Electric heating – Metal heating – By arc
Reexamination Certificate
2001-04-05
2003-04-15
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130210, C219S1370PS
Reexamination Certificate
active
06548784
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the art of welding. More specifically, it relates to providing a reduced OCV and for terminating the arc at the end of a weld.
BACKGROUND OF THE INVENTION
There are a number of different welding processes. Two known welding processes are stick welding and TIG. Each welding process has its own characteristics, objectives and problems that often make it difficult to adapt a control scheme from one process to be used with another process.
Stick welding (also called SMAW or Shielded Metal Arc Welding) is an arc welding process which uses a consumable stick electrode, and may be performed using an ac or a dc output. The stick welding process is often touch or scratch started, wherein 80 volts (typically) is the open circuit voltage (OCV). The user scratches or touches the electrode to the workpiece and the OCV causes current to flow. The user pulls the electrode away from the workpiece, drawing an arc therebetween. This starting process may be adequate for skilled welders, but is often difficult for less experienced welders. Also, 80 volts OCV may be sufficient for a user to feel an electric shock if they touch the electrode and the workpiece, or the output studs of the power supply. While this might not injure the worker, it can result in downtime while the worker receives attention making sure they are not injured, and/or the power supply is serviced.
Tungsten-inert gas (TIG) welding is a welding process where a tungsten electrode is used, and the electrode does not become part of the completed weld. One known way of starting the TIG welding process is the Miller LiftArc®, which provides a reduced OCV in a standby state, and then monitors the output to detect a short-circuit between the electrode and the workpiece. A detected short (which occurs when the user touches the electrode to the workpiece) indicates the users intent to start the weld. The power supply then enters a preheating state, where a low current is provided to heat the electrode. After the electrode is heated the user needs to lift the electrode away from the workpiece a second time to cause the power supply to enters a welding state where the desired welding current is provided. Such a starting scheme will not function properly in stick welding because the stick will adhere to the workpiece after preheating.
Accordingly, a welding power supply that allows for easy starting of a stick welding process, yet provides for a reduced OCV when not welding is desirable. Also, the power supply should provide a start that does not cause the electrode to adhere to the workpiece.
Other problems occur when the arc welding process is terminated. The simplest way to terminate a weld is to pull the electrode away from the workpiece, drawing a longer arc. The machine output voltage increases, in an attempt to maintain the arc. Eventually, the arc length is so great that the power supply cannot provide voltage sufficient to maintain the arc, and the arc is extinguished. One problem with terminating the arc in this fashion is that when the arc is relatively long it has a tendency to whip across the surface of the workpiece, leaving undesired weld tracks. This problem is particularly prevalent in ending a TIG process.
Another known way to terminate a welding process is to provide a remote switch that the user can reach while welding. The switch terminates the weld process abruptly. It is remote so that the user can access it without taking his attention from the arc, lest the arc stray across the workpiece. Unfortunately, a remote control adds cost and complexity to a welding power supply.
A known way to terminate a stick welding process is to monitor the current in an inverter in the power supply, and to terminate power when the current crosses below a threshold. However, this process is not well suited for TIG because the arc can become undesirably long before the current drops below a threshold that avoids false detection of the end of the arc. Thus, the arc can whip across the workpiece.
Accordingly, a welding power supply that ends a TIG welding process easily, cleanly, and without adding excess cost is desirable.
SUMMARY OF THE PRESENT INVENTION
According to a first aspect of the invention a welding power supply includes a source of power and a controller. An output feedback circuit provides feedback to the controller. The controller includes a comparator that compares the fedback signal to a threshold. A standby/welding control is responsive to the comparator.
According to a second aspect of the invention an apparatus for starting a welding process includes an output feedback circuit and a short-circuit detect circuit, that receives the fedback signal. A controller provides a standby state output to a power supply if the electrode is not initially touching the workpiece. It provides a welding state output if the electrode subsequently touches the workpiece.
According to a third aspect of the invention a welding power supply includes a source of power and a controller for the source of power. An output feedback circuit provides feedback to the controller, The controller includes a comparator that receives a signal indicative of a threshold and the feedback signal. The controller further includes an arc end control that responds to the comparator.
The controls include at least a portion of a digital circuit, and is a microprocessor with instruction in various embodiments. The instructions control the output voltage to a first level an initial time the comparator has a first output. Then, they control the output voltage to a second level when the comparator has a second output in another embodiment. The instructions control the output of the power supply to a welding status in the event the comparator has a third output and that control the output of the power supply to an arc end status in the event the comparator has a fourth output for a period of time.
The feedback circuit includes a voltage feedback circuit and/or a current feedback circuit in alternative embodiments.
The comparator is a delay comparator in one embodiment.
According to a fourth aspect of the invention a method of providing welding power includes sensing an output parameter and comparing the sensed parameter to a threshold. A power supply is controlled to be in a standby or welding state in response to the comparison.
According to a fifth aspect of the invention a method of providing welding power includes sensing at least one output parameter and comparing it to a threshold. A power supply is controlled to be in a welding state or an arc end state in response to the comparison.
According to a sixth aspect of the invention a method of ending a welding process includes sensing an output parameter and determining if an arc length exceeds a threshold. A power supply is controlled to be in a welding state if the arc length does not exceed the threshold, or in an arc end state if the arc length does exceed the threshold.
The output voltage is controlled to a first level or a second level in response to the comparison in one alternative.
The sensed output parameter is voltage and/or current, and the comparison is output voltage and/or current to the threshold in other alternatives.
The welding state includes a first output voltage and the standby state includes terminating the output after a delay in one embodiment.
According to a seventh aspect of the invention a method of starting a welding process includes sensing an output parameter, and determining if an electrode touches a workpiece in response to the sensed parameter. A power supply is controlled to be in a standby state if the electrode is initially not touching the workpiece, and then controlled to be in a welding state if the electrode is touching the workpiece.
According to an eighth aspect of the invention an apparatus for ending a welding process includes an output feedback circuit and an arc length detect circuit. A controller provides a welding state output if the arc length is less than a threshold and provides an arc end state outpu
DeCoster Albert
Ferkel Stephen
Kadlec Mark
Sammons Michael
Corrigan George R.
Illinois Tool Works Inc.
Shaw Clifford C.
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