Estimating method and estimating device for nugget diameter

Abstract

To provide a technique that can determine whether a gap has occurred between materials to be welded and whether welding is defective.SOLUTION: An estimating method for a nugget diameter of materials welded by spot welding includes: an expansion amount calculating step of calculating expansion amounts of the welded materials being energized, using amounts of displacement and amounts of applied voltages in an electrode; an inclination calculating step of dividing a characteristic line showing a temporal change having the expansion amounts set as a vertical axis and time set as a lateral axis into a plurality of time sections, and calculating respective inclinations of the characteristic line in the plurality of time sections; a discriminating step of discriminating whether spot welding is defective or not, when a minimum expansion amount of the calculated expansion amounts is below or equal to a predetermined threshold, using a discriminant created from historic data showing a temporal change of expansion amounts in spot welding performed in the past including the inclination in an input variable; and an estimating step of estimating a nugget diameter by correcting the expansion amounts, when it is discriminated that the spot welding is not defective.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] There is known a technique for estimating the diameter of a nugget formed in a workpiece by resistance spot welding using the expansion amount of the workpiece. Patent Document 1 discloses a technique for estimating the nugget diameter using a regression line obtained from the inter-electrode displacement amount during welding current flow and a saturation point of the displacement amount. In addition, there is known a technique for estimating the nugget diameter by correcting the expansion amount of the workpiece when it is estimated that a gap occurs between the workpieces. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2000-79482 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the case of poor welding, it may be erroneously determined that a gap has occurred between the workpieces, and the nugget diameter may be estimated using the corrected amount of expansion. [Means for solving the problem]

[0005] The present disclosure has been made to solve the above-mentioned problems, and can be realized in the following forms.

[0006] (1) According to one embodiment of the present disclosure, there is provided a method for estimating a nugget diameter of workpieces joined by spot welding, the method including: an expansion amount calculation step of calculating an expansion amount of the workpieces during current flow using a displacement amount and a pressure amount of an electrode; a slope calculation step of dividing a characteristic line showing a change over time, with the expansion amount as the vertical axis and time as the horizontal axis, into a plurality of time intervals and calculating a slope of each of the characteristic lines in the plurality of time intervals; a determination step of determining whether the spot welding was defective when a minimum expansion amount among the calculated expansion amounts is equal to or less than a predetermined threshold value, using a discriminant created from past data showing a change over time of the expansion amount in the past spot welding, the discriminant including the slope as an input variable; and an estimation step of correcting the expansion amount to estimate the nugget diameter when it is determined that the spot welding is not defective. According to this estimation method, when the minimum expansion amount is equal to or less than a predetermined threshold and it is determined that the spot welding is not defective, the expansion amount is corrected to estimate the nugget diameter. Therefore, when the welding is defective, it is possible to prevent an erroneous determination that a gap has occurred between the welded materials and to prevent an estimation of the nugget diameter using the corrected expansion amount. (2) In the estimation method of the above aspect, the discriminant is represented by the following formula (1):

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[0007] The present disclosure can be realized in various forms, for example, in the form of a resistance spot welding apparatus, an expansion amount correction method for a resistance spot welding apparatus, or the like. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing a configuration of a resistance spot welding device. [Diagram 2] 4 is a flowchart showing an example of a method for estimating a nugget diameter. [Diagram 3] FIG. 13 is a diagram showing an example of past data of past expansion amounts. [Figure 4] This is a determination result indicating that the determination as to whether or not the spot welding was defective was performed correctly. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] A. First embodiment: Fig. 1 is a schematic diagram showing the configuration of a resistance spot welding apparatus 100 in this embodiment. In the following description, the up-down direction is a direction parallel to the ascending and descending direction of an upper electrode tip 21, which is a movable electrode and will be described later, as shown in Fig. 1. The resistance spot welding apparatus 100 joins a workpiece W, which is composed of two or more overlapping metal plates, by resistance spot welding. The resistance spot welding apparatus 100 includes a welding gun 10, a power supply device 30, a welding force measuring unit 51, a displacement amount measuring unit 52, a current sensor 53, a voltage sensor 54, and a control unit 80.

[0010] The workpieces W are steel plates. In this embodiment, the workpieces W are exemplified by a first metal plate W1 and a second metal plate W2 that are overlapped with each other.

[0011] The welding gun 10 is attached to the tip of a robot arm (not shown). The gun body 11 is moved by the robot arm to a target welding point of the workpieces W. The welding gun 10 has the gun body 11, a moving mechanism 20, a pair of opposing electrode tips 21, 22, and a pressure device 40. The upper electrode tip 21 is a movable electrode and is attached to the upper part of the gun body 11. The lower electrode tip 22 is a fixed electrode and is attached to the lower part of the gun body 11 at a position opposite to the upper electrode tip 21. Note that the upper electrode tip 21 may be a fixed electrode and the lower electrode tip 22 may be a movable electrode.

[0012] The moving mechanism 20 raises and lowers the upper electrode tip 21. The moving mechanism 20 has a servo motor (not shown), converts the rotational force of the servo motor into a linear movement force in the raising and lowering direction, and transmits the converted linear movement force to the upper electrode tip 21 to raise and lower the upper electrode tip 21. The power supply unit 30 supplies a welding current of a target current value between the pair of electrode tips 21, 22. The pressure device 40 has a cylinder (not shown), and presses the electrode tips 21, 22 against the workpieces W to apply pressure. Due to the shape of the workpieces W and manufacturing variations, etc., the metal plates W1, W2 may not be crushed completely, resulting in a gap where they do not come into contact with each other. This gap is also called a "plate gap."

[0013] The pressure measuring unit 51 measures the pressure applied by the electrode tips 21, 22 to the workpieces W, and transmits a signal indicating the pressure to the control unit 80 at predetermined time intervals. The displacement measuring unit 52 measures the displacement of the electrode tips 21, 22, and transmits a signal indicating the displacement to the control unit 80 at predetermined time intervals. The displacement measuring unit 52 obtains the position of the upper electrode tip 21 by detecting the rotation angle position of the output shaft of the servo motor of the movement mechanism 20, for example, by an encoder (not shown). The displacement measuring unit 52 also obtains the position of the lower electrode tip 22 by measuring the amount of strain of the robot arm, and multiplying the amount of strain by a coefficient that is predetermined as a characteristic of the welding gun 10.

[0014] The current sensor 53 detects the current value of the welding current supplied from the power supply device 30, and transmits a signal indicating the current value to the control unit 80 at predetermined time intervals. The current sensor 53 is realized using, for example, a toroidal coil. The voltage sensor 54 detects the voltage value between the upper electrode tip 21 and the lower electrode tip 22 at predetermined time intervals, and transmits a signal indicating the voltage value to the control unit 80.

[0015] The control unit 80 is composed of a microcomputer or the like constituted by a central processing unit (CPU) and storage devices such as RAM and ROM. The control unit 80 controls the operation of the resistance spot welding apparatus 100 by the microcomputer executing a program installed in advance. More specifically, the control unit 80 comprehensively controls the current value, the current application time, the electrode pressure, the current application timing, and the pressure application timing. The control unit 80 also estimates the nugget diameter of the workpieces W that are joined by spot welding. The control unit 80 is also called an estimation device. However, some or all of the functions may be realized by a hardware circuit. The storage device also stores in advance various data such as the amount of expansion under standard conditions with no plate gap, and a discrimination formula to be described later.

[0016] FIG. 2 is a flow chart showing an example of a method for estimating the nugget diameter. This process may be performed after the completion of spot welding, or may be performed in parallel with spot welding. In step S100, the expansion amount of the workpieces W during current flow is calculated. More specifically, the expansion amount of the workpieces W during current flow is calculated using the displacement amount and pressure amount of the upper electrode tip 21. The expansion amount of the workpieces W is the expansion amount in the perpendicular direction to the surface direction of the workpieces W. This process is also called an "expansion amount calculation process." In the control unit 80, the function of performing this process is also called an "expansion amount calculation unit."

[0017] In step S110, a characteristic line showing a change over time with the expansion amount on the vertical axis and the time on the horizontal axis is divided into a plurality of time intervals, and the slope of the characteristic line in each time interval is calculated. This step is also referred to as a "slope calculation step." In the control unit 80, a function that performs this step is also referred to as a "slope calculation unit." The number and positions of the time intervals to be divided are determined by a discriminant used in step S130, which will be described later. In this embodiment, the slope of a time interval is a value calculated by dividing the difference between the expansion amount at the start timing of the time interval and the expansion amount at the end timing of the time interval by the length of the time interval.

[0018] In step S120, it is determined whether the minimum expansion amount Emin is equal to or less than a predetermined threshold value Eth. The threshold value Eth is an expansion amount that can determine that there is a high possibility that a gap was formed between the workpieces when the current was started. The threshold value Eth is a value smaller than 0, and is determined experimentally and empirically. The threshold value Eth is, for example, -0.1 mm. If the minimum expansion amount Emin is equal to or less than the threshold value Eth, that is, if there is a high possibility that a gap was formed between the workpieces when the current was started, the process proceeds to step S130. On the other hand, if the minimum expansion amount Emin is greater than the threshold value Eth, that is, if there is a low possibility that a gap was formed between the workpieces when the current was started, the process proceeds to step S145, and the nugget diameter is estimated using the expansion amount calculated in step S100. For example, the nugget diameter is estimated using the resistance waveform and expansion amount calculated from the current value and voltage value of the current in spot welding.

[0019] In step S130, the slope calculated in step S110 and a discriminant created from past data are used to determine whether the spot weld was defective. The past data is information indicating changes over time in the amount of expansion in spot welds performed in the past. The discriminant is an equation that includes the slope as an input variable. This step is also referred to as the "discrimination step." In the control unit 80, the function that performs this step is also referred to as the "discrimination unit."

[0020] In this embodiment, the discriminant is an equation that is created by performing a discriminant analysis on past data that is determined to be highly likely to have formed a gap between the workpieces when current is started, using a well-known SQC (Statistical Quality Control) method. The past data includes information that indicates a change in the amount of expansion over time when the spot welding is good, and information that indicates a change in the amount of expansion over time when the spot welding is poor. For example, the discriminant is expressed by the following equation (1).

[0021]

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[0022] Here, C1, C2, Ck, and Ca are constants, Emax is the maximum value of the expansion amount, Emin is the minimum value of the expansion amount, and Rk is the slope in interval k.

[0023] FIG. 3 is a diagram showing an example of past data on the amount of expansion in the past. FIG. 3 shows characteristic lines indicating changes over time with the amount of expansion on the vertical axis and time on the horizontal axis. Characteristic line g1 is a characteristic line when the spot welding is good and a gap is formed. Characteristic line g2 is a characteristic line when the spot welding is defective. In all of the characteristic lines g1 shown in FIG. 3, the minimum amount of expansion is equal to or less than the threshold value Eth. In other words, whether the spot welding is defective cannot be determined only based on whether the minimum amount of expansion is equal to or less than the threshold value Eth.

[0024] Timing T0 is the timing at which the expansion amount is the smallest for the first time after the start of energization in past data when a gap is formed. The timing at which the expansion amount is the smallest for the first time is the timing at which the formed gap disappears. In this embodiment, timing T0 is a timing used by using a statistical method, such as an average value of the timing at which the expansion amount is the smallest for the first time after the start of energization in past data when a gap is formed, which is used to generate a discriminant. Timing T1 is a timing at which a predetermined threshold time has elapsed from timing T0. The threshold time is, for example, 100 ms. Timing T3 is a timing at which energization is terminated in spot welding. Timing T2 is a timing after timing T1 and before timing T3.

[0025] The start position of the section in the discriminant is preferably located after the time when the expansion amount is first smallest after the start of energization in past data when a gap is formed. That is, the start position of the section in the discriminant is preferably located after timing T0. In this embodiment, timing T1 is the start position of the section. The length of time section A3 from timing T1 to timing T2 is shorter than the length of time section A4 from timing T2 to timing T3.

[0026] In this embodiment, formula (1) is created so that y is 0 or more when the spot weld is good, and y is less than 0 when the spot weld is poor. In formula (1), for example, C1 is 65.559, C2 is -1104.583, n is 4, C3 is -83662.495, C4 is -82449.156, and Ca is 259.281. Section R3 is time section A3. Section R4 is time section A4.

[0027] In step S130 (see FIG. 2), if it is determined that the spot welding is not a failure, more specifically, if y in formula (1) is equal to or greater than 0, the process proceeds to step S140. On the other hand, if it is determined that the spot welding is a failure, more specifically, if y in formula (1) is less than 0, the nugget diameter estimation process ends.

[0028] In step S140, the expansion amount is corrected to estimate the nugget diameter. More specifically, the nugget diameter is estimated using the corrected expansion amount. This step is also referred to as the "estimation step." In the control unit 80, the function that performs this step is also referred to as the "estimation unit." In this embodiment, the expansion amount is corrected using a correction amount that is determined using the difference between the timing T0 at which the minimum expansion amount Emin is reached and a reference timing that is stored in advance. The reference timing is the timing at which the expansion amount is minimum when no gap is formed between the workpieces W when current flow begins and the spot welding is good.

[0029] The correction amount can be calculated, for example, by the following formula (2): The expansion amount after correction can be calculated by the following formula (3).

[0030] Cm = α × (Tth - T0) + β … (2)

[0031] Here, Cm is the correction amount, Tth is the reference timing, and α and β are constants.

[0032] Ec = Ep + Cm…(3)

[0033] Here, Ec is the corrected expansion amount, and Ep is a value obtained by performing zero point correction by vertically shifting the characteristic line so that the minimum expansion amount Emin becomes 0.

[0034] In step S140 of this embodiment, the nugget diameter is estimated by replacing the expansion amount in the method for estimating the nugget diameter in step S145 with the corrected expansion amount. More specifically, the nugget diameter is estimated using the resistance waveform calculated from the current value and voltage value of the current supplied in spot welding and the corrected expansion amount.

[0035] FIG. 4 shows the discrimination result indicating that the discrimination of whether the spot welding was defective or not was correctly performed in step S130 (see FIG. 2). FIG. 4 shows the discrimination result using the above-mentioned formula (1) for 1510 joined members W whose minimum expansion amount is equal to or less than the threshold as a predicted number, and the discrimination result discriminated by visual inspection as an actual number. The discriminated 1510 joined members W are the actual number of joined members W whose spot welding was good, which is 1500, and the actual number of joined members W whose spot welding was defective, which is 10. By the discriminant formula, 1500 joined members W were discriminated to have good spot welding, and 10 joined members W were discriminated to have defective spot welding. That is, the accuracy rate of the discrimination result using formula (1) is 100%.

[0036] According to the method for estimating the nugget diameter in the present embodiment described above, when the minimum expansion amount is equal to or less than a predetermined threshold, it is determined whether or not the spot weld is defective using a judgment formula. When it is determined that the spot weld is not defective, the expansion amount is corrected to estimate the nugget diameter. Therefore, when the weld is defective, it is possible to prevent a mistaken judgment that a gap has occurred between the welded materials and to estimate the nugget diameter using the corrected expansion amount. Furthermore, if the judgment formula is created in advance, it is possible to determine whether or not the weld is defective, and therefore the estimation method can be easily introduced.

[0037] In addition, the start position of the section in the discriminant is located after the time when the expansion amount is first smallest after the start of energization in the past data when a gap is formed. In the past data when a gap is formed, the characteristic line in the time section before the time when the expansion amount is first smallest after the start of energization is approximate to the characteristic line of that time section in the past data when the spot weld is defective. Therefore, the accuracy of determining whether the spot weld is defective is improved compared to when the start position of the section in the discriminant is located before the time when the expansion amount is first smallest after the start of energization.

[0038] B. Other embodiments: (B1) In the above-described embodiment, the lengths of the multiple time intervals in the discriminant formula (1) are different. This is not limiting, and the lengths of two or more of the multiple time intervals in the formula (1) may be equal. Furthermore, the multiple time intervals do not have to be consecutive time intervals and can be determined arbitrarily.

[0039] (B2) In the above embodiment, the slope of a time segment is a value calculated by dividing the difference between the expansion amount at the start timing of the time segment and the expansion amount at the end timing of the time segment by the length of the time segment. The slope of a time segment may be calculated, for example, by using the least squares method.

[0040] (B3) In the above-described embodiment, the discriminant may be an equation for determining whether the slope of the increase after the expansion rate has decreased is equal to or greater than a predetermined threshold value. As shown in Fig. 3, a characteristic line g1 indicating a case where the spot welding is not a failure increases significantly after the expansion rate has decreased. A characteristic line g2 indicating a case where the spot welding is a failure does not increase as much as the characteristic line g1 after the expansion rate has decreased. Therefore, it is possible to determine whether the spot welding is a failure based on the slope of the increase after the expansion rate has decreased.

[0041] (B4) In the above-described embodiment, in step S140, the nugget diameter is estimated by replacing the expansion amount in the method for estimating the nugget diameter in step S145 with a corrected expansion amount. That is, the method for estimating the nugget diameter in step S140 and the method for estimating the nugget diameter in step S145 differ in that the expansion amount is corrected. However, the method for estimating the nugget diameter in step S140 and the method for estimating the nugget diameter in step S145 may differ in other ways besides the expansion amount being corrected.

[0042] 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 in the embodiments corresponding to the technical features in each form described in the Summary of the Invention column can be appropriately replaced or combined in order to solve the above-mentioned problems or to achieve some or all of the above-mentioned effects. Furthermore, if the technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0043] 10...welding gun, 11...gun body, 20...movement mechanism, 21...upper electrode tip, 22...lower electrode tip, 30...power supply, 40...pressure device, 51...pressure measuring section, 52...displacement measuring section, 53...current sensor, 54...voltage sensor, 80...control section, 100...resistance spot welding device, W...workpiece, W1...first metal plate, W2...second metal plate

Claims

1. A method for estimating a nugget diameter of welded materials joined by spot welding, comprising the steps of: an expansion amount calculation step of calculating an expansion amount of the workpiece during current flow using a displacement amount and a pressure amount of the electrode; a slope calculation step of dividing a characteristic line showing a change over time, the characteristic line being a vertical axis of the expansion amount and a horizontal axis of time, into a plurality of time intervals, and calculating a slope of each of the characteristic lines in the plurality of time intervals; a discrimination step of discriminating whether or not the spot welding was defective when a minimum expansion amount among the calculated expansion amounts is equal to or less than a predetermined threshold value, using a discriminant created from past data indicating a change over time in the expansion amount in the past spot welding, the discriminant including the slope as an input variable; and when it is determined that the spot weld is not defective, correcting the amount of expansion to estimate the nugget diameter.

2. 2. The estimation method according to claim 1, The discriminant is represented by the following formula (1): [0030] C1, C2, Ck, and Ca are constants, Emax is the maximum value of the expansion amount, Emin is the minimum value of the expansion amount, and Rk is the slope in section k. the determining step determining that the spot weld was not defective if y in formula (1) is 0 or more, and determining that the spot weld was defective if y in formula (1) is less than 0.

3. The estimation method according to claim 1 or 2, a start position of the section in the discriminant is positioned after a time at which the expansion amount first becomes smallest after the start of energization in the past data when the spot welding was not defective.

4. An apparatus for estimating a nugget diameter of a workpiece joined by spot welding, comprising: an expansion amount calculation unit that calculates an expansion amount of the workpiece during current flow using a displacement amount and a pressure amount of the electrode; a slope calculation unit that divides a characteristic line showing a change over time, the characteristic line being a vertical axis of the expansion amount and a horizontal axis of time, into a plurality of time intervals, and calculates a slope of each of the characteristic lines in the plurality of time intervals; a discrimination unit that discriminates whether or not the spot welding was defective when a minimum expansion amount among the calculated expansion amounts is equal to or less than a predetermined threshold value, using a discriminant created from past data indicating a change over time of the expansion amount in the past spot welding, the discriminant including the slope as an input variable; an estimation unit that, when it is determined that the spot weld is not defective, corrects the amount of expansion to estimate the nugget diameter.

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