Electric heating – Metal heating – By arc
Reexamination Certificate
2001-02-16
2002-03-19
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S1370PS
Reexamination Certificate
active
06359258
ABSTRACT:
This application relates to the art of electric arc welding and more particularly to a method and system for determining the actual inductance of cables used in the welding installation.
BACKGROUND OF INVENTION
Electric arc welding involves passing a current between an electrode and a workpiece in a manner to perform a desired arc welding process. To control the current during the welding process, the controller is set up to assume a certain impedance, including inductance and resistance, of the cables connecting the power supply with the electrode and workpiece at the welding station. Consequently, it is desirable to know the cable inductance and resistance to diagnose grounding problems and to improve the set up of the welding operation. Further, the actual inductance must be known to dynamically control the current flowing during the welding operation. In the field, the inductance of the cables used in the welding operation varies drastically. Welding processes are generally intolerant to poor circuit conditions that cause changes in the inductance of the cables. Further, some highly optimized welding processes require welding impedance within a narrow range of values. Consequently, the ability to measure and report or store inductance and/or resistance of the welding circuit is a very useful tool for application engineers in the field of electric arc welding. If the resistance or inductance values are outside specific ranges for acceptable operation, an application engineer can take corrective actions to adjust the welding circuit or factory installed welding programs, that are developed assuming certain cable impedance to obtain acceptable welding performance.
If the resistance of a cable is known to be beyond a certain limit, corrective action can be taken, such as replacing the cable or replacing moving parts in the grounding circuit. In a like manner, if the actual inductance of the welding circuit is known and is found to be too low, a length of cable can be coiled to achieve the desired inductance during set up before the process is implemented. In a like manner, if the actual circuit inductance is beyond a certain value, the welding cables can be shortened or gathered together to reduce the physical area enclosed by the welding cables to lower the inductance of the cables. To perform these corrective actions, the resistance and impedance of the welding cables must be determined on a real time basis or during set up. The value of such parameters may not be the same in prior set ups even if no discernable changes have been made to the installation.
One arrangement for measuring the actual inductance of the welding operation involves the use of leads at the power supply terminals. However, such leads have certain limitations. If one of the leads is connected to the electrode side of the installation, near the arc, and the other lead is attached to the power supply work connection, the impedance of the work lead itself is measured; however, this is not the impedance of the total circuit. In a like manner, connection of one lead to the work circuit near the arc and the other lead to the power supply electrode connection, the inductance measured is the electrode lead inductance. The use of these leads can identify certain problems; however, the actual inductance and resistance of the welding operation is not obtained. These systems have become necessary as welding control technology has advanced and become quite complex. Thus, accurate high speed voltage sensing has been used for the purpose of accurately controlling the dynamic arc voltage and measuring the transient events that occur in the arc. These systems involve a method for determining impedance of the welding circuit itself. The knowledge of the welding circuit inductance is used to optimize dynamic performance for the welding power supply. Thus, it is a substantial value to know the welding circuit impedance including inductance and resistance for use by the welding engineer. Also, this information is beneficial for machine set up, maintenance, troubleshooting, and diagnostics with respect to tooling and grounding problems. Consequently, systems have been developed in an effort to measure and record the inductance and resistance of the welding circuit. To obtain the inductance, a digital processing technique is employed. This technique involves increasing the current in a ramped straight line. By measuring the sensed voltage at a first current, awaiting a time dt and then recording a second sensed voltage at a second current, the di/dt can be calculated by the digital processor by the difference in current number over the time delay in digital format. The average voltage is determined by dividing the sum of the two sensed voltage by two. In a like manner, the average current is determined by dividing the sum of the two measured currents by two. Having these determinations made by the digital processor in the second phase of the calculation technique, the circuit inductance is obtained by digitally subtracting the average voltage from the product of the average current and circuit resistance to obtain a digital number or value divided by the digital representation of di/dt. Thus, a digital representation of circuit inductance is obtained. This technique produces a circuit resistance during the first stage and a circuit inductance during the second stage. However, these two stages are done successively while the electrode is shorted against the workpiece. The accuracy of the measurements is compromised by the variations which can occur from time to time during the testing procedure with the electrode shorted. The use of the two stage measuring operation to determine resistance and inductance of the welding operation still depends upon a measuring device, such as leads and the stability of the resulting values is less than optimum.
THE INVENTION
The digital processing procedure used for determining resistance and inductance of the welding process whereby the electrode is shorted and two measuring stages are performed is merely background to the present invention; however, it is not necessary prior art to the present invention. However, such prior two stage digital processing procedure has been improved by the present invention consisting of a method for determining the inductance of the network connecting the electrical power supply to an electric arc welding station. The network is normally a cable and gun, or a pair of cables, between the power supply and the electrode contact tip and workpiece at which the welding process is implemented. In accordance with the invention, the electrode is shorted to the workpiece so that the arc voltage is zero. Then, a series of waveforms are created. Each of these waveforms includes a first state operated by the power supply at a constant current of a selected value. In practice, this value is at least 300 amperes, which is maintained for approximately 10 ms. The waveform then includes a second state wherein the power supply is operated at a low value of open circuit voltage so that the current through the shorted circuit decays from the selected value along a decay path having a curve determined by the value of the impedance of the circuit. In practice, the open circuit voltage in the second state is zero; however, it could be at a value less than about 2.0 volts. While the electrode is shorted, many of these waveforms are sampled in succession. During each of the waveforms, the first state is digitally processed to obtain the average current and the average resistance. This operation involves taking several samples of instantaneous voltage and instantaneous current. These values are converted to digital numbers. The average resistance is the summation of the arc voltage at a sampling time divided by the instantaneous current at a sampling time. The summation is then divided by the number of samples to obtain the resistance of the cable or connecting network. This digital process provides the average resistance for the network. At the same time, during the first state the sam
Blankenship George D.
Hsu Christopher
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
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