Current control method and control device in resistance spot welding

Abstract

Provided is a current control method for resistance spot welding in which a plurality of stacked metal plates are clamped with a pair of electrodes, and electricity is passed therethrough while pressure is applied, to weld the metal plates. The current control method includes: a step for successively acquiring the electrical resistance between a pair of electrodes during welding; a step for successively calculating the expansion amount caused by welding, using the strain calculated from the pressure applied from the electrodes and the stroke of the electrodes; a step for successively calculating a calculated nugget diameter, which is the diameter of a nugget during welding, using the electrical resistance and the expansion amount; and a step for successively determining the current to be applied between the pair of electrodes during welding, using the difference between the calculated nugget diameter and a master calculated nugget diameter that is calculated from a master weld, which is a weld in a state where no gap exists between the metal plates and from which a target diameter is obtained as a result of the welding.

Description

Cross - reference to related applications

[0001] This application claims priority based on Japanese Patent Application No. 2022 - 71996 filed on April 26, 2022, and the entire disclosure thereof is incorporated herein by reference.

Technical Field

[0002] This disclosure relates to resistance spot welding.

Background Art

[0003] WO 2014 / 156290 discloses a resistance spot welding system. In this resistance spot welding system, during test welding, the time change of the instantaneous heat generation amount calculated from the electrical characteristics between electrodes during welding when energizing by constant current control to form an appropriate nugget is calculated. Based on the time change of this instantaneous heat generation amount, with an external input after test welding, the energization pattern is divided into a plurality of steps, and the time change and cumulative heat generation amount of the instantaneous heat generation amount for each step are stored as target values. During actual welding, welding is started based on the time change curve of the instantaneous heat generation amount stored as the target value. If, in any step, the amount of change in the time of the instantaneous heat generation amount deviates from the target time change curve, the difference is compensated within the remaining energization time of that step, and the welding current or voltage is adjusted during welding so that the instantaneous heat generation amount and the cumulative heat generation amount are generated to match the target cumulative heat generation amount of that step.

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in such a conventional resistance spot welding method, when there is a disturbance such as a gap between the two plates to be welded, it has been difficult to expand the nugget diameter to the target value.

Means for Solving the Problems

[0005] This disclosure can be realized in the following forms. One aspect of the present disclosure is a method for controlling the current in resistance spot welding in which a plurality of stacked metal plates are sandwiched between a pair of electrodes and energized while being pressurized to weld the metal plates. The method includes: (A) a step of sequentially obtaining the electrical resistance between the pair of electrodes during welding; (B) a step of sequentially calculating the amount of expansion generated by welding using the strain calculated from the pressing force applied by the electrodes and the stroke of the electrodes; (C) a step of sequentially calculating a calculated nugget diameter, which is the diameter of a nugget during welding, using the electrical resistance and the amount of expansion; and (D) a step of sequentially determining, during welding, the current flowing between the pair of electrodes using the difference between the calculated nugget diameter and a master calculated nugget diameter calculated in a master welding in which there is no gap between the metal plates and a nugget having a target diameter is obtained by welding. In step (D), the current is sequentially determined based on the following formula I 1 (t 2 )=I M (t 2 ) + C3·ΔANI(t 1 )+C5 Or I 2 (t 2 )=I M (t 2 ) + C3·ΔANI(t 1 )·ANI C4 (t 1 )+C5 where I 1 (t 2 ) or I 2 (t 2 ) is sequentially determined as the current. In the formula, ΔANI(t 1 ) is the difference between the master calculated nugget diameter and the calculated nugget diameter at timing t 1 , ANI(t 1 ) is the calculated nugget diameter at timing t 1 , I 1 (t 2 ) and I 2 (t 2 ) are the currents at timing t 1 + Δt after timing t 2 , Δt is the interval for sequentially determining the current, I M (t 2 ) is the master current at timing t 2 during master welding, and C3, C4, and C5 are predetermined constants. This is a method for controlling the current. The present disclosure can also be realized in the following forms.

[0006] (1) According to a first aspect of the present disclosure, there is provided a current control method for resistance spot welding in which a plurality of stacked metal plates are sandwiched between a pair of electrodes and energized while being pressurized to weld the metal plates. This current control method includes: (A) a step of sequentially obtaining the electrical resistance between the pair of electrodes during welding; (B) a step of sequentially calculating the amount of expansion generated by welding using the strain calculated from the pressing force by the electrodes and the stroke of the electrodes; (C) a step of sequentially calculating a calculated nugget diameter, which is the diameter of a nugget during welding, using the electrical resistance and the amount of expansion; and (D) a step of sequentially determining, during welding, the current flowing between the pair of electrodes using the difference between the calculated nugget diameter and a master calculated nugget diameter calculated in a master welding in which there is no gap between the metal plates and a nugget having a target diameter is obtained by welding. According to the current control method of this aspect, even when there is a disturbance such as a gap existing between the metal plates to be welded, the diameter of the nugget can be expanded to the target value.

[0007] (2) In the above aspect, prior to the steps (A) to (D), a step of determining the current flowing between the pair of electrodes based on a predetermined current waveform from the start of energization of the pair of electrodes to a predetermined timing may be included.

[0008] (3) In the above aspect, the predetermined timing may be the timing when the nugget starts to be generated.

[0009] (4) In the above aspect, in the step (D), the current may be sequentially determined using the product of the value obtained by raising the calculated nugget diameter to a predetermined exponent and the difference.

[0010] (5) According to a second aspect of the present disclosure, there is provided a control device for resistance spot welding in which a plurality of stacked metal plates are sandwiched between a pair of electrodes and energized while being pressed to weld the metal plates. This control device includes a resistance acquisition unit that sequentially acquires the electrical resistance between the pair of electrodes during welding, an expansion amount calculation unit that sequentially calculates the expansion amount generated by welding using the strain calculated from the pressing force by the electrodes and the stroke of the electrodes, a nugget diameter calculation unit that sequentially calculates a calculated nugget diameter that is the diameter of the nugget during welding using the electrical resistance and the expansion amount, and a current determination unit that sequentially determines the current flowing between the pair of electrodes during welding using the difference between the calculated nugget diameter and the master calculated nugget diameter calculated in a master welding in which there is no gap between the metal plates and a nugget of a target diameter is obtained by welding.

[0011] The present disclosure can also be realized in various forms other than the above-described current control method and control device. For example, it can be realized in the form of a resistance spot welding method, a resistance spot welding system, a computer program, a non-transitory tangible recording medium recording the computer program, and the like.

Brief Description of the Drawings

[0012]

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Mode for Carrying Out the Invention

[0013] A. First Embodiment: FIG. 1 is a diagram showing the configuration of a resistance spot welding system 100 according to the first embodiment. The resistance spot welding system 100 includes a spot welding power source 10, a control unit 20, a pair of electrodes 30 and 32, a pressurizing cylinder 40, a lower arm 50, an ammeter 60, and a voltmeter 70.

[0014] The metal plates 200 and 210 to be welded are sandwiched between the pair of electrodes 30 and 32. The electrode 30 is provided on the pressurizing cylinder 40. The electrode 32 is provided on the lower arm 50. The pressurizing cylinder 40 presses the electrode 30 toward the metal plate 200 with a pressing force F. The pressurizing cylinder 40 is provided with a pressure sensor 42. The pressure sensor 42 measures the pressing force F and the stroke S of the electrode 30 during pressurization. The spot welding power source 10 is connected to the electrodes 30 and 32. The ammeter 60 measures the current I flowing between the electrodes 30 and 32. The voltmeter 70 measures the voltage V applied between the electrodes 30 and 32.

[0015] When a current I flows between two electrodes 30 and 32, Joule heat is generated due to the contact resistance R between the two metal plates 200 and 210. When the pressing force F is large, the contact resistance R is small, and when the pressing force F is small, the contact resistance R is large. The generated Joule heat melts a part of the metal plates 200 and 210 at the interface between the two metal plates 200 and 210. Then, when the energization is stopped, the melted metal is cooled and solidified, and a nugget 220 is generated at the boundary between the two metal plates 200 and 210. The nugget 220 has a flat shape along the interface between the two metal plates 200 and 210, and adheres the two metal plates 200 and 210 by welding. The strength of the welding depends on the nugget diameter. The larger the nugget diameter of the nugget 220, the stronger the welding strength between the two metal plates 200 and 210.

[0016] The control unit 20 is a control device that controls the current I flowing between the electrodes 30 and 32 during welding. The control unit 20 is constituted by a computer including a processor 21 and a memory 22. The processor 21 functions as a resistance acquisition unit 23, a dilation amount calculation unit 24, a nugget diameter calculation unit 25, and a current determination unit 26 by executing a program stored in the memory 22. The program may be recorded on a non-transitory tangible computer-readable recording medium.

[0017] The resistance acquisition unit 23 sequentially acquires the electrical resistance between a pair of electrodes 30 and 32 during welding. The dilation amount calculation unit 24 sequentially calculates the dilation amount caused by welding using the strain calculated from the pressing force by the electrodes 30 and 32 and the stroke S of the electrode 30. The nugget diameter calculation unit 25 sequentially calculates the calculated nugget diameter, which is the diameter of the nugget during welding, using the electrical resistance and the dilation amount. The current determination unit 26 sequentially determines the current flowing between the pair of electrodes during welding using the difference between the calculated nugget diameter and the master calculated nugget diameter. The master calculated nugget diameter is the nugget diameter calculated in master welding, which is welding in a state where there is no gap between the metal plates and a nugget of the target diameter is obtained by welding. Note that these functional units may be realized by a circuit.

[0018] FIG. 2 is a control flowchart of the master welding process sequentially performed by the control unit 20. Master welding means ideal welding in which there is no gap between two metal plates 200 and 210 and a target nugget diameter can be obtained by welding. In the master welding process, the control unit 20 acquires various parameters for controlling the welding used in this welding, such as the master current I M , the applied voltage V, the electrical resistance, the pressing force F, the stroke S, etc., and records them as a master pattern.

[0019] In step S100, the control unit 20 starts master welding by passing a current I between the electrodes 30 and 32 while applying pressure. In step S110, the control unit 20 sequentially measures the voltage V between the two electrodes 30 and 32, the current I flowing between the two electrodes 30 and 32, the pressing force F of the pressing cylinder 40, and the stroke S of the electrode 30 in the master welding.

[0020] In step S120, the control unit 20 calculates the electrical resistance between the two metal plates 200 and 210 and the expansion amount E of the nugget in the plate thickness direction. The resistance acquisition unit 23 of the control unit 20 sequentially calculates and acquires the electrical resistance from the measured current I and voltage V according to Ohm's law. The electrical resistance is approximately equal to the contact resistance R. The expansion amount calculation unit 24 of the control unit 20 converts the change amount ΔF of the pressing force F into a strain amount and adds the change amount ΔS of the stroke S to sequentially calculate and acquire the expansion amount E. The formula (1) for calculating the expansion amount E is as follows.

[0021] E = ΔS + a·ΔF …(1)

[0022] In formula (1), the coefficient a is a strain amount conversion coefficient. The second term of formula (1) is the strain amount. As an example, the strain amount conversion coefficient a is 0.00048, and in this case, formula (1) becomes the following formula (2).

[0023] E = ΔS + 0.00048·ΔF …(2)

[0024] Figure 3 is a graph showing the pressing force F. In Figure 3, in addition to the pressing force F in the master welding, the pressing force F in the present welding described later is also shown. In the master welding, metal plates without gaps are welded to each other at the welding position. In the present welding, two metal plates having a 2-mm gap are welded to each other at the welding position. The pressing force F is a larger value for the master welding where there is no gap between the metal plates than for the present welding where there is a 2-mm gap between the metal plates. The pressing force F in the present welding varies depending on the state of the two metal plates 200 and 210 to be welded, for example, the size of the gap between the two metal plates 200 and 210 at the welding position and the surface state of the interface of the metal plates 200 and 210.

[0025] Figure 4 is a graph showing the stroke S. In Figure 4, similar to Figure 3, in addition to the stroke S in the master welding, the stroke S in the present welding is also shown. The stroke S is a larger value for the present welding where there is a 2-mm gap between the metal plates than for the master welding where there is no gap between the metal plates. Similar to the pressing force F, the stroke S in the present welding varies depending on the state of the two metal plates 200 and 210 to be welded, for example, the size of the gap between the two metal plates 200 and 210 at the welding position and the surface state of the interface of the metal plates 200 and 210.

[0026] Figure 5 is a graph showing the expansion amount E. In Figure 5, similar to Figures 3 and 4, in addition to the expansion amount E in the master welding, the expansion amount E in the present welding is also shown. The expansion amount E is a larger value for the master welding where there is no gap between the metal plates than for the present welding where there is a 2-mm gap between the metal plates. The expansion amount E in the plate thickness direction correlates with the nugget diameter along the interface between the metal plates.

[0027] In step S130 of Figure 2, the nugget diameter calculation unit 25 of the control unit 20 sequentially calculates the calculated nugget diameter ANI. The nugget diameter calculation unit 25 sequentially calculates the calculated nugget diameter ANI at an arbitrary timing in the current control section, for example, according to the following formula (3).

[0028] ANI = C1·ΔR + C2·E max+C3·∫E + C4 …(3)

[0029] In Equation (3), ΔR is the change in electrical resistance. E max is the maximum value of the expansion amount E. ∫E is the integral value of the expansion amount E. C1, C2, C3, and C4 are constants obtained by experiments or multiple regression analysis. The change in electrical resistance ΔR, the maximum value E max of the expansion amount E, and the integral value ∫E of the expansion amount E will be described in detail with reference to FIG. 6. Since the calculated nugget diameter ANI is the calculated nugget diameter in master welding, it is also called the master calculated nugget diameter ANI M The control unit 20 designates the current I passed in master welding as the master current I M and records the master calculated nugget diameter ANI M and the master current I M as a master pattern.

[0030] FIG. 6 is a graph showing the master current I M After energization starts, the control unit 20 passes a relatively large master current I M about three times until timing D. The reason for passing a relatively large master current I M about three times is to form an alloy layer in a short time. From timing D to timing C, the control unit 20 passes a master current I M of substantially constant magnitude. By passing a master current I M of substantially constant magnitude, the contact state between the metal plates can be adjusted. Thereafter, from timing C to timing B, the control unit 20 gradually increases the master current I M . From timing B to timing A, the control unit 20 passes a master current I M of substantially constant magnitude. The control unit 20 stops energization at timing A.

[0031] The section from the start of energization to timing D is a preparation section for generating a jet. In the section from the start of energization to timing D, the control unit 20 determines the current flowing through the pair of electrodes 30 and 32 according to a predetermined current waveform. The section from timing C to timing A is a control section for growing the jet to the target size. Timing C, which is the start timing of the control section, is the timing when the jet starts to be generated. In other words, timing C is the timing when the metal plates start to melt with each other. The above-mentioned timings D, C, B, and A are predetermined according to the combination of metal plates. For example, depending on the combination of metal plates, the jet may start to be generated at timing D. In such a case, the control unit 20 makes timing C coincide with timing D. When timing C is made to coincide with timing D, the master current I M gradually increases from timing D(=timing C) toward timing B, so there is no section where a master current I M of substantially constant magnitude flows between timing D and timing B.

[0032] The above-mentioned change amount ΔR of the electrical resistance is the integrated value of the resistance change from the electrical resistance between the two electrodes 30 and 32 at timing D to the electrical resistance between the two electrodes 30 and 32 at the time of energization stop. The maximum value E max of the expansion amount E is the maximum value of the expansion amount E during the period from the start of energization to the stop of energization. As can be seen from FIG. 5, since the expansion amount E increases monotonically from the start of energization, the maximum value E max of the expansion amount E is the expansion amount E at the time of energization stop. The integrated value ∫E of the expansion amount E is the result of integrating the expansion amount E from the start of energization to the stop of energization.

[0033] FIG. 7 is a graph comparing the calculated net diameter ANI calculated using Equation (3) with the measured net diameter MNI. The circular marks indicate the case where two metal plates 200 and 210 without a gap at the welding position are energized up to timing A. The square marks indicate the case where two metal plates 200 and 210 having a 2-mm gap at the welding position are energized up to timing A. The triangular marks indicate the four cases of (1) two metal plates 200 and 210 without a gap being energized up to timing B, (2) two metal plates 200 and 210 without a gap being energized up to timing C, (3) two metal plates 200 and 210 having a 2-mm gap being energized up to timing B, and (4) two metal plates 200 and 210 having a 2-mm gap being energized up to timing C. In Equation (3) for calculating the graph of FIG. 7, each constant in Equation (3) is determined using information (such as electrical resistance and expansion amount) of samples energized not only up to timing A but also up to timings B and C.

[0034] The calculated net diameter ANI shown in FIG. 7 falls within a value within ±10% of the measured net diameter MNI. Therefore, by determining Equation (3) using information at various timings, Equation (3) becomes a valid equation for estimating the net diameter ANI regardless of the state of the two metal plates 200 and 210 at the welding position and the energization stop timing. Note that "t" in FIGS. 7, 8, 12, and 14 represents the thickness of the thinner of the two metal plates to be welded.

[0035] FIG. 8 is a graph comparing the calculated nugget diameter ANI calculated by Expression (3) determined from only the information at the completion of energization, i.e., timing A, with the measured nugget diameter MNI. The size of the gap and the energization stop timing at the welding positions of the points indicated by circles, squares, and triangles are the same as those in FIG. 7. When Expression (3) is determined from only the information at timing A, in the case of welding two metal plates 200 and 210 without a gap and in the case of welding two metal plates 200 and 210 with a 2-mm gap, the calculated nugget diameter ANI is almost equal to the measured nugget diameter MNI. However, when energization is performed only up to timing B or timing C, in either the case of welding two metal plates 200 and 210 without a gap or the case of welding two metal plates 200 and 210 with a 2-mm gap, the measured nugget diameter MNI is smaller than the calculated nugget diameter ANI. That is, it was found that when Expression (3) is determined from only the information at timing A, the estimation accuracy of the calculated nugget diameter ANI in the middle of the control section is lower than when Expression (3) is determined using the information at timings B and C.

[0036] FIG. 9 is a control flowchart of the main welding process sequentially performed by the control unit 20. In step S200, the control unit 20 starts welding by energizing while applying pressure with reference to the master current I M recorded during the master welding.

[0037] In step S210, the control unit 20 sequentially measures the voltage V, current I, pressing force F of the pressing cylinder 40, and stroke S of the electrode 30 between the two electrodes 30 and 32 in the same manner as in step S110 (see FIG. 2) in the master welding. Until timing C when the nugget starts to be generated, the control unit 20 passes a current according to a predetermined current waveform in the same way as the master current I M . Therefore, until timing C, the current flowing between the pair of electrodes 30 and 32 is determined based on the predetermined current waveform. After timing C, the control unit 20 passes a current I 1Flow. In step S220, similar to step S120 in the master welding (see FIG. 2), the control unit 20 sequentially acquires the electrical resistance between the two metal plates 200 and 210 and sequentially calculates the expansion amount E. In step S230, the control unit 20 sequentially calculates the calculated nugget diameter ANI using Equation (3), similar to step S130 in the master welding (see FIG. 2).

[0038] In step S240, the current determination unit 26 of the control unit 20 uses the difference ΔANI between the calculated nugget diameter ANI in the master welding and the calculated nugget diameter ANI in this welding during the control period from timing C to timing A in FIG. 6 to determine the current I 1 to be passed sequentially during this welding and controls the current. Here, the calculated nugget diameter in the master welding is referred to as the master calculated nugget diameter ANI M and the calculated nugget diameter in this welding is referred to as the calculated nugget diameter ANI 1 . The control formula (4) used for the current control executed by the control unit 20 is as follows. The control formula (4) is a formula obtained through experiments.

[0039] I 1 (t 2 ) = I M (t 2 ) + C3·ΔANI(t 1 ) + C5 …(4)

[0040] In the control formula (4), ΔANI(t 1 ) is the difference between the master calculated nugget diameter ANI 1 at timing t M and the calculated nugget diameter ANI 1 in this welding. I 1 (t 2 ) is the current at timing t 1 after Δt from timing t 2 , and I M (t 2 ) is the master current at timing t 2 . C3 and C5 are constants and are obtained through experiments and multiple regression analysis. Δt is the time when the control unit 20 adjusts the current I 1(t 2 ) is the control interval when sequentially calculating, for example, 2 ms. That is, the control unit 20 calculates the current I 1 (t) every 2 ms. Note that the control interval Δt may be an interval other than 2 ms, such as 0.5 ms, 1 ms, 3 ms, etc.

[0041] As an example, the constant C3 in the control formula (4) is 540, and the constant C5 is 0. In this case, the control formula (4) becomes the following control formula (5).

[0042] I 1 (t 2 ) = I M (t 2 ) + 540·ΔANI(t 1 ) …(5)

[0043] FIG. 10 is an explanatory diagram showing the master current I M (t) passed during master welding and the current I 1 (t) passed during this welding. The control unit 20 controls the current in this welding based on the master current I M up to the timing C in FIG. 6, and between the timing C and the timing A, the current I 1 (t) in this welding is sequentially determined by the control formula (5) to control the current.

[0044] FIG. 11 is an explanatory diagram showing the master calculation nugget diameter ANI M and the calculation nugget diameter ANI 1 in this welding. At the timing C when the control unit 20 starts control with the current I 1 (t), the calculation nugget diameter ANI 1 in this welding is smaller than the master calculation nugget diameter ANI M . As a result, the ΔANI(t 1 ) in the second term of the control formula (5) is large. The control unit 20 makes the current I 1 (t) in this welding increase more rapidly than the master current I M (t) in master welding. Welding proceeds in this state, and the calculation nugget diameter ANI in this welding1 When it increases, the second term ΔANI(t of the control formula (5), which is the difference from the master calculation naget diameter ANI M ) becomes smaller. The control unit 20 makes the current I 1 (t) in this welding approach the master current I 1 (t M (t 2 ).

[0045] Figure 12 is a comparison diagram of the measured naget diameters. The example in (A) shows the case where two metal plates 200 and 210 with no gap are welded with the master current I M flowing through. The example in (B) shows the case where two metal plates 200 and 210 with a 2 mm gap are welded with the master current I M flowing through. The example in (C) shows the case where two metal plates 200 and 210 with a 2 mm gap are welded with the current by the conventional current control. The conventional current control is a control that determines the current based on the difference between the electrical resistance in the master welding and the electrical resistance in this welding. The example in (D) shows the case where two metal plates 200 and 210 with a 2 mm gap are welded with the current sequentially calculated by the control formula (5) in the first embodiment.

[0046] In the example (B) where two metal plates 200 and 210 with a 2 mm gap are welded with the master current I M flowing through, the measured naget diameter MNI is 5.3√t, which is insufficient for the target value of the measured naget diameter MNI of 6.6√t. Also, in the case (C) where two metal plates 200 and 210 with a 2 mm gap are welded with the current by the conventional current control, the measured naget diameter MNI is 5.4√t, and similarly, it is insufficient for the target value of the measured naget diameter MNI of 6.6√t. In contrast, the measured naget diameter MNI in the first embodiment (D) where two metal plates 200 and 210 with a 2 mm gap are welded with the current sequentially calculated by the control formula (5) is 6.1√t, and the master current I MWhen welding two metal plates 200 and 210 with a 2 mm gap by passing a current (Case B), or when welding two metal plates 200 and 210 with a 2 mm gap by passing a current under conventional current control (Case C), it is approaching the measured nugget diameter of 6.6√t in the target master welding (A).

[0047] As described above, according to the first embodiment, the control unit 20 uses the electrical resistance and the expansion amount E during energization to sequentially calculate the calculated nugget diameter ANI 1 and, using the difference between the calculated nugget diameter ANI 1 and the calculated nugget diameter ANI M in the master welding, sequentially calculates and controls the current I 1 (t) in this welding. As a result, in the first embodiment, even when there is a disturbance such as a gap existing between the metal plates to be welded, the measured nugget diameter MNI M can be brought close to a value close to the measured nugget diameter MNI 1 in the target master welding.

[0048] B. Second Embodiment: In the first embodiment, the current determination unit 26 of the control unit 20 sequentially determines the current to be passed during energization using the difference between the calculated nugget diameter and the master calculated nugget diameter. In contrast, in the second embodiment, the current determination unit 26 sequentially determines the energization current using the product of the value obtained by raising the calculated nugget diameter to a predetermined exponent and the difference between the calculated nugget diameter and the master calculated nugget diameter. In the second embodiment, the configuration of the resistance spot welding system 100 and the calculation formula for the calculated nugget diameter are the same as those in the first embodiment.

[0049] The control formula (6) used for the current control sequentially executed by the control unit 20 in the second embodiment is as follows. The control formula (6) is a formula obtained through experiments.

[0050] I 2 (t 2 ) = I M (t 2 ) + C3·ΔANI(t 1 )·ANIC4 (t 1 ) + C5 …(6)

[0051] In the control formula (6), I M (t 2 ) is the master current. ANI(t 1 ) is the calculated net diameter, ΔANI(t 1 ) is the master calculated net diameter ANI at timing t 1 and the calculated net diameter ANI in this welding M The difference between them. C3, C4, and C5 are constants, which are obtained by experiments or multiple regression analysis. 1

[0052] As an example, the constant C3 of the control formula (6) is 90, the constant C 4 is 2, and the constant C5 is 0. In this case, the control formula (6) becomes the following control formula (7).

[0053] I 2 (t 2 ) = I M (t 2 ) + 90·ΔANI(t 1 )·ANI 2 (t 1 ) …(7)

[0054] Figure 13 is an explanatory diagram showing the current I 2 (t) passed by the control unit 20 during this welding in the second embodiment, compared with the master current I M (t) and the current I 1 (t) passed by the control unit 20 during this welding in the first embodiment. Also in the second embodiment, similar to the first embodiment, the control unit 20 passes a current according to a predetermined current waveform until timing C. From timing C to timing A, the control unit 20 passes the current sequentially calculated according to the above control formula (7). The current I 2 (t) passed by the control unit 20 during this welding at the initial stage of control for some time after timing C in the second embodiment is smaller than the current I 1 (t) passed by the control unit 20 during this welding in the first embodiment. On the other hand, in the later stage of control, the current I passed by the control unit 20 during this welding in the second embodiment2 I(t) is the current passed during the main welding in the first embodiment 1 is larger than I(t).

[0055] Figure 14 is a comparison diagram of the measured nugget diameters. In Figure 14, Figure 12 shows the case where two metal plates 200 and 210 with a 2-mm gap are welded by the current I 2 (t) (Second Embodiment (E)) is added. The measured nugget diameter MNI 2 in the second embodiment (E) is 6.6√t, which is the same value as the measured nugget diameter 6.6√t in the master welding (A), and the measured nugget diameter MNI 1 in the first embodiment (D) is closer to the measured nugget diameter 6.6√t in the master welding, which is the target value.

[0056] As described above, according to the second embodiment, the measured nugget diameter MNI 2 can be made closer to the target value than in the first embodiment.

[0057] Also, according to the second embodiment, since the information on the nugget diameter (calculated nugget diameter ANI(t)) at each timing is added to the control formula (7), at the initial stage of control where the contact diameter is small, the current I 2 (t) can be suppressed from rising rapidly, and the current I 2 can be increased according to the expansion of the nugget diameter and the contact diameter. As a result, the spatter limit current value can be increased, the control range of the current is increased, and the measured nugget diameter MNI M in the master welding, which is the target value, can be made closer.

[0058] C. Other Embodiments: (C1) In each of the above-described embodiments, the timing C, which is the start timing of the current control section, is assumed to be the timing when the nugget starts to be generated. In contrast, the timing C may be the timing when the nugget grows to a predetermined size in the initial stage of welding. Also, the timing C may be the timing immediately before the nugget starts to be generated.

[0059] (C2) In each of the above-described embodiments, an example in which two metal plates 200 and 210 are overlapped and welded has been described. However, the number of metal plates to be welded may be two or more. Further, the members to be welded are not limited to metal plates and may be metal blocks.

[0060] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the gist thereof. For example, the technical features of the embodiments corresponding to the technical features in each of the forms described in the summary of the invention can be appropriately replaced or combined in order to solve some or all of the above-described problems or to achieve some or all of the above-described effects. Further, if the technical feature is not described as essential in this specification, it can be appropriately deleted.

Claims

1. A method for controlling the current in resistance spot welding, which involves sandwiching a plurality of overlapping metal plates between a pair of electrodes, energizing them while applying pressure to weld the metal plates, comprising: (A) a step of sequentially obtaining the electrical resistance between the pair of electrodes during welding; (B) a step of sequentially calculating the amount of expansion generated by welding using the strain calculated from the pressing force applied by the electrodes and the stroke of the electrodes; (C) a step of sequentially calculating the calculated nugget diameter, which is the diameter of the nugget during welding, using the electrical resistance and the amount of expansion; (D) a step of sequentially determining the current flowing between the pair of electrodes during welding using the difference between the calculated nugget diameter and the master calculated nugget diameter calculated in master welding, which is welding in a state where there is no gap between the metal plates and a nugget of the target diameter is obtained by welding; including: In the step (D), the following formula I1(t2) = IM(t2) + C3·ΔANI(t1) + C5 or I2(t2) = IM(t2) + C3·ΔANI(t1)·ANI C4(t1) + C5 is used to sequentially determine I1(t2) or I2(t2) as the current, wherein ΔANI(t1) is the difference between the master calculated nugget diameter and the calculated nugget diameter at timing t1, ANI(t1) is the calculated nugget diameter at timing t1, I1(t2) and I2(t2) are the currents at timing t2 after Δt from timing t1, Δt is the interval for sequentially determining the current, IM(t2) is the master current at timing t2 during master welding, and C3, C4, and C5 are predetermined constants. A current control method.

2. The current control method according to Claim 1, further comprising: a step of determining the current flowing between the pair of electrodes based on a predetermined current waveform from the start of energization of the pair of electrodes to a predetermined timing prior to the steps (A) to (D).

3. The current control method according to Claim 2, wherein the predetermined timing is the timing when the nugget starts to be generated.

4. A control device for resistance spot welding, which sandwiches a plurality of overlapping metal plates between a pair of electrodes, energizes them while applying pressure to weld the metal plates, A resistance acquisition unit that sequentially acquires the electrical resistance between the pair of electrodes during welding; An expansion amount calculation unit that sequentially calculates the expansion amount generated by welding using the strain calculated from the pressing force by the electrodes and the stroke of the electrodes; A nugget diameter calculation unit that sequentially calculates a calculated nugget diameter, which is the diameter of the nugget during welding, using the electrical resistance and the expansion amount; A current determination unit that sequentially determines the current flowing between the pair of electrodes during welding using the difference between the calculated nugget diameter and the master calculated nugget diameter calculated in master welding, which is welding in a state where there is no gap between the metal plates and a nugget with a target diameter is obtained by welding; Comprising; The current determination unit uses the following formula I1(t2) = IM(t2) + C3·ΔANI(t1) + C5 Or, I2(t2) = IM(t2) + C3·ΔANI(t1)·ANIC4(t1) + C5 Based on this, I1(t2) or I2(t2) is sequentially determined as the current, In the formula, ΔANI(t1) is the difference between the master calculated nugget diameter and the calculated nugget diameter at timing t1, ANI(t1) is the calculated nugget diameter at timing t1, I1(t2) and I2(t2) are the currents at timing t2 after Δt from timing t1, Δt is the interval for sequentially determining the current, IM(t2) is the master current at timing t2 during master welding, and C3, C4, and C5 are predetermined constants. Control device.

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