Welding gun with an actuator on a fixed electrode

Abstract

A welding gun for resistance spot welding includes a first electrode; a second electrode having a fixed part and a movable part movable toward the first electrode, an electromechanical linear actuator of the first electrode fixedly attached to the fixed part of the second electrode and having a first force sensor and a rod connected to the first electrode; an electromechanical linear actuator of the movable part of the second electrode having a housing, an electric motor, an inverted roller-screw gear transforming rotary motor movement into rod translation, a position sensor determining a position of the movable part of the second electrode, and a bevel gear transferring electric motor rotation to the inverted roller-screw gear. The rod of the electromechanical linear actuator is connected to the movable part of the second electrode. The welding gun further includes a second force sensor configured to measure force on the second electrode.

Claims

1. A welding gun comprising: a first electrode; a second electrode having a fixed part and a movable part, wherein the first electrode and the movable part of the second electrode are movable toward each other; an electromechanical linear actuator of the first electrode, wherein the electromechanical linear actuator of the first electrode comprises a first force sensor, wherein a rod of the electromechanical linear actuator of the first electrode is connected to the first electrode; and an electromechanical linear actuator of the movable part of the second electrode, the electromechanical linear actuator of the movable part comprising: a housing; an electric motor; an inverted roller-screw gear for transforming rotary movement of the electric motor into translational movement of the rod of the electromechanical linear actuator of the movable part of the second electrode; a bevel gear (for transferring rotation of the electric motor to the inverted roller-screw gear, wherein an axis of the inverted roller-screw gear coincides with an axis of the rod of the electromechanical linear actuator of the movable part of the second electrode and with the axis of the inverted roller-screw gear angled relative to an axis of the electric motor; and a position sensor configured to determine a position of the movable part of the second electrode, wherein the electromechanical linear actuator of the movable part of the second electrode is fixedly attached on the fixed part of the second electrode with a part of the housing of the electromechanical linear actuator of the movable part of the second electrode, in which the electric motor is located, wherein the rod of the electromechanical linear actuator of the movable part of the second electrode is connected to the movable part of the second electrode, and wherein the welding gun further comprises a second force sensor configured to measure force on the second electrode.

2. The welding gun according to claim 1, wherein an angle between the axes of the inverted roller-screw gear and the electric motor is right, obtuse or acute.

3. The welding gun according to claim 1, wherein the rod of the electromechanical linear actuator of the movable part of the second electrode is coaxial or parallel to the movable part of the second electrode.

4. The welding gun according to claim 1, wherein the movable part of the second electrode is angled relative to the rod of the electromechanical linear actuator of the movable part of the second electrode.

5. The welding gun according to claim 1, further comprising an anti-rotation assembly of the movable part of the second electrode and a channel for cooling fluid that cools the movable part of the second electrode, wherein the channel for cooling fluid that cools the movable part of the second electrode is at least partially located in the anti-rotation assembly.

6. The welding gun according to claim 5, wherein the anti-rotation assembly comprises tubes slidably contacting sleeves through which the tubes are arranged to pass and the channel for cooling fluid is at least partially located in the tubes and openings in the housing, and wherein the tubes and the openings are parallel to the rod.

7. The welding gun according to claim 5, wherein the anti-rotation assembly comprises two projections slidably contacting the housing and the channel for cooling fluid is partially located in the tubes and openings in the projections, wherein the tubes and the openings are parallel to the rod.

8. The welding gun according to claim 1, wherein the electromechanical linear actuator of the first electrode is fixedly attached relative to the fixed part of the second electrode.

9. The welding gun according to claim 1, wherein the fixed part of the second electrode is angled relative to the movable part of the second electrode.

10. The welding gun according to claim 1, wherein the movable part of the second electrode is coaxial or parallel to the first electrode.

11. The welding gun according to claim 1, wherein the movable part of the second electrode is angled relative to the first electrode.

12. The welding gun according to claim 1, wherein the movable part of the second electrode comprises a mounting base having a cylindrical projection, and wherein the movable part of the second electrode and the fixed part of the second electrode are connected to each other with the mounting base.

13. The welding gun according to claim 1, wherein the second force sensor comprises a resistive-strain sensor or a piezo-electric force sensor mounted to a surface of the fixed part of the second electrode.

14. The welding gun according to claim 1, further comprising a control system having a computation unit to calculate positions of the first electrode and the movable part of the second electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 shows a welding gun in a closed and opened state (side view) according to an embodiment of the present invention.

[0036] FIG. 2 shows a second electrode and an EMA (cross-sectional side view) of a movable part of a second electrode of the welding gun according to an embodiment of the present invention.

[0037] FIG. 3 shows an isometric view of the second electrode and the EMA of the movable part of the second electrode of the welding gun according to an embodiment of the present invention.

[0038] FIG. 4 shows channels for cooling a cap of the second electrode and the EMA of the movable part of the second electrode of the welding gun (part-sectioned side view) according to an embodiment of the present invention.

[0039] FIG. 5 shows an embodiment of the EMA of the movable part of the second electrode (cross-sectional side view) with an anti-rotation assembly having a spline shaft.

[0040] FIG. 6 shows an embodiment of the EMA of the movable part of the second electrode (part-sectioned side view) with water channels partially arranged in the anti-rotation assembly.

[0041] FIG. 7 shows an embodiment of the EMA of the movable part of the second electrode (part-sectioned side view) with the anti-rotation assembly in the form of projections on a transition piece to which the movable part of the second electrode is attached.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0042] FIG. 1 shows a welding gun comprising a frame 1, a first electrode 2 (hereinafter, the electrode 2) and a second electrode having a fixed part 3 of the second electrode and a movable part 4 of the second electrode (hereinafter, the electrode 4). The welding gun is fixedly attached on a movable arm of a six-axis robot (not shown in the drawings) by a flange 5 and mounting screws (not shown in the drawings). An EMA 8 of the first electrode, to a rod 9 of which the first electrode 2 is fixedly attached, is fixedly mounted on the frame 1 of the welding gun. The EMA 8 of the first electrode comprises an electric motor (not shown in the drawings), a roller-screw gear (not shown in the drawings) and a position sensor (not shown in the drawings). It should be noted that any other transmission that transforms rotational movement of the electric motor into translation movement of the EMA rod can be used instead of the roller-screw gear, for example, a ball screw. Also, the EMA 8 of the first electrode can be a linear electric motor. The rod of the EMA 8 of the first electrode is connected to a moving reciprocating element of the roller-screw gear—a roller-screw gear screw (not shown in the drawings), which achieves reciprocating movement of the electrode 2. The EMA 8 of the first electrode comprises a force sensor (not shown in the drawings) configured to measure force on the first electrode 2.

[0043] Caps 6 and 7 are mounted at the ends of the electrodes of the welding gun, and these caps 6 and 7 contact the parts 54 to be welded. For simplicity, the contact between the caps 6, 7 and the parts 54 to be welded is referred to as the contact of the electrode 2 and the electrode 4 with the parts 54 to be welded. The second electrode has the fixed part 3 fixedly mounted on the frame 1 of the welding gun. The EMA 10 of the second electrode is fixedly attached on the fixed part 3 of the second electrode. The rod 11 of the EMA 10 of the second electrode (FIG. 2) is fixedly attached to the electrode 4 coaxial to the first electrode 2. In the suggested embodiment, the movable part 4 of the second electrode is connected to the rod 11 of the EMA through a transition piece 12 to make it easy to replace the movable part 4 of the second electrode due to wear of a retaining cone on which the cap 7 is mounted. The electrode 4 is connected to the transition piece 12 with mounting screws 13. The transition piece 12 is connected to the rod 11 with a mounting screw 14 coaxial to the rod 11. To improve robustness of the connection between the rod 11 and the transition piece 12, longitudinal teeth that cut into the smooth surface of the cylindrical opening in the transition piece 12 causing this surface to deform are provided on the rod 11. In the suggested embodiment, the rod 11 is coaxial to the electrode 4. However, to weld the parts 54 to be welded in locations near to the bent portions of the parts 54 to be welded at a right angle, the electrode 4 may be slightly eccentric relative to rod 11, which eccentricity will allow for the desired operating life of the EMA 10 of the second electrode. It should be noted that in order to reduce working apertures in vehicle body parts, the electrodes 2 and 4 may be non-coaxial to each other, but, for example, be angled relative to each other. The electrodes 2 and 4 can be also not coaxial relative to each other if the welding spot of the parts 54 to be welded is too close to the bent portions of the parts 54 to be welded to prevent the welding gun from hitting the parts 54 to be welded and damaging the latter. In this case, the rod 11 is angled with respect to the electrode 4. The EMA 10 of the second electrode, the cross-section of which is shown in FIG. 2, comprises an electric motor, an inverted roller-screw gear, a bevel gear and a position sensor 17 within housings 15 and 16. The electric motor is comprised of a stator 18 fixed on an inner surface of an opening in the housing 16 and a rotor 19 having permanent magnets 20 which are fixedly attached on a surface of the rotor 19. The rotor 19 is mounted on two bearing supports 21 and 22, which are radial bearings but also can be other known bearings configured to receive load, such as needle bearings. A shaft of the position sensor 17, which is an inductive encoder, is fixed on the rotor 19. The position sensor also can be an optical encoder or a resolver, or any other known position sensor, which is used to determine the angular position of the rotor 19 of the electric motor. The position sensor 17 is fixed on a transition part 23, which is fixed to the housing 16. The position sensor 17 and the stator 18 are connected to the control system of the EMA 10 of the second electrode by cables (not shown in the drawings), and the control system can be performed as a frequency converter (not shown in the drawings). The frequency converter receives signals from the position sensor 17 and forms supply voltage of the stator 18.

[0044] The roller-screw gear comprises a nut 24 having internal threading, rollers 25 with external threading, and a part of the rod 11 on which a portion with external threading is positioned. In another embodiment, the rod 11 can be comprised of two separate parts, a roller-screw gear screw with a portion of external thread and a rod. When the nut 24 rotates, the rollers 25 roll around the rod 11 in planetary motion and move together with the rod 11 along the axis of the nut 24. The rod 11 of the EMA 10 of the second electrode is fixedly connected to the screw. Thus, the electrode 4 and the rod 11 moves in a reciprocating manner within the stroke restricted by the length of the internal threading of the nut 24. The nut 24 is arranged to rotate in a bearing support, which represents two radial thrust bearings 26. A bevel gear is configured to transfer the rotary movement from the rotor 19 of the electric motor to the nut 24. The bevel gear is configured to transfer rotary movement and torque from the rotor 19 of the electric motor to the nut 24 and also to position the nut 24 and the rotor 19 such that their axes are perpendicular to each other. Generally, the fixed electrode of the welding gun has several bends. Arranging the roller-screw gear and the electric motor at an angle to each other allows placing the EMA in the bent portion of the fixed electrode without substantially increasing the length of the welding gun. In other words, this arrangement of the roller-screw gear and the electric motor reduces the length of the EMA 10 of the second electrode in the direction along the axis of the electrode 4, which reduces the distance from the end of the electrode 4 to an outer protruding portion of the fixed part 3 of the second electrode. This feature reduces the space the welding gun takes up near a welding spot, which lowers the risk of one welding gun colliding with another welding gun performing welding in adjacent areas on a vehicle body in one welding wok cell. The toothed gears 27 and 28 are engaged with each other and transfer the rotary movement and torque from the rotor 19 of the electric motor to the nut 24. The reduction ratio of the bevel gear is selected such as to provide the desired linear speed of movement of the electrode 4 and the force on that electrode, while maintaining a small size of the electric motor and small dimensions of the EMA 10 of the second electrode. It is important that, depending on the configuration of parts 54 to be welded, the welding gun has a corresponding shape of the fixed part 3 of the second electrode, e.g., arranged at an acute or obtuse angle to the movable part of the second electrode. In this case, the toothed gears 27 and 28 have the same angle between the axes as the fixed part 3 of the second electrode and the electrode 4. It minimizes the size of the protruding portion of the welding gun.

[0045] It should be noted that the stroke of the rod 11 of the EMA 10 of the second electrode is significantly less than the stroke of the EMA 8 of the first electrode, since the EMA 10 of the second electrode moves the second electrode 4 only over a small distance. This is the distance that is formed between the end of the electrode 4 and one of the parts 54 to be welded when the robot brings the welding gun to the parts 54 to be welded. The distance is selected with consideration of the tolerance for the parts 54 to be welded in the equipment.

[0046] The welding current is supplied to the electrodes 2 and 4 using a welding transformer 29, which is controlled by a welding controller (not shown in the drawings). Current from the welding transformer 29 is supplied to the first electrode 2 via a current lead 30, the ends of which are fixed on the first electrode and on a current-conducting element (not shown in the drawings) connected to the welding transformer 29. The electrode 4 receives the welding current through the fixed part 3 of the second electrode connected to the welding transformer 29 and along a current lead 31, the ends of which are fixed on a mounting base 32 and the transition piece 12. The current leads 30 and 31 are made of a set of thin copper sheets for providing flexibility and avoiding additional load on the EMA 8 of the first electrode and the EMA 10 of the second electrode when both electrodes move. The mounting base 32 is configured to be connected with the fixed part 3 of the second electrode. In the suggested embodiment, the mounting base 32 has a cylindrical projection. This connection of the parts of the fixed electrode is typical for welding guns of known constructions that have a single movable electrode 2. This feature provides for modernization of this welding gun by further fitting them with the EMA for moving the second electrode 4 without changing the main part of the welding gun construction. The mounting base 32 is clamped with a clamp 33 incorporated into the fixed part 3 of the second electrode of the welding gun. The clamp 33 is compressed with mounting screws 34, which fixedly fasten the mounting base 32 relative to the fixed part 3 of the second electrode accordingly. Due to the frictional fixation of the housing of the EMA 10 of the second electrode relative to the fixed part 3 of the second electrode, the electrode 4 can be placed coaxially to the first electrode 2 during assembly. There is a projection on the fixed part 3 of the second electrode instead of on the mounting base 32 of a flattened surface coupled to this projection. Due to this, the electrode 4 takes a precise angular position during welding gun assembly. The mounting base 32 can be made without the projection such that the angular position of the electrode 4 can be adjusted so that the end thereof coincides with the end of the electrode 2. The mounting base 32 is connected to the housing 16 with mounting screws (not show in the drawings) and is replaceable to allow mounting the electrode 4 with the EMA 10 of the second electrode on the welding gun, which has the mounting area for the electrode 4 of different diameters and forms. It should be noted that the electrode 2, the fixed part 3 of the second electrode, the electrode 4, the caps 6 and 7, and the transition piece 12 are made of a current-conducting material, such as copper.

[0047] There is a force sensor 37, which is a resistive-strain sensor adhered to the surface of the fixed part 3 of the second electrode so that the former deforms and the resistance thereof changes when the fixed part 3 of the second electrode bends. The force sensor 37 is calibrated on the welding gun by compressing an outer force sensor, which is placed between the electrodes 2 and 4 during calibration, and plots the ratio of the force on the outer force sensor to a signal of the force sensor 37. The force sensor 37 can be also presented as a piezoelectric force sensor mounted to the fixed part 3 of the second electrode with mounting screws. The signal of the force sensor 37 is transmitted through a cable (not shown in the drawings) to a frequency converter that controls the EMA 10 of the second electrode. In another embodiment, the force sensor 37 is positioned within the EMA.

[0048] The caps 6 and 7 heat up during welding of the parts 54 to be welded. The caps are cooled with water cooling. For this, within the electrode 4, there is a channel in the form of a longitudinal aperture in the electrode 4 and a tube 35, through which water passes. There is also a similar channel and a tube 36 for water inside the first electrode 2. A channel and a cooling liquid flow tube pass within the mounting base 32. To cool the cap 7, the water enters through the first channel formed inside the fixed part 3 of the second electrode (not shown in the drawings), then through the first opening in the mounting base 32 (FIG. 4), then through one of the tubes 42 in the housing 16, then through the first opening in the housing 15, then through a tube 38 which is in communication with the first opening of the housing 15, then through the first opening in the transition piece 12 (not shown in the drawings), then through the tube 35. After that, the water enters the second opening of the transition piece 12 through the internal volume of the cap 7 along the channel in the opening of the electrode 4. Then, heated by the heat of the cap 7, the water passes through the second tube 38 which is in communication with the second opening in the transition piece 12, then through the second opening in the housing 15, then through the second tube 42 (not shown in the drawings) in the housing 16, then through the second opening in the mounting base 32 into the second channel inside the fixed part 3 of the second electrode. To improve the ease of manufacturing of the water flow openings, the housing 15 can be comprised of a main part and a cover 43, between which rubber sealing rings 44 and 45 are arranged to seal the channels. The cover 43 is attached to the housing 15 with mounting screws (not shown in the drawings). The joint of the housings 15 and 16 is protected from leaks in the area of the tube 42 by rubber sealing rings 46 and 47. The tubes 38 are protected from leaks by rubber rings 48. The tubes 38 are fixedly mounted on the transition piece 12 e.g., with a press fit achieved by providing the tubes 38 with a diameter slightly exceeding the diameter of the opening for receiving them in the transition piece 12. The tubes 38 move together with the transition piece 12 and the electrode 4 relative to the housing 15. To reduce friction when the tube 38 moves, sleeves 40 are provided through which the tubes 38 are arranged to pass. It should be noted that the channels arranged for water to flow to the cap 7 and back from it are laid inside the tubes 38 that do not exceed the dimensions of the EMA 10 of the second electrode, which eliminates the need to provide the water flow tubes outside the EMA 10 of the second electrode, which tubes can be damaged when the welding gun collides with an obstacle during robot training.

[0049] Optionally, in order to prevent the nut 24 and the rod 11 of the EMA 10 of the second electrode from rotating together and to achieve linear movement of the rod 11, the EMA 10 of the second electrode comprises an anti-rotation assembly. In one of the embodiments, the anti-rotation assembly is a current lead 31, which can bend in the direction along the axis of the electrode 4 but is sufficiently rigid in the direction across the axis of the electrode 4, which causes the rod 11 to remain in the same axial position and achieves translation movement of the rod 11. In another embodiment shown in FIG. 5, the anti-rotation assembly is comprised of a spline shaft 49 fixedly attached on the cover 43, and longitudinal slots 50 in the opening of the rod 11. The slots 50 mate with the longitudinal slots on the spline shaft 49. Thus, linear movement of the rod 11 and, consequently, of the electrode 4 is achieved. It is important that, according to another embodiment (FIG. 6), when the tubes 38 are sufficiently robust, i.e., if the tubes have a sufficient diameter and a wall thickness to receive torque applied to the rod 11, the anti-rotation assembly is comprised of two tubes 38 and sleeves 40. The tubes slidably contact the sleeves 40. The tubes 38 are fixedly attached to the transition piece 12 and are parallel to the rod 11. The water passing through the tubes 38 improves the cooling of the sleeves 40, which reduces the amount of wear thereof and increases the operating life of the anti-rotation assembly and, therefore, the operating life of the welding gun.

[0050] To increase the maximum force of the EMA 10 of the second electrode, the anti-rotation assembly has to be reinforced, since the rod 11 receives increased torque. For this, in another embodiment, cylindrical rods 41 configured to be received in the openings of the sleeves 39 are fixedly attached on the housing 15. The sleeves 39 are pressed in the openings in the transition piece 12, and the rods 41 have the form of pins and are pressed into the housing 15. At the same time, the diameter of the rods 41 is slightly less than the internal diameter of the sleeves 39 to allow them to move freely relative to each other parallel to the axis of the rod 11. In another embodiment, the rods 41 may be connected to the housing 15 through threading on the surface thereof and a threaded opening in the housing 15. The transition piece 12 moves together with the tubes 38 and sleeves 39 relative to the sleeves 40 and the rods 41, respectively, wherein the rods 38 and the sleeves 39 contact the sleeves 40 and the rods 41 across the entire stroke of the rod 11 and of the transition piece 12 and the electrode 4, respectively. Thus, the rod 11, the transition piece 12 and the electrode 4 are fixed around their axis relative to the housing of the EMA 10 of the second electrode, and their reciprocating movement is achieved.

[0051] As stated above, in an embodiment, the electrode 4 can be arranged at an angle to the electrode 2, and the rod 11 can be arranged at an angle to the electrode 4, correspondingly. In this case, when the parts are welded, a radial (lateral) load occurs that acts on the electrode 4. The radial load acting on the electrode 4 also occurs when the welding gun collides with an obstacle, if the collision impacts the electrode 4, for example, when training the robot. The tubes 38 and the rods 41 in corresponding embodiments take the radial load to prolong the operating life of the roller-screw gear. Due to the fact that the short length of the electrode 4 prevents forming of a large bending moment acting on the electrode 4, the tubes 38, the sleeves 40, the rods 41, the sleeves 39 and parts of the roller-screw gear, the size of said parts and units, as well as the size of coupling elements of the transition piece 12 and the electrode 4 can be reduced and, therefore, the size of the EMA 10 of the second electrode can be reduced. In another embodiment shown in FIG. 7, the transition piece 12 on the end surface facing the housing 15 has two projections 51. Each projection 51 has an opening parallel to the tubes 52 fixed in the housing 15, wherein said opening is slightly larger than the tube 52 to allow them to move freely relative to each other parallel to the axis of the rod 11. The water is supplied to and drawn from the cap 7 through the tubes 52. The tubes are sealed with sealing rings 53. The projections 51 slidably contact the housing 15 so that the transition piece 12 can freely move parallel to the axis of the rod 11 but cannot rotate around own axis. Thus, the transition piece 12 and the electrode 4 in this embodiment are prevented from rotating to enable linear movement of the electrode 4. The wear-resistant pads (not shown in the drawings) are placed between projections 51 and the housing 15. The improved cooling of the pads because of water passing through the openings in projections 51 decreases the wear of said pads. In case of increased torque required for increased force on the electrode 4, rods 41 fixed on the housing 15 and the sleeves 39 fixed on the transition piece 12 are further provided, where the rods 41 and the sleeves 39 are linearly movable relative to each other.

[0052] The EMA 8 of the first electrode is controlled by a separate frequency converter but can be controlled by the same frequency converter as the EMA 10 of the second electrode, if it is capable of connecting to and controlling at least two EMAs, for example, in a two-channel implementation. A signal from the force sensor of the EMA 8 of the first electrode is supplied to the frequency converter.

[0053] The suggested welding gun operates as follows. When configuring the robot, an operator, using a control unit of the robot controller, moves the welding gun to a testing plate (a steel plate fixedly mounted relative to the robot) such that the electrode 4 is facing the testing plate, is perpendicular to the testing plate and is at a distance from the nearest surface not exceeding the stroke length of the rod of the EMA 10 of the second electrode. At the same time, the rod 11 is fully retracted, which minimizes the risk of damaging the EMA 10 of the second electrode when the welding gun collides with an obstacle, such as the equipment with parts 54 to be welded or another welding gun. With the rod 11 fully retracted, the gap between the housing 15 and the transition piece 12 disappears, which causes the force that may be applied to the electrode 4 upon impact to be transferred to the housing 15 and not to the threaded portion of the rod 11, other details of the roller-screw gear or bearings 26. In response to a command from the operator, the controller records the current spatial coordinate of the welding gun in a controller memory. Then the operator sends a command to move the electrode 4 via the control unit. The stator 18 receives supply voltage from the frequency converter, wherein a rotating magnetic field is generated in the stator, which engages the rotor 19 and, thus, rotates the toothed gear 27. The toothed gear 27 drives the toothed gear 28 and the nut 24. When the nut 24 rotates, the rollers 25 engaged with the internal threading thereof roll in planetary motion around the threaded portion of the rod 11 and extend together with the rod 11 and the electrode 4. The electrode 4 moves towards the testing plate until it contacts the latter, wherein the time of contact is determined based on a signal from the force sensor 37, which changes when the force starts acting on the electrode 4. Using the position sensor 17, a coordinate of the electrode 4 is determined, in which the signal from the force sensor 37 achieved a pre-determined force, the value of which can be used to conclude that the electrode 4 touched the testing plate. When this force is reached, the electrode 4 stops. The distance which the electrode 4 has passed from the coordinate of the welding gun is added to the coordinate of the welding gun recorded earlier along the axis along which the electrode 4 has moved. The resulting coordinate is a coordinate of the end of the electrode 4 (Tool Center Point) with the rod 11 fully retracted relative to the robot. Thus, a spatial position of the welding gun relative to the robot becomes known, and a zero position of the electrode 4 is determined. Then, the spatial coordinate of the welding gun needs to be determined when the welding gun is brought to each welding spot intended to be welded by this welding gun. The process is performed in the same way as the process of determining the coordinate of the end of the electrode 4 and the zero position thereof, except the electrode 4 touches one of the parts 54 to be welded instead of the testing plate. By moving the electrode 4 instead of the welding gun and due to the small weight of the electrode 4, the configuration process of the welding gun is performed faster. It reduces the cost of configuring the welding gun, which is particularly relevant for vehicle factories, which use hundreds of welding guns.

[0054] To weld the parts 54 to be welded, a welding workcell robot brings the welding gun to the welding spot placing the welding gun such that the electrode 4 is facing one of the parts 54 to be welded, wherein the electrode 4 is perpendicular to the parts 54 to be welded. The stator 18 receives supply voltage from the frequency converter, wherein a rotating magnetic field is generated in the stator, which engages the rotor 19 and, thus, rotates the toothed gear 27. The toothed gear 27 drives the toothed gear 28 and the nut 24. When the nut 24 rotates, the rollers 25 engaged with the internal threading thereof roll in a planetary motion around the threaded portion of the rod 11 and extend together with the rod 11 and the electrode 4. The electrode 4 moves towards the parts 54 to be welded until it contacts the latter, and this time of contact is determined based on a signal from the force sensor 37, which changes when the force starts acting on the electrode 4. However, the electrode 4 can begin moving before the full stop of the robot that moves the welding gun in order to reduce the welding cycle duration. After this, the frequency converter moves the electrode 2 coupled to the EMA 8 of the first electrode towards the parts 54 to be welded, at first, at a high speed and, starting from a certain coordinate at which the electrode 2 still does not contact the parts 54 to be welded, at a reduced speed that prevents damage to the parts 54 to be welded upon a collision with the electrode 2.

[0055] It should be noted that it is possible to start moving the electrode 2 to the parts 54 to be welded when the electrode 4 is moving to the part to be welded to shorten the welding cycle duration. Then, the electrode 2 contacts the parts 54 to be welded and the clamping of the parts 54 to be welded by the welding gun between the electrode 4 and the electrode 2 begins. When the parts 54 to be welded are clamped, both electrodes are moved at the same speed. In response to a signal from the position sensors of the EMA 8 of the first electrode and the EMA 10 of the second electrode, the ratio of the force on the electrodes of the welding gun to the position of these electrodes (a signal from the position sensors) is determined, and the coordinate of each electrode, at which the preset force will be reached in the welding cycle, is predicted. The electrodes 2 and 4 stop at the preset coordinates, and the control system checks if the force is reached based on the signal from the force sensor 37 and the force sensor of the EMA 8 of the first electrode. Then, the welding controller supplies current to the electrodes 2 and 4 through the welding transformer 29, and the welding of the parts 54 to be welded is performed. After the welding is complete, the EMA 8 of the first electrode and the EMA 10 of the second electrode move the electrodes 2 and 4 away from the welded parts. The robot then moves the welding gun to the next welding spot.

[0056] In the course of welding, the caps 6 and 7 wear out and a procedure for restoring the shape thereof, such as sharpening, is performed. After several sharpening procedures, the caps 6 and 7 become extremely short, and the caps are replaced. In this case, the procedure of determining the coordinate of the end of the electrode 4 should be repeated. Therefore, not only the configuration at startup time of the welding gun is reduced, but also time on the welding gun maintenance is saved, therefore, inefficient stand still time of the welding workcell is reduced.

[0057] Also, an embodiment is possible, in which a control system is presented by a controller (not shown in the drawings) and a two-channel frequency converter or two frequency converters. A computation unit is integrated in the controller to calculate a position of the electrodes 2 and 4. This control system avoids redundancy of the calculation functions in the frequency converters and simplifies said frequency converters. The functionality of this controller may be also performed by a robot controller.

[0058] It should be noted that the suggested welding gun can be stationary arranged, i.e., fixedly attached to, e.g., a base fixed on the floor of the manufacturing facility, and the parts 54 to be welded may be brought to them using a robot having equipment, on the moving axis thereof, for capturing these parts to be welded.

[0059] The present invention is not meant to be limited by the particular embodiments disclosed in the description by way of example and includes all possible modifications and alternative embodiments falling within the spirit and scope of the present invention as defined in the enclosed claims.