Electric heating – Metal heating – By arc
Reexamination Certificate
2000-11-28
2002-02-26
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121570, C219S121500
Reexamination Certificate
active
06350960
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to improvements in plasma-arc torches and, particularly, to 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.
Plasma-arc torches, also known as electric arc torches, are commonly used for cutting, welding, and spray bonding. Such torches typically operate by directing a plasma consisting of ionized gas particles toward a workpiece. In general, a pressurized gas to be ionized is supplied to the front end of the torch and flows past an electrode before exiting through an orifice in the torch tip. The electrode has a relatively negative potential and operates as a cathode. The torch tip, which is adjacent to the end of the electrode at the front end of the torch, constitutes a relatively positive potential anode. When a sufficiently high 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. 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.
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 is applied to the electrode (relative to the tip) to establish a pilot arc between the electrode and the tip. As mentioned above, when the torch head is moved toward the workpiece, the arc transfers to the workpiece. By way of contrast, in contact start torches, 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 biases the tip and/or electrode such that the tip and electrode are in electrical contact. When the operator squeezes the torch trigger, a voltage is applied to the electrode and pressurized 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.
The electrode and torch tip are consumable items that eventually need replacement. As such, the torch head allows for removal and replacement of both the tip and the electrode. It is normally undesirable, however, to operate a plasma-arc torch when either the tip or electrode is not properly installed. Therefore, plasma-arc torches can benefit from detection circuits that determine whether the tip and/or electrode are properly installed. In non-contact start torches these detection circuits may include a circuit for sensing tip installation, electrode installation, and that the tip and electrode are not in electrical contact. Such a parts-in-place system used for non-contact start torches may not work for contact start torches because in a contact start torch, electrical contact between the tip and electrode is necessary during pilot arc initiation.
Moreover, conventional plasma torch parts-in-place detection systems typically use additional electrical or mechanical means within the torch head or torch leads, such as contact pins, microswitches and the like. This, of course, increases the cost and complicates the design and manufacture of the torch and lead assembly.
Further, conventional plasma torches require a relatively high open circuit voltage (e.g., −300 VDC) at the electrode in order to ionize the flowing gas. If the tip were not installed, the electrode would be exposed. An exposed electrode with such a high voltage potential presents an undesirable condition.
One solution to the presence of a relatively high open circuit voltage during the parts-in-place check is described in commonly owned U.S. Pat. No. 5,961,855, the entire disclosure of which is incorporated herein by reference. The solution described therein relies on the use of a relatively low voltage source for conducting the parts-in-place check. Upon satisfactory completion of the check, the power supply transitions to a higher voltage source for normal operations. Such a system, however, requires switching between two voltage outputs.
There is a need, therefore, for a parts-in-place detection system for use with a contact start plasma-arc torch that requires no additional electrical or mechanical means or wiring within the torch head or torch leads. There is a further need to accomplish the parts-in-place function without the need for two separate voltage outputs, but rather uses the existing power supply output capabilities.
For these reasons, an improved system and method is desired for ensuring that the tip and/or electrode are properly installed in a contact start torch.
SUMMARY OF THE INVENTION
The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved circuit and method for determining whether the tip and electrode of a contact start plasma-arc torch are properly installed. This is accomplished by monitoring selected parameters indicative of proper tip and electrode installation. Advantageously, the tests described herein are completed before the power supply output reaches its final value, and preferably before it reaches approximately 40 VDC. Thus, the safety reset circuit and method described herein do not require an additional low-voltage power supply output to accomplish the parts-in-place test.
Briefly described, a system embodying aspects of the invention includes a plasma arc torch for use in connection with a work piece. The torch has an electrode. A tip is in electrical relationship with the electrode. A power supply has a power output. A housing supports the tip and the electrode. The housing allows electrical continuity between the tip and the electrode. A soft start circuit is in electrical combination with the power supply. The combination of the soft start circuit and the power supply provide a power signal to the electrode that transitions from an initial level to an operating level. A monitor circuit monitors the electrical relationship between the tip and the electrode. The monitor circuit provides a status signal indicative of the electrical relationship between the tip and the electrode. A reset circuit is responsive to the status signal. The reset circuit resets the soft start circuit so that the power signal returns to the initial level when the status signal indicates an unacceptable electrical relationship between the tip and the electrode.
Another embodiment of the invention is a plasma arc torch for use with a workpiece. The torch includes an electrode. A tip has an electrical relationship with the electrode. A power supply has a power output. A housing supports the tip and the electrode. The housing allows electrical continuity between the tip and the electrode. A soft start circuit is in electrical communication with the power supply. The soft start circuit supplies a power signal to the electrode via a soft start process. The soft start process transitions the power signal from an initial level to an operating level. A monitor circuit monitors the electrical relationship between the tip and the electrode. The monitor circuit provides a status signal indicative of the electrical relationship between the tip and the electrode. A reset circuit is responsive to the status signal. The reset signal resets the soft start circuit so that the power signal returns to the initial level when the status signal indicates an unacceptable electrical relationship between the tip and the electrode.
A further embodiment of the invention is a method of operating a contact start torch for use in connection with
Paschall Mark
Senniger Powers Leavitt & Roedel
Thermal Dynamics Corporation
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