Method for monitoring a resistance spot welding device

Abstract

The invention relates to a method for monitoring a resistance spot welding device comprising a transformer and a welding gun which are electrically connected to each other via welding strips. According to the invention, an impending failure of at least one of the welding strips is predicted by monitoring a phase angle (PA) during short-circuit welding (KS) and continuously comparing two mean values ​​(MW1, MW2) of the phase angle (PA) from a predetermined total number (A) of past short-circuit welding (KS), wherein the first mean value (MW1) is formed from a predetermined first number (X) of phase angle values ​​of the most recently performed short-circuit welding (KS) and the second mean value (MW2) is formed from a predetermined second number (Y) of phase angle values ​​from the short-circuit welding (KS) performed first within the predetermined total number (A), wherein a difference value (DW) is formed by subtracting the second mean value (MW2) from the first mean value (MW1), wherein the determined difference value (DW) is compared with a predetermined reference value (VW).and wherein an impending failure of at least one of the welding bands is predicted if the determined difference value (DW) exceeds the specified reference value (VW).

Description

[0001] The invention relates to a method for monitoring a resistance spot welding device according to the features of the preamble of claim 1.

[0002] A method for operating a welding device is known from the prior art, as described in DE 10 2006 038 786 A1. The welding device has a welding electrode that is operated with a variable electrical reference variable. This electrical reference variable is controlled by a control device taking into account a reference data set that is characteristic of a welding process to be performed. The reference data set is determined based on a raw data set that is characteristic of the welding process to be performed.

[0003] The invention is based on the object of providing a method for monitoring a resistance spot welding device which is improved compared to the prior art.

[0004] The object is achieved according to the invention by a method for monitoring a resistance spot welding device having the features of claim 1.

[0005] Advantageous embodiments of the invention are the subject of the subclaims.

[0006] In a method according to the invention for monitoring a resistance spot welding device, in particular a short-circuit welding device, which has a transformer and a welding gun, which are electrically conductively connected to one another via welding bands, an impending failure of at least one of the welding bands is predicted according to the invention by monitoring a phase angle during short-circuit welding operations carried out by means of the resistance spot welding device, ie in particular resistance spot welding operations carried out as short-circuit welding operations, and continuously comparing two mean values ​​of the phase angle from a predetermined total number of previous short-circuit welding operations,wherein the first mean value is formed from a predetermined first number of phase control values ​​of the last short-circuit welds performed, and the second mean value is formed from a predetermined second number of phase control values ​​from the first short-circuit welds performed within the predetermined total number, wherein a difference value is formed by subtracting the second mean value from the first mean value, wherein the determined difference value is compared with a predetermined comparison value, and wherein the impending failure of at least one of the welding bands is predicted if the determined difference value exceeds the predetermined comparison value.

[0007] Advantageously, an alarm is triggered when the impending failure of at least one of the welding bands is predicted in the manner described, i.e., when the determined difference value exceeds the specified comparison value. This allows, for example, a check or other measure to be carried out in the production facility, in particular in the resistance spot welding device. In particular, the welding bands can be replaced upon such an alarm, or a replacement can be planned and then carried out at a convenient time.

[0008] In particular, it is intended that only successfully completed short-circuit welds are taken into account. This means that faulty short-circuit welding processes are advantageously not considered, as their values ​​would falsify the calculation and thus also trigger false alarms. These faulty short-circuit welding processes are therefore not taken into account in the specified total number of past short-circuit welds and thus also in the first number of phase control values ​​and the resulting calculation of the first mean value, as well as in the second number of phase control values ​​and the resulting calculation of the second mean value.

[0009] The phase angle values, the mean values, the difference value and the specified comparison value are each given in particular as a number of scale divisions.

[0010] In one embodiment, the specified comparison value for steel component welding carried out using the resistance spot welding device is 2.7.

[0011] In one embodiment, the specified comparison value for aluminum component welding carried out using the resistance spot welding device is 1.5.

[0012] In one embodiment, for steel component welding carried out using the resistance spot welding device, the specified total number of previous short-circuit welds is 100.

[0013] In one embodiment, for aluminum component welding carried out using the resistance spot welding device, the specified total number of previous short-circuit welds is 500.

[0014] In one embodiment, the predetermined first number of phase control values ​​is 20.

[0015] In one embodiment, the predetermined second number of phase control values ​​is 20.

[0016] The respective welding band is also referred to as a secondary cable or secondary band. It is designed, in particular, as a flexible copper cable.

[0017] The described solution enables failure prediction of welding bands for resistance spot welding.

[0018] In the solution described, it is particularly provided that the phase angle is monitored during short-circuit welding after a milling process.

[0019] The process can also be adapted for devices that are constructed differently, for example by monitoring a current during short-circuit welding.

[0020] Resistance spot welding requires a connection between the transformer and the welding tongs. Since welding tongs are movable, the connection must be designed to be flexible. This is achieved via welding bands, which are typically designed as flexible copper cables. These welding bands are subject to high levels of wear. Beyond a certain level of wear, they fail, resulting in a previously unforeseeable system downtime, particularly a previously unforeseeable downtime of the resistance spot welding device. Correcting such a fault takes an average of approximately 25 minutes and thus represents a significant disruption to the overall bodyshell production process.

[0021] This problem is solved by the method described, because it predicts the impending failure of at least one of the welding belts, so that a timely replacement can be planned and carried out at a time favorable for production, thus avoiding an unforeseen downtime.

[0022] The described solution solves this problem by using certain welding control parameters, particularly the phase control, to detect the failure of a welding band early. This allows the welding band to be replaced during non-production periods, such as breaks or weekends, before it fails, i.e., without causing a disruption that impacts production. The core of the described solution is the methodology for predicting an imminent welding band failure, which predicts the impending failure with a very high degree of reliability.

[0023] A welding control system for a resistance spot welding device, particularly designed as a short-circuit welding device, is a controlled system that periodically performs a short-circuit weld with a constant current in a service program, particularly for short-circuit welding. The constant current is controlled via the phase control parameter.

[0024] The solution described here is based on the findings of an analysis of the defective welding strips, which established a causal relationship between an increase in the phase angle during short-circuit welding and the welding strip defect. This relationship is based on the physical principle of Ohm's law. As soon as the electrical resistance of the welding strip is affected by wear on the welding strip, in particular by the breaking off of individual copper strands, charred copper strands, and / or other impairments to the welding strip, the system must adjust the specified current via the phase angle when performing the short-circuit welding. Since surrounding current-carrying components are subject to negligible wear, a direct relationship can be established between the phase angle and current strip wear.

[0025] The method comprises, in particular, a calculation method which takes this relationship into account. In the method described, this calculation method is particularly provided for comparing two average values, i.e. mean values, of the phase angle from a plurality of short-circuit welds. The first mean value is calculated from the predetermined first number of phase angle values ​​(X) of the most recently performed short-circuit welds. The second mean value is calculated from the predetermined second number of phase angle values ​​(X) from short-circuit welds performed in the past. The two predetermined numbers are part of the predetermined total number of short-circuit welds performed. The number of phase angle values ​​X or Y can each be, independently of one another, in the range from 15 to 30 and preferably between 18 and 22, in particular 20.

[0026] The process continuously and retrospectively considers the specified total number of short-circuit welds. In the steel process, i.e., when welding steel components using a resistance spot welder, at least the last 100 successfully completed short-circuit welds are preferably used as the basis for calculation, i.e., as the specified total number. Faulty processes are not considered, as these values ​​would falsify the calculation and thus trigger false alarms. In the aluminum process, i.e., when welding aluminum components using a resistance spot welder, at least the last 500 successfully completed short-circuit welds are preferably considered, i.e., used as the specified total number.

[0027] Due to the significantly higher energy requirements of the aluminum welding process, four welding bands are installed on an aluminum welding gun. Only two welding bands are installed on a steel welding gun. Furthermore, short-circuit welding occurs much more frequently in the aluminum welding process, approximately every three to five components, compared to only every 10 to 15 components in the steel welding process. This means that the welding bands wear less between two short-circuit welds in the aluminum welding process, and thus more data is required for the calculation.

[0028] In the next step, the second mean value is calculated from a predetermined initial range of the predetermined total number of retrospectively considered values, i.e., from the predetermined second number of phase control values ​​from the short-circuit welds first performed within the predetermined total number, preferably from the first 20 short-circuit welds performed. The first mean value is calculated from a predetermined end range of the predetermined total number of retrospectively considered values, i.e., from the predetermined first number of phase control values ​​of the last short-circuit welds performed, preferably from the last 20 short-circuit welds performed. For steel welds, the predetermined total number is advantageously 100 to 150, and for aluminum welds, advantageously 500 to 650.

[0029] The two mean values ​​are then offset against each other as described above, i.e., the second mean value is subtracted from the first. Based on the tests and evaluations conducted, it has been shown that in the steel welding process, a determined difference value of > 2.7 is suitable for predicting the impending failure of at least one of the welding bands and thus advantageously triggering an alarm. In the aluminum welding process, however, a value of > 1.5 has proven advantageous for predicting the impending failure of at least one of the welding bands and thus for triggering an alarm.

[0030] The process therefore involves a continuous retrospective review of the specified total number of the last successfully completed short-circuit welds, in the steel welding process in particular the last 100 successfully completed short-circuit welds and in the aluminum welding process in particular the last 500 successfully completed short-circuit welds.

[0031] The method, in particular the calculation of the mean values, begins only after the specified total number of short-circuit welds, in particular the successfully completed short-circuit welds, has been reached, in particular after the welding bands or at least one of the welding bands has been replaced. The described calculation is then carried out continuously, as described, i.e., in particular continuously, in particular in a rolling manner, in particular again after each further short-circuit weld performed, in particular a successfully completed short-circuit weld.

[0032] In particular, 20 phase control values ​​are used for each averaging operation, i.e. the specified first number of phase control values ​​for calculating the first averaging operation is 20 and the specified second number of phase control values ​​for calculating the second averaging operation is 20. The first averaging operation is thus calculated in particular from the phase control values ​​of the last 20 successfully performed short-circuit welds of the specified total number of short-circuit welds and the second averaging operation is calculated in particular from the phase control values ​​of the first 20 successfully performed short-circuit welds of the specified total number of short-circuit welds.

[0033] The described solution makes it possible to apply "predictive maintenance"—i.e., condition-based maintenance—as a maintenance method for welding strips used for resistance spot welding, particularly in vehicle body construction, thus preventing production disruptions. The calculation method used in the described procedure eliminates the need for individual reference values ​​for new welding strips. Thus, welding strip monitoring can be implemented quickly and easily in an ongoing production facility.

[0034] Embodiments of the invention are explained in more detail below with reference to a drawing.

[0035] It shows: Fig. 1 schematically shows a phase control as a function of the number of short-circuit welds performed.

[0036] Fig.Figure 1 shows schematically recorded phase angle values ​​of a phase angle PA as a function of the number of short-circuit welds KS performed. The phase angle values ​​are given as a number of scale divisions.

[0037] The KS short-circuit welds are carried out using a resistance spot welding device that has a transformer and a welding gun that are electrically connected to each other via welding bands.

[0038] In a method for monitoring this resistance spot welding device, an impending failure of at least one of the welding strips is predicted by monitoring the phase angle PA during the short-circuit welds KS and continuously comparing two mean values ​​MW1, MW2 of the phase angle PA from a predetermined total number A of previous short-circuit welds KS. In particular, only successfully completed short-circuit welds KS are taken into account.

[0039] In the simplified schematic example, the specified total number A of previous, particularly successfully performed, short-circuit welds KS is 39, i.e., the 39th short-circuit weld KS is the short-circuit weld KS that was successfully performed at the time the two mean values ​​MW1 and MW2 were currently being determined. This is therefore the time of the current calculation of the mean values ​​MW1 and MW2 and the additional calculations described below. In the example shown, this is also the time of the change of the welding bands, or at least one of the welding bands.

[0040] In the method described here, the first mean value MW1 is calculated from a predetermined first number X of phase angle values ​​of the most recently performed short-circuit welds KS, in the example shown from the last 10 phase angle values. The first mean value MW1 in the example shown is 35.6.

[0041] In the method described here, the second mean value MW2 is formed from a predetermined second number Y of phase control values ​​from the short-circuit welds KS carried out first within the predetermined total number A, in the example shown from the first 10 phase control values ​​of the predetermined total number A. The second mean value MW2 is 5.7 in the example shown.

[0042] In the method described here, a difference value DW is now formed by subtracting the second mean value MW2 from the first mean value MW1. DW=MW1−MW2

[0043] For the example shown, the difference value DW is: DW=35.6−5.7=29.9

[0044] In the method described here, the determined difference value DW is compared with a specified reference value VW. An impending failure of at least one of the welding bands is predicted if the determined difference value DW exceeds the specified reference value VW, ie if DW>VW

[0045] For example, when welding steel components using the resistance spot welding device, the specified comparison value VW is 2.7.

[0046] For welding of aluminum components using the resistance spot welding device, the specified comparison value VW is, for example, 1.5.

[0047] In this case, both the reference value VW for the steel component welds and the reference value VW for the aluminum component welds would have been significantly exceeded. Thus, both when the short-circuit welds KS were performed as steel component welds and when they were performed as aluminum component welds, the impending failure of at least one of the welding bands is predicted. This advantageously triggers an alarm. This allows the replacement of the welding bands to be planned in a timely manner and then carried out, for example, in such a way that production processes are not delayed. For example, the replacement can then be carried out during an already scheduled production break.

[0048] For the sake of clarity, a simplified example is shown here to illustrate the basic procedure.

[0049] Deviating from the example shown here, the specified total number A of previous short-circuit welds KS is, for example, 100 for steel component welding carried out using the resistance spot welding device and 500 for aluminum component welding carried out using the resistance spot welding device.

[0050] Deviating from the example shown here, the specified first number X of phase control values ​​and the specified second number Y of phase control values ​​are each 20, for example. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2006 038 786 A1

[0002]

Claims

[1] Method for monitoring a resistance spot welding device comprising a transformer and a welding gun which are electrically connected to each other via welding bands, characterized bythat an impending failure of at least one of the welding strips is predicted by monitoring a phase angle (PA) during short-circuit welds (KS) and continuously comparing two mean values ​​(MW1, MW2) of the phase angle (PA) from a predetermined total number (A) of previous short-circuit welds (KS), wherein the first mean value (MW1) is formed from a predetermined first number (X) of phase angle values ​​of the most recently performed short-circuit welds (KS) and the second mean value (MW2) is formed from a predetermined second number (Y) of phase angle values ​​from the short-circuit welds (KS) performed first within the predetermined total number (A), wherein a difference value (DW) is formed by subtracting the second mean value (MW2) from the first mean value (MW1), wherein the determined difference value (DW) is compared with a predetermined comparison value (VW),and wherein an impending failure of at least one of the welding bands is predicted if the determined difference value (DW) exceeds the specified comparison value (VW). [2] Method according to claim 1, characterized by that only successfully completed short-circuit welds (KS) are taken into account. [3] Method according to one of the preceding claims, characterized by that the phase angle values, the mean values ​​(MW1, MW2), the difference value (DW) and the specified comparison value (VW) are each given as a number of scale divisions. [4] Method according to one of the preceding claims, characterized by that for steel component welding carried out using the resistance spot welding device, the specified comparison value (VW) is 2.

7. [5] Method according to one of the preceding claims, characterized bythat for welding of aluminium components carried out using the resistance spot welding device, the specified comparison value (VW) is 1.

5. [6] Method according to claim 4, characterized by that for steel component welding carried out using the resistance spot welding device, the specified total number (A) of previous short-circuit welds (KS) is 100 to 150. [7] Method according to claim 5, characterized by that for welding of aluminium components carried out using the resistance spot welding device, the specified total number (A) of previous short-circuit welds (KS) is 500 to 650. [8] Method according to one of the preceding claims, characterized by that the predetermined first number (X) of phase control values ​​and / or the predetermined second number (Y) of phase control values ​​are each independently in the range from 15 to 30.

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