Method for estimating nugget diameter

Abstract

To provide a method for estimating a nugget diameter, that can suppress deterioration in estimation accuracy although a spot position of an electrode deviates from a position specified in a design drawing, in resistance welding.SOLUTION: A method for estimating a nugget diameter in resistance welding includes the steps of: specifying a contraction amount and an expansion amount of an object to be welded at the time of resistance welding; utilizing the specified contraction amount to estimate a distance along a contact surface from an edge end of a contact surface in contact with an electrode on the object to be welded, to a spot position of the electrode; estimating an expansion amount of the object to be welded, from the estimated distance; comparing the estimated expansion amount with a reference value of the expansion amount; calibrating the specified expansion amount at the time of resistance welding by using the comparison result; and estimating a nugget diameter by using the calibrated expansion amount.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a method for estimating a nugget diameter. [Background technology]

[0002] In resistance welding, the diameter of a nugget formed by welding is estimated in order to evaluate the reliability of the welding. For example, Patent Document 1 discloses a technique for estimating the nugget diameter using the resistance value of a preliminary current applied before welding. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-171942 A Summary of the Invention [Problem to be solved by the invention]

[0004] The inventors of the present application have found that there is a correlation between the nugget diameter and the amount of expansion of the workpiece during welding, and that the nugget diameter can be estimated using this amount of expansion. The amount of expansion during welding is correlated with the position of the welding point between the workpiece and the electrode. However, the welding point position may deviate from the position specified in the design drawing. If the welding point position deviates, the amount of expansion varies from the assumption at the time of design, so if the nugget diameter is estimated using this amount of expansion, there is a problem that the estimation accuracy decreases. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] According to one embodiment of the present disclosure, there is provided a method for estimating a nugget diameter in resistance welding, the method comprising the steps of: identifying an amount of contraction and an amount of expansion of a workpiece during resistance welding; estimating a distance along a contact surface of the workpiece from an edge of the contact surface that contacts an electrode to a position of a welding point of the electrode using the identified amount of contraction; estimating an amount of expansion of the workpiece from the estimated distance; comparing the estimated amount of expansion with a reference value of the amount of expansion; correcting the amount of expansion identified during resistance welding using a result of the comparison; and estimating the nugget diameter using the corrected amount of expansion. According to this embodiment, the distance from the edge of the contact surface of the workpiece that contacts the electrode to the welding point of the electrode is estimated, and the expansion amount of the workpiece is estimated from the estimated distance. The estimated expansion amount is then compared with a reference value of the expansion amount, and the expansion amount during resistance welding is corrected using the comparison result. Therefore, even if the welding point of the electrode deviates from the position specified in the design drawing, at least a part of the fluctuation in the expansion amount caused by the deviation of the welding point position is corrected before estimating the nugget diameter, so that a decrease in estimation accuracy can be suppressed. The present disclosure may be realized in various forms other than the method for estimating a nugget diameter, for example, a computer program for implementing the method for estimating a nugget diameter, a non-transitory recording medium on which the computer program is recorded, or the like. [Brief description of the drawings]

[0007] [Figure 1] 1 is a diagram showing a schematic configuration of a resistance welding device according to an embodiment of the present invention; [Diagram 2] FIG. 4 is a process diagram showing a method for estimating a nugget diameter in the first embodiment. [Diagram 3] 1 is a graph showing the correspondence between the average amount of shrinkage of the workpieces during resistance welding and the distance along the contact surface from the edge of the contact surface of the workpieces in contact with the electrode to the position of the electrode's welding point. [Figure 4] 1 is a graph showing the amount of expansion and contraction versus time during resistance welding. [Diagram 5] 1 is a graph showing the correspondence relationship between the distance along the contact surface from an edge of a workpiece to a contact point of an electrode during resistance welding and the integral value of the amount of expansion of the workpiece; [Figure 6] 13 is a graph showing estimated values ​​and actual measured values ​​of nugget diameters when the nugget diameter is estimated according to a comparative example. [Figure 7] 1 is a graph showing estimated values ​​and actual measured values ​​when the nugget diameter is estimated by the method of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] A. First embodiment: A1: Equipment configuration: 1 is a diagram showing a schematic configuration of a resistance welding apparatus 100 in this embodiment. The resistance welding apparatus 100 is an apparatus that applies a current to a plurality of overlapping workpieces W1, W2 to generate Joule heat and join the workpieces W1, W2. The resistance welding apparatus 100 includes a welding gun 10, a robot 20, a resistance measuring device 30, and a control unit 40.

[0009] The workpieces W1 and W2 are metal materials such as aluminum or steel. The workpieces W1 and W2 may be the same type of metal material or different types of metal materials. The workpieces W1 and W2 are not limited to two sheets, and may be three or more sheets.

[0010] The welding gun 10 includes a gun body 1, a pair of electrodes, that is, an upper electrode 2 and a lower electrode 3, and an actuator 4.

[0011] The upper electrode 2 and the lower electrode 3 include shanks 5a, 5b, tips 6a, 6b, and strain sensors 7a, 7b. The upper electrode 2 is attached to the upper part 1a of the gun body 1 via an actuator 4. The lower electrode 3 is disposed in the lower part 1b of the gun body 1. In this embodiment, the upper electrode 2 is a moving electrode, and the lower electrode 3 is a fixed electrode. The tips 6a, 6b are attached to the tips of the shanks 5a, 5b, respectively. The tips 6a, 6b come into contact with the workpieces W1, W2 when resistance welding is performed. The strain sensors 7a, 7b are attached to the sides of the shanks 5a, 5b, respectively. The strain sensors 7a, 7b measure the amount of expansion and contraction of the workpieces W1, W2 during resistance welding. The strain sensors 7a, 7b are sensors that can optically detect the amount of displacement of the workpieces W1, W2, for example. The measured amounts of expansion and contraction are transmitted to the control unit 40.

[0012] The actuator 4 is attached to the upper part 1a of the gun body 1, and moves the upper electrode 2 in a direction to contact the lower electrode 3 or in a direction to move the upper electrode 2 away from the lower electrode 3. As a result, the workpieces W1, W2 are pressed and clamped between the upper electrode 2 and the lower electrode 3 when resistance welding is performed. The actuator 4 is, for example, an electric actuator, a hydraulic actuator, or a pneumatic actuator.

[0013] The robot 20 is, for example, an articulated robot, etc. The robot 20 is operated to move the welding gun 10 in the horizontal and vertical directions and to rotate the welding gun 10 about an arbitrary rotation axis, thereby changing the position and angle of the welding gun 10.

[0014] The resistance measuring device 30 measures the circuit resistance value in the resistance welding apparatus 100. In this embodiment, the circuit resistance value is the resistance value of the entire system including the welding gun 10, the robot 20, the resistance measuring device 30, and the control unit 40. The circuit resistance value measured by the resistance measuring device 30 is transmitted to the control unit 40.

[0015] The control unit 40 controls the operation of the welding gun 10 and the robot 20. Specifically, the control unit 40 adjusts the value of the current flowing through the upper electrode 2 and the lower electrode 3, adjusts the pressure applied to the workpieces W1, W2 by the operation of the actuator 4, and adjusts the positions of the welding points of the upper electrode 2 and the lower electrode 3 on the workpieces W1, W2. The control unit 40 is realized by, for example, a computer having a processor and a memory. The processor is, for example, a central processing unit (CPU), and the memory is, for example, a read only memory (ROM), a random access memory (RAM), and a storage.

[0016] When welding the workpieces W1, W2 using the resistance welding apparatus 100 described above, the upper electrode 2 and the lower electrode 3 clamp the workpieces W1, W2 and apply pressure while passing current between them. The workpieces W1, W2 melt due to resistance heat generated by the current passing through them and then solidify, thereby joining the workpieces W1, W2. A nugget is formed at the joint interface of the joined workpieces W1, W2. The nugget is a portion where the molten parts of the workpieces W1, W2 are cooled and solidified. The nugget has a go stone shape with the joint interface as the center plane.

[0017] A2. Method for estimating nugget diameter: 2 is a process diagram showing a method for estimating a nugget diameter in the first embodiment. The method for estimating a nugget diameter is performed to evaluate the reliability of resistance welding. For example, if the nugget diameter obtained by this estimation method is within a normal range, the reliability of the resistance welding is evaluated as high. On the other hand, if the nugget diameter is out of the normal range, the reliability of the resistance welding is evaluated as low.

[0018] The amounts of contraction and expansion of the workpieces W1, W2 during resistance welding are identified (step S105). In this embodiment, the amount of contraction is the amount of displacement of the workpieces W1, W2 after current flow from the electrodes to the workpieces W1, W2 is terminated. On the other hand, the amount of expansion is the amount of displacement of the workpieces W1, W2 from the start of current flow from the electrodes to the workpieces W1, W2 to the end of current flow. The amounts of contraction and expansion can be identified by measuring using strain sensors 7a, 7b when resistance welding the workpieces W1, W2.

[0019] Using the shrinkage amount determined in step S105, the distance along the contact surface from the edge of the contact surface of the workpieces W1 and W2 that contact the electrodes to the electrode welding point is estimated (step S110). FIG. 3 is a graph showing the correspondence between the average shrinkage amount of the workpieces W1 and W2 during resistance welding and the distance along the contact surface from the edge of the contact surface of the workpieces W1 and W2 that contact the electrodes to the electrode welding point. The horizontal axis shows the average shrinkage amount of the workpieces W1 and W2, and the vertical axis shows the distance along the contact surface from the edge of the contact surface of the workpieces W1 and W2 that contact the electrodes to the electrode welding point. As shown in FIG. 3, when resistance welding is performed by contacting the electrodes at a welding point position closer to the edge than the welding point position specified in the drawing, i.e., the nominal position, i.e., when welding is performed by edge welding, the shrinkage amount is small. This is because when the electrode contact point is close to the edge, the workpieces W1 and W2 are easily crushed when pressed by the electrode, resulting in a small amount of shrinkage. On the other hand, the farther the electrode contact point is from the edge, the greater the amount of shrinkage. This is because the workpieces W1 and W2 are less likely to be crushed by the pressure applied by the electrode compared to when the electrode contact point is close to the edge.

[0020] Such a correspondence relationship between the amount of shrinkage and the welding point position is obtained by performing a simple regression analysis with the distance from the edge to the electrode welding point position as the objective variable and the average value of the amount of shrinkage as the explanatory variable. Specifically, resistance welding is performed on the workpieces W1 and W2 at various welding points, and data on the distance along the contact surface to the electrode welding point position and the amount of shrinkage is accumulated. Next, the accumulated data is plotted and a least squares method is performed to obtain a regression equation. Using such a correspondence relationship and the amount of shrinkage determined in step S105, the distance along the contact surface from the edge of the contact surface of the workpieces W1 and W2 that contacts the electrode to the electrode welding point position is estimated.

[0021] In this embodiment, the average amount of shrinkage refers to the average amount of shrinkage of the workpieces W1, W2 from 600 ms to 700 ms after the start of current flow. Fig. 4 is a graph showing time and the amount of expansion and contraction during resistance welding. The horizontal axis represents time, with the time when current flow started being 0. The vertical axis represents the amount of expansion and contraction. Time t1 is the time when current flow is ended. In other words, the amount of displacement of the workpieces W1, W2 from time 0 to time t1 represents the amount of expansion, and the amount of displacement of the workpieces W1, W2 after time t1 represents the amount of contraction.

[0022] The expansion amounts of the workpieces W1 and W2 are estimated from the distance estimated in step S110 (step S115). FIG. 5 is a graph showing the relationship between the distance along the contact surface from the edge of the contact surface of the workpieces W1 and W2 that contacts the electrode to the electrode contact point during resistance welding, and the integral value of the expansion amount of the workpieces W1 and W2. The horizontal axis shows the distance along the contact surface from the edge of the workpieces W1 and W2 to the electrode contact point, and the vertical axis shows the integral value of the expansion amount of the workpieces W1 and W2. "Nominal" in FIG. 5 shows the expansion amount corresponding to the distance from the edge of the contact point position specified in the nominal drawing. "Edge welding" shows an example of the expansion amount when welding is performed at a position where the contact point position is closer to the edge than the position specified in the nominal drawing. As shown in FIG. 5, the expansion amount increases as the distance to the contact point position is farther from the edge. The reason for this is the same as that explained in step S110 regarding the correspondence between the distance to the impact point position and the contraction amount.

[0023] Such a correspondence relationship between the distance to the welding point position and the amount of expansion is obtained by performing a simple regression analysis with the integral value of the amount of expansion as the objective variable and the distance to the welding point position as the explanatory variable. Specifically, resistance welding is performed on the workpieces W1 and W2 at various welding point positions, and data on the distance from the edge to the electrode welding point position and the amount of expansion is accumulated. Next, a regression equation is obtained by plotting the accumulated data and performing the least squares method. The amount of expansion of the workpieces W1 and W2 is estimated using such a correspondence relationship and the distance from the edge to the electrode welding point position estimated in step S110.

[0024] The expansion amount estimated in step S115 is compared with a reference value of the expansion amount (step S120). In this embodiment, the reference value of the expansion amount refers to the expansion amount measured when the electrodes are brought into contact with the workpieces W1 and W2 to be welded at the welding point positions defined in the design drawings and resistance welding is performed. The reference value can be obtained from the correspondence shown in FIG. 5 and the distance from the edge of the workpieces W1 and W2 to the welding point positions of the electrodes defined in the design drawings. In step S120, the estimated expansion amount is divided by the reference value to calculate the ratio of the estimated expansion amount to the reference value (estimated expansion amount / reference value).

[0025] Using the result of the comparison in step S120, the expansion amount determined during resistance welding is corrected (step S125). In step S125, the expansion amount determined in step S105 is corrected. Specifically, the determined expansion amount is multiplied by the inverse of the ratio of the estimated expansion amount to the reference value calculated in step S115 (reference value / estimated expansion amount).

[0026] The nugget diameter is estimated using the expansion amount corrected in step S125 (step S130). The nugget diameter is estimated by a multiple regression equation with the nugget diameter as the objective variable and the expansion amount, contraction amount, and circuit resistance value of the welded objects W1 and W2 during resistance welding as explanatory variables. Here, the larger the expansion amount and contraction amount, the larger the estimated nugget diameter, and the larger the resistance value, the smaller the estimated nugget diameter. Such a multiple regression equation is obtained by accumulating the expansion amount, contraction amount, and circuit resistance value when resistance welding is performed as experimental data and performing multiple regression analysis. When estimating the nugget diameter using such a multiple regression equation, if the expansion amount measured by the strain sensors 7a and 7b is used as it is, the fluctuation in the expansion amount caused by the fluctuation in the electrode contact point position is not taken into consideration, so there is a risk that the estimation accuracy of the nugget diameter will decrease. Therefore, the expansion amount is corrected as described in step S125, and the nugget diameter is estimated using the corrected expansion amount.

[0027] According to the method for estimating the nugget diameter in the embodiment described above, even if the electrode welding point positions of the workpieces W1 and W2 deviate from the positions specified in the design drawings, the nugget diameter is estimated by correcting the fluctuation in the expansion amount caused by the fluctuation in the electrode welding point positions. Therefore, even if the electrode welding point positions deviate from the positions specified in the design drawings, it is possible to suppress a decrease in the estimation accuracy of the nugget diameter.

[0028] FIG. 6 is a graph showing the estimated value and the actual measurement value of the nugget diameter when the nugget diameter is estimated by the comparative example. In the comparative example, the expansion amount is not corrected and used in the multiple regression equation for estimating the nugget diameter. FIG. 7 is a graph showing the estimated value and the actual measurement value when the nugget diameter is estimated by the method of the embodiment. In FIG. 6 and FIG. 7, the horizontal axis shows the estimated value of the nugget diameter, and the vertical axis shows the actual measurement value of the nugget diameter. In addition, in FIG. 6 and FIG. 7, the solid line L1 is a line where the estimated value and the actual measurement value match, that is, a line showing that the estimation accuracy is 100%. The broken line L2 is a line showing that the estimated value is 10% short of the actual measurement value. The broken line L3 is a line showing that the estimated value is 10% over the actual measurement value. Here, the estimation accuracy refers to the ratio of the estimated value based on the actual measurement value. As shown in FIG. 6, in the comparative example, the estimation accuracy was within the range of -25% to 9%. The reason why there are many data in which the estimated value is smaller than the actual measurement value is as follows. If the electrode welding point position is closer to the edge than the position specified in the design drawing, the workpieces W1 and W2 pressed by the electrode are more likely to be crushed. This results in a smaller expansion amount, and the estimated value is estimated to be smaller than the actual value. This results in a large amount of data in which the estimated value is smaller than the actual measured value. As shown in FIG. 7, according to the method of the embodiment, the nugget diameter is estimated after correcting the amount of expansion fluctuation due to the deviation of the electrode welding point position, so that the estimation accuracy was within the range of -14% to +11%. This shows that the method of estimating the nugget diameter of the embodiment suppresses the decrease in estimation accuracy.

[0029] In addition, according to the method of the present embodiment, since the decrease in the estimation accuracy of the nugget diameter can be suppressed, the reliability of the weld can be calculated while maintaining a certain level. Therefore, the ultrasonic inspection process performed by an inspector to check the reliability of the weld can be omitted, and the manufacturing cost can be reduced.

[0030] B. Other embodiments: (B1) In step S110 of the first embodiment, the average value of the contraction amount from 600 ms to 700 ms after the start of energization is used, but the average value of the contraction amount in any time range may be used. Also, instead of the average value of the contraction amount, the maximum value or the integral value of the contraction amount may be used.

[0031] (B2) In step S120 of each embodiment, the reference value of the amount of expansion is the amount of expansion measured when resistance welding is performed by bringing the electrodes into contact with the workpieces W1 and W2 at welding points specified in the design drawings. Alternatively, the reference value may be the amount of expansion measured when resistance welding is performed by bringing the electrodes into contact with the workpieces W1 and W2 at any welding point.

[0032] (B3) In each embodiment, the nugget diameter may be estimated by the control unit 40. In such a case, the control unit 40 may estimate the nugget diameter by executing a program recorded in memory. Also, the correspondence relationship between the amount of contraction and the distance to the hitting point position in step S110, and the correspondence relationship between the distance to the hitting point position and the amount of expansion in step S115 may be recorded in the memory of the control unit 40.

[0033] (B4) In each embodiment, the resistance welding apparatus 100 has been described in which the upper electrode 2 is a moving electrode and the lower electrode 3 is a fixed electrode, but instead of this configuration, the upper electrode 2 may be a fixed electrode and the lower electrode 3 may be a moving electrode. Also, both the upper electrode 2 and the lower electrode 3 may be moving electrodes.

[0034] (B5) In each embodiment, the amount of expansion and contraction is measured by the strain sensors 7a, 7b. However, instead of this, the amount of expansion and contraction may be measured using the amount of displacement of the movable electrode.

[0035] (B6) In each embodiment, the amount of expansion used to estimate the nugget diameter may be the maximum or average value of the displacement of the workpieces W1 and W2 in the time range from the start of energization to the end of energization. Also, instead of the time range from the start of energization to the end of energization, any time range may be used.

[0036] (B7) In each embodiment, if the distance from the edge to the electrode welding point position estimated in step S110 deviates from the distance from the edge to the electrode welding point position specified in the design drawing, the control unit 40 may issue a notification to correct the electrode welding point position to the position specified in the design drawing. The control unit 40 may also control the welding gun 10 and the robot 20 to correct the positions of the workpieces W1 and W2.

[0037] The present disclosure is not limited to the above-mentioned embodiment, and can be realized in various configurations without departing from the spirit of the present disclosure. For example, the technical features of the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention can be appropriately replaced or combined to solve some or all of the above-mentioned problems or to achieve some or all of the above-mentioned effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0038] W1, W2...workpiece, 1...gun body, 1a...upper part, 1b...lower part, 2...upper electrode, 3...lower electrode, 4...actuator, 5a, 5b...shank, 6a, 6b...tip, 7a, 7b...strain sensor, 10...welding gun, 20...robot, 30...resistance measuring device, 40...control unit, 100...resistance welding device

Claims

[Claim 1] A method for estimating a nugget diameter in resistance welding, comprising: determining an amount of contraction and an amount of expansion of a workpiece during the resistance welding; estimating a distance along a contact surface from an edge of a contact surface of the workpiece that contacts the electrode to a striking point of the electrode using the identified amount of shrinkage; estimating an expansion amount of the workpiece from the estimated distance; comparing the estimated expansion amount with a reference value of the expansion amount; using the results of the comparison to correct the amount of expansion determined during the resistance welding; estimating the nugget diameter using the corrected expansion amount; The method for estimating a nugget diameter comprises:

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