Spot welding device, welding method, and spot welding quality monitoring system

Abstract

[Problem] To provide a spot welding device and a welding method that can be used to accurately determine whether the weld zone (nugget) is good or bad even if the plate thickness changes, and a spot welding quality monitoring system that can guarantee weld quality. [Solution] The device includes a welding gun 10, the welding gun 10 comprising a pair of electrodes 12A, 12B and a pair of electrode supporting parts 11A, 11B for holding the electrodes 12A, 12B, respectively, wherein the pair of electrodes 12A, 12B are moved in mutually approaching directions by moving the electrode supporting parts 11A, 11B by means of an electrode contacting / separating means 15, and a member 30 to be welded is pressurized and energized for welding. The device includes a strain detector 40 provided to the welding gun 10, and a determination means 28 for determining the quality of welding on the basis of a change in polarity of a strain displacement amount, which is a temporal change in the amount of strain detected by the strain detector 40.

Description

Spot welding device, welding method, and spot welding quality monitoring system

[0001] The present invention relates to a spot welding apparatus, a welding method, and a spot welding quality monitoring system that perform spot welding and are capable of determining the quality of a welded portion (nugget) in spot welding and providing welding assurance.

[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 members 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 in the electrode movement direction of the electrode drive part due to the expansion and contraction of the welded portion (nugget) during welding and the amount of deflection of the welding gun due to the pressure applied from the electrode to the workpiece.

[0004] Patent No. 3593981

[0005] However, while Patent Document 1 estimates the expansion / contraction of the nugget based on the correlation between the inter-electrode movement amount and time, this expansion / contraction of the nugget deforms the welding device and is 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 device, a welding method, and a spot welding quality monitoring system that can accurately determine the quality of a welded portion (nugget), for example, in-line.

[0008] The above-described problems are solved in the following embodiments: (First Aspect) A spot welding device that opens and closes a pair of electrodes attached to an electrode support portion of a welding gun by moving a drive portion of an electrode drive means having a drive portion connected to at least one of the pair of electrodes, and applies pressure to and passes current through the pair of electrodes to weld members, the device comprising: a strain detector provided on the electrode support portion, and determination means that determines the quality of welding based on a change in polarity of a strain displacement amount, which is a time-varying change in the amount of strain detected by the strain detector.

[0009] (Second aspect) A spot welding method in which a pair of electrodes attached to an electrode support portion of a welding gun are moved toward each other by moving a drive portion of an electrode drive means having a drive portion connected to at least one of the pair of electrodes, and the pair of electrodes pressurize and energize the welded members, wherein a strain detector is provided on the electrode support portion, and the quality of the weld is determined based on a change in polarity of a strain displacement amount, which is a change over time in the amount of strain detected by the strain detector.

[0010] (Third aspect) 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 opens and closes the electrode support parts using an electrode contact / separation means to move the pair of electrodes toward each other and weld the workpieces by applying pressure and passing current through them, the system comprising: an input unit that acquires a strain amount detected by a strain detector provided on the electrode support parts; a calculation unit that calculates a strain displacement amount and its polarity, which is a change over time in the strain amount detected by the strain detector; and a determination unit that determines the welding quality for each welding point based on the change in polarity of the strain displacement amount, and monitors production that associates the welding points with welding quality.

[0011] According to the present invention, by focusing on the change in the amount of strain displacement of the electrode support part over time during expansion and contraction of the nugget part in spot welding and determining the change in polarity of the amount of strain displacement, it is possible to determine the quality of the weld, even in an in-line welding process. Also provided are a spot welding device, a welding method, and a spot welding quality monitoring system that can accurately determine the quality of the weld part (nugget) even if the plate thickness changes.

[0012] 1 is a schematic diagram of a spot welding device and a spot welding quality monitoring system;

[0023] FIG. 1 is a graph showing changes in distortion and distortion displacement in relation to the explanation of terms;

[0024] FIG. 2 is a graph showing an example of changes over time in distortion and distortion displacement detected by a distortion detector and welding current in the case of a good weld;

[0025] FIG. 3 is a graph showing an example of changes over time in distortion and distortion displacement detected by a distortion detector and welding current in the case of a poor weld;

[0026] FIG. 4 is a graph showing an example of changes over time in distortion and distortion displacement detected by a distortion detector and welding current in the case of a spattering;

[0027] FIG. 5 is an explanatory diagram showing changes over time in the welding condition of a good weld;

[0028] FIG. 6 is an explanatory diagram showing changes over time in the welding condition of a poor weld;

[0029] FIG. 7 is an explanatory diagram showing a case where tip dressing or tip replacement is required;

[0029] FIG. 8 is a schematic perspective view of a product and an example of a welding point;

[0030] FIG. 9 is a cross-sectional view;

[0031] FIG. 10 is a schematic perspective view of a support part of a product;

[0032] FIG. 11 is a schematic perspective view of a product, a welding point, and an example of a clamp position;

[0033] FIG. 12 is a schematic diagram of an electrode;

[0034] FIG. 13 is an explanatory diagram of the configuration of a spot welding quality monitoring system;

[0035] FIG. 14 is an explanatory diagram of an example of information processing;

[0036] FIG. 15 is an explanatory diagram of an example of threshold setting.

[0013] Next, embodiments of the present invention will be described in detail below with reference to the drawings.

[0014] (Overview of Welding Apparatus) Fig. 1 shows the main components of an example welding apparatus, in which the welding apparatus 10 has a pair of a first electrode support 11A and a second electrode support 11B. For details of the welding apparatus 10, please also refer to Fig. 14. The first electrode support 11A and the second electrode support 11B hold a first electrode 12A and a second electrode 12B at their tips, respectively.

[0015] The first electrode support 11A and the second electrode support 11B are connected at their intermediate portions by a support shaft 13. The first electrode support 11A and the second electrode support 11B are connected to a holding arm 14 which is connected to the tip of a moving means, for example, a robot arm (not shown), and the tip of the holding arm 14 holds both the first electrode support 11A and the second electrode support 11B so that they can swing around the support shaft 13. Note that although the electrode support in the embodiment is a gun arm of a welding gun, it may also be an electrode support arm of a stationary resistance welding machine.

[0016] 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 of the extension / contraction mechanism 15 is connected to the base end of the second electrode support part 11B. As a result, as the extension / contraction mechanism 15 extends or contracts, the first electrode support part 11A and the second electrode support part 11B rotate about the support shaft 13, and the first electrode 12A of the first electrode support part 11A and the second electrode 12B of the second electrode support part 11B move toward and away from each other (opening and closing). When the first electrode 12A and the second electrode 12B are moved in a direction toward each other, a predetermined pressure is applied to the target workpieces 30 to be welded (e.g., overlapping automotive steel sheets) via the tips at the ends of the first electrode 12A and the second electrode 12B.

[0017] 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.

[0018] The central processing unit 22 outputs commands to the extension / retraction 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, thereby controlling the welding pressure required for welding. The central processing unit 22 also controls the current applied to the electrodes. Note that although the welding device in the embodiment maintains the welding pressure and welding current values ​​constant, they may also be changed.

[0019] 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.

[0020] 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 the current supply device.

[0021] When the welding device 10 is provided on an articulated robot, the control device 20 can be built into the robot control device (not shown) or can be independently arranged in parallel.

[0022] On the other hand, the welding apparatus 10 is provided with an electrode tip state confirmation means 28 for the first electrode 12A and the second electrode 12B. 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 mode of the imaging device is not shown. The welding apparatus 10 is also provided with a display device 30.

[0023] In the 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.

[0024] Spot welding quality monitoring system 50 in the embodiment includes welding device 10 and welding quality assurance determination means 60 for welding device 10. Various types of information during welding by welding device 10 are input to input section (acquisition section) 61 of welding quality assurance determination means 60. For example, signals from distortion amount output means configured by distortion detector 40, current value output means 25, welding force output means 26, resistance value output means 27, and electrode tip state confirmation means 28 are input to input section (acquisition section) 61 of welding quality assurance determination means 60.

[0025] The first information (amount of strain) input from the strain amount output means to input unit (acquisition unit) 61 is processed by calculation unit 62, and the amount of strain displacement and its polarity are passed to judgment criterion creation unit 64 in judgment storage unit 63, where judgment unit 65 judges whether the welding is good or bad according to the judgment criterion. The result of judgment unit 65 on the quality of the welding is displayed on display device 30 of welding device 10, and if the welding is defective, measures to deal with the defect are also displayed.

[0026] On the other hand, second information, namely, the current value from the current value output means 25, the pressure force from the pressure force 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 memory unit 63, and the judgment criteria in the judgment unit 65 are updated based on this second information.

[0027] In addition, 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 groups, 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.

[0028] The results of the assessment made by the learning unit 66 are displayed on an overall system management monitor attached to the spot welding quality monitoring system 50 from the output unit 68 via the quality control unit 67, 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 the production history. This production data includes various information such as the polarity, current value, and pressure for each product.

[0029] 6 is a schematic diagram illustrating the process of forming a nugget from the time when the first electrode 12A and the second electrode 12B of the welding gun 10 are moved toward each other (closed) and a predetermined pressure is applied to the two overlapping workpieces 30 to be welded, to the time when the welding gun 10 starts to be opened.

[0030] When current begins to flow, resistance heating begins around the interface of the overlapping parts to be welded 30, the base material is heated and expands toward the electrodes (stage S1), and with this expansion, a 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).

[0031] After that, while the base material continues to expand, its temperature 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 pressing force takes precedence over the expansion force (Step S3). When this force reaches its maximum, it is determined that a sufficient welded portion (nugget) Na has been formed, and current application is terminated (Step S4). Finally, when cooling is completed, the first electrode 12A and the second electrode 12B are separated from each other, and the welding gun 10 is opened (Step S5). After going through the above steps, a sufficient welded portion (nugget) Na has been formed, and it can be determined that the welding has been performed well.

[0032] 7, the welded portion (nugget) Na is not sufficiently formed, and the welding can be determined to be defective. That is, even though current is turned on and resistance heating of the workpieces 30 begins (Step S1), sufficient heat is not generated and the welded portions of the base metals only expand slightly (Step S2). Therefore, the reaction force acting from the base metal to the first electrode 12A and the second electrode 12B in the opening direction, separating them from each other, increases slightly, but to a small extent (Step S3).

[0033] Thereafter, although the welded portion (nugget) Na continues to expand, the current flow is terminated without sufficient melting or softening of the base material (step S4). When cooling is finally completed, the first electrode 12A and the second electrode 12B are separated from each other, and the welding gun 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 occurs between the two layers of the welded member 30.

[0034] (Determining the Quality of Welding) In order to determine whether the welding is good when the process shown in FIG. 6 is observed or whether the welding is poor when the process shown in FIG. 7 is observed, in this embodiment, a strain detector 40 is provided in the welding device 10. The strain detector 40 may be installed in either the first electrode support 11A or the second electrode support 11B. It is also possible to provide the strain detector 40 in both the first electrode support 11A and the second electrode support 11B, but since this would complicate the processing of the strain signal, it is sufficient to provide it in one of the gun arms (electrode support parts). In this embodiment, the strain detector is provided in the first electrode support part 11A. Note that providing the strain detector in the fixed electrode support part allows for a more stable waveform to be obtained.

[0035] As shown in Figure 2, the strain detector 40 defines the difference between two consecutively sampled strain amounts Lt1 and Lt2 as a strain displacement amount ΔLt2 = Lt2 - Lt1. The polarity of the strain displacement amount is positive (+) due to the expansion process. Meanwhile, during the pressing process, the difference between the two strain amounts Lt3 and Lt4 is defined as a strain displacement amount ΔLt4 = Lt4 - Lt3, and the polarity of the strain displacement amount is negative (-). In this way, the quality of the weld is determined based on the change over time in the polarity of the strain displacement amount under current and pressure.

[0036] The change over time in the amount of strain detected by the strain detector 40 is shown in both Figures 6 and 7. In the case of a "good weld" shown in Figure 6, when current is applied, the electrode support part opens, so the amount of strain shows a change in positive displacement, reaches a maximum in a relatively short period of time, and then shows a gradual change in negative displacement as the base material melts and the electrode support part is pressed in. When current is applied and cooling is started, the rate of change in the amount of negative displacement of the strain increases until cooling is complete.

[0037] In the case of the "poor welding" shown in Figure 7, the amount of strain shows a change in positive displacement from the start of energization until the completion of energization, but the rate of change in the amount of strain is slow even over a long period of time. Once energization is completed and cooling begins, the rate of change in the amount of negative displacement of the strain increases until cooling is completed.

[0038] 6 and 7, it can be inferred that capturing the change over time in the amount of strain displacement detected by strain detector 40 during pressure application and current flow is effective for determining the quality of the spot weld, i.e., the formation of a nugget. A determination means 28 for determining the quality of this nugget formation, i.e., the quality of the spot weld, is included in control device 20 shown in FIG.

[0039] An example of the output of the strain detector 40 in the case of a "good weld" is shown in Figure 3. This is an example of the output of the strain detector 40 per sampling unit time (e.g., 20 milliseconds). When current begins to flow, expansion occurs due to resistance heating. The amount of strain increases. The overlapping welded members WM are heated, and although the change in the amount of strain subsequently slows, at a certain point, the base material (the welded members WM) begins to melt, mainly at the overlapping interface. As the current continues to flow and heating continues, the base material melts and softens, resulting in the growth and softening of a nugget where the base material melts. As a result, the pressure of the first electrode 12A and the second electrode 12B becomes dominant over the expansion, and a period occurs in which a force is generated in a direction that moves the first electrode 12A and the second electrode 12B closer to each other (the period marked "pressure applied" in Figure 3).

[0040] The term "push" refers to the melting and softening of the base material, which causes the pressure of the first electrode 12A and the second electrode 12B to prevail over the expansion force and push the workpiece WM into place. When the pressure of the first electrode 12A and the second electrode 12B prevails over the expansion force and this prevails for a predetermined period of time, it can be determined that a sufficient weld zone (nugget) Na has been formed. The welding current is then reduced to zero, and the current flow 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.

[0041] During the welding process described above, the welded portion (nugget) continues to expand while the base metal begins to melt and soften. As a result, the pressure applied by the first and second electrodes 12A and 12B becomes dominant over the expansion, and a force is generated in the direction that the first and second electrodes 12A and 12B approach each other (the "push-in" period). This period begins midway through the period of steady current flow. This indicates that sufficient welded portion (nugget) Na is being formed. As shown in the example output of the strain detector 40 in Figure 3, during the "push-in" period, the "polarity" of the strain amount at a given sampling point, which indicates whether the "displacement" is positive or negative relative to the strain amount at the previous sampling point, is negative. Therefore, if multiple consecutive samples with negative "polarity" occur midway through the period of steady current flow, the weld can be determined to be "good."

[0042] On the other hand, Figure 4 shows an example of the output of the strain detector 40 in the case of a "poor weld." This occurs when the electrode contacts the workpiece at an angle, the electrode tip is worn, the current density is low, or debris or other foreign matter adheres to the overlapping interface of the workpieces, preventing the appropriate current density for forming a nugget. In this case, resistance heating is low, so the pressure of the first electrode 12A and the second electrode 12B does not prevail over the expansion. Therefore, the "no push-in" state persists throughout the entire period of steady current flow. Furthermore, throughout the entire period of steady current flow, the "polarity" remains positive and never becomes negative. This expansion / pushing phenomenon is independent of the thickness of the workpieces. Therefore, welding judgment based on the amount of strain displacement is faster than conventional welding judgment based on the amount of strain per thickness, enabling welding inspection during automatic welding on a mass production line.

[0043] As described above, when the polarity of the strain displacement changes over time to a negative value, the welding can be determined to be good. However, although a good welding can be determined when the polarity of the strain displacement changes over time to a negative value only once, the determination may lack stability. For example, even if the polarity of the strain displacement changes to a negative value only once, it may later change to a positive value due to the vibration of the welding gun or the reaction force of the plunge. Therefore, it is desirable to determine a good welding result when the polarity is negative or zero repeatedly. For example, as shown in Figure 3, the first, second, and third samples, which indicate the start of the "good welding" period, show three consecutive negative values. In such cases, a good welding result can be determined with stability. Furthermore, a good welding result can also be determined when the polarity repeatedly changes from negative to zero.

[0044] FIG. 5 shows the changes in strain and strain displacement when spatter occurs. Even when spatter occurs, multiple instances of negative polarity can occur. When spatter occurs, molten metal is scattered, causing a sudden indentation, which can suddenly change the polarity to the negative side, and multiple instances of negative polarity can occur afterward. Therefore, when the polarity suddenly changes to the negative side and multiple instances of negative polarity occur afterward, it can be determined that spatter has occurred. The spatter occurrence record can also be used as data for adjusting the next welding conditions. Comparing such instances of negative polarity with instances of spatter occurrence further clarifies the significance of determining a weld as good based on the criteria described in paragraph 0043 above.

[0045] On the other hand, in order to determine whether the welding is good or bad, it is desirable to incorporate a monitoring device (not shown) for the primary current value during welding, in consideration of the possibility of unexpected factors occurring, and to determine that 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.

[0046] When determining the quality of spot welding, it is desirable to be able to use the device shown in Figure 1, for example, even when the thickness and number of welded parts are changed in multiple combinations. Fortunately, according to the above embodiment, it is possible to determine that the weld is good when the polarity is negative, or when the polarity is negative or zero multiple times. Even if the thickness of the welded parts changes or the degree of strain displacement differs, the polarity does not change, so the same determination method can be used.

[0047] (Spot welding quality monitoring system) The quality of the above-mentioned welding can be determined by a determination unit 65 of a welding quality assurance determination means 60 shown in Fig. 14. In this embodiment, the quality of the welding is determined based on the fact that the change in polarity of the strain displacement amount of the electrode support part during pressure application and current flow changes from positive to negative over time during expansion and contraction of the nugget part of the spot welding. The determination unit 65 can determine that the welding is defective if, for example, the proportion of negative polarity 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.

[0048] The welding quality assurance judgment means 60 is configured to not only judge the quality of welding, but also monitor the welding state and incorporate various elements to ensure long-term welding stability. This welding quality assurance judgment means 60 stores the time-varying polarity changes in the amount of strain displacement for each welding point of each product, each product, and each production lot in a judgment memory unit 63, and outputs and saves this as production data, thereby ensuring production quality assurance. In other words, traceability can be established because the quality of each individual product in a production lot is guaranteed before delivery.

[0049] The first function of the welding quality assurance determination 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 period thereafter, as shown in Figures 6 and 8, which are reproductions of Figure 3, and whether the expansion process is occurring and whether the material to be welded has subsequently begun to melt.

[0050] On the other hand, since the electrode tip wears out with multiple welding operations, tip dressing or tip replacement has generally been performed periodically depending on the number of welding points. Tip dressers and their use for this purpose are disclosed in Japanese Patent Application Laid-Open No. 2021-79399 and Japanese Patent Application Laid-Open No. 2018-103200. The above-described embodiments can be used as indicators of when to dress or replace the tip.

[0051] For this purpose, the second function of the welding quality assurance determination means 60 is to predetermine an upper limit for the distortion displacement in the positive polarity region in the upslope process when the polarity is positive as shown in Figures 3 and 8, in the case of a good chip at its usable limit, and to predetermine a lower limit threshold value S+ (see Figure 16) for the distortion displacement at, for example, "plus 7".

[0052] At a certain point when the wires are actually stacked at the impact point, if the change in polarity of the strain displacement detected by the strain detector 40 over time 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 can be determined that the tip has deteriorated or that irregular welding has caused the target portion of the material to be welded to expand but not melt sufficiently, and the tip can be dressed or replaced using a tip dresser, or the irregularity can be resolved.

[0053] 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 a good tip would have started to be pushed in, the welding quality assurance judgment means 60 can determine that no push-in is occurring and that tip dressing or tip replacement is required.

[0054] 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) for the strain displacement can be set to, for example, "minus 10."

[0055] 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.

[0056] 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.

[0057] 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.

[0058] The input unit 61 of the welding quality assurance determination means 60 preferably inputs (acquires) not only the distortion amount of the first information but also at least one of the current value, pressure, and resistance value when forming the welding point, and electrode tip state information based on image capture information (second information), and uses this information to monitor production by correlating the welding point and the like with the welding quality. The second information is a factor that causes changes in welding quality over time as production progresses.

[0059] The welding quality assurance judgment means 60 preferably 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 for each welding point (third information) 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.

[0060] Specifically, the learning unit 66 can use the percentage (%) of the amount of strain displacement with negative polarity for each welding point during the process (push-in process) when the polarity is negative, as a threshold for determining whether the welding is good or bad. Here, the percentage (%) of negative polarity refers to the frequency of negative polarity during the push-in process, expressed as a percentage.

[0061] When setting various thresholds, for example, as shown in Table 1, for the strain displacement, a threshold S+ is set for determining pass / fail in the process where the polarity is positive (expansion process) (for example, +7 or more is determined as a pass / fail judgment), and a threshold S- is set for determining pass / fail in the process where the polarity is negative (pressing process) (for example, -10 or more is determined as a pass / fail judgment), and a threshold is also set for determining pass / fail for the proportion (%) of negative polarity (for example, 70% or more is determined as a pass / fail judgment). For other elements, thresholds for pass / fail judgment are also set as shown in Table 1.

[0062]

[0063] After setting the thresholds in this way, if the rate (%) of negative polarity for the welding points of a certain sampled product is 70% as shown in Table 1, the current judgment threshold (70% or more) is satisfied, so the current judgment threshold (70% or more) can be maintained (it does not need to be updated). On the other hand, if the thresholds are satisfied for the current value and pressure, but the nugget diameter is small in the sampling inspection, it is considered that the pressing force is insufficient, and to resolve this, the current judgment threshold (70% or more) can be updated to, for example, 71% or more as a pass judgment.

[0064] A threshold value can also be set for the distortion displacement amount and used for determining whether the product is good or bad. In this case, a threshold value for the distortion displacement amount in a process in which the polarity is positive or a threshold value for the distortion displacement amount in a process in which the polarity is negative is set and used for determining whether the product is good or bad. If necessary, the learning unit 66 can learn based on the sampling inspection information (third information) and update the determination criteria.

[0065] 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 the welding point of a certain sampled product is +7, the current judgment threshold value (+7) is equal to or greater than the current judgment threshold value (+7), so the current judgment threshold value (+7) can be maintained (it does not need to be updated). On the other hand, if the current value and pressure satisfy the threshold values ​​but the nugget diameter is small in the sampling inspection, it is considered that the base material is not melted sufficiently, and to resolve this, the current judgment threshold value (+7) is updated so that, for example, +8 or more is a pass judgment.

[0066]

[0067] 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 the welding point of a certain sampled product is -10, this is the current judgment threshold value (-10 or more), 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 current value and pressure satisfy the threshold values ​​but the nugget diameter is small in the sampling inspection, it is considered that spatter has occurred, and to resolve this, the current judgment threshold value (-10 or more) is updated to, for example, -9 or more as a pass judgment.

[0068]

[0069] When this replacement was made, random inspection revealed that the electrode diameter was large (i.e., wear was quite advanced), which suggested that the electrode had not been pressed in tightly enough, and this confirmed the appropriateness of the replacement.

[0070] 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 the quality of the weld in the judgment unit 65.

[0071] The welding quality assurance determination means 60 further includes a quality control unit 67, which can display on the display device 30 of the welding device 10 a message indicating that tip dressing or tip replacement is required when the proportion of negative polarity strain displacement is on a downward trend and the electrode tip condition information indicates wear (the tip diameter φ is increasing), as shown in Table 4. This allows tip dressing or tip replacement to be performed appropriately, and can also be used to predict electrode wear.

[0072]

[0073] 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 is increased), the quality control unit 67 can also display on the display device 30 of the welding device 10 that tip dressing or tip replacement is required. Even in this case, tip dressing or tip replacement can be performed appropriately, and the information can also be used to predict electrode wear.

[0074] When the polarity of the strain displacement amount does not change from positive to negative, the calculation unit 62 can issue a command to stop the welding device 10. This prevents the production of defective products and enables management to produce only non-defective products.

[0075] 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.

[0076] 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), and only good products can be produced without continuously producing defective products. On the other hand, by displaying the welding defect results on the overall system management monitor, even if a defect is found, defective products can be identified (and addressed) without stopping production (without having to inspect all products), thereby improving productivity.

[0077] (Use of Information Related to Welding Points of a Product) The memory device 23 of the welding device (welding robot) 10 stores the electrode movement path when welding while moving the electrode in accordance with the welding program for each welding point of the product. A complex movement path affects the quality of welding for each welding point.

[0078] This example will be described for the product shown in Figures 9 to 11. The product shown in the figures is an automobile side sill in which long reinforcing plates 36 are fixed to multiple bulkheads 35 by spot welding. This product will ultimately be fixed in place while covered by a side sill stiffener (not shown). The spot welding points (marked with "x") are indicated by a series of symbols P1, P2, P3, etc.

[0079] In this example, the movement path from welding point P1 to welding point P2 is short, so the vibrations caused by the welding device (including the welding robot) 10 are small. In contrast, when moving from welding point P2 to welding point P3, the welding gun must move a long distance while avoiding the cornered ridge of the reinforcing plate 36, resulting in significant vibrations of the welding gun. Components with negative polarity have smaller distortion displacements than components with positive polarity, and are therefore more susceptible to vibrations along the movement path. Therefore, the memory unit 23 stores the product information and movement path information between each welding point. During the pressing process at each welding point, for example, when the polarity is negative, the threshold value for pass / fail judgment in the judgment unit 65 can be used to set or update the judgment criteria based on the sampling inspection information.

[0080] For example, for products similar to those shown in Figures 9 to 11, the relationship between the clamp position and the impact point (see Figure 12) will result in differences in the vibration pattern of the welding gun. That is, at impact points 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 vibration is less likely to be absorbed. On the other hand, at impact points far from the clamp Cr, the product is not supported, so the product is less rigid near the impact point, so the product is more likely to bend and the welding gun vibration is more likely to be absorbed.

[0081] Therefore, welding device 10 has a robot function for moving the clamp of the product (workpiece) and the pair of electrodes to each welding point, and has a memory unit (storage device 23) for storing the clamp Cr position and each welding point position, and based on this stored information, learning unit 66 of welding quality assurance judgment means 60 can use the relationship between the welding quality information for each welding point of the product and the clamp Cr position and each welding point position as information for setting or updating thresholds in judgment unit 65. As a result, the accuracy of quality judgment is improved, making it possible to produce only good products while preventing the production of defective products.

[0082] FIG. 15 explains again the functions of the above-mentioned elements of the spot welding quality monitoring system.

[0083] DESCRIPTION OF SYMBOLS 10... Welding device 11A, 11B... Electrode support portion 12A, 12B... Electrode 15... Extension 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 means

Claims

1. A spot welding device comprising a pair of electrodes and a pair of electrode support parts that respectively hold the pair of electrodes, wherein the electrode support parts are opened and closed by an electrode contact / separation means to move the pair of electrodes in a direction toward each other, and weld the workpieces by applying pressure and passing current through them, the device comprising: a strain detector provided on the electrode support parts; and a judgment part that judges the quality of welding based on a change in polarity of the strain displacement amount, which is a change over time in the amount of strain detected by the strain detector.

2. The spot welding device according to claim 1, wherein said determining unit determines that the welding is good when said polarity changes from positive to negative during pressure application and current flow.

3. The spot welding device according to claim 1, wherein the determining unit determines that the welding is good when the polarity changes from positive to negative and the negative, positive, or zero polarity is repeated multiple times.

4. The spot welding device of claim 1, wherein the judgment unit judges the welding to be good when both of the following conditions are met: the polarity changes from positive to negative and the negative polarity is repeated multiple times with a positive or zero polarity; and the welding current is constant.

5. The spot welding device according to claim 1, wherein said determining unit records the change in polarity of the strain displacement amount over time for each production lot in a storage device and outputs the record.

6. A spot welding method comprising a pair of electrodes and a pair of electrode support parts that respectively hold the pair of electrodes, wherein the electrode support parts are opened and closed by an electrode contact / separation means to move the pair of electrodes in a direction toward each other, and then pressurize and energize the workpieces to weld, characterized in that a strain detector is provided on the electrode support parts, and the quality of the welding is judged based on a change in polarity of the strain displacement amount, which is a change over time in the amount of strain detected by the strain detector.

7. The spot welding method according to claim 5, wherein the method judges any of the following (1) to (4): (1) when the polarity of the strain displacement amount, which is the change in the amount of strain over time detected by the strain detector, is only positive, it is judged that the melting of the workpiece is insufficient and that the workpiece is in the expansion process; (2) when the polarity of the strain displacement amount, which is the change in the amount of strain over time detected by the strain detector, is positive and the positive amount of the strain displacement amount is below a predetermined threshold, it is judged that tip dressing or tip replacement is required, or that irregular welding is occurring; (3) when the polarity of the strain displacement amount, which is the change in the amount of strain over time detected by the strain detector, is positive and the duration of the positive polarity is longer than a predetermined duration, it is judged that tip dressing or tip replacement is required; (4) when the polarity of the strain displacement amount, which is the change in the amount of strain over time detected by the strain detector, is negative and the negative amount of the strain displacement amount exceeds a predetermined threshold, it is judged that tip dressing or tip replacement is required.

8. A spot welding method according to claim 6, wherein the criteria for determining whether the welding is good or bad are the same even when the plate thicknesses of the workpieces to be welded are different.

9. A welding quality monitoring system comprising a welding device comprising a pair of electrodes and a pair of electrode support parts that respectively hold the pair of electrodes, and which moves the pair of electrodes toward each other by opening and closing the electrode support parts using an electrode contact / separation means, and which welds the workpieces by applying pressure and passing current through them, the system comprising: a strain detector provided on the electrode support parts; an input part that acquires the amount of strain detected by the strain detector; a calculation part that calculates the polarity of the amount of strain displacement that is a change over time in the amount of strain detected by the strain detector; a determination part that determines the welding quality for each welding point based on the change in polarity of the amount of strain displacement; and a spot welding quality monitoring part that monitors production in which the welding points are associated with welding quality.

10. The spot welding quality monitoring system of claim 9, wherein the judgment unit judges the welding to be defective when the negative ratio is below a predetermined threshold during the pushing process after the polarity of the strain displacement amount changes from positive to negative.

11. A spot welding quality monitoring system as described in claim 9 or 10, wherein the input unit further acquires at least one of the current value, pressure, resistance value, and tip state information of the electrode based on image capture information when the welding point is formed, and uses this information as information for monitoring production that associates the welding point with welding quality.

12. A spot welding quality monitoring system as described in claim 10, further comprising a learning unit that takes in welding quality information for each welding point in a produced spot-welded product, and the learning unit uses the welding quality information for each welding point as information for setting or updating the threshold value in the judgment unit.

13. A spot welding quality monitoring system as described in claim 10, further comprising a learning unit that takes in welding quality information for each welding point in a spot-welded product produced, and the learning unit uses the information to set or update a threshold value for determining whether the welding is good or bad for the amount of distortion displacement during a process in which the polarity is positive, or a threshold value for determining whether the welding is good or bad for the amount of distortion displacement during a process in which the polarity is negative, for each welding point.

14. A spot welding quality monitoring system as described in claim 10, wherein the electrode tip condition information based on the imaging information is compared with the electrode tip condition information for each welding point in the produced spot-welded product, and the comparison results are used as information for setting or updating a threshold value for determining whether the welding is good or bad in the determination unit.

15. The spot welding quality monitoring system of claim 9, further comprising a quality control unit, which, when the proportion of negative polarity in the strain displacement amount is on a decreasing trend and the electrode tip condition information based on the image information indicates wear, causes a display device on the welding device to display a message indicating that tip dressing or tip replacement is required.

16. The spot welding quality monitoring system of claim 10 further comprises a quality control unit, wherein the quality control unit displays on the display device of the welding device a message indicating that tip dressing or tip replacement is required when, when the maximum values ​​of the distortion displacement amount are arranged in chronological order for each welding point during a polarity positive process in which the polarity of the distortion displacement amount is positive, the maximum value of the distortion displacement amount shows a decreasing trend and the electrode tip condition information based on the image capture information indicates wear.

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

18. The spot welding quality monitoring system according to claim 10, further comprising an output section, which performs at least one of displaying a welding defect result when the welding quality is determined to be poor by the judgment section on a display device on the welding device side and displaying it on an overall system management monitor.

19. A spot welding quality monitoring system as described in claim 10, further comprising a learning unit that takes in welding quality information for each welding point in the spot-welded product produced, wherein the welding device has a robot function that moves the pair of electrodes to each welding point and a memory unit that stores the movement path, and 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 judgment unit.

20. A spot welding quality monitoring system as described in claim 10, further comprising a learning unit that takes in welding quality information for each welding point in the spot-welded product produced, wherein the welding device has a robot function that moves the clamp of the product to be welded and the pair of electrodes to each welding point, and has a memory unit that stores the clamp position and each welding point position, and 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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