WELDING CONTROL

• US 3,558,849 A
• Filed: 04/10/1968
• Issued: 01/26/1971
• Est. Priority Date: 04/11/1967
• Status: Expired due to Term

• Current Assignee: Schlumberger Electronics UK Limited (Thales SA)
• Original Assignee: Catherall Reginald, Levinge Rodney Walter, Ley Anthony John
• Inventors: CATHERALL REGINALD, LEY ANTHONY JOHN, LEVINGE RODNEY WALTER
• Application Number:
  US04/720,208
• Time in Patent Office: 1,021 Days
• US Class Current: 219/110
• CPC Class Codes: B23K 11/256 the measured parameter being the inter-electrode electrical resistance

Abstract

The duration of a resistance-welding operation is controlled by deriving two successions of electrical signals which depend upon voltage across and the current through the electrodes respective, deriving from the first and second signals a succession of third signals dependent upon the impedance of the workpiece, detecting the difference between each third signal and the next succeeding third signal, and stopping the welding operation when the difference reaches a predetermined amount.

Claims

1. A method of resistance welding comprising the steps of positioning a workpiece between two electrodes, initiating current flow through the electrodes and the workpiece, deriving a succession of first electrical signals each being representative of the voltage between the electrodes during a predetermined portion of each successive half-cycle of said current, deriving a succession of second electrical signals each being representative of the current through the electrodes during the same predetermined portions of successive half-cycles, deriving a succession of third electrical signals from respective ones of said first and second signAls, each third signal being representative of the impedance of the workpiece throughout the respective ones of the predetermined portions, detecting the difference in magnitude between each third signal and the next succeeding third signal, and stopping the current flow when the difference reaches a predetermined amount.

2. A method according to claim 1, wherein each first signal is substantially proportional to the voltage between the electrodes throughout the respective portion, each corresponding second signal is substantially proportional to the current through the workpiece throughout the same portion, and the step of deriving each third signal comprises deriving the ratio of the respective first and second signals.

3. A method according to claim 2, wherein each predetermined period embraces a peak of current flow.

4. A method according to claim 3, wherein each step of detecting the difference between each third signal and the next succeeding third signal comprises generating a succession of fourth signals each of which is proportional to a function of one of the third signals, charging a capacitor with one of the fourth signals, applying a succeeding fourth signal representative of the next succeeding third signal to the capacitor, detecting a discharge from the capacitor, and generating a signal when the discharge from the capacitor exceeds a predetermined value.

5. A method according to claim 4, wherein each fourth signal is proportional to an exponential function of the respective third signal.

6. Weld control apparatus comprising first input circuit means connected to weld electrodes for deriving a succession of first electrical signals, each being representative of a predetermined portion of successive periods of the voltage across said weld electrodes;

second input circuit means for deriving a succession of second electrical signals, each being representative of a predetermined portion of successive periods of the current through said weld electrodes;

means for deriving a succession of third electrical signals, each being representative of the ratio between one of said succession of first signals and the one of said succession of second signals which was derived during the same period as the first signal;

detector means for receiving said succession of third signals and for producing a control signal representative of the difference of one of said succession of third signals and the next succeeding one of said third signals; and

means connected to said detector means for terminating weld current when said control signal reaches a predetermined magnitude.

7. Apparatus according to claim 6, wherein the said successive periods are successive half-cycles of an alternating current.

8. Apparatus according to claim 7, wherein each successive period embraces a peak of current flow.

9. Apparatus according to claim 8, wherein said first input circuit means for deriving a succession of first electrical signals includes a first gate, and said second input circuit means for deriving a succession of second electrical signals includes a second gate, said first and second gates being open throughout the said successive periods, the apparatus further comprising a timer adapted to open the gates throughout the said predetermined periods and to close the gates throughout the periods between the said predetermined periods.

10. Apparatus according to claim 9, wherein the timer comprises an oscillator adapted to run at an integral multiple of the frequency of the alternating current flow and in predetermined phase relation thereto, a series of binary dividers connected to the output of the oscillator, and gating means connected between first and second gates and the output of the oscillator and at least one of the binary dividers to provide signals for opening and closing the said first and second gates.

11. Apparatus according to claim 6 wherein said detector means includes means for storing a fourth signal dependent upon one of said succession of third signals and for applying to said means for storing a fourth signal a succeeding fourth signal dependent upon the next succeeding third signal, means for detecting a discharge from the means for storing a fourth signal and for generating the control signal when the discharge from the means for storing exceeds a predetermined value.

12. Apparatus according to claim 11, wherein the means for storing is adapted to derive each fourth signal as an exponential function of the corresponding third signal.

13. Apparatus according to claim 6, wherein the means for deriving first signals includes a first integrator, the means for deriving second signals includes a second integrator, and the means for deriving the third signals includes a gate connected between the output of one of the integrators and an input of the other integrator, the last said gate being opened after each predetermined period and closed when the output of the first integrator is reduced to zero, whereby the time for which the last said gate is open is dependent upon the ratio of corresponding first and second signals.

14. Resistance welding apparatus comprising two electrodes adapted for contacting a workpiece, means for initiating current flow through the electrodes and the workpiece, means for deriving a succession of first electrical signals each being representative of the voltage between the electrodes throughout a respective one of a succession of predetermined periods, means for deriving a succession of second electrical signals each being representative of the current through the electrodes throughout a respective one of the said predetermined periods, means for deriving a succession of third electrical signals from respective first and second signals, each third signal being dependent upon the impedance of the workpiece during the respective one of the predetermined periods, a detector adapted for detecting the difference between each third signal and the next succeeding third signal, and means for stopping the current flow when said difference reaches a predetermined amount.

15. Apparatus according to claim 14, wherein each first signal is substantially proportional to the voltage between the electrodes throughout the respective period, each corresponding second signal is substantially proportional to the current through the workpiece throughout the same period, and the means for deriving a succession of third electrical signals comprises means for deriving the ratio of the corresponding first and second signals.

16. Apparatus according to claim 14, wherein the said current flow is an alternating current flow, and the said predetermined periods are arranged to occur during successive half-cycles of the alternating current.

17. Apparatus according to claim 16, wherein each predetermined period embraces a peak of current flow.

18. Apparatus according to claim 16, wherein the means for deriving a succession of first electrical signals includes a first gate, and the means for deriving a succession of second electrical signals includes a second gate, the first and second gates being open throughout the said predetermined periods, the apparatus further comprising a timer adapted to open the gates throughout the said predetermined periods and to close the gates throughout the periods between the said predetermined periods.

19. Apparatus according to claim 18, wherein the timer comprises an oscillator adapted to run at an integral multiple of the frequency of the alternating current flow and in predetermined phase relation thereto, a series of binary dividers connected to the output of the oscillator, and gating means connected between first and second gates and the output of the oscillator and at least one of the binary dividers to provide signals for opening and closing the said first and second gates.

20. Apparatus according to claim 14, wherein the detector includes means for storing a fourth signal dependent upon a third signal and for applying to said means for storing a succeedinG fourth signal dependent upon the next succeeding third signal, means for detecting a discharge from said means for storing and for generating a control signal when the discharge from said means for storing exceeds a predetermined value.

21. Apparatus according to claim 20, wherein said means for storing is adapted to derive each fourth signal as an exponential function of the corresponding third signal.

22. Apparatus according to claim 14, wherein the means for deriving first signals includes a first integrator, the means for deriving second signals includes a second integrator, and the means for deriving the third signals includes a gate connected between the output of one of the integrators and an input of the other integrator, the last said gate being opened after each predetermined period and closed when the output of the first integrator is reduced to zero, whereby the time for which the last said gate is open is dependent upon the impedance of the workpiece.