Feedback welder control system

• US 4,459,457 A
• Filed: 09/30/1982
• Issued: 07/10/1984
• Est. Priority Date: 10/16/1980
• Status: Expired due to Term

• Current Assignee: Square D Company (Schneider Electric SE)
• Original Assignee: Square D Company (Schneider Electric SE)
• Inventors: Jurek, Dennis J.
• Primary Examiner(s): Shaw, C. C.
• Application Number:
  US06/430,448
• Time in Patent Office: 649 Days
• Field of Search: 219/108, 219/110, 219/114, 323/205, 323/211, 323/212, 323/244, 323/246, 323/345
• US Class Current: 219/110
• CPC Class Codes: B23K 11/24 Electric supply or control circuits therefor

Abstract

A control system for resistance welders of the type in which welding current is supplied to a workpiece through a transformer. The system assures weld quality by monitoring the change in resistance across the weld during the welding operation without the need for auxillary leads or separate sensing transducers at or near the workpiece and automatically terminates the welding current when a quality weld is made as determined by weld resistivity characteristics. The system monitors discrete changes in the characteristics of the current in the welding transformer which occur as a result of changes in weld resistance. Immunity to line voltage disturbances which cause charges in current characteristics unrelated to resistance change in the weld is achieved by the use of a modeling circuit which is subjected to the same line voltage disturbances as the actual welder circuit and which simulates the current characteristics of the actual welder but is unresponsive to the resistive changes which occur across the weld during the welding operation. The system compares the current characteristics of the modeling circuit with those of the actual welding current and terminates the welding current in response to the true resistance change which occurs during welding formation.

Claims

1. A control system for controlling the quality of a weld as formed in a resistive welding apparatus wherein the apparatus includes a pair of welding electrodes engagable with workpieces to be welded together and connectable through a means to an AC source for energizing the electrodes with alternating polarity half cycles of welding current comprising:

means providing a weld signal indicative of the actual welding current through the electrodes and workpieces;

means providing a model signal indicative of a simulated welding current through the electrodes when workpieces are absent and the electrodes, are engaging each other;

and means responsive to a selected difference between the weld signal and the model signal for terminating the welding current when the difference satisfies a preselected criteria.

2. In a control system for controlling the quality of a weld in a resistive welding apparatus including a pair of welding electrodes that are engagable with workpieces to be welded together and connectable to an AC source for energizing the electrodes with alternating polarity half cycles of welding current the combination comprising:

means providing a model simulative of a welding current through the electrodes when work pieces are absent from a position between the electrodes and a model signal indicative of an instant when each half cycle of the simulated model welding current is extinguished,

means providing a weld signal indicative of the actual welding current through the electrodes and the workpieces and the instant when each half cycle of welding current is extinguished,

and means for measuring intervals between the model signal and the weld signal and interrupting the welding current when a measured interval is at least equal to a preselected value.

3. A control system for controlling the quality of a weld in a resistance welding apparatus with the apparatus including a pair of welding electrodes that are engagable with workpieces to be welded together and a welding transformer having a secondary winding connected to the electrodes and a primary winding connectible to an AC source for energizing the primary and the secondary windings with alternating polarity half cycles of welding current comprising:

means providing a model signal simulative of alternating polarity half cycles of a simulated welding current through the electrodes when workpieces are absent from a position between the electrodes and indicative of the instant when each half cycle of the model signal is extinguished;

means providing a weld signal indicative of the actual welding current through the electrodes and the workpieces and the instant when each half cycle of actual welding current is extinguished; and

means for measuring an interval between the model signal and the weld signal and interrupting the energizing circuit between the source and the primary winding when the interval is at least equal to a preselected value.

4. The control system as recited in claim 3 wherein the primary winding is connected to the source by a pair of solid state switches connected in inverse parallel and the signal indicative of the welding current is provided by a means connected across the switches.

5. The control system as recited in claim 4 wherein the means for providing a model of the welding current and the model signals includes a solid state switch that is switched to a conductive state concurrently with the switching of the inverse connected solid state switches to a conductive state.

6. The control system as recited in claim 3 wherein the means providing the model of the welding current, the means providing the weld signal, the means for measuring the time difference between the model signal and the actual weld signals and the means for interrupting the energizing circuit are controlled by a microprocessor.

7. A control system for controlling the quality of a weld in a resistance welding apparatus with the apparatus including a pair of welding electrodes that are engagable with workpieces to be welded together and a welding transformer having a secondary winding connected to the electrodes and a primary winding connectible to an AC source for energizing the primary and the secondary winding with alternating polarity half cycles of actual welding current comprising:

means providing a model indicative of a welding current through the electrodes when workpieces are absent from a position between the electrodes and a model signal indicative of the instant when each half cycle of the simulated welding current is extinguished;

means providing a weld signal indicative of the actual welding current through the electrodes and the workpieces and the instant when each half cycle of the welding current is extinguished;

means for measuring a time difference between the model signal and the weld signal during a half cycle when the value of the time difference between the signals is minimum and storing the value of the minimum difference; and

means for interrupting the actual welding current when the difference between the minimum difference value and a signal indicative of the difference between the model signal and the actual weld signal during a half cycle subsequent to the half cycle producing the minimum difference value is greater than or equal to a preprogrammed value.

8. A method of controlling the quality of a weld as the weld is formed by a resistive welder apparatus having a welding transformer including a secondary winding connected in a circuit to a pair of electrodes engagable with workpieces that are to be welded together and a primary winding connectable to an alternating current source for energizing the primary and the secondary windings with alternating polarity half cycles of welding current comprising:

sensing the instant of extinction of each half cycle of weld current pulses through the workpieces and generating a signal indicative of the instants of extinction;

simulating the electrical characteristics of the secondary winding circuit excluding the workpieces by a modeling circuit that is connected to the AC source to generate a model signal that is indicative of the instants simulated weld current pulses are extinguished in the simulated secondary winding circuit,

determing the order of occurence of the weld current signal and the model signal and generating and accumulating counted clock pulses that are indicative of the duration of the interval between the signals,

programming a memory with preselected count values indicative of a quality weld for comparison with the counted pulses,

and terminating the weld current pulses when the accumulated counted pulses equals the preselected value.

9. A control system for controlling the quality of a weld in a resistance welding apparatus with the apparatus including a pair of welding electrodes and a welding transformer having a secondary winding connected in a circuit with the electrodes and a primary winding connectable to an AC source for energizing the primary and the secondary winding with alternating polarity half cycles of welding current comprising:

means providing a model of a simulated welding current in the primary winding when workpieces are absent from a position between the electrodes and a signal indicative of the power factor of the model during each half cycle of the simulated welding current;

means providing a weld signal indicative of the power factor of the actual welding current in the primary winding when workpieces are present between the electrodes;

means for measuring a time difference between the power factors of the model and the weld signal during a half cycle when the value of the difference between the signals is minimum and storing the value of the minimum difference; and

means for interrupting the welding current in the primary winding when the difference between the minimum difference value and a signal indicative of the difference between the power factor of the model and the power factor of the weld signal during a half cycle subsequent to the half cycle producing the minimum difference value is greater than or equal to a preprogrammed value.