Sputtering detection method

Abstract

The problem to be solved by the present invention is to provide a spot welding spatter detection method that can detect spatter generation without resetting the voltage detection line. To solve the above problem, in the spot welding method, a pulsed waveform welding current is provided to the workpiece by alternately performing the following steps in multiple cycles: a step of maintaining the welding current within a set peak current range; a step of decreasing the welding current from the peak current range towards the bottom current, and then increasing the welding current towards the peak current range when the effective value of the welding current reaches a set target range. In the spatter detection method, the pulse widths IW(1), IW(2), ... of the pulsed waveform in each cycle are measured. If the pulse width difference D(M) = IW(M) - IW(M-1) between the pulse width IW(M) in the target cycle (the Mth cycle) and the pulse width IW(M-1) in the previous cycle exceeds a width threshold Dth, then the generation of spatter is detected.

Description

Technical Field

[0001] This invention relates to a spatter detection method. More specifically, it relates to a spatter detection method for detecting the generation of spatter in spot welding. Background Technology

[0002] When welding multiple metal plates, spot welding is performed using a spot welding device. Spot welding involves clamping multiple metal plates, which serve as workpieces, between a pair of electrode chips, and then using an electric current between the pair of electrode chips to create a weld nugget between the multiple metal plates to weld them together.

[0003] In the spot welding method proposed by the applicant in Patent Document 1, multiple metal plates are held in a state by a pair of electrodes, and a pulsed waveform welding current is provided in multiple cycles, thereby welding multiple metal plates.

[0004] [Previous Technical Documents]

[0005] (Patent Documents)

[0006] Patent Document 1: International Publication No. 2020 / 050011 Summary of the Invention

[0007] [The problem the invention aims to solve]

[0008] However, in spot welding, a phenomenon known as "splatter" occurs during the application of welding current to the metal sheet, where a portion of the metal sheet melts and scatters. Since spatter can reduce weld strength, it is preferable to detect it immediately when it occurs.

[0009] Furthermore, if such a splash occurs, the voltage between the electrode chip pairs will drop sharply at that instant. Therefore, most conventional splash detection methods rely on voltage changes between the electrode chips to detect splash generation. However, in this case, a voltage detection line needs to be placed near the electrode chips to detect the voltage, which may increase costs.

[0010] The purpose of this invention is to provide a method for detecting spatter in spot welding, which can detect the generation of spatter without resetting the voltage detection line.

[0011] [Technical means to solve the problem]

[0012] (1) The spatter detection method of the present invention detects the generation of spatter in a spot welding method, which joins a workpiece (e.g., workpiece W described later) by providing a welding current. In the aforementioned spot welding method, the aforementioned welding current with a pulsed waveform is provided by alternately performing the following steps in multiple cycles: a step of maintaining the aforementioned welding current within a set peak current range; a step of decreasing the aforementioned welding current from the aforementioned peak current range toward a bottom current, and then increasing the welding current toward the aforementioned peak current range when the effective value of the aforementioned welding current reaches a set target range. In the aforementioned spatter detection method, the pulse width (e.g., pulse width IW(N) described later) of the aforementioned pulsed waveform in each cycle is measured, and if the difference between the pulse width in the target cycle and the pulse width in the previous cycle of the target cycle (e.g., pulse width difference D(M) described later) exceeds a predetermined width (e.g., width threshold Dth described later), the generation of spatter is detected.

[0013] (2) Preferably, in the aforementioned splash detection method, when the generation of splash is detected, the quality of the product manufactured by welding the aforementioned workpiece is estimated based on the timing of the detection of the splash generation.

[0014] (3) Preferably, in the aforementioned spot welding method, the slope time of the current rise time from the aforementioned bottom current to the aforementioned peak current range is fixed by adding the peak maintenance time of the aforementioned welding current within the aforementioned peak current range.

[0015] (The effect of the invention)

[0016] (1) If spatter occurs during spot welding, the inter-plate resistance of the workpiece will drop sharply. Therefore, if spatter occurs during the process of joining the workpiece with a pulsed welding current, the inter-plate resistance of the workpiece will drop sharply, and even the time constant for the welding current to drop from the peak current range to the bottom current will become drastically longer. Therefore, when the effective value of the welding current reaches the target range, current control is performed to provide a pulsed welding current that increases the welding current towards the peak current range. At this time, the difference between the pulse width of the welding current in the period in which spatter occurs and the pulse width of the welding current in the previous period before spatter occurs becomes larger. In this invention, by utilizing this phenomenon, the generation of spatter can be detected without resetting the voltage detection line. Moreover, in this invention, the generation of spatter is detected based on the comparison between the pulse width in the target period and the pulse width in the previous period before the target period. Thus, the generation of spatter can be detected without prior knowledge of the differences in the material properties of the workpiece to be welded or the influence of the welding equipment.

[0017] (2) If spatter is detected at the initial stage of welding, the weld diameter is expected to increase due to subsequent energization, thus ensuring sufficient weld strength for the product. Conversely, if spatter is detected at the later stage of welding, the weld diameter cannot be expected to increase due to subsequent energization, therefore, sometimes insufficient weld strength for the product cannot be ensured. In this invention, the quality of the product is estimated based on the timing of spatter detection. Therefore, it is unnecessary to re-inspect the quality of all products that experienced spatter during welding by visual inspection, thus reducing post-weld inspection time.

[0018] (3) If spatter occurs as described above, the inter-plate resistance will drop sharply, and the current fall time when the welding current drops from the peak current range to the bottom current will become longer. Therefore, in this invention, the slope time of the sum of the current rise time from the bottom current to the peak current range and the peak holding time when the welding current is maintained within the peak current range is fixed throughout the entire cycle. As a result, the change in current fall time caused by spatter can be reflected in the change in pulse width, and thus, the generation of spatter can be detected with higher accuracy. Attached Figure Description

[0019] Figure 1 This is a diagram illustrating the structure of a welding system that applies a spot welding method according to an embodiment of the present invention.

[0020] Figure 2 It is a diagram illustrating the circuit structure of a welding power supply circuit.

[0021] Figure 3 It is a diagram showing the relationship between the AC voltage input from the inverter circuit to the transformer and the welding current applied to the counter electrode chip in the welding power supply circuit.

[0022] Figure 4 It is a schematic diagram showing a cross-section of a workpiece during welding.

[0023] It is a diagram illustrating the state in which the workpiece is clamped by the upper electrode chip and the lower electrode chip, pressure is applied, and welding current is applied to the workpiece.

[0024] Figure 5 It is a flowchart illustrating the specific steps of welding current control in the control device.

[0025] Figure 6 It is a drawing application Figure 5 The waveform of the welding current is shown in the figure.

[0026] Figure 7 It is a flowchart illustrating the specific steps of the current control process.

[0027] Figure 8 It is a flowchart illustrating the specific steps of the effective value control process.

[0028] Figure 9 It is a flowchart illustrating the specific process of splash detection and treatment.

[0029] Figure 10 It is a diagram showing the pulse waveform of welding current over nine cycles.

[0030] Figure 11 It is a schematic illustration by Figure 5 The waveform of the welding current for one cycle is obtained by controlling the welding current.

[0031] Figure 12 It is a graph showing the relationship between the weld diameter formed when welding current is continuously supplied for a specified period and the timing of spatter generation. Detailed Implementation

[0032] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0033] Figure 1 This is a diagram illustrating the structure of a welding system S that applies the spot welding method and the spatter detection method of this embodiment.

[0034] The welding system S includes: a spot welding device 1, which is a welding gun; a workpiece W, which is a laminate of metal plates joined by the spot welding device 1; and a robot 6 for supporting the spot welding device 1.

[0035] Workpiece W is a laminate formed by stacking multiple metal plates. In this embodiment, the case where a laminate formed by stacking three metal plates sequentially from top to bottom is described as workpiece W, but the present invention is not limited to this. The three metal plates are the first metal plate W1, the second metal plate W2, and the third metal plate W3. The number of metal plates constituting workpiece W can be two or more. Furthermore, in the following description, the case where the thickness of the first metal plate W1 is thinner than the thickness of the second metal plate W2 and the third metal plate W3, and the first metal plate W1, the second metal plate W2, and the third metal plate W3 are made of the same metal, is described, but the present invention is not limited to this. It is sufficient that at least one of these metal plates W1 to W3 has a different rigidity than the other metal plates.

[0036] The robot 6 comprises: a robot body 60 mounted on the ground; an articulated arm 61 for axially supporting the robot body 60; and a robot control device 62 for controlling the robot 6. The articulated arm 61 comprises: a first arm portion 611, whose base end is axially supported by the robot body 60; a second arm portion 612, whose base end is axially supported by the first arm portion 611; a third arm portion 613, whose base end is axially supported by the second arm portion 612; and a fourth arm portion 614, whose base end is axially supported by the third arm portion 613, and a spot welding device 1 is mounted on its top end.

[0037] The robot control device 62 drives each arm 611 to 614 by driving multiple motors mounted on the robot body 60 and the articulated arm 61, and controls the position and orientation of the spot welding device 1 mounted on the fourth arm 614, so that the electrode chips 21, 26 (described later) mounted on the spot welding device 1 are moved to the joint of the workpiece W.

[0038] The spot welding device 1 includes: a welding power circuit 3, which is the source of welding current; a gun body 2, which is equipped with an upper electrode chip moving mechanism 4 and a part of the welding power circuit 3; an upper electrode chip 21 and a lower electrode chip 26 as a pair of electrodes; an upper electrode chip support 22; an upper adapter body 23; a gun arm 25; a lower electrode chip support 27; and a lower adapter body 28.

[0039] The upper electrode chip support 22 is a rod-shaped part extending vertically, with the upper electrode chip 21 mounted at its top end. The upper adapter body 23 is columnar, connecting the gun body 2 to the upper electrode chip support 22. The upper adapter body 23 is slidably disposed relative to the gun body 2 along a sliding direction parallel to the axis of the upper electrode chip support 22.

[0040] The gun arm 25 extends downwards and bends vertically from the gun body 2 towards the upper electrode chip 21. The lower electrode chip support 27 is a rod-shaped part coaxial with the upper electrode chip support 22, and the lower electrode chip 26 is mounted at its top end. The lower adapter body 28 is cylindrical and connects the top end of the gun arm 25 to the lower electrode chip support 27. Figure 1 As shown, the lower electrode chip 26 is supported by the lower electrode chip support 27 at a predetermined interval relative to the upper electrode chip 21 along the axis of the chip support portions 22, 27.

[0041] The upper electrode chip moving mechanism 4 includes a cylinder and its control device, which causes the upper adapter body 23, the upper electrode chip support 22, and the upper electrode chip 21 to slide forward and backward together in the sliding direction. Therefore, the upper electrode chip 21 can be pressed against the upper surface of the workpiece W while the lower electrode chip 26 is pressed against the lower surface of the workpiece W. Furthermore, these electrode chips 21 and 26 can be used to clamp the workpiece W for pressure application.

[0042] Figure 2 This is a diagram illustrating the circuit structure of the welding power supply circuit 3. The welding power supply circuit 3 includes: a welding control circuit 3a; a direct current (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 chip 21 and the lower electrode chip 26 via power lines L1 and L2. Additionally, as... Figure 1 As shown, the DC welding transformer 3b and current sensor 3d of the welding power circuit 3, configured as described above, are mounted on the gun body 2. Furthermore, the welding control circuit 3a of the welding power circuit 3 is mounted on a base separate from the gun body 2 and is connected to the DC welding transformer 3b via a power cable 3c. This reduces the weight of the gun body 2.

[0043] The welding control circuit 3a includes a converter circuit 31, an inverter circuit 32, and a control device 33. Additionally, the DC welding transformer 3b includes a transformer 34 and a rectifier circuit 35.

[0044] The converter circuit 31 converts the three-phase power input from the three-phase power supply 30 into DC power through full-wave rectification, and supplies this DC power to the inverter circuit 32.

[0045] Inverter circuit 32 converts the direct current input from converter circuit 31 into single-phase alternating current and outputs it to transformer 34 via power cable 3C. More specifically, inverter circuit 32 has four bridged switching elements. Inverter circuit 32 converts direct current into single-phase alternating current by turning these switching elements on or off according to a gate drive signal sent from the gate drive circuit mounted on control device 33.

[0046] Transformer 34 transforms the AC power input from inverter circuit 32 and outputs it to rectifier circuit 35. Rectifier circuit 35 rectifies the AC power input from transformer 34 and outputs DC power to the electrode chips 21 and 26 respectively connected to power lines L1 and L2. This rectifier circuit 35 uses, for example, a known full-wave rectifier circuit, which is constructed by combining two rectifier diodes 351 and 352 with a center tap 353.

[0047] The current sensor 3d detects the welding current supplied from the welding power supply circuit 3 to the chips 21 and 26. The current sensor 3d is provided on, for example, the power line L1 that connects the rectifier circuit 35 to the upper electrode chip 21, and sends a current detection signal corresponding to the magnitude of the welding current flowing through this power line L1 to the control device 33.

[0048] The control device 33 includes: a microcomputer that uses the current detection signal sent from the current sensor 3d to perform welding current control and spatter detection processing as described later; and a gate drive circuit that generates a gate drive signal based on the calculation result of the microcomputer and sends it to the inverter circuit 32.

[0049] Figure 3 This diagram illustrates 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 electrode chips 21 and 26 in the welding power supply circuit 3 described above.

[0050] After driving inverter circuit 32, such as Figure 3 As shown, a rectangular wave AC voltage Vt is output from inverter circuit 32. The AC voltage output from inverter circuit 32 is transformed in transformer 34 and further rectified in rectifier circuit 35. The DC welding current Iw is applied to workpiece W via electrode chips 21 and 26.

[0051] Here, as Figure 3 As shown, relative to the predetermined carrier period T, the larger the pulse width PW ratio (duty cycle) of the AC voltage Vt during Hi or Lo, the larger the welding current Iw. (See below for reference.) Figure 5 and Figure 6 As explained, the control device 33 determines the pulse width PW according to known feedback control rules such as PI control, so that the output current of the welding power supply circuit 3 detected by the current sensor 3d becomes the target current set by a process not shown, and uses PWM control to turn on or off multiple switching elements in the drive inverter circuit 32 under the duty cycle set by the pulse width PW.

[0052] Next, the sequence of the spot welding method for joining workpiece W using the welding system S described above will be explained.

[0053] First, such as Figure 1 As shown, 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 as to place the workpiece W between the upper electrode chip 21 and the lower electrode chip 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 chip 26 abuts against the lower surface of the third metal plate W3 of the workpiece W.

[0054] Next, as Figure 4 As shown, the upper electrode chip moving mechanism 4 is used to slide the upper adapter body 23, thereby bringing the upper electrode chip 21 closer to the lower electrode chip 26. After the upper electrode chip 21 approaches the lower electrode chip 26 and abuts against the upper surface of the first metal plate W1, the workpiece W is clamped by the upper electrode chip 21 and the lower electrode chip 26, and pressure is applied to it.

[0055] Next, the control device 33 of the welding power circuit 3 maintains the state of applying pressure to the workpiece W from both sides using electrode chips 21, 26, using a reference... Figure 5 The welding current control is executed in the specified order, and a pulsed welding current flows between the upper electrode chip 21 and the lower electrode chip 26. Thus, as... Figure 4 As shown, a first fusion nugget N1 is formed between the first metal plate W1 and the second metal plate W2, and a second fusion nugget N2 is formed between the second metal plate W2 and the third metal plate W3, and the first to third metal plates W1 to W3 are welded together.

[0056] Figure 5 This is a flowchart illustrating the specific steps of welding current control in control device 33. Figure 6 It is a drawing application Figure 5 The waveform of the welding current achieved by welding current control is shown in the figure. Figure 6 As shown, by performing Figure 5 The welding current generated by the welding current control has a pulsed waveform that alternates between peak and non-peak states. The peak state reaches or is maintained within a set peak current range, and the non-peak state decreases from the peak current range to a bottom current (e.g., 0) and then rises back towards the peak current range. That is, in Figure 5 Under the control of welding current, the following steps are performed alternately in multiple cycles (at least two cycles) to provide the workpiece W with the following... Figure 6 The pulsed waveform of the welding current shown refers to the process of maintaining the welding current within the peak current range; and the process of raising the welding current back towards the peak current range after the welding current has decreased from the peak current range toward the bottom current range.

[0057] Initially, in S1, the control device 33 sets the value of the counter N, which is used to count the number of cycles (pulse number) of the welding current pulses supplied to the workpiece W, to the initial value of 0, and then moves to S2.

[0058] Next, in S2, the control device 33 causes the counter N to increment by only 1 and then proceeds to S3 (N = N + 1).

[0059] Next, in S3, the control device 33 starts the pulse width timer T, which is used to measure the pulse width IW(N) of the welding current pulse in the Nth cycle, from the value of 0, and then proceeds to S4.

[0060] Next, in S4, control device 33 performs current control processing and then proceeds to S5. Then, as referred to... Figure 7As detailed in the description, in this current control process, after the control device 33 causes the welding current to rise from the bottom current to the peak current range, it maintains the peak state for a predetermined time.

[0061] Next, in S5, the control device 33 determines whether the predetermined slope time has elapsed. For example... Figure 5 As shown, this slope time is a preset time that combines the current rise time and the peak holding time. The current rise time is the time it takes for the welding current to rise from the bottom current to the upper limit of the peak current range, and the peak holding time is the time it takes for the welding current to remain within the peak current range. In other words, this slope time is fixed by the welding current pulses of all cycles. When the determination result of S5 is "no", the control device 33 returns to S4 to continue executing the current control process; and when the determination result of S5 is "yes", it proceeds to S6.

[0062] Next, in S6, control device 33 performs effective value control processing and then proceeds to S7. Then, as referred to... Figure 8 As detailed above, in this effective value control process, the control device 33 waits for a period of time (refer to) based on the effective value of the welding current. Figure 6 The current control process is awaited within the specified time.

[0063] Next, in S7, control device 33 stops the pulse width timer T started in S3, stores the pulse width timer T at this time as the pulse width IW(N) of the welding current pulse in the Nth cycle in a memory not shown, and proceeds to S8. That is, the pulse width IW(N) corresponds to the time from the start of the current control process in the Nth cycle to the end of the effective value control process in the Nth cycle (i.e., until the start of the current control process in the (N+1)th cycle). Figure 6 In the example, this corresponds to the time between time t1 and t5.

[0064] Next, in S8, the control device 33 determines whether the counter N has reached a preset predetermined number of cycles Nset. This predetermined number of cycles Nset corresponds to the number of welding current pulses required to join a point of the workpiece W by the spot welding device 1, and is preset according to the thickness and material properties of the workpiece W. When the determination result of S8 is "no", the control device 33 returns to S2 to start the next current control process. Moreover, when the determination result of S8 is "yes", the control device 33 moves to S9. Furthermore, in this embodiment, the case where the welding current is continued to be provided until the number of welding current pulse cycles reaches the preset predetermined number of cycles Nset has been described, but the present invention is not limited to this. For example, the welding current can be continued from the start of the first cycle of welding current control until a preset energizing time has elapsed.

[0065] Next, in S9, after performing the splash detection process, the control device 33 ends. Figure 5 The process begins to initiate the joining of workpiece W to the next point or another workpiece W. Then, as referred to... Figure 9 The details describe how, in this spatter detection process, it is determined whether spatter occurs during the period when welding current is supplied, the timing of spatter occurrence, and the quality of the product as a result of spatter occurrence.

[0066] As described above, in welding current control, the control device 33 repeatedly executes current control processing (refer to S4) and effective value control processing (refer to S6) during the energizing time to control the welding current. Figure 6 The pulsed welding current shown is applied between electrode chips 21 and 26.

[0067] Figure 7 It is a flowchart illustrating the specific steps of the current control process.

[0068] Initially, in S11, the control device 33 acquires the current current value Ipv using the current detection signal sent from the current sensor 3d, and then proceeds to S12. The current current value Ipv is the current value of the welding current. In S12, the control device 33 sets a target current value Isp, which is equivalent to the target value for the welding current, and then proceeds to S13. Figure 6 As shown, the target current value Isp is set between a predetermined current rise slope or between the upper and lower limits of the peak current range.

[0069] In S13, the control device 33 calculates the effective value of the welding current Irms using the current current value Ipv obtained in S11, and then proceeds to S14. More specifically, the control device 33 calculates the effective value of the welding current Irms by using the current current value Ipv obtained in S11. Figure 5 The effective value Irms is calculated by taking the square root of the mean square of the current value Ipv during the time elapsed since the welding current was controlled.

[0070] In S14, the control device 33 calculates the current deviation Idev by subtracting the current current value Ipv obtained in S11 from the target current value Isp set in S12, and then transfers the result to S15.

[0071] In S15, the control device 33 calculates the pulse width PW based on the feedback control rule (specifically, for example, PI control rule) calculated in S14 with the current deviation Idev equal to 0, and then proceeds to S16. More specifically, the control device 33 calculates the pulse width PW by adding the value obtained by multiplying the current deviation Idev by a predetermined proportional gain Kp to the integral value of the current deviation Idev multiplied by a predetermined integral gain Ki.

[0072] In S16, control device 33 starts PW counting and proceeds to S17. In S17, control device 33 opens the switching element provided on inverter circuit 32 and proceeds to S18. In S18, control device 33 determines whether the PW count value is 0, that is, whether a time equivalent to the pulse width PW has elapsed since PW counting started in S16. If the determination result of S18 is "no", control device 33 returns to S17 and continues to open the switching element; if the determination result of S18 is "yes", it proceeds to S19.

[0073] In S19, control device 33 shuts down the switching element installed on inverter circuit 32 and proceeds to S20. In S20, control device 33 determines whether a set carrier cycle has elapsed since the switching element was turned on in S17. If the determination result of S20 is "no", control device 33 returns to S19 and continues to shut down the switching element; if the determination result of S20 is "yes", it proceeds to... Figure 5 S5.

[0074] Figure 8 It is a flowchart illustrating the specific steps of the effective value control process.

[0075] In S31, the control device 33 begins a waiting time count to measure the power-on waiting time (refer to) equivalent to the execution time of the effective value control processing. Figure 6 Then, the process proceeds to S32. In S32, the control device 33 acquires the current current value Ipv using the current detection signal sent from the current sensor 3d, and then proceeds to S33. In S33, the control device 33 calculates the effective value Irms of the welding current using the current current value Ipv acquired in S32, following the same order as in S13, and then proceeds to S34.

[0076] In step S34, the control device 33 determines whether the effective value Irms calculated in S33 falls within the target effective value range (Itrg_min ≤ Irms ≤ Itrg_max) set between the predetermined lower limit value Itrg_min and the predetermined upper limit value Itrg_max. Furthermore, when the determination result of S34 is "yes", the control device 33 begins the current control process of the next cycle, causing the welding current to rise again towards the peak current range and shifting to... Figure 5 The S7.

[0077] Furthermore, if the determination result of S34 is "No", the control device 33 proceeds to S35 to determine whether the value of the waiting time count that started in S31 is 0; in other words, it determines whether the power-on waiting time has exceeded the predetermined time. If the determination result of S35 is "No", the control device 33 returns to S32, reduces the effective value Irms, and waits to start the current control process of the next cycle until it reaches the effective value target range. Conversely, if the determination result of S35 is "Yes", that is, if the effective value Irms has not reached the effective value target range within the predetermined time, the control device 33 proceeds to S36, notifying the operator of an abnormality by, for example, illuminating a warning light, and ending the welding current control.

[0078] Next, regarding the waveform of the welding current generated by performing the welding current control as described above, refer to... Figure 6 Please provide a detailed explanation.

[0079] Initially, between times t1 and t3, control device 33 repeatedly executes... Figure 7 The current control process shown continues until a preset slope time has elapsed. (Refer to...) Figure 7 As explained, in this current control process, a target current value Isp is set, and PI control is used to determine the pulse width PW so that the current current value Ipv obtained by the current sensor 3d becomes the target current value Isp. Under this pulse width PW, PWM control is used to drive the inverter circuit 32. Thus, as... Figure 6 As shown, after time t1, the welding current rises from the bottom current towards the peak current range, reaching the upper limit of the peak current range at time t2. Then, after time t2, the welding current is maintained within the peak current range using PI control in control device 33. Then, at time t3, corresponding to a preset slope time elapsed after the current control process started at time t1 (refer to S5), control device 33 ends the current control process (refer to S4) and begins the effective value control process (refer to S6).

[0080] By performing the current control process described above, a welding current is applied to the workpiece W and maintained within the peak current range. Thus, as... Figure 4 As shown, the growth of molten nuclei N1 and N2 is promoted between the first metal plate W1 and the second metal plate W2, and between the second metal plate W2 and the third metal plate W3, respectively. Here, as... Figure 4As shown, since the thickness of the first metal plate W1 is thinner than that of the second metal plate W2 and the third metal plate W3, the first metal plate W1 is more easily deformed under pressure. Therefore, the contact area between the first metal plate W1 and the second metal plate W2 is greater than the contact resistance between the second metal plate W2 and the third metal plate W3. Consequently, the contact resistance between the first metal plate W1 and the second metal plate W2 is smaller than that between the second metal plate W2 and the third metal plate W3. Therefore, the Joule heat generated between the first metal plate W1 and the second metal plate W2 due to the welding current flowing through the contact resistance is greater than that between the second metal plate W2 and the third metal plate W3. Therefore, at peak conditions, the growth rate of the weld nugget N2 formed between the second metal plate W2 and the third metal plate W3 is faster than the growth rate of the weld nugget N1 formed between the first metal plate W1 and the second metal plate W2.

[0081] return Figure 6 Between time t3 and t5, control device 33 executes the reference... Figure 8 The effective value control process is described below. In this effective value control process, the control device 33 calculates the effective value Irms of the welding current (refer to S33) and stops driving the inverter circuit 32 until this effective value Irms reaches the effective value target range. Therefore, after time t3, the welding current rapidly decreases towards the bottom current, reaching the bottom current at time t4. Then, at time t5, corresponding to the effective value Irms decreasing and reaching the effective value target range, the control device 33 ends the effective value control process and begins the current control process for the next cycle. Therefore, after time t5, the welding current rises again from the bottom current towards the peak current range.

[0082] By performing the effective value control process described above, the inverter circuit 32 is stopped during the power-on waiting period before the effective value Irms of the welding current reaches the effective value target range. Therefore, during the effective value control process, the welding current is maintained below the lower limit of the peak current range, and the weld nuggets N1 and N2 generated between the metal plates are cooled by heat dissipation. Here, as described above, the thickness of the first metal plate W1 is thinner than the thickness of the second metal plate W2 and the third metal plate W3. Therefore, the heat dissipation between the second metal plate W2 and the third metal plate W3 is less than the heat dissipation between the first metal plate W1 and the second metal plate W2. When maintaining the welding current below the peak current range, the cooling amount achieved by the heat dissipation of the weld nugget N2 is greater than the cooling amount achieved by the heat dissipation of the weld nugget N1. As described above, since the growth rate of weld nugget N2 in the peak state is faster than that of weld nugget N1, by maintaining the welding current below the peak current range during the energizing waiting time and promoting the cooling of weld nugget N2, spatter generation between the second metal plate W2 and the third metal plate W3 can be suppressed. Furthermore, in this effective value control process, by starting the next cycle of current control processing when the effective value Irms corresponding to the welding current reaches the effective value target range, the welding current increases again, and the energy required for the growth of weld nuggets N1 and N2 between each metal plate W1 to W3 can be controlled. Therefore, as described above, spatter generation can be suppressed and the workpieces can be firmly bonded.

[0083] Figure 9 It is a flowchart illustrating the specific steps of splash detection and treatment.

[0084] Figure 10 It is a diagram showing the pulsed waveform of the welding current over nine cycles. Furthermore, Figure 10 The image shows the splashing that occurred in the fourth cycle.

[0085] Initially, in S51, the control device 33 sets the counter M, which counts the number of welding current pulses, to the initial value of 1, and then proceeds to S52.

[0086] Next, in S52, the control device 33 causes the counter M to increment by only 1, and then proceeds to S53 (M = M + 1).

[0087] Next, in S53, the control device 33 takes the Mth cycle as the target cycle, calculates the difference between the pulse width IW(M) of the welding current pulse in the target cycle and the pulse width IW(M-1) of the welding current pulse in the previous cycle (i.e., the M-1 cycle), which is the pulse width difference D(M) of the target cycle, and enters S54 (D(M)=IW(M)-IW(M-1)).

[0088] Next, in S54, the control device 33 determines whether the counter M has reached the specified number of cycles Nset. When the determination result of S54 is "no", the control device 33 returns to S52; and when the determination result of S54 is "yes", it proceeds to S55.

[0089] Next, in S55, the control device 33 determines whether there is a difference in the pulse width difference D(2),...,D(Nset) of Nset-1 cycles that exceeds the preset width threshold Dth, in order to determine whether splashing occurs.

[0090] Figure 11 It is a schematic illustration by Figure 5 The waveform of the welding current, achieved through welding current control, is shown in a diagram representing one cycle. Furthermore, Figure 11 The waveform of the welding current in the target cycle is represented by dashed lines, and the waveform of the welding current in the previous cycle is represented by solid lines.

[0091] As mentioned above, in Figure 5 In welding current control, the slope time of the sum of the current rise time between t11 and t12 and the peak duration between t12 and t13 is used to perform current control processing (see reference). Figure 7 The time for this is fixed throughout the entire cycle; therefore, the target cycle is equal to the previous cycle. In contrast, due to the execution of effective value control processing (refer to...), Figure 8 The effective value of the welding current Irms in this cycle will continue until it reaches the effective value target range. Therefore, the time required from the start of the effective value control process to the end is different between the target cycle (time t13 to t15) and the previous cycle (time t13 to t14).

[0092] In the effective value control process, which involves waiting for the current to be controlled based on the effective value of the welding current, the welding current decreases exponentially under a time constant τ (τ∝1 / R) that is inversely proportional to the inter-plate resistance R of the workpiece W. Therefore, if, for example, spatter occurs in the target cycle and the inter-plate resistance R decreases sharply, the current fall time (times t13 to t15) required for the effective value of the welding current to decrease to the target effective value range is longer than the current fall time (times t13 to t14) in the previous cycle before the spatter occurred. Therefore, whether spatter occurs and the timing of its occurrence can be specified by comparing the pulse width of the welding current in the target cycle with the pulse width of the welding current in the previous cycle.

[0093] Return to Figure 9When the determination result of S55 is "No", that is, the pulse widths D(2),...,D(Nset) of Nset-1 cycles are all less than the width threshold Dth, the control device 33 moves to S56, determines that no splashing is generated, and moves to S57. In S57, the control device 33 determines that the quality of the product manufactured by joining workpiece W is good, and ends the process. Figure 9 The processing.

[0094] Furthermore, when the determination result of S55 is "yes", that is, at least one of the pulse widths D(2), ..., D(Nset) of Nset-1 cycles exceeds the width threshold Dth, the control device 33 moves to S58, detects the generation of splash, and moves to S59.

[0095] In S59, the control device 33 calculates the splash generation cycle P corresponding to the timing of the splash, and proceeds to S60. More specifically, the control device 33 acquires the cycles that exceed the width threshold Dth among the pulse widths D(2),...,D(Nset) of Nset-1 cycles, and sets this cycle as the splash generation cycle P. If there are multiple cycles that exceed the width threshold Dth among the pulse widths D(2),...,D(Nset) of Nset-1 cycles, it is preferable to set the last cycle that exceeds the width threshold Dth as the splash generation cycle P.

[0096] In S60, the control device 33 determines whether the splash generation cycle P is less than the cycle threshold Pth set between 2 and Nset, so as to determine the quality of the product based on the timing of splash generation.

[0097] Figure 12 This is a graph showing the relationship between the weld diameter (i.e., weld strength) and the timing of spatter generation when a welding current is continuously supplied for a specified period Nset. For example... Figure 12 As shown, the later the spatter occurs, the smaller the final weld diameter and therefore the lower the weld strength. This is generally believed to be because if the spatter occurs in the early stages of welding, subsequent energization may lead to an increase in the weld diameter; while if the spatter occurs in the later stages of welding, subsequent energization may result in insufficient increase in the weld diameter.

[0098] Return to Figure 9 When the determination result of S60 is "yes", that is, the splash generation cycle P is earlier than the cycle threshold Pth, the control device 33 moves to S57, and determines that the product quality is good and ends. Figure 9 The processing is as follows. Furthermore, when the determination result of S60 is "no," meaning the splash generation cycle P is later than the cycle threshold Pth, the control device 33 moves to S61, determines the product quality is defective, and ends the process. Figure 9 The processing.

[0099] Furthermore, preferably, for products that are determined to be of poor quality through the above-mentioned splash detection process, their quality is reconfirmed by visual inspection or the like.

[0100] The splash detection method according to this embodiment achieves the following effects:

[0101] (1) If spatter occurs during spot welding, the inter-plate resistance of the workpiece W will drop sharply. Therefore, if spatter occurs during the process of joining the workpiece W with a pulsed welding current, the inter-plate resistance of the workpiece W will drop sharply, and even the time constant for the welding current to drop from the peak current range to the bottom current will become drastically longer. Therefore, when the effective value of the welding current reaches the target range, current control is performed to provide a pulsed welding current that increases the welding current towards the peak current range. At this time, the difference between the pulse width of the welding current in the period in which spatter occurs and the pulse width of the welding current in the previous period before spatter occurs becomes larger. In this embodiment, by utilizing this phenomenon, the generation of spatter can be detected without resetting the voltage detection line. Moreover, in this embodiment, the generation of spatter is detected based on the comparison between the pulse width in the target period and the pulse width in the previous period before the target period. Thus, the generation of spatter can be detected without prior knowledge of the differences in the material properties of the workpiece W to be welded or the influence of the welding equipment.

[0102] (2) If the spatter generation period P is in the early stage of welding before the period threshold Pth, the weld diameter is expected to increase due to subsequent energization, thus ensuring sufficient weld strength of the product. Conversely, if the spatter generation period P is detected in the later stage of welding after the period threshold Pth, the weld diameter cannot be expected to increase due to subsequent energization, therefore, sometimes it is not possible to ensure sufficient weld strength of the product. In this embodiment, the quality of the product is determined based on the spatter generation period P. Therefore, it is not necessary to re-inspect the quality of all products that generated spatter during the welding process by visual inspection, thus reducing post-weld inspection time.

[0103] (3) If spatter occurs as described above, the inter-plate resistance will drop sharply, and the current fall time when the welding current drops from the peak current range to the bottom current will become longer. Therefore, in this embodiment, the slope time of the sum of the current rise time from the bottom current to the peak current range and the peak maintenance time when the welding current is maintained within the peak current range is fixed throughout the entire cycle. As a result, the change in current fall time caused by spatter can be reflected in the change in pulse width, and thus, the generation of spatter can be detected with higher accuracy.

[0104] The above description illustrates one embodiment of the present invention, but the invention is not limited thereto. Appropriate modifications to the details can be made within the scope of the invention's intent.

[0105] Furthermore, in the above embodiments, the case where spatter detection processing is performed after the welding current is provided within a predetermined period Nset to detect the generation of spatter has been described. However, the timing of performing spatter detection processing is not limited to this. As described above, in the spatter detection processing of the present invention, spatter generation is detected by comparing the pulse width of the welding current in the target period with the pulse width of the welding current in the previous period, thereby enabling spatter detection processing to be performed simultaneously with the provision of welding current.

[0106] Figure Labels

[0107] S: Welding system

[0108] W: Workpiece

[0109] 1: Spot welding equipment

[0110] 2: Welding torch body

[0111] 21: Upper electrode chip

[0112] 26: Lower electrode chip

[0113] 3: Welding power supply circuit

[0114] 3a: Welding control circuit

[0115] 3b: DC welding transformer

[0116] 3D: Current Sensor

[0117] 31: Converter Circuit

[0118] 32: Inverter circuit

[0119] 33: Control device

[0120] 34: Transformer

[0121] 35: Rectifier circuit

[0122] L1, L2: Power lines

[0123] Nset: Specified period

[0124] IW(1),…,IW(Nset): Pulse width

[0125] D(2),…,D(Nset): Pulse width difference

[0126] Dth: Width threshold

Claims

1. A spatter detection method for detecting the generation of spatter in a spot welding method, wherein the spot welding method joins workpieces by providing a welding current. In the aforementioned spot welding method, the aforementioned welding current with a pulsed waveform is provided by alternately performing the following steps in multiple cycles: a step of maintaining the aforementioned welding current within a set peak current range; a step of decreasing the aforementioned welding current from the aforementioned peak current range toward a bottom current, and then increasing the welding current toward the aforementioned peak current range when the effective value of the aforementioned welding current reaches a set target range. In the aforementioned spot welding method, the slope time of the current rise time from the aforementioned bottom current to the aforementioned peak current range and the peak maintenance time of the aforementioned welding current within the aforementioned peak current range are fixed. The pulse width of the aforementioned pulsed waveform in each period is measured, and the generation of splash is detected when the difference between the pulse width in the target period and the pulse width in the previous period exceeds a predetermined width.

2. The splash detection method according to claim 1, wherein, When spatter is detected, the quality of the product manufactured by welding the aforementioned workpiece is estimated based on the timing of the detection of the spatter.

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