator  34  may be kept in an ON state as long as the pilot current is less than a preset level, where the ON state may be defined by a closed switch position that allows current flow through the pilot arc circuit, establishing the pilot arc  38 . If the current exceeds the preset level, the chopper switch toggles to an OFF state, where the OFF state is defined by an open switch position that prohibits additional current flow through the pilot arc circuit. When the switch is in an OFF state, the current decays to a lower limit at which point the switch closes to an ON state to maintain the pilot arc  38 . In this way, the current regulator  34  switches ON and OFF to control the current through the pilot arc circuit and maintain the pilot arc  38 . 
     After a current flow and a pilot arc  38  have been established in the pilot arc circuit, the setpoint of the current source  28  can be incrementally increased until the chopper switch in the current regulator  34  starts to switch ON and OFF to maintain a preset level of current in the pilot arc circuit by limiting the amount of current allowed to flow from the current source  28  through the pilot arc circuit. In one embodiment, when the current source  28  receives feedback indicating that limiting is occurring, no substantial transfer of the pilot arc  38  to the workpiece  22  is occurring. However, when the chopper switch in the current regulator  34  stays ON as the current  30  from the current source  28  is increased, this indicates that transfer of the pilot arc  38  to the workpiece  22  is occurring since the current  30  from the source  28  is configured to flow either to the workpiece  22  or to the current regulator  34  when exiting the node  32 . The current regulator  34  may be left in the current path while the current output  30  is increased to a preset level without the pilot arc circuit going into limit. At this point, the cutting arc  40  is established between the electrode  26  and the workpiece  22 , the current regulator  34  may be removed from the current path, and cutting may occur. During the plasma cutting operation, if imminent cutting arc  40  outage is detected, the current regulator  34  may be placed back in the current path, re-enabling current flow through the pilot arc circuit and reestablishing the pilot arc  38 . 
     The combined use of the pilot arc circuit with the cutting arc circuit in accordance with aspects of the present disclosure offers distinct benefits. For instance, there is no need for a current sensor at the workpiece  22  for detection of arc transfer to the workpiece  22 . The exact preset current limit in the current regulator  34  need not be known. Instead, when no arc transfer from the pilot arc  38  to the cutting arc  40  occurs, the programmable current output  30  from the current source  28  may be manipulated to find a threshold for the limit value of the current regulator  34 . Additionally, any time the current regulator  34  is in limit, an improved voltage potential  44  is established between the tip  24  and the workpiece  22 , leading to an advantageous transfer height. 
       FIG. 3  illustrates one embodiment of the arc control circuit shown in  FIG. 2 . In this embodiment, a first current path through the pilot arc circuit may be established when the programmable current source  28  outputs the first current  30  that flows through the node  32 , a first transistor switch  48 , a first current sensor  50 , a first inductor  52 , the tip  24 , and the electrode  26 , forming the pilot arc  38  between the tip  24  and the electrode  26  with the resulting second current  36  (i.e., pulse width modulated current). The current  36  then passes through a second current sensor  54 , which provides a feedback signal  56  to the current source  28 . The current continues through a second inductor  58  to return to the current source  28 , thus completing the first current path through the pilot arc circuit. The pilot arc circuit also contains a diode  60  and a second transistor switch  62 , which breaks the path through the diode  60 . The diode  60  and the first transistor switch  48  combine with the first inductor  52  to form and function as a buck converter. The intrinsic property of the first inductor  52  that attempts to keep current flow constant is exploited. A current feedback signal  64  from the first current sensor  50  communicates with a pulse width modulation control  66 , which switches the first transistor  48  ON and OFF to maintain the pilot arc  38 . When the first transistor  48  is ON (i.e. in a closed position), the first inductor  52  resists increases in current flow and energy builds in the first inductor  52 . When the first transistor switch is OFF (i.e. in an open position), the current through the pilot arc circuit is forced by the first inductor  52  to freewheel through the tip  24 , the electrode  26 , and the second inductor  58 , up through the diode  60  and the second transistor  62 , and back through the first inductor  52 . In this way, the buck converter (i.e. the first transistor  48 , the diode  60 , first inductor  52 , and the second inductor  58 ) controls the current flow in the tip  24 , preventing current overshoots and subsequent tip  24  damage. As previously described with respect to  FIG. 2 , a second current path through a cutting arc circuit may be established when the programmable current source  28  outputs a first current  30  that flows through a node  32  into the workpiece  22  and the electrode  26 , establishing a cutting arc  40  between the workpiece  22  and the electrode  26 . The first current  30  then returns to the current source  28  through a second current sensor  54  and a second inductor  58 , completing the second current path through the components that define the cutting arc circuit. A first capacitor  68 , a second capacitor  70 , and a ground  72  minimize circuit noise. Additionally, the first capacitor  68  and the second capacitor  70  may provide a high speed path for current flow when the first transistor  48  is switching. 
       FIG. 4  is a block diagram illustrating exemplary processing logic that may be used to control the functioning of the arc control circuit  20  by controlling the current source output  30  and the pilot control circuitry. In the illustrated embodiment, a controller  74  comprises a pilot controller  76 , a main controller  78 , and a processor  80 , which receive feedback signals from and deliver commands to the plasma cutting operation. The pilot controller  76  and the main controller  78  may comprise software, memory, circuitry, and so forth. The pilot controller  76  may receive signals regarding the functioning of the pilot arc circuit, such as a limiting status  82  of the current regulator  34 , and output a control signal  84  based on its inputs. Similarly, the main controller  78  may receive signals regarding the current source  28 , such as a level of the current output  86 , and output a control signal  88  based on its inputs. The processor  80  receives the control signals  84 ,  88  from the pilot controller  76  and the main controller  78  and integrates the information with any additional auxiliary input signals  90 . The processor then generates output control signals that drive the operation of the arc control circuit  20 . The pilot arc circuit is controlled by a signal  94  from the processor  80  that enables or disables the pilot control circuit. A signal  96  from the processor  80  drives the increase or decrease of output current from the current source  28 . Additionally, the processor may output one or more auxiliary signals  98  that drive peripheral functions related to the plasma cutting operation. 
       FIG. 5  is a block diagram illustrating exemplary logic behind one embodiment of the present disclosure that may be used to establish the pilot arc  38  and the cutting arc  40 . Each block in  FIG. 5  may represent a function or step. First, in the illustrated embodiment, the controller  74  initiates the arc start, as represented by block  100 , and enables the pilot control circuitry, as represented by block  102 . Initially, the main controller  78  outputs a signal  88  that commands a low output current level, as represented by block  104 . The processor then outputs an auxiliary control signal  98  to enable torch pressure, as represented by block  106 . Feedback regarding whether or not the pilot control is limiting is then sought from the arc control circuit  20 , as represented by block  108 . If the pilot control is limiting, the controller  74  maintains a constant output current level. If the pilot control is not limiting, the main controller  78  outputs a control signal  88  that incrementally increases the current output of the current source  28 , as represented by block  110 . Feedback regarding whether the main controller  78  has reached a defined current setpoint is then sought, as represented by block  112 . If the main controller  78  has not reached the defined setpoint, feedback is once again sought regarding whether or not the pilot control is limiting, as represented by block  108 . If the main controller  78  has reached the defined setpoint, current is flowing through the work piece  22 , the pilot controller  76  is disabled, and the main control ramps up the current output to a level sufficient for the plasma cutting operation, as represented by block  114 . At this point, as represented by block  116 , the cutting arc  40  is cutting. The described pilot-to-work transfer, as represented by block  118 , illustrates the logic behind the circuit illustrated in  FIG. 2 . 
       FIGS. 6 and 7  illustrate exemplary current and voltage potential waveforms, respectively, from when the pilot arc  38  is initially struck until arc transfer from between the electrode  26  and the tip  24  to between the electrode  26  and the workpiece  22 .  FIG. 6  illustrates a tip current waveform  120 , an electrode current waveform  122 , and a work current waveform  124 .  FIG. 7  illustrates a work-electrode potential waveform  126 , a tip-electrode potential waveform  128 , and a work-tip potential waveform  130 . The arc control circuit  20  begins operation at an initial start time  132 , beginning current flow through the tip  24  and the electrode  26 . At a later time  134 , air flow through the welding system begins, giving rise to a work-electrode potential and a tip-electrode potential. Subsequently, at a time  136 , the pilot circuit goes into limit, leveling out current flow through the tip  24  and giving rise to an increase in the work-electrode potential and an initiation of a tip-work potential. At a later time  138 , tip current decreases while work current increases due to a diversion of current from the tip  24  to the workpiece  22 . At this time  138 , the work-electrode potential decreases to a new steady state value, and the tip-work potential falls back to zero. Subsequently, at a time  140 , the pilot circuit is disengaged by the controller  74 , triggering a falloff of tip current down to zero and a corresponding increase in work current. 
       FIG. 8  is a block diagram illustrating exemplary logic behind one embodiment of the present disclosure that may be used to transfer the cutting arc  40  back from between the workpiece  22  and the electrode  26  to between the tip  24  and the electrode  26  during instances when an imminent arc outage may be detected. In this diagram, the logic  118  behind the pilot-to-work arc transfer remains the same with respect to  FIG. 5 . However, once the cutting arc is cutting, as represented by block  116 , a feedback signal regarding whether or not arc outage is imminent is sought, as represented by block  142 . If arc outage is not imminent, cutting continues, as represented by block  116 . If arc outage is imminent, the pilot controller is enabled, as represented by block  102 , to reestablish the pilot control circuitry as part of the current path. The main controller  78  outputs a control signal  88  to incrementally reduce the output current, as represented by block  144 . A feedback signal is then sought from the arc control circuit  20  regarding whether or not the pilot control is limiting, as represented by block  108 . If the pilot control is limiting, the main controller  78  outputs a control signal  88  to incrementally reduce the output current, as represented by block  144 . If the pilot control is not limiting, the pilot to work transfer logic represented by block  118  is employed to once again transfer the pilot arc  38  from between the electrode  26  and the tip  24  to between the electrode  26  and the workpiece  22 . 
       FIGS. 9 and 10  illustrate exemplary current and voltage potential waveforms, respectively, from when the cutting arc  40  is cutting to when cutting arc outage is imminent to when the pilot arc  38  is reestablished.  FIG. 9  illustrates a tip current waveform  146 , an electrode current waveform  148 , and a work current waveform  150 .  FIG. 10  illustrates a work-electrode potential waveform  152 , a tip-electrode potential waveform  154 , and a work-tip potential waveform  156 . Initially, the cutting current is established and flowing through the electrode and the workpiece and a work-electrode and a tip-electrode potential exist as indicated by arrow  158 . However, when an arc outage becomes imminent, as indicated by an increase in the work-electrode and the tip-electrode potentials designated by arrow  160 , the pilot circuit is reengaged, as indicated by arrow  162 , to prevent loss of the plasma arc. When the pilot circuit is reengaged, as indicated by arrow  162 , the tip current increases until the pilot circuit goes into limit and the tip current becomes limited, as indicated by arrow  164 . Additionally, when the pilot circuit goes into limit, the work-electrode potential and the tip-electrode potential decrease from a peak while the main output ramps down, as indicated by arrow  166  and the electrode current spikes downward. For a short time duration, the output current briefly undershoots the tip limit, as indicated by arrow  168 . When the pilot circuit is back in limit, the work-electrode and the tip-electrode potential fall back to a steady state value. 
     While only certain features of the present disclosure 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 present disclosure.