Electric heating – Metal heating – By arc
Reexamination Certificate
2002-03-04
2003-12-30
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121540, C219S121550, C219S121590
Reexamination Certificate
active
06670572
ABSTRACT:
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 w
Hewett Roger W.
Norris Stephen W.
Tatham David A.
Paschall Mark
Thermal Dynamics Corporation
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