Electric heating – Metal heating – By arc
Reexamination Certificate
2000-03-14
2001-06-19
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130330, C219S1370PS
Reexamination Certificate
active
06248976
ABSTRACT:
The present invention relates to the art of electric arc welding and more particularly to a method of controlling arc welding processes using a unique control parameter and a welder having a controller for developing and using such unique parameter.
BACKGROUND OF INVENTION
Electric arc welding is a complex electrical phenomenon having many process control variables affecting the quality of the weld, whether using spray transfer, pulse transfer, DC GMAW welding or AC welding. In constant voltage welding processes, the welding process is normally controlled by the welding current I
a
. When the voltage is held constant, changes in the CTWD cause current changes as well as other process variations; therefore, control arrangements using the volts/ampere characteristics curve for the process have inherent limitations. The contact tip to work distance CTWD has a major influence on current since the current increases as the CTWD decreases and vice versa. Attempts to regulate the DC GMAW welding process based upon electrode stickout or CTWD have presented difficulties, since these parameters are difficult to determine on a real time basis. There is no weld process parameter heretofore measurable that provides a control signal essentially unaffected by arc current. Even more importantly, there has been no parameter to create a control signal to directly regulate on a real time basis the arc length. It is known that maintaining this distance between the bottom of the electrode and the workpiece at a fixed value will result in superior and repeatable weld quality.
THE INVENTION
The present invention overcomes prior difficulties in controlling arc welding processes by measuring a parameter to create a unique control signal that is representative of the resistance of the arc, but is not dependent upon the welding current. Consequently, the control signal created by using the invention is not affected by the large number of variables associated with control based upon welding current. In accordance with the present invention, the derivative of the welding voltage with respect to the welding current is measured to provide a parameter value used as a control signal for maintaining a weld condition, such as the arc length, during a welding process. The derivative of the welding voltage with respect to the welding current is a constructed, unique control parameter distinctly different than the load resistance obtained by merely dividing the voltage by the current. This unique parameter has not been constructed and/or used in electric arc welding for creating a signal to control the welding process.
The invention is primarily concerned with a parameter indicative of the property of a welding process that can generate a real time control signal for maintaining arc length at a selected distance. The complimentary electrode stickout (ESO) combined with the arc length equals the contact tip to work dimension CTWD.
The arc voltage V
arc
of a welding process includes a component equal to a constant A plus a constant B multiplied by the arc length L
arc
. The first constant A is the work function at the cathode plus the anode and cathode voltage drops. The anode and cathode voltage drops are known to be approximately 4.15 volts for ferrous materials. The preferred embodiment of the invention is a process for welding steel; however, other metals, such as aluminum, are welded using the invention. Thus, the anode drop and cathode drop is a constant which equals to 8.3 volts. The work function is a cathode phenomenon and is normally about 3.7. Consequently, the first constant A to the formula providing the arc voltage is in the general range of 12.0, i.e. 8.3+3.7. The second constant B that is multiplied by the arc length L
arc
is related to the shielding gas being used in the welding process. For a gas with the composition of 85% argon and 15% carbon dioxide, this constant is 18.7. Consequently, the V
arc
varies in accordance with the first constant (12.0) plus the second constant (18.7) multiplied by the arc length L
arc
in centimeters. The other component of the arc voltage V
arc
is the welding current I
a
multiplied by the arc resistance R
arc
. In summation, V
arc
=BL
arc
+I
a
R
arc
. Consequently, if V
arc
is known, an appropriate algorithm is used to determine arc length. Then arc length can be maintained. Since the welding current I
a
can be sensed, the only unknown is the arc resistance R
arc
. The present invention provides a signal representative of arc resistance. Thus, the arc length can be maintained using the present invention for developing a signal or value representative arc resistance.
By using the present invention, a unique parameter, voltage relating to current, (dV/dI) is used as a control signal as a representation of arc resistance for use in maintaining arc length. This derivative function relates to the resistance of the arc itself. Indeed, it relates to the total resistance R
ESO
+R
arc
across CTWD. Due to the low relative magnitude of R
ESO
, the inventive dV/dI signal is primarily a representation of the arc resistance in an arc welding process. The inventive signal does not equal the load resistance obtained by merely dividing welding voltage by welding current on a real time basis. The invention relates to the concept of sensing the derivative of voltage with respect to current to give a resistance function that ignores current. This parameter is used to determine the voltage across the arc as well as maintaining the length of the arc.
In accordance with the invention, there is provided a method of generating a real time control signal for use in an electric arc welding process having a process welding voltage and a process welding current. This method comprises determining a derivative of the welding voltage with respect to the welding current to generate a control signal. This control signal is referred to as resistance or impedance since it involves dividing current into voltage. Thus, the novel control signal varies with the magnitude of arc resistance and is used as a direct correlation to the voltage of the arc. The pseudo resistance (“incremental resistance”) is the control signal and constitutes a unique parameter that is the basic concept of the present invention. This control signal is multiplied by the actual welding current to produce a value to which the arc length value is added to obtain a signal level representing the voltage across the arc. The known control function exists where a first element (a) is the sum of a first constant (about 8-20, 12 for ferrous metals), a second constant (about 10-35, 18.5 for 90% argon, 10% CO
2
) times arc length L
arc
is compared with a second element (b) which is arc voltage minus welding current times the arc resistance to (c) maintain arc length. The invention allows implementation of this control function by creating a control signal representing or varying with arc resistance. These control systems for maintaining arc length are accomplished by and are aspects of the present invention where a novel control signal is created as a derivative of the total voltage in the welding process with respect to the total current in the welding process.
In accordance with another aspect of the present invention, the derivative forming the primary feature of the invention is obtained by a small dither of the welding current. A dither pattern is applied to the welding current to determine the “incremental impedance” or “incremental resistance” R
z
of the welding circuit. Before the dither, the voltage and current are sensed. The dither pattern then increases current and waits for a time delay. This delay removes distortion caused by inductive reactance. The voltage and current is again measured and recorded. By subtracting the voltage readings and subtracting the current readings the derivative of voltage to current is obtained by dividing the voltage difference by the current difference. By using a derivative of the relationship between voltage and current, a high resolution circuit is used because the
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
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