Patent Publication Number: US-6670572-B2

Title: Solenoid control and safety circuit system and method

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to plasma-arc torch systems and power supplies. In particular, the invention relates to systems, circuits, and methods for controlling contact starting and operating plasma-arc torches, including controlling a gas control solenoid, a power supply, and contact start elements. 
     Plasma-arc torches, also known as electric arc torches, are commonly used for cutting, welding, and spray bonding workpieces. Such torches typically operate by directing a plasma consisting of ionized gas particles toward a workpiece. An example of a conventional gas plasma-arc torch is disclosed in U.S. Pat. No. 3,813,510, the entire disclosure of which is incorporated herein by reference. 
     In general, a pressurized gas to be ionized is supplied to the front end of the torch (also referred to as the torch head) and flows past an electrode before exiting through an orifice in a torch tip. The electrode has a relatively negative potential and operates as a cathode. The torch tip, which is adjacent the electrode at the front end of the torch, constitutes a relatively positive potential anode. When a sufficiently high magnitude voltage is applied to the electrode, an arc is established across the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as a flame extending externally from the tip. The arc so established is commonly referred to as a pilot arc. A typical pilot arc circuit may provide, for example, 5-50 amps, at 100-200 volts across the electrode to tip gap. 
     Plasma-arc torches may be found in both “non-contact start” and “contact start” varieties. In non-contact start torches, the tip and electrode are normally maintained at a fixed physical separation in the torch head. Typically, a high voltage high frequency signal (HVHF) is applied to the electrode (relative to the tip) to establish a pilot arc between the electrode and the tip. This may be referred to as HF starting. HF starting generally requires additional circuitry that can cause undesirable electromagnetic interference (EMI) conditions. Regardless of how a pilot arc is established, when the torch head is moved toward the workpiece, the arc transfers to the workpiece-assuming a conductive (e.g., metal) workpiece that is connected to the positive return. 
     In a typical contact start torch, the tip and/or electrode make electrical contact with each other (e.g., along a longitudinal axis of the electrode). For example, a spring or other mechanical means may be used to bias the tip and/or electrode such that the tip and electrode are normally in electrical contact when gas is not flowing. When the operator squeezes the torch trigger (also referred to as a torch activation switch), a voltage is applied to the electrode and pressurized gas (the plasma gas and/or a secondary gas) flows. The gas causes the tip and electrode to overcome the bias and physically separate. As the tip and electrode separate, a pilot arc is established therebetween. 
     There are several ways, mechanically speaking, to create the electrical contact necessary to employ a contact starting process. For example, a fixed electrode and translatable tip configuration is possible. In such a configuration, a spring or other means biases the tip into contact with the electrode. When a gas control solenoid opens and supplies plasma and/or secondary gas, the gas flow overcomes the bias force and separates the tip from the electrode, thereby establishing a pilot arc. This configuration is typically referred to as a blow forward contact start torch. Another example involves a fixed tip and translatable electrode that is biased into electrical contact with the tip. In such a configuration, the flow of plasma and/or secondary gas overcomes the bias and separates the electrode from the tip to establish the pilot arc. This configuration is typically referred to as a blow back contact start torch. Both of these exemplary configurations may be referred to as blow apart torches because they employ gas pressure to separate the tip and electrode during the contact start process. Mechanical and/or electromechanical contact starting means are also possible. 
     Commonly owned U.S. patent application Ser. No. Ser. No. 09/724984, filed Nov. 28, 2000, the entire disclosure of which is incorporated herein by reference, describes contact start torch operations in the context of a circuit and method for ensuring that the parts of a contact start plasma-arc torch are properly in place before allowing the output voltage to ramp up to its final value. Commonly owned U.S. Pat. No. 5,961,855, the entire disclosure of which is incorporated herein by reference, describes a contact start torch in context of a low-voltage source for conducting a parts-in-place check. 
     In order to use a plasma-arc torch with a workpiece, a main or cutting arc must normally be established between the electrode and the workpiece. As the torch head or front end is brought toward the workpiece, the arc transfers between the electrode and the workpiece because the impedance of the workpiece to negative is typically lower than the impedance of the torch tip to negative. During this “transferred arc” operation, the workpiece serves as the anode. 
     Once the arc transfer is sensed, it is generally preferred to cease current flow between the electrode and the tip. One method of terminating current flow between the electrode and the tip is to open circuit the pilot arc current path. This may be accomplished by sensing the presence of a current flowing in the workpiece and open circuiting a switch between the tip and ground (positive return). Commonly owned U.S. Pat. Nos. 5,170,030, and 5,530,220, the entire disclosures of which are incorporated herein by reference, describe an arc transfer process. 
     After arc transfer occurs, the output current is typically increased to a higher, cutting level. The power supply preferably is current controlled so that the cutting current is maintained at or near a constant current level. If the transferred arc is stretched beyond the capacity of the power supply it can extinguish. The arc may stretch, for example, when cutting a discontinuous workpiece (e.g., a metal grate), when cutting near the end of a workpiece, or when the torch is moved away from the workpiece. Once the arc has been extinguished, the torch starting process must typically be repeated. As can be appreciated, it is often desirable to restart the torch as quickly as possible. Commonly owned U.S. patent application Ser. No. 09/870,272, filed May 30, 2001, the entire disclosure of which is incorporated herein by reference, describes systems and methods for re-attaching the pilot arc before the transferred arc completely extinguishes, thereby reducing the likelihood of having to restart the torch. 
     In plasma arc torch systems employing HF starting, plasma and/or secondary gasses are usually turned on and allowed to run for a brief time before striking an arc. This allows the flow to reach a maximum level before the pilot arc is ignited. A gas pressure switch may be positioned between a gas control solenoid and the torch head to prevent pilot arc ignition until sufficient pressure is sensed, thereby ensuring the availability of plasma gas and that the solenoid properly opened. In prior art contact start torch systems, however, where gas pressure may be necessary to separate the tip and electrode, the gas control solenoid is normally opened at substantially the same time that a DC voltage is applied to the tip and electrode in the starting process. Accordingly, the pressure switch arrangement employed in HF starting systems does not provide an indication that the solenoid has operated properly (and allowed gas flow) before voltage is applied to the torch parts. 
     FIG. 1 illustrates a prior art contact start process. In particular, FIG. 1 illustrates the contact start process associated with a prior art blow apart torch. Blocks  102 - 112  reflect the contact start process up to the point at which the electrode and tip make contact with power applied. Thereafter, at block  114 , a time delay accounts for the expected worst-case time for the electrode and tip to make contact in a blow apart torch. Blocks  116 - 126  generally reflect normal torch operations after a pilot arc is established during the contact start process. 
     As can be appreciated, the prior art time delay safety margin approach (block  114 ) is less than optimal and will slow down operation of a torch that is capable of restarting faster than the predefined worst-case time. For example, if an operator pulls the torch trigger, a burst of air separates the parts. If the operator thereafter releases the trigger (e.g., because an arc was not established or for other reasons) it takes a finite amount of time for the air pressure to dissipate and allow the parts to come back into contact. If the operator pulls the trigger before the parts come together, the system will fail to arc (there was no contact to execute the contact start process). In prior art systems, it may require more than one second for a fifty foot hose to bleed sufficient air to allow the parts to come back into contact. Thus, such systems require a delay of about 1.5 seconds or more to account for this delay. Such a delay is inefficient in torch systems exhibiting a shorter dissipation time (e.g., because of a shorter hose). Similarly, if a particular torch requires more time than the expected worst case time, such prior art systems may prevent that torch from restarting at all because they would allow the torch operator to initiate a start process before the parts return into contact. 
     It is also known that a failure of the gas control solenoid can prevent the flow of gas. For example, if the gas control solenoid fails to allow gas flow to the torch during the starting process (e.g., because of a component failure of an obstruction in the solenoid), DC voltage may be applied to both the tip and electrode. These elements, however, will not separate if air flow is required to blow the components apart. In some prior art systems, this voltage can exceed −48 VDC (plasma arc torch systems generally operate on a negative voltage basis; the voltage applied at the electrode is negative and the positive power supply output is connected to ground). This can occur in prior art torch systems that employ a pilot resistor (e.g., 1 ohm) between the tip and the positive ground. Further, the tip may be exposed so that the voltage applied to it is likewise exposed. 
     Another possible problem in blow forward torches can occur during transferred arc operations. If the torch tip is pushed into contact with the workpiece with sufficient force to overcome the gas pressure that normally keeps the tip and electrode separated (i.e., the tip is forced into contact with the electrode) the arc could extinguish but current would continue to flow. 
     For these reasons, an improved contact start plasma-arc torch system is desired. Such an improved plasma-arc torch system benefits from an improved solenoid control circuit and method that improves the efficiency of the contact start process. Also, a torch system is desired that provides benefits from an improved circuit and method for ensuring that the contact start elements separate as expected upon the presence of flowing gas. Such an improved system allows greater efficiency, for example, by improving reliability and restart capabilities. 
     SUMMARY OF THE INVENTION 
     The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved contact start plasma-arc torch system and method. In one aspect, the invention relates to a method of operating a contact-start plasma arc torch system. The torch system includes a torch activation switch indicating desired operational states of the torch, a power supply having first and second output terminals and selectively supplying an output voltage therebetween, a plurality of contact start elements, with a first one of the plurality of contact start elements being in selective electrical communication with the first output terminal, and with a second one of the plurality of contact start elements being in selective electrical communication with the second output terminal. The method includes monitoring the torch activation switch and determining if the desired operational state of the torch transitions from an off state to an operating state. The method also includes causing a condition allowing a closed electrical circuit to be established between the plurality of contact start elements. The method further includes sensing a closed electrical circuit condition between the plurality of contact start elements. The method also includes opening a gas control switch and providing a flowing gas to be ionized upon sensing the closed electrical circuit condition between the plurality of contact start elements. The method further includes sensing an open electrical circuit condition between the plurality of contact start elements after opening the gas control switch, closing the gas control switch and removing the flowing gas if the open electrical circuit condition is sensed between the plurality of contact start elements after the flowing gas has been provided. 
     In another aspect, the invention relates to a method of operating a contact-start plasma arc torch system. The torch system includes a torch actuation switch indicating desired operational states of the torch, a power supply having first and second output terminals and selectively supplying an output voltage therebetween, a plurality of contact start elements, with a first one of the plurality of contact start elements being in selective electrical communication with the first output terminal, and with a second one of the plurality of contact start elements being in selective electrical communication with the second output terminal. The method includes monitoring the torch activation switch and determining if the desired operational state of the torch transitions from an off state to an operating state. The method further includes closing a gas control switch to allow the plurality of contact start elements to form a closed electrical circuit between said plurality of contact start elements. The method also includes monitoring the output voltage and comparing the output voltage to a low voltage threshold. The method further includes opening the gas control switch and providing a flowing gas to be ionized if the output voltage is less than the low voltage threshold. The method also includes comparing the output voltage to a high voltage threshold after opening the gas control switch and thereafter closing the gas control switch and removing the flowing gas if the output voltage is greater than the high voltage threshold. 
     In still another aspect, the invention relates to a method of operating a contact-start plasma arc torch system. Such a torch system includes a torch activation switch having a first state and a second state, a power supply responsive to the state of the torch actuation switch and selectively providing an output voltage, and a solenoid selectively allowing a gas to be ionized to flow into the torch. The method includes determining the state of the torch actuation switch; determining a value indicative of the output voltage; determining the position of the solenoid; opening the solenoid if the torch actuation switch is in the second state and the solenoid is closed; and closing the solenoid if the torch actuation switch is in the second state and the solenoid is open and the output voltage is greater than an open circuit threshold. 
     In yet another aspect, the invention relates to a contact start plasma-arc torch system for use by a torch operator in connection with a workpiece. The torch system includes a power source for selectively providing an output voltage. The output voltage transitions from a first value to a second value when a contact start operation is initiated. A torch head includes an electrode and a tip. The electrode is positioned in a circuit path with the power source and receives the output voltage. The tip is adjacent the electrode. The system also includes a source of gas to be ionized, a gas supply line, and a gas control solenoid associated with the gas supply line. The gas control solenoid is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. A gas monitor circuit provides a gas pressure signal having a parameter indicative of a gas pressure in the gas supply line at a point between the gas control solenoid and the torch head. A control circuit monitors the output voltage and the gas pressure signal. Te control circuit sets a time delay period when the output voltage transitions from the first value to the second value and causes the output voltage to reset from the second value to a third value if after the time delay period the gas pressure signal is less than a gas threshold. 
     In still another aspect, the invention relates to a contact start plasma-arc torch system for use by a torch operator in connection with a workpiece. The torch system includes a power source for selectively providing an output voltage. The torch system also includes a torch head including an electrode and a tip. The electrode is positioned in a circuit path with the power source and receives the output voltage. The tip is adjacent the electrode. The torch system further includes a source of gas to be ionized, a gas supply line, and a gas control solenoid associated with the gas supply line. The gas control solenoid is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. A gas monitor circuit provides a gas pressure signal having a parameter indicative of a gas pressure in the gas supply line at a point between the gas control solenoid and the torch head. A control circuit monitors a differential voltage selectively established between the electrode and the tip. The differential voltage transitions from a first value to a second value when a contact start operation is initiated. The control circuit also monitors the gas pressure signal. The control circuit sets a time delay period when the differential voltage transitions from the first value to the second value and causes the power source to remove the output voltage from the electrode if after the time delay period the gas pressure signal is less than a gas pressure threshold. 
     In another aspect, the invention relates to a contact start plasma-arc torch for use by a torch operator in connection with a workpiece. The torch system includes a power source for selectively providing an output voltage that transitions from a first value to a second value when a contact start operation is initiated by the torch operator. A torch head includes an electrode and a tip. The electrode is positioned in a circuit path with the power source and receives the output voltage. The tip is adjacent the electrode. A control circuit monitors a differential voltage between the electrode and the tip. The control circuit causes the output voltage of the power source to transition to a third value if the differential voltage between the electrode and the tip remains less than a contact start threshold after a time period sufficient to allow the contact start operation to complete has elapsed. 
     In yet another aspect, the invention relates to a power supply suitable for use in connection with a contact start plasma-arc torch system. Such a torch system includes a torch head, a source of gas to be ionized, a gas supply line supplying gas to the torch head, and a gas control switch associated with the gas supply line. The gas control switch is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. A gas pressure switch provides a gas pressure signal indicative of a gas pressure in the gas supply line at a point between the gas control solenoid and the torch head. The power supply includes a power source for selectively providing an output voltage to the torch head. The output voltage transitions from a first value to a second value when a contact start operation is initiated. A control circuit monitors the output voltage and the gas pressure signal. The control circuit sets a time delay period when the output voltage transitions from the first value to the second value and causes the output voltage to reset from the second value to a third value if after the time delay period the gas pressure signal is less than a gas pressure threshold. 
     In still another aspect, the invention relates to a power supply suitable for use in connection with a contact start plasma-arc torch system. The torch system includes a torch head having an electrode and a tip, a source of gas to be ionized, a gas supply line supplying gas to the torch head, and a gas control switch associated with the gas supply line. The gas control switch is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. A gas pressure switch provides a gas pressure signal indicative of a gas pressure in the gas supply line at a point between the gas control solenoid and the torch head. The power supply includes a power source for selectively providing an output voltage to the electrode. The output voltage transitions from a first value to a second value when a contact start operation is initiated. A control circuit monitors a differential voltage selectively established between the electrode and the tip. The differential voltage transitions from a first value to a second value when the contact start operation is initiated. The control circuit also monitors the gas pressure signal. The control circuit sets a time delay period when the differential voltage transitions from the first value to the second value and causes the power source to remove the output voltage from the electrode if after the time delay period the gas pressure signal is less than a gas pressure threshold. 
     In another aspect, the invention relates to a power supply suitable for use in connection with a contact start plasma-arc torch system. The torch system includes a torch head having an electrode and a tip, a source of gas to be ionized, a gas supply line supplying gas to the torch head, and a gas control switch associated with the gas supply line. The gas control switch is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. The power supply includes a power source for selectively providing an output voltage to the electrode. The output voltage transitions from a first value to a second value when a contact start operation is initiated. A control circuit monitors a differential voltage between the electrode and the tip. The control circuit causes the output voltage of the power source to transition to a third value if the differential voltage between the electrode and the tip remains less than a contact start threshold after a time period sufficient to allow the contact start operation to complete has elapsed. 
     Alternatively, the invention may comprise various other methods, circuits, and systems. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart that illustrates a prior art method of controlling a gas solenoid to ensure sufficient time to allow a contact start process to initiate. 
     FIG. 2 is a flow chart of an improved method of controlling a gas solenoid during a contact start process, in accordance with aspects of the present invention. 
     FIG. 3 is a flow chart of a method of operating a contact start plasma-arc torch that detects when there may be insufficient gas pressure to separate the contact start elements. 
     FIG. 4 is a flow chart of a method of operating a contact start plasma-arc torch that detects when the contact start elements fail to separate as expected. 
     FIG. 5 is a state transition diagram that illustrates a method of controlling a gas solenoid in a blow apart, contact start torch. 
     FIG. 6 is a block diagram of a contact start torch system suitable for use in implementing the method of controlling a gas solenoid of FIG.  2 . 
     FIG. 7 is a block diagram of a contact start torch system suitable for use in implementing the methods of FIGS. 3 and 4. 
     FIG. 8 is a logic diagram that illustrates particular aspects of preferred control circuitry suitable for use in the system of FIG.  7 . 
     FIGS. 9A and 9B provide a detailed schematic of a preferred configuration of the control circuitry of FIG.  8 . 
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 is a flow chart illustrating aspects of a method  200  of controlling a gas control solenoid in accordance with aspects of the present invention. The method illustrated in FIG. 2 is suited for use with, for example, blow apart contact start plasma-arc torches. As explained above, with respect to FIG. 1, some prior art blow apart torches require a preset time delay to account for the expected worst case time needed for the contact start elements to come into contact upon the removal of gas (e.g., removal of pre-flow gas). Several factors cause this time delay to vary between torches, including, for example, the age/wear of the parts, the length and size of the gas hose, the type of torch, and operating conditions such as temperature. Advantageously, the method illustrated in FIG. 2 eliminates a need for such a preset time delay. Rather, the method of FIG. 2 provides for more accurate and reliable operations. 
     At block  202 , main torch power is turned on and gas pressure is available. As is typical in the art, a torch activation switch (e.g., a trigger switch in the torch handle to be actuated by the torch user) is used to render the torch operational. At block  204 , the torch activation switch is actuated and, thereafter, at block  206 , a gas control solenoid is opened to allow gas from the gas supply to flow into the torch head assembly. This is referred to as pre-flow gas. With a blow apart torch, the presence of such pre-flow gas should cause the contact start elements to separate, as illustrated at block  208 . If such capability is desired, other logic (discussed in greater detail herein) may be used to determine whether the contact start elements in fact separated. At block  210 , the gas control solenoid is closed, thereby removing the supply of flowing gas. With the gas removed, the contact start elements should make electrical contact, as shown at block  212 . This is expected because, as explained above, in a typical blow apart torch, the contact start elements are biased into electrical contact; air pressure is used to overcome the bias and separate the elements. At block  214 , control logic senses whether the contact start elements have, in fact, made contact. This can be accomplished, for example, by monitoring a differential voltage between the contact start elements. By way of further example, if the electrode and tip serve as the contact start elements, a relatively low differential voltage should appear between the electrode and tip when the elements are in electrical contact. This may be referred to as a short circuit, electrode-tip voltage even though the value may have a magnitude in excess of 0 VDC. 
     In the alternative, the method of the invention could monitor the output voltage of the torch system power supply for an indication of whether the contact start elements have made contact. There are differences between monitoring the differential electrode-tip voltage (Ve-t) and the output voltage (Vout). For example, in torch systems employing a pilot switch to selectively connect the tip into an electrical circuit with the electrode, there will be times when Ve-t is a completely open circuit while Vout is a lower magnitude voltage. Similarly, the presence of a pilot resistor in the path between the tip and ground reference can result in differences between the values of Ve-t and Vout. There are other differences between monitoring Ve-t and Vout (as those terms are generally used herein) which include timing differences associated with establishing settled values and the presence of other circuitry and functionality used for other purposes in a particular torch or torch system. 
     If the control logic determines that the contact start elements are in contact, a signal is provided at block  216  to open the gas control solenoid. This allows gas to flow into the torch head. The gas pressure should cause the contact start elements to again separate (block  218 ). As is known in the art, when the elements separate, the high voltage potential between the elements creates a spark and causes the flowing gas to ionize and produce a pilot arc. 
     Block  220  illustrates that, in many cutting operations, the pilot arc is transferred to a workpiece. Block  222  shows that if the torch operator releases the torch switch, post flow gas is allowed to flow for a brief period but power is removed from the torch head (block  224 ). If the operator re-actuates the torch switch (block  226 ), the gas solenoid is again turned off to allow the contact start process to begin anew. 
     FIG. 3 is a flow chart that illustrates a method of operating a contact start torch, in accordance with aspects of the present invention. More particularly, FIG. 3 illustrates a method  300  of detecting when the contact start elements do not properly separate as expected. In such case, the power is removed from the torch. At block  302 , main torch power is on and gas pressure is available. At block  304 , a torch operator actuates the torch switch to begin torch operations. At block  306 , a contact start process is initiated. This contact start process is preferably substantially similar to the contact start process illustrated in FIG. 2 (e.g., pre-flow gas, if present, is removed to allow the starting elements to make electrical contact). At block  308 , the contact start elements are in electrical contact. Control logic may be employed to ensure sufficient electrical contact between the elements. For example, a differential amplifier, a comparator, or a circuit providing suitable similar functionality can detect a relatively low differential voltage between the elements, indicating continuity therebetween. As a further example, if the electrode and tip comprise the contact start elements, although the full output of the power supply is provided to the electrode, the low impedance path between the electrode and the tip results in a low differential voltage between these elements. A pilot resistor, preferably located between the tip and positive ground, may be used to limit current. It should also be noted that continuity can be detected by monitoring the output voltage rather than the differential voltage between the tip and electrode. It should further be noted that a continuity check at block  308  is not necessarily required in this embodiment; such a test is preferred, as discussed with respect to FIG. 2 above. A simplified method could rely on biasing forces alone to ensure that the contact start elements make contact upon removal of gas flow and then turn gas back on after a brief time delay. 
     At block  310 , the gas solenoid is opened to allow flowing gas to separate the contact start elements and strike a pilot arc. At block  312 , control logic determines whether sufficient gas pressure exists to blow the contact start elements apart. Preferably, this control logic includes a delay (e.g., 100-500 ms) to allow the requisite gas pressure to build up. Thus, after the gas control solenoid is turned on, the control logic waits a brief period. If, after that brief period, the gas pressure is below a threshold value needed to separate the contact start elements, the power supply output is removed from the torch at block  314 . 
     As will be explained in greater detail below, one preferred way of implementing the gas pressure check of block  312  is to use a gas pressure switch to sense gas pressure at a point between the solenoid and the torch head. Pressure switches, if used at all in the prior art, are typically located between the gas solenoid and the gas supply. There are several distinct advantages of using a pressure sensing process such as that depicted in FIG.  3 . One of the typical reasons that contact start elements fail to separate is a lack of sufficient gas pressure to cause separation. There can be a variety of reasons for a lack of gas pressure. Advantageously, and unlike the prior art, locating a pressure switch between the solenoid and torch head (as opposed to locating it between the gas supply and the solenoid) detects a solenoid failure (e.g., the solenoid fails to open fully or opens only partially). It is believed that the prior art systems are unable to provide such failure detection. 
     FIG. 4 illustrates a method  400  that reflects an alternative and/or addition to the method  300  illustrated and described with regard to FIG.  3 . It should be understood that the methods illustrated herein (including methods  200 ,  300 , and  400 ) are not mutually exclusive and may be selectively combined to provide enhanced functionality. Blocks  402 - 410  and  414  preferably provide the same functionality as blocks  302 - 310  and  314  of FIG. 3, respectively. Block  412  illustrates a primary difference between methods  300  and  400 . Rather than (or in addition to) using pressure sensing to detect a likelihood that the elements will not separate, method  400  monitors a voltage indicative of whether the elements remain in continuity. For example, after the gas solenoid is turned on (block  410 ), the control logic delays for a short period (e.g., 500-1000 ms) to allow the parts to separate. After that delay, the control logic compares the differential voltage between the electrode and tip to a low voltage threshold to determine if a short circuit still remains. If a short circuit remains, the elements have not separated and power is removed (block  414 ). 
     FIG. 5 is a state transition diagram that illustrates one preferred method of operating a blow apart, contact start torch in accordance with aspects of the present invention. In particular, FIG. 5 illustrates a method of controlling a gas solenoid by monitoring the status of the torch activation switch and a voltage. The monitored voltage is preferably the output voltage of the power supply, but other voltages can be monitored (e.g., a differential voltage between contact start elements such as an electrode and a tip). It should be understood that the output of a typical power supply for a plasma-arc torch system is substantial. Thus, it is normally desirable to use voltage dividers or other means that provide an indication of the output voltage. 
     As used in FIG. 5, the variable “SW” refers to the torch activation switch state and is illustrated as having two values—ON and OFF. The variable “Vout” refers to the output voltage of the power supply. For purposes of illustration and understanding, Vout is shown as having three values—OCV (open circuit voltage); SC (short circuit voltage); and OP (operating voltage). Of course, there are other possible values for the output voltage of a power supply used in a plasma-arc torch system. Such values include, for example, no voltage. It should also be understood that plasma-arc torch power supplies normally operate with a positive reference/return. In other words, the positive output terminal of the power supply is tied to ground and the negative output terminal is selectively connected to the electrode. For simplicity, voltages are generally referred to herein in terms of their respective magnitudes. As used in FIG. 5, OCV refers to a relatively high voltage condition which would occur, for example, when the power supply is on but relatively little or no current is flowing. OCV can be, for example, 200 or 300 VDC or more. As used in FIG. 5, SC refers to a relatively low voltage condition which would occur when the contact start elements are in electrical contact (i.e., a low impedance path between the electrode and the positive reference). SC may be below 10 VDC, for example. As used in FIG. 5, OP refers to an operating voltage condition between OCV and SC which would typically occur when an arc is present. The values of OCV, SC, and OP depend upon torch type, and exact values are not generally required to understand the aspects of the invention identified and illustrated herein. 
     In a first state  502 , the gas solenoid is off. This may be the case if the torch switch is not actuated, or for other reasons which are explained herein or are understood in the art. If the gas solenoid is off and torch switch state SW is OFF, the solenoid remains off, regardless of the value of Vout, as shown by line  504  (i.e., Vout is not applicable in such case). On the other hand, if the gas solenoid is currently off and torch switch state SW transitions to ON, as shown by line  506 , the solenoid is turned on (state  508 ), regardless of the value of Vout. For example, when the torch switch is turned on, pre-flow gas may be supplied (see FIG. 2 above). 
     If the solenoid is on and the operator releases the torch switch, torch switch state SW is OFF and the solenoid is turned off (line  510 ), regardless of the value of Vout. When the solenoid is on and torch switch state SW remains ON and Vout is SC (indicating a short circuit), the system proceeds along line  512  and enters a short wait state (state  514 ), leaving the solenoid on. This wait state  514  corresponds to the time needed to allow the contact start elements to separate (i.e., to allow Vout to transition from SC to OP). If, upon completion of the wait state, Vout is OP (indicating that an arc has been established), the solenoid remains on as shown by line  516 . If, however, upon completion of the wait state Vout is SC (indicating the contact start elements remain in contact) or OCV (indicating that although the parts are no longer in contact, no arc was struck), the solenoid is turned off, as shown by line  518 . 
     So long as the torch switch is on (SW=ON), and the output voltage indicates an arc (Vout=OP), the solenoid remains on, as shown by line  520 . If, however, the arc is lost, the output voltage will transition to a high level (Vout=OCV) and the solenoid is turned off, as shown by line  522 . 
     State diagram  500  also illustrates how a system operating according to that diagram accounts for the contact start elements failing to make sufficient contact to strike an arc. With the solenoid off, when the operator depresses the torch switch (SW=ON), the system follows line  506  and turns on the solenoid. Recall that the contact start elements are normally biased into contact. Thus, when power is applied and gas is turned on, an arc should be struck when gas pressure builds up. This is illustrated by traversing lines  506 ,  512 , and  516 . If the output voltage is high after the airflow begins (Vout=OCV), however, the parts never made contact or no arc was struck and the system proceeds along line  522  to turn off the solenoid. Thus, so long as the operator holds the switch down and the parts fail to make contact, the solenoid will cycle on and off (e.g., along lines  506  and  522 ), giving an indication of a contact start failure. 
     FIG. 6 is a block diagram of a contact start plasma-arc torch system  600  embodying aspects of the present invention. The system  600  illustrated in FIG. 6 is suitable for implementing the method of controlling the gas solenoid illustrated and described with respect to FIG. 2, and may also be adapted for use with a system operating in accordance with the state diagram illustrated and described with respect to FIG.  5 . 
     FIG. 6 illustrates a positive reference power supply  602  having two output terminals—a positive terminal connected to ground, and a negative terminal that is selectively connected to a torch head  604 . A power control switch  606  is illustrated to show that the power supply  602  may be cut off from the torch head  604 . More specifically, the negative terminal of power supply  602  is selectively connected to an electrode  608 . The positive terminal of power supply  602  is selectively connected to a torch tip  610 , via a pilot resistor  612  and a pilot switch  614 . Power supply  602  controls the operation of the pilot switch  614  by a pilot switch control signal on line  616 . It should be appreciated that the power control switch  606  is shown as being in-line with the negative terminal for illustrative purposes only and could preferably be within the power supply or elsewhere (e.g., one or more power control switches, relays, contactors, or the like). 
     In the embodiment illustrated in FIG. 6, the torch comprises a blow apart torch and the electrode  608  and the tip  610  comprise the contact start elements. Electrode  608  and tip  610  are normally biased into contact. The presence of flowing gas of sufficient pressure overcomes the bias and separates electrode  608  and tip  610 . 
     A supply of gas  620  is selectively connected to torch head  604  via a gas control solenoid  622  and a gas supply line  624 . A control circuit  626  monitors the output voltage (Vout) of power supply  602  via lines  628  and  630 . The control circuit  626  also monitors the state of a torch activation switch  632 . Control circuit  626  provides a solenoid control signal  634  on a line  636  to the gas control solenoid  622 . Control circuit  626  also provides a power control signal  640  to power control switch  606 . In a preferred embodiment, control circuit  626  is located within the power supply; it is illustrated external to facilitate a detailed explanation of the system. 
     In operation, when an operator first depresses torch activation switch  632 , control circuit  636  causes gas control solenoid  622  to open (via solenoid control signal  634 ), thereby allowing pre-flow gas to flow from gas supply  620  into torch head  604  to separate electrode  608  and tip  610  (see also FIG. 2 above). Thereafter, control circuit  636  closes gas control solenoid  622 , which removes the pre-flow and allows electrode  608  and tip  610  to make contact. At this point, the pilot switch  614  should be closed so that tip  610  is connected to the positive return of power supply  602  (preferably via optional pilot resistor  612 ), and power supply output Vout is applied to electrode  608 . 
     Control circuit  626  monitors a voltage to determine when electrode  608  and tip  610  are in contact. As illustrated in FIG. 6, control circuit  626  monitors Vout to make this determination. If Vout is below a low voltage threshold, control circuit  626  determines that electrode  608  and tip  610  have made contact and thereafter opens gas solenoid  622  (if contact is detected). With solenoid  622  open, gas again flows into torch head  604 . If the torch is operating as expected, the flow of gas causes electrode  608  and tip  610  to separate. As separation occurs, the voltage potential between electrode  608  and tip  610  causes a spark and ionizes the flowing gas, thereby creating an arc. If, on the other hand, control circuit  626  senses a high voltage (e.g., open circuit condition between electrode  608  and tip  610 ) rather than the expected low voltage condition, it turns off the gas solenoid. As mentioned above, it should be understood, with the benefit of the present disclosure, that control circuit  626  could also be configured to monitor a differential voltage Ve-t established between the electrode and tip to determine when electrode  608  and tip  610  are in electrical contact. There are, however, differences between Ve-t and Vout which must be considered. 
     FIG. 7 is a block diagram of a contact start plasma-arc torch system  700  embodying aspects of the present invention. The system  700  illustrated in FIG. 7 is preferably substantially similar to system  600  of FIG. 6. A primary difference between the systems  600  and  700  is the addition of a pressure switch and monitoring of a differential voltage between the electrode and the tip. With the benefit of the present disclosure, the systems of FIGS. 6 and 7 could be combined. System  700  is suitable for implementing the methods illustrated in FIGS. 3 and 4. 
     FIG. 7 illustrates a positive reference power supply  702  having two output terminals—a positive terminal connected to ground, and a negative terminal that is selectively connected to a torch head  704 . A power control switch  706  is illustrated to show that the power supply  702  may be cut off from the torch head  704 . More specifically, the negative terminal of power supply  702  is selectively connected to an electrode  708 . The positive terminal of power supply  702  is selectively connected to a torch tip  710 , via a pilot resistor  712  and a pilot switch  714 . Power supply  702  controls the operation of the pilot switch  714  by a pilot switch control signal on line  716 . It should be appreciated that the power control switch  706  is shown as being in-line with the negative terminal for illustrative purposes only and could preferably be within the power supply or elsewhere (e.g., one or more power control switches, relays, contactors, or the like). 
     In the embodiment illustrated in FIG. 7, the torch comprises a blow apart torch and the electrode  708  and the tip  710  comprise the contact start elements. Electrode  708  and tip  710  are normally biased into contact. The presence of flowing gas of sufficient pressure overcomes the bias and separates electrode  708  and tip  710 . 
     A supply of gas  720  is selectively connected to torch head  704  via a gas control solenoid  722  and a gas supply line  724 . A control circuit  726  monitors a differential voltage developed between electrode  708  and tip  710  (Ve-t) via lines  728  and  730 . The control circuit  726  also monitors the state of a torch activation switch  732 . Control circuit  726  provides a solenoid control signal  734  on a line  736  to open and close the gas control solenoid  722 . Control circuit  726  also provides a power control signal  740  to power control switch  706 . In a preferred embodiment, control circuit  726  is located within the power supply. It is illustrated external to facilitate a detailed explanation of system  700 . 
     A pressure switch  750  is preferably located such that it senses a pressure in the gas line  724  at a point between gas control solenoid  722  and torch head  704 . Several of the advantages of sensing pressure at such a location are described above. The pressure switch  750  preferably provides a pressure status signal  752  to control circuit  726  via a line  754 . The pressure status signal  752  has a parameter indicative of a pressure in gas line  724 . It should be appreciated that pressure switch  750  can provide a simple binary signal (e.g., on/off) having a first value if the sensed pressure is less than a low pressure threshold and a second value if the sensed pressure exceeds the threshold. More complicated switches and pressure sensing techniques are also possible. For example, a pressure switch could provide an analog or digital signal having a parameter indicative of a relative gas pressure within the gas supply line. 
     In operation, when an operator first depresses torch activation switch  732 , control circuit  736  causes gas control solenoid  722  to open (via solenoid control signal  734 ) and provide pre-flow gas, thereby allowing gas to flow from gas supply  720  into torch head  704  to separate electrode  708  and tip  710 . Thereafter, control circuit  736  closes gas control solenoid  722 , which removes the flow of gas and allows electrode  708  and tip  710  to make contact. At this point, the pilot switch  714  should be closed so that tip  710  is connected to the positive return of power supply  702  (preferably via optional pilot resistor  712 ), and the power supply output is applied to electrode  708 . 
     Control circuit  726  monitors a voltage to determine when electrode  708  and tip  710  are in contact. Preferably, as illustrated in FIG. 7, control circuit  726  monitors Ve-t to make this determination. If Ve-t is below a low voltage threshold (e.g., 6.2 VDC), control circuit  726  determines that electrode  708  and tip  710  have made contact and thereafter opens gas solenoid  722 . With solenoid  722  open, gas again flows into torch head  704 . If the torch is operating as expected, the flow of gas causes electrode  708  and tip  710  to separate and create a pilot arc. If an arc exists, Ve-t will be substantially higher than the low voltage threshold. 
     Advantageously, control circuit  726  can cut off power in the event that electrode  708  and tip  710  remain in electrical contact for a time period after solenoid  722  has been turned on and gas is expected to be flowing. A first way of accomplishing this shut down function is to monitor pressure status signal  752  from pressure switch  750 . If the reason that electrode  708  and tip  710  remain in contact is an insufficient supply of gas pressure, pressure switch  750  provides an indication to control circuit  726 . If sufficient time has passed since gas control solenoid  722  was turned on (e.g., 100-500 ms), control circuit  726  cuts off power to the torch (e.g., power control signal  740  causes power control switch  706  to remove power from the torch head). Preferably, control circuit  726  also turns off gas control solenoid  722  at this time. 
     A second way of accomplishing the shut down function is by monitoring Ve-t. If sufficient time has passed since gas control solenoid  722  was turned on (e.g., 500-1000 ms), and Ve-t remains below a low voltage threshold, control circuit  726  cuts off power to the torch (e.g., power control signal  740  causes power control switch  706  to remove power from the torch head). Preferably, control circuit  726  also turns off gas control solenoid  722  at this time. 
     FIG. 8 is a logic diagram that illustrates pertinent aspects of a preferred embodiment of control circuit  726 . The output “Q” of a D-type flip-flop  802  is used to control a pilot contactor switch  804 , a gas control solenoid (e.g., solenoid  722 ), and a main contactor switch  806 . The output Q is referred to as a gas control signal or simply GC herein. The data input “D” of the flip-flop  802  is tied to a logic 1. The clock input “CK” receives a START signal, which is indicative of the status of the torch activation switch  732 . The set input “S” is tied to a logic 0. The reset input “R” is connected to additional logic that will be described below. The inverted output (Qnot) is connected to a time delay circuit  810 . 
     As shown in FIG. 8, Ve-t is sensed by a comparator  812 . The output  814  of the comparator  812  provides an indication of the status of Ve-t. Output  814  of comparator  812  is logically ANDed with a time delay status signal supplied on the output  816  of time delay circuit  810 . This AND operation is illustrated in FIG. 8 by AND gate  818 . The output  820  of this AND operation comprises a contact start status signal which is supplied as a first input to an OR operation illustrated in FIG. 8 by OR gate  822 . The other input of the OR operation is an inverted version of the START signal referenced above. 
     When the torch is activated (i.e., the operator presses the torch activation switch), the START signal rises from a logic 0 to a logic 1. This rising edge on the START signal clocks flip-flop  802 . Because the data input “D” of flip-flop  802  is tied to a logic 1, the clock signal causes the  1  to appear at the flip-flop output Q. Thus, gas control signal GC is asserted, turning on the pilot contactor  804 , gas control solenoid  722 , and the main contactor  806 . At this point, therefore, power is applied and a short circuit should exist between the contact start elements. When gas pressure builds up, the contact start elements should separate. 
     The comparator  812  monitors the differential electrode-tip voltage Ve-t. If Ve-t is less than a low voltage threshold (e.g., 6.2 VDC), indicating a short circuit condition between the contact start elements, output  814  of comparator  812  is a logic 1. If Ve-t is not less than the low voltage threshold, the output  814  is a logic 0, indicating that no short circuit condition is sensed. Output  814  of comparator  812  is logically ANDed with the output  816  of time delay circuit  810  to provide the contact start status signal. The time delay accounts for the expected amount of time it should take for the contact start elements to separate (e.g., 500-1000 ms). Time delay circuit  810  outputs a logic 1 if the delay period has completed and a logic 0 otherwise. The AND operation ( 818 ) ensures that the contact start status signal is a logic 1 only when the time delay is complete and comparator  812  senses electrical continuity between the contact start elements. Consequently, the contact start status signal is a logic 0 if there is no contact between the elements or the time delay has not been satisfied. Stated differently, the contact start status signal is a logic 1 if the parts failed to separate as expected within the time delay period. 
     The reset input “R” of flip-flop  802  is used to cut off power and turn off the gas when the contact start elements fail to properly separate or if the START signal is removed. Thus, if a logic 1 appears on either input of the AND operator  822 , flip-flop  802  is reset, causing GC (output Q) to a logic 0 which turns off pilot contractor  804 , gas solenoid  722 , and main contactor  806 . Another clock signal (START transitions from logic 0 to 1) is required before GC is again a logic 1. Accordingly, if the time delay has completed and comparator  812  indicates a short circuit between the contact start elements, a logic 1 at the input of OR operator  822  resets flip-flop  802 . Otherwise, power and airflow remain on and torch operations continue. 
     It should be appreciated that other circuitry and logic herein described could be combined with the capabilities illustrated and described with respect to FIG.  8 . 
     FIGS. 9A and 9B provide a detailed schematic, illustrating one set of preferred circuitry for implementing the control functionality of FIG.  8 . The voltage on the tip (PCB connector pin  22 ) and electrode (PCB connector pin  24 ) can have a high magnitude (e.g., 300-400 volts) which cannot be measured directly by a standard comparator. Consequently, the voltage is divided down and level shifted from negative to positive by the dividers comprising R 26  and R 27  (tip) and R 32 , D 3 , and R 31 . The divided down tip voltage is applied to the negative comparator input (U 1 -C pin  8 ) and the divided down electrode voltage is applied to the positive comparator input (U 1 -C pin  9 ). When the tip voltage is less than 6.2 volts from the electrode voltage, as determined by the zener voltage of zener diode D 3  (note that other values could be used, the present being exemplary only), the voltage on the positive input (U 1 -C pin  9 ) is positive relative to the voltage on the negative input (U 1 -C pin  8 ), causing the output of the comparator (U 1 -C pin  14 ) to be high. 
     To start a pilot arc, a low level start signal is applied to the connector pin  19 . This low level signal passes through U 5 -A, U 5 -B and U 6 -A to a flip-flop (U 3 -A) clock input (pin  3 ), setting the flip-flop to a logic high on its output (U 3 -A pin  1 ). This turns on relay K 3  and the main contactor to supply DC power to the torch. It should be understood that instead of a relay, a logic level signal could be used to enable an inverter power supply. The high on U 3 -A pin  1  also passes through Dl, U 7 -D, U 5 -C, U 5 -D, and U 6 -C to turn on the gas solenoid. 
     At the time the start signal is applied, the tip and the electrode should be in contact and the voltage between them should be less than the zener voltage of zener diode D 3  (e.g., less than 6.2 volts) so that the output of the comparator (U 1 -C) is high. The high output, after passing through U 7 -C to U 3 -A pin  4 , (the flip-flip&#39;s reset pin), would hold the flip-flop reset and prevent a start. However, the output (pin  2 ) of comparator U 1 -A is configured as a time delay. The output of the time delay is initially low, holding pin  14  low as well. Thus, U 3 -A can be set to start the pilot. 
     At the same time that pin  1  of U 3 -A goes high, pin  2  of U 3 -A goes low, discharging capacitor C 22  (which was initially charged to the +12 volt logic supply) through resistor R 45 . In approximately 500 ms (those skilled in the art will appreciate that the time constant providing the delay can be varied across a wide range, depending upon needs) capacitor C 22 , which is connected to U 1 -A pin  4 , is discharged below the voltage on pin  5  of U 1 -A, causing the output of U 1 -A to go high. If at the end of the 500 ms the torch parts have separated (as expected), the output of U 1 -C will have gone low, keeping the reset signal low. If, however, the tip and electrode remain in contact after the 500 ms delay period, a high on both U 1 -A and U 1 -C&#39;s outputs will, through U 7 -C, reset the flip-flop and shut off the K 3  relay (or logic signal) and remove power from the torch. 
     Although several of the embodiments illustrated herein have been described in terms of contact starting using an electrode and a tip that are biased into contact for starting purposes, other arrangements are possible. For example, contact starting can be achieved by causing a condition that allows a closed electrical circuit to be established between two or more contact start elements. By way of further example, one or more of these contact start elements can be made movable for causing the closed electrical circuit to be established and/or terminated, as needed. Further, one or more of these elements can be monitored for voltage sensing, such as sensing a voltage established between two or more contact start elements. 
     When introducing elements of the present invention or preferred embodiments thereof, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is further to be understood that the steps described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated. It is also to be understood that additional or alternative steps may be employed with the present invention.