Method for estimating nugget diameter in resistance spot welding

Abstract

To provide an estimation method for a nugget diameter that can improve accuracy in estimating the nugget diameter.SOLUTION: An estimation method for a nugget diameter, which estimates the nugget diameter using a prescribed estimating formula in which a welding parameter contributing to a size of a nugget diameter which is formed in resistance spot welding is adopted as explanatory variables and an estimated value of the nugget diameter is adopted as objective variables, comprises a resistance value obtaining step of obtaining a latter-half resistance value which is an electric resistance value in a latter half of an energization period at the time of welding, which is an electric resistance value of an entire energizing circuit including an object to be welded, and a nugget diameter calculating step of calculating the nugget diameter, by plugging the obtained resistance value in the latter-half into the estimation formula.SELECTED DRAWING: Figure 3

Description

[Technical field]

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

[0002] Conventionally, resistance spot welding has been used as a means for joining multiple metal plates together when manufacturing vehicle body frames and the like. In this resistance spot welding, a current is passed between a pair of electrodes while the workpieces (multiple metal plates) are sandwiched between them, and the metal plates are melted and joined together using Joule heat generated by the electrical resistance of the workpieces themselves. In order to increase the joint strength of the welded portion between the metal plates, it is necessary to ensure a sufficient weld nugget diameter obtained by this resistance spot welding. For this reason, a method for estimating whether a sufficient weld nugget diameter has been ensured is required.

[0003] For example, in the method of estimating the nugget diameter in a resistance spot welding apparatus described in Patent Document 1, a preliminary current is applied to register a master pattern under preset welding conditions. Then, when performing main welding under the preset welding conditions, the weld nugget diameter obtained in main welding is estimated based on the deviation between the electrical resistance value in the master pattern and the electrical resistance value when main welding is performed. In this estimation method, the electrical resistance value is calculated using all information during the current application period, for example, from the start of current application to the end of current application. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-171942 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, when there are various disturbances at the welding site, such as gaps between steel sheets, inclination of the steel sheet, wear of the tip of the electrode pressed against the steel sheet, etc., the correlation between the electrical resistance value and the nugget diameter is not high throughout the entire current flow period, resulting in a problem of poor accuracy in estimating the nugget diameter. [Means for solving the problem]

[0006] The present disclosure can be realized in the following forms.

[0007] (1) According to one aspect of the present disclosure, there is provided a method for estimating a nugget diameter, which estimates a nugget diameter by using a predetermined estimation equation having welding parameters that contribute to the size of a nugget diameter formed in resistance spot welding as explanatory variables and an estimated value of the nugget diameter as a response variable, and includes a resistance value acquisition step of acquiring, as the welding parameter, a latter-half resistance value that is an electrical resistance value in a latter half of a current-carrying period during welding and is an electrical resistance value of an entire current-carrying circuit including a workpiece to be welded, and a nugget diameter calculation step of substituting the acquired latter-half resistance value into the estimation equation to calculate the nugget diameter. The current conduction period during the welding includes an initial current conduction period and a main current conduction period that is performed at an interval after the initial current conduction, the latter half resistance value is a resistance value in the latter half of the main current conduction period, and the estimation formula is nugget diameter = C1 x [later half resistance value] + C2 (C1 and C2 are predetermined constants). do. According to this embodiment, the nugget diameter is estimated using the latter resistance value, which is the resistance value in the latter half of the current flow, as a welding parameter. The inventors of the present application have found that the latter resistance value has a higher correlation with the nugget diameter than the resistance value in the first half of the current flow. The accuracy of estimating the nugget diameter can be improved by using the latter resistance value, which has a high correlation with the nugget diameter, as a welding parameter for estimation. In addition, when the current flow period includes initial current flow and main current flow, the accuracy of estimating the nugget diameter can be further improved by using the resistance value in the latter half of the main current flow period, which is a period with higher correlation, as a welding parameter. (2) In the above form, The estimation formula may be a linear equation calculated by performing linear regression on plot points in which the latter resistance value and the nugget diameter are associated with each other. According to this embodiment, the correlation between the latter resistance value and the nugget diameter can be uniquely expressed. Also, the estimation formula can be easily created by using the plot points in which the latter resistance value and the nugget diameter are associated with each other. [Brief description of the drawings]

[0008] [Figure 1] 1 is a schematic configuration diagram showing a nugget diameter estimation system according to a first embodiment of the present disclosure. FIG. [Diagram 2] FIG. 2 is a block diagram showing a schematic configuration of a control device. [Diagram 3] 4 is a flowchart showing the steps of a method for estimating a nugget diameter. [Figure 4] FIG. 13 is a graph showing the change in resistance value over time. [Diagram 5] 5 is an enlarged view of a portion of the graph in FIG. 4 surrounded by a two-dot chain line. [Figure 6] FIG. 13 is a diagram showing the relationship between the resistance value and the nugget diameter in the latter half of energization. [Figure 7] FIG. 13 is a diagram for explaining the effect when the nugget diameter is estimated using the latter resistance value as a welding parameter. [Figure 8] FIG. 13 is a diagram for explaining the effect when the nugget diameter is estimated using the latter resistance value as a welding parameter. [Figure 9] FIG. 11 is a diagram showing a change in welding current in a second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] A. First embodiment: A1. Overall configuration of nugget diameter estimation system 1: In describing a method for estimating a nugget diameter in a first embodiment of the present disclosure, first, a configuration of a nugget diameter estimation system 1 (hereinafter also simply referred to as "estimation system 1") will be described. FIG. 1 is a schematic configuration diagram showing a nugget diameter estimation system 1 according to a first embodiment of the present disclosure. The estimation system 1 is a system that estimates a nugget diameter of a weld formed on a workpiece W by resistance spot welding performed by a resistance spot welding apparatus 10. The estimation system 1 includes the resistance spot welding apparatus 10, a control device 100, and a measurement mechanism 9.

[0010] The resistance spot welding apparatus 10 is an apparatus that melts and joins a plurality of overlapping metal plates W1, W2 to be welded together W. The resistance spot welding apparatus 10 includes a welding gun G and a robot arm RA.

[0011] The welding gun G includes a gun body 11, a pair of electrodes, an upper electrode 2 and a lower electrode 3, an electrode lifting device 4, a pressure device 5, and a current adjusting device 6. The gun body 11 is held by a robot arm RA. The lower electrode 3 is a fixed electrode arranged in a fixed state on the lower part 11b of the gun body 11. The upper electrode 2 is a movable electrode that is movable in a direction along the opposing direction of the upper electrode 2 and the lower electrode 3. The upper electrode 2 is attached to the upper part 11a of the gun body 11 via the electrode lifting device 4. The upper electrode 2 and the lower electrode 3 each have a flow path (not shown) formed therein for circulating cooling water.

[0012] The electrode lifting device 4 is an electric device that holds and lifts the upper electrode 2. The electrode lifting device 4 is attached to the tip of the upper part 11a of the gun body 11. The electrode lifting device 4 includes a servo motor 41 and a lifting member 42 that is coupled to the drive shaft of the servo motor 41. The electrode lifting device 4 lifts and lowers the lifting member 42 by operating the servo motor 41 in accordance with a lifting command from the control device 100.

[0013] The pressure device 5 presses the upper electrode 2 and the lower electrode 3 in a direction in which the upper electrode 2 and the lower electrode 3 approach each other. Specifically, the pressure device 5 is a device that applies a force to each of the upper electrode 2 and the lower electrode 3 in order to press the welded member W with a predetermined pressure force in a state in which the welded member W is sandwiched between the upper electrode 2 and the lower electrode 3. The pressure device 5 applies a predetermined force to each of the upper electrode 2 and the lower electrode 3 in response to a pressure command from the control device 100.

[0014] The current adjusting device 6 adjusts the value of the welding current (hereinafter, referred to as the current value) flowing between the upper electrode 2 and the lower electrode 3 in response to a current command transmitted from the control device 100. The current adjusting device 6 is, for example, a device equipped with a variable resistor or a device equipped with a converter.

[0015] 2 is a block diagram showing a schematic configuration of the control device 100. In this embodiment, the control device 100 has a function of controlling the operation of the resistance spot welding device 10, as well as a function as an estimation device for estimating a nugget diameter in resistance spot welding. Note that the estimation device may be configured separately from the control device 100 and may be configured to communicate data via wire or wirelessly.

[0016] The control device 100 includes a communication unit 30, a display 40, an input operation unit 50, a storage unit 60, and a CPU 20. The control device 100 is, for example, a computer including each of the components 20 to 60. The communication unit 30 connects the resistance spot welding device 10 and the measurement mechanism 9 to the control device 100 so that they can communicate with each other. The display 40 is, for example, a liquid crystal display, and displays information in response to commands from the CPU 20. The input operation unit 50 includes, for example, a keyboard and a mouse, and receives instructions from a user.

[0017] The storage unit 60 stores various programs for controlling the operation of the resistance spot welding device 10 and various data including an estimation formula 670. The storage unit 60 includes a RAM, a ROM, a rewritable non-volatile memory, and the like. The estimation formula 670 is a relational expression prepared in advance for calculating an estimated value of the nugget diameter. Specifically, the estimation formula 670 is a relational expression in which a welding parameter contributing to the size of the nugget diameter is an explanatory variable, and an estimated value of the nugget diameter is an objective variable. In the first embodiment, the estimation formula 670 is expressed by the following linear equation (1) using a latter resistance value, which is an electric resistance value in the latter half of a current application period during welding and is an electric resistance value of the entire current application circuit including the welding object, as a welding parameter. The "later resistance value" will be described in detail later. Nugget diameter = C1 × [later resistance value ]+C2 (C1 and C2 are given constants) Equation (1)

[0018] The CPU 20 executes various programs stored in the storage unit 60, thereby functioning as an operation control unit 200, an acquisition unit 210, and an estimation unit 290.

[0019] The operation control unit 200 controls the operation of the resistance spot welding apparatus 10. The operation of the resistance spot welding apparatus 10 is controlled in accordance with parameters (hereinafter, welding parameters) that are preset for welding. That is, the welding parameters are welding conditions that are preset when welding is performed. The welding parameters are, for example, a current value, a voltage value, a resistance value, an amount of displacement between electrodes, an amount of expansion, and a welding force of the electrodes 2, 3. The operation control unit 200 controls these operations in an integrated manner. The operation control unit 200 controls the operation of the resistance spot welding apparatus 10 in accordance with the set values ​​of the welding parameters that are preset by the user via the input operation unit 50, for example.

[0020] The acquisition unit 210 acquires the latter resistance value during welding described above. The estimation unit 290 calculates an estimated value of the nugget diameter by substituting the latter resistance value into the estimation formula (the above formula (1)). Details of each function will be described later together with a method of estimating the nugget diameter. At least a part of the functions of the CPU 20 may be realized by a hardware circuit.

[0021] Measurement mechanism 9 is a mechanism that measures actual values ​​of welding parameters as physical quantities required for welding using resistance spot welding apparatus 10. The "actual values ​​of welding parameters" referred to here are the values ​​of the welding parameters that are actually observed when welding is performed according to commands related to the welding conditions transmitted from operation control unit 200, i.e., preset set values ​​of the welding parameters. The actual values ​​of the welding parameters measured by measurement mechanism 9 are transmitted to CPU 20 via communication unit 30.

[0022] In this embodiment, the measurement mechanism 9 includes a current value measuring unit 91, a voltage value measuring unit 92, a resistance value calculating unit 93, a pressure measuring unit 95, an inter-electrode displacement measuring unit 96, and an inter-electrode displacement calculating unit 97. Note that the configuration and functions of the measurement mechanism 9 are not limited to this. For example, the measurement mechanism 9 may further include a mechanism for measuring other welding parameters. Also, at least a part of the functions of the measurement mechanism 9 may be realized as one function of the CPU 20.

[0023] Each of the components 91-97 included in the measurement mechanism 9 measures the actual values ​​of the welding parameters at predetermined measurement times during welding. The current value measurement unit 91 measures the value of a current flowing between the upper electrode 2 and the lower electrode 3. The current value measurement unit 91 is, for example, a current sensor. The voltage value measurement unit 92 measures the voltage value (potential difference) between the upper electrode 2 and the lower electrode 3. The voltage value measurement unit 92 is, for example, a voltage sensor.

[0024] The resistance value calculation unit 93 calculates the electrical resistance value (hereinafter, resistance value) using the actual current value and voltage value measured when electricity is applied. Specifically, the resistance value calculation unit 93 calculates the resistance value by dividing the voltage value by the current value. At this time, the current value and voltage value at the same measurement point are used to calculate the resistance value.

[0025] The pressing force measuring unit 95 measures the pressing force of each of the electrodes 2, 3 against the welded members W. The pressing force measuring unit 95 is, for example, a load cell housed inside the electrode lifting device 4. The inter-electrode displacement amount measuring unit 96 includes a first displacement amount measuring unit 961 that measures the displacement amount of the upper electrode 2 (hereinafter, referred to as the first displacement amount), and a second displacement amount measuring unit 962 that measures the displacement amount of the lower electrode 3 (hereinafter, referred to as the second displacement amount).

[0026] The first displacement amount measuring unit 961 measures, for example, the elevation position of the upper electrode 2 at a first measurement time point and the elevation position of the upper electrode 2 at a second measurement time point a predetermined measurement time after the first measurement time point, and calculates the first displacement amount from the difference between the measurement values. The first displacement amount measuring unit 961 is, for example, an encoder housed inside the electrode lifting device 4 and detecting the rotation angle position of the output shaft of the servo motor 41 to measure the elevation position of the upper electrode 2.

[0027] The second displacement amount measuring unit 962 measures the second displacement amount. Specifically, it is fixed to the lower part 11b of the gun body 11. Therefore, when a force is applied due to thermal expansion, the lower electrode 3 distorts without moving. Therefore, the second displacement amount measuring unit 962 measures the force (strain value) applied to the lower electrode 3 as the second displacement amount. The second displacement amount measuring unit 962 is, for example, a strain sensor that measures the strain value of the lower electrode 3.

[0028] The inter-electrode displacement amount calculation unit 97 calculates the amount of displacement between the upper electrode 2 and the lower electrode 3 (hereinafter, inter-electrode displacement amount) using the first displacement amount measured and calculated by the first displacement amount measurement unit 961 and the second displacement amount measured by the second displacement amount measurement unit 962. At this time, the inter-electrode displacement amount is calculated using values ​​at the same measurement time point. The inter-electrode displacement amount calculation unit 97 calculates the inter-electrode displacement amount during current application, for example, by adding up the first displacement amount and the second displacement amount during the current application period. Note that the method of calculating the inter-electrode displacement amount is not limited to this.

[0029] A2. Method for estimating nugget diameter: Next, a method for estimating a nugget diameter in the first embodiment of the present disclosure using the estimation system 1 described above in detail will be described. Fig. 3 is a flowchart showing the steps of the method for estimating a nugget diameter. As shown in Fig. 3, the method for estimating a nugget diameter includes a resistance value acquisition step (S101) and a nugget diameter calculation step (S102).

[0030] In a resistance value acquisition process (S101), a latter-stage resistance value is acquired by the acquisition unit 210. In a resistance value estimation process (S101), an estimation unit 290 substitutes the latter-stage resistance value into equation (1), which is an estimation equation, to calculate an estimated value of the nugget diameter.

[0031] Next, the latter resistance value used as the welding parameter in the first embodiment will be described in more detail. Fig. 4 is a graph showing the change in resistance value over time. Fig. 5 is an enlarged view of the portion surrounded by the two-dot chain line in the graph in Fig. 4. Fig. 5 shows the change in resistance value over time at the end of the current application period. In Figs. 4 and 5, the current value is shown by a thick solid line, and the resistance values ​​of test patterns 1 to 4 in multiple (four in this embodiment) test weldings are shown by a thin solid line, a dashed line, a one-dot chain line, and a two-dot chain line, respectively.

[0032] As shown in FIG. 4, the current supply start time T1 is 100 ms, and the current supply stop time T4 at which the current supply to the welded member W is stopped is 300 ms. The current supply middle time T2 is an intermediate time between the current supply start time T1 and the current supply stop time T4, and is 200 ms. Here, the "second half of the current supply period" means from the current supply middle time T2 to the current supply stop time T4. The "second half of the current supply period" is also simply referred to as the "second half of the current supply period". The current supply end time T3 is between the current supply middle time T2 and the current supply stop time T4, and is set, for example, several tens of ms before the current supply stop time T4. This current supply end time T3 corresponds to, for example, a time 10 to 20% back in the current supply period from the current supply stop time T4. The period from the current supply end time T3 to the current supply stop time T4 is referred to as the "current supply end period S". In other words, the current supply end period S is included in the second half of the current supply period.

[0033] The inventors of the present application have investigated features of resistance value that are correlated with nugget diameter, and found that the resistance value in the latter half of the current-flow period has a higher correlation with the nugget diameter. The larger the nugget diameter, the smaller the resistance value. In particular, the correlation between the resistance value and the nugget diameter in the final current-flow period S was high. This is considered to be because, although various disturbances may occur at the welding site, the influence of the disturbances is less noticeable in the first half of the current-flow period, and the influence of the disturbances is more noticeable in the final current-flow period S. Examples of disturbances include gaps between steel sheets, inclination of the steel sheets, and wear of the tip of the electrode pressed against the steel sheets. As shown in FIG. 5, in the final current-flow period S, the change in resistance value due to the influence of disturbances can be read more clearly than in other periods of the current-flow period.

[0034] The inventors of the present application have considered that the accuracy of estimating the nugget diameter can be improved by using the resistance value during the period when the correlation is greater. In the latter half of the current flow period, a molten pool is formed by welding, and the molten pool reaches a size close to the target nugget diameter, and the molten state becomes more stable compared to the initial period of current flow.

[0035] FIG. 6 is a diagram showing the relationship between the resistance value and the nugget diameter in the second half of the current flow. When creating the above estimation formula, a first current value that creates a target nugget diameter and a second current value that is smaller than the first current value and deliberately lowers the current value to create an excessively small nugget diameter were set, and the second half resistance value and the nugget diameter were measured at each current value to obtain multiple data. In FIG. 6, the obtained multiple data are plotted. In addition, the average resistance value in the final period S of the current flow was used as the second half resistance value. From these data, a simple regression analysis was performed to create the above estimation formula. The estimation formula is a linear equation calculated by linear regression of the plot points corresponding to the second half resistance value and the nugget diameter, and shows a straight line in FIG. 6.

[0036] 7 and 8 are diagrams for explaining the effect of estimating the nugget diameter using the latter resistance value as a welding parameter. In FIG. 7, the horizontal axis is the actually measured nugget diameter (hereinafter also referred to as the "actual diameter"), and the vertical axis is the estimated nugget diameter (hereinafter also referred to as the "estimated diameter"), and the results are plotted. In FIG. 8, the horizontal axis is the estimation accuracy, and the vertical axis is the actually measured diameter, and the results are plotted. In FIG. 7 and FIG. 8, when the estimated diameter and the actually measured diameter match, the plot points are located on the ideal line L1 shown by the dashed line in the figures. In FIG. 7 and FIG. 8, all of the plot points are located on or near the ideal line L1. In addition, all of the plot points are located within the range of the estimation error of ±20% or less, as shown by the dashed lines L2 and L3 in each figure. In other words, it was confirmed that the estimated nugget diameter has a high accuracy.

[0037] According to the method for estimating a nugget diameter of the first embodiment, the latter resistance value, which is the resistance value in the latter half of the current supply, is used as a welding parameter to estimate the nugget diameter. The latter resistance value has a higher correlation with the nugget diameter than the resistance value in the first half of the current supply. By using the latter resistance value, which has a higher correlation with the nugget diameter, as a welding parameter for estimation, the accuracy of estimating the nugget diameter can be improved compared to, for example, the case where the entire resistance value during the current supply period is used.

[0038] Furthermore, the estimation formula used in the method for estimating the nugget diameter of the first embodiment is a linear equation calculated by linear regression of the plot points in which the latter resistance value and the nugget diameter are associated. Therefore, the correlation between the latter resistance value and the nugget diameter can be uniquely expressed. In addition, the estimation formula can be easily created by using the plot points in which the latter resistance value and the nugget diameter are associated.

[0039] B. Second embodiment: Next, a second embodiment will be described with reference to FIG. 9. In the second embodiment, only the energization form is different from the first embodiment, and other configurations and estimation methods are similar, so detailed description will be omitted. FIG. 9 is a diagram showing changes in welding current I in the second embodiment of the present disclosure. As shown in FIG. 9, in resistance spot welding in the second embodiment, initial energization is performed for a predetermined time, energization is stopped for a predetermined time, and then main energization is performed. In other words, the energization period includes a period of initial energization and a period of main energization that is performed with an interval after the initial energization.

[0040] In the second embodiment, the latter resistance value is an average value of the resistance values ​​in the latter half (times T5 to T6) of the main current supply period. In the second embodiment, the same effects as in the first embodiment can be achieved.

[0041] C. Other embodiments: (C1) In each of the above embodiments, the estimation formula includes only the latter resistance value as the welding parameter as shown in formula (1), but it may also include other welding parameters such as the amount of expansion.

[0042] (C2) In the first embodiment, the latter resistance value is the average resistance value in the final current supply period S, but it is not limited to the final current supply period S, and may be the average resistance value of any period in the latter current supply period after the current supply midpoint T2. Also, it does not have to be the average value, and an integral value of the resistance value over any period in the latter current supply period may be used. Any of these are called "later resistance values".

[0043] The present disclosure is not limited to the above-mentioned embodiments, and can be realized in various configurations without departing from the spirit of the present disclosure. For example, the technical features in each embodiment 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 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]

[0044] 1...nugget diameter estimation system, 2...upper electrode, 3...lower electrode, 4...electrode lifting device, 5...pressure device, 6...current adjustment device, 9...measurement mechanism, 10...resistance spot welding device, 11...gun body, 11a...upper, 11b...lower, 20...CPU, 30...communication unit, 40...display, 41...servo motor, 42...lifting member, 50...input operation unit, 60...memory unit, 91...current value measurement unit, 92...voltage value measuring unit, 93...resistance value calculating unit, 95...applied pressure measuring unit, 96...inter-electrode displacement measuring unit, 97...inter-electrode displacement calculating unit, 100...control device, 200...motion control unit, 210...acquisition unit, 290...estimation unit, 670...estimation formula, 961...first inter-electrode displacement measuring unit 962...second displacement measuring unit, G...welding gun, I...welding current, RA...robot arm, S...final current application period, W...welded member, W1...metal plate

Claims

1. 1. A method for estimating a nugget diameter by using a predetermined estimation equation having welding parameters that contribute to a size of a nugget diameter formed in resistance spot welding as explanatory variables and an estimated value of the nugget diameter as a response variable, comprising: a resistance value acquisition step of acquiring, as the welding parameter, a latter resistance value which is an electrical resistance value in a latter half of a current-carrying period during welding and is an electrical resistance value of an entire current-carrying circuit including a workpiece to be welded; a nugget diameter calculation step of calculating the nugget diameter by substituting the acquired latter resistance value into the estimation formula; Equipped with The energization period during the welding includes an initial energization period and a main energization period that is performed at an interval after the initial energization, The latter resistance value is a resistance value in a latter part of the main current supply period, The estimation formula is: Nugget diameter = C1 x [second half resistance value] + C2 (C1 and C2 are predetermined constants) A method for estimating the nugget diameter.

2. The estimation formula is: The method for estimating a nugget diameter according to claim 1 , wherein the second half resistance value and the nugget diameter are plotted as corresponding plot points, and the first equation is calculated by performing linear regression on the plot points.

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