Method for operating a resistance spot welding device, resistance spot welding device and computer program product

Abstract

A method for operating a resistance spot welding device (1), wherein a welding process is carried out at a welding point (5) of a workpiece (2) by applying electric current to a spot welding electrode (3) of the resistance spot welding device (1), wherein during the welding process at least one parameter of the electric current flowing through the spot welding electrode (3) is detected, wherein a state value describing a state of the spot welding electrode (3) is determined on the basis of the at least one parameter and a conditioning of the spot welding electrode (3) is carried out as a function of the state value, characterized in that one of the following values ​​is used as the state value or the state value is determined from one of the following values: time of a first maximum (11) of a dynamic resistance, resistance value of the dynamic resistance at the first maximum (11),Time of a first minimum (12) of the dynamic resistance, resistance value of the dynamic resistance in the first minimum (12), time of a second maximum (13) of the dynamic resistance, resistance value of the dynamic resistance in the second maximum (13), time of a second minimum (14) of the dynamic resistance, resistance value of the dynamic resistance in the second minimum (14), variance of the time of the second maximum (13), weighted difference between a curve (10) and a reference curve of the dynamic resistance and time shift that maps the curve (10) and the reference curve of the dynamic resistance to one another in a time-normalized manner, wherein the reference curve is determined during processing.

Description

The invention relates to a method for operating a resistance spot welding device, wherein a welding process is carried out at a welding point of a workpiece by applying electric current to a spot welding electrode of the resistance spot welding device, wherein at least one parameter of the electric current flowing through the spot welding electrode, which parameter changes via the welding process, is detected during the welding process, wherein a state value describing a state of the spot welding electrode is determined on the basis of the at least one parameter and a conditioning of the spot welding electrode is carried out as a function of the state value. The invention further relates to a resistance spot welding device and a computer program product.The prior art discloses, for example, the publication DE 103 31 617 A1. This describes a method and a device for detecting and monitoring the properties of components or parts in a welding circuit of spot welding systems, in particular for detecting and monitoring the milling result of the electrode caps of resistance spot welding systems. Here, two reference measurements are performed in the welding circuit by a welding in a short circuit without workpieces in order to obtain as a reference value two welding current values between a specific number of welds. These two welding current values are compared with one another using a control unit using empirical or numerically-theoretically determined criteria and the property of the component is derived therefrom. The reference measurements are made with a constant primary current and electrode pressing force so that the changes in welding current value are due to the change in electrical resistance of the components.The document DE 10 2009 056 234 A1 relates to a method for monitoring and / or controlling a device for applying a welding point in each case for a multiplicity of temporally successive joining events. In order to be able to better detect aging of the device, provision is made for a measurement of a current and a voltage of the joining events, a determination of a resistance of the joining events by means of the current and the voltage, a determination of a time series of the resistances and a determination of a prognosis for aging of the device as a function of a characteristic of the time series.It is the object of the invention to propose a method for operating a resistance spot welding device, which has advantages over known methods, in particular enables more efficient use of the spot welding electrode.This is achieved according to the invention with a method for operating a resistance spot welding device having the features of claim 1. It is provided that one of the following values is used as the state value or the state value is determined from one of the following values: time of a first maximum of the dynamic resistance, resistance value of the dynamic resistance in the first maximum, time of a first minimum of the dynamic resistance, resistance value of the dynamic resistance in the first minimum, time of a second maximum of the dynamic resistance, resistance value of the dynamic resistance in the second maximum, time of a second minimum of the dynamic resistance, resistance value of the dynamic resistance in the second minimum, variance of the time of the second maximum, weighted difference between a profile and a reference profile of the dynamic resistance and time shift which maps the profile and the reference profile of the dynamic resistance to one another in a time-normalized manner, wherein the reference profile is determined during the conditioning.In principle, it is provided that during the welding process at least one parameter of the electric current flowing through the spot welding electrode, which parameter changes via the welding process, wherein a state value describing a state of the spot welding electrode is determined on the basis of the at least one parameter and a conditioning of the spot welding electrode is carried out as a function of the state value.Advantageous embodiments with expedient developments of the invention are specified in the dependent claims. It is pointed out that the exemplary embodiments explained in the description are not restrictive; rather, any variations of the features disclosed in the description, the claims and the figures can be realized.The method explained serves for operating the resistance spot welding device, which is provided and designed for carrying out resistance spot welding during the welding process. The resistance spot welding device has the spot welding electrode for performing the welding operation. During the welding process, the spot welding electrode is pressed onto the workpiece at the welding location in order to produce an electrically conductive connection between the spot welding electrode and the workpiece. Resistance spot welding is thus a resistance pressure welding method.The electric current used for welding may be either a direct current or an alternating current. During the welding process, at least one parameter of the electric current is set by means of a welding controller. For example, the current intensity of the electric current is kept constant or regulated to a constant setpoint value over the welding process.The workpiece at which the welding point is to be produced can basically be configured in any desired manner. For example, the workpiece is composed of a plurality of elements, for example of a plurality of sheets or steel sheets. The metal sheets are particularly preferably present as galvanized steel sheets which are joined by welding during the welding process.To perform the welding operation, it is necessary to establish a closed electrical circuit across the workpiece. For this purpose, two electrical poles are connected to the workpiece. A first of the electrical poles is electrically connected to the spot welding electrode so that it is electrically connected to the workpiece via the spot welding electrode during the welding process, namely by the application of the spot welding electrode to the workpiece or the pressing of the spot welding electrode against the workpiece. A second of the electrical poles is electrically connected to the workpiece in any desired manner.For example, the spot welding electrode is present as a blind electrode and the second pole is electrically connected directly to the workpiece away from the spot welding electrode. Preferably, however, in addition to the spot welding electrode, a further spot welding electrode is present, which is electrically connected to the second pole, so that the second pole is electrically connected to the workpiece via the further spot welding electrode, namely by applying or pressing the further spot welding electrode to the workpiece. The spot welding electrode is also referred to as a first spot welding electrode and the further spot welding electrode as a second spot welding electrode in the context of this description. Particularly preferably, the two spot welding electrodes are of identical construction, but differently configured spot welding electrodes can also be used.The spot welding electrodes are preferably pressed against the workpiece for carrying out the welding process, so that the workpiece is held between them in a clamping manner. The spot welding electrodes are particularly preferably arranged on opposite sides of the workpiece and are further preferably situated directly opposite one another. The explained method of operating the resistance spot welding device will be explained with respect to the first spot welding electrode. However, the above-described method can naturally also be used for the second spot welding electrode or each of the two spot welding electrodes.During the welding operation, electric current flows through the spot welding electrode. This causes the workpiece to be heated and thus the welding process to be carried out. A total resistance occurring during the welding process is composed of at least an electrode resistance of the spot welding electrode, a junction resistance between the spot welding electrode and the workpiece, and a workpiece resistance of the workpiece. The workpiece resistor can likewise be composed of a plurality of partial resistors, in particular of a material resistor of a first element, a contact resistor between the first element and a second element, and a material resistor of the second element.The total resistance also comprises a further contact resistance, namely between the workpiece and the second pole. In the case of the second spot welding electrode, the contact resistance is thus present between the workpiece and the second spot welding electrode. In addition, an electrode resistance of the second spot welding electrode is added. Overall, therefore, a plurality of electrode resistances, a plurality of transition resistances and the workpiece resistance or a plurality of partial resistances of the workpiece form the overall resistance. The total resistance can also be referred to as dynamic resistance.The dynamic resistance changes over time during the welding process, so that the welding process can be divided into different welding phases. In a first of the welding phases, the dynamic resistance increases sharply on account of contaminants and unevennesses on a surface of the workpiece, in order subsequently to fall down again in a second welding phase on account of degradation of the contaminants and the unevennesses. At the end of the first welding phase or at the beginning of the second welding phase, the dynamic resistance reaches a first maximum, usually a global maximum, over the entire welding process. In the second welding phase, the dynamic resistance reaches a first minimum, usually a local minimum.Starting from the first minimum, the dynamic resistance rises again, namely during a third welding phase and during a fourth welding phase. The increase in the third welding phase is caused at least in part by an increase in the temperature of the workpiece. In the fourth welding phase, the material of the workpiece begins to melt and the dynamic resistance increases further, namely up to a second maximum, usually a local maximum. Starting from the second maximum, the dynamic resistance falls, likewise during the fourth welding phase. The fourth welding phase is followed by a fifth welding phase, in which, for example, weld spots form and the spot welding electrode penetrates into the workpiece due to the melting of the workpiece.At one end of the fifth welding phase, the welding process is completed. At the end of the welding operation, the dynamic resistance reaches a second minimum, usually a global minimum. A resistance value of the dynamic resistor is greater in the first maximum than in the first minimum, than in the second maximum, and than in the second minimum. In the first minimum, the resistance value of the dynamic resistor is smaller than in the first maximum and than in the second maximum, but larger than in the second minimum. In the second maximum, the resistance value of the dynamic resistor is smaller than in the first maximum, but larger than in the first minimum and than in the second minimum. In the second minimum, the resistance value of the dynamic resistor is smaller than in the first maximum, than in the first minimum and than in the second maximum.Alloying elements of the workpiece to be joined or welded are deposited on the spot welding electrode. As a result, a bearing surface of the spot welding electrode on the workpiece increases and the need for conditioning the spot welding electrode, which preferably takes place in a material-removing manner. During the material-removing processing, a tip of the spot welding electrode facing the workpiece during the welding process is processed in a material-removing, in particular chip-removing, manner, for example by milling and / or grinding. This usually takes place after a certain number of welding operations, for example in the case of welding galvanized steel sheet after approximately 120 welding operations.However, the applicant has found that the state of the spot welding electrode allows a higher number of welding operations before the conditioning must be performed, at least in some cases. Surprisingly, it has been found here that the state of the spot welding electrode can be determined on the basis of the electric current flowing through the spot welding electrode during the welding process or its parameter. For this reason, it is provided to record the at least one parameter of the electric current during the welding process. The state value is determined from this parameter. Depending on the state value, it is then determined whether or not the conditioning of the spot welding electrode needs to be performed.The parameter of the electric current used is a parameter which changes at least temporarily during the welding process. A current intensity and / or a voltage of the electric current appear particularly suitable. Alternatively, the dynamic resistance can be detected, in particular indirectly, that is to say by detecting the current intensity and the voltage and calculating the dynamic resistance from these. The acquisition of the at least one parameter can be carried out during the welding process once, repeatedly or continuously. It is particularly preferred to record the parameter repeatedly at equal time intervals during the welding process. The determination of the state value from the at least one parameter can be carried out during the welding process, at the end of the welding process or after the welding process, but preferably before a further welding process following the welding process.If the state value lies within a (first) setpoint value range, it is assumed that the spot welding electrode is in order and neither the conditioning nor the replacement is necessary. Accordingly, after the welding operation, the further welding operation can be performed immediately without preparing or replacing the spot welding electrode. However, if the state value is outside the target value range, it is assumed that the spot welding electrode has a state in which it cannot be used for a further welding operation. Accordingly, after the welding process, in particular before the further welding process following the welding process, the conditioning of the spot welding electrode is carried out.The preparation of the spot welding electrode comprises the mechanical machining of the spot welding electrode, in particular the material-removing machining of the spot welding electrode. As an alternative to the conditioning, the spot welding electrode can be replaced or at least partially replaced, in particular if the spot welding electrode has already been conditioned a number of times. Particularly preferably, the spot welding electrode is mechanically machined if the state value lies outside the first setpoint value range and within a second setpoint value range. If, on the other hand, the state value lies outside both the first setpoint value range and the second setpoint value range, then the spot welding electrode is exchanged, i.e. replaced by another spot welding electrode.With the described procedure, the resistance spot welding device is operated in the proper state, namely depending on the state of the spot welding electrode. This makes it possible to carry out a higher number of welding processes for at least some of a plurality of spot welding electrodes than was the case hitherto, since the spot welding electrode has hitherto been supplied to the conditioning after a fixed number of welding processes and has been exchanged after a fixed number of conditioning processes. Furthermore, a higher production throughput of the resistance spot welding device results, since more welding processes can be carried out per unit time. In addition, the costs for replacing the spot welding electrode are reduced. Particularly preferably, a course of the at least one parameter is detected during the welding process, i.e. a value of the respective parameter is determined, in particular measured, at different times. The state value is then determined from the plurality of values for the parameter or parameters, for example indirectly via the dynamic resistance.A further development of the invention provides that a current intensity and / or a voltage of the electric current is used as the at least one parameter, an intermediate variable, in particular a dynamic resistance of the resistance spot welding device, is determined from the at least one parameter, and the state value is determined from the intermediate variable. The state value therefore does not result directly from the at least one parameter, but is determined only indirectly from the latter, namely via the intermediate variable. The intermediate variable used is, for example, the dynamic resistance which has already been discussed. Preferably, both the current intensity and the voltage of the electric current are detected and the dynamic resistance is calculated from these in the usual manner. The use of the dynamic resistance enables a particularly accurate determination of the state of the spot welding electrode.A further development of the invention provides that the at least one parameter, the intermediate variable and / or the state value is calculated with a reference variable and the conditioning of the spot welding electrode is carried out as a function of a result of the calculation. The intermediate variable is understood to mean a variable which is determined from the at least one parameter and from which the state value is determined. The state value thus results from the at least one parameter about the bypass of the intermediate variable. The parameter, the intermediate variable or the state value is calculated with the reference variable. The calculation can be carried out, for example, by ascertaining a difference or by standardization or division.Preferably, the reference variable or a profile of the reference variable over time is thus subtracted from the at least one parameter or its profile over time or the parameter or its profile is normalized by means of the reference variable or its profile. The procedure can be analogous for the intermediate variable or the state value. From the result of the calculation, i.e. for example from the result of the standardization or subtraction, it is subsequently determined whether the conditioning is to be carried out. In particular, the processing is omitted if the result of the calculation lies within the first setpoint value range; conversely, it is carried out if the result lies outside the first setpoint value range and-preferably-within the second setpoint value range. This achieves the required conditioning of the spot welding electrode.The invention provides that one of the following values is used as the state value or the state value is determined from one of the following values: time of a first maximum of the dynamic resistance, resistance value of the dynamic resistance in the first maximum, time of a first minimum of the dynamic resistance, resistance value of the dynamic resistance in the first minimum, time of a second maximum of the dynamic resistance, resistance value of the dynamic resistance in the second maximum, time of a second minimum of the dynamic resistance, resistance value of the dynamic resistance in the second minimum, variance of the time of the second maximum, weighted difference between a profile and a reference profile of the dynamic resistance and time shift which maps the profile and the reference profile of the dynamic resistance to one another in a time-normalized manner.The characteristic maxima and minima of the dynamic resistance have already been discussed. The time of the respective maximum or the respective minimum is to be understood as the time in which the respective maximum or minimum occurs starting from a start of the welding process. The respective resistance value denotes the value of the dynamic resistance in the respective maximum and minimum. It can be provided that one of the aforementioned points in time or one of the aforementioned resistance values is used directly as a state value. This is the case in particular for the time and the resistance value of the dynamic resistor in the second maximum. Surprisingly, it has been found that both the time of the second maximum of the dynamic resistance and the resistance value present therein respectively describe the state of the spot welding electrode with high accuracy and are consequently suitable as the state value.However, it can also be provided that the state value is determined from one of the values mentioned, for example as a difference between two of the points in time or as a difference between two of the resistance values. For example, the difference between the time of the second maximum on the one hand and the time of the first maximum or of the first minimum on the other hand is used as a state value or alternatively a difference between the resistance value in the second maximum on the one hand and the resistance value in the first maximum or in the first minimum on the other hand.Alternatively, the variance of the time of the second maximum is used as the state value. Variance describes the spread of a number of values around their mean. The values are in particular values of the time for a plurality of welding processes, i.e. of the welding process and of at least one further welding process. If the variance reaches or exceeds a limit value, then the conditioning of the spot welding electrode is carried out. Alternatively, the weighted difference between the curve and the reference curve of the dynamic resistance is used as the state value. The course of the dynamic resistance is to be understood as its course during the welding process, in particular over the entire welding process. The reference course is a stored course, preferably the reference course is permanently stored or it is determined as a reference course during a first welding process after the spot welding electrode has been prepared or replaced. The weighted difference is calculated according to the relationship, wherein a plurality of values of the history and a plurality of values of the reference history correspond. The variable n here denotes the number of values. Values with the same index i are preferably present for the profile and the reference profile at the same point in time.The weighting factor is calculated from the relationship. It can be seen that the weighting factor increases with increasing duration of the welding process and consequently with increasing index i. Values of the dynamic resistance present at the beginning of the welding process are thus at least partially masked out.A further alternative for the state value is the time shift, by means of which the profile and the reference profile of the dynamic resistance are mapped to one another in a time-normalized manner. The time shift is determined by means of dynamic time warping (dynamic time warping), an algorithm which maps value sequences of different lengths to one another.Each of the above values enables to determine with high accuracy whether the conditioning of the spot welding electrode needs to be performed. In particular, the processing is omitted if the respective stated value lies within the first setpoint value range. Conversely, it is carried out if the value lies outside the first setpoint value range, preferably only if it additionally lies within the second setpoint value range. If the value is outside the second setpoint value range and also the first setpoint value range, the spot welding electrode is particularly preferably exchanged. This realizes the operation of the resistance spot welding device according to requirements.A further development of the invention provides that, in addition to the welding process, at least one further welding process is carried out at a further welding location and, during the further welding process, the at least one parameter of the electric current flowing through the spot welding electrode, which parameter changes via the welding process, wherein the state value describing the state of the spot welding electrode is determined on the basis of the at least one parameter during the welding process and, in addition, on the basis of the at least one parameter during the at least one further welding process.The at least one further welding process preferably follows directly the welding process, in particular without conditioning and without replacing the spot welding electrode. Preferably, a plurality of further welding processes are carried out, for example at least four, at least six or at least eight. The explanation applies analogously to each of these further welding processes. The procedure for the at least one further welding operation is as for the welding operation, i.e. the at least one parameter of the electric current is detected.The state value is determined not only from the parameter for the welding process, but additionally from the parameter or the parameters for the further welding process or the further welding processes. For example, a moving average value is determined for this purpose from the parameters; in particular, the parameters of a plurality of successive welding processes or the corresponding curves are therefore calculated with one another by averaging and the state value is determined from this average value. This reliably masks random errors.A further development of the invention provides that the spot welding electrode has an electrode cap which is attached to an electrode body in such a way that it can be changed, and the state of the spot welding electrode describes the electrode cap, wherein the electrode cap is processed as a function of the state value. The spot welding electrode is thus a multipart electrode that comprises at least the electrode body and the electrode cap. During the preparation of the spot welding electrode, the electrode cap, preferably only the electrode cap, is mechanically processed. The replacement of the spot welding electrode is performed by replacing the electrode cap, and the electrode body is thus maintained. This ensures a particularly efficient execution of the welding process.The invention provides that the reference profile is determined during the processing. For example, it is provided that a reference welding process is carried out at the end of the processing, i.e. after the mechanical machining of the spot welding electrode, preferably without a workpiece.During the reference welding process, the spot welding electrode is directly connected to the second electrical pole, for example, by directly applying the first spot welding electrode to the second spot welding electrode.During the reference welding process, the at least one parameter of the electric current flowing through the spot welding electrode is detected and the state of the spot welding electrode is deduced therefrom. If the resulting state is outside a predetermined state, it may be provided to repeat the mechanical machining of the spot welding electrode until this is the case. Alternatively, the spot welding electrode or the electrode tip is replaced. This ensures that the conditioning of the spot welding electrode has been successfully carried out.A further development of the invention provides that the spot welding electrode or the electrode cap is exchanged depending on the state value. Preferably, as already described, the spot welding electrode or its electrode cap is processed if the state value lies within a specific setpoint value range, namely in the second setpoint value range. However, if the state value is outside the second setpoint value range, then it is preferably provided that no further conditioning is carried out, but instead the spot welding electrode or the electrode cap is exchanged directly. This avoids unnecessary effort during the processing.The invention further relates to a resistance spot welding device, in particular for carrying out the method according to the statements within the scope of this description, wherein the resistance spot welding device is provided and configured to carry out a welding process at a welding point of a workpiece by applying electric current to a spot welding electrode of the resistance spot welding device. The resistance spot welding device is further provided and configured to detect at least one parameter of the electric current flowing through the spot welding electrode that changes during the welding process, wherein a state value describing a state of the spot welding electrode is determined on the basis of the at least one parameter and a conditioning of the spot welding electrode is carried out as a function of the state value. It is provided that one of the following values is used as the state value or the state value is determined from one of the following values: time of a first maximum of the dynamic resistance, resistance value of the dynamic resistance in the first maximum, time of a first minimum of the dynamic resistance, resistance value of the dynamic resistance in the first minimum, time of a second maximum of the dynamic resistance, resistance value of the dynamic resistance in the second maximum, time of a second minimum of the dynamic resistance, resistance value of the dynamic resistance in the second minimum, variance of the time of the second maximum, weighted difference between a profile and a reference profile of the dynamic resistance and time shift which maps the profile and the reference profile of the dynamic resistance to one another in a time-normalized manner, wherein the reference profile is determined during the conditioning.The advantages of such a configuration of the resistance spot welding device and such a procedure have already been pointed out. Both the resistance spot welding device and the method for operating it can be further developed according to the explanations within the scope of this description, so that reference is made to these in this respect.The invention also relates to a computer program product comprising instructions for causing the resistance spot welding device according to the statements of this description to execute the explained method. With regard to the advantages and possible advantageous developments, reference is made in its entirety to the description.The features and combinations of features described in the description, in particular the features and combinations of features described in the following description of the figures and / or shown in the figures, can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention. Embodiments are therefore also to be considered as encompassed by the invention which are not explicitly shown or explained in the description and / or the figures, but which emerge from the explained embodiments or can be derived from them.The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing, without any restriction of the invention being effected. The following shows: FIG. 1 shows a schematic illustration of a resistance spot welding device and of a workpiece arranged therein, and FIG. 2 is a graph in which a dynamic resistance occurring in the resistance spot welding device during a welding process is plotted over time.FIG. 1 shows a highly schematic representation of a resistance spot welding device 1 and a workpiece 2 located in the resistance spot welding device 1. The resistance spot welding device 1 serves to carry out a welding process at one or more welding points 5 of the workpiece 2. The workpiece 2 has two elements 6 and 7 to be connected to one another by welding, which are designed, for example, as steel sheets, preferably as galvanized steel sheets.The spot welding electrodes 3 and 4 each have a welding tip 8 and 9, respectively, which is formed on an electrode cap. The electrode caps of the spot welding electrodes 3 and 4 are interchangeably disposed on each electrode body of the spot welding electrodes 3 and 4. From time to time, it is necessary to prepare the spot welding electrodes 3 and 4 or their electrode caps. This is preferably done by mechanical processing and in particular depending on a state value describing the state of the respective spot welding electrode 3 or 4.FIG. 2 shows a diagram in which a curve 10 shows a dynamic resistance R occurring in the resistance spot welding device 1 across the spot welding electrodes 3 and 4 and the workpiece 2 over time t. The curve 10 can be divided into a plurality of welding phases, namely a first welding phase (t 0 ≤ t < t 1), a second welding phase (t 1 ≤ t < t 2), a third welding phase (t 2 ≤ t < t 3), a fourth welding phase (t 3 ≤ t < t 4) and a fifth welding phase (t 4 ≤ t < t 5). It can be seen that the dynamic resistance has a first maximum 11 at the time t 1 a first minimum 12 in the second welding phase, a second maximum 13 in the fourth welding phase and a second minimum 14 at the end of the welding process, i.e. at the end of the fifth welding phase.The dynamic resistance is detected during the welding process, particularly by measuring a current and a voltage of the electric current flowing through the spot welding electrodes 3 and 4. From the dynamic resistance, the state value for the spot welding electrode 3 or the spot welding electrodes 3 and 4 is then calculated. In this case, the dynamic resistances of a plurality of welding processes are particularly preferably used, in particular by averaging.Particularly preferably, a time of the second maximum 13 is used as the state value. Alternatively, a variance of the time of the second maximum 13, a weighted difference between the curve 10 and a reference curve of the dynamic resistance or a time shift which maps the curve 10 and the reference curve of the dynamic resistance to one another in a time-normalized manner is used as the state variable. Within the scope of investigations carried out by the applicant, it has surprisingly been found that the state of the spot welding electrodes 3 and 4 can be determined with good accuracy in this way.LIST OF REFERENCE CHARACTERS:1 Resistance spot welding device 2 Workpiece 3 1 Spot welding electrode 4 2 Spot welding electrode 5 Welding point 6 Element 7 Element 8 Welding tip 9 Welding tip 10 Course 11 1 Maximum 12 1 Minimum 13 2 Maximum 14 2 Minimum

Claims

Method for operating a resistance spot welding device (1), wherein a welding process is carried out at a welding location (5) of a workpiece (2) by applying electric current to a spot welding electrode (3) of the resistance spot welding device (1), wherein at least one parameter of the electric current flowing through the spot welding electrode (3) which changes via the welding process is detected during the welding process, wherein a state value describing a state of the spot welding electrode (3) is determined on the basis of the at least one parameter and a conditioning of the spot welding electrode (3) is carried out as a function of the state value, characterized in that one of the following values is used as the state value or the state value is determined from one of the following values: time of a first maximum (11) of a dynamic resistance, resistance value of the dynamic resistance in the first maximum (11), Time of a first minimum (12) of the dynamic resistance, resistance value of the dynamic resistance in the first minimum (12), time of a second maximum (13) of the dynamic resistance, resistance value of the dynamic resistance in the second maximum (13), time of a second minimum (14) of the dynamic resistance, resistance value of the dynamic resistance in the second minimum (14), variance of the time of the second maximum (13), weighted difference between a profile (10) and a reference profile of the dynamic resistance and time shift which maps the profile (10) and the reference profile of the dynamic resistance to one another in a time-normalized manner, wherein the reference profile is determined during the conditioning.Method according to Claim 1, characterized in that the at least one parameter used is a current intensity and / or a voltage of the electric current, an intermediate variable is determined from the at least one parameter and the state value is determined from the intermediate variable.Method according to one of the preceding claims, characterized in that the at least one parameter, the intermediate variable and / or the state value is calculated with a reference variable and the conditioning of the spot welding electrode is carried out as a function of a result of the calculation.Method according to one of the preceding claims, characterized in that, in addition to the welding operation, at least one further welding operation is carried out at a further welding point (5) and, during the further welding operation, the at least one parameter of the electric current flowing through the spot welding electrode (3) which parameter changes via the welding operation is detected, wherein the state value describing the state of the spot welding electrode (3) is determined on the basis of the at least one parameter during the welding operation and additionally on the basis of the at least one parameter during the at least one further welding operation.Method according to one of the preceding claims, characterized in that the spot welding electrode (3) has an electrode cap which is attached to an electrode body in such a way that it can be changed, and the state of the spot welding electrode (3) describes the electrode cap, the electrode cap being processed as a function of the state value.Method according to one of the preceding claims, characterized in that the spot welding electrode (3) or the electrode cap is exchanged as a function of the state value.Resistance spot welding device (1), in particular for carrying out the method according to one or more of the preceding claims, wherein the resistance spot welding device (1) is provided and configured to carry out a welding process at a welding point (5) of a workpiece (2) by applying electric current to a spot welding electrode (3) of the resistance spot welding device (1), wherein the resistance spot welding device (1) is further provided and configured to detect at least one parameter of the electric current flowing through the spot welding electrode (3) which parameter changes via the welding process, wherein a state value describing a state of the spot welding electrode (3) is determined on the basis of the at least one parameter and a conditioning of the spot welding electrode (3) is carried out as a function of the state value, characterized in that, using as the state value one of the following values or determining the state value from one of the following values: time of a first maximum (11) of a dynamic resistance, resistance value of the dynamic resistance in the first maximum (11), time of a first minimum (12) of the dynamic resistance, resistance value of the dynamic resistance in the first minimum (12), time of a second maximum (13) of the dynamic resistance, resistance value of the dynamic resistance in the second maximum (13), time of a second minimum (14) of the dynamic resistance, resistance value of the dynamic resistance in the second minimum (14), variance of the time of the second maximum (13), weighted difference between a curve (10) and a reference curve of the dynamic resistance, and time shift mapping the curve (10) and the reference curve of the dynamic resistance to one another in a time-normalized manner, wherein the reference profile is determined during the processing.A computer program product comprising instructions for causing the resistance spot welding device (1) according to claim 7 to carry out the method steps according to one or more of claims 1 to 6.

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