Method for adaptive control of a welding gun traveling electrode

Abstract

A method for adaptive control of the traveling electrode of a spot welding gun includes causing the traveling electrode of the welding gun to move at a first speed when performing a welding (working) cycle and to switch to a second speed at a point, the coordinate of which is calculated based on the point of contact of the coordinate of the electrode obtained at the configuration cycle performed before the working cycle. The method allows for a reduction in welding cycle time.

Claims

1. A method for adaptive control of a traveling electrode of a spot welding gun, the method comprising: providing a welding gun comprising: a pair of electrodes, at least one of which is the traveling electrode; a linear actuator for the traveling electrode, the linear actuator being arranged for linear movement of the traveling electrode toward another electrode of the pair of electrodes; a position sensor arranged for sensing a position of the traveling electrode; a force sensor arranged for sensing a force acting on the traveling electrode; input/output means for working parameters input; a controller configured for receiving data via the input/output means, and data from the position sensor and the force sensor, for storing and processing said data and for outputting control signals for controlling the linear actuator of the traveling electrode; and performing a cycle of a spot welding gun configuration, wherein the cycle comprises: setting working parameters including values of a first speed of movement of the traveling electrode, and a second speed of movement of the traveling electrode; first and second compressive forces of the electrodes; target force; a distance equal to a maximum thickness of parts to be welded, including tolerance, wherein the second compressive force is less than a target force, at which welding of the parts to be welded is performed, and wherein the first compressive force is greater than a force, at which force fluctuation, caused by a touching of the electrodes to each other, finishes; bringing the traveling electrode to the other electrode of the pair of electrodes at the second speed without contacting the electrodes with each other; bringing the electrodes in contact with each other and further moving the electrodes toward each other to reach the first compressive force; building up a compressive force by moving the traveling electrode at the second speed until the second compressive force is reached, after which a speed of movement of the traveling electrode is lowered; and lowering the speed of movement of the traveling electrode until the target force is reached, the method further comprising: after the second compressive force is reached, determining a coordinate of a base point, at which the electrodes touch each other, as a point of intersection of a linear function defining correspondence between an electrode compression force and the coordinate of the traveling electrode during build-up of a force from the first compressive force to the second compressive force, with a level of zero compressive force; performing a working cycle of a welding gun compression, when the parts to be welded are positioned between the electrodes of the welding gun, the working cycle comprising: bringing the traveling electrode to the parts to be welded at the first speed without contacting the electrodes with the parts to be welded; lowering the speed of the traveling electrode movement from the first speed; bringing the traveling electrode in contact with a nearest part to be welded; lowering the speed of the traveling electrode movement to the preset second speed at a distance A from a base point at which the electrodes touch each other; reaching the first compressive force; building up a part compression force with the movement of the traveling electrode at the second speed to reach the second compressive force, after which the speed of the traveling electrode movement is lowered; lowering the speed of the traveling electrode until the target force is reached; after the second compressive force is reached, a coordinate of a point at which the traveling electrode touches the part to be welded is determined as a point of intersection of a linear function defining correspondence between the part compression force and a coordinate of the traveling electrode during the build-up of force from the first compressive force to the second compressive force, with a level of zero compression force; and setting a coordinate of the point at which the traveling electrode begins movement at the first speed during a next working cycle of compression, the coordinate of the point being equal to a coordinate of the point at which the traveling electrode touches the part to be welded, determined during the performed working cycle of the welding gun compression.

2. The method of claim 1, wherein performing the working cycle of the welding gun compression further comprises: determining a point of contact of the pair of electrodes before setting the working parameters, wherein before to the contact of the electrodes to each other the method further comprises: determining a point based on data on the point of contact, at which the electrodes, contacting each other, is a point at which the first speed of the movement of the traveling electrode lowers to the second speed of the movement of the traveling electrode; bringing the traveling electrode to the other electrode of the pair of electrodes at the first speed without contacting of the electrodes with each other; and lowering the speed of the movement of the traveling electrode to the second speed.

3. The method of claim 1, wherein the distance A from the base point, at which the electrodes touch each other, to the coordinate of the point of beginning of the movement of the traveling electrode at the second speed further includes a distance equal to a maximum possible gap between the parts to be welded.

4. The method of claim 1, wherein the distance A from the base point, at which the electrodes touch each other, to the coordinate of the point of beginning of the movement of the traveling electrode at the second speed further includes a distance equal to a maximum displacement of the parts to be welded along an axis of the traveling electrode.

5. The method of claim 1, wherein the distance A from the base point, at which the electrodes touch each other, to the coordinate of the point of beginning of the movement of the traveling electrode at the second speed further includes a distance equal to a maximum possible thickness of the parts to be welded, which are configured for installation in a tooling that holds the parts to be welded during welding.

6. The method of claim 1, wherein the first compressive force is corrected during the working cycle.

7. The method of claim 1, wherein the first compressive force and the second compressive force are preset during calibration of the spot welding gun.

8. The method of claim 1, wherein the first speed equals to a maximum speed of the linear actuator of the traveling electrode of the spot welding gun.

9. The method of claim 1, wherein a limiting value for the coordinate of the point, at which the traveling electrode touches the part to be welded, is set according to a maximum permissible cap wear.

10. The method of claim 1, wherein the linear actuator of the traveling electrode is implemented as an electromechanical actuator with a roller screw drive or a ball screw drive.

11. The method of claim 1, wherein the linear actuator of the traveling electrode is implemented as a linear electric motor.

12. The method of claim 1, wherein the position sensor is implemented as an encoder or resolver.

13. The method of claim 10, wherein the controller is arranged in an electromechanical actuator.

14. The method of claim 1, wherein the welding gun comprises another force sensor configured to sense a force arisen on the electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] FIG. 1 shows plots of force and speed against the position in a welding cycle according to a prior art method, at portions where the traveling electrode moves at speed (V.sub.0), at speed (V.sub.1), and where compression of the electrodes reaching the target force is carried out.

[0068] FIG. 2 schematically shows a welding gun.

[0069] FIG. 3 schematically shows a robotic welding cell with industrial manipulators and a welding gun fixed thereto.

[0070] FIG. 4 shows plots of force and speed against the position in the configuration cycle and in the working cycle at portions where the traveling electrode moves at speed (V.sub.1), compression of the electrodes and maintaining the target force.

[0071] FIG. 5 shows plots of force and speed against the position in the welding cycles according to proposed method and prior art method.

DETAILED DESCRIPTION OF THE INVENTION

[0072] FIG. 2 shows a welding gun 1 of C-type which may be controlled by the proposed method. Similar C-type welding gun is described, for example, in EP1221351A1. The welding gun 1 comprises a stationary electrode 2 and a traveling electrode 3. A transformer 4 is mounted on the welding gun for supplying the electrodes 2 and 3 with the current for welding. A traveling electrode linear displacement mechanism, such as an electromechanical actuator (EMA) 5 with a roller screw drive is fixedly mounted on the welding gun 1 for moving the electrode 3. Similar EMA is described in U.S. Pat. No. 9,520,756B2. The electrode 3 is fixed on a shaft 6. The EMA 5 comprises an electric motor (not shown) having a rotor and a stator (not shown), a position sensor 7, a force sensor 8 and a mechanism for transforming rotary movement of the rotor into translational movement of the shaft 6—a roller screw drive, although the suggested method can be implemented with the EMA 5 used with a ball screw drive or any other known mechanism of transforming rotary movement of the rotor into translational movement of the shaft. In order to prevent the shaft 6 and the electrode 3 from rotating along with the EMA 5 rotor during roller screw operation, the EMA 5 is provided with an anti-rotation assembly for reducing the mass and dimensions of the welding gun 1.

[0073] Instead of an integrated anti-rotation assembly, the welding gun may be provided with an anti-rotation assembly in the form of a guide (not shown), for example. It should be noted that in addition to the disclosed example, the EMA 5 may be provided as a linear electric motor that does not require a mechanism for transforming rotary movement into translational movement of the shaft or as any other known EMA 5 configuration. The EMA 5 contains a controller 9 for controlling the EMA 5. Controller 9 comprises the processing unit (not shown) to process a data received from different sensors e.g., the force data from the force sensor 8 and the position data from the position sensor 7 in order to calculate the electrode 3 motion parameters (coordinates, speed and force) which are described further according to the present invention. The position sensor 7 may be an encoder or a resolver attached to the EMA 5 rotor and enables the controller 9 to track the position of the traveling electrode 3. In other embodiments, a controller 9 may be positioned as a separate unit, or the controller 9 may be combined with a robot controller 10 for controlling a robot 11. However, positioning the controller 9 within the EMA 5 beneficially reduces the number of conductors for electric installation when placing power and communication lines across the robot 11, since it is not required to place cables for a force sensor 8 across the robot 11 and, in other embodiments of the welding gun, for a force sensor 12 as well.

[0074] FIG. 3 schematically shows a robotic welding cell (hereinafter—a welding cell) comprising a robot 1 (an industrial manipulator) having 6 degrees of freedom, and a welding gun 1 fixed on the last movable arm of the robot. The welding cell further comprises a welding controller 13 for controlling a transformer 4 for supplying the electrodes 2 and 3 with the welding current. The controller 9, the robot controller 10 and the welding controller 13 are interconnected by communication lines (not shown) for exchanging “ready for operation” signals, operation initiation commands and operation completion commands. The robot 11 moves the welding gun 1 bringing the stationary electrode 2 to the parts 14 to be welded. The robot 11 brings the welding gun 1 such that the stationary electrode 2 touches at a right angle one of the parts 14 fixed in the tooling 15. The parts 14 to be welded may be in the form of at least two metal parts of an item, for example, parts of a vehicle body. There can be two or more parts 14 depending on the construction of the product. In response to a command from the controller 9, the EMA 5 moves the electrode 3 towards the parts 14 (i.e., the welding gun 1 closes) and compresses the parts 14 between the electrodes 2 and 3 with predetermined force.

[0075] It should be noted that in response to a command from the robot controller 10, the robot 11 moves the welding gun 1 as the parts 14 are being compressed, and a distance of the movement equals the elastic deformation of the welding gun 1, which corresponds to the predetermined force, to avoid deforming the parts 14. In other words, the robot performs equalizing—a process of equalizing forces on both electrodes. In an embodiment, an EMA (not shown) is located between a movable arm of the robot 11 and the welding gun 1, wherein the EMA shaft axis (not shown) coincides with the axes of the electrodes 2 and 3. The EMA is fixed on the robot 11 arm, and the shaft of the EMA is fastened to the welding gun, e.g., to the frame thereof. The EMA moves the welding gun 1 a distance equal to elastic deformation of the welding gun 1 so that the robot 11 does not have to do this. The speed of the welding gun movement during equalizing shall coincide with speed (V.sub.1). The distance passed during equalizing is called an equalizing amount. Equalizing helps to speed up a welding cycle because said EMA operates faster than the robot 11 because of a much shorter response time. To increase equalizing accuracy, a force sensor 12 may be mounted on the welding gun 1 stationary electrode. In this case, the welding gun 1 can be moved up to a moment when force on the force sensor 8 and the force on the force sensor 12 are equal. After the parts 14 are compressed, a welding controller 13 supplies welding current through a transformer 4 mounted on the welding gun 1 to the electrodes 2 and 3, and the welding current melts the metal of the part 14 in the contact zone of the electrodes 2 and 3 and forms a weld nugget (not shown). After the welding current is no longer supplied by the EMA 3, the weld nugget cools down and the parts 14 become joined. Then, the EMA 5 moves the electrode 3 in a direction away from the parts 14 (i.e., welding gun opens), and the robot 11 moves the welding gun 1 to a next welding spot. In an embodiment of the welding cell, the welding gun 1 is fixedly attached, for example, on the floor, and the robot 1 has means for capturing the parts 14 and is configured for moving the parts 7 so as to position them between the electrodes 5 and 6. The welding cycle is performed in the same order as disclosed above, except that the robot 1, and not the welding gun 2, moves the parts 14.

[0076] FIG. 4 shows plots of force and speed against the position of the traveling electrode 3 in a welding cycle without a welding gun 1 opening portion, because the present invention does not relate to this part of a welding cycle. It is important, that to initially determine the point of touch of the electrodes 2 and 3 contact point, at a distance from which a point where the movement at low speed begins in the first working cycle including the welding of the parts 14 (see below), a welding gun 1 configuration cycle is performed (configuration cycle). Prior to the welding gun configuration cycle, the limiting coordinates of the electrode 3 movement are determined. The current coordinate of the electrode 3 is determined based on the position sensor 7 signal. The closing limiting coordinate is defined as a coordinate obtained by the addition of certain displacement to the point coordinate, where the electrodes 2 and 3 contact without generating force. This displacement exceeds the assumed curvature (deformation) of the welding gun 1 when force is generated between the electrodes 2 and 3 by the assumed cap wear value and some allowance which is necessary since the welding gun 1 curvature is slightly different in each part due to manufacturing tolerances. Thus, the closing limiting coordinate is beyond the point of contact of the electrodes 2 and 3 without exerting any force. Determination of the limiting coordinates is necessary to make possible for the controller 9 to stop the welding cycle if, for example, a cap (not shown) comes off the electrodes 2 or 3 (for example, because of its loose fit due to wear of the retaining cones on the electrodes 2 and 3), and these electrodes will travel a longer distance than expected before colliding with each other.

[0077] The point of contact of the electrodes without exerting any force is determined by the operator moving the electrode 3 with an input/output means, such as a teach pendant or through another known human-machine interface (HMI) of the controller 9 at a low speed until the electrodes contact each other. After the electrodes contact, the operator assumes the current coordinate as the point from which the operator later sets the distance at which speed should be reduced to the speed (V.sub.1) in the configuration cycle. This distance depends on the accuracy of determination of the point of contact of the electrodes 2 and 3 without generating force, and it is subtracted from the coordinate of the point of contact. Thus, the coordinate of the point (SV.sub.1) (the point at which the speed is reduced to the lower speed (V.sub.1) in the welding gun configuration cycle) is determined, wherein the controller 9 calculates the coordinate of the point (SV.sub.1) automatically when the point of contact of electrodes with no force exerted and the specific shift value are input into the HMI of the controller 9. Then, a coordinate of the maximum opening of the welding gun 1 is set by inputting this coordinate, which can be limited by the maximum stroke of the electrode 3, into the controller 9 HMI. The coordinate may be determined by moving the electrode 3 in a direction away from the electrode 2 until the electrode 3 reaches its end position, or it may be determined from the properties of the welding gun 1. It is noted that it is possible to automatically determine limiting coordinates, such as by the controller 9, which moves the electrode 3 for opening and closing the welding gun until the moment the consumed current is exceeded or certain force values are reached.

[0078] Then, the operator sets a maximum value of speeds (V.sub.0) (first speeds) and (V.sub.1) (second speeds) with the HMI of the controller 9, after which the configuration cycle is performed. The speed (V.sub.0) value depends on the opening distance of the welding gun 1. For example, it is pointless to set the speed (V.sub.0) to be equal to the maximum speed of the EMA for a small distance, since the EMA will not build up this speed within this distance. Therefore, the speed (V.sub.0) value can be equal to the speed (V.sub.1). The speed (V.sub.0) can be automatically calculated by the controller 9 based on the distance the electrode 3 will have been moved and based on the acceleration of the electrode 3 speed increase and decrease.

[0079] Then, the configuration cycle is performed, wherein the electrode 3 is moved by the EMA toward the electrode 2, i.e., the welding gun 1 closes at the speed (V.sub.0) which is preferably equal to the maximum speed of the EMA, and then, starting from a point (SV.sub.1), at the speed (V.sub.1), or the electrode 3 travels the entire welding gun 1 closing distance at the speed (V.sub.1), if this distance is too small for reaching the speed (V.sub.0).

[0080] After the speed is lowered in the point (SV.sub.1), the electrodes 2 and 3 contact in the point (S.sub.cont) (the actual point of contact of the electrodes 2 and 3 in the configuration cycle), after which the electrode 2 and 3 compression force is generated on the electrodes. The force on the electrodes increases until it reaches a target force (F.sub.trg). When the electrodes 2 and 3 are compressed, the force value gradually reaches the value (F.sub.1) (force at which the fluctuations related to the contact deformation of the electrodes stop) and (F.sub.2) (force at which the target force (F.sub.trg) is approached and the speed (V.sub.1) begins to lower), until the target force (F.sub.trg) is reached, wherein the electrodes move at the constant speed (V.sub.1) in the portion before reaching the force (F.sub.2).

[0081] In the configuration cycle, coordinates S(F.sub.1), S(F.sub.2) are determined for preset forces (F.sub.1) and (F.sub.2), accordingly. The force is controlled with the force sensor 8. The forces (F.sub.1) and (F.sub.2) are selected based on analyzing the force build-up beginning portion—the duration of force fluctuations—and a transition process portion when the target force (F.sub.trg) is reached—the duration of approaching the force (F.sub.trg). The force (F.sub.1) should be greater than the force at which the force fluctuation caused by the contact of the electrodes 2 and 3 in the point (S.sub.cont1) ends. The coordinate of the point (S.sub.cont1) can be determined based on a signal of the position sensor 7 received by the controller 9 when the force on the electrode 3 begins to change. It is important for the force (F.sub.1) to be also greater than the force at which a non-linear portion of the force change due to the electrode 3 movement, which portion is caused by a gap between the parts 14, ends. The force (F.sub.2) is chosen so that there is a portion of movement at constant speed (V.sub.1) between the coordinates S(F.sub.1) and S(F.sub.2). To obtain the first data for analysis, the cycle of compression of the welding gun 1 up to the force (F.sub.trg) can be performed before setting the forces (F.sub.1) and (F.sub.2). The operator inputs the forces (F.sub.1) and (F.sub.2) into the controller 9. Based on the duration of the force fluctuations caused by the electrode 3 and the electrode 2 collision, and on the duration of approaching the target force (F.sub.trg), the speed (V.sub.1) is corrected relative to the speed set for the first compression, if such correction is required. The speed (V.sub.1) is also chosen such that there is a linear portion at the electrode 3 and 2 compression step, which is free from force fluctuations, at which portion the coordinates S(F.sub.1) and S(F.sub.2) can be determined for the forces (F.sub.1) and (F.sub.2). Also, the speed (V.sub.1) should be less that the speed at which the collision of the electrodes can cause the welding gun 1 or the EMA 5 to break down (for example, deformation of the electrodes 2 and 3). The higher the speed (V.sub.1), the longer the force fluctuations decay after the electrode 3 touches the parts 14, and the sooner deceleration is required before reaching the target force (F.sub.trg). It should be noted that the forces (F.sub.1), (F.sub.2) and speed (V.sub.1) can be calculated automatically by the controller 9 based on the automatic analysis of the duration of force fluctuations and of approaching the target force (F.sub.trg) by estimating successive force data and dispersing the latter relative to one another and relative to the target force.

[0082] Further, as a part of the configuration cycle, a linear function is determined, wherein the linear function represents a force change during the build-up of the parts compression force from the force (F.sub.1) to the force (F.sub.2) when the coordinate of the traveling electrode is changed from S(F.sub.1) to S(F.sub.2) (hereinafter—the linear function). It should be noted that the force data is received with a delay relative to the determination of the coordinate corresponding to this force, i.e., for example, data on the force (F.sub.1) is actually received with a certain time shift (delay) relative to the moment when the data on the coordinate S(F.sub.1) is received. At the portion of transition from the force (F.sub.1) to the force (F.sub.2), the electrode 3 moves with a constant speed (V.sub.1) so that said shift is the same at both the point S(F.sub.1) and the point S(F.sub.2), which allows for lowering the linear function definition error. Linear extrapolation is performed on the linear function before intersecting the level of force (F.sub.trg), i.e., a coordinate (S.sub.trg1), which is the coordinate of reaching the target force (F.sub.trg) in the configuration cycle, is determined. After the force (F.sub.2), the electrode 3 moves with its speed being reduced to zero at reaching the target force (F.sub.trg) (hereinafter—the force (F.sub.trg)). It is noted that the values of accelerations and decelerations of the electrode 3 are chosen such that undesirable dimensional fluctuations of the welding gun 1 and movable arms of the robot 11 are avoided. In order to minimize the duration of the compressive force build-up, it is important for the electrode 3 not to stop in the process of welding gun 1 compression up to the coordinate (S.sub.trg1).

[0083] After the force (F.sub.2) is reached, the controller 9 performs linear extrapolation on the linear function until intersecting the zero force level, and the electrode contact point in the configuration cycle is assumed as a point (S.sub.base) of intersection between the extrapolated linear function and the zero force level. For convenience, hereinafter the electrode contact point in the configuration cycle is referred to as (S.sub.base). Then, the electrode 3 moves away from the electrode 2 to its initial position, i.e. the welding gun 1 opens.

[0084] In the first working cycle, a point from which the electrode 3 starts to move with the speed (V.sub.1) is set by the controller 9 at a distance A from (S.sub.base) (i.e., the value A is subtracted from the (S.sub.base) coordinate). This point is called a point (S.sub.touch) of touch (hereinafter—the point (S.sub.touch)) in a working welding cycle. The point (S.sub.touch) is characterized as an extreme possible coordinate, where the speed of the electrodes can exceed the speed (V.sub.1). The distance A between the points (S.sub.base) and (S.sub.touch) is set by the operator to be no less than the combined thickness of the parts 14 including tolerance. Additionally, a maximum gap between all parts 14 and tolerances for spatially arranging the parts 14 may be added to the distance A if it is needed to weld the parts 14 taking into account special aesthetic requirements, and it is needed to reduce the speed at which the electrode 3 contacts the parts 14 to avoid dents on the surface thereof. The parts arrangement tolerance should also be added to the distance A to prevent the electrode 3 from contacting the parts 14 at a speed that would damage them or damage the EMA 5 and/or the welding gun 1. It will prevent the electrodes 3 from collision with the parts 14 at a speed close to the maximum speed (V.sub.0). Due to the deceleration of the electrode 3 to the speed (V.sub.1) at the point (S.sub.touch), the energy of the electrode 3 collision with the parts 14 can be reduced and damage to the latter is avoided. If it is possible to place more parts 14 than it is envisioned by the technological process for a specific welding cycle, to prevent the electrode 3 from colliding with the parts 14 at a speed close to the maximum speed (V.sub.0), the distance between the points (S.sub.touch) and (S.sub.base) should be increased by the overall thickness of possible excessive parts 14.

[0085] The operation of initial determination of the points (S.sub.base), (S.sub.touch), the forces (F.sub.1) and (F.sub.2) (welding gun 1 configuration) may be combined with welding gun 1 calibration to reduce labor intensity of the configuration. The welding gun 1 calibration usually includes determining a ratio between the force measured by the force sensor (not shown) located in the EMA 3 and the force measured by a calibration force sensor (not shown) located between the electrodes 2 and 3. The forces obtained from the force sensor and the calibration force sensor are fed to the controller 9. The rigidity of the force sensor is significantly greater than the rigidity of the welding gun 1 and, thus, it has little effect on the point (S.sub.base) calculation accuracy. If the welding gun 1 configuration is combined with calibration thereof, the thickness of the force sensor should be subtracted from the distance A, i.e. the force sensor thickness should be added to the point (S.sub.touch) taking into account the thickness of the parts, the gap, the position tolerance (if they need to be accounted for). To weld the parts 14, the robot 11 moves the welding gun 1 such that the electrode 3 touches the parts 14. In a first working cycle (shown in FIG. 4) the welding gun 1 is closed at the speed (V.sub.0) which is chosen based on configured accelerations and decelerations and the stroke to the point (S.sub.touch) where the movement with the speed (V.sub.1) begins. The actual contact of the electrode 3 and one of the parts 14 (hereinafter referred to as the contact of the electrode 3 with the parts 14) takes place in the first working cycle at a speed from (V.sub.1) to (V.sub.0) in the (S.sub.cont2) point. The coordinate of the (S.sub.cont2) point may be determined in the same manner as the (S.sub.cont1) point. The speed (V.sub.0) in the first working cycle, as well as in the configuration cycle, may be equal to the maximum speed of the EMA 3 to reduce the welding cycle duration, and may be calculated automatically by the controller 9. Starting from the point (S.sub.touch), the electrode 3 moves with the speed (V.sub.1) up to the moment of reaching the force (F.sub.2). Lowering the electrode 3 speed starts no sooner than the force (F.sub.2) is reached.

[0086] The linear function is determined in the same manner as in the configuration cycle, but it will be understood that new coordinates S(F.sub.1)′ and S(F.sub.2)′ are determined, since there are parts 14 located between the electrodes 2 and 3. According to the signal from the force sensor 8, the controller 9 checks if the force (F.sub.1) is greater than the force at which the force fluctuations due to collision of the electrode 3 and the parts 12 occurs, and is greater than the force at which non-linear portion of the force change due to the electrode 3 position ends. If it is wrong, the force (F.sub.1) increases, and S(F.sub.1)′ coordinate is determined again. In the following welding cycles with the same part 14 thicknesses and (F.sub.trg) force value, the force (F.sub.1) remains unchanged. Linear extrapolation is performed on the linear function before its intersection with the level of force (F.sub.trg), i.e., a coordinate (S.sub.trg2) is determined ((S.sub.trg2) is the coordinate of the electrode 3, where (F.sub.trg) force is reached in the working cycle). The electrode 3 stops at the coordinate (S.sub.trg2) corresponding to the force (F.sub.trg). The point (S.sub.touch) is calculated based on parameters S(F.sub.1)′, S(F.sub.2)′ that are determined at a portion where the electrode moves with the constant speed (V.sub.1) after force fluctuations, which increases the accuracy of point (S.sub.touch) calculation. Wherein the point (S.sub.touch) calculation may be carried out by the controller 9 simultaneously with the welding process, and not prior thereto, therefore the change of the force (F.sub.1) does not result in the increase in the welding cycle duration. It is noted that during the disclosed working welding cycle, the caps on the electrodes 2 and 3 are shortened due to material removal from their ends contacting with the parts 14 in the process of heating and pressing against the parts 14. In each welding cycle, the controller 9 calculates the point (S.sub.touch) again and sets the coordinate of the point at which the electrode 3 starts to move with the speed (V.sub.1) in the next welding cycle equal to the coordinate of calculated point (S.sub.touch), which reduces the duration of movement at a low speed and the duration of the welding cycle regardless of the length of the caps. It should be noted that the difference between the coordinates of the point (S.sub.touch) in adjacent welding cycles includes not only the wear of the electrodes 2 and 3 caps, but also variation of thickness of the parts 14. However, after several cycles, for example, a number of cycles in which the average difference between the coordinates of the points (S.sub.touch) in these several configuration cycles and welding cycles exceeds the sheet thickness tolerance, the difference between the coordinates of the points (S.sub.touch) in the current working cycle and the first working cycle may be indicative of the degree of the cap wear between them.

[0087] It is important that, to account for wear, the coordinates of the point (S.sub.touch) in the current welding cycle and in the first welding cycle be compared after configuration with the same nominal thickness of the parts 14. Said difference is recorded in the memory (such as EEPROM or flash memory) which can be arranged within the controller 9, for example, and the resulting value is compared by the controller 9 to the predetermined permissible cap wear value. When the predetermined wear value is reached, a signal is generated (by one of the control systems—of the EMA 5 or of the robot 11) about the necessity to restore the shape (the longitudinal section profile), for example, by sharpening, molding or replacing the caps. At the same time, the number of welding cycles until the cap shape needs to be restored can be predicted.

[0088] FIG. 5 shows graphs of welding cycles according to the claimed method (bottom) and according to the method known from the prior art (top), provided that the welding cycles are performed with the same equipment, i.e., the welding gun 1, the EMA 5 and the rest of the elements of the welding cell are unchanged. Accordingly, the force (F.sub.trg*), the coordinate (S.sub.trg*) corresponding to this force, the speeds (V.sub.0*) and (V.sub.1*) have the same values in both welding cycles. Thus, the difference in the distance the electrode 3 travels at the speed (V.sub.0*) between the two methods is apparent. In the claimed method, by setting the point at which the movement with the speed (V.sub.1*) begins at the point (S.sub.touch*), the distance that the electrode 3 travels at the speed (V.sub.0*) can be increased, which, evidently, reduces the duration of the welding cycle. Determining the point (S.sub.touch*) through a linear function allows determining this point with high consistency and eliminates the need to lower the speed of the electrode 3 to the speed (V.sub.1*) before the point (S.sub.touch*) due to a low error in determining the point (.sub.Stouch*) in comparison with the known methods.

[0089] It should be noted that the method is suitable for controlling a welding gun traveling electrode comprising an EMA, which can be any known form of an EMA construction. For example, it can be provided as a linear electric motor, the output link of which is a shaft. The shaft interacts with a magnetic field generated by a stator and moves translationally. Also, the EMA can be provided as a rotary EMA comprising an electric motor and a reductor, an input link of which is fixedly attached to the electric motor rotor, and to the output link of which an arm is fixedly attached, which arm functions as an EMA output link. A welding gun traveling electrode is attached to the output link of the EMA such that they cannot move relative to each other. The welding gun of such construction is disclosed in patent DE102011054376A1. In such welding gun configuration, when the EMA comprises an output link configured as an arm, a traveling electrode is fixed on an end of said arm. It is important to note that in such configuration, a torque sensor can be provided instead of a force sensor to determine the force on the traveling electrode.