Spot welding method

Abstract

To provide a spot welding method capable of performing current control that makes a rising slope of waveform constant even in a case where pulse-shaped waveforms overlap with each other.SOLUTION: The spot welding method includes, for a welding current L10: setting a target rise current value P for each predetermined PWM pulse when the welding current increases toward a peak current range R1; and setting, as a first target current value A, a target rise current value P that is larger than and the closest to a rise start current value S in a case where an effective value L14 of the welding current reaches a set welding current L23 while the welding current is decreasing toward a bottom current PB.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a spot welding method. [Background technology]

[0002] When welding metal plates, spot welding is sometimes performed using a spot welding device. In spot welding, a plurality of metal plates are clamped between a pair of electrode tips and a current is passed between the pair of electrode tips to generate a nugget between the plurality of metal plates, thereby welding the plurality of metal plates together.

[0003] In the invention disclosed in Patent Document 1, a plurality of metal plates are welded by applying a plurality of micropulses to the plurality of metal plates while the plurality of metal plates are sandwiched between a pair of electrodes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. WO2020 / 050011 Summary of the Invention [Problem to be solved by the invention]

[0005] The invention disclosed in Patent Document 1 provides a spot welding method that can reliably join workpieces while suppressing spatter generation. This spot welding method uses a welding current having a predetermined pulse waveform. This pulse waveform includes a peak state, where the welding current is a predetermined value, and a non-peak state, which is a state other than the peak state. When the effective value of the welding current reaches a predetermined target range during the non-peak state, current control is initiated to increase the welding current so that the peak state is achieved. This current control realizes that the welding current supplies energy to the workpiece so that a nugget of appropriate size is formed, while current control is initiated at a timing that suppresses spatter generation. Furthermore, the increase in welding current toward the peak state is controlled by a predetermined rise time, in other words, the time it takes to reach the peak current.

[0006] In the invention disclosed in Patent Document 1, when the effective value of the welding current becomes high, the next waveform may rise during the fall of a non-peak waveform, resulting in overlapping of pulse-like waveforms. In the invention disclosed in Patent Document 1, the welding current is controlled by a preset rise time. Therefore, when overlapping of pulse-like waveforms occurs, the slope of the rise of the waveform may not be the same for the first and second waves. If the slope of the rise of the waveform changes, the heat input conditions to the welding point will not be constant, leading to a decrease in welding quality.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a spot welding method capable of current control that keeps the rising slope of the waveform constant even when pulse-shaped waveforms overlap. [Means for solving the problem]

[0008] (1) A spot welding method according to the present invention controls a welding current by PWM control, and the welding current has a pulse waveform that alternates between a peak state in which the welding current reaches or is maintained within a set peak current range and a non-peak state in which the welding current drops from the peak current range toward a bottom current and then rises again toward the peak current range. In the non-peak state, when the effective value of the welding current reaches a set target range, current control is started to raise the welding current toward the peak current range to join workpieces. In this spot welding method, a rise target current value is set for each predetermined PWM pulse as the welding current rises toward the peak current range, and when the effective value of the welding current reaches the set target range while descending toward the bottom current, the rise target current value that is greater than and closest to the rise start current value is set as a first target current value.

[0009] According to the above-described method, when the effective value of the welding current reaches a set target range while descending toward the bottom current, the closest rising target current value to the rising current value is set as the first target current value. Therefore, even when the welding current is increased toward the peak current range while descending toward the bottom current, the rising slope of the welding current can be kept constant, and the heat input can be stabilized. This prevents deterioration of welding quality.

[0010] (2) In the above spot welding method, the rising target current value is set to a second target current value that is the next larger target value than the first target current value, and an intermediate target current value is set between the first target current value and the second target current value.

[0011] According to the above-mentioned method, when a pulse waveform rises while the next pulse waveform is falling, i.e., when the pulse waveforms overlap, the pulse width immediately after the rise changes significantly, resulting in a steep rise, which may induce biased magnetization in the welding transformer. However, by setting an intermediate target current value between the first target current value and the second target current value, the sudden change in pulse width can be suppressed.

[0012] (3) In the spot welding method, in controlling the effective value of the welding current, the target rise current value that is lower than the rise start current value at the start of current control for increasing the welding current is not included in the calculation.

[0013] According to the above-described method, the control load can be reduced by eliminating the rise target current value that is lower than the rise start current value.

[0014] (4) In the spot welding method, the intermediate target current value is calculated by the following formula: A' = A + coefficient × (BA) A: First target current value A': Intermediate target current value B: Second target current value Coefficient: Any value between 0% and 100%

[0015] According to the above method, by adjusting the coefficient values ​​using the above formula, it is possible to set the intermediate target value so as to enable a more stable start-up. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a spot welding method capable of current control that keeps the rising slope of the waveform constant even when pulse-shaped waveforms overlap. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing the configuration of a welding system to which a spot welding method according to one embodiment of the present invention is applied. [Figure 2] FIG. 2 is a diagram showing the circuit configuration of a welding power supply circuit. [Figure 3] FIG. 3 is a diagram showing the relationship between the AC voltage input to the transformer from the inverter circuit and the welding current applied to the electrode tip pair in the welding power supply circuit. [Figure 4] FIG. 4 is a diagram showing a cross section of a workpiece during welding. [Figure 5]FIG. 5 is a flow chart showing the procedure for controlling the welding current in the control device. [Figure 6] FIG. 6 is a diagram showing the waveform of the welding current realized by the welding current control of FIG. [Figure 7] FIG. 7 is a diagram showing the relationship between different set welding currents and the waveform of the welding current. [Figure 8] FIG. 8 is a diagram showing the waveform of the welding current in this embodiment. [Figure 9] FIG. 9 is an enlarged view of the framed area R50 in FIG. [Figure 10] FIG. 10 is an enlarged view of the box R50 in FIG. 8, which shows the intermediate target current value. [Figure 11] FIG. 11 is a flowchart showing the process flow of the spot welding method of this embodiment. [Figure 12] FIG. 12 is a diagram showing the welding current when the intermediate target current is not set. [Figure 13] FIG. 13 is a diagram showing the welding current when an intermediate target current is set. DETAILED DESCRIPTION OF THE INVENTION

[0018] (First embodiment) A spot welding method according to one embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a diagram showing the configuration of a welding system 100 to which the spot welding method according to this embodiment is applied.

[0019] (welding system) The welding system 100 includes a spot welding device 1 which is a welding gun, a workpiece W which is a stack of metal plates to be joined by the spot welding device 1, and a robot 6 which supports the spot welding device 1.

[0020] The workpiece W is a laminated body formed by stacking multiple metal plates. In this embodiment, the workpiece W is described as a laminated body formed by stacking two metal plates, a first metal plate W1 and a second metal plate W2, in order from top to bottom, but the present invention is not limited to this. The number of metal plates forming the workpiece W may be three or more. Furthermore, the thicknesses of the stacked metal plates may be the same or different.

[0021] The robot 6 includes a robot body 60 attached to the floor, an articulated arm 61 pivotally supported by the robot body 60, and a robot control device 62 that controls the robot 6. The articulated arm 61 includes a first arm section 611 pivotally supported at its base end by the robot body 60, a second arm section 612 pivotally supported at its base end by the first arm section 611, a third arm section 613 pivotally supported at its base end by the second arm section 612, and a fourth arm section 614 pivotally supported at its base end by the third arm section 613 and having a spot welding device 1 attached to its tip end.

[0022] The robot control device 62 drives each arm section 611 to 614 by driving multiple motors provided on the robot body 60 and the articulated arm 61, controls the position and orientation of the spot welding device 1 attached to the fourth arm section 614, and moves the upper electrode tip 21 and lower electrode tip 26 described below provided on the spot welding device 1 to the joint portion of the workpiece W.

[0023] The spot welding device 1 includes a welding power supply circuit 3 which is a source of welding current, a gun body 2 on which an upper electrode tip moving mechanism 4 described below and a part of the welding power supply circuit 3 are mounted, an upper electrode tip 21 and a lower electrode tip 26 which are a pair of electrodes, an upper electrode tip support part 22, an upper adapter body 23, a gun arm 25, a lower electrode tip support part 27, and a lower adapter body 28.

[0024] The upper electrode tip support part 22 is rod-shaped and extends vertically, and the upper electrode tip 21 is attached to the tip of the upper electrode tip support part 22. The upper adapter body 23 is columnar and connects the gun body 2 and the upper electrode tip support part 22. The upper adapter body 23 is provided so as to be able to slide relative to the gun body 2 in a direction parallel to the axis of the upper electrode tip support part 22.

[0025] The gun arm 25 curves and extends from the gun body 2 vertically downward of the upper electrode tip 21. The lower electrode tip support part 27 is rod-shaped and coaxial with the upper electrode tip support part 22, and the lower electrode tip 26 is attached to its tip. The lower adapter body 28 is columnar and connects the tip of the gun arm 25 to the lower electrode tip support part 27. As shown in FIG. 1 , the lower electrode tip 26 is supported by the lower electrode tip support part 27 so as to face the upper electrode tip 21 at a predetermined distance along the axes of the upper electrode tip support part 22 and the lower electrode tip support part 27.

[0026] The upper electrode tip moving mechanism 4 includes a cylinder and a control device therefor, and moves the upper adapter body 23, together with the upper electrode tip support portion 22 and the upper electrode tip 21, forward and backward in a direction parallel to the axis of the upper electrode tip support portion 22. This allows the upper electrode tip 21 to abut against the upper surface of the workpiece W while the lower electrode tip 26 is abutting against the lower surface of the workpiece W, and further allows the upper electrode tip 21 and lower electrode tip 26 to clamp and pressurize the workpiece W.

[0027] (welding power circuit) FIG. 2 is a diagram showing the circuit configuration of the welding power supply circuit 3. The welding power supply circuit 3 includes a welding control circuit 3a, a DC welding transformer 3b, a power cable 3c, and a current sensor 3d. The welding power supply circuit 3 is connected to the upper electrode tip 21 and the lower electrode tip 26 via a first power line L1 and a second power line L2. As shown in FIG. 1, the gun body 2 is equipped with the DC welding transformer 3b and the current sensor 3d of the welding power supply circuit 3 configured as described above. The welding control circuit 3a of the welding power supply circuit 3 is mounted on a base separate from the gun body 2 and is connected to the DC welding transformer 3b via the power cable 3c. This allows the weight of the gun body 2 to be reduced.

[0028] The welding control circuit 3a includes a converter circuit 31, an inverter circuit 32, and a control device 33. The DC welding transformer 3b includes a transformer and a rectifier circuit .

[0029] The converter circuit 31 converts the three-phase power input from the three-phase power supply 30 into DC power by full-wave rectifying it, and supplies this DC power to the inverter circuit 32.

[0030] The inverter circuit 32 converts the DC power input from the converter circuit 31 into single-phase AC power and outputs it to the transformer 34 via the power cable 3c. Specifically, the inverter circuit 32 includes four bridge-connected switching elements. The inverter circuit 32 converts the DC power into single-phase AC power by turning these switching elements ON or OFF in response to a gate drive signal transmitted from a gate drive circuit mounted on the control device 33.

[0031] The transformer 34 transforms the AC power input from the inverter circuit 32 and outputs the transformed AC power to the rectifier circuit 35. The rectifier circuit 35 rectifies the AC power input from the transformer 34 and outputs DC power between the upper electrode tip 21 and the lower electrode tip 26 connected to the first power line L1 and the second power line L2, respectively. For this rectifier circuit 35, a known full-wave rectifier circuit configured by combining a first rectifier diode 351, a second rectifier diode 352, and a center tap 353 is used, for example.

[0032] The current sensor 3d detects the welding current supplied to the upper electrode tip 21 and the lower electrode tip 26 from the welding power supply circuit 3. The current sensor 3d is provided, for example, on the first power line L1 connecting the rectifier circuit 35 and the upper electrode tip 21, and transmits a current detection signal to the control device 33 according to the magnitude of the welding current flowing through this first power line L1.

[0033] The control device 33 includes a microcomputer that executes the welding current control described below using the current detection signal transmitted from the current sensor 3d, and a gate drive circuit that generates a gate drive signal according to the calculation result of the microcomputer and transmits it to the inverter circuit 32.

[0034] (AC voltage Vt and welding current Iw) Fig. 3 is a diagram showing the relationship between the AC voltage Vt input from the inverter circuit 32 to the transformer 34 and the welding current Iw applied to the upper electrode tip 21 and the lower electrode tip 26 in the above-described welding power supply circuit 3. The horizontal axis of the graph in Fig. 3 represents time (t). Line L15 in Fig. 3 represents the AC voltage Vt input to the transformer 34, and line L16 represents the welding current Iw applied to the upper electrode tip 21 and the lower electrode tip 26. The vertical axis for line L15 in Fig. 3 represents voltage (V), and the vertical axis for line L16 represents current (A).

[0035] When the inverter circuit 32 is driven, it outputs an AC voltage Vt having a rectangular waveform as shown in Fig. 3. The AC voltage output from the inverter circuit 32 is transformed in a transformer 34 and further rectified in a rectifier circuit 35, and a DC welding current Iw is applied to the workpiece W via the upper electrode tip 21 and the lower electrode tip 26.

[0036] As shown in Fig. 3, the welding current Iw increases as the duty ratio increases. The duty ratio is the ratio of a pulse width PW, which is the period during which the AC voltage Vt is Hi or Lo, to a predetermined carrier period T. The control device 33 determines the pulse width PW in accordance with a known feedback control law such as PI control so that the output current of the welding power supply circuit 3 detected by the current sensor 3d becomes a target current determined by processing (not shown), and drives the multiple switching elements in the inverter circuit 32 to turn ON / OFF using PWM control under the duty ratio determined by this pulse width PW.

[0037] (Spot welding method procedure) Next, the steps of a spot welding method for joining workpieces W using the above-described welding system 100 will be described.

[0038] 1 , the robot control device 62 controls the position and posture of the spot welding device 1 by driving the robot body 60 and the articulated arm 61 so that the workpiece W is placed between the upper electrode tip 21 and the lower electrode tip 26. At this time, the robot control device 62 controls the position and posture of the spot welding device 1 so that the lower electrode tip 26 abuts against the lower surface of the second metal plate W2 of the workpiece W.

[0039] Next, as shown in Fig. 4, the upper adapter body 23 is slid using the upper electrode tip moving mechanism 4, and the upper electrode tip 21 is brought closer to the lower electrode tip 26. Fig. 4 is a diagram showing a state in which a welding current is applied to the workpiece W while the workpiece W is clamped and pressurized between the upper electrode tip 21 and the lower electrode tip 26. When the upper electrode tip 21 approaches the lower electrode tip 26 and abuts against the upper surface of the first metal plate W1, the workpiece W is clamped and pressurized between the upper electrode tip 21 and the lower electrode tip 26.

[0040] Next, while maintaining the state in which the workpiece W is pressed from both sides by the upper electrode tip 21 and the lower electrode tip 26, the control device 33 of the welding power supply circuit 3 executes welding current control according to the procedure described with reference to Fig. 5, and causes a pulsed welding current to flow between the upper electrode tip 21 and the lower electrode tip 26. As a result, as shown in Fig. 4, a nugget N is formed between the first metal plate W1 and the second metal plate W2, and the first metal plate W1 and the second metal plate W2 are welded together.

[0041] (Welding current control procedure) The welding current control procedure and the welding current waveform will be described with reference to Figures 5 and 6. Figure 5 is a flow chart showing a specific procedure for welding current control in control device 33. Figure 6 is a diagram showing the welding current waveform achieved by the welding current control of Figure 5.

[0042] As described above, in the welding current control, a pulsed welding current is passed. The relationship between adjacent pulse waveforms of the welding current can be considered to be either a case where the pulse waveforms do not overlap or a case where the pulse waveforms do overlap. Figure 6 shows a case where adjacent pulse waveforms of the welding current do not overlap.

[0043] Before explaining the procedure for controlling the welding current, the graph in Figure 6 will be explained. The X axis of the graph is time (t) and the Y axis is current (A). The X axis, i.e., current 0, indicates the bottom current PB. Line L10 indicates the welding current, line L11 indicates the first pulse waveform of the welding current, and line L12 indicates the second pulse waveform of the welding current. The first pulse waveform L11 is the first wave of the continuous pulse wave, and the second pulse waveform L12 is the second wave. Furthermore, range R2 indicates the PWM pulse.

[0044] In the example shown in Figure 6, the first pulse waveform L11 and the second pulse waveform L12 of the welding current L10 do not overlap. After the current value of the first pulse waveform L11 drops to 0 V, i.e., the bottom current PB, the second pulse waveform L12 rises. The point in time when the current value of the first pulse waveform L11 drops to 0 V is indicated by time t4, and the point in time when the second pulse waveform L12 begins to rise is indicated by time t5. In the example shown in Figure 6, time t5 occurs after time t4.

[0045] (peak and non-peak conditions) As shown in Fig. 6, the welding current generated by performing the current control process of Fig. 5 has a pulsed waveform that alternates between a peak state in which the welding current reaches or remains within a preset peak current range R1 and a non-peak state in which the welding current decreases from within the peak current range R1 toward a bottom current PB (e.g., 0 V) ​​and then begins to increase again toward the peak current range R1. In Fig. 6, the peak state range is indicated by range R11, and the non-peak state range is indicated by range R12. Note that the peak current range R1 is the range in which the welding current is equal to or greater than a predetermined lower limit value L22 and equal to or less than a predetermined upper limit value L21.

[0046] (effective value of welding current) Line L14 in Figure 6 indicates the effective value of the welding current. Using the current detection signal transmitted from current sensor 3d, control device 33 can acquire the current current value Ipv, which is the current value of the welding current. Control device 33 calculates the effective value Irms of the welding current using the current current value Ipv. Specifically, control device 33 can calculate the effective value Irms by calculating the square root of the mean square of the current current value Ipv over the time that has elapsed since the start of welding current control up to the present time.

[0047] (Rising target current value) A target rising current value Isp, which is a target current value for the welding current, is preset in the control device 33. Some of the target rising current values ​​Isp are shown in Fig. 6 and other figures as points P1 to P4. A plurality of target rising current values ​​Isp are set within the current rising slope and the peak current range R1.

[0048] Point P1 shown in FIG. 6 is the rising target current value Isp set for the first pulse. Point P2 is the rising target current value Isp set for the second pulse. The same applies to points P3 and P4 below. The rising target current value Isp shown at point P1 will be referred to as the first rising target current value P1. The same applies to the other rising target current values ​​Isp.

[0049] The procedures for the current control process and the effective value control process included in the welding current control will be described with reference to Figure 5. In Figure 6, the period during which the current control process is performed is indicated by range R21, and the period during which the effective value control process is performed is indicated by range R22. In the following description and drawings, S1 represents step 1. The same applies to S2 and subsequent steps.

[0050] (S1) S1 is a step in which the controller 33 executes a current control process. The current control process is a process in which the controller 33 increases the welding current from a bottom current PB toward a peak current range R1, and then maintains the peak state R11 for a predetermined time.

[0051] (S2) In step S2, the control device 33 determines whether a predetermined slope time T4 has elapsed. If the determination result in step S2 is NO, the control device 33 returns to step S1 and continues to execute the current control process. If the determination result in step S2 is YES, the control device 33 proceeds to step S3.

[0052] As shown in FIG. 6, slope time T4 is the sum of current rise time T1 and peak maintenance time T2, and is a preset time. Current rise time T1 is the time it takes for the welding current to reach the upper limit L21 of peak current range R1 from bottom current PB, i.e., the time from time t1 to time t2. Peak maintenance time T2 is the time the welding current is maintained within peak current range R1, i.e., the time from time t2 to time t3. Slope time T4 is the time from time t1 to time t3.

[0053] (S3) S3 is a step in which control device 33 executes effective value control processing. Effective value control processing is processing in which control device 33 waits to execute current control processing R21 for a waiting time determined based on the effective value of the welding current. The waiting time determined based on the effective value of the welding current corresponds to current supply waiting time T3 shown in Figure 6. Current supply waiting time T3 is the time from time t3 when peak maintenance time T2 ends to time t5 when second pulse waveform L12 begins to rise.

[0054] (S4) S4 is a step in which the control device 33 determines whether a set current application time has elapsed since the start of welding current control. The current application time corresponds to the time required for the spot welding device 1 to weld one point on the workpiece W, and is a preset time. If the determination result in S4 is NO, the control device 33 returns to S1 and executes the current control process again. If the determination result in S4 is YES, the control device 33 ends the process of FIG. 5 to start welding the next point on the workpiece W.

[0055] As described above, in the welding current control, the control device 33 repeatedly executes the current control process (see S1) and the effective value control process (see S3) over the current flow time, thereby applying a welding current having a pulse waveform as shown in FIG. 6 between the upper electrode tip 21 and the lower electrode tip 26.

[0056] (When the first pulse waveform and the second pulse waveform overlap) The current control process and the welding current waveform have been outlined above with reference to FIG. 6, which shows an example in which the first and second pulse waveforms L11 and L12 of the welding current do not overlap. When the effective value of the welding current increases, the next pulse waveform may rise during the falling edge of the welding current pulse waveform. In this case, the pulse waveforms overlap. Below, we will explain the case in which the pulse waveforms overlap with reference to the drawings.

[0057] (Set welding current) Fig. 7 shows the change in welding current when the set welding current is changed. The set welding current is the reference current value at which current control is initiated to increase the welding current toward the peak current range. When the effective value of the welding current falls to the set welding current, current control is initiated to increase the welding current toward the peak current range.

[0058] Graph 71 on the left side of Fig. 7 shows the case where the pulse waveforms do not overlap. Graph 72 on the right side of Fig. 7 shows the case where the pulse waveforms overlap. In Fig. 7 and other figures, line L14 indicates the effective value of the welding current, and line L23 indicates the set welding current. Arrow D indicates the time when the next chopping waveform starts.

[0059] In the spot welding method of this embodiment, when the pulse waveform is in a non-peak state, current control is initiated to increase the welding current toward the peak current range when the effective value of the welding current reaches a set target range. The point at which the effective value of the welding current reaches the set target range is time t5, i.e., the point at which the second pulse waveform L12 begins to rise. The set welding current L23 is the upper limit of the above-mentioned target range. The range below the set welding current L23 is the above-mentioned target range.

[0060] Time t5 is the time when the effective value L14 corresponding to the first pulse waveform L11 drops to the set welding current L23. At time t5, welding current control is initiated to increase the welding current toward the peak current range. The welding current in graph 71 on the left side of FIG. 7 is the same as the welding current shown in FIG. 6. In the example shown in graph 71 on the left side, time t5 occurs after time t4, when the current value of the first pulse waveform L11 drops to 0. Therefore, the second pulse waveform L12 does not overlap with the first pulse waveform L11. At time t5, the second pulse waveform L12 rises from the bottom current PB toward the peak current range.

[0061] The welding current control shown in graph 72 on the right side of Figure 7 will now be described. In the example shown in graph 72 on the right side, the set welding current L23 is set to a higher current value than in the example of graph 71 on the left side. Therefore, time t5, at which the effective value of the welding current reaches the set target range, occurs before time t4, at which the current value of the first pulse waveform L11 is expected to drop to 0 V. Therefore, the first pulse waveform L11 and the second pulse waveform L12 of the welding current overlap. As such, when the set welding current value increases, adjacent waveforms may overlap. When the set effective value of the welding current increases, adjacent waveforms are more likely to overlap.

[0062] (Rising slope of pulse waveform) Next, we will explain the rising slope of the pulse waveform. The time it takes for the welding current to reach the peak current range is called the rise time T1. The welding current is controlled by a preset rise time T1. Therefore, when overlapping pulse waveforms occurs, the rising slope of the second pulse waveform L12, which is the shape of the second wave, may not be the same as the rising slope of the first pulse waveform L11, which is the shape of the first wave.

[0063] 7, the ranges R31 to R34 show the portions corresponding to the slope of the rising edge of the pulse waveform. In the graph 71 on the left, the slope of the rising edge of the waveform is the same for the first wave shown in range R31 and the second wave shown in range R32.

[0064] In contrast, in graph 72 on the right, the slope of the rising waveform differs between the rising edge of the first wave shown in range R33 and the rising edge of the second wave shown in range R34. The slope of the rising edge of the first wave shown in range R33 is the same as the slope of the rising edge shown in range R31 and the slope of the rising edge of the second wave shown in range R32 in graph 71 on the left. On the other hand, the rising edge of the second wave shown in range R34 differs from these slopes.

[0065] As described above, the welding current is controlled by the rise time T1. Therefore, when overlapping pulse waveforms occurs, the slope of the rise of the waveform changes between the first and second waves. Unlike the left graph 71, the current value at time t5 is not zero in the right graph 72. Therefore, in order to increase the welding current to the peak current range at the rise time T1 in the right graph 72, the slope must be gentler than when increasing the welding current to the peak current range at the rise time T1 in the left graph 71. This is because the current value to be increased is smaller in the right graph 72 than in the left graph 71.

[0066] If the slope of the rising edge of the waveform changes, the heat input condition to the welding point changes, resulting in a decrease in welding quality.

[0067] (Setting the first target current value) Fig. 8 is a diagram showing the waveform of the welding current in this embodiment. In Fig. 8, similar to the graph 72 on the right side of Fig. 7, time t5 at which the effective value L14 of the welding current falls within the set target range, i.e., at which the set welding current L23 is reached, occurs before time t4 at which the current value of the first pulse waveform L11 is expected to drop to 0 V.

[0068] In the spot welding method of this embodiment, a rising target current value is set for the welding current for each predetermined PWM pulse as the welding current rises toward the peak current range. Points P1 to P4 in Fig. 8 are some of the rising target current values. In the spot welding method of this embodiment, when the effective value of the welding current reaches a set target range while the welding current is decreasing toward the bottom current PB, a rising target current value that is greater than the rising start current value (rising start current) and closest to the rising start current value is set as the first target current value.

[0069] When the effective value L14 of the welding current reaches the set welding current L23 while the welding current L10 is decreasing toward the bottom current PB, the current value of the welding current at that time is not 0. The current value of the welding current at this time is set as the rise start current value S. This rise start current value S is set to be greater than at least one of the preset rise target current values. In this case, the rise target current value that is greater than the rise start current value S and closest to the rise start current value S is set as the first target current value.

[0070] Figure 9 is an enlarged view of the box R50 in Figure 8. As shown in Figure 9, the rise start current value S is greater than the first rise target current value P1. Therefore, a second rise target current value P2, which is greater than the rise start current value S and is the rise target current value closest to the rise start current value S, is set as the first target current value A. After time t5, the welding current value rises toward the peak current range in accordance with the second rise target current value P2 and the third rise target current value P3, which are the first target current value A.

[0071] Range R41 in FIG. 8 indicates a portion corresponding to the rising slope of the first pulse waveform L11. Range R42 indicates a portion corresponding to the rising slope of the second pulse waveform L12. In the spot welding method of this embodiment, unlike the example shown in graph 72 on the right side of FIG. 7, the rising slope of the second pulse waveform L12 shown in R42 is the same as the rising slope of the first pulse waveform L11 shown in R41. This is because, like the waveform of the first pulse waveform L11, the waveform of the second pulse waveform L12 also rises in accordance with a predetermined rising target current value.

[0072] In the spot welding method of this embodiment, when the effective value of the welding current reaches a set target range while decreasing toward the bottom current PB, the closest rising target current value to the rising start current value is set as the first target current value. This makes it possible to maintain a constant rising slope of the welding current even when the welding current is increased toward the peak current range while decreasing toward the bottom current PB. In other words, current control is possible to maintain a constant rising slope of the waveform even when pulse-shaped waveforms overlap. As a result, the heat input is stabilized, thereby preventing deterioration of welding quality.

[0073] (Processing flow) The process flow of the spot welding method of this embodiment will be described with reference to Fig. 11. Fig. 11 is a flowchart showing the process flow of the spot welding method of this embodiment.

[0074] (S11) In step S11, the control device acquires the effective value of the welding current. For example, the control device acquires the current value based on the current detection signal from the current sensor, and calculates the effective value of the welding current using the acquired current value.

[0075] (S12) S12 is a step in which, when the effective value of the welding current reaches the target range, it is determined whether the welding current value has decreased to the current value of the bottom current. When the effective value of the welding current acquired in S11 reaches a preset set welding current, the control device determines whether the value of the welding current is the current value of the bottom current. If the determination result is YES, the first pulse waveform and the second pulse waveform do not overlap. If the determination result is YES, the process proceeds to step S15. If the determination result is NO, the first pulse waveform and the second pulse waveform overlap. If the determination result is NO, the process proceeds to step S13.

[0076] (S13) In step S13, the welding current value when the effective value of the welding current reaches the target range is set to the rising current value, and the second pulse waveform starts rising from this rising current value toward the peak current range.

[0077] (S14) In S14, the first target current value is set to a rising target current value that is greater than the rising start current value and closest to the rising start current value among the preset rising target current values. Instead of setting the rising target current value of the first pulse among the preset rising target current values ​​as the first target current value of the second pulse waveform, the first target current value is set to the rising target current value that is closest to the rising start current value among the preset rising target current values. This makes the rising slope of the second pulse waveform the same as the rising slope of the first pulse waveform. After S4, the process ends.

[0078] (S15) In step S15, the current value of the bottom current is set to the rising start current value. If the determination result in step S12 is YES, the welding current value has decreased to the current value of the bottom current. Therefore, the current value of the bottom current becomes the rising start current value of the second pulse waveform.

[0079] (S16) In step S16, a predetermined target current value for the rise of the first pulse, i.e., the first pulse target current value, is set as the first target current value. If the first pulse waveform and the second pulse waveform do not overlap, the second pulse waveform also rises toward the peak current range using the predetermined target current value for the rise, just like the first pulse waveform. After step S16, the process ends.

[0080] (Second embodiment) A spot welding method according to a second embodiment of the present invention will be described. Regarding the second embodiment, differences from the first embodiment will be mainly described. Regarding the second embodiment, matters not specifically described can be the same as those in the first embodiment. The first and second embodiments differ in the way the target current value is set. In the first embodiment, a new target current value is set using only a preset target current value. In the second embodiment, a new current value is created, and the target current value is set using the preset target current value and the newly created current value. This makes it possible to suppress sudden changes in the pulse width.

[0081] (pulse width fluctuation) The fluctuation of the pulse width will now be described. In the spot welding method of this embodiment, the target current value is set to a second target current value C, which is the next largest rising target current value after the first target current value B, and an intermediate target current value is set between the first target current value and the second target current value. As shown in FIG. 9, since the first target current value A is smaller than the rising start current value S, the second target current value B is the second rising target current value P2. The third target current value C, which is the target current value next to the second target current value B, can also be set to a third rising target current value P3, which is the rising target current value next to the second rising target current value P2. However, in this case, the pulse width may change suddenly. This will be explained below.

[0082] Time T21 in FIG. 9 is the pulse width from the rising start current value S to the first target current value B (second rising target current value P2). Times T11 and T12 are the PWM pulse period. As shown in FIG. 9, when the preset rising target current values ​​are set in order as the first target current value B and the second target current value C, the pulse width may change suddenly. In other words, the difference between time T21 and time T22 may become large. Also, the second target current value C may not be reached.

[0083] Therefore, in the spot welding method of this embodiment, the second target current value C is set as the rising target current value next to the first target current value B, and an intermediate target current value B' is set between the first target current value B and the second target current value C. In FIG. 10, the intermediate target current value B' is indicated by point P21. Point P21, which is the intermediate target current value B', is located between the second rising target current value B and the third rising target current value C.

[0084] Setting the intermediate target current value B' can prevent the pulse width from changing suddenly. Time T31 in FIG. 10 is the pulse width from the rising start current value S to the first target current value B (second rising target current point P2). Time T32 is the pulse width from the first target current value B (second rising target current point P2) to the intermediate target current value B' (point P21). Time T33 is the pulse width from the intermediate target current value B' (point P21) to the second target current value C (third rising target current point P3). As shown in FIGS. 9 and 10, the difference between time T31 and time T32 and the difference between time T32 and time T33 are both smaller than the difference between time T21 and time T22. Also, while the second target current value C (point P3) is not reached in FIG. 9, P3 reaches the second target current value C in FIG. 10. By setting the intermediate target current value B' in this way, it is possible to suppress a sudden change in the pulse width and improve the performance of reaching the target current.

[0085] (Calculation of intermediate target current value) The intermediate target current value A' can be calculated by the following formula. A' = A + coefficient × (BA) A: First target current value A': Intermediate target current value B: Second target current value Coefficient: Any number greater than 0% and less than 100%

[0086] By adjusting the coefficient values ​​using the above formula, it is possible to further suppress abrupt changes in pulse width.

[0087] The difference in pulse width depending on whether or not an intermediate target current value is set will be described with reference to Figures 12 and 13. Figure 12 is a diagram showing the welding current when an intermediate target current is not set. Figure 13 is a diagram showing the welding current when an intermediate target current is set. In the example shown in Figure 13, the coefficient in the above formula for calculating the intermediate target current value is set to 30%. In the graphs shown in Figures 12 and 13, the X axis represents time (ms) and the Y axis represents welding current (A).

[0088] 12 and 13, the rise start current value S is between a second rise target current value P2, which is one of the preset rise target current values, and a third rise target current value P3, which is also one of the preset rise target current values. That is, FIGS. 12 and 13 show a case where the second wave is restarted near the third rise target current value P3.

[0089] The first target current value A is set to the third rising target current value P3, and the second target current value B is set to the fourth rising target current value P4.

[0090] In both the examples of FIGS. 12 and 13, the first target current value A is set to 10251 A, which is the third pulse target current value P3, and the second target current value B is set to 12807 A, which is the fourth pulse target current value P4.

[0091] (When the target current value is not set) First, a case where the target current value A' is not set will be described with reference to Fig. 12. In the graph shown in Fig. 12, time T31 is the pulse width from the rising start current value S to the first target current value A. Time T32 is the pulse width from the first target current value A to the second target current value B. The pulse width increased by 364 µs, from 116 µs at time T31 to 480 µs at time T32.

[0092] Regarding the current value, the current value reached at the first target current value A was 10,264 A, which was a difference of 13 A from the target current value of 10,251 A. The current value reached at the second target current value B was 11,944 A, which was a difference of 863 A from the target current value of 12,807 A. In other words, if the intermediate target current value A' is not set, the maximum difference between the target current value and the reached current value until the second target current value B is reached is 863 A.

[0093] (When setting the target current value) Next, setting of the intermediate target current value A' will be described with reference to Fig. 13. In Fig. 13, the intermediate target current value A' is indicated by point P31. The first target current value A and the second target current value B are the same as those in the example shown in Fig. 12. The intermediate target current value A' is set to 11018A. 11018A is a value calculated using a coefficient of 30% in the above-mentioned calculation formula for the intermediate target current value A'.

[0094] In the graph shown in FIG. 13, time T41 represents the pulse width from the rising start current value S to the first target current value A. Time T42 represents the pulse width from the first target current value A to the intermediate target current value A'. Time T43 represents the pulse width from the intermediate target current value A' to the second target current value B. The pulse width increases by 221 μs, from 116 μs at time T41 to 337 μs at time T42. The pulse width thereafter increases by 96 μs, from 337 μs at time T42 to 433 μs at time T43. Both the 221 μs and 96 μs increases are smaller than the 364 μs increase when the intermediate target current value A' is not set. By setting the intermediate target current value A', the difference between the first and second pulse widths is reduced by approximately 60%, from 364 μs to 221 μs.

[0095] Regarding the current value, the current value reached at the first target current value A was 10,264 A, which was a difference of 13 A from the target current value of 10,251 A. The current value reached at the intermediate target current value A' was 11,022 A, which was a difference of 4 A from the target current value of 11,018 A. The current value reached at the second target current value B was 12,814 A, which was a difference of 7 A from the target current value of 12,807 A. When the intermediate target current value A' was set, the maximum difference between the target current value and the reached current value until the second target current value B was reached was 13 A. By setting the intermediate target current value A', the maximum difference between the target current value and the reached current value until the second target current value B was reached was improved by approximately 66 times, from 863 A to 13 A.

[0096] It was also confirmed that the maximum difference between the target current value and the achieved current value could be improved by setting an intermediate target current value A', even for all pulses up to the time when the welding current reaches the peak current range.

[0097] When a pulse waveform rises while the next pulse waveform is falling, i.e., when the pulse waveforms overlap, the pulse width may fluctuate significantly immediately after the first pulse rises, resulting in a steep rise. In such cases, biased magnetization of the welding transformer may occur. In the spot welding method of this embodiment, the sudden fluctuation in pulse width can be suppressed by setting an intermediate target current between the first target current and the second target current.

[0098] (Control load reduction) The control device 33 performs various calculations, such as calculations for generating a gate drive signal and transmitting it to the inverter circuit 32. In the spot welding method of each of the above-described embodiments, the calculation for controlling the effective value can be configured not to include a rise target current value lower than a rise start current value, which is the current value at which current control for increasing the welding current is started. This will be described with reference to FIG. 8.

[0099] As shown in Fig. 8, when the first pulse waveform L11 and the second pulse waveform L12 overlap, among the preset rise target current values, a rise target current value lower than the rise start current value S is not used to control the rise of the second wave. In the example shown in Fig. 8, the first pulse target current value P1 is not used to control the rise of the second wave. Therefore, the first pulse target current value P1 is not included in the calculations of the control device 33.

[0100] In this way, by eliminating the rising target current value lower than the rising start current value S from the calculation targets, the control load can be reduced.

[0101] As described above, in a spot welding method according to one embodiment of the present invention, a target value for the rising current is set for each PWM pulse. When a chopping waveform is applied and the next chopping waveform is started during the falling edge of the previous chopping waveform, the target value that is larger than and closest to the current at which chopping starts is set as the first target value (A). Next, a target value (A') is added between the above target value and the next target value (B). The target value of the added A' is set as A' = A + coefficient × (BA). The coefficient is set to an arbitrary value greater than 0% and less than 100%. This makes it possible to realize a control method that prevents changes in the rising slope of the welding current and suppresses sudden changes in the PWM pulse, even when pulse waveforms overlap.

[0102] Although one embodiment of the present invention has been described above, the present invention is not limited to this, and the detailed configuration may be modified as appropriate within the scope of the spirit of the present invention. [Explanation of symbols]

[0103] L10 Welding current L11 First pulse waveform L12 Second pulse waveform L14 Effective value of welding current L23 Set welding current (target range) P Rising target current value R1 Peak current range R11 Peak state R12 Non-peak condition PB Bottom Current S Rising current value A First target current value B Second target current value A' intermediate target current value

Claims

1. The welding current is controlled by PWM control. the welding current has a pulse-like waveform in which a peak state in which the welding current reaches or is maintained within a set peak current range and a non-peak state in which the welding current drops from the peak current range to a bottom current and then rises again toward the peak current range, In spot welding in which, in the non-peak state, when the effective value of the welding current reaches a set target range, current control is started to increase the welding current toward the peak current range to join workpieces, The welding current is When rising towards the peak current range, a rising target current value is set for each predetermined PWM pulse; a spot welding method in which, when the effective value of the welding current reaches a set target range while descending toward a bottom current, a rise target current value that is greater than and closest to the rise start current value is set as a first target current value.

2. The rising target current value is a second target current value that is the next largest target current value after the first target current value; The spot welding method according to claim 1 , wherein an intermediate target current value is set between the first target current value and the second target current value.

3. In controlling the effective value of the welding current, The spot welding method according to claim 1 or 2, wherein the target rise current value lower than a rise start current value at the start of current control for increasing the welding current is not included in the calculation.

4. The intermediate target current value is calculated by the following formula: A' = A + coefficient × (B - A) A: First target current value A': Intermediate target current value B: Second target current value Coefficient: Any value between 0% and 100% The spot welding method according to claim 2 .

Images