Electric heating – Metal heating – By arc
Reexamination Certificate
2002-01-25
2003-05-27
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130010
Reexamination Certificate
active
06570130
ABSTRACT:
This invention relates to the art of electric arc welding and more particularly to a device for monitoring the voltage sense leads of the power source during a welding operation. In addition, the invention involves the novel method of monitoring the continuity of the voltage leads using the unique monitoring device.
BACKGROUND OF INVENTION
In the electric arc welding technology, a power source passes a current between an electrode and a work piece. Often, the electrode is a continuous welding wire drawn from a supply of welding wire, such as a drum or reel, and passed through a contact tip on its way to being melted and deposited onto the work piece. In this type of welding procedure, the power source of the welder includes a first stud connected to the electrode, usually through the contact tip, and a second stud connected to the work piece Connections are by welding cables, which cables may be quite long and include a variety of impedance variables, such as inductive reactance based upon length, position and shape of the cables. When performing a welding process, the power supply receives a current command to create a particular pulse wave between the electrode and work piece. One of the more common power sources is the Power Wave sold by The Lincoln Electric Company of Cleveland, Ohio. Such welder must accurately control the pulse shape or waveform by controlling the voltage to a pulse width modulator operated at a frequency exceeding about 20 kHz. To assure the desired welding operation constituting specific waveforms between the electrode and work piece, the command signal is created based upon a feedback from the actual welding operation. This feedback involves the arc current and/or the arc voltage. Measurement of the arc current for feedback control presents minor distortions, since the current is in a series circuit and is zero when the cables are not connected. However, the arc voltage between the electrode and work piece can not be determined by the voltage between the output studs of the power source. Voltage is affected by not only the impedance of the cables, but also the choke and other impedance creating components in the welding operation. To assure an accurate feedback of arc voltage, it is common practice to use remote voltage sensing leads directed from the controller of the power source to the electrode or contact tip and the work piece. The voltage of these leads determines the command signal to the power source from the controller. Consequently, disastrous results affecting the quality of the weld can occur if the voltage sense leads become disconnected from the welding circuit. Since the voltage command to the power source is determined by the voltage feedback signal, a broken or disconnected sense lead will provide a decreased feedback signal indicating a drop in the arc voltage. Consequently, the voltage command signal to the power source must be increased to compensate for the presumed decrease in arc voltage. This can cause the electrode to melt too rapidly for the advancing wire feed speed so the incoming wire melts into the contact tip. Such event is especially undesirable in automatic or robotics welding. The melt back of the electrode into the contact tip melts the contact tip to produce molten copper that migrates into the weld pool and results in an undesirable metallurgy for the resulting weld. Consequently, it is essential for automatic welding, such as by robot, that the operator be alerted when a voltage sense lead is disconnected or otherwise interrupted. In the past this problem has been addressed by an external device or box that is connected to the studs of the welder and receives remote voltage readings from the leads. If the voltage at the studs is greater than the remote reading plus a normal voltage drop associated with the welding cables, the power source is turned off or the operator is flagged to indicate problems with the welding operation demanding immediate attention. The most common defect is when the voltage lead to the work is broken or disconnected. This external circuitry for verifying the proper status of the voltage sense lead has disadvantages. It requires an additional component or box that is mounted onto the external portion of the power supply and requires skill by the end user for proper installation. It is expensive and time consuming to connect the box to the voltage leads and to the studs. In addition, this component must be monitored which is also expensive. Welder customers do not desire add-on components which therefore drastically reduces the desirability of the prior attempts to determine when the voltage sense leads are not properly connected.
THE INVENTION
The present invention overcomes the disadvantages of the prior art wherein an external box is needed for monitoring the voltage sense leads. In accordance with this invention, a signal representing the voltage which should appear across the leads or at the two power source studs is calculated by knowing the primary voltage, secondary current, turns ratio of output transformer and the duty cycle of the switching network of the power source. After this theoretical voltage is determined for a given time during a specific weld cycle, the synchronized theoretical voltage signal is then compared with the sensed actual voltage at either the studs or the voltage leads for this same given time. Referring specifically to the voltage leads, if one lead becomes disconnected the control circuit will see one-half the actual voltage of the welding operation. If both leads are disconnected, the control circuit will see no voltage. Consequently, by comparing the theoretical calculated voltage with the actual voltage at the voltage sense leads, the proper connection of the sense leads can be monitored on a continuous basis. If the voltage provided by monitoring the voltage sense leads is drastically under the theoretical voltage, the invention terminates operation of the welder. The invention is an executive program that is performed by the digital signal processor in a control board of the type used in most modern welders. The Power Wave welder has such control. The Power Wave is shown in Blankness U.S. Pat. No. 5,278,390, incorporated by reference herein. Thus, by merely calculating the desired theoretical voltage at a given time and reading the voltage at the sense leads at this same time, the program of the present invention sets a status bit in the control program of the welder to terminate operation of the welder immediately. To prevent burn back of the electrode into the contact tip, the total test sequence must be implemented to determined whether the status bit is to terminate operation of the welder within a time less than the time for electrode burn back into the tip. In most welding applications, burn back requires over 50 ms depending upon the type of wire, the wire diameter and the amount of original stick out. Consequently, the algorithm constituting the test sequence is completed within a time substantially less than 50 ms. In practice, this cycle time is less than 5.0 ms. In addition, the invention performs the total program for the test sequence periodically during the welding process to give a real time reading of the voltage at the leads. A total executive program is implemented in a time substantially less than the burn back time and the period of implementing the test sequence is substantially greater than the burn back time. In other words, periodically the test sequence of the present invention is performed. This implementation of the sequence involves a cycle time substantially less than the burn back time. Thus, if there is a problem with the continuity of the sense leads, the welder power source will be terminated prior to the time necessary for electrode burn back. To immediately detect a lead separation the repeat period of the lead sequence is greater than 5 ms, but less than 50 ms.
In practice, there is an exception to the general operation described above. When there is a short circuit operation, there is not enough power available to c
Kooken Todd E.
Myers Russell K.
Fay Sharpe Fagan Minnich & McKee
Lincoln Global Inc.
Shaw Clifford C.
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