Spot welding system and spot welding method

Abstract

To provide a spot welding system and a spot welding method that are able to accurately detect a defect of a welding dot.SOLUTION: A spot-welding system 100 includes: a spot-welding device 10 that performs spot-welding at a plurality of to-be-welded spots P1 in such a manner that electrical current is supplied between a pair of electrodes 4 while the to-be-welded spots P1 of a to-be-welded material W1 composed of a plurality of stacked metal plates is held and pressed between the pair of electrodes 4, thereby welding the to-be-welded material W1; and a welding management device 40 that determines that the to-be-welded spot P1 is spot-welded a plurality of times when an amount of shrinkage of a plate thickness of the to-be-welded spot P1 decreases below a determination threshold through a period of time from when the supply of the electrical current is started to when the pair of electrodes 4 separates from the to-be-welded material W1.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Spot welding is known, in which a workpiece made of multiple overlapping metal plates is welded by clamping the welding target points between a pair of electrodes and applying pressure between the pair of electrodes to weld the workpiece. For example, when manufacturing an automobile body, spot welding is performed on multiple welding target points, with more than 2000 welding points per vehicle. For this reason, there is a risk of missing welding points due to a programming error in the welding robot program, for example, when starting up a new model or when reassigning welding points. Since missing welding points is a serious quality issue, there is a strong demand for detecting missing welding points and preventing the outflow of vehicles with missing welding points.

[0003] Patent Document 1 discloses a welding management system having a welding control device that controls a welding machine arranged on a production line for workpieces, and a welding management device capable of communicating with each welding control device on the production line, wherein the welding control device comprises: a control means that controls the welding machine in accordance with welding teaching data including welding point positions and welding point numbers that are preset for each specification of the workpiece; a control number acquisition means that acquires a control number assigned to each workpiece; and a transmission means that transmits welding data including the welding point numbers of the welding point positions where welding has been completed and the control number assigned to the workpiece, and the welding management device comprises: a receiving means that receives the welding data transmitted from the welding control device, a welding point number list holding means that holds a welding point number list for each workpiece, and a defect determination means that identifies a workpiece on the basis of the control number included in the welding data received by the receiving means, and compares the welding point numbers included in the welding data with the welding point number list for the workpiece held by the welding point number list holding means to determine whether or not there is a defect in a welding point. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2006-95542 A Summary of the Invention [Problem to be solved by the invention]

[0005] According to the technology described in Patent Document 1, it is possible to reliably check for defects at each welding point, and if a defect is found, it is possible to identify the location of the defect. However, the technology described in Patent Document 1 has a problem in that it is unable to detect defects at welding points that may occur when the same welding target location is welded multiple times, resulting in a decrease in detection accuracy.

[0006] The present invention has been made to solve such problems, and has an object to provide a spot welding system and a spot welding method that are capable of detecting defects in a weld point with high accuracy. [Means for solving the problem]

[0007] A spot welding system according to one embodiment includes a spot welding device that performs spot welding at multiple welding locations, in which the welding locations of a workpiece consisting of multiple stacked metal plates are sandwiched between a pair of electrodes and pressure is applied while current is passed between the pair of electrodes to weld the workpiece, and a welding management device that determines that the same welding location has been spot welded multiple times if the amount of contraction of the plate thickness of the welding location from the start of current flow to the time the pair of electrodes separates from the welded material falls below a determination threshold.

[0008] In addition, a spot welding method according to one embodiment includes a welding process for performing spot welding at a plurality of welding target locations, in which the welding target locations of a workpiece consisting of a plurality of stacked metal plates are sandwiched between a pair of electrodes and pressure is applied while current is passed between the pair of electrodes to weld the workpiece, and a determination process for determining that the same welding target location has been welded multiple times if the amount of contraction of the plate thickness of the welding target location from the start of current passage to the time when the pair of electrodes separate from the welded material falls below a determination threshold. Effect of the Invention

[0009] According to the present invention, it is possible to provide a spot welding system and a spot welding method capable of detecting defects in a weld point with high accuracy. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram showing a configuration of a spot welding system according to a first embodiment. [Diagram 2] FIG. 13 is a diagram illustrating an example of a defect at a welding point. [Diagram 3] 1A to 1C are diagrams illustrating a spot welding method including detection of a defect at a welding point using an expansion amount waveform during welding. [Figure 4] 1 is a diagram illustrating the amount of contraction in plate thickness when the same welding location is spot-welded multiple times; FIG. [Diagram 5] 1A to 1C are diagrams illustrating a spot welding method including detection of a defect at a welding point using a resistance waveform during welding. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] First embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the present disclosure is not limited to the following embodiment. In addition, in order to clarify the description, the following description and drawings are appropriately simplified. In the following description, the same or equivalent elements are given the same reference numerals, and duplicate descriptions are omitted.

[0012] FIG. 1 is a diagram showing a configuration of a spot welding system according to a first embodiment. The spot welding system 100 will be described with reference to FIG. 1. The spot welding system 100 shown in FIG. 1 includes a spot welding apparatus 10 and a welding management apparatus 40. The spot welding apparatus 10 includes a spot welding robot 20 including a robot body R and a welding gun G attached to the tip of the robot body R. The spot welding apparatus 10 further includes a robot control apparatus (not shown) that controls the robot body R and a welding control apparatus 30 (welding timer) that controls the welding gun G. The welding management apparatus 40 is communicably connected to the welding control apparatus 30 via a network.

[0013] The robot body R is a multi-joint arm having multiple rotation axes 1 driven by servo motors for driving the rotation axes. The servo motors for driving the rotation axes are driven by control signals from a robot control device. The robot body R can change the position and posture of the welding gun G by driving the servo motors for driving the rotation axes under the control of the robot control device. The base end of the robot body R is supported on the ground or on a pedestal fixed to the ground.

[0014] The welding gun G has a mounting part 2 attached to the tip of the robot body R, a U-shaped gun arm 3 fixed to the lower part of the mounting part 2, a pair of electrodes 4 consisting of a movable electrode 5 and a fixed electrode 6, an electrode lifting device 7, and a current adjusting device 8.

[0015] The movable electrode 5 is disposed above the workpiece W1. The fixed electrode 6 is disposed below the workpiece W1. The movable electrode 5 and the fixed electrode 6 are disposed coaxially facing each other. The workpiece W1, which is made up of metal plates W2 and W3, is disposed between the movable electrode 5 and the fixed electrode 6 in a stacked manner. In this embodiment, the workpiece W1 is made up of two metal plates, but may be made up of three or more metal plates. The movable electrode 5 and the fixed electrode 6 each come into contact with the workpiece W1 during welding.

[0016] The electrode lifting device 7 is an electric device that holds and lifts the movable electrode 5. The electrode lifting device 7 has a servo motor 71 and a lifting member 72 that is connected to the output shaft of the servo motor 71 via a gear. The servo motor 71 is driven by a control signal from the welding control device 30. The electrode lifting device 7 can lift and lower the lifting member 72 by driving the servo motor 71 under the control of the welding control device 30. The movable electrode 5 is attached to the lifting member 72 so as to protrude downward. The fixed electrode 6 is attached to the lower part of the gun arm 3 so as to protrude upward. The lifting member 72 is lowered by the servo motor 71, so that the welded material W1 is sandwiched between the pair of electrodes 4. The electrode lifting device 7 applies a pressure force in the plate thickness direction to the welded material W1 arranged between the pair of electrodes 4 by moving the movable electrode 5 up and down.

[0017] The current adjusting device 8 applies a current to the workpiece W1 held between the pair of electrodes 4. The current adjusting device 8 adjusts the current (hereinafter, welding current) applied between the pair of electrodes 4 in response to a control signal transmitted from the welding control device 30.

[0018] The spot welding device 10 sandwiches and pressurizes the workpiece W1 between a pair of electrodes 4, passes current between the pair of electrodes 4 to melt the workpiece W1, and then stops passing current and maintains the workpiece W1 in a pressurized state, thereby forming a weld having a nugget N in the workpiece W1. A plurality of such spot welding robots 20 are provided, for example, near a production line for assembling automobile bodies. A dedicated welding control device 30 is connected to each of the plurality of spot welding robots 20.

[0019] Each of the robot control device and the welding control device 30 is configured by a computer including a CPU, a memory including a ROM and a RAM, an input / output interface, etc. (not shown). The robot control device is connected to the welding control device 30 so as to be able to communicate with each other.

[0020] The memory of the robot control device stores, for example, teaching data input from a teaching controller (not shown) in a rewritable format. The teaching data includes welding teaching information, which is the position and posture of the robot body R and the welding gun G when sequentially spot welding a plurality of welding target locations P1 preset for each workpiece W1, and welding condition information preset for each workpiece W1. The welding conditions include a pressure pattern of the pressure applied by the pair of electrodes 4 to the workpiece W1, a current pattern of the welding current applied to the workpiece W1 via the pair of electrodes 4, a squeeze time T1, a pre-current time T2, a main current time T3, a holding time T4, etc.

[0021] A welding control program is stored in the memory of welding control device 30. A CPU of welding control device 30 functions as a resistance value calculation unit 31 and an expansion amount calculation unit 32 by executing the welding control program pre-stored in the memory.

[0022] Welding control device 30 is electrically connected to voltage measurement unit 81, current measurement unit 82, electrode position measurement unit 83, and pressure measurement unit 84. Measurement values ​​from voltage measurement unit 81, current measurement unit 82, electrode position measurement unit 83, and pressure measurement unit 84 are input to welding control device 30. The memory of welding control device 30 temporarily stores the measurement values ​​from each measurement unit (voltage measurement unit 81, current measurement unit 82, electrode position measurement unit 83, and pressure measurement unit 84) and information such as the plate thickness of workpiece W1.

[0023] The voltage measuring unit 81 measures the voltage between the pair of electrodes 4 (hereinafter, welding voltage). The voltage measuring unit 81 is, for example, a voltage sensor attached near the movable electrode 5 and the fixed electrode 6. The current measuring unit 82 measures the welding current passed through the workpiece W1. The current measuring unit 82 is, for example, a current sensor attached to a current path through which the welding current flows. The electrode position measuring unit 83 measures the stroke by detecting the position of the movable electrode 5. The stroke is the amount of the movable electrode 5 pushed in and pushed back after coming into contact with the workpiece W1 (more specifically, the metal plate W2). The electrode position measuring unit 83 is, for example, an encoder that detects the rotation angle position of the output shaft of the servo motor 71. The pressing force measuring unit 84 measures the pressing force applied by the pair of electrodes 4. The pressing force measuring unit 84 is, for example, a strain sensor attached to the gun arm 3.

[0024] Additionally, the welding control device 30 is electrically connected to the electrode lifting device 7 and the current adjusting device 8. The welding control device 30 outputs a control signal to each of the electrode lifting device 7 and the current adjusting device 8. The welding control device 30 controls the electrode lifting device 7 and the current adjusting device 8 based on inputs from each measuring unit (voltage measuring unit 81, current measuring unit 82, electrode position measuring unit 83, and welding force measuring unit 84).

[0025] The resistance value calculation unit 31 calculates the resistance value using the welding voltage measured by the voltage measurement unit 81 and the welding current measured by the current measurement unit 82. The resistance value is obtained by dividing the welding voltage value by the welding current value. Furthermore, the resistance value calculation unit 31 generates a resistance waveform indicating the change in the resistance value over time based on the calculated resistance value. Note that the resistance value calculation unit 31 may be omitted as necessary.

[0026] The expansion amount calculation unit 32 calculates the expansion amount using the stroke measured by the electrode position measurement unit 83 and the pressing force measured by the pressing force measurement unit 84. In a preferred embodiment, the expansion amount of the plate thickness of the welding target location P1 in the workpiece material W1 is calculated by the following formula (1). E = S + a × F (1)

[0027] Here, E is the amount of expansion, S is the stroke, a is a strain conversion coefficient, and F is the applied pressure. The strain conversion coefficient a is a coefficient for converting applied pressure to strain, and can be any constant. In this way, by using the stroke and applied pressure, the amount of expansion can be calculated more accurately than by calculating them independently.

[0028] Next, Fig. 2 is a diagram for explaining an example of a loss of a welding point. Fig. 2 shows each welding point P2 and its welding point number after spot welding is completed at four welding target points P1 set on a workpiece W1 using the spot welding device 10. For example, the welding control device 30 can obtain the welding point number from a robot control device having a function of assigning a welding point number to each welding point P2.

[0029] As shown on the left side of FIG. 2, when there is no loss of the welding points P2, the welding point numbers 1 to 4 are assigned to each of the four welding points P2. On the other hand, as shown on the right side of FIG. 2, when there is a loss of the welding points P2, the welding point numbers 1 to 4 may be assigned to three welding points P2. In the example shown in FIG. 2, the workpiece W1 is subjected to the first spot welding at the first welding target point P1, and the second and third spot welding are performed at the second welding target point P1. Therefore, when the first to fourth spot welding are completed at the four welding target points P1, the welding point P2 corresponding to the third welding target point P1 is missing.

[0030] Therefore, the welding management device 40 shown in FIG. 1 can use the expansion amount waveform to determine whether or not the same welding point P1 has been spot-welded a plurality of times. The welding management device 40 is configured by a computer equipped with a CPU, a memory including a ROM and a RAM, and an input / output interface, not shown. A welding management program is stored in the memory of the welding management device 40. The CPU of the welding management device 40 executes the welding management program stored in advance in the memory, thereby functioning as a waveform acquisition unit 41, a waveform analysis unit 42, a determination unit 43, and a notification unit 44. A determination threshold value related to the amount of shrinkage for determining whether or not there is a defect in the welding point P2 is stored in the memory of the welding management device 40. In addition, information such as data of the expansion amount waveform acquired from the welding control device 30 is temporarily stored in the memory of the welding management device 40.

[0031] The waveform acquisition unit 41 acquires the expansion amount waveform by receiving the expansion amount waveform transmitted from the welding control device 30. The waveform analysis unit 42 calculates the amount of contraction of the plate thickness of the welding target portion P1 from the start of current application to the time when the pair of electrodes 4 separate from the workpiece W1 by analyzing the expansion amount waveform acquired by the waveform acquisition unit 41. In a preferred embodiment, the contraction amount is calculated as the difference between the maximum value of the expansion amount of the plate thickness during the current application time from the start to the end of current application and the minimum value of the expansion amount of the plate thickness during the holding time T4 during which the pair of electrodes 4 maintain the pressurized state until they separate from the workpiece W1 after the end of current application.

[0032] The determination unit 43 compares the shrinkage amount of the plate thickness calculated by the waveform analysis unit 42 with a determination threshold value related to the shrinkage amount read from the memory to determine whether or not the same welding target location P1 has been spot welded multiple times. When the shrinkage amount of the plate thickness is below the determination threshold value, the determination unit 43 determines that the same welding target location P1 has been spot welded multiple times. On the other hand, when the shrinkage amount of the plate thickness is equal to or greater than the determination threshold value, the determination unit 43 determines that one welding target location P1 has been spot welded once. When the determination unit 43 determines that the same welding target location P1 has been spot welded multiple times, the notification unit 44 issues an external warning that there has been a loss of the welding point P2. When the determination unit 43 determines that one welding target location P1 has been spot welded once, the notification unit 44 may notify the outside that there has been no loss of the welding point P2.

[0033] Next, a spot welding method using the above-described spot welding system 100 will be described. The spot welding method according to this embodiment includes a welding step, a waveform acquiring step, a waveform analyzing step, a determining step, and a notifying step. The spot welding method according to this embodiment will be described in detail with reference to Fig. 3. Fig. 3 is a diagram for explaining the spot welding method including detection of a defect at a welding point using an expansion amount waveform during welding.

[0034] The welding process is a process of spot welding a plurality of welding target points P1 of the workpiece W1, which is composed of a plurality of overlapping metal plates W2 and W3, by sandwiching the welding target points P1 between a pair of electrodes 4 and applying pressure to the workpiece W1 while passing current between the pair of electrodes 4. First, in the welding process, the robot control device outputs a control signal to each servo motor for driving the rotating shaft according to the teaching data to operate the robot, and controls the position and attitude of the welding gun G relative to the workpiece W1 to place the workpiece W1 between the pair of electrodes 4. As a result, the fixed electrode 6 abuts against the metal plate W3 of the workpiece W1 that is placed between the pair of electrodes 4. Next, the welding control device 30 outputs a control signal to the electrode lifting device 7 according to the teaching data to operate the movable electrode 5. As a result, the movable electrode 5 descends and abuts against the metal plate W2 of the workpiece W1.

[0035] Then, the welding control device 30 outputs a control signal to the electrode lifting device 7 in accordance with the teaching data to operate the movable electrode 5, controls the pressure applied by the pair of electrodes 4 in accordance with the pressure pattern, and outputs a control signal to the current adjusting device 8 to control the welding current supplied to the pair of electrodes 4 in accordance with the current pattern, thereby performing spot welding at a predetermined welding target location P1.

[0036] The welding process according to this embodiment includes a squeeze process, a pre-energization process, a main energization process, and a holding process. The squeeze process is a process in which the workpiece W1 is sandwiched between a pair of electrodes 4, and pressure is applied by the pair of electrodes 4 from the start of the pressure application by the pair of electrodes 4 to the energization of the pair of electrodes 4. The time from the start of the pressure application by the pair of electrodes 4 to the energization of the pair of electrodes 4 is referred to as a squeeze time T1.

[0037] The pre-current passing process is a process of passing a preheating current between a pair of electrodes 4 while sandwiching and pressurizing the workpiece W1 between the pair of electrodes 4. By performing pre-current passing, at least the opposing surfaces of the metal plates W2, W3 are reacted at high temperature, and an oxide film, which is a film with high electrical resistance, can be removed or reduced in size. Such pre-current passing is performed to facilitate melting of the workpiece W1 in the main current passing described later. The time during which the preheating current is passed between the pair of electrodes 4 is called the pre-current passing time T2.

[0038] The main current application process is a process of applying main current between the pair of electrodes 4 while sandwiching and pressurizing the workpiece W1 between the pair of electrodes 4. By applying main current, a nugget N, which is a portion where the workpiece W1 melts and solidifies, is formed between the metal plates W2 and W3. As a result, the workpiece W1 is welded. The main current application is performed to promote the growth of the nugget N. The time during which the welding current is applied between the pair of electrodes 4 is referred to as the main current application time T3.

[0039] The holding process is a process of maintaining the pressurized state of the pair of electrodes 4 and cooling it from the time when the passage of current to the pair of electrodes 4 is terminated until the pair of electrodes 4 separates from the workpiece W1. The time during which the pressurized state is maintained from the time when the passage of current is terminated until the pair of electrodes 4 separates from the workpiece W1 is called the holding time T4. By providing the holding time T4 immediately after the termination of the passage of current, the nugget N has time to cool, and therefore the tensile stress in the plate thickness direction due to the thermal contraction of the nugget N can be reduced.

[0040] A series of spot welding is completed by sequentially repeating the above-mentioned squeeze process, pre-energization process, main energization process, and holding process for all welding target locations P1 (four welding target locations P1 in this embodiment) of the workpiece W1.

[0041] Next, in the waveform acquisition step, the waveform acquisition unit 41 acquires an expansion amount waveform from the welding control device 30. As shown in the expansion amount waveforms L1 and L2 in FIG. 3, the expansion amount during welding behaves as follows. First, during the squeeze time T1, the expansion amount is zero because the current flow has not started. Next, when the pre-current flow time T2 starts, the expansion amount increases slightly by passing a preheating current between the pair of electrodes 4, and then the expansion amount decreases to near zero by temporarily suspending the current flow. Next, when the main current flow time T3 starts, the expansion amount gradually increases as the nugget N formed by passing a welding current between the pair of electrodes 4 grows, and is saturated at a maximum value. Then, when the holding time T4 starts after the current flow ends (after the main current flow time T3 ends), the expansion amount decreases rapidly as the nugget N is cooled by the pair of electrodes 4.

[0042] Next, in a waveform analysis step, the expansion amount waveform is analyzed by the waveform analyzer 42. In this step, the expansion amount waveform is analyzed by the waveform analyzer 42 to calculate the amount of contraction of the plate thickness of the welding target part P1 from the start of current flow until the pair of electrodes 4 separate from the workpiece W1.

[0043] Here, the amount of shrinkage is calculated, for example, by the difference between the maximum amount of expansion of the plate thickness during the current flow time from the start to the end of current flow and the minimum amount of expansion of the plate thickness during the holding time T4 during which the pair of electrodes 4 maintain the pressurized state until they separate from the workpiece W1 after the current flow ends. More specifically, the amount of shrinkage is calculated by the difference between the maximum amount of expansion of the plate thickness during the current flow time T3 and the minimum amount of expansion of the plate thickness during the holding time T4. In this embodiment, this amount of shrinkage is used to detect defects at each welding point P2.

[0044] The expansion amount waveform L1 shown in FIG. 3 is an expansion amount waveform obtained during normal welding in which one welding target point P1 is spot welded only once. The minimum value of the expansion amount in the expansion amount waveform L1 during the holding time T4 is smaller than the expansion amount in the squeeze time T1 before the start of current flow. On the other hand, the expansion amount waveform L2 shown in FIG. 3 is an expansion amount waveform obtained during a multiple (second) welding in which one welding target point P1 is spot welded multiple times (twice). The minimum value of the expansion amount in the expansion amount waveform L2 during the holding time T4 is larger than the expansion amount in the squeeze time T1.

[0045] Here, FIG. 4 is a diagram for explaining the amount of shrinkage of the plate thickness when the same welding target portion P1 is spot-welded a plurality of times. As shown in the left side of FIG. 4, the plate thickness of the welding target portion P1 of the workpiece W1 before spot welding is equal to the sum of the plate thickness of the metal plate W2 and the plate thickness of the metal plate W3. As shown in the center of FIG. 4, when the first spot welding is performed on the workpiece W1 before spot welding, a dent D is formed on both the upper and lower end faces of the workpiece W1, so that the plate thickness of the welding target portion P1 is reduced compared to before spot welding. Therefore, the plate thickness shrinkage amount X1 of the welding target portion P1 after the first spot welding (see FIG. 3) is equal to the value obtained by adding the dent amount of the dent D to the shrinkage amount of the nugget N. In this way, after the first spot welding, the dent amount of the dent D is reflected, so the plate thickness shrinkage amount X1 is larger than the shrinkage amount of the nugget N.

[0046] On the other hand, as shown on the right side of Fig. 4, the second spot welding is performed on the workpiece W1 after the first spot welding has been performed, with the workpiece W1 already having a dent D formed therein, so the amount of dent caused by the dent D does not change before and after the second spot welding. Therefore, the amount of shrinkage X2 (see Fig. 3) of the sheet thickness of the welded portion P1 after the second spot welding is equal to the amount of shrinkage of the nugget N. In this way, since the amount of dent caused by the dent D is not added after the second spot welding, the amount of shrinkage X2 of the sheet thickness after the second spot welding is smaller than the amount of shrinkage X1 of the sheet thickness after the first spot welding.

[0047] Therefore, in the determination step, by comparing the amount of contraction calculated from the expansion amount waveform for each spot welding with a determination threshold, it is possible to determine whether or not the same welding target location P1 has been spot welded a plurality of times.

[0048] In the determination step, if the amount of shrinkage of the plate thickness of the welding target portion P1 from the start of energization until the pair of electrodes 4 separate from the welded material W1 falls below a determination threshold, the determination unit 43 determines that the same welding target portion P1 has been spot-welded once if the amount of shrinkage of the plate thickness of the welding target portion P1 is equal to or greater than the determination threshold. This determination step makes it possible to detect defects in each welding point P2.

[0049] In the notification step, if it is determined in the determination step that the same welding point P1 has been spot-welded multiple times, a warning is issued to the outside that there has been a defect in the welding point P2. Also, in the notification step, if it is determined in the determination step that one welding point P1 has been spot-welded once, it may be notified to the outside that there has been no defect in the welding point P2.

[0050] As described above, according to the present embodiment, it is possible to provide the spot welding system 100 and the spot welding method that are capable of detecting a defect in the welding point P2 with high accuracy.

[0051] The present invention is not limited to the above embodiment, and can be modified as appropriate without departing from the spirit and scope of the present invention. For example, the welding management device 40 can determine whether the same welding target point P1 has been spot welded multiple times by using a resistance waveform instead of an expansion amount waveform.

[0052] In this case, the memory of the welding management device 40 stores a judgment threshold value related to the resistance value for judging whether or not there is a defect in the welding point P2. The waveform acquisition unit 41 of the welding management device 40 acquires the resistance waveform by receiving the resistance waveform transmitted from the welding control device 30. The waveform analysis unit 42 of the welding management device 40 calculates the resistance value between the pair of electrodes 4 during the pre-energization time T2 or the beginning of the energization time by analyzing the resistance waveform acquired by the waveform acquisition unit 41. Furthermore, the judgment unit 43 of the welding management device 40 judges whether or not the same welding target location P1 has been spot welded multiple times by comparing the resistance value between the pair of electrodes 4 calculated by the waveform analysis unit 42 with the judgment threshold value related to the resistance value read from the memory. Specifically, when the resistance value between the pair of electrodes 4 falls below the judgment threshold, the judgment unit 43 judges that the same welding target location P1 has been spot welded multiple times. On the other hand, when the resistance value between the pair of electrodes 4 is equal to or greater than the judgment threshold, the judgment unit 43 judges that one welding target location P1 has been spot welded once.

[0053] In the waveform acquisition step of the spot welding method, a resistance waveform is acquired from welding control device 30 by waveform acquisition unit 41. Resistance waveform L3 shown in Fig. 5 is a resistance waveform during pre-energization time T2 obtained during normal welding in which one welding target location P1 is spot-welded only once. Meanwhile, resistance waveform L4 shown in Fig. 5 is a resistance waveform during pre-energization time T2 obtained during a multiple-time (second-time) welding in which one welding target location P1 is spot-welded multiple times (twice).

[0054] Next, in the waveform analysis process, the waveform analyzer 42 analyzes the resistance waveform, and calculates, for example, the resistance value between the pair of electrodes 4 during the pre-energization time T2. Here, the larger the contact area between the pair of electrodes 4 and the workpiece W1, the wider the current path becomes in the radial direction of the nugget N perpendicular to the plate thickness direction, and therefore the smaller the resistance value between the pair of electrodes 4 becomes.

[0055] When performing a first spot welding on the workpiece W1 before spot welding, the pair of electrodes 4 contact both flat end surfaces of the workpiece W1. On the other hand, when performing a second spot welding on the workpiece W1 on which the first spot welding has been performed, the pair of electrodes 4 contact the dents D formed on both end surfaces of the workpiece W1 by the first spot welding. Therefore, the contact area during the second spot welding is larger than the contact area during the first spot welding. In other words, the resistance value between the pair of electrodes 4 is smaller during the pre-current application time T2 of the second spot welding than during the pre-current application time T2 of the first spot welding.

[0056] Therefore, in the determination step, by comparing the resistance value obtained from the resistance waveform for each spot welding with a determination threshold, it is possible to determine whether or not the same welding target location P1 has been spot welded multiple times. Note that, since the contact area between the pair of electrodes 4 and the welded material W1 increases and the current path expands radially outward as the current conduction time elapses, it is preferable to perform the determination step using the resistance value during the pre-current conduction time T2 or at the beginning of the current conduction time.

[0057] In the determination step, when the resistance value between the pair of electrodes 4 during the pre-energization time T2 or at the beginning of the energization time falls below a determination threshold, the determination unit 43 determines that the same welding target location P1 has been spot-welded multiple times. Also, in the determination step, when the resistance value between the pair of electrodes 4 is equal to or greater than the determination threshold, the determination unit 43 determines that one welding target location P1 has been spot-welded once. This determination step can also detect defects in each welding point P2. [Explanation of symbols]

[0058] 1 Rotating shaft 2 Mounting part 3 Gun arm 4 Electrodes 5 Movable electrodes 6 Fixed electrodes 7 Electrode lifting device 8 Current adjustment device 10 Spot welding device 20 Spot welding robot 30 Welding control device 31 Resistance value calculation section 32 Expansion amount calculation section 40 Welding control device 41 Waveform acquisition section 42 Waveform analysis section 43 Judgment section 44 Notification section 71 Servo motor 72 Lifting member 81 voltage measuring section 82 current measuring section 83 electrode position measuring section 84 applied pressure measuring section 100 Spot Welding System D Impact mark G Welding gun L1, L2 Expansion amount waveform L3, L4 Resistance waveform N Nugget P1 Welding target area P2 Welding point R Robot body T1 Squeeze time T2 Pre-energization time T3 Main energization time T4 Hold time W1 Welding material W2, W3 Metal plate X1, X2 Shrinkage amount

Claims

1. a spot welding device that performs spot welding at a plurality of welding target locations of a workpiece formed of a plurality of overlapping metal plates by sandwiching the welding target locations of the workpiece between a pair of electrodes and applying pressure to the workpiece while passing current between the pair of electrodes; a welding management device that determines that the same welding target portion has been spot-welded a plurality of times when a contraction amount of a plate thickness of the welding target portion from the start of the current flow to the separation of the pair of electrodes from the workpiece falls below a determination threshold value; A spot welding system having

2. 2. The spot welding system according to claim 1, wherein the amount of shrinkage is calculated by the difference between a maximum amount of expansion of the sheet thickness during the current flow time from the start to the end of the current flow and a minimum amount of expansion of the sheet thickness during a holding time during which the pair of electrodes maintain a pressurized state until they separate from the workpiece after the current flow ends.

3. The expansion amount is E=S+a×F...(1) 3. The spot welding system according to claim 2, wherein E is an expansion amount, S is a stroke, a is a distortion amount conversion coefficient, and F is a welding force.

4. a welding process for performing spot welding at a plurality of welding target locations of a workpiece constituted by a plurality of overlapping metal plates by sandwiching the welding target locations between a pair of electrodes and applying pressure to the workpiece while passing current between the pair of electrodes; a determination step of determining that the same welding target portion has been welded a plurality of times when a contraction amount of a plate thickness of the welding target portion from the start of the current flow to the time when the pair of electrodes are separated from the welded material falls below a determination threshold value; A spot welding method comprising the steps of:

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