METHOD FOR RESISTANCE WELDING OF WORKPIECES

Description

[0001] The present invention relates to a method for resistance welding of workpieces, as well as a welding device and a computer program for carrying it out. State of the art

[0002] Welding processes, such as resistance welding, allow workpieces to be joined together using a material-bonded process. For example, in automated body-in-white production, robot-guided welding guns weld different workpieces, such as sheet metal, together using resistance welding.

[0003] In resistance welding, two welding electrodes of a welding gun are first pressed against a welding point on the workpieces using an electrode drive during a so-called force build-up phase until a predetermined electrode force is reached. After reaching the predetermined electrode force, the welding electrodes can be pressed against the welding point without current for a so-called hold phase. The actual welding process then begins, during which the welding electrodes are energized with a welding current for the duration of a welding time or current time. This causes resistance heating of the two workpieces to be welded between the welding electrodes, heating them to the required welding temperature. After the welding time has ended and the current to the welding electrodes has ceased, the welding process is called the "cooling phase."The follow-up phase, during which the welding electrodes are still pressed against the welding point of the workpieces by the electrode force, but are no longer energized by the welding current.

[0004] For example, DE 102 24 402 A1 describes a welding gun for electric resistance welding with two electrodes, at least one of which is attached to an electrode arm, means for moving at least one electrode arm, wherein each electrode arm is received at its end by an electrode arm holder, means for generating the welding current and a force sensor for detecting an actual force value between the two electrodes, wherein the force sensor is arranged on one of the electrode arm holders.

[0005] For example, DE 101 44 286 C1 describes a method for assessing the quality of a weld joint of at least two sheet metal parts during electric resistance welding using a welding gun with two electrodes, an electrically driven, non-self-locking spindle for moving at least one of the two electrodes, means for generating the welding current, a gun control, a position control for the electric drive, means for detecting the spindle position, and a power section comprising a speed control and a power switch for the electric drive, wherein the power section has a speed setpoint input, a current limiting input, a current actual value output, a spindle position actual value output, and an output connected to the drive for the motor power cable.where, at least during welding, the position control is activated and the clamp control detects and evaluates changes in the actual value of the current at the current output of the power section, which is proportional to the drive torque. Disclosure of the invention

[0006] Against this background, a method for resistance welding of workpieces, as well as a welding device and a computer program for its execution, are proposed, comprising the features of the independent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

[0007] Welding electrodes are pressed against a welding point on the workpieces by an electrode drive using electrode force. During the welding process, the welding electrodes are energized with a welding current for the duration of the welding time. The welding current can be increased to a predetermined value at the beginning of the welding process during a so-called current build-up phase and reduced to zero at the end of the welding process during a so-called current decay phase.

[0008] After the welding time has ended, i.e., after the welding current has ceased, the welding electrodes are no longer energized during a follow-up phase, but are still pressed against the weld point by the electrode drive using electrode force. During this time, the weld point can cool down.

[0009] Within the framework of the present procedure, during this follow-up phase, a time course of a welding electrode parameter characterizing the position of one or both welding electrodes is determined, and it is checked whether this time course of the welding electrode parameter fulfills at least one characteristic feature. Depending on this check, it is determined when the follow-up phase ends and thus when the welding electrodes can be removed or moved away from the welding point of the workpieces.

[0010] After the workpieces are heated by the welding current during the welding process, they cool down after the welding current is switched off. During the hold-open phase, the workpieces should ideally be pressed together at the weld point until the weld has cooled sufficiently to achieve the necessary strength and the workpieces are firmly joined. If the hold-open phase is too short and the welding electrodes are removed from the weld point too soon, the weld can fail due to the existing stresses, and the workpieces can spring apart at the weld point.

[0011] The present method provides a way to automatically and optimally determine the duration or end of the follow-up phase for each individual welding point. The present invention is based on the understanding that the thermal expansion of the weld metal due to the current flow during welding, as well as the converse cooling behavior after the current flow ceases, are reflected in the simultaneous course of the electrode movement. Therefore, the cooling process of the weld point can be inferred, in particular, from the temporal course of the electrode movement.

[0012] The welding electrode parameter, which characterizes the position of the welding electrode(s), specifically defines the current position of one or both welding electrodes or of the drive moving the welding electrode(s). The cooling of the weld spot and the associated reduction in thermal expansion cause the welding electrodes to move towards each other at the weld spot. This is reflected in a corresponding change in this welding electrode parameter over time, from which the current position of the welding electrodes can be deduced.

[0013] The cooling process is particularly advantageously reflected in a characteristic way in the time course of the welding electrode parameter. If the weld spot cools for a sufficiently long time, during which the weld joint reaches the necessary strength, the corresponding characteristic features are particularly usefully observed in the time course of the welding electrode parameter. Therefore, if the corresponding characteristic feature(s) are identified during the verification of the time course of the welding electrode parameter, the follow-up phase is expediently terminated.

[0014] The characteristic feature is expediently understood as a feature characterizing the cooling process of the weld point, or a feature characteristic of this cooling process. In particular, the characteristic feature is expediently understood as a feature that characterizes sufficient cooling of the weld point so that the necessary strength of the weld joint is achieved.

[0015] The present method allows the welding machine, control unit, or welding controller to automatically perform the hold-hold period until the weld joint has reached the necessary strength, thus preventing the weld joint from loosening or the workpieces from popping open at the weld point. In particular, no additional parameterization or workpiece measurement is required; the sufficient duration is determined automatically and online by the welding machine itself during the hold-hold phase.

[0016] The sufficient duration of the holding phase generally depends primarily on the specific materials of the workpieces to be welded, their sheet thickness combination (TDC), and, in particular, on the weld spot position—that is, the orientation of the welding electrodes relative to the weld spot and the workpiece surfaces. For example, this can result in weld spots with the same sheet thickness combination but different weld spot positions requiring different holding phases to achieve sufficient strength. Conventionally, parameterizing the holding phase duration is often difficult or time-consuming, as these individual factors, such as sheet thickness combination and weld spot position, must be explicitly considered for each weld spot. Therefore, the same default value is often set for all weld spots.The present method automatically and implicitly takes such factors into account, without requiring them to be explicitly considered when determining the sufficiently long retention period. This method is therefore particularly advantageous, for example, in processes where numerous weld points with complex, varying sheet thickness combinations must be welded together, each with its own individual weld point position, such as in car body manufacturing.

[0017] Furthermore, the present method allows, for example, even with suboptimal or incorrectly aligned welding electrodes or suboptimal workpiece or sheet metal pressures, which can lead to a gap between the workpieces to be joined, a sufficiently long holding phase to achieve sufficient strength of the weld point.

[0018] According to the invention, the temporal profile of the welding electrode parameter fulfills at least one characteristic feature when a value of the welding electrode parameter, in particular an absolute value or an amplitude, reaches a parameter threshold. In particular, this parameter threshold characterizes a thickness of the weld spot, and more advantageously, a predetermined or predefinable final value of the reduction in thermal expansion. When the weld spot has cooled sufficiently and the thermal expansion has decreased sufficiently, the thickness of the weld spot falls below a certain thickness threshold, which is particularly advantageously reflected in the parameter threshold for the welding electrode position.

[0019] Alternatively or additionally, according to the invention, the time course of the welding electrode parameter fulfills at least one characteristic feature if the slope of the time course of the welding electrode parameter reaches a gradient threshold. This gradient threshold of the slope particularly characterizes the reduction in the rate of thermal shrinkage of the weld spot. When the weld spot has cooled sufficiently and cured sufficiently, the welding electrodes move towards each other more and more slowly, which is expediently characterized by this gradient threshold for the welding parameter slope.

[0020] According to the invention, at least one characteristic feature is predetermined, at least partially, depending on a holding phase or on a value of the welding electrode parameter during the holding phase. During this holding phase, the welding electrodes are pressed against the weld point of the workpieces with the electrode force for a predetermined duration before the welding process begins, and are not yet energized with the welding current. This holding phase advantageously takes place between the force build-up phase and the actual welding process. Advantageously, the weld point can be examined before the welding process begins, and conclusions can be drawn about how the cooling process after welding should behave to achieve sufficient strength at this specific weld point. It is particularly advantageous to implicitly or explicitly determine this.automatically takes into account the special properties of the respective welding point, in particular the special material of the workpieces as well as the respective sheet thickness combination and furthermore in particular the respective welding point position.

[0021] According to the invention, a temporal profile of the welding electrode parameter is determined during the pre-welding phase, particularly in a manner corresponding to the temporal profile during the post-welding phase. The at least one characteristic feature is determined, at least partially, according to the invention as a function of this temporal profile of the welding electrode parameter determined during the pre-welding phase. In particular, it is possible to learn or investigate during the pre-welding phase how the welding electrode position or the welding electrode parameter behaves relative to the weld spot before the welding process at the corresponding electrode force. Based on these findings, it is advantageous to conclude or extrapolate how the welding electrode parameter should behave after the welding process and during the cooling of the weld spot.It is advantageous to learn individual and characteristic features for the welding parameter profile during the holding phase for the respective welding point.

[0022] Preferably, the parameter threshold for the welding electrode parameter is determined as a function of the time-dependent profile of the welding electrode parameter as determined during the holding phase. It is particularly advantageous to use the welding electrode parameter and the associated welding electrode position during the holding phase to investigate the thickness of the workpieces at the weld point before the start of the welding process. Based on this knowledge, it is then useful to deduce how the thickness of the weld point should behave during the welding process and during the cooling process.

[0023] In particular, it is possible to determine the maximum thickness the weld spot should have after sufficient cooling, and thus the threshold value of the welding electrode parameter at which the follow-up phase can be terminated. The weld spot location, sheet thickness combination, and workpiece material are automatically and conveniently taken into account.

[0024] Advantageously, during the follow-up phase, it is also checked whether the duration of the follow-up phase reaches a predefined minimum value and / or a predefined maximum value. Therefore, it is useful to monitor, as an additional safety criterion, whether the follow-up phase ends at a plausible time based on the verification of the welding electrode parameter.

[0025] A predetermined measure is particularly advantageous if the follow-up phase is terminated, or would be terminated, based on the monitoring of the welding electrode parameter's time course before its duration reaches the predetermined minimum value. This specifically indicates that the follow-up phase was chosen too short and the weld spot has not yet cooled sufficiently to exhibit adequate strength. In this case, it is expedient to continue the follow-up phase as a predetermined measure at least until its duration reaches the predetermined minimum value.

[0026] Furthermore, a predetermined measure is particularly advantageous if the follow-up phase, based on the monitoring of the welding electrode parameter's time course, is not terminated, or would not be terminated, before the duration of the follow-up phase reaches the predetermined maximum value. In particular, this maximum value is chosen to be sufficiently large so that the weld spot should be adequately cooled and cured after this maximum duration. Specifically, in this case, the follow-up phase is terminated as a predetermined measure after the duration has reached the predetermined minimum value.

[0027] In these two cases, an error may have occurred, such as the follow-up phase being too short or not being terminated in time. Therefore, preferably, a corresponding error message can be issued as a predefined measure in each case.

[0028] According to the invention, a drive parameter of the electrode drive is defined as the welding electrode parameter. In this context, drive parameters are understood to be, in particular, quantities or parameters that are determined during the regulation or control of the electrode drive in order to move the welding electrodes in a predetermined manner and press them against the workpieces. Specifically, the drive parameters characterize the current position or movement of the welding electrodes. Therefore, the current position of the welding electrodes can be conveniently deduced from the current values ​​or time profiles of the drive parameters, and from this, the welding point of the workpieces can be determined. Thus, the drive parameters are particularly suitable for monitoring the cooling process. The reduction in the thermal expansion of the workpieces after the welding process causes the welding electrodes to move towards each other.to move towards the welding point, which is reflected in the temporal progression of certain drive parameters, from which the current position of the welding electrodes can be deduced.

[0029] Preferably, the welding electrode parameter is determined as the actual position of one or both electrodes of the electrode drive, i.e., in particular the current position of one or both welding electrodes. For example, the actual position can be determined relative to the welding point or the workpieces, or expediently also in relation to a reference point of the welding machine, in particular to a rest position of the welding electrodes. In particular, the position or actual position characterizes the position of the electrode caps in space. Furthermore, in particular, only one axis is considered, expediently an axis in the direction of movement of the electrodes, which can be defined by an actual position of a drive that opens and closes the electrodes.

[0030] Alternatively or additionally, a torque value of the electrode drive is preferably determined as the welding electrode parameter. A motor of the electrode drive, for example a servo motor, transmits a torque to a drive spindle of the electrode drive. Depending on mechanical parameters, this results in a certain force, the actual value of which is expediently controlled to a target value, for which the torque value is adjusted accordingly. The electrode force is a quantity derived from this torque.

[0031] Alternatively or additionally, drive parameters such as an actual speed value and / or a target speed value and / or a torque-generating current and / or an actual acceleration value and / or a clamping speed of the electrode drive can also be determined as welding electrode parameters. In particular, the position of the welding electrode(s) and thus the weld point during the cooling process can also be deduced from such drive parameters.

[0032] The present method is particularly well-suited for body-in-white production, especially for automated welding processes in body-in-white manufacturing, preferably in the context of vehicle production. Specifically, sheet metal panels are welded together to manufacture a vehicle body. During the manufacturing process of a single body, up to several thousand weld points with several hundred different sheet thickness combinations can be processed automatically; for example, approximately 5,000 weld points with approximately 700 different sheet thickness combinations for a mid-range vehicle. The method automatically determines the optimal follow-up phase for each of these individual weld points online, conveniently without requiring manual intervention or additional parameter settings. This ensures that the individual weld points are welded with the highest possible quality.

[0033] A control unit (processing unit), e.g., a welding control unit of a welding machine, is configured, particularly in terms of programming, to carry out a method according to the invention. The control unit or welding control unit can, for example, be designed as a PLC (programmable logic controller), as an NC (numerical control), or as a CNC (computerized numerical control).

[0034] A welding machine according to the invention for resistance welding comprises a welding gun with welding electrodes and an electrode drive for moving the welding electrodes. Furthermore, the welding machine includes a control unit.

[0035] Implementing a method according to the invention in the form of a computer program with program code for carrying out all process steps is also advantageous, as this incurs particularly low costs, especially if an executing control unit is already used for other tasks and is therefore already available. Suitable data carriers for providing the computer program are, in particular, magnetic, optical, and electrical storage media, such as hard drives, flash memory, EEPROMs, DVDs, etc. Downloading a program via computer networks (Internet, intranet, etc.) is also possible.

[0036] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.

[0037] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or on their own, without leaving the scope of the present invention.

[0038] The invention is schematically illustrated in the drawing using exemplary embodiments and is described in detail below with reference to the drawing. Character description

[0039] Figure 1 schematically shows a preferred embodiment of a welding device according to the invention, which is configured to carry out a preferred embodiment of a method according to the invention. Figure 2 schematically shows a preferred embodiment of a method according to the invention as a block diagram. Figure 3 schematically shows time-dependent profiles of welding electrode parameters, which can each be determined in the course of a preferred embodiment of a method according to the invention. Figure 4 schematically shows time-dependent profiles of actual position values ​​of welding electrodes, which can each be determined in the course of a preferred embodiment of a method according to the invention. Detailed description of the drawing

[0040] In Figure 1 A welding machine for resistance welding is shown schematically and labelled 100.

[0041] The welding machine 100 can be used to join workpieces 120 together by resistance welding. In particular, the workpieces 120 are welded together during the construction of a car body shell, specifically for the production of a motor vehicle body. For example, two aluminum sheets 121 and 122 are welded together as workpieces.

[0042] The welding machine 100 has a welding gun 110 with two welding electrodes 111 and 112. An electrode drive 130 is provided to move the welding electrodes 111 and 112. Figure 1 The welding gun 110 is, for example, represented as a servo-electric welding gun with an electrode drive 130 designed as a servo motor. It is also conceivable that the electrode drive 130 could be designed, for example, as an electric motor, hydraulic motor or pneumatic motor.

[0043] During resistance welding, during a so-called force build-up phase, the welding electrodes 111 and 112 are pressed against the sheets 121 and 122 at a welding point 125 by means of the electrode drive 130. Subsequently, during the actual welding process, the welding electrodes 111 and 112 are energized with a welding current for the duration of a welding cycle, resulting in resistance heating of the sheets 121 and 122 at the welding point 125.

[0044] Furthermore, the welding machine 100 has a control unit (welding controller) 140, which can, for example, be designed as a PLC (programmable logic controller). The control unit 140 is configured to control the electrode drive 130, indicated by reference numeral 151, and also the welding process, indicated by reference numeral 152. For this purpose, a corresponding drive control 141 and a corresponding welding process control 142 are implemented in the control unit 140, which can each be implemented as separate control programs or as a single, combined control program.

[0045] The control unit 140 is furthermore, in particular in terms of programming, configured to carry out a preferred embodiment of a method according to the invention, which in Figure 2 schematically represented as a block diagram and subsequently in relation to the Figure 1 and 2 will be explained.

[0046] In step 201, the force build-up phase takes place, during which the welding electrodes 111 and 112 are pressed against the sheets 121 and 122 at the welding point 125 with the help of the electrode drive 130 until the specified electrode force is reached.

[0047] Step 202 refers to a so-called holding phase, during which the welding electrodes 111, 112 are pressed against the welding point 125 of the workpieces 121, 122 by the electrode force before the start of the welding process, but are not yet energized with the welding current. During this holding phase, a time course of a welding electrode parameter characterizing the position of one or both welding electrodes is determined, for example, a time course of a position value of the electrode drive 130, which characterizes the current position of the welding electrodes 111, 112.

[0048] In step 203, a threshold value for an absolute value of the actual position is determined as a parameter threshold, depending on this temporal progression of the actual position value. For example, the parameter threshold can be determined depending on a current actual position value, which in particular characterizes the current position of the welding electrodes 111, 112 relative to the welding point 125 and thus, in particular, the thickness of the workpieces at the welding point 125. For example, the parameter threshold can be determined as the sum of the current actual position value and a tolerance value, where this tolerance value can, for example, be a predefinable percentage of the current actual position value.

[0049] In step 204 the welding process takes place, during which the welding electrodes 111, 112 are supplied with the welding current for the duration of the welding time.

[0050] After the welding time has ended and the current supply to the welding electrodes 111, 112 has ceased, the so-called follow-up phase takes place in step 205. During this phase, the welding electrodes 111, 112 are still pressed against the weld point 125 of the workpieces 121, 122 by the electrode force, but are no longer energized by the welding current. Similar to the pre-hold phase 202, a time-dependent profile of a welding electrode parameter characterizing the position of one or both welding electrodes is also determined during the follow-up phase. For example, the time-dependent profile of the actual position value of the electrode drive 130 is also determined as this welding electrode parameter during the follow-up phase.

[0051] Step 206 checks whether this time course of the actual position value fulfills at least one characteristic feature. In particular, it is checked whether an absolute value of the actual position value reaches the parameter threshold determined in step 203 and / or whether, furthermore, a slope of the time course reaches a predefined gradient threshold.

[0052] The threshold value for the actual position value represents in particular a maximum value for the thickness of the cooled weld spot 125, whereby falling below the maximum value indicates in particular that the weld spot 125 has cooled and hardened sufficiently and that the weld joint has achieved a necessary strength.

[0053] The gradient threshold for the slope can, for example, be predefined and stored in the control unit 140. In particular, the slope of the actual position value characterizes the reduction in the rate of thermal shrinkage and thus the cooling process of the weld spot 125. When the slope reaches the corresponding gradient threshold, this indicates, in particular, that the welding electrodes are moving towards each other only slowly or hardly at all, and that the thickness of the weld spot is decreasing slowly or hardly at all. Reaching this gradient threshold also indicates sufficient cooling of the weld spot 125.

[0054] If, in step 207, both the actual position value and the slope of the time course of the actual position value reach their respective threshold values, this indicates that the weld joint has achieved sufficient strength and the follow-up phase can be ended.

[0055] In this case, step 208 checks whether the duration of the follow-up phase has reached a predefined minimum value. If so, the follow-up phase is ended in step 209 and the welding electrodes 111 and 112 are moved away from the welding point.

[0056] However, if the duration of the follow-up phase has not yet reached the specified minimum value, even though the follow-up phase can be terminated based on check 206 of the time course of the actual position value, this indicates an error. In this case, an error message is issued in step 210, and the follow-up phase continues at least until the specified minimum value is reached.

[0057] Furthermore, an error can also lead to the follow-up phase not being terminated based on the check of the actual position value over time, even though weld point 125 has already cooled and cured sufficiently. Therefore, in step 206, in addition to checking the actual position value over time, it is also verified whether the duration of the follow-up phase reaches a predefined maximum value. If the duration reaches this maximum value even though the actual position value and its slope have not yet reached their respective thresholds, this indicates a corresponding error. In this case, a corresponding error message is issued in step 211, and the follow-up phase is terminated.

[0058] Figure 3 schematically shows the time courses of welding electrode parameters of the electrode drive 130 during resistance welding.

[0059] Curve 310, for example, shows the time course of the actual position value, curve 320 of the actual torque value, and curve 330 of the actual electrode force value. The time courses 310, 320, and 330 are shown during the pre-welding phase 301, during the actual welding process 302, and during the post-welding phase 303.

[0060] As in Figure 3 As can be seen, the temporal profile of the actual position value 310 changes continuously during a cooling phase 311 of the weld point during the follow-up phase 303, until the actual position value reaches an essentially constant value and the slope of the temporal profile essentially drops to zero. The actual position value, or rather its temporal profile, is therefore particularly suitable for inferring the cooling process of the weld point.

[0061] Figure 4Figure 410 schematically shows a multitude of time-dependent profiles of actual position values ​​of welding electrodes during a variety of different resistance welding processes and thus at a multitude of different welding points. It should be noted that the scale of the y-axis in Figure 4 compared to the curve 310 from Figure 3 The relevant scale is reversed.

[0062] These various resistance welding processes were carried out, for example, on the same sheet thickness combination. The sheet thickness combination, and thus the total sheet thickness, was the same for all curves 410 shown. For example, curve 411 denotes the weld spot with the largest spot diameter or the largest spot dimension in the direction of the electrode axes, and curve 412 denotes the weld spot with the smallest spot diameter. For the multitude of time profiles 410, in Figure 4Furthermore, the corresponding parameter threshold 420 for the absolute value of the positional value is shown, as well as the gradient threshold 430 for the slope of the time profiles.

Claims

1. Method for resistance welding of workpieces (121, 122), wherein welding electrodes (111, 112) are pressed against a welding point (125) of the workpieces (121, 122) with the aid of an electrode drive (130) with an electrode force (201, 202), wherein, in the course of a welding process (302), the welding electrodes (111, 112) are energized with a welding current for the duration of a welding time (204), and wherein the welding electrodes (111, 112) are pressed against the welding point (125) of the workpieces (121, 122) with the electrode force after the end of the welding time in the course of a post-holding phase (303) (205), wherein, in the course of the post-holding phase (303), a temporal profile (310, 320, 330, 410) of a welding electrode parameter characterizing a welding electrode position of one or both welding electrodes (111, 112) is determined (205), wherein a drive parameter of the electrode drive is determined as the welding electrode parameter, wherein a check is carried out to determine whether the temporal profile (310, 320, 330, 410) of the welding electrode parameter satisfies at least one characteristic feature (206), wherein the temporal profile (310, 320, 330, 410) of the welding electrode parameter satisfies the at least one characteristic feature if the welding electrode parameter reaches a parameter threshold value (420) and / or if a gradient of the temporal profile (310, 320, 330, 410) of the welding electrode parameter reaches a gradient threshold value (430), wherein, on the basis of this check, it is determined (206) when the post-holding phase (303) is ended (209), wherein the at least one characteristic feature is predefined at least partially on the basis of a pre-holding phase (202, 301), wherein the welding electrodes (111, 112), in the course of this pre-holding phase (202, 301), are pressed against the welding point (125) of the workpieces (121, 122) with the electrode force before the start of the welding process (302) and are not energized with the welding current, wherein, in the course of the pre-holding phase (202, 301) a temporal profile (310, 320, 330, 410) of the welding electrode parameter is determined (202), characterized in that the at least one characteristic feature is determined at least partially on the basis of the temporal profile of the welding electrode parameter, which temporal profile is determined in the course of the pre-holding phase (202, 301) (203).

2. Method according to Claim 1, wherein the parameter threshold value (420) for the welding electrode parameter is determined on the basis of the temporal profile of the welding electrode parameter, which temporal profile is determined in the course of the pre-holding phase (301) (203).

3. Method according to either of the preceding claims, wherein, furthermore, in the course of the post-holding phase (303), a check is carried out to determine whether a duration of the post-holding phase (303) reaches a predefined minimum value and / or reaches a predefined maximum value (206, 208).

4. Method according to Claim 3, wherein a predefined measure is carried out, in particular a fault notification is output, if, on the basis of the check of the temporal profile (310, 320, 330, 410) of the welding electrode parameter, the post-holding phase (303) is ended before the duration of the post-holding phase (303) reaches the predefined minimum value (210), or if, on the basis of the check of the temporal profile (310, 320, 330, 410) of the welding electrode parameter, the post-holding phase (303) is not ended before the duration of the post-holding phase (303) reaches the predefined maximum value (211).

5. Method according to one of the preceding claims, wherein a position actual value (310, 410) of one or both electrodes (111, 112) and / or a torque actual value (320) of the electrode drive (130) are / is determined as the drive parameter of the electrode drive.

6. Welding device (100) for resistance welding, wherein the welding device (100) has a welding gun (110) comprising welding electrodes (111, 112), an electrode drive (130) for moving the welding electrodes (111, 112) and a control unit (140), characterized in that the control unit (140) is configured in such a way that the welding device carries out a method according to one of Claims 1 to 5.

7. Computer program comprising commands that cause the welding device (100) according to Claim 6 to carry out the method steps according to one of Claims 1 to 5.

8. Machine-readable storage medium with a computer program according to Claim 7 stored thereon.