Method for Checking Quality when Resistance-Welding Workpieces

Abstract

A method for checking quality when resistance-welding workpiece includes pressing welding electrodes with an electrode force against a weld spot of the workpieces using an electrode drive and energizing the welding electrodes with a welding current for a duration of a welding time in order to liquefy a surface of the workpieces. The method further includes determining, at a first time before a beginning of the liquefaction, a first value of a welding electrode parameter identifying a position of one or both electrodes, and determining, at a second time after the beginning of the liquefaction, a second value of the welding electrode parameter identifying a position of one or both electrodes. The method further includes comparing the first value and the second value and, evaluating a quality of the welding process based on the comparison.

Claims

1. A method for checking quality when resistance-welding workpieces, the method comprising: pressing welding electrodes with an electrode force against a weld spot of the workpieces using an electrode drive; energizing the pressed welding electrodes with a welding current for a duration of a welding time in order to liquefy a surface of the workpieces; determining, at a first time before a beginning of the liquefaction, a first value of a welding electrode parameter identifying a position of one or both of the welding electrodes, wherein the welding electrodes are pressed with the electrode force against the weld spot of the workpieces at the first time; determining, at a second time after the beginning of the liquefaction, a second value of the welding electrode parameter identifying a position of one or both of the welding electrodes, wherein the welding electrodes are pressed with the electrode force against the weld spot of the workpieces at the second time; comparing the first value and the second value; and evaluating a quality of a welding process based on the comparison.

2. The method according to claim 1, further comprising: calculating a signal parameter by calculating an energy of a signal in a spectral sub-range.

3. The method according to claim 1, further comprising: determining a sampling frequency by determining a frequency of a sub-range.

4. The method according to claim 1, further comprising: determining a sampling frequency as a function of a comparison of a signal parameter or a value derived therefrom with a threshold value.

5. The method according to claim 4, further comprising: calculating the signal parameter using the threshold value dependent on a quantization parameter of an analog-to-digital converter.

6. The method according to claim 1, further comprising: calculating a signal parameter using at least one of a band-pass filter and a controllable high-pass filter.

7. The method according to claim 1, further comprising: executing repeatedly a calculation of a signal parameter and a determination of a sampling frequency by: repeatedly calculating an additional signal parameter in an additional spectral sub-range of a signal to be converted; and repeatedly determining a sampling frequency by determining a sampling rate of an analog-to-digital converter using the additional signal parameter and operating an analog-to-digital converter using a determined sampling rate.

8. The method according to claim 7, wherein the repeated determination of the sampling frequency further includes: determining the additional signal parameter in the additional spectral sub-range, which has a center frequency, which corresponds to half a center frequency of a spectral sub-range within a tolerance band.

9. The method according to claim 1, wherein a calculation of a signal parameter and a determination of a sampling frequency are executed repeatedly in succession.

10. The method according to claim 1, further comprising: determining a sampling frequency by: determining a sampling rate using at least one of a frequency synthesizer and a frequency divider; and operating an analog-to-digital converter with the determined sampling rate.

11. A device comprising: a control unit configured to: calculate a signal parameter in a spectral sub-range of a signal to be converted, wherein the spectral sub-range includes a frequency range of a potential sampling frequency range of an analog-to-digital converter, which does not include frequencies of at least one other sub-range of the potential sampling frequency range; determine a sampling frequency of the analog-to-digital converter based on the signal parameter; and operate the analog-to-digital converter using the determined sampling frequency.

12. The method according to claim 1, wherein a computer program is configured to at least one of execute and control the method.

13. The method according to claim 12, wherein the computer program is stored in a machine-readable storage medium.

14. The method according to claim 2, further comprising: executing the calculation of the signal parameter using at least one of a rectifier and a low-pass filter.

15. The method according to claim 3, further comprising: determining a cutoff frequency of the sub-range as the sampling frequency.

16. The method according to claim 15, further comprising: determining a maximum frequency of the sub-range as the sampling frequency.

17. The method according to claim 4, further comprising: determining a frequency of a sub-range as the sampling frequency if the signal parameter or a value derived therefrom is greater than the threshold value.

18. The method according to claim 6, further comprising: calculating the signal parameter in a spectral sub-range based on the calculation of the signal parameter using at least one of the band-pass filter and the controllable high-pass filter.

19. The method according to claim 9, further comprising: performing the calculation of the signal parameter and the determination of the sampling frequency in a repeated cycle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 schematically shows one preferred configuration of a welding device according to the disclosure that is configured so as to perform one preferred embodiment of a method according to the disclosure.

[0042] FIG. 2 schematically shows one preferred embodiment of a method according to the disclosure in the form of a block diagram.

[0043] FIG. 3 schematically shows time sequences of welding electrode parameters that are able to be determined in the course of one preferred embodiment of a method according to the disclosure.

[0044] FIG. 4 schematically shows time sequences of welding electrode parameters that are able to be determined in the course of one preferred embodiment of a method according to the disclosure.

DETAILED DESCRIPTION

[0045] FIG. 1 schematically illustrates a welding device for resistance welding, this device being denoted 100.

[0046] The welding device 100 is able to connect workpieces 120 to one another in a bonded manner through resistance welding. The workpieces 120 are in particular welded to one another in the course of body in white production, wherein a body of a motor vehicle is in particular manufactured. Two metal sheets 121 and 122 made from aluminum are welded to one another as workpieces here, by way of example.

[0047] The welding device 100 has a welding gun 110 having two welding electrodes 111 and 112. An electrode drive 130 is provided in order to move the welding electrodes 111, 112. In FIG. 1, the welding gun 110 is illustrated for example in the form of a servo-electric welding gun with an electrode drive 130 designed as a servo-motor. It is likewise conceivable for the electrode drive 130 to be able to be designed for example in the form of an electric motor, hydraulic motor or pneumatic motor.

[0048] In the course of the resistance welding, the welding electrodes 111 and 112 are pressed with an electrode force on a weld spot 125 against the metal sheets 121 and 122 by way of the electrode drive 130 during what is known as a force generation phase. The welding electrodes 111 and 112 are then energized with a welding current during the actual welding process for the duration of a welding time, as a result of which resistance heating of the metal sheets 121 and 122 takes place at the weld spot 125 and the surface of the workpieces 121, 122 is liquefied.

[0049] The welding device 100 furthermore has a control unit (welding controller) 140 that may be designed for example as a PLC (programmable logic controller). The control unit 140 is configured so as to regulate the electrode drive 130, indicated by reference sign 151, and also the welding process, indicated by reference sign 152. For this purpose, corresponding drive regulation 141 and corresponding welding process regulation 142 are performed in the control unit 130, each of which may be implemented as corresponding control programs or else as a joint control program.

[0050] The control unit 140 is furthermore configured so as to evaluate a quality of the welding process or of the generated weld spot 125. For this purpose the control unit 140 is configured, in particular in terms of programming, so as to perform a preferred embodiment of a method according to the disclosure, this being illustrated schematically in FIG. 2 as a block diagram and being explained below with reference to FIGS. 1 and 2.

[0051] Step 201 denotes the force generation phase, in the course of which the welding electrodes 111 and 112 are pressed against the metal sheets 121 and 122 on the weld spot 125 by way of the electrode drive 130 until the predefined electrode force is reached.

[0052] Step 202 denotes what is known as a pre-holding phase, during which the welding electrodes 111, 112 are pressed with the electrode force against the weld spot 125 of the workpieces 121, 122 before the beginning of the welding process, but are not yet energized with the welding current. During this pre-holding phase, in the present example, a first value of a position actual value of the electrode drive 130 that characterizes the current position of the welding electrodes 111, 112 is determined as welding electrode parameter at a first time before a beginning of the liquefaction of the surfaces of the workpieces 121, 122.

[0053] In step 203, the welding process then takes place, in the course of which the welding electrodes 111, 112 are energized with the welding current for the duration of the welding time.

[0054] It is also conceivable to determine the first value of the position actual value after the beginning of the welding process during the first phases, as long as no liquefaction of the workpiece surfaces has yet taken place. By way of example, the first value may then be determined at a first time that may lie within up to 10% of the welding duration, for example during what is known as a preconditioning phase.

[0055] After the end of the welding time and after the energization of the welding electrodes 111, 112 has been ended, in step 204, what is known as the post-holding phase takes place, during which the welding electrodes 111, 112 are still pressed with the electrode force against the weld spot 125 of the workpieces 121, 122, but are no longer energized with the welding current. During this post-holding phase, in the present example, a second value of the welding electrode parameter, that is to say of the position actual value of the electrode drive 130, is determined at a second time after the beginning of the liquefaction of the surfaces of the workpieces 121, 122. The second time in particular lies here already after the end of the solidification.

[0056] As an alternative, the second value of the position actual value may also be determined during the last phases before the end of the welding process, when a small welding current is still flowing. By way of example, the second value may be determined at a second time that may lie after 90% of the complete welding duration, for instance during what is known as a post-heating phase.

[0057] The first value and the second value of the position actual value are then compared with one another and a quality of the welding process or of the generated weld spot 125 is evaluated depending on a comparison result. For this purpose, a difference between the first value and the second value is calculated in step 205. This difference characterizes in particular how the position actual value has changed during the welding process. If weld spatters occur during the welding process, liquid metal on the weld spot 125 is removed from the workpieces 121, 122, the welding electrodes 111, 112 move further toward one another and the position actual value increases. This difference thus constitutes a particularly advantageous variable for being able to evaluate and quantify the effects of weld spatters on the quality of the welding process.

[0058] In step 206, the difference is then compared with a threshold value. The threshold value may be selected for example depending on a sheet-metal thickness combination of the workpieces 121, 122 to be welded. If the difference does not reach the threshold value, this indicates that the position of the welding electrodes 111, 112 has not changed by more than a permissible value during the welding process, and that no weld spatters have therefore occurred or that at least no quality impairment has occurred in the event of occurrence of a weld spatter. In step 207, it is evaluated in this case that a high quality of the welding process or of the weld spot 125 has been achieved.

[0059] If on the other hand the difference reaches or exceeds the threshold value, the position of the welding electrodes 111, 112 has changed by more than the permissible value during the welding process, since a weld spatter has occurred. In step 208, it is evaluated in this case that the remaining wall thickness of the weld spot 125 is not high enough to guarantee a desired strength and that sufficient quality of the welding process or of the weld spot 125 has not been achieved. In this case, a corresponding measure may be performed in step 209, by way of example a fault notification may be output that the weld spot 125 has been formed, but the required strength has not been achieved.

[0060] FIG. 3 schematically shows a graph 300 of welding electrode parameters plotted against time, which parameters may be determined in the course of one preferred embodiment of the method according to the disclosure.

[0061] Curve 310 shows the temporal profile of the position actual value. FIG. 3 furthermore illustrates the temporal profile 320 of the electrode force and the temporal profile 330 of the rotational speed actual value of the electrode drive 130. The curves 310, 320, 330 illustrated in FIG. 3 in particular characterize a high-quality welding process in which no weld spatter has occurred.

[0062] FIG. 3 furthermore illustrates the difference 1 between the first value of the position actual value, determined at the first time t1, and the second value of the position actual value, determined at the second time t2. The difference 1 in the example that is shown has a positive value, which indicates that the electrodes have moved apart from one another and no weld spatter has thus occurred.

[0063] In the same way as FIG. 3, FIG. 4 also schematically shows a graph 400 of welding electrode parameters plotted against time, which parameters may be determined in the course of one preferred embodiment of the method according to the disclosure. FIG. 4 also illustrates the temporal profiles of the position actual value 410, of the electrode force 420 and of the rotational speed actual value 430 of the electrode drive 130. FIG. 4 however shows the case of an insufficient-quality welding process in which a weld spatter has occurred.

[0064] FIG. 4 furthermore shows, in the same way as FIG. 3, the difference 2 between the first value of the position actual value, determined at the first time t3, and the second value of the position actual value, determined at the second time t4. The difference 1 has a negative value in this example, which indicates that the electrodes have moved toward one another and thus indicates a weld spatter.