METHOD OF DETECTION OF WELDING WORKPIECE POSITION USING MOVABLE ELECTRODE

• US 20110089146A1
• Filed: 09/16/2010
• Published: 04/21/2011
• Est. Priority Date: 10/21/2009
• Status: Active Grant

• Current Assignee: Fanuc Ltd (Fanuc Corporation)
• Original Assignee: Fanuc Ltd (Fanuc Corporation)
• Inventors: Nishimura, Akinori, TAKAHASHI, Hiromitsu, AOKI, Toshimichi
• Granted Patent: US 8,426,761 B2
• Time in Patent Office: Days
• US Class Current: 219/91.2
• CPC Class Codes: B23K 11/115 by means of two electrodes placed opposite one another on both sides of the welded parts

Abstract

A spot welding system including spot welding gun having a movable electrode driven by a servo motor, a counter electrode arranged facing it and with a multiarticulated robot, holding one of the welding workpiece and spot welding gun which moves welding workpiece and spot welding gun relative to each other and thereby make the movable electrode and the welding workpiece approach each other from a separated state or vice-versa, monitoring the current or torque of the servo motor, and detecting the surface position of the welding workpiece from the position of the movable electrode and the position of the multiarticulated robot when the trend of the current or torque changes. The precision of detection of the surface position of the welding workpiece by a movable electrode in a spot welding system can now be improved without lengthening the time required for detection of the surface position of the welding workpiece.

Claims

1. A method of detection of a welding workpiece position using a movable electrode to detect a surface position of a welding workpiece in a spot welding system provided with a spot welding gun having a movable electrode driven by a servo motor and a counter electrode arranged facing the movable electrode and a multiarticulated robot holding one of the welding workpiece and the spot welding gun, the servo motor being used to make the movable electrode and the counter electrode approach and move away from each other and clamp the welding workpiece between the counter electrode and the movable electrode of the spot welding gun for spot welding the welding workpiece,

which method comprises steps of using the multiarticulated robot to make the welding workpiece and the spot welding gun move relative to each other so that the movable electrode and the welding workpiece are made to approach each other from a separated state or to move away from each other from a contact state, while doing this, monitoring a current or torque of the servo motor and, when a trend of change of the current or torque changes, judging that the movable electrode has contacted the welding workpiece or that the movable electrode has separated from the welding workpiece, and detecting the surface position of the welding workpiece from the position of the movable electrode and the position of the multiarticulated robot when the trend of change of the current or torque has changed.

2. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 1, further comprising, when monitoring the current or torque of said servo motor to detect the position of said welding workpiece, using said servo motor to drive said movable electrode at a speed Vg while using said multiarticulated robot to make said spot welding gun and said welding workpiece move relative to each other.

3. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 2, wherein the speed Vg is a speed of an extent enabling elimination of static friction of a mechanism for driving said movable electrode.

4. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 1, further comprising steps of monitoring the current or torque of said servo motor to detect the position of said welding workpiece while using said multiarticulated robot to make said spot welding gun and said welding workpiece move relative to each other in a direction making the movable electrode and the welding workpiece approach each other from a separated state and judging said movable electrode has contacted said welding workpiece when the actual value or amount of change per unit time of the current or torque of said servo motor changes to a trend of increase compared with the value or amount of change of the current or torque of said servo motor when assuming a reference state.

5. A method of detection of a welding workpiece position detecting a surface position of a welding workpiece which a counter electrode contacts in a spot welding system provided with a spot welding gun having a movable electrode driven by a servo motor and a counter electrode arranged facing the movable electrode and a multiarticulated robot holding one of the welding workpiece and the spot welding gun and making it move relative to the other, the servo motor being used to make the movable electrode and the counter electrode approach and separate from each other and insert the welding workpiece between the movable electrode and the counter electrode of the spot welding gun for spot welding the welding workpiece,

which method comprises steps of positioning the movable electrode so as to be contiguous with the surface of the welding workpiece, then using the servo motor to make the movable electrode move by a predetermined speed Vg in a direction making it approach the counter electrode, simultaneously using the multiarticulated robot to make the spot welding gun and the welding workpiece move relative to each other at a speed the same as the speed Vg so as to make the counter electrode and the welding workpiece approach each other, while doing so, monitoring at least one of the speed of movement and acceleration of the movable electrode with respect to the counter electrode, and thereby detecting contact of the counter electrode and the welding workpiece and detecting the surface position of the welding workpiece, to which the counter electrode contacts, from the detected position of the counter electrode.

6. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 5, wherein the servo motor for driving said movable electrode with respect to said counter electrode is set with a torque limit.

7. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 6, wherein the torque limit of said servo motor is set based on the value of the torque of said servo motor when making said spot welding gun and said welding workpiece move by the same speed as said speed Vg.

8. A method of detection of a welding workpiece position detecting respective surface positions of a welding workpiece to which a movable electrode and a counter electrode contact in a spot welding system provided with a spot welding gun having a movable electrode driven by a servo motor and a counter electrode arranged facing the movable electrode and a multiarticulated robot holding one of the welding workpiece and the spot welding gun and making it move relative to the other, the servo motor being used to make the movable electrode and the counter electrode approach and move away from each other and clamp the welding workpiece between the counter electrode and the movable electrode of the spot welding gun for spot welding the welding workpiece,

which method comprises steps of using the multiarticulated robot to make the welding workpiece and the spot welding gun move relative to each other so that the movable electrode and the welding workpiece are made to approach each other from a separated state or to separate from each other from a contact state, while doing so, monitoring a current or torque of the servo motor and, when a trend of change of the current or torque changes, judging that the movable electrode has contacted the welding workpiece or that the movable electrode has moved away from the welding workpiece, detecting the surface position of the welding workpiece, to which the movable electrode contacts, from the position of the movable electrode and the position of the multiarticulated robot when the trend of change of the current or torque has changed, and

detecting the surface position of the welding workpiece which the counter electrode contacts by the method of detection of the welding workpiece position of claim 5.

9. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 8, further including a step of finding a thickness of the welding workpiece from the detected surface position of the welding workpiece to which said movable electrode contacts and the detected surface position of the welding workpiece to which said counter electrode contacts.

10. A method of detection of a welding workpiece position detecting a surface position of a welding workpiece to which a movable electrode contacts in a spot welding system provided with a spot welding gun having a movable electrode driven by a servo motor and a counter electrode arranged facing the movable electrode and a multiarticulated robot holding one of the welding workpiece and the spot welding gun and making it move relative to the other, the servo motor being used to make the movable electrode and the counter electrode approach and move away from each other and clamp the welding workpiece between the counter electrode and the movable electrode of the spot welding gun for spot welding the welding workpiece,

which method comprises steps of making the movable electrode and the welding workpiece move relative to each other so as to approach each other from a separated state, monitoring at least one of the speed of movement and acceleration of the movable electrode with respect to the counter electrode during the relative movement of the movable electrode and the welding workpiece, judging that the movable electrode has contacted the welding workpiece when the speed of movement or acceleration of the movable electrode with respect to the counter electrode has changed, and finding a surface position of the welding workpiece from the position of the movable electrode and the position of the multiarticulated robot when the speed of movement or acceleration changed.

11. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 10, further comprising a step of setting said servo motor driving said movable electrode with a torque limit and in that state making said movable electrode and said welding workpiece move relative to each other so that they approach each other from a separated state.

12. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 11, wherein the torque limit of said servo motor is set to at least a value of a torque required for maintaining the speed of movement of said movable electrode with respect to said counter electrode at a constant level.

13. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 12, wherein the torque limit of the servo motor is set based on the value of the torque of said servo motor when said movable electrode is moving in a preparatory operation zone after the start of relative movement of said movable electrode and said welding workpiece and before contact of said movable electrode and said welding workpiece.

14. A method of detection of the welding workpiece position using a movable electrode as set forth in claim 13, wherein the relative movement of said movable electrode and said welding workpiece is performed by using said servo motor to drive said movable electrode so as to make said movable electrode approach said counter electrode.

15. A method of detection of the welding workpiece position as set forth in claim 1 wherein said method of detection of the welding workpiece position is performed by executing welding program instructions.