Spot welding quality monitoring system

Abstract

To provide a spot welding quality monitoring system that can accurately determine whether a weld zone (nugget) is good or bad and then guarantee the welding quality.SOLUTION: A spot welding quality monitoring system 50 comprises a welding device 10. The welding device comprises a pair of electrodes, and a pair of electrode support units to respectively hold the pair of electrodes, and welds a member to be welded by pressurizing and energizing it. The system includes: an input unit 61 that acquires an amount of strain detected by a strain detector 40; a calculation unit 62 that calculates a strain displacement amount, which is a temporal change in the amount of strain detected by the strain detector 40, and a polarity thereof; and a determination unit 65 that determines welding quality for each welding spot on the basis of a change in the polarity of the strain displacement amount. The system monitors production by associating the welding spots with the welding quality.SELECTED DRAWING: Figure 14

Description

[Technical Field]

[0001] The present invention relates to a spot welding quality monitoring system that can determine the quality of a spot welded portion (nugget) and guarantee the quality of welding. [Background technology]

[0002] In the field of resistance welding, typified by spot welding, a method for determining the quality of welding in real time (during welding) has been known in the past, in which the properties of the welded portion (nugget) formed on the welded material are determined.

[0003] For example, Patent Document 1 proposes a method for detecting the amount of movement between welding electrodes, which derives the amount of movement between welding electrodes by adding the amount of movement of the electrode drive part in the electrode movement direction due to expansion and contraction of the welded part (nugget) during welding and the amount of deflection of the welding gun due to the pressure applied from the electrode to the workpiece. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3593981 Summary of the Invention [Problem to be solved by the invention]

[0005] However, while Patent Document 1 estimates the expansion and contraction of the nugget based on the correlation between the inter-electrode movement amount and time, this expansion and contraction of the nugget deforms the welding equipment and is therefore not detected as the inter-electrode movement amount.In addition, since there are many variable and irregular factors such as the material, plate thickness, and welding conditions of the welded parts, and welding speed is also required in mass production sites, it is thought that this method is not suitable for detecting the quality of welding in-line.

[0006] Specifically, when welding workpieces of different thicknesses, it is necessary to prepare multiple pieces of data correlating the inter-electrode movement amount and time in order to determine whether the nugget is good or bad. Since the current measurement value is compared and determined based on these multiple correlation data, it takes a long time to make the determination, which results in a slow welding speed.

[0007] Therefore, a main object of the present invention is to provide a spot welding quality monitoring system that can accurately determine the quality of a welded portion (nugget) and ensure welding quality. [Means for solving the problem]

[0008] The embodiment that solves the above problem is as follows. (Basic aspects) A welding quality monitoring system including a welding device that includes a pair of electrodes and a pair of electrode support parts that respectively hold the pair of electrodes, and that moves the pair of electrodes in a direction toward each other by opening and closing the electrode support parts with an electrode contact / separation means, and applies pressure and current to weld members to weld them, an input unit that acquires the amount of strain detected by a strain detector provided on the electrode support unit; a calculation unit that calculates a strain displacement amount, which is a time change in the strain amount detected by the strain detector, and its polarity; a determination unit that determines the welding quality for each welding point based on a change in polarity of the strain displacement amount, monitoring production in association with the welding point and welding quality; A spot welding quality monitoring system characterized by: [Effects of the Invention]

[0009] According to the present invention, a spot welding quality monitoring system is provided that can accurately determine whether a weld is good or bad and ensure welding quality. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of a spot welding quality monitoring system. [Figure 2] 10 is a graph showing changes in strain amount and strain displacement amount relating to the explanation of terms. [Figure 3] 10 is a graph showing an example of the change over time in the amount of strain and the amount of strain displacement detected by a strain detector, and the welding current in the case of a good weld. [Figure 4] 10 is a graph showing an example of changes over time in the amount of strain and the amount of strain displacement detected by a strain detector, and in the welding current in the case of a defective weld. [Figure 5] 10 is a graph showing an example of changes over time in the amount of strain and the amount of strain displacement detected by a strain detector, and in the welding current when spatter occurs. [Figure 6] FIG. 10 is an explanatory diagram illustrating the transition of the welding state over time in the case of a good weld. [Figure 7] FIG. 10 is an explanatory diagram illustrating the transition of the welding state over time in the case of a defective weld. [Figure 8] FIG. 10 is an explanatory diagram illustrating a case where tip dressing or tip replacement is required. [Figure 9] FIG. 10 is a schematic perspective view of a product and an example of welding points. [Figure 10] FIG. [Figure 11] FIG. 10 is a schematic perspective view of a support for a product. [Figure 12] FIG. 10 is a schematic perspective view of an example of a product, welding points, and clamp positions. [Figure 13] FIG. 1 is a schematic diagram of an electrode. [Figure 14] FIG. 1 is a diagram illustrating the configuration of a spot welding quality monitoring system. [Figure 15] FIG. 10 is an explanatory diagram of an example of information processing. [Figure 16] FIG. 10 is an explanatory diagram of an example of threshold setting. DETAILED DESCRIPTION OF THE INVENTION

[0011] Next, embodiments of the present invention will be described in detail below with reference to the drawings.

[0012] (Welding equipment overview) 1 shows the main parts of an example of a welding device 10, which has a pair of a first electrode support portion 11A and a second electrode support portion 11B. Regarding the welding device 10, please also refer to FIG. First electrode support portion 11A and second electrode support portion 11B hold first electrode 12A and second electrode 12B at their tip portions, respectively.

[0013] The middle portions of the first electrode support portion 11A and the second electrode support portion 11B are connected by a support shaft 13. The first electrode support portion 11A and the second electrode support portion 11B are connected to a holding arm 14, which is connected to the tip of a robot arm (not shown), as a means for moving them. The tip of the holding arm 14 holds both the first electrode support portion 11A and the second electrode support portion 11B so that they can swing around the support shaft 13. Although the electrode support portion in the embodiment is the gun arm of a welding gun, it may also be the electrode support arm of a stationary resistance welding machine.

[0014] An extension / contraction mechanism 15 of the electrode approaching / separating means is provided at the base end of the first electrode support part 11A, and the tip of the rod thereof is connected to the base end of the second electrode support part 11B. As a result, as the extension / contraction mechanism 15 extends and contracts, the first electrode support portion 11A and the second electrode support portion 11B rotate around the support axis 13, and the first electrode 12A of the first electrode support portion 11A and the second electrode 12B of the second electrode support portion 11B move closer to and away from each other (open and close). When the first electrode 12A and the second electrode 12B are moved in a direction approaching each other, a predetermined pressure is applied to the target welded members (e.g., overlapping automotive steel plates) 30 via the tips at the ends of the first electrode 12A and the second electrode 12B.

[0015] The welding apparatus in the embodiment is equipped with a control device 20, which can include a central processing unit (CPU) 22 that performs various signal processing related to the robot and the above-mentioned welding apparatus 10, an auxiliary processing unit 24, a memory device 23, etc.

[0016] The central processing unit 22 controls the welding pressure required for welding by outputting commands to the extension mechanism 15 to apply or release pressure to the first electrode 12A and the second electrode 12B in accordance with a welding program previously stored in the storage device 23. Furthermore, the central processing unit 22 also controls the current applied to the electrodes. Although the welding device in the embodiment maintains the welding pressure and welding current values ​​constant, they may be changed.

[0017] The storage device 23 stores a welding program and welding conditions (welding pressure, welding current value, current application time, current application interval, resistance value), etc.

[0018] Here, the value of the welding pressure under the welding conditions can be obtained from a known amplifier (not shown) connected to the strain detector 40 . The welding current value and the resistance value during welding can be obtained from a current supply device.

[0019] When the welding device 10 is provided on an articulated robot, the control device 20 can be built into a robot control device (not shown) or can be independently arranged in parallel.

[0020] On the other hand, there is provided an electrode tip state confirmation means 28 for the first electrode 12A and the second electrode 12B in the welding device 10. The electrode tip state confirmation means 28 is, for example, an imaging device such as a camera, and is used to confirm the state of the electrode tip (for example, measuring the electrode tip diameter φ shown in FIG. 13) before welding begins or after welding (for a certain product) is completed. The specific installation manner of the imaging device is not shown. The welding device 10 is also provided with a display device 30 .

[0021] In an embodiment of the present invention, a spot welding quality monitoring system 50 having the configuration example shown in FIG. 14 is provided, which is equipped with the welding device 10 of the above example, and this spot welding quality monitoring system 50 ensures welding quality.

[0022] The spot welding quality monitoring system 50 in this embodiment includes a welding device 10 and a welding quality assurance means 60 for the welding device 10. Various pieces of information during welding in welding device 10 are configured to be input to input unit (acquisition unit) 61 of welding quality assurance means 60. For example, signals from the strain amount output means constituted by the strain detector 40, the current value output means 25, the pressure output means 26, the resistance value output means 27, and the electrode tip state confirmation means 28 are input to the input section (acquisition section) 61 of the welding quality assurance means 60.

[0023] The first information (strain amount) input from the strain amount output means to the input unit (acquisition unit) 61 is processed by the calculation unit 62, and the strain displacement amount and its polarity are passed through the judgment criteria creation unit 64 of the judgment memory unit 63 to judge whether the welding is good or bad in accordance with the judgment criteria in the judgment unit 65. The welding quality result from the judging unit 65 is displayed on the display device 30 of the welding device 10, and if the welding is defective, measures to deal with the defect are also displayed.

[0024] On the other hand, second information, namely, the current value from the current value output means 25, the pressure from the pressure output means 26, the resistance value from the resistance value output means 27, and the (tip) electrode diameter from the electrode tip state confirmation means 28, is given to the judgment criterion memory unit 63, and the judgment criterion in the judgment unit 65 is updated based on this second information.

[0025] Furthermore, the pass / fail judgment results from the judgment unit 65 are provided to the learning unit 66 as sampling inspection information (third information) from actual product batches, and the learning unit performs machine learning on the results, which are then fed back to the judgment criteria creation unit 64 to improve the accuracy of the pass / fail judgment results.

[0026] The judgment results from the learning unit 66 are sent via the quality control unit 67 to the output unit 68, which displays them on the overall system management monitor attached to the spot welding quality monitoring system 50, and in the event of a welding defect, measures to address the defect are also displayed. The output unit 68 stores production data in a storage device (not shown) to preserve production history. This production data includes various information such as the polarity, current value, and pressure force for each product.

[0027] (Transition of welding condition) The transition of the welding condition over time will be outlined below. Figure 6 is a schematic diagram illustrating the process of producing a nugget in the period from the point at which the first electrode 12A and the second electrode 12B are moved toward each other (closed) in the welding gun of the welding device 10 and a predetermined pressure force is applied to the two overlapping welded members WM to the point at which the welding gun begins to open.

[0028] When current begins to flow, resistance heating begins around the interface where the welded parts WM overlap, the base material is heated and expands toward the electrodes (stage S1), and with this expansion, the reaction force (expansion force) in the opening direction from the base material side to the first electrode 12A and the second electrode 12B moving away from each other increases (stage S2).

[0029] After that, the base material continues to expand, but on the other hand, the temperature of the base material rises due to resistance heating and it begins to melt and soften. As a result, the force (push-in force) that moves the first electrode 12A and the second electrode 12B closer to each other due to the applied pressure takes precedence over the expansion force (stage S3). When this force reaches its maximum, it is determined that the welded portion (nugget) Na has been sufficiently formed, and the current flow is terminated (stage S4). When cooling is finally completed, the first electrode 12A and the second electrode 12B are separated from each other, and the welding gun is opened (Step S5). Once the above steps have been completed, sufficient welded portion (nugget) Na will be produced, and it can be determined that the welding is performed well.

[0030] In contrast, if the stage shown in FIG. 7 is passed, the welded portion (nugget) Na is not sufficiently formed, and it can be determined that the welding is defective. That is, even though current is turned on and resistance heating of the workpieces WM begins (stage S1), sufficient heat is not generated and the expansion of the base metal to be welded is slight (stage S2). Therefore, the reaction force in the opening direction that separates the first electrode 12A and the second electrode 12B from each other from the base metal side increases slightly, but the magnitude is small (stage S3).

[0031] Thereafter, although the welded portion (nugget) Na continues to expand, the current flow is completed without sufficient melting or softening of the base material (stage S4). When cooling is finally completed, the first electrode 12A and the second electrode 12B are separated from each other, and the welding device 10 is opened (Step S5). In this type of case, sufficient formation of the welded portion (nugget) is not observed, and the welded portion (nugget) Na is small, or peeling of the two overlapping welded members WM occurs.

[0032] (Weld quality judgement) In this embodiment, a strain detector 40 is provided in welding device 10 to determine whether the welding is good when the process shown in FIG. 6 is observed and whether the welding is poor when the process shown in FIG. The strain detector 40 may be installed at either the first electrode support portion 11A or the second electrode support portion 11B. Although it is possible to provide a strain detector 40 on both the first electrode support part 11A and the second electrode support part 11B, this would complicate the processing of strain signals, so it is sufficient to provide it on one of the electrode support parts. In this embodiment, it is provided on the first electrode support part 11A. Note that providing it on the fixed electrode support part allows for a more stable waveform to be obtained.

[0033] As shown in Figure 2, the strain detector 40 defines the difference between two consecutively sampled strain amounts Lt1 and Lt2 as strain displacement ΔLt2 = Lt2 - Lt1. The polarity of the strain displacement is positive (+) due to the expansion process. On the other hand, during the pressing process, the difference between the two strain amounts Lt3 and Lt4 is defined as strain displacement ΔLt4 = Lt4 - Lt3, and the polarity of the strain displacement is negative (-). In this way, the quality of the weld can be determined from the change over time in the polarity of the strain displacement under current and pressure.

[0034] The change over time in the amount of strain detected by the strain detector 40 is shown in both FIG. 6 and FIG. In the case of "good welding" shown in Figure 6, when current begins to flow, the electrode support part opens, so the strain amount shows a change in the amount of displacement that is positive, and after reaching a maximum in a relatively short period of time, the base material melts and the electrode support part is pressed in, gradually showing a change in the amount of displacement that is negative. When the energization is completed and cooling is started, the degree of change in the negative displacement of the strain increases until the cooling is completed.

[0035] In the case of the "poor welding" shown in Figure 7, the strain amount shows a positive change in displacement from the start of energization until the end of energization, but the rate of change in the strain amount is slow even after a long time has passed. When the energization is completed and cooling is started, the degree of change in the negative displacement of the strain amount increases until the cooling is completed.

[0036] From the comparative explanation of Figures 6 and 7, it can be seen that capturing the change over time in the amount of strain displacement detected by the strain detector 40 during pressure and current application is effective for determining the quality of the spot weld, i.e., the formation of a nugget.

[0037] An example of the output of the strain detector 40 in the case of a "good weld" is shown in Fig. 3. This is an example of the output of the strain detector 40 for each sampling unit time (for example, 20 milliseconds). When the current is first applied, resistance heating causes expansion, increasing the amount of strain. The overlapping welded parts WM are heated, and although the change in strain slows down after that, at a certain point, the base material (the welded parts WM) begins to melt, mainly around the overlapping interface. As the current continues to flow and heating continues, the base material continues to melt and soften, and as a result, the nugget in which the base material melts grows and softens, so the pressure applied by the first electrode 12A and the second electrode 12B becomes dominant over the expansion, and a force is generated in the direction that brings the first electrode 12A and the second electrode 12B closer to each other (this is the process marked "with pressing" in Figure 3).

[0038] The term "pushing in" means that as a result of the base material melting and softening, the pressure of the first electrode 12A and the second electrode 12B becomes dominant over the expansion force, and the workpiece WM is pushed in. When the pressure applied by the first electrode 12A and the second electrode 12B becomes dominant over the expansion and this process continues for a predetermined time, it can be determined that a sufficient amount of welded portion (nugget) Na has been formed. Thereafter, the welding current is reduced to zero and the current supply is terminated. Once cooling is complete, the first electrode 12A and the second electrode 12B are separated from each other, and the welding device 10 is opened.

[0039] In the above-described welding process, the expansion of the welded portion (nugget) continues, while the base material begins to melt and soften. As a result, the pressure applied by the first electrode 12A and the second electrode 12B becomes dominant over the expansion, and a force is generated in the direction in which the first electrode 12A and the second electrode 12B approach each other (the "push-in" process). This process occurs midway through the period of current flow due to steady current flow, which means that the welded portion (nugget) Na has been sufficiently formed. As shown in the example output of the strain detector 40 in FIG. 3, in the "indentation" process, the "polarity" indicating whether the "displacement amount" of the strain amount at a certain sampling point relative to the strain amount at the previous sampling point is positive or negative is negative. Therefore, if multiple consecutive samples with negative "polarity" are taken partway through the period of energization with steady current, it can be determined that the welding is good.

[0040] 4 shows an example of the output of the strain detector 40 in the case of a "poor weld." This occurs when the electrode abuts on the workpiece at an angle, the electrode tip is worn, the current density is low, or dirt or other debris adheres to the overlapping interface of the workpieces, making it impossible to obtain an appropriate current density to produce a nugget. In this case, the resistance heat is small, so the pressure applied to the first electrode 12A and the second electrode 12B does not prevail over the expansion. Therefore, it is shown that the "no pressing" state continues throughout the entire energization period with a steady current. Moreover, throughout the entire energization period of the steady energization current, the "polarity" is positive and never becomes negative. This expansion and push-in phenomenon is unrelated to the thickness of the welded parts, and welding judgment based on this amount of strain displacement is faster than conventional welding judgment based on the amount of strain for each plate thickness, making it possible to inspect welding during automatic welding on a mass production line.

[0041] As described above, when the change in polarity of the strain displacement over time is negative, it can be determined that the weld is good. However, although a good weld can be determined if the polarity of the strain displacement changes negatively only once, the determination may be unstable. For example, even if the polarity of the strain displacement changes negatively only once, it may later change to positive due to the vibration of the welding gun or the reaction force of the plunger. Therefore, it is desirable to judge the welding to be good when the polarity is negative or zero repeatedly. For example, as shown in Figure 3, there are three consecutive negative readings, namely the first sampling, which marks the beginning of the "good welding" period, the second sampling, and the third sampling. In such a case, it can be determined with stability that the welding is good. Furthermore, if the polarity alternates between negative and zero, the welding can be determined to be good.

[0042] FIG. 5 shows the change in the amount of strain and the change in the amount of strain displacement when sputtering occurs. When sputtering occurs, there may be multiple times when the polarity is negative. When sputtering occurs, the molten metal splashes, causing a sudden push-in, which can result in a sudden change in polarity to the negative side, followed by multiple periods of negative polarity. Therefore, if the polarity suddenly changes to the negative side and then there are multiple instances of the polarity being negative, it can be determined that spatter has occurred. The record of spatter occurrence can also be used as data for adjusting the welding conditions for the next welding. When compared with the case where spatter occurs when the polarity is negative, the meaning of determining that the weld is good based on the criteria explained in the previous paragraph 0041 becomes even clearer.

[0043] On the other hand, in view of the possibility of unexpected factors occurring, it is desirable to incorporate a monitoring device (not shown) for the primary current value during welding and to determine whether the welding is good when both of the following conditions are met: when the polarity is negative, or when negative or zero is repeated multiple times, and when the welding current from the monitoring device for the primary current value is constant.

[0044] When determining the quality of spot welding, it is desirable that the device shown in FIG. 1, for example, can be used in common even when the thickness and number of welded members change in a plurality of combinations. Fortunately, according to the above embodiment, a good weld can be determined when the polarity is negative, or when the polarity is negative or zero multiple times. Even if the plate thickness of the welded member is changed, or even if the degree of strain displacement differs, the polarity remains the same, and the same determination method can be used to make the determination.

[0045] (Spot welding quality monitoring system) The quality of the welding can be judged by the judgment unit 65 of the welding quality assurance means 60 shown in FIG. In the embodiment, the quality of welding is judged based on the fact that the polarity of the strain displacement of the electrode support part changes over time from positive to negative during pressure and current application in the expansion and contraction of the nugget part of the spot welding. The determining unit 65 can determine that the welding is defective when, for example, the proportion of negative values ​​is equal to or less than a predetermined threshold value during the pushing process after the polarity of the strain displacement amount changes from positive to negative.

[0046] The welding quality assurance means 60 is configured to incorporate various elements not only to judge whether the welding is good or bad, but also to monitor the welding state or to ensure the long-term stability of the welding. This welding quality assurance judgment means 60 stores the time-varying change in polarity of the amount of distortion displacement for each welding point of each product, each product, and each production lot in the judgment memory unit 63, and outputs and saves it as production data, thereby ensuring production quality assurance. In other words, traceability can be established because the quality of each product in a production lot is guaranteed before delivery.

[0047] The first function of the welding quality assurance judgment means 60 is to monitor whether the polarity of the strain displacement is positive during the upslope process in which the welding current is gradually increased and the subsequent process, as shown in Figures 8 and 6, which are reproductions of Figure 3, indicating that the expansion process is underway and that the material to be welded has subsequently begun to melt.

[0048] On the other hand, since the electrode tip wears due to multiple welding operations, tip dressing or tip replacement has generally been performed periodically depending on the number of welding points. Tip dressers and the use of tip dressers for this purpose are disclosed in Japanese Patent Application Laid-Open No. 2021-79399 and Japanese Patent Application Laid-Open No. 2018-103200, etc. The above-described embodiment can be used as an indicator of tip dressing or tip replacement timing.

[0049] For this purpose, as a second function of the welding quality assurance judgment means 60, in the upslope process, when the polarity is positive as shown in Figures 3 and 8, an upper limit value for the distortion displacement in the positive polarity region in the case of a good chip at the limit of use can be determined in advance, and a lower limit threshold value S+ (see Figure 16) for the distortion displacement can be set to, for example, "plus 7".

[0050] At a certain point when the wire is actually stacked at the weld point, if the time-dependent change in polarity of the strain displacement detected by the strain detector 40 changes from positive to negative and the positive displacement falls below the threshold value S+ (i.e., does not exceed the threshold value S+), it is determined that the tip has deteriorated or that irregular welding has caused the target portion of the welded material to expand but not melt sufficiently, and the tip can be dressed or replaced using a tip dresser, or the irregularity can be resolved.

[0051] Furthermore, when the polarity of the strain displacement detected by the strain detector 40 is positive and the duration of the positive polarity is longer than a predetermined duration (as shown by Ca1 in Figure 8), that is, when it is the time when a good tip would have started to be pushed in, the welding quality assurance judgment means 60 can determine that tip dressing or tip replacement is required, as no push-in has occurred.

[0052] Furthermore, even if the polarity of the strain displacement detected by the strain detector 40 changes over time to negative, but the degree of negative polarity is extremely large (as shown by Ca2 in FIG. 8), it can be determined that tip dressing or tip replacement is necessary. In the case of Ca2, welding is insufficient, and there is a risk of peeling. Therefore, the threshold value S- (see FIG. 16) can be set to, for example, "minus 10" for the strain displacement amount.

[0053] Furthermore, instead of periodically dressing or replacing the tip, the wear state of the electrode can be detected while it is in use and the timing for dressing or replacing the tip can be determined, thereby improving environmental friendliness by extending the life of the electrode.

[0054] The distortion detector can output a distortion detection signal at intervals of 20 milliseconds or less. If the interval is too long, it is difficult to determine whether the polarity is correct. If the interval is too short, noise may be mixed in, making the determination unstable.

[0055] It is preferable to provide a device for monitoring the resistance and voltage between the tips of the first electrode 12A and the second electrode 12B and combine this with the welding quality assurance determination means 60 described above.

[0056] The input section 61 of the welding quality assurance judgment means 60 preferably inputs (acquires) not only the distortion amount of the first information, but also at least one of the current value, pressure, resistance value, and electrode tip condition information (second information) based on imaging information when forming the welding point, and uses this information to monitor production by associating the welding point, etc. with welding quality. The second information is a factor that causes changes in welding quality over time as production progresses.

[0057] It is desirable that the welding quality assurance judgment means 60 further includes a learning unit 66 that acquires welding quality information for each welding point in the "product" produced by spot welding the welded member WM. The welding quality information for each welding point in the product can be third information based on a sampling inspection. The learning unit 66 can provide the welding quality information (third information) for each welding point to the judgment criteria creation unit 64 to use as information for updating the judgment criteria in order to set a threshold value in the judgment unit 65.

[0058] Specifically, the learning unit 66 can use the percentage (%) of the amount of strain displacement that has a negative polarity for each welding point, for example, during the process (push-in process) in which the polarity is negative as shown in Figure 16, as a threshold for determining whether the welding is good or bad. Here, the rate (%) of negative polarity is the frequency of negative polarity during the pressing process expressed as a percentage.

[0059] When setting various thresholds, for example, as shown in Table 1, for the strain displacement amount, the threshold for determining pass / fail in the process where the polarity is positive (expansion process) is set as S+ (for example, +7 or more is a pass) and the threshold for determining pass / fail in the process where the polarity is negative (pressing process) is set as S- (for example, -10 or more is a pass), and also the threshold for determining pass / fail for the percentage (%) where the polarity is negative (for example, 70% or more is a pass). For other elements, thresholds for determining pass / fail are set as shown in Table 1.

[0060] [Table 1]

[0061] After setting the threshold in this way, if the percentage (%) of negative polarity for the dots of a sampled product is 70% as shown in Table 1, the current judgment threshold (70% or more) is met, so the current judgment threshold (70% or more) can be maintained (there is no need to update it). On the other hand, if the threshold values ​​for the current value and pressure are met but the nugget diameter is small in the random inspection, it is considered that the pressing force is insufficient. To resolve this, the current judgment threshold (70% or more) can be updated to, for example, 71% or more as a pass judgment.

[0062] A threshold value can also be set for the amount of strain displacement, and this can be used to determine whether the product is good or bad. In this case, a threshold value for the amount of distortion displacement during the process when the polarity is positive, or a threshold value for the amount of distortion displacement during the process when the polarity is negative, is set and used for determining whether the product is good or bad. If necessary, the learning unit 66 can be configured to learn based on the sampling inspection information (third information) and update the determination criteria.

[0063] For example, when the updated judgment threshold value in Table 1 is set as the "current judgment threshold value" as shown in Table 2, if the maximum strain displacement amount during the process in which the polarity is positive for a certain sample product's impact point is +7, then since this is greater than or equal to the current judgment threshold value (+7), the current judgment threshold value (+7) can be maintained (it does not need to be updated). On the other hand, if the threshold values ​​for the current value and pressure are met but the nugget diameter is small in the random inspection, it is considered that the base material is not melted enough, and to resolve this, the current judgment threshold value (+7) is updated to, for example, +8 or above as a good judgment.

[0064] [Table 2]

[0065] Furthermore, for example, when the updated judgment threshold value in Table 2 is set as the "current judgment threshold value" as shown in Table 3, if the minimum strain displacement amount during the process in which the polarity is negative for a certain sample product's impact point is -10, then this is the current judgment threshold value (-10 or more), and so the current judgment threshold value (-10 or more) can be maintained (it does not need to be updated). On the other hand, if the threshold values ​​for the current value and pressure are satisfied but the nugget diameter is small in the sampling inspection, it is considered that spatter is occurring. To resolve this, the current judgment threshold (-10 or more) is updated to, for example, -9 or more as a good judgment.

[0066] [Table 3]

[0067] When this replacement was made, random inspections revealed that the electrode diameter was large (i.e., wear was quite advanced), which made it reasonable to assume that the electrode had not been pressed in tightly enough, and this ensured the appropriateness of the replacement.

[0068] On the other hand, as shown in FIG. 13, tip condition information based on the imaging information of the electrodes 12A, 12B, for example, the tip diameter φ, can be compared with the tip condition information of the electrodes for each welding point in the produced spot-welded product, and the comparison results can be used as information for setting or updating a threshold value for determining whether the welding is good or bad in the judgment unit 65.

[0069] The welding quality assurance judgment means 60 further has a quality control unit 67, which can display on the display device 30 on the welding device 10 side that tip dressing or tip replacement is required when the proportion of negative polarity in the amount of strain displacement is on a decreasing trend as shown in Table 4 and the electrode tip condition information indicates wear (tip diameter φ becomes larger). This allows for proper tip dressing or tip replacement, and can also be used to predict electrode wear.

[0070] [Table 4]

[0071] Furthermore, as also shown in Table 4, when the positive polarity of the strain displacement is on a downward trend and the electrode tip condition information based on the imaging information indicates wear (the tip diameter φ becomes larger), the quality control unit 67 can also display on the display device 30 on the welding device 10 side that tip dressing or tip replacement is required. In this case, too, the tip can be appropriately dressed or replaced, and the information can also be used to predict electrode wear.

[0072] The calculation unit 62 can issue a command to stop the welding device 10 if the polarity of the strain displacement amount does not change from positive to negative. This makes it possible to prevent the occurrence of defective products and to control the production of only non-defective products.

[0073] In addition, the output unit 68 can be configured to display the welding failure result when the welding quality is poor in the judgment unit 65 on the display device 30 on the welding device 10 side, or on the overall system management monitor, or both.

[0074] By displaying the welding defect results on the display device 30 on the welding device 10 side, the worker can take immediate action (for example, by stopping the welding device), thereby preventing the continuous production of defective products and allowing the production of only non-defective products. On the other hand, by displaying the results of defective welding on the overall system management monitor, even if there are defects, defective products can be identified (and addressed) without stopping production (without having to inspect all products), thereby improving productivity.

[0075] (Use of information related to the impact points of manufactured products) The memory device 23 of the welding device (welding robot) 10 stores the electrode movement path when welding is performed while moving the electrode in accordance with a welding program for each welding point of the product. The complex movement path affects the quality of the welding at each welding point.

[0076] This example will be described in the case of a product shown in Figures 9 to 11. The product shown in the figures is a side sill of an automobile, in which a long reinforcing plate 36 is fixed to a plurality of bulkheads 35 by spot welding. This product is ultimately fixed in place while covered with a side sill stiffener (not shown). The spot weld points (marked with an x) are indicated by a series of symbols P1, P2, P3...

[0077] In this example, the movement path from the hitting point P1 to the hitting point P2 is short, so the vibration caused by the welding device (including the welding robot) 10 is small. In contrast, when moving from the hitting point P2 to the hitting point P3, the movement is long while avoiding the ridge portion having the corners of the reinforcing plate 36, and the welding gun vibrates greatly. Since the amount of strain displacement for negative polarity components is smaller than that for positive polarity components, they are more susceptible to the influence of vibrations along the movement path. Therefore, the product information and movement path information between each contact point are stored in the memory unit 23, and during the pressing process at each contact point where the polarity becomes negative, for example, the pass / fail judgment threshold value in the judgment unit 65 can be used to set or update the judgment criteria based on the sampling inspection information.

[0078] For example, for a product similar to the product shown in FIGS. 9 to 11, the vibration pattern of the welding gun will differ depending on the relationship between the clamp position and the welding point position (see FIG. 12). That is, at the impact point close to the clamp Cr, the product (workpiece) near the impact point is supported, so the product is less likely to bend and the welding gun vibrations are less likely to be absorbed. On the other hand, at impact points far from the clamp Cr, the product is not supported, so the rigidity of the product near the impact point is low, the product is prone to bending, and the vibrations of the welding gun are easily absorbed.

[0079] Therefore, the welding device 10 is equipped with a robot function that moves the clamp of the product (workpiece) and a pair of electrodes to each welding point, and has a memory unit (memory device 23) that stores the clamp Cr position and each welding point position, and based on this stored information, the learning unit 66 of the welding quality assurance judgment means 60 can use the relationship between the welding pass / fail information for each welding point of the product and the clamp Cr position and each welding point position as information for setting or updating a threshold in the judgment unit 65. As a result, the accuracy of quality determination is improved, making it possible to produce only good products while preventing the production of defective products.

[0080] FIG. 15 explains again the functions of each of the elements of the spot welding quality monitoring system described above. [Explanation of symbols]

[0081] 10...Welding equipment 11A,11B...Electrode support part 12A,12B…electrode 15…Expansion mechanism (electrode contact / separation means) 20...Control device 28...Electrode tip state confirmation means 30…Display device 40...Distortion detector 50...Spot welding quality monitoring system 60...Welding quality assurance judgment method

Claims

1. A welding quality monitoring system including a welding device that includes a pair of electrodes and a pair of electrode support parts that respectively hold the pair of electrodes, and that moves the pair of electrodes in a direction toward each other by opening and closing the electrode support parts using an electrode contact / separation means, and applies pressure and current to members to be welded to weld them, an input unit that acquires the amount of strain detected by a strain detector provided on the electrode support unit; a calculation unit that calculates a strain displacement amount, which is a time change in the strain amount detected by the strain detector, and its polarity; a determination unit that determines the welding quality for each welding point based on a change in polarity of the strain displacement amount, monitoring production in association with the welding point and welding quality; A spot welding quality monitoring system characterized by:

2. 2. The spot welding quality monitoring system according to claim 1, wherein the determination unit determines that the welding is defective when the negative ratio is equal to or less than a predetermined threshold value during the pushing process after the polarity of the strain displacement amount changes from positive to negative.

3. 3. The spot welding quality monitoring system according to claim 1, wherein the input unit further acquires at least one of a current value, a pressure force, a resistance value, and information on the state of the tip of the electrode based on image capture information when the welding point is formed, and uses the acquired information as information for monitoring production that associates the welding point with welding quality.

4. The method further includes a learning unit that acquires welding quality information for each welding point in the spot-welded product produced, 3. The spot welding quality monitoring system according to claim 2, wherein the learning unit uses the welding quality information for each welding spot as information for setting or updating the threshold value in the judging unit.

5. The method further includes a learning unit that acquires welding quality information for each welding point in the spot-welded product produced, 3. The spot welding quality monitoring system according to claim 2, wherein the learning unit uses the information for setting or updating a threshold value for determining whether a welding is good or bad for the distortion displacement amount during the process in which the polarity is positive, or a threshold value for determining whether a welding is good or bad for the distortion displacement amount during the process in which the polarity is negative, for each welding spot.

6. 3. The spot welding quality monitoring system according to claim 2, wherein the electrode tip condition information based on the imaging information is compared with electrode tip condition information for each welding point in a produced spot-welded product, and the comparison result is used as information for setting or updating a threshold value for determining whether the welding is good or bad in the determination unit.

7. 3. The spot welding quality monitoring system of claim 2, further comprising a quality control unit, wherein the quality control unit displays, on a display device on the welding device, a message indicating that tip dressing or tip replacement is required when the proportion of negative polarity in the amount of strain displacement is on a decreasing trend and the electrode tip state information based on the imaging information indicates wear.

8. 3. The spot welding quality monitoring system according to claim 2, further comprising a quality control unit, wherein the quality control unit displays, on a display device on the welding device, a message indicating that tip dressing or tip replacement is required when the positive polarity of the strain displacement amount tends to decrease and when the tip state information based on the image capture information indicates wear.

9. The spot welding quality monitoring system according to claim 1 , wherein the calculation unit issues a stop command to the welding device when the polarity of the strain displacement amount does not change from positive to negative.

10. 3. The spot welding quality monitoring system according to claim 2, further comprising an output unit, wherein the output unit performs at least one of displaying a defective welding result when the welding quality is determined to be poor by the determination unit on a display device on the welding device side and displaying the defective welding result on an overall system management monitor.

11. The method further includes a learning unit that acquires welding quality information for each welding point in the spot-welded product produced, the welding device has a robot function for moving the pair of electrodes to each welding point and a memory unit for storing a movement path; 3. The spot welding quality monitoring system according to claim 2, wherein the learning unit uses the relationship between the welding quality information for each welding point and the movement path as information for setting or updating the threshold value in the determination unit.

12. The method further includes a learning unit that acquires welding quality information for each welding point in the spot-welded product produced, The welding device has a robot function for moving a clamp of the workpiece to be welded and the pair of electrodes to each welding point, and has a memory unit for storing the clamp position and each welding point position, 3. The spot welding quality monitoring system according to claim 2, wherein the learning unit uses the welding quality information for each welding point and the relationship between the clamp position and each welding point position as information for setting or updating the threshold value in the judgment unit.

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