Electric heating – Metal heating – By arc
Reexamination Certificate
2001-10-30
2004-09-21
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130210, C219S130310
Reexamination Certificate
active
06794608
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a system and method of controlling a weld current for use in arc welding processes, and more particularly to a software-based system and method which permits generation of a wide variety of weld current profiles, including weld current profiles that can dynamically adapt to changes in welding conditions simply by changing the timing of segments of the weld current profile having predetermined (but not necessarily constant) amplitude profiles.
Unlike prior systems utilizing waveform generation to control a weld current power supply, the waveform generation system and method of the invention requires neither conventional hardware-implemented arc current or voltage feedback loops, nor complex digital signal processor-based controllers.
The reference waveforms used to generate the weld current profiles may be in the form of tables containing amplitude and trigger or duration values for each segment of the waveform. The waveforms may either be applied without feedback directly to the weld current power supply, or digitally modified based on feedback from an arc current and/or voltage sensor. Modification of the profiles is carried out by modifying a trigger or duration parameter for a selected amplitude segment in the table used to represent the reference waveform, preferably based on current and/or voltage data gathered over multiple weld cycles, simplifying programming and reducing required processor resources.
Despite its simplicity, the software-based weld current control of the invention can carry-out, without limitation, any of the following relatively complex weld-current control functions:
in short-arc welding, synchronizing of the waveform to the start of a short circuit,
in short-arc welding, re-starting the short circuit portion of the waveform if premature re-arcing occurs and/or if the re-establishment of another short circuit is detected, indicating that the original short circuit was only an incipient short,
in short-arc welding, adjusting the duration of the short-circuit phase based on analysis of previous cycles, including adjustment to prevent stub-outs if re-arcing fails to occur,
in any consumable electrode process, generating a current amplitude profile for each of a plurality of electrode diameters, feed-rates, material types and welding gas types,
in any consumable electrode process, interpolating between previously generated current amplitude profiles for different electrode feed-rates to facilitate operation at electrode feed-rates other than those associated with a particular amplitude profile or table, and
in any pulsed arc process, using the software to also provide synchronized sensing and control of the torch-to-workpiece voltage by adjusting the torch-to-workpiece distance or, in pulsed arc consumable electrode processes only, using the software to provide synchronized sensing and control the torch-to-workpiece voltage by adjusting the duration of any one of the current amplitude segments.
It will of course be appreciated by those skilled in the art that the system and method of the invention may be applied to welding processes other than those specified above, including all types of manual, semiautomatic, and automatic welding, as well as all types of out-of-position and pipe welding processes.
2. Description of Related Art
In recent years, substantial effort has been devoted to improving control of weld currents in arc welding systems. Until recently, most of this effort has involved design of ever more sophisticated feedback circuits which modulate the output of the weld current generator in response to changes in currents or voltages measured between the electrode and the workpiece. With the development of improved microprocessors, however, attempts have been made to digitize many welding control functions, including control of the arc welding current. Transfer of hardware-implemented functions to software has the advantage of enabling a particular apparatus to be adapted for a greater variety of different welding processes or conditions, without the need to design specific circuits for each change in the welding sequence.
U.S. Pat. No. 4,973,814 illustrates a microprocessor-based weld current controller which, like the present invention, uses pre-determined reference waveforms as a basis for control of a weld current power supply. However, the output of the waveform generator described in this patent is applied as the command signal to a current amplitude feedback loop of the welding apparatus, rather than directly to the power supply, the current amplitude feedback loop including a comparator for comparing the applied waveform to the actual waveform and generating an error signal. The only adjustments made to the waveform itself are made through a display of the difference between the reference and actual waveforms, and a keyboard for adjusting the applied reference waveform as necessary to achieve a desired actual waveform.
In the system illustrated in U.S. Pat. No. 4,973,814, only the amplitude and not the duration of the various parts of the weld current cycle are dynamically adjusted, and the adjustment is carried out by an analog servo, rather than by modifying the reference waveform. Adjustment of the reference waveform can only be carried out manually. As a result, while systems of the type illustrated in this patent can, through the analog servo, provide automatic control of a relatively simple and stable weld cycle, once the appropriate waveform has been manually adjusted, they cannot be used to provide automatic control for welding processes that are highly dependent on the timing of events that occur during the cycle, such as short-circuit arc welding, in which the cycle consists of at least two segments of variable length.
U.S. Pat. No. 5,278,390, on the other hand, discloses a version of the controller disclosed in U.S. Pat. No. 4,973,814, in which the hardware-implemented closed loop servo function is transferred to a digital signal processor capable of modifying the reference “waveform” expressed in a “state table.” In the system disclosed in this patent, the current profile is stored in state tables that can be selected for a particular weld process. Feedback is achieved, as explained in col. 13, lines 13 et seq. of U.S. Pat. No. 5,278,390, by manipulating the current amplitude values contained in the active state table, resulting in a modified waveform which is applied to the output of a weld current power source.
The system disclosed in U.S. Pat. No. 5,278,390, which takes what may be referred to as a “brute force” approach to feedback, appears to be capable of controlling a wide variety of welding processes, but suffers from the disadvantages that the response to current or voltage feedback values must be separately programmed for each part of the current cycle, and the closed loop control circuit must be adaptable enough to recognize and respond appropriately to each different part of the weld cycle by continually adjusting the current amplitude values according to the different requirements for each portion of the weld current cycle. In a highly variable environment such a short-circuit arc welding system, without the assistance of hardware-based feedback loops, the current amplitude control circuit of U.S. Pat. No. 5,278,390 would essentially be required to re-write the state table during each cycle, with the weld current profile essentially being determined by programming of the controller rather than of the state table.
The present invention, in contrast, does not attempt to instantaneously modify a waveform to conform to changes in sensed arc voltage or current, but rather takes a simpler and yet more universally adaptable approach in which the waveform is divided into predetermined amplitude segments, and only the timing of the segments is varied, thereby eliminating the need to continually adjust amplitude values for each weld cycle.
In a sense, the invention is based on recognition that, for cyclical welding processes, differen
Flood Dale A.
Rothermel Ronald R.
Bacon & Thomas PLLC
Shaw Clifford C.
Tri Tool Inc.
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