Robot assisted resistance spot welding system and method for resistance spot welding using such system

Abstract

Disclosed is a robot assisted resistance spot welding system. The system comprises a robot; a spot welding gun (1) secured to said robot; a sensor system (6, 7, 8, 9); a control system comprising a means for installing and executing a computer program and a welding program for controlling the movement of said robot and the welding controller and a storage means for storing the computer program, the welding program and data received by the control system; a welding controller connected to the control system to exchange control data with the control system, and a welding transformer. The control system is connected to the sensor system to exchange data. The spot welding system is characterized in that said sensor system is configured for determining the relative spatial position of said welding gun (1) and the workpiece bundle (4) to be welded at the estimated welding spot (5). Said welding gun (1) has two independently movable electrodes (2, 3), wherein the electrodes (2, 3) allow measuring the force on each electrode and the position of each electrode by means of said sensor system (6, 7, 8, 9). The computer program and the welding program are configured to determine the reference values of welding parameters before welding, receive current values of welding parameters during welding, compare the above values and control the robot, movable electrodes and welding controller based on the comparison. Increase of the welding quality and productivity is achieved.

Description

ROBOT ASSISTED RESISTANCE SPOT WELDING SYSTEM AND METHOD FOR RESISTANCE SPOT WELDING USING SUCH SYSTEMField of the Invention

[0001] The present invention relates to welding, in particular to robot-assisted resistance spot welding.Background of the Invention

[0002] Resistance spot welding is used for joining metal parts. In resistance spot welding, a bundle is placed between two electrodes, the bundle containing at least two conductive workpieces having the same or different thickness and made of the same material or different materials. The electrodes compress the bundle using a certain force which ensures the elimination of a potential air gap between the workpieces, as well as the necessary degree of engagement between the contact surfaces of the electrodes and the surface of the workpieces. After the workpieces are compressed, electric current is passed through the electrodes to provide heating of the workpieces between the electrodes, melting thereof and formation of a welding nugget connecting the workpieces. The point on the surface of the workpieces at which the electrodes contact the workpieces is called the welding spot. Technological parameters such as the force with which the electrodes must compress the workpiece bundle, the strength of the electric current, electric current duration, in some cases the dynamics of electric current strength over time, force maintaining duration, in some cases the dynamics of the required force over time are determined by the engineer based on the properties of workpiece materials in the bundle, the thicknesses of the workpieces and the properties of the required welding nugget. Accurate provision of said parameters during the welding process is necessary to ensure high-quality welding at the welding spot. For the purposes of the present disclosure, high-quality welding is understood to mean the formation of a welding nugget of the required size and shape, and the required physical properties, without bum-throughs or expulsions of workpiece material. When bundling the workpieces, a plurality of welding spots are formed on the bundle. The position of the welding spots on the surface of the workpieces and the sequence of their formation are determined by the engineer based on the geometric configuration of the workpieces and the required bundling properties. The quality of bundling of the workpieces depends on accuracy of the position of welding spots on the surface of the workpieces.

[0003] Robot-assisted resistance spot welding is common in the automotive industry and is used for joining vehicle body parts. In robot-assisted resistance spot welding, an embodiment of which is discussed in the present application, the electrodes form a part of a welding gun mounted on a robot, and the workpiece bundle is mounted and secured on a tooling arranged within reach of the robot. The workpieces are manually or automatically arranged on the tooling. The robot moves the welding gun relative to the workpiece bundle according to its control program, stopping the movement at welding spots and ensuring a predetermined position of the welding gun with respect to the workpieces. When stopped, the robot must ensure that one of the electrodes (the fixed electrode) of the welding gun touches the surface of the workpiece. Due to potential inaccuracies in positioning the tooling with respect to the robot, positioning the workpiece bundle on the tooling, changes in the geometry of the workpieces under the influence of gravity or due to heat expansion caused by performing previous welding spots, as well as wear of the components of the robot and the electrodes, when the robot reaches the spot according to its control program, the position of the spot can differ from that required by the technology, which further leads to formation of a welding nugget in an incorrect position, and to potential problems in terms of workpiece bundling quality. Furthermore, the fixed electrode can fail to touch the workpieces, or can touch them but continue moving, thus causing elastic or plastic deformation of the workpieces, which negatively affects the quality of the welded product. Further, when controlling the second (movable) electrode, the robot moves it towards the workpieces and the first electrode, compressing the workpiece bundle between the electrodes and providing the required force gain. The movement of the electrode is most often controlled based on its position (gaining the required force requires extending the electrode to a certain distance), or based on the value of the current on the electrode drive motor (a certain value of the current corresponds to a certain force gain). Due to the previously mentioned potential inaccuracies in the position of the first electrode during movement and application of force by the second electrode, elastic or plastic deformation of the workpieces can occur, negatively affecting the quality of the welded product. When controlling the second electrode based on its position, the accuracy of creating the required force is affected by the wear of the electrode drive, heat expansions, rigidity of the welding gun, and the force created during welding can differ from the required value, leading to inadequate welding nugget parameters and deviation in quality at the welding spot. When controlling the second electrode based on the value of the current on its drive motor, the same factors (drive wear, heat expansion) lead to the ratio between thecurrent on the motor and the force not being constant, and periodic calibration is required, which takes time and reduces productivity due to suspension of work for performing calibration, or the force is provided incorrectly, and, consequently, quality of welding at the spot ends up being inadequate, if calibration is not performed in a timely manner. However, even with timely calibration, temperature changes and gear torque of the drive motor, which are variable over the technological cycle, cannot be taken into account, which also leads to an inadequate force value and potential problems with welding quality at the spot.

[0004] During moving the electrode and gaining force, two steps are distinguished: moving at high speed up to the estimated moment of contact with the workpiece, and moving at reduced speed up to gaining the required force. The transition to a reduced speed is carried out to prevent hitting and deforming the workpiece. Due to the aforementioned potential positional inaccuracies, the transition to a reduced speed is carried out prior to contact with the workpiece, with an established tolerance for the inaccuracy of its position. This reduces operational performance. Furthermore, the movable electrode is usually controlled as an additional axis of the robot and has travel speed and dynamics (acceleration) limitations associated with robot control system settings, which also reduces operational performance.

[0005] After compressing the workpieces and gaining the required force, an equalization process is carried out, the process including displacement of the robot with the welding gun to a pre-calibrated distance determined by the rigidity of the welding gun and the required force, in order to compensate for the displacement of the first electrode and the deformation of the workpiece bundle caused by the second electrode gaining force. The equalization performed by moving the robot is time-consuming and reduces operational performance.

[0006] Further, high-quality welding requires the largest possible contact spot between the electrodes and the surface of the workpieces. Said spot is provided when the end surfaces of the electrodes are parallel to the workpiece surface at the welding spot. Due to the aforementioned potential positional inaccuracies, this parallel arrangement can be compromised. Reducing the area of the contact spot can lead to welding defects, such as metal expulsion, and lead to a deviation of welding spot quality and the quality of bundling the workpieces in general.

[0007] After performing welding at the spot, the robot proceeds to the next spot according to its control program. For this purpose, the robot retracts the second electrode from the surface of the workpieces by controlling its drive, opening the welding gun to an extent sufficient for moving to the next spot without any potential collision with the workpieces. Due to theaforementioned positional inaccuracies, said distance includes tolerance for potential inaccuracies, i.e., it exceeds the minimum sufficient distance, thus reducing operational performance. Furthermore, not only the second electrode, but also the first (fixed) electrode has to be retracted from the surface of the workpieces. Said retraction is carried out by moving the robot, i.e., the robot moves between welding spots according to the control program along a trajectory different from the shortest possible path, which also reduces operational performance.

[0008] Therefore, the conventional robot-assisted resistance spot welding process can be improved in terms of productivity and quality, said potential improvements require accurately ensuring the required relative position of the welding gun and the workpieces, accurately providing the required force, and effectively controlling the robot and the electrode. The present disclosure is directed to exploring said improvements.

[0009] Patent document KR100588505B1 (filed on August 26, 2005, IPC B23K 11 / 11) discloses a robot position compensation control device and a corresponding method applied to resistance spot welding. The disclosed device and method determine the actual position of the tooling with respect to the welding gun by means of three ultrasonic transceivers mounted on the tooling and one ultrasonic reflector mounted on the welding gun. A computing device determines deviation in the position of the tooling from the position set in the robot control program according to the readings of the sensors, and a corresponding adjustment of all welding spots is carried out. The disclosed device requires additional actions of mounting the sensors on the tooling and removing them thereafter, and the proposed method is used once during welding equipment setup, i.e., it compensates for a potential mismatch between the relative position of the robot and the tooling but does not take into account potential inaccuracies in the position of the workpiece bundle. Further, said patent document discloses a method and apparatus for adjusting the angular position of electrodes with respect to the surface of workpieces. 4 laser rangefinders mounted on both the first and the second electrode determine the distances to the outer surfaces of the workpieces in a bundle, 2 planes are plotted, and the plane equidistant from said two planes is taken as the actual plane of the workpiece bundle. Angular adjustment is determined to ensure that the axes of the electrodes are perpendicular to said plane. The above method is also used once during equipment setup, i.e., it does not take into account potential inaccuracies in the position of each individual workpiece bundle in the automatic operational cycle of the equipment. Furthermore, the method provides perpendicularity of the electrode axes to the plane, which is not universal,since the end surface of the electrodes is not always perpendicular to their axes.

[0010] Patent application JPS63112076A (filed on May 17, 1988, IPC B23K 11 / 24) discloses a control device for positioning in resistance spot welding. The disclosed device is aimed at correcting the positional deviation of the welding spot caused by temperature effects and wear during the welding operation. The device comprises a welding spot detection unit comprising an infrared photo camera and two rangefinders equipped with a drive. The device is fixedly mounted at the facility above the workpieces to be welded. After performing welding at the spot and retracting the robot and the gun, the infrared camera determines the position of the welding nugget (having a temperature greater than that of the surrounding workpiece material), and the rangefinders determine two distances from the position of the welding nugget to the edge of the workpiece in the plane of the workpiece surface by moving., Adjustment for the next welding spot in the plane of the surfaces of the workpieces is calculated based on the measured distances and deviation thereof from those set during setup of the welding robot for the workpieces and transmitted to the robot. The disclosed device and method provide correction in only one plane, that is, potential defects caused by deviation in the placement of the workpiece between the electrodes will not be determined. Furthermore, the device and method are suitable for adjusting the position of the welding spot on flat workpieces, the surface of which is arranged parallel or at a slight angle with respect to the device. The invention is not applicable for workpieces of complex shape, whereon the position of the welding spots has a more complex spatial configuration. Correction of the angular position aimed at ensuring the maximum contact spot between the electrodes and the workpiece is not performed. Furthermore, the position cannot be adjusted without performing welding at a spot, i.e., for the first welding spot, the position will always be incorrect.

[0011] Patent document US8513560B2 (filed on May 26, 2011, IPC B23K 11 / 11) discloses a spot welding system configured for automatically adjusting the position of the welding spot. Two modes of operation of the welding system are established: the spot welding mode or the position correction mode. In the spot welding mode, welding is performed according to the control program. In the position correction mode, when the second (movable) electrode is moved towards the workpieces, the moment of contact between the electrode and the workpieces is determined based on the change in its speed (or the torque on its drive motor, or the current on its drive motor) and the position according to the readings of the position sensor. Required adjustment of the fixed electrode position, which is carried out by the robot moving the welding gun, is determined based on the current distance between the end surfacesof the electrodes and the thickness of the workpieces. Thus, an increase in welding quality and productivity is ensured (by eliminating manual calibration of the position of the spots). However, in order to adjust the position, the welding system needs to be switched to a separate operating mode, which, even taking into account the fact that the operating mode can be preset in the robot control program, leads to a loss of performance. In addition, the above correction method allows to adjust the position of the spot in one direction only (along the electrode closure axis). Further, the above method does not allow for correcting the angular position and ensuring the maximum contact spot between the end surface of the electrode and the workpieces.

[0012] The invention disclosed in the present application is aimed at providing a method and system for robot-assisted resistance spot welding of a workpiece bundle that will increase productivity of resistance spot welding (i.e. increase a number of welding operations per time unit) in comparison with known analogues. In addition, the invention is aimed at improving the quality (i.e. at increasing a number of welding joints meeting the required parameters) in comparison with known analogues. Another object of the present invention is to reduce a number of energy-intensive operations in a spot welding cycle and reduce energy consumption when performing spot welding.Summary of the Invention

[0013] To solve the above problems, a method and system for robot-assisted resistance spot welding of workpieces are proposed. The robot-assisted resistance spot welding system comprises a robot, a welding gun secured to said robot, a sensor system, a control system, and a welding controller, wherein the control system comprises a means for installing and executing a computer program for controlling the movement of said robot and said welding gun and a welding program for controlling the welding controller and storage means for storing the computer program, the welding program and data receiving by control system; the welding controller is connected to the control system to exchange control data with the control system and is configured to execute said welding program, and a welding transformer for supplying welding current to the welding gun. The control system is connected to the sensor system to exchange data with the sensor system, and the sensor system is configured for determining the relative spatial position of said welding gun and a workpiece bundle to be welded at the estimated welding spot. Furthermore, said welding gun comprises two electrodes configured for independent movement, wherein the force on each electrode and theposition of each electrode are measurable by means of said sensor system, and each electrode is connected to a separate electromechanical drive. Said control system computer program and welding program stored in the storage means are configured to allow the control system to determine reference values of welding parameters before welding, receive actual values of welding parameters during welding, to compare the above values and to control the robot, the movable electrodes of the welding gun and the welding transformer based on said comparison. Claimed spot welding system allows ensuring the spot welding quality due to provision more exact position of the electrodes relative to each other and relative to the workpiece bundle, and more exact position of the welding point relative to the workpiece bundle leading to decrease in probability of deformation the workpiece bundle and to maximization of contact spot between the electrodes and the workpiece bundle. Moreover, productivity of resistance spot welding is increased as far as the independently movable electrodes provide the possibility not to move the robot when welded is the workpiece bundle of complex shape, in addition, equalization of the electrodes is not required.

[0014] In an embodiment, independent moving of the electrodes is ensured by the presence of electromechanical drives mounted on the welding gun and configured to change the position of the electrodes.

[0015] In an embodiment, the welding parameters determined by the sensor system includes one or more of: a position of the point of contact between the electrode and the workpiece bundle at the estimated welding spot, an angle between the electrodes and the surface of the workpiece bundle at the point of contact, and a distance between the electrodes and the surface of the workpiece bundle. Said control system is configured for, based on the data obtained by said sensor system: determining the position of the point of contact between the electrode and the workpiece bundle at the estimated welding spot and the deviation of the point of contact position from the reference position for the same welding spot; calculating the required displacement of the robot to ensure the coincidence of the positions of the point of contact and the reference point of contact. In addition, said control system is configured for determining, based on the data obtained by said sensor system, the angle between the electrodes and the surface of the workpiece bundle at the point of contact and deviation of the angle from the reference angle for the same welding spot; calculating the required displacement of the robot to ensure the required angle. In addition, said control system is configured for determining, based on the data obtained by said sensor system, the distance between the electrodes and the surface of the workpiece bundle at the welding spot,determining the target electrode compression point. In addition, said control system is configured for determining, based on the data obtained by said sensor system, the possibility of performing welding without significant deformation of the workpiece bundle to be welded at the current relative position of the welding gun and the workpiece bundle with the current shape of the workpiece bundle at the welding spot and at the target electrode compression point by moving the movable electrodes without changing the position of the robot and determining the need of displacement of the robot with the welding gun mounted on the robot. In addition, said control system is configured for controlling the robot based on the data obtained by said sensor system,: transmitting the calculated displacements and commands to perform the movement to the robot; controlling the movable electrodes of the welding gun by ensuring their movement to gain, maintain and adjust the compression force on the workpiece bundle at a set target compression point in accordance with parameters required by the welding program; controlling said welding controller with initiating the welding program, and supplying welding current by means of the welding transformer.

[0016] It should be noted that the phrase "without substantial deformation" as used herein means "without deformation significant to the properties of the workpiece bundle to be welded". Those skilled in the art will understand that at some level, any contact causes deformation of the surface of the contacting materials, but in general, it is important for the material or the product made therefrom that no deformation significant in terms of its properties occurs.

[0017] In an embodiment, the spot welding system is characterized in that the control system is configured for storing reference values. In particular, the control system can store the reference position of the point of contact between the electrodes and the workpiece bundle on the surface of the workpiece bundle prior to initiating closure of the electrodes. Further, the control system can store the reference angle between the electrodes and the surface of the workpiece bundle prior to initiating closure of the electrodes. Further, the control system can store reference tolerance for the values of the position of the point of contact between the electrodes and the workpiece on the workpiece surface prior to initiating closure of the electrodes and for the angle values between the electrodes and the surface of the workpiece bundle prior to initiating closure of the electrodes. Further, the control system can store the reference position of the workpieces between the electrodes prior to initiating closure of the electrodes. Further, the control system can store the reference angle between the electrodes and the surface of the workpiece bundle after gaining the required force, and the referenceposition of the electrodes after gaining the required force.

[0018] In another embodiment, the sensor system is configured for measuring and transmitting to the control system the actual values of the main welding process parameters, including at least one of the following: current between the electrodes, voltage between the electrodes, resistance between the electrodes, temperature at the welding spot. The control system is configured for accumulating, systematizing and storing data regarding the dynamics of said parameters during the welding process.

[0019] In another embodiment, the sensor system is configured for measuring and obtaining data for determining the technical state of at least one of the welding gun, electromechanical drives, the robot, the welding controller, including at least one of the following parameters: drive motor current, drive motor temperature, drive motor movement speed, drive motor torque, and diagnostic signals from the robot and the welding controller. The control system is configured, based on the data received from the sensor system, to diagnose the current technical state of the system hardware components and generate a signal indicating a malfunction or the need for calibration. In a particular embodiment, the control system is configured for accumulating, storing and systematizing data regarding the parameters necessary for determining the technical state of the system hardware components. The control system is configured for predictive diagnostics of the technical state of the system hardware components and for generating signals indicating required preventive maintenance based on the accumulated data.

[0020] In some embodiments, the means for installing and executing the computer program in the control system is a separate computing device. In other embodiments, the means for installing and executing the computer program and the welding program in the control system is a process controller of the spot welding system or the welding controller of the robot, forming part of the control system and acting as a means for installing and executing the computer program and the welding program.

[0021] The present disclosure further relates to a method for performing resistance spot welding by said robot-assisted resistance spot welding system. The method comprises setting up the robot-assisted resistance spot welding system, as well as implementing a welding program for each welding spot in an automatic cycle. In the process of setting up the spot welding system, reference values of the welding parameters and their tolerance are obtained and stored in the storage means of the control system for each welding spot. Further, automatic welding cycle, for each welding spot, is performed based on comparison of thereference values of the welding parameters and the values of the welding parameters received from the sensor system during the welding. The automatic welding cycle comprises the following sequence of steps. Step 1 : placing the welding gun by the robot to the welding operation area according to the computer program. Step 2: receiving information on the relative position of the welding gun and the workpiece bundle and on the shape of the workpieces by the sensor system. Step 3: determining the welding parameters based on the data received from the sensor system; by comparing the obtained values with the reference values stored for the welding spot during the setup process, the control system determines whether the welding spots fall within the established tolerance and determines the feasibility of welding with the current welding parameters. If high-quality welding is unfeasible, the adjustment value for the position of the target electrode closure point for the current welding spot is calculated and transmitted to the robot which carries out the adjustment, after which steps 2, 3 are repeated. Step 4: based on the values of the welding parameters received from the sensor system, the control system calculates the values of electrode movement using the position and the electrode movement speed control profile ensuring maximum operating speed thereof. Step 5: the electrodes are moved in accordance with the calculated parameters; upon completion of said movement, the movement of the electrodes is controlled based on the force value and the required force is gained equally on both electrodes. Step 6: after gaining the required force, the sensor system receives information regarding the welding parameters. Step 7: the control system calculates, based on the data regarding the previously performed adjustment and comparison of the values of the welding parameters for the current and all previously operated welding spots on the workpiece bundle for the current workpiece bundle with their reference values, the correction value for the position of the subsequent welding spots and transmits it to the robot controller which applies the adjustment to the coordinates of the subsequent spots. Step 8: the control system initiates the execution of the welding program by the welding controller; and the welding transformer generates welding current with the required parameters. Step 9: the electrodes are opened and moved away from the workpiece bundle to the reference position. After performing step 9, in-between two welding spots in the welding program, the control system transmits a command to the robot to move to the next welding spot, after which steps 1-9 are repeated. Those skilled in the art will understand that for the last welding spot, steps 1 to 9 are carried out, after which a transition to standby mode can be performed, the workpiece bundle can be released or another action can be performed on the workpiece bundle, said action not falling within the scope of the claimed method forperforming robot-assisted resistance spot welding of a workpiece bundle. The control program is configured such that, when moving to each successive welding spot in a series of welding spots, the robot moves the target electrode closure point within the workpiece bundle in accordance with the configuration of the workpiece bundle to be welded, taking into account potential adjustments transmitted in step 7 (those skilled in the art will understand that the control program can include other points in addition to the welding spots, e.g., reference points serving to ensure safe movement of the system hardware, and said points can be arranged within the workpiece bundle or outside it; if there are such points in the control program after step 9, the robot moves the welding gun with the electrodes to such reference point (or sequentially through several reference points), and then proceeds to step 1 from the last reference point; in step 1, when transporting the welding gun to the intended welding area, the target electrode closure point is arranged within the workpiece bundle). The proposed method for performing resistance spot welding by the proposed robot-assisted resistance spot welding system further provides an increase in the quality and productivity of resistance spot welding. Improving of the spot welding quality is provided by more exact position of the electrodes relative to each other and relative to the workpiece bundle, and more exact position of the welding point relative to the workpiece bundle leading to decrease in probability of deformation the workpiece bundle and to maximization of contact spot between the electrodes and the workpiece bundle. Moreover, productivity of resistance spot welding is increased as far as the independently movable electrodes provide the possibility not to move the robot when welded is the workpiece bundle of complex shape, in addition, equalization of the electrodes is not required. Moreover, the productivity of resistance spot welding is increased due to specifying of the welding spots position not only in setting up process, but also in the automatic welding cycle that leads to providing operating speed of the electrodes.

[0022] In a particular embodiment, the method is characterized in that the welding parameters includes position of the workpieces, the position of the electrodes, and the angle between the electrodes and the surface of the workpiece bundle.

[0023] In a particular embodiment, during the setup process, the reference workpiece bundle thickness value after compression is stored for each welding spot and the tolerance is set to this value, and when performing step 7, based on the data received from the sensor system in step 6, the control system determines thickness of the workpiece bundle after compression, compares the obtained value with the reference value and, in case of deviation beyond theestablished tolerance, stops the execution of the automatic welding cycle. This allows to prevent low-quality welding.

[0024] In another embodiment, when performing step 9, the control system monitors, based on the sensor system data, the position and force on the electrodes during their movement, determines, based on the dynamics of the position and force, sticking of the electrodes to the surface of the workpiece bundle and, when sticking of the electrodes to the surface of the workpieces is detected, the system generates a signal indicating the required restoration of the surface of the electrodes, further ensuring prevention of low-quality welding. Furthermore, since both electrodes in the present application are movable, it is feasible to determine the sticking of each individual electrode to the surface of the workpiece bundle.

[0025] In yet another embodiment, during the setup process, a force control profile is determined for each welding program during program execution. When performing step 8 simultaneously with the execution of the welding program by the welding controller, the control system controls the movement based on the force value on the electrodes according to the set force control profile.

[0026] In yet another embodiment, the sensor system is configured for measuring and transmitting to the control system the actual values of the main welding process parameters, including at least one of the following set of parameters: current between the electrodes; voltage between the electrodes; resistance between the electrodes; and temperature at the welding spot, wherein the control system is configured for accumulating, systematizing and storing data regarding the dynamics of said parameters during the welding process. In the process of setting up the spot welding system, reference data on the dynamics of process parameters during welding are stored in the storage means of the control system for each welding spot, and when performing step 8, the control system analyzes deviations in process parameters from the reference values and, if the tolerance set for such deviation is exceeded, the system generates a signal regarding issues with welding spot quality, based on which the operator or the upper level control system can decide whether inspection of the workpiece bundle is required.

[0027] In yet another embodiment, when performing step 8, upon detection of a deviation from the reference values of the process parameters, the control system calculates compensating adjustment of the electrodes and the electrodes are moved as required. In another embodiment, when performing step 8, upon detection of a deviation from the reference values of the process parameters, the welding program calculates welding currentcompensating adjustment aimed at bringing the parameters closer to the reference values, transmits a signal to the welding controller, and the welding controller provides the required parameter change.

[0028] In yet another embodiment, control programs for moving the robot, controlling the welding gun electrodes, and controlling the welding controller are generated prior to initiating setup of the spot welding system using the computer program and a personal computer based on 3D models of the workpiece bundle, tooling, the robot, and the welding gun. Said control programs include at least the following parameters for each welding spot: the opening values for each electrode, the angle between the electrodes and the surface of the workpiece bundle, the distance from the end surface of the electrode to the surface of the workpieces, the position of the target electrode closure point in the coordinate system of the robot or the workpiece bundle, the distance for moving the electrodes when controlling based on the position, the required compression force for the workpiece bundle, and the thickness of the workpiece bundle. In the process of setting up the spot welding system, the following sequence of steps is carried out for each welding spot. Step i: the robot performs the movement according to the coordinates of the point set by its computer program. Step ii: the sensor system receives information on the relative position of the welding gun and the workpiece bundle. Step iii: the control system analyzes the calculated values of the relative position of the welding gun and workpieces based on the data from the control program and the actual values received from the sensor system, and if the actual values deviate from the calculated values by more than the established tolerance, the control system calculates the necessary adjustment of the position of the robot and transmits the calculated value and a movement signal thereto, and after the robot performs the movement, the new coordinates are stored as the coordinates of the welding spot in the control program, and the steps (ii-iii) are repeated. Step iv: the control system stores the processed data from the sensor system as reference values prior to initiating closure of the electrodes. Step v: the workpiece bundle is compressed and the required force is gained. Step vi: the sensor system receives information regarding the relative position of the welding gun and the workpiece bundle. Step vii: the control system processes the sensor system data and stores said data as reference values after gaining the required force. Step viii: the electrodes are opened. Then the control system transmits a signal to the robot indicating the required movement to the next welding spot, and steps i-viii are performed up to completion of all welding spots.Brief Description of the Drawings

[0029] The present invention will now be described in detail with reference to the accompanying drawings, in which:

[0030] FIG. 1 schematically shows a workpiece bundle, a welding gun, and a series of welding spots to be operated;

[0031] FIG. 2 schematically shows an orthogonal image and a projection along the axis of the electrodes of an optical system included in the sensor system according to one embodiment of the method for implementing the present invention;

[0032] FIG. 3 schematically shows an example image allowing to estimate the displacement of the welding gun with respect to the workpieces in the surface plane of the workpiece bundle.Detailed Description of the Invention

[0033] FIG. 1 schematically shows an image of a workpiece bundle 4, a welding gun 1, and a series of welding spots 5 operated during robot-assisted resistance spot welding. The robot- assisted resistance spot welding system is a hardware- software complex to perform resistance spot welding. The system comprises a robot (not shown in the figures) comprising a number of links, in particular an output (last) link, and a controller (robot controller), a welding gun 1 secured to the last link of said robot, a sensor system, a control system configured for installing a computer program and a welding program and provided with a storage means; a welding controller and a welding transformer.

[0034] Furthermore, the welding gun 1 has two independently movable electrodes: the first electrode 2 and the second electrode 3, wherein force on each electrode 2 and 3 and positions thereof can be measured by means of said sensor system. Said electrodes are configured for independent movement using electromechanical drives configured for switching between movement control based on position and based on force value. The drives of each electrode 2 and 3 provide movement control based on position and based on force value. When controlling the movement of electrodes 2 and 3 based on position, the control program sets the position towards which each electrode 2 or 3 moves. When controlling the movement of electrodes 2 or 3 based on force value, the control program sets the force value to be achieved on each electrode 2 or 3 by moving it. The electrodes 2 and 3 can move in a direction towards each other or away from each other, the electrodes are driven to move by electromechanical drives based on a roller drive (not shown in the figures). The drives, as in the prior art, aresecured to the frame of the welding gun 1 and are connected to the electrodes directly or via levers. The drives are stationary with respect to the welding gun 1, and when a drive rod is moved, the corresponding electrode 2 or 3 moves therewith. The electromechanical drives comprise integrated force sensors measuring the force on the corresponding rod, as well as position sensors determining rotation of the roller drive, i.e., taking into account the value of the transmission function of the drive reducer, allowing to determine the movement value of the corresponding rod; and a temperature sensor. The electromechanical drives are controlled by frequency converters (not shown in the figures) configured for controlling movement based on position and based on force value. For this purpose, the sensors integrated in the drives are connected to frequency converters via communication channels. The frequency converter is a computing device executing algorithms, calculating and controlling the drive (current supply to the drive) based on the set required position or force values, operational properties of the drive and actual values obtained from the sensors. Two frequency converters are connected to each other via a communication channel for synchronizing the required control function. The robot controller and frequency converters are connected to the process controller of the spot welding system (not shown in the figures) implementing the functionality and algorithms described in the present application via communication channels. The values obtained from the sensors are also transmitted to the controller of the spot welding system via communication channels. When performing welding, the robot moves the welding gun 1 with respect to a bundle 4 of workpieces to be welded, said bundle mounted on the tooling (not shown in the figures) along a path formed by the segments connecting welding spots 5. The coordinates of each welding spot are described in the control program of the robot stored in its controller during the process of setting up the spot welding system for performing welding.

[0035] Each of the welding spots in the coordinate system of the robot is described by six values: the values of displacement along the X, Y, Z axes, and the values of Euler's angles characterizing the rotation (or inclination angle) of the tool (the welding gun 1) at the spot. In addition to the coordinates of the welding spots, the control program of the robot contains the parameters for movement between the spots (speed, allowing or disallowing rotation of individual links and other parameters). The process controller of the spot welding system can receive the parameters of the robot control program for a given welding spot, receive a signal indicating the completion of movement, transmit changes in the parameters of the control program for a given welding spot, transmit changes in the coordinate system, and transmit asignal indicating the possibility of movement to a given welding spot via the communication channel with the controller of the robot using a data exchange protocol. The robot stops the welding gun 1 at each of the welding spots according to its control program, after which it transmits a signal indicating entry into the welding spot, and welding is performed using the method described in the present disclosure.

[0036] Further, in the robot control program, reference points of robot movement trajectory can be set, which are necessary for unhindered passage of the welding gun 1 along the workpiece bundle 4 if the shape of the workpieces in the bundle is complex. Welding is not performed at the reference points: after the welding gun 1 is moved to a reference point, the robot initiates movement of the welding gun to the next point set in its control program.

[0037] The control system of the spot welding system is configured for installation of computer program and welding program. In various embodiments, the control system can be a computing device or a combination of computing devices, a process controller of the spot welding system and frequency converters, and can further comprise, e.g., of a robot controller and a process controller of the spot welding system.

[0038] The control system is configured for storing data regarding the reference position of the point of contact between the electrode and the workpiece at the estimated welding spot, for storing data regarding the reference angle between the electrodes and the surface of the workpieces at the point of contact, and for storing welding parameters required by the set welding process. Said control system can perform a set of actions based on the data received by the sensor system. For instance, the control system can determine the position of the point of contact between the electrode and the workpiece at the estimated welding spot and the deviation of the position of the point of contact from the reference position for the same welding spot. Further, the control system can calculate the required displacement of the robot to ensure that the positions of the point of contact and the reference point of contact coincide. Further, the control system can determine the angle between the electrodes and the surface of the workpieces at the point of contact and its deviation from the reference angle for the same welding spot, and calculate the required displacement of the robot to ensure the required angle. Furthermore, the control system can determine the distance between the electrodes and the surface of the workpiece at the welding spot, determine the target electrode compression point providing minimal deformation of the workpieces to be welded; determine the possibility of performing welding with minimal deformation of the workpieces to be welded at the current relative position of the welding gun 1 and the workpiece bundle 4 consideringcurrent shape of the workpiece bundle 4 in the vicinity of the welding spot 5, at the target electrode compression point, by moving the movable electrodes 2 and 3 without changing the position of the robot; if welding is unfeasible, the control system can calculate the required displacement to enable welding. The control system can also control the robot: transmit calculated displacements and commands to perform movement to the robot; control the movable electrodes 2 and 3 of the welding gun 1, ensuring movement thereof for gaining, maintaining and adjusting the compression force of the workpieces 4 at said target compression point in accordance with the parameters required by the welding process; control said welding controller by initiating the execution of programs required by the welding process and the supply of welding current by the welding transformer.

[0039] The sensor system is configured for determining the relative spatial position of said welding gun 1 and the workpieces to be welded, and for determining the shape of the workpieces 4 to be welded at the estimated welding spot. In particular, for this purpose, it can be equipped with a video camera, a 3D camera, a laser or an acoustic rangefinder, and with any other suitable means. Furthermore, a sensor system in the present disclosure is also understood to be a combination of sensors integrated into the electromechanical drives of electrodes 2 and 3. To determine the relative position of the welding gun 1 and the workpiece bundle 4, the present method further includes the optical system shown in FIG. 2 as part of the sensor system.

[0040] For the sake of simplicity, FIG. 2 shows a coordinate system co-directed on its Z axis with the movement axis of the electrode 2. A computer vision camera 6 is mounted on the second electrode 2. At the request of the control system, the camera 6 can output a black and white image. The camera 6 is secured to the electrode 2 with an attachment, and thus moves therewith, i.e. it is arranged at a fixed distance from the contact surface of the electrode 2. The change in the length of the electrode 2 in the process of restoring the contact surface of its cap is measured, stored in the control system and taken into account in further calculations. The optical axis of the camera 6 is directed along the Z axis of the coordinate system, i.e., parallel to the movement axis of the electrode 2.

[0041] Further, two laser rangefinders 7 and 8 are mounted on the second electrode 2. The rangefinders are secured to the electrode 2 with an attachment, and thus move therewith, i.e. they are arranged at a fixed distance from the contact surface of the electrode 2. The optical axes of the rangefinders are also directed along the Z axis. The rangefinder 7 is mounted with an offset along the X axis with respect to the movement axis of the electrode 2, and therangefinder 8 is mounted with an offset along the Y axis with respect to the movement axis of the electrode 2.

[0042] A laser rangefinder 9 with an optical axis coaxial to the optical axis of the rangefinder 8 is mounted on the first electrode 3. Thus, based on the readings of the rangefinders 7 and 8, the distances to the workpieces from the side of the second electrode 2 at two points (A and B) can be measured, and based on the readings of the rangefinder 9, the distance to the workpieces from the side of the first electrode 3 at one point can be measured. The rangefinders 7, 8, 9 and the camera 6 are connected to the control system via communication lines.

[0043] Robot-assisted resistance spot welding is performed using the disclosed spot welding system. The method includes setting up the spot welding system, as well as implementing an automatic welding cycle for each welding spot.

[0044] In the process of setting up the spot welding system, reference values are stored in the storage means of the control system for each welding spot. The reference values can include data regarding the position of the welding spot, which is then used in the automatic welding cycle for determining deviations occurring under the influence of various factors, for adjusting the position of the welding gun 1 at the current welding spot 5 (if necessary) and for adjusting movement trajectory of the gun 1 performed by the robot (coordinates of subsequent spots). Further, during the setup process, the target point of closure of the electrodes 2 and 3 is determined, which is the point located between the end planes of the electrodes 2 and 3 along the direction of their closure, the movement of which determines the movement trajectory of the last link and the tool (welding gun 1) by the robot. The optimal position of said point is determined based on the opening value of the electrodes 2 and 3 necessary for unimpeded transition between successive welding spots 5, the dynamic parameters of the electromechanical drives of the electrodes 2 and 3 (in particular, the developed speeds and accelerations, the available stroke length from the opened position), and the thickness of the workpiece bundle 4 to be welded. During setup for each welding spot 5, the robot and the welding gun 1 are placed in the required position with respect to the workpieces to be welded, including ensuring the required angle between the end surfaces of the electrodes and the surface of the workpieces at the point of contact of the electrodes therewith, after which the sensor system receives reference position data. Then, the workpiece bundle is compressed by the electrodes, and after gaining the required force, reference position data are obtained again. Both datasets are stored for subsequent use. During the welding process, the workpieces in thebundle may bend due to own weight and the structural properties of the tooling on which they are arranged, as well as due to heat expansion. The sensor system data obtained prior to compressing the electrodes which can be obtained, e.g., by means of a video camera, 3D camera, laser or acoustic rangefinders (the above list does not in any way limit potential technical solutions for implementing the sensor system of the present invention), will take into account the aforementioned changes in the shape of the workpieces, in particular, the gaps and curvatures of the workpieces eliminated after compression. The sensor system data obtained after compressing the electrodes will contain the reference position of the workpieces without the aforementioned changes.

[0045] In one embodiment of the present invention, during the setup process, for each welding spot after compressing the workpiece bundle 4 and gaining the required force, based on the data received from the sensor system, the thickness of the workpiece bundle 4 at the welding spot is also calculated, stored as a reference value for said spot, and a tolerance is set for said value. It is assumed that the setup process is carried out on the correct workpiece bundle 4, i.e., the bundle comprises the correct number of workpieces with the correct thickness, without the presence of foreign objects between the workpieces. In the automatic cycle, due to external circumstances, a deviation in the thickness of the workpieces can occur, which, when welding with set modes, can lead to low-quality welding. A deviation can also occur in the number of workpieces supplied (the bundle is composed of fewer or more workpieces than required), which leads to a loss of process performance, since performing welding on the wrong bundle leads to rejection of the entire part and loss of welding line performance. Further, foreign objects can be found between the workpieces, which, when welding with the required parameters, can lead to low-quality welding. The bundles to be welded can have a complex configuration (some of the spots are worked with 2 workpieces to be welded, and some with 3, 4 workpieces and so on, within one bundle 4), and therefore the values are determined and stored for each spot separately. In the automatic welding cycle, in step 7, the actual thickness of the workpiece bundle is calculated, and its deviation from the stored reference value being greater than the set tolerance value means that: if the deviation value is too small, then the thickness of the workpieces is smaller than expected or the required number of workpieces in the bundle is smaller than expected, and if the deviation value is too high, the thickness of the workpieces is greater than expected or the required number of workpieces in the bundle is greater than expected, or there are foreign objects present between the workpieces. When such deviation is detected, the automatic cycle isstopped, which further improves the quality and productivity of welding (by eliminating the detected deviation in a timely manner without performing low-quality welding and without losing process rhythm of the welding line).

[0046] After setting up the spot welding system, a sequence of at least nine steps is performed in the automatic welding cycle for each welding spot.

[0047] Step 1 : the robot transports the welding gun 1 to the intended welding area according to its control program, i.e., according to the coordinates of the target closure point of the electrodes 2 and 3 stored in the control program of the robot during the setup process, taking into account potential adjustments. When transporting between two consecutive spots in the same series (a series is understood as a sequence of spots forming a single section of a welded joint, where the spots are located at a short distance from each other and are performed sequentially; switching to another part of the workpieces stops the series), the robot moves the target electrode closure point within the workpiece bundle (between the surfaces of the bundle) along a trajectory similar to the shape of the workpieces (in a straight line or along a smoothed curve matching the shape of the workpiece bundle). It should be noted that in the context of the present application, since both electrodes 2 and 3 are movable and independently controllable, the target point of their compression is not tied to the surface of the fixed electrode, as is the case in the prior art, but is located within the workpiece bundle 4. The shape of the movement trajectory is set during setup for welding the workpiece bundle, and the trajectory parameters (transition parameters between spots, intermediate reference points on the trajectory without welding) are stored in the robot control program. Such movement of the robot reduces movement time and, accordingly, increases the productivity of the operation and reduces energy consumption compared to the conventional movement trajectory, where prior to moving to the next point, the robot retracts the first electrode from the workpiece, and after reaching the spot, brings the first electrode thereto.

[0048] Step 2: after the movement of the welding gun by the robot is completed, the sensor system receives information regarding the relative position of the welding gun and the workpieces, and regarding the shape of the workpieces. The set of obtained data and the mechanisms for obtaining said data are identical to those used in the process of setting up for welding, which allows to detect deviations from the stored reference values and measurable magnitudes thereof.

[0049] Step 3: the control system determine, based on the data received from the sensor system, the position of the point of contact between the electrodes 2 and 3 and the workpieceson the surface of the workpieces during welding gun 1 closure at the current point, the angle between the electrodes and the workpiece bundle, and the distance between the electrodes and the workpiece bundle; by comparing the obtained values with the reference values stored for the welding spot during the setup process, the control system determines whether the values fall within the established tolerance and determines the possibility of performing high-quality welding at the current position of the welding gun with respect to the workpieces. If high- quality welding is unfeasible, the adjustment value for the position of the target electrode closure point for the current welding spot is calculated and transmitted to the robot which carries out the adjustment, after which steps 2, 3 are repeated. This ensures that the spot at which welding is to be performed coincides with its required position, regardless of errors in placement of the workpiece bundle on the tooling and other positional errors. Adjusting the position for the current spot increases the probability of providing the required parameters (position of the welding spot on the surface of the parts), and, consequently, increases welding quality. The target closure point of the electrodes coinciding with its actual position allows to gain and maintain the force by the two movable electrodes 2 and 3 effectively and quickly and, consequently, increases welding quality and productivity.

[0050] Step 4: the control system calculates, based on the position value of the workpieces between the electrodes received from the sensor system and the position of the target electrode closure point, the values of electrode movement in the control loop based on position and the electrode movement speed control profile ensuring maximum operating speed thereof.

[0051] Step 5: the electrodes are moved in accordance with the calculated parameters in the position control loop. Prior to the estimated contact with the workpieces, the electrodes 2 and 3 are moved in the control loop according to the position with maximum speed, reducing said speed prior to the moment of contact so as to initiate contact at a speed that ensures the absence of significant deformation of the workpieces and oscillatory processes, as well as the possibility of gaining force when controlling the movement based on force value without excessive correction. Recalculation of displacement values in the position control loop allows travelling the maximum possible distance at high speed, which provides an increase in welding performance. After the movement is completed, the electrodes 2 and 3 are switched to control movement based on force value and ensure gaining the required force equally on both electrodes without displacement and significant deformation of the workpieces. Compression at the target closure point of the electrodes 2 and 3 (taking into account potentialcorrection if it deviates from the required point within tolerance) and control of the electrodes 2 and 3 when compressing according to the readings of the force sensors integrated into electrode drives provide the required force gain without significant deformation of the workpieces, thus improving welding quality. The lack of displacement and significant deformation of the workpieces in this process eliminates the need for equalization which requires time and movement of the welding gun 1 by the robot, thus improving welding performance and reducing energy consumption.

[0052] Step 6: after gaining the required force, the sensor system receives information regarding the position of the workpieces, the position of the electrodes, and the angle between the electrodes and the surface of the workpiece bundle. The set of obtained data and the mechanisms for obtaining said data are identical to those used in the process of setting up for welding, which allows to detect deviations from the stored reference values and measurable magnitudes thereof. Previously obtained data (prior to compression), in particular the data regarding the angle between the electrodes and the surface of the workpiece bundle, could contain errors caused by a bend in the workpieces under own weight or due to heat deformation during welding. The data obtained after compressing the workpieces at the target closure point of the electrodes reflects their actual position without taking bending into account.

[0053] Step 7: the control system calculates, based on the data regarding the previously performed adjustment of the position of the target electrode closure point and comparison of the position of the electrodes and the angle between the electrodes and the surface of the workpiece bundle for the current and all previously operated welding spots on the workpiece bundle for the current workpiece bundle with their reference values, the correction value for the position of the subsequent welding spots and transmit it to the robot which applies the adjustment to the coordinates of the subsequent spots. The calculation is carried out taking into account the previously operated welding spots, the deviations obtained therefor and the corrections made. A correction value is determined, based on arrays of spots, providing a minimum average deviation, and therefore, with each subsequent spot added to the array, an increase in the reliability of predicting subsequent spots is provided. The calculated correction value is transmitted to the robot which applies it to the subsequent spots. The correction value can be transmitted as a change in the coordinate system of the robot, as corrections to the coordinate values of the spots, or as new values for the coordinates of the spots. Correctingrobot trajectory allows to implement movement along the trajectory described earlier in the description of step 1, which increases welding performance, and to provide the minimum necessary electrode opening values to perform the movement of the welding gun along the workpieces without colliding with them (i.e., during setup for processing a workpiece bundle, it allows to avoid increasing the opening of the electrodes to account for potential inaccuracies in the positioning of the workpieces), which increases welding productivity and quality. Furthermore, as the data accumulates in the array of processed spots and the reliability of the prediction of the coordinates of subsequent spots increases, the likelihood of needing to perform corrections in step 3 decreases, thus increasing welding performance.

[0054] Step 8: the control system initiates the execution of the welding program by the welding controller; the welding controller executes the required program, and the welding transformer generates welding current with the required parameters, resulting in melting of the metal material of the workpieces and the formation of a welding nugget joining them.

[0055] Step 9: the electrodes 2 and 3 are opened and moved away from the workpieces to the reference position. The opening value at this step does not include tolerances for inaccuracies in the positioning of workpieces, which reduces opening distance, thus increasing welding performance.

[0056] After performing step 9, in-between two welding spots 5 in the welding program, the control system transmits a command to the robot to move to the next welding spot, after which steps 1-9 are repeated until welding is performed at all welding spots 5 for the current workpiece bundle. After performing welding at the last welding spot 5, a transition to standby mode can be performed, the workpiece bundle 4 can be released or another action can be performed on the workpiece bundle 4, said action not falling within the scope of the claimed method for performing robot-assisted resistance spot welding of a workpiece bundle. That includes the robot possibly returning to its original position, and the workpiece bundle can be replaced.

[0057] In addition, it will be understood by those skilled in the art that the control program can include other points in addition to the welding spots 5, e.g., reference points serving to ensure safe movement of the system hardware, and said points can be arranged within the workpiece bundle 4 or outside it. If there are such points in the control program after step 9, the robot moves the welding gun with the electrodes to such reference point (or sequentially through several reference points), and then proceeds to step 1 therefrom; in step 1, when transporting the welding gun to the intended welding area, the target electrode closure point isarranged within the workpiece bundle.

[0058] More particularly, when performing an automatic welding cycle after stopping the robot to perform welding at a welding spot (the intended welding spot), the control system (e.g., a computing device) receives readings from the sensor system indicating distances measured by the rangefinders 7, 8, and 9. Then the control system compares said readings with the corresponding readings stored in the storage means of the control system for the welding spot during the process of setting up the spot welding system. It is important that the measurements are carried out at the same opening value (in the same position of the drives) for the electrodes 2 and 3, or the difference in the position of the electrodes 2 and 3 is taken into account when estimating the distance using the control system. It is assumed that the readings stored during the setup process are reference values; that is, using said readings, welding will be performed with the required quality at the required position of the welding spot 5 on the surface of the workpieces 4. In the event that the values measured in the automatic cycle deviate from the reference values by more than the tolerance value, which is also set during the setup process, the control system stops the automatic welding cycle and sends an error signal indicating critical deviation in the position of the workpiece bundle 4 which cannot be corrected automatically. In this case, the equipment operator must correct the position of the workpieces and start the automatic cycle again. In general, such a deviation can occur only for the very first welding spot5 of the workpiece bundle 4, such that low-quality welding will not occur.

[0059] In the event that the distance readings of the rangefinders 7, 8 or 9 deviate from the reference values within set tolerance, upon request from the control system, the computer vision camera 6 obtains a black and white image of its field of view and transmits it to the control system. The same operation was performed in the process of setting up the spot welding system. The image obtained during the setup process can be processed by the control system, and at least 2 reference elements can be selected thereon based on the difference in image tint, the distance from which to the border of the image frame in pixels can be converted into millimeters based on resolution data, the focal length of the camera 6 and the height above the surface of the workpieces according to the readings of the rangefinders 7 and 8. The same processing can be performed and the same distances can be calculated for the image obtained during the automatic cycle. In the event that the height of the camera 6 above the surface of the workpieces according to the readings of the rangefinders 7 and 8 during the automatic cycle differs from the height value obtained during the setup process, the image ispre-processed by the control system: scaled accordingly based on the difference in the reference readings and the actual readings from the rangefinders. In the event of not establishing any reference elements in the image during the setup process (the surface of the workpieces is flat, without sharp bends or protruding elements), the reference element can be the edge a of the bundle to be welded (FIG. 3), since resistance spot welding is most often used for bundling workpieces along the edge. In this case, distances b and c to 2 points on the edge a of the bundle 4 equidistant from the center of the frame can be measured, for example.

[0060] Thus, with respect to the coordinate system shown in FIG. 2 for reference, the following can be noted. The uniform difference between the reference readings and the actual readings from the rangefinders 7 and 8 indicates displacement of the welding gun 1 and the workpiece bundle 4 with respect to the reference position along the Z axis. The unequal difference between the reference readings and the actual readings from the rangefinders 7 and 8 indicates inclination of the welding gun 1 and the workpiece bundle 4 with respect to the reference position in the XZ and YZ planes. The displacement of the welding gun 1 and the workpiece bundle 4 with respect to the reference position along the X and Y axes, as well as rotation of the welding gun 1 in the XY plane, are determined based on the comparison of the distances obtained from processing the image from the camera 6.

[0061] In the event of said deviations exceeding the tolerances set therefor in the process of setting up the spot welding system, the control system calculates the correction for the linear and angular coordinates required to bring the values into alignment. This correction is transmitted via a communication channel to the robot controller, and then a command is transmitted to perform the movement. The robot moves the welding gun 1 with respect to the workpiece bundle 4, after which the measurement cycle is repeated again.

[0062] If the value deviations are within the tolerance values, the relative position of the welding gun 1 and the workpiece bundle 4 is assumed to correspond to the reference used for the setup and, therefore, high-quality welding is considered feasible in this position.

[0063] The control system calculates, based on the distances to the surface of the workpiece bundle 4 measured by the rangefinders 7 and 8, the distance that the second electrode 2 can travel at maximum speed so as to reduce it to a speed allowing to gain the required force without excessive correction. Similarly, the control system calculates, based on the distance to the surface of the workpiece bundle 4 measured by the rangefinder 9, the distance that the first electrode 3 can travel at maximum speed so as to reduce it to a speed allowing to gain the required force without excessive correction. The electrodes 2 and 3 travel the calculateddistance in the control loop based on the position at maximum speed, reducing the speed at the end of movement to a speed suitable for gaining force, after which the required force is gained by controlling the movement based on force value.

[0064] After gaining force, the sensor system transmits distance readings of the rangefinders 7, 8 and 9 to the control system, and similarly to the process described above, the control system compares said readings with the distances measured in the process of setting up the spot welding system and stored in the storage means of the control system as reference values, the control system calculates, based on the distances measured by the rangefinders 8 and 9, and the distances from the rangefinders to the contact surfaces of the electrodes 2 and 3, respectively, the actual thickness of the workpiece bundle 4 after compensating for all potential bends and gaps between the workpieces in the bundle 4. In the event of this thickness value deviating from the corresponding value stored for the welding spot in the storage means of the control system during the setup process by more than the tolerance specified for this deviation, the control system concludes that the workpieces 4 are incorrectly supplied (for example, the number of workpieces is incorrect or the workpieces are structurally different from the required workpieces), stops the automatic welding cycle and sends an error signal indicating critical deviation in the thickness of the workpiece bundle 4 which cannot be corrected automatically. In this case, the equipment operator must correct the position of the workpieces and start the automatic cycle again, or continue the automatic cycle in the event that the number of workpieces to be welded in the bundle is not the same for all spots of the bundle. Thus, low-quality welding is avoided.

[0065] In Fig. 2, the third point C is added to the readings of the rangefinders 7 and 8 shown at points A and B, respectively. The position of this point along the Z axis is determined by the control system based on the movement value for the electrode 2 received from the sensor system. The use of three points A, B and C allows the control system to build a plane therethrough. The deviation of this plane from the same plane built by the control system during the setup process and stored in the storage means of the control system as a reference value allows the control system to calculate the Euler angles: the deviation of the angular position of the welding gun 1 from the reference position. The control system possessing the reference values of the coordinates of each previously performed welding spot stored in the storage means of the control system and obtained via the communication line from the robot controller, as well as the actual values of the linear coordinates of the welding spot 5 (determined through the performed corrections of the position of the welding gun 1 prior tocommencing compression of the workpiece bundle) and the actual values of the angular coordinates of the welding spot 5 (Euler angles) for all previously operated spots, calculates the required displacement and rotation of the coordinate system of the robot, minimizing the overall error (the total difference in the coordinates). This displacement and rotation is transmitted to the robot controller which applies them to its own coordinate system before moving to the next spot, thus minimizing the likelihood of position adjustments described above being required after reaching the spot.

[0066] In one embodiment of the present invention, when performing the automatic welding cycle, step 9 includes controlling the force on the electrodes 2 and 3 and the position of the electrodes 2 and 3 during their opening. According to the dynamics of these parameters, the sticking of electrodes 2 and 3 to the surface of the workpieces after welding can be determined. Usually, welding caps are put on the ends of the electrodes, through which contact is made with the workpiece bundle and which pass welding current therethrough. During the welding process, carbon or oxide film can form on the contact surfaces of the caps. This can lead to a change in their conductivity, and, as a result, to a change in the current therebetween, a change in the temperature of the workpieces in the melt zone and the formation of a welding nugget that does not meet set requirements, which negatively affects the quality of the welding spot. This can also lead to sticking of the electrodes to the surface of the workpieces after welding, and in the process of opening the electrodes, the workpiece will follow the electrode 2 or 3 until detached, which can lead to its deformation, which also negatively affects welding quality. In order to avoid these effects, the procedure for restoring the surface of the electrode caps is performed periodically (once per a set number of welding spots, or once per a set number of workpiece bundles). When performing this procedure, the contact surface of the caps is ground or polished, i.e., a layer of material is removed therefrom. Consequently, the caps are periodically worn out to the extent that it is impossible to restore their surface and they need to be fully replaced. These procedures take time but are often synchronized with the operational cycles of the welding line so that they are performed at the same time with the procedure of replacing the workpiece bundle and do not reduce productivity.

[0067] In the present invention, upon detection of sticking of the electrodes to the surface of the workpieces, a signal is generated indicating the need to restore the surface of the caps which can be performed, depending on welding line settings, immediately before the next spot or after completing the processing of the current workpiece bundle. Thus, the restoration ofthe surface of the caps is carried out according to their actual condition, leading to an additional increase in welding quality (in the event that the periodic restoration was set up with insufficient frequency, and the sticking of caps with deformation of workpieces was allowed), an increase in productivity and a decrease in costs (in the event that the periodic restoration was set up with excessive frequency). The use of this embodiment of the invention is not limited to the above: monitoring the sticking of the electrodes to the workpieces can be used during the setup process to determine the required frequency of restoration of the surface of the caps. In this case, during setup, welding is also performed (in fact, an automated cycle is performed) on a number of workpiece bundles sufficient to obtain reliable statistical data regarding the sticking of the caps. It is important that the present invention allows to determine the formation of carbon residue or oxide film on each of the electrodes separately.

[0068] In one embodiment of the present invention, during the setup process, a force control profile is determined for each welding program (for each individual welding spot 5, or for spots on which the same welding program is performed), the profile comprising force changes over time during the welding process. When welding a different number of workpieces of different thicknesses made from different materials, the welding technology can require a change in force at different welding stages. For instance, in the process of heating the material and melting, it may be necessary to gradually reduce the force, and then increase the force again and maintain it for a certain period of time until the welding nugget cools. Several welding pulses may also be required, e.g., when welding aluminium workpieces, two welding pulses with different force values may be required: the first pulse is performed to bum the oxide film formed on the surface of the workpieces, and the second pulse is performed to form the welding nugget. Accurate adherence to the required force control profile is critical for quality welding. In the automatic cycle, when performing step 8, the electrodes 2 and 3, when controlling the movement based the force value, ensure the adherence to the required force control profile with greater accuracy and independence from external factors compared to controlling the force based on the position of the electrodes, current or torque on the electrode drive motor, thus additionally improving welding quality. Controlling movement based on the force value due to the operation of two movable electrodes 2 and 3 ensures its adjustment without significant deformation of the workpiece bundle 4, in contrast to the conventional solution with one movable electrode, thus further improving welding quality.

[0069] In one embodiment of the present invention, in addition to the parameters characterizing relative position of the welding gun 1 and the workpiece bundle 4, the sensorsystem is configured for measuring the actual values of the main welding process parameters, which may include, e.g., current between the electrodes, voltage between the electrodes, resistance between the electrodes, temperature of the workpiece bundle in the area of contact with the electrodes, and other parameters, as well as compression force of the workpieces by the electrodes. For this purpose, the sensor system can be equipped with conventional technical means allowing measurements of said parameters. It is important to be able to also measure said parameters during welding current operation. The measured values of the parameters in relation to the welding spot 5 and time are stored in the storage means of the control system, and an archive of the parameters of the welding spots 5 is formed. The archive can be provided in the form of a set of files, or in the form of a database. Analysis of the influence of parameter values on welding quality and productivity can be performed based on the archive data by the process operator or by machine tools, and the parameters can be optimized for improving quality and productivity.

[0070] In one embodiment of the present invention, the main technological welding parameters are measured during the welding process by the sensor system and stored in the storage means of the control system in relation to the welding spot 5 and time as reference dynamics of the parameters for the welding spot. In the automatic welding cycle, when performing step 8, the sensor system obtains the actual values of the same parameters, and the control system compares actual dynamics thereof over time with the stored reference values. If the deviation exceeds the established tolerance for one or more parameters, low-quality welding can be potentially performed. Thus, burn-throughs, metal expulsions, insufficient material melt and, accordingly, potential inadequate welding quality at the spot can be determined. Typically, welding quality control is performed in a non-destructive manner for each welding spot 5 of each bundle 4, or selectively with a set sample size, as well as in a destructive manner selectively with a set sample size. This leads to additional time costs, and, accordingly, to a decrease in welding line productivity, and to the destruction of a certain portion of the product, that is, to an increase in costs. In an embodiment of the present invention, when deviation in the main technological parameters over time is determined, a signal is generated regarding potential issues with the quality of the welding spot. Consequently, non-destructive or destructive testing can be performed according to the actual condition and only for the welding spots 5 marked as having potential quality problems, which would increase welding performance (with a stable technological process and excessive testing frequency), reduce the cost of destructive testing (with excessive frequency ofdestructive testing), and further ensure an increase in product quality (since with an initially insufficient frequency of non-destructive testing, individual welding spots of inadequate quality may not be identified).

[0071] In one embodiment of the present invention, the formation of an oxide film on the surface of the cap of any of the electrodes 2 and 3, as well as damage to the surface of the electrode cap can be determined based on deviation from the reference values of resistance between the electrodes at the welding spot. In this case, similarly to the process of determining the need to restore the surface condition of the caps causing sticking of the cap when opening the electrodes, the control system generates a signal indicating the need to restore the condition of the cap.

[0072] In one embodiment of the present invention, in the automatic welding cycle, when performing step 8 and detecting deviations of the main technological parameters from reference values, the compensating adjustment of the electrodes 2 and 3 is calculated, aimed at changing compression force of the workpiece bundle 4 in order to bring the parameters closer to reference values, and the electrodes 2 and 3 perform the required movement. Key for the quality of the welding nugget is the required degree of material melt which depends on welding current and electrical resistance of the workpiece bundle 4 between the electrodes 2 and 3. If necessary, the resistance in the workpiece bundle 4 can be adjusted by changing compression force. Increasing the force generally reduces resistance, and decreasing the force increases resistance. Thus, by controlling the movement of the electrodes based on force value, the required electrical parameters can be achieved and, accordingly, the quality of the weld joint at the welding spot 5 can be further improved. It is important that, when controlling the movement based on force value, the fast-acting drives of the two movable electrodes 2 and 3 allow to quickly and accurately implement the desired control.

[0073] In one embodiment of the present invention, in the automatic welding cycle, when performing step 8 and detecting deviations of the main technological parameters from reference values, the required degree of material melt can be ensured by changing welding current strength. The required current change is calculated and transmitted to the welding controller. The welding controller implements the required change. Effectiveness of changes (the degree of compliance of the actual parameters with reference values) is monitored and, if necessary, additional corrections are made based on data obtained from the sensor system. Thus, by controlling the current, the quality of the weld joint at the welding spot 5 can befurther improved. It is important that the welding current correction can be used in conjunction with the workpiece compression force correction, allowing to compensate for deviations faster, and, consequently, increasing the likelihood of producing a high-quality weld joint.

[0074] In one embodiment of the present invention, in addition to data on the relative position of the welding gun 1 and the workpiece bundle 4 and / or the main technological welding parameters, the sensor system obtains data regarding the parameters required for determining the technical state of the hardware equipment components, including the welding controller, the robot, the welding gun 1, and the electromechanical drives for moving the electrodes 2 and 3. These data can include both diagnostic signals from the equipment and measurements of physical values, such as, e.g., the current of the electromechanical drive motor of the electrode, drive motor temperature, drive movement speed, drive motor torque and other values. Said physical values can be measured using conventional technical means in this field of technology. The control system performs diagnostics of the current technical state of the hardware components of the equipment based on the values of one or more parameters, their dynamics and combinations, and upon detection of deviations based on the diagnostic results, generate a signal indicating a malfunction or the need for calibration. Typically (in the prior art), system hardware components such as, e.g., the robot or the welding controller, comprise self-diagnostic systems reporting faults. At the same time, the welding gun and the electrode drive in such systems lack self-diagnostics, and possible malfunction or the need for calibration, can be diagnosed only upon producing inadequate product. The proposed embodiment of the invention allows to diagnose a malfunction or the need for calibration when it occurs, and to perform the necessary corrective measures in a short time period, thus reducing the potential volume of inadequate product, i.e., further improving welding quality and increasing productivity of the welding line.

[0075] In one embodiment of the present invention, data regarding the parameters required for determining the technical state of the hardware components of the equipment are accumulated, systematized and stored in the storage means of the control system, and specialized software performs, based on the accumulated data and their dynamics, predictive diagnostics of the technical state of the hardware components of the equipment and generates signals indicating required preventive maintenance. This allows to plan preventive measures and implement them before a malfunction occurs or the need to calibrate components arises, thus preventing production of inadequate product due to a malfunction or incorrect equipmentcalibration, thus further improving product quality and productivity of the welding line.

[0076] In any embodiment of the present invention, setting up the spot welding system for welding the workpiece bundle 4 can be implemented as follows:

[0077] 1. A welding strategy is formed based on 3D models of the workpiece bundle, the tooling and the surroundings of the welding line cell, as well as on kinematic 3D models of the robot and the welding gun 1, using a personal computer or other means on which said models can be created or stored, and specialized computer program. The operator positions the 3D models with respect to each other, determines movement trajectory parameters (reference points of the trajectory for the robot, the parameters for movement between the points, the welding spots), the parameters of each welding spot (opening values for each of the electrodes, the required compression force for the workpiece bundle, the number of the welding controller program, the required force control profile during welding (if required)). The program determines deviation tolerances for the parameters. Using the welding strategy, the 3D model of the workpiece bundle, and the parameters of the set points, the program calculates the target electrode closure point for each spot, corrects the electrode opening values, determines the angle between the electrodes and the surface of the workpiece bundle, calculates the distances from the end surface of the electrodes to the surface of the workpieces, calculates the distances for moving the electrodes in the control loop based on position, and determines the thickness of the workpiece bundle. The program generates control programs for the robot, the welding controller and the control system of the spot welding system based on the welding strategy, and the calculated and determined parameters. The generated control programs are loaded into the corresponding control devices.

[0078] 2. The setup of the spot welding system for performing welding of the workpiece bundle 5 is carried out based on the control programs loaded into the control devices. For each welding spot, the following sequence of steps is performed:

[0079] i: the robot performs the movement according to the coordinates of the point set by its control program.

[0080] ii. The sensor system receives information regarding the relative position of the welding gun 1 and the workpiece bundle 4.

[0081] iii. The control system compares the calculated data regarding the relative position of the welding gun 1 and the workpiece bundle 4 stored in the control program with the data received from the sensor system. The control program generated by the program is"idealized", that is, it does not take into account potential deviations in the positioning between the robot and the tooling, and between the tooling and the workpiece bundle, and generally does not take into account potential deformation of the workpieces arising from their own weight, flexibility and geometric properties of the tooling. Thus, the actual relative position of the welding gun 1 and the workpiece bundle 4, after executing the control program formed based on the welding strategy, will always differ from the "ideal" version recorded in the control program. Conventionally, an operator / setter is involved in the process at this stage, said operator using the robot training console to make manual adjustments to the position of the welding gun 1 and store said adjustments in the control program. This requires a highly qualified operator and a sufficiently long time period, due to the need to carefully align the position of the welding gun using manual measuring means. In the present method, in case of deviation of the actual values from the calculated values by more than the established tolerance, the control system calculates the necessary adjustment of the position of the robot and transmits the calculated value and a movement recommendation thereto. The movement recommendation can be executed automatically by the robot, or after confirmation of the need for adjustment by the operator / setter. After the robot performs the movement, the new coordinates are stored as the coordinates of the welding spot in the control program, and the steps ii-iii are repeated. Thus, the duration of positional setup for each welding spot 5 is reduced and the qualification requirements for the operator / setter are lowered.

[0082] iv. The control system stores the processed data from the sensor system as reference values prior to initiating closure of the electrodes 2 and 3. These data are further used when performing the automatic welding cycle.

[0083] v. After the welding gun 1 assumes the required position, the workpiece bundle 4 is compressed and the required force is gained.

[0084] vi. After gaining the required force, the sensor system re-acquires information on the relative position of the welding gun 1 and the workpiece bundle 4.

[0085] vii. The control system saves the processed sensor system data as reference values after gaining the required force. These data are further used when performing the automatic welding cycle.

[0086] viii. The electrodes 2 and 3 are opened.

[0087] Then the control system transmits a signal to the robot indicating the possibility of transition to the next welding spot 5, and steps i-viii are repeated until setup is performed at all welding spots 5. The signal is processed automatically by the robot, or after confirmationof the need for adjustment by the operator / setter.

[0088] The proposed method reduces duration of positional setup for each welding spot 5, i.e. increases welding performance, and further reduces qualification requirements for the operator / setter.

[0089] In any embodiment of the present invention, the functions of the control system regarding the computer program and welding program execution and data storage can be performed by a computing device or they can be distributed between the robot controller and a separate process controller of the spot welding system.

[0896] The proposed invention was tested in real production conditions at an automobile plant. When using the proposed system for welding a bundle of 3 parts at 12 welding spots according to the proposed method, an increase in productivity of 10% was achieved compared to the welding technology used in an automobile plant and corresponding to the known one, for example, from US8513560B2. By optimizing the movement path and eliminating equalization, without attempting to optimize the distance traveled by the electrodes, welding was completed in 42 seconds when carried out by the proposed spot welding system; at the same time, a similar welding process used in the automobile plant was completed in 47 seconds. The quality of the welding produced was similar.

[0091] According to calculations, depending on the complexity of the geometry of the workpiece bundle being welded, the position of the welding points on it, taking into account the optimization of the distances traveled by the electrodes, the productivity increase can be up to 30%.

[0092] As for the welding quality improvement, the following should be noted. During testing, when 20 of same workpiece bundles were welded sequentially as described above using welding technology used in the automobile plant, there were 2 instances of material sloshing. At the same time, when using the proposed spot welding system and the proposed method under similar conditions, not a single instance of material splashing occurred.

Claims

CLAIMS:

1. A robot-assisted resistance spot welding system comprising:• a robot;• a spot welding gun secured to said robot;• a sensor system;• a control system, and• a welding controller, wherein the control system comprises o a means for installing and executing a computer program for controlling the movement of said robot and said welding gun and a welding program for controlling the welding controller and o a storage means for storing the computer program, the welding program and data received by the control system; and wherein o the welding controller is connected to the control system to exchange control data with the control system and is configured to execute said welding program, the spot welding system further comprises• a welding transformer for supplying welding current to the welding gun, wherein the control system is connected to the sensor system to exchange data with the sensor system; and• said sensor system is configured for determining the relative spatial position of said welding gun and a workpiece bundle to be welded, and for recognizing the shape of the workpiece bundle to be welded at the estimated welding spot;• said welding gun comprises two independently movable electrodes configured for independent movement using electromechanical drives mounted on the welding gun and configured for changing the position of the electrodes, wherein the force on each electrode and the position of each electrode are measurable by means of said sensor system, and each electrode is connected to a separate electromechanical drive;• the computer program and the welding program stored in the storage means are configured to allow the control system to determine reference values of the welding parameters before welding, to receive actual values of the welding parameters during the welding, to compare the above values, and to control the robot, the movable electrodes of the welding gun and the welding transformer based on said comparison.

2. The spot welding system according to claim 1, characterized in that• welding parameters determined by the sensor system include one or more of: a position of the point of contact between the electrode and the workpiece bundle at the estimated welding spot, an angle between the electrodes and the surface of the workpiece bundle at the point of contact, and a distance between the electrodes and a surface of the workpiece bundle;• said control system is configured for, based on the data obtained by said sensor system: o determining the position of the point of contact between the electrode and the workpiece bundle at the estimated welding spot and deviation of the point of contact position from the reference position for the same welding spot; calculating the required displacement of the robot to ensure the coincidence of the positions of the point of contact and the reference point of contact; o determining the angle between the electrodes and the surface of the workpiece bundle at the point of contact and the deviation of said angle from the reference angle for the same welding spot; calculating the required displacement of the robot to ensure the required angle; o determining the distance between the electrodes and the surface of the workpiece bundle at the welding spot, determining the target electrode compression point; o determining the possibility of performing welding without significant deformation of the workpieces to be welded at the current relative position of the welding gun and the workpiece bundle with the current shape of the workpiece bundle at the welding spot and at the target electrode compression point by moving the movable electrodes without changing the position of the robot, and further determining the need for displacement of the robot with the welding gun with electrodes, the welding gun secured thereon; o controlling the robot by transmitting to said robot calculated displacements and commands to perform movement; o controlling the movable electrodes of the welding gun by ensuring their movement to gain, maintain and adjust the compression force on the workpiece bundle at said target compression point in accordance with parameters required by the welding program; o controlling said welding controller by initiating the welding program, and supplying welding current by means of the welding transformer.

3. The spot welding system according to claim 1 or 2, characterized in that the controlsystem is configured for storing at least the following reference values: o position of the point of contact between the electrodes and the workpiece bundle on the surface of the workpiece bundle prior to initiating closure of the electrodes; o angle between the electrodes and the surface of the workpiece bundle prior to initiating closure of the electrodes; o tolerance for the value of the position of the point of contact between the electrodes and the workpiece bundle on the workpiece bundle surface prior to initiating closure of the electrodes and for the angle value between the electrodes and the surface of the workpiece bundle prior to initiating closure of the electrodes; o position of the workpiece bundle between the electrodes prior to initiating closure of the electrodes; o angle between the electrodes and the surface of the workpiece bundle after gaining the required force; o position of the electrodes after gaining the required force.

4. The spot welding system according to any of claims 1-3, characterized in that:• the sensor system is configured for measuring and transmitting to the control system the actual values of the main welding process parameters, including at least one of the following: o current between the electrodes; o voltage between the electrodes; o resistance between the electrodes; o temperature at the welding spot,• wherein the control system is configured for accumulating, systematizing and storing data regarding change in said parameters during the welding process.

5. The spot welding system according to any of claims 1-4, characterized in that:• the sensor system is configured for measuring and obtaining data for determining the technical state of at least one of the welding gun, electromechanical drives, the robot, the welding controller, including at least one of the following parameters: o drive motor current; o drive motor temperature; o drive movement speed; o drive motor torque; o diagnostic signals from the robot and the welding controller,• the control system is configured, based on the data received from the sensor system, to diagnose the current technical state of the system hardware components and to generate a signal indicating a malfunction or the need for calibration.

6. The spot welding system according to claim 1, characterized in that:• the control system is configured for accumulating, storing and systematizing data regarding the parameters necessary for determining the technical state of the system hardware components;• the control system is configured for predictive diagnostics of the technical state of the system hardware components and for generating signals indicating required preventive maintenance based on the accumulated data.

7. The spot welding system according to any one of claims 1-6, characterized in that the means for installing and executing the computer program and the welding program in the control system is a computing device.

8. The spot welding system according to any one of claims 1-7, characterized in that the means for installing and executing the computer program and the welding program in the control system is a process controller of the spot welding system or the welding controller of the robot, forming part of the control system and acting as a means for installing and executing the computer program and the welding program.

9. A method for performing resistance spot welding by the robot-assisted resistance spot welding system according to any one of claims 1-8, the method comprises:• setting up the robot-assisted spot welding system using the sensor system to obtain reference values of the welding parameters and to store them in the storage means of the control system,• implementing resistance spot welding for each welding spot based on comparison of the reference values of the welding parameters and the values of the welding parameters received from the sensor system during the welding.

10. The method according to claim 9, characterized in that implementing resistance spot welding for each welding spot comprises the following sequence of steps: o (Step 1) placing the welding gun by the robot to the welding area according to the computer program; o (Step 2) receiving information on the relative position of the welding gun and the workpiece bundle and on the shape of the workpiece bundle; o (Step 3) determining the welding parameters based on the data received from thesensor system; by comparing the obtained values with the reference values stored for the welding spot during the setup process, determining whether the position of welding spots fall within the established tolerance and the feasibility of welding with the current welding parameters; if welding is unfeasible, calculating the adjustment value for the position of the target electrode closure point for the current welding spot, transmitting the value to the robot, carrying out the adjustment by the robot and repeating steps 2, 3; o (Step 4) using the position and the electrode movement speed control profile, calculating the electrode movement value by the control system based on the welding parameters values received from the sensor system; o (Step 5) moving the electrodes in accordance with the calculated parameters; upon completion of said movement, controlling the movement of the electrodes based on the force value on the electrodes and gaining the required force equally on both electrodes; o (Step 6) receiving information on the welding parameters by the sensor system after gaining the required force; o (Step 7) based on the data regarding the previously performed adjustment and based on comparison of the welding parameters values for the current welding spot and all previously operated welding spots on the workpiece bundle for the current workpiece bundle with their reference values, calculating by the control system the correction value for the position of the subsequent welding spots and transmitting it to the robot controller which applies the adjustment to the coordinates of the subsequent spots; o (Step 8) initiating by the control system the execution of the welding program by the welding controller, and generating welding current with the required parameters by the welding transformer; o (Step 9) opening and moving the electrodes away from the workpiece bundle to the reference position; wherein after performing step 9, in-between two welding spots in the welding program, the control system transmits a command to the robot to move to the next welding spot, after which steps 1-9 are repeated, and wherein the control program is configured such that, when moving to each successive welding spot in a series of welding spots, the robot moves the target electrode closure point within the workpiece bundle in accordance with the configuration of the workpiece bundle to be welded, taking into account potential adjustments transmitted in step 7.

11. The method according to claim 10, characterized in that the welding parameters include: a position of the point of contact between the electrode and the workpiece bundle at the estimated welding spot, an angle between the electrodes and the surface of the workpiece bundle at the point of contact, and a distance between the electrodes and a surface of the workpiece bundle.

12. The method according to claim 10 or 11, characterized in that:• during the setup process, the reference workpiece bundle thickness value after compression is stored for each welding spot and the tolerance is set to this value;• when performing step 7, the control system: o determines, based on the data received from the sensor system in step 6, the thickness of the workpiece bundle after compression; o compares the obtained value with the reference value; o and, in case of deviation beyond the established tolerance, stops the execution of the automatic welding cycle.

13. The method according to any of claims 10-12, characterized in that, when performing step 9, the control system:• monitors, based on the sensor system data, the position and force on the electrodes during their movement;• determines, based on the dynamics of the position and force, sticking of the electrodes to the surface of the workpiece bundle;• and, when sticking of the electrodes to the surface of the workpiece bundle is detected, a signal is generated indicating the required restoration of the surface of the electrodes.

14. The method according to any one of claims 10-13, characterized in that:• during the setup process, a force control profile is determined for each welding program during program execution;• when performing step 8 simultaneously with the execution of the welding program by the welding controller, the control system controls the movement based on the force value on the electrodes according to the set force control profile.

15. The method according to any one of claims 10-14, wherein in the spot welding system, the sensor system is configured for measuring and transmitting to the control system the actual values of at least one of the following parameters: o current between the electrodes; o voltage between the electrodes;o resistance between the electrodes; o temperature at the welding spot, wherein the control system is configured for accumulating, systematizing and storing data regarding the dynamics of said parameters during the welding process, wherein the method is characterized by that• in the process of setting up the spot welding system, reference data on the dynamics of process parameters during welding are stored in the storage means of the control system for each welding spot;• when performing step 8, the control system analyzes deviations in process parameters from the reference values and, if the tolerance set for such deviation according to at least one parameter is exceeded, generates a signal.

16. The method according to claim 15, characterized in that when performing step 8, upon detection of a deviation from the reference values of the process parameters, the control system calculates compensating adjustment of the electrodes and the electrodes are moved as required.

17. The method according to claim 15 or claim 16, characterized in that, when performing step (8), upon detection of a deviation from the reference values of the process parameters, the welding program calculates welding current compensating adjustment aimed at bringing the parameters closer to the reference values, transmits a signal to the welding controller, and the welding controller provides the required parameter change.

18. The method according to any of claims 10-17, characterized in that:• control programs for moving the robot, controlling the welding gun electrodes, and controlling the welding controller are generated prior to initiating setup of the spot welding system based on 3D models of the workpiece bundle, tooling, the robot, and the welding gun, wherein the control programs include at least the following parameters for each welding spot: o opening values for each electrode; o angle between the electrodes and the surface of the workpiece bundle; o distance from the end surface of the electrode to the surface of the workpiece bundle; o position of the target electrode closure point in the coordinate system of the robot or the workpiece bundle; o distances for moving the electrodes when controlled based on position; o required compression force of the workpiece bundle; o thickness of the workpiece bundle;• in the process of setting up the spot welding system, the following sequence of steps is carried out for each welding spot: o (Step i) the robot performs the movement according to the coordinates of the point set by the control program; o (Step ii) the sensor system receives information regarding the relative position of the welding gun and the workpiece bundle; o (Step iii) the control system analyzes the calculated values of the relative position of the welding gun and the workpiece bundle based on the data from the control program and the actual values received from the sensor system, and if the actual values deviate from the calculated values by more than the established tolerance, the control system calculates the necessary adjustment of the position of the robot and transmits the calculated value and a movement signal thereto, and after said robot performs the movement, the new coordinates are stored as the coordinates of the welding spot in the control program, and the steps (ii-iii) are repeated; o (Step iv) the control system stores the processed data from the sensor system as reference values prior to initiating closure of the electrodes; o (Step v) the workpiece bundle is compressed and the required force is gained; o (Step vi) the sensor system receives information regarding the relative position of the welding gun and the workpiece bundle; o (Step vii) the control system processes the sensor system data and stores it as reference values after gaining the required force; o (Step viii) the electrodes are opened; o the control system then transmitting a signal to the robot indicating the required movement to the next welding spot, and steps i-viii are performed up to completion of all welding spots.

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