Method, device, and system for estimating nugget diameter in resistance spot welding

Abstract

To provide a technique capable of improving estimation accuracy of a nugget diameter when resistance spot welding is executed sequentially for a plurality of welding points present at different positions of a base material.SOLUTION: A method for estimating a nugget diameter comprises: an acquisition step of acquiring time-sequential data including data before a start of energization for each welding point regarding two or more welding parameters including an expansion amount calculated on the basis of displacement of upper and lower electrodes; a difference calculation step of calculating (i) a first difference between an expansion amount at a first point before the start of energization and an expansion amount at an N-th point (N is integer of 2-M) before the start of energization and (ii) a second difference between a welding parameter at the first point at the start time of energization and after and a welding parameter at the N point at the start time of energization and after; and an estimation step of estimating a nugget diameter at a P point (P is integer of 1-M) by using the welding parameters.SELECTED DRAWING: Figure 11

Description

[Technical field]

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

[0002] Conventionally, there is known a technique for estimating the nugget diameter in resistance spot welding by creating an approximation equation for the expansion and contraction changes from the amount of thermal expansion of the base material while the welding current is flowing and the amount of contraction of the base material after the welding current is stopped (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2003-181649 A Summary of the Invention [Problem to be solved by the invention]

[0004] In conventional techniques, when resistance spot welding is performed sequentially on a plurality of weld points that are located at different positions on a base material, there is a risk that the accuracy of estimating the nugget diameter will decrease. [Means for solving the problem]

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

[0006] (1) According to a first aspect of the present disclosure, there is provided a method for estimating a nugget diameter in resistance spot welding of workpieces clamped between an upper electrode and a lower electrode, the resistance spot welding is performed on M (M is an integer of 2 or more) welding points located at different positions on the workpieces, in order from a first point to an Mth point, and the estimation method includes the steps of: (i) acquiring, for each of the welding points, time-series data including data before start of application of a welding current for two or more welding parameters that contribute to the size of the nugget diameter and include an expansion amount calculated based on a displacement between the upper electrode and the lower electrode; (ii) calculating the expansion amount of the first point before start of application of current and the expansion amount of the Nth point (N The method includes a difference calculation step of calculating (i) a first difference which is a difference between the expansion amount at the first welding point after the start of energization and the expansion amount at the Nth welding point after the start of energization, and (ii) a second difference which is a difference between the expansion amount at the first welding point after the start of energization and the expansion amount at the Nth welding point after the start of energization, and an estimation step of estimating the nugget diameter at the Pth point (P is an integer of 1 to M) using the welding parameters, and the estimation step corrects the calculated welding parameters at the Pth point using the expansion amount at the Pth point before the start of energization and a correlation between the first difference and the second difference, and estimates the nugget diameter using the corrected welding parameters. According to this embodiment, when welding is performed in order from the 1st point to the Mth point, the tendency of the welding parameters changes for each welding point due to the effect of heat generation of the electrode. In contrast, according to this embodiment, the nugget diameter is estimated after correcting the welding parameters at the Pth point, which are used to estimate the nugget diameter, using the expansion amount before the start of energization, which is correlated with the temperature of the electrode. This makes it possible to estimate the nugget diameter while taking into consideration the difference in welding parameters for each welding point due to the difference in electrode temperature when resistance spot welding is performed in sequence on multiple welding points located at different positions on the welded material, thereby preventing a decrease in the estimation accuracy of the nugget diameter when resistance spot welding is performed in sequence on multiple welding points located at different positions on the welded material. (2) The above embodiment may further include a correction formula creating step of creating a correction formula indicating the correlation, and the estimation step may include a step of calculating a correction value of the welding parameter by substituting a third difference, which is a difference between the expansion amount at the first point before the start of energization and the expansion amount at the Pth point before the start of energization, into the correction formula when estimating the nugget diameter at the Pth point, and a step of calculating the estimated value of the nugget diameter by substituting the corrected welding parameter, which is corrected by adding the correction value to the calculated welding parameter at the Pth point, into a predetermined estimation formula having the welding parameter as an explanatory variable and the estimated value of the nugget diameter as an objective variable. According to this embodiment, a correction formula for calculating a correction amount of the welding parameter to be substituted into the estimation formula for the nugget diameter can be created. As a result, the estimation accuracy of the nugget diameter can be improved by substituting the corrected welding parameter corrected by the correction formula into the estimation formula. (3) In the above embodiment, the correction formula may be a linear equation calculated by linear regression of each of the values ​​determined by a combination of the first difference and the second difference. According to this embodiment, the correlation between the first difference and the second difference can be uniquely expressed. Also, the correction formula can be easily created from each of the values ​​determined by a combination of the first difference and the second difference. (4) In the above embodiment, the correction equation may be a higher-order equation calculated by nonlinear regression of each of the values ​​determined by a combination of the first difference and the second difference. According to this embodiment, it is possible to create a correction equation that reflects the correlation between the first difference and the second difference in more detail than when the correction equation is created by linear regression. Therefore, it is possible to further improve the estimation accuracy of the nugget diameter. (5) In the above aspect, when the expansion amounts at a plurality of time points before the start of energization are calculated for the same welding point, the first difference may be calculated using an average value of the expansion amounts before the start of energization. According to this aspect, when an extreme change is observed in a welding parameter before the start of energization, it is possible to avoid calculating the first difference based only on an abnormal value. (6) According to a second aspect of the present disclosure, there is provided an apparatus for estimating a nugget diameter in resistance spot welding of workpieces clamped between an upper electrode and a lower electrode, the resistance spot welding is performed on M (M is an integer of 2 or more) welding points located at different positions on the workpieces, in order from a first point to an Mth point, and the estimation apparatus includes an acquisition unit that acquires, for each of the welding points, time-series data including data before start of application of a welding current for two or more welding parameters that contribute to the size of the nugget diameter and include an expansion amount calculated based on a displacement between the upper electrode and the lower electrode, (i) a calculation unit that calculates the expansion amount of the first point before the start of application of current and an expansion amount of the Nth point before the start of application of current (ii) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (iii) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (iv) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (v) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (vi) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (vii) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (v ...v) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (v) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (v) a calculation unit that calculates the expansion amount of the Nth point before the start of application of current (v) a calculation unit that calculates the expansion amount of the Nth point before the start of application The welding apparatus further includes a difference calculation unit that calculates (i) a first difference, which is a difference between the expansion amount at the first welding point after the start of current application and the expansion amount at the Nth welding point after the start of current application, and (ii) a second difference, which is a difference between the expansion amount at the first welding point after the start of current application and the expansion amount at the Nth welding point after the start of current application, and an estimation unit that estimates the nugget diameter at a Pth point (P is an integer of 1 to M) using the welding parameters, and the estimation unit corrects the calculated welding parameters at the Pth point using the expansion amount at the Pth point before the start of current application and a correlation between the first difference and the second difference, and estimates the nugget diameter using the corrected welding parameters. According to this embodiment, when welding is performed in order from the 1st point to the Mth point, the tendency of the welding parameters changes for each welding point due to the effect of heat generation of the electrode. In contrast, according to this embodiment, the nugget diameter is estimated after correcting the welding parameters at the Pth point, which are used to estimate the nugget diameter, using the expansion amount before the start of current application, which is correlated with the temperature of the electrode. This makes it possible to estimate the nugget diameter while taking into consideration the difference in welding parameters for each welding point due to the difference in electrode temperature when resistance spot welding is performed in sequence on multiple welding points located at different positions on the welded material, thereby preventing a decrease in the estimation accuracy of the nugget diameter when resistance spot welding is performed in sequence on multiple welding points located at different positions on the welded material. (7) According to a third aspect of the present disclosure, there is provided a system for estimating a nugget diameter in resistance spot welding. In the system for estimating a nugget diameter in resistance spot welding of a workpiece clamped between an upper electrode and a lower electrode, the resistance spot welding is performed in order from a first point to an Mth point for M welding points (M is an integer of 2 or more) that are located at different positions on the workpiece, and the estimation system includes the estimation device according to the above aspect and a measurement mechanism that generates time-series data of welding parameters. According to this aspect, the nugget diameter can be estimated using the time-series data generated by the measurement mechanism. The present disclosure may be realized in various forms other than the above-described estimation method, estimation device, and estimation system for resistance spot welding, for example, in the form of a manufacturing method for the estimation device and the estimation system, a control method for the estimation device and the estimation system, a computer program for implementing the control method, a non-transitory recording medium on which the computer program is recorded, etc. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram showing a schematic configuration of an estimation system. [Diagram 2] Schematic diagram showing the flow of resistance spot welding. [Diagram 3] FIG. 1 is a block diagram showing a schematic configuration of an estimation system. [Figure 4] Figure 1 shows the time series data of interelectrode displacement for each welding point. [Diagram 5] 5 is a graph showing the relationship between the expansion amount and the electrode temperature in a section before the start of current flow. [Figure 6] Figure 2 shows the time series data of interelectrode displacement for each welding point. [Figure 7] FIG. 13 is a graph showing time series data of resistance values ​​for each welding point during the energized section. [Figure 8] FIG. 11 is a diagram for comparing the estimation error of the nugget diameter before and after correction. [Figure 9] 4 is a flowchart showing a method for estimating a nugget diameter. [Figure 10]11 is a flowchart showing details of a difference calculation step. [Figure 11] FIG. 13 is a diagram showing an example of a correction formula. [Figure 12] 11 is a flowchart showing details of an estimation process. [Figure 13] FIG. 13 is a diagram for comparing welding parameters before and after correction using a correction formula. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] A. First embodiment: 1 is a schematic diagram showing a general configuration of an estimation system 1. The estimation system 1 is a system that estimates a nugget diameter when resistance spot welding (hereinafter, welding) is performed in order from a first point to an Mth point at M welding points (M is an integer of 2 or more, the same applies below) that are located at different positions on a workpiece W (base metal). In other words, the estimation system 1 estimates a nugget diameter when welding is performed two or more times consecutively at different welding points. The estimation system 1 includes a resistance spot welding apparatus 10, a control device 100, and a measurement mechanism 9.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 2 is a schematic diagram showing the flow of resistance spot welding. In this embodiment, a case in which two metal plates W1 and W2 are welded as the workpieces W will be described as an example.

[0015] First, in a state in which the tip 3a of the lower electrode 3 and the second metal plate W2 are in contact with each other and the first metal plate W1 is placed on the second metal plate W2, the control device 100 transmits a descent command to the electrode lifting device 4 to lower the upper electrode 2 toward the first metal plate W1. As a result, the upper electrode 2 descends from a predetermined initial position toward the workpieces W, and the workpieces W are sandwiched between the upper electrode 2 and the lower electrode 3. Then, the control device 100 transmits a command to start pressing to the pressing device 5. As a result, the workpieces W are pressed by the pair of electrodes 2, 3 with a predetermined pressing force.

[0016] Next, in a state where the workpieces W are clamped and pressed between the upper electrode 2 and the lower electrode 3 (hereinafter, the holding state), a welding current is passed between the upper electrode 2 and the lower electrode 3. The control device 100 transmits a current command related to the start of current flow to the current adjustment device 6, and continues the flow of the welding current for a predetermined current flow time. As a result, the workpieces W melt near the interface WS between the first metal plate W1 and the second metal plate W2 due to resistance heat generated while the welding current is being passed (hereinafter, the current is being passed). In FIG. 2, the melted portion M of the workpieces W is shown with rough hatching. Then, with the increase in the amount of resistance heat generated and the formation of the melted portion M, the workpieces W thermally expand three-dimensionally, pushing back the electrodes 2 and 3.

[0017] After a predetermined current time has elapsed since the start of current flow, the control device 100 transmits a current command to stop current flow to the current adjustment device 6, and stops the flow of the welding current while maintaining the holding state. Then, after the predetermined holding time has elapsed, the control device 100 transmits a command to stop applying pressure to the pressure device 5, and stops applying pressure to the workpieces W to be welded. In other words, after the current flow is stopped, the holding state is continued for the predetermined holding time. Next, the control device 100 transmits a lift command to the electrode lifting device 4, and separates the upper electrode 2 from the first metal plate W1. This releases the workpieces W from the electrodes 2, 3.

[0018] At this time, due to the effect of resistance heating generated during current flow, the hardness (e.g., Brinell hardness) of the welded members W at the time current flow is stopped is lower than that at the time current flow started. In other words, the welded members W at the time current flow is stopped are softer than the welded members W at the time current flow started. Furthermore, during the period from the time current flow is stopped to the time when a predetermined holding time has elapsed, the welded members W are continuously pressurized by the pair of electrodes 2, 3. Therefore, the welded members W contract three-dimensionally as they are pushed and spread in an extension direction D2 along a plane perpendicular to the pressure direction D1 of the electrodes 2, 3.

[0019] In reality, the thermal expansion and contraction of the welded components W occur in three-dimensional directions, but in this disclosure, the degree of thermal expansion of the welded components W (hereinafter, component expansion amount) is calculated as the amount of change in the dimensions of the welded components W in the expansion direction D3. The "expansion direction D3" referred to here refers to a direction perpendicular to the interface WS of the welded components W that pushes back the pair of electrodes 2, 3, and is opposite to the pressure direction D1 of the electrodes 2, 3. In this disclosure, the degree of contraction of the welded components W (hereinafter, component contraction amount) is the amount of change in the dimensions of the welded components W in the pressure direction D1, and is calculated as the amount of change from the point in time when the current flow is stopped.

[0020] Then, when the current is stopped, the amount of resistance heat is reduced, and the temperature of the welded members W drops, so that the molten portion M of the welded members W solidifies. As a result, the metal plates W1 and W2 are joined. A nugget N is formed at the joint interface WS where the metal plates W1 and W2 are joined. The nugget N is the joint portion where the metal plates W1 and W2 melt and then solidify. In FIG. 2, the nugget N is shown with dense hatching. The shape of the nugget N is roughly a go stone shape with the center plane passing through the center O of the joint portion and along the extension direction D2. Hereinafter, the dimension of the nugget N in the direction along the joint interface WS of the welded members W is referred to as the nugget diameter LN.

[0021] 3 is a block diagram showing a schematic configuration of the estimation system 1. In this embodiment, the control device 100 has a function of controlling the operation of the resistance spot welding device 10, and also a function as an estimation device for estimating the nugget diameter LN 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.

[0022] 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.

[0023] The storage unit 60 stores various programs for controlling the operation of the resistance spot welding apparatus 10, and various data including a correction formula 650 and an estimation formula 670. The estimation formula 670 is a relational formula prepared in advance for calculating an estimated value of the nugget diameter LN. Specifically, the estimation formula 670 is a relational formula in which the welding parameters that contribute to the size of the nugget diameter LN after the start of current application are explanatory variables and the estimated value of the nugget diameter LN is a response variable. The correction formula 650 is a relational formula created for calculating a correction value of the welding parameters to be substituted into the estimation formula 670. The storage unit 60 includes a RAM, a ROM, a rewritable nonvolatile memory, and the like.

[0024] The CPU 20 functions as an operation control unit 200, an acquisition unit 210, a difference calculation unit 220, a correction formula creation unit 230, a correction value calculation unit 240, and an estimation unit 290 by deploying various programs stored in the memory unit 60.

[0025] 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, an amount of contraction, and a pressure applied to the electrodes 2 and 3. The operation control unit 200 comprehensively 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.

[0026] The acquisition unit 210 acquires time series data for each welding point, which will be described later. The difference calculation unit 220 calculates difference data, which will be described later, using the time series data. The correction equation creation unit 230 creates a correction equation 650 using the difference data. The correction value calculation unit 240 calculates a correction value of a welding parameter to be substituted into the estimation equation 670, using the correction equation 650. The estimation unit 290 includes a parameter correction unit 291 and an estimation value calculation unit 292. The parameter correction unit 291 calculates a corrected parameter in which the correction value is reflected in the welding parameter. The estimation value calculation unit 292 calculates an estimation value of a nugget diameter LN at a P-th point (P is an integer between 1 and M, the same applies below) using the corrected welding parameter. Details of each function will be described later together with a method of estimating a nugget diameter LN shown in FIG. 9 and subsequent figures. At least a part of the functions of the CPU 20 may be realized by a hardware circuit.

[0027] Measurement mechanism 9 is a mechanism for measuring actual values ​​of welding parameters as physical quantities required for welding using resistance spot welding device 10. The "actual values ​​of welding parameters" referred to here are measurements made when welding is performed according to commands relating to welding conditions transmitted from operation control unit 200, i.e., preset values ​​of welding parameters, or values ​​calculated from the measurements. When welding is performed on workpieces W in order from the first point to the Mth point, the actual values ​​of the welding parameters are measured for each welding point. The actual values ​​of the welding parameters measured by measurement mechanism 9 are transmitted to CPU 20 via communication unit 30.

[0028] 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 welding force measuring unit 95, a displacement amount measuring unit 96, and an inter-electrode displacement amount 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.

[0029] Each of the components 91-97 included in the measurement mechanism 9 measures the actual values ​​of the welding parameters at predetermined measurement times while welding is being performed (hereinafter, during welding). Here, "during welding" refers to the period from when the workpieces W are clamped and pressurized by the pair of electrodes 2, 3 to a held state (hereinafter, welding start time) to when the pressure on the workpieces W is released and the workpieces W are no longer held (hereinafter, non-held state) (hereinafter, welding end time). In this embodiment, the time when the measurement of the welding parameters starts (hereinafter, measurement start time) is the same as the welding start time. Also, the time when the measurement of the welding parameters ends (hereinafter, measurement end time) is the same as the welding end time.

[0030] The current value measuring unit 91 measures the value of a current flowing between the upper electrode 2 and the lower electrode 3. The current value measuring unit 91 is, for example, a current sensor.

[0031] The voltage value measuring unit 92 measures the voltage value (potential difference) between the upper electrode 2 and the lower electrode 3. The voltage value measuring unit 92 is, for example, a voltage sensor.

[0032] 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.

[0033] The pressure measuring unit 95 measures the pressure of the electrodes 2, 3 applied to the workpieces W. The pressure measuring unit 95 is, for example, a load cell accommodated inside the electrode lifting device 4.

[0034] The 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).

[0035] 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.

[0036] 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.

[0037] 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, a value at the same measurement time point is used to calculate the inter-electrode displacement amount. The inter-electrode displacement amount calculation unit 97 calculates the inter-electrode displacement amount in the current-on section, for example, by adding up the first displacement amount and the second displacement amount in the current-on section. Note that the method of calculating the inter-electrode displacement amount is not limited to this.

[0038] FIG. 4 is a first diagram showing time series data DH1 to DH6 of inter-electrode displacement amount for each welding point. The horizontal axis of FIG. 4 represents the elapsed time from the start time T1 of measurement of the inter-electrode displacement amount. The vertical axis of FIG. 4 represents the actual measurement value of the inter-electrode displacement amount. FIG. 4 shows time series data DH1 to DH6 of inter-electrode displacement amount at each welding point from the first point to the sixth point which are welded in sequence. The "time series data" referred to here is data in which the actual measurement values ​​of the welding parameters are arranged in chronological order. In other words, the time series data represents the transition of multiple actual measurement values ​​for each welding parameter during welding. Note that the set values ​​of the welding parameters at each welding point from the first point to the sixth point are the same. The time series data DH1 to DH6 may be generated as a graph as in FIG. 4, or may be generated as a collection of numerical data such as a table.

[0039] In this embodiment, the current supply start time T2 at which the current supply to the welded member W starts is a time when 100 milliseconds have elapsed since the measurement start time T1 of the welding parameter. The current supply stop time T3 at which the current supply to the welded member W stops is a time when 400 milliseconds have elapsed since the measurement start time T1 of the welding parameter. The measurement end time T4 at which the measurement of the welding parameter ends is a time when a predetermined holding time has elapsed since the current supply stop time T3. In the example shown in FIG. 4, the measurement end time T4 is a time when about 900 milliseconds have elapsed since the current supply stop time T3. Therefore, in this embodiment, the section from 0 milliseconds to 100 milliseconds after the measurement start time T1 of the welding parameter is the "pre-current supply start section", and the section from 100 milliseconds to 400 milliseconds before is the "current supply period". In addition, the section from 400 milliseconds after the measurement start time T1 of the welding parameter is the "post-current supply stop section". Hereinafter, the inter-electrode displacement amount in the pre-energization period and the inter-electrode displacement amount in the current-on period from the initial positions of the electrodes 2 and 3 will also be referred to as the "expansion amount." Also, the inter-electrode displacement amount in the post-energization period from the positions of the electrodes 2 and 3 at time T3 when current is stopped will also be referred to as the "contraction amount."

[0040] When welding is performed continuously from the first point to the Mth point, the multiple welding points are located at different positions on the workpiece W. Therefore, the workpiece W near each welding point does not thermally expand at a time point before the current start time T2, and the amount of material expansion in the pre-current start section is considered to be approximately zero. In other words, the amount of material expansion does not contribute to the amount of expansion (electrode displacement amount) in the pre-current start section. Therefore, if it is assumed that an increase in the number of consecutive welding operations does not affect the actual measured values ​​of the welding parameters, it is considered that the amount of expansion in the pre-current start section shows an approximately constant value regardless of the welding point. However, in reality, as shown in FIG. 4, the time series data DH1 to DH6 of the amount of expansion in the pre-current start section shows differences between the first point, the second point, and the third point and thereafter. From this, it is understood that an increase in the number of consecutive welding operations affects the actual measured values ​​of the welding parameters.

[0041] FIG. 5 is a graph showing the relationship between the amount of expansion and the electrode temperature in the pre-energization section shown in FIG. 4. The horizontal axis of FIG. 5 represents the actual measured value of the amount of expansion in the pre-energization section. The vertical axis of FIG. 5 represents the actual measured value of the electrode temperature (upper electrode 2, 22). As shown in FIG. 5, the electrode temperature is higher at the second point than at the first point, and higher at the third point and thereafter than at the second point. This is because the electrode temperature that rose during the previous welding has not yet fully decreased. From the third point onwards, the electrode temperature at each welding point is in the range of 100 degrees or more and 150 degrees or less, and the rise in the electrode temperature remains high. In other words, when welding each welding point from the first point to the third point, the electrode temperature rises in accordance with the increase in the number of consecutive welding operations. In contrast, when welding each welding point from the fourth point onwards, the electrode temperature at the third point is generally maintained. Therefore, there is a correlation between the welding point (the number of consecutive welding operations) and the electrode temperature. In this embodiment, the increase in the electrode temperature due to the increase in the number of consecutive welding operations remains high from the fourth point onwards, but the manner in which the electrode temperature changes is not limited to this.

[0042] As shown in FIG. 2, when the electrode temperature rises, the upper electrode 2 and the lower electrode 3 thermally expand. Specifically, the higher the electrode temperature of the upper electrodes 2, 22, the longer the lengths L10, L20 (dimension in the longitudinal direction) of the upper electrodes 2, 22. In other words, the upper electrodes 2, 22 extend in a direction approaching the workpieces W with an increase in electrode temperature. In FIG. 2, the outer shape of the upper electrode 2 at the first welding point is illustrated by a solid line. Also, the outer shape of the upper electrode 22 at the Nth welding point (N is an integer of 2 to M) after the first welding point is illustrated by a dashed line. Due to the increase in electrode temperature, the length L20 of the upper electrode 22 at the Nth welding point becomes longer than the length L10 of the upper electrode 2 at the first welding point, and the separation distance L2 at the Nth welding point becomes shorter than the separation distance L1 at the first welding point. The "separation distances L1, L2" referred to here are the shortest linear distance between the tips 2a, 22a of the upper electrodes 2, 22 and the workpieces W, and are the moving distances from the initial positions of the upper electrodes 2, 22 to the workpieces W. Therefore, the difference between the separation distance L1 at the first point and the separation distance L2 at the Nth point represents the degree of thermal expansion of the upper electrodes 2, 22 caused by the rise in electrode temperature due to resistance heating (hereinafter, the first electrode expansion amount). In this embodiment, the separation distances L1, L2 are determined by the difference in the first displacement amount.

[0043] Also, similarly to the upper electrode 2, the higher the electrode temperature of the lower electrodes 3, 33, the longer the length (dimension in the longitudinal direction) of the lower electrode 3. In FIG. 2, the outer shape of the lower electrode 3 at the first welding point is shown by a solid line. Also, the outer shape of the lower electrode 33 at the Nth point after the first welding point is shown by a dashed line. In this way, the length of the lower electrode 3 at the Nth point becomes longer than the length of the lower electrode 3 at the first point, so that distortion occurs in the lower electrode 3 at the Nth welding point. Therefore, the difference between the second displacement amount at the first point and the second displacement amount at the Nth point represents the degree of thermal expansion of the lower electrodes 3, 33 caused by the rise in electrode temperature due to resistance heating (hereinafter, referred to as the second electrode expansion amount).

[0044] From the above, it is considered that the electrode temperature rises and the lengths L10, L20 of the electrodes 2, 3 change when welding is performed continuously at a plurality of different welding points, and therefore the tendency of the time series data DH1 to DH6 of the inter-electrode displacement amount differs for each welding point, as shown in FIG. 4. In fact, as shown by the straight line GS in FIG. 5, a correlation is observed between the expansion amount in the pre-current-flow section and the electrode temperature. Specifically, the expansion amount in the pre-current-flow section increases as the electrode temperature rises. Therefore, there is a correlation between the welding point (the number of consecutive welding operations) and the expansion amount in the pre-current-flow section.

[0045] As described above, in the pre-energization section, the amount of member expansion does not contribute to the amount of inter-electrode displacement. Therefore, the amount of inter-electrode displacement in the pre-energization section represents the degree of thermal expansion of the two electrodes 2, 3 accompanying the rise in electrode temperature (hereinafter, the amount of electrode expansion). The "amount of electrode expansion" here refers to the sum of the amount of first electrode expansion and the amount of second electrode expansion.

[0046] FIG. 6 is a second diagram showing the time series data DH1, DH25 of the interelectrode displacement amount for each welding point. FIG. 7 is a diagram showing the time series data DT1, DT25 of the resistance value in the current-on section for each welding point. The horizontal axis of FIG. 6 and the horizontal axis of FIG. 7 represent the elapsed time from the measurement start time T1 of the welding parameters. The vertical axis of FIG. 6 represents the actual measurement value of the interelectrode displacement amount. The vertical axis of FIG. 7 represents the actual measurement value of the resistance value. In FIG. 6 and FIG. 7, in order to easily compare the difference in the actual measurement value of the welding parameters that occurs after the current-on start time T2 due to an increase in the number of continuous weldings, the graphs are shown that are adjusted in advance so that the actual measurement value of the welding parameters in the section before the current-on start is approximately the same between the 1st point and the 25th point. The set values ​​of the welding parameters when the time series data shown in FIG. 6 and FIG. 7 are obtained are the same as those in FIG. 4.

[0047] As shown in Fig. 6, the time-series data DH1, DH25 of the inter-electrode displacement amount have different trends between the 1st point and the 25th point. The following reasons are considered as the reasons why the trends differ depending on the welding point.

[0048] As shown in Fig. 5, when welding the first to third points, the electrode temperature rises with an increase in the number of consecutive welds. That is, when welding the first point, the electrode temperature rises continuously during the current-on section, which is considered to increase the amount of electrode expansion during the current-on section. Therefore, in the time-series data DH1 of the amount of inter-electrode displacement during welding the first point shown in Fig. 6, it is considered that both the amount of member expansion and the amount of electrode expansion contribute to the amount of expansion during the current-on section.

[0049] As shown in FIG. 5, the expansion amount in the pre-energization section at the second point is larger than that in the pre-energization section at the first point. Since resistance heating is caused by energization and the amount of resistance heating does not increase while energization is stopped, it is unlikely that the electrode temperature will rise significantly in the post-energization section. Therefore, it is considered that the electrode temperature at the time T3 when energization is stopped is generally maintained in the post-energization section during welding of the first point. Therefore, in the time-series data DH1 of the inter-electrode displacement amount during welding of the first point shown in FIG. 6, it is considered that both the member contraction amount and the electrode contraction amount contribute to the contraction amount in the post-energization section.

[0050] On the other hand, as shown in Fig. 5, when welding the fourth point and thereafter, the electrode temperature remains roughly the same as that at the third welding, regardless of the increase in the number of consecutive weldings. Therefore, when welding the 25th point, it is considered that no change is observed in the amount of electrode expansion in both the current-on section and the section after current-off. Therefore, in the time-series data DH25 of the amount of electrode-to-electrode displacement at the 25th welding point shown in Fig. 6, it is considered that the amount of expansion in the current-on section is mainly contributed by the amount of member expansion, and the amount of contraction in the section after current-off is mainly contributed by the amount of member contraction.

[0051] In fact, as shown in Fig. 6, during the energization period, for example, at a time point immediately before the energization stop time T3 where the expansion amounts at the 1st point and the 25th point are significantly different, the expansion amount at the 25th point is smaller than the expansion amount at the 1st point. Moreover, throughout the entire period after the energization stop, the contraction amount at the 25th point is smaller than the contraction amount at the 1st point.

[0052] Furthermore, as shown in FIG. 7, the time series data DT1 and DT25 of the resistance value have different trends between the first point and the 25th point. In other words, it is considered that the difference in the electrode temperature in the pre-energization section causes a difference in the resistance value. In this case, the estimation formula 670 of the nugget diameter LN is a relational expression assuming that welding is performed using the electrodes 2 and 3 having a temperature close to room temperature, that is, that the nugget diameter LN at the first welding point is estimated. Therefore, if the number of consecutive welding times is not taken into consideration, that is, if the welding parameter values ​​are substituted into the estimation formula 670 without correcting the values, the estimation accuracy of the nugget diameter LN may decrease.

[0053] FIG. 8 is a diagram for comparing the estimation error of the nugget diameter LN before and after the correction using the correction formula 650. The "estimation error" here refers to the degree of deviation between the actual measurement value of the nugget diameter LN and the estimated value of the nugget diameter LN calculated by the estimation formula 670. The left diagram of FIG. 8 shows the estimation error of the nugget diameter LN when the nugget diameter LN is estimated without correcting the welding parameters substituted into the estimation formula 670. The right diagram of FIG. 8 shows the estimation error of the nugget diameter LN when the nugget diameter LN is estimated by substituting the welding parameters after the correction using the correction formula 650 into the estimation formula 670. The horizontal axis in the left diagram and the right diagram of FIG. 8 is the estimation error of the nugget diameter LN. The vertical axis in the left diagram and the right diagram of FIG. 8 is the actual measurement diameter of the nugget diameter LN.

[0054] In FIG. 8, data relating to the estimated error when welding is performed under four different welding conditions are plotted. The welding under the first condition is performed on the first welding point in a state where 2.0 liters of cooling water is circulated inside the electrodes 2 and 3. The welding under the second condition is performed on the 25th welding point in a state where 2.0 liters of cooling water is circulated inside the electrodes 2 and 3. The welding under the third condition is performed on the first welding point in a state where 1.3 liters of cooling water, which is less than the first condition, is circulated inside the electrodes 2 and 3. The welding under the fourth condition is performed on the 25th welding point in a state where 1.3 liters of cooling water, which is less than the first condition, is circulated inside the electrodes 2 and 3. In each condition, the welding point to be welded and the conditions other than the flow rate of the cooling water (for example, the set values ​​of welding parameters such as the current value) are the same. For each condition, FIG. 8 shows multiple data for each condition.

[0055] As shown in the left diagram of Fig. 8, when the nugget diameter LN is estimated without correcting the welding parameters substituted into the estimation formula 670, the estimation error of the nugget diameter LN for the 25th welding point is 10% or more regardless of the flow rate of the cooling water. In other words, when the nugget diameter LN is estimated without taking into account the rise in electrode temperature that occurs when welding is performed continuously from the 1st point to the Mth point, the estimation accuracy of the nugget diameter LN at the 25th point is lower than that at the 1st point.

[0056] Therefore, the inventors of the present application considered calculating a correction equation 650 for correcting the welding parameters to be substituted into the estimation equation 670, using the amount of expansion in the pre-energization section, which is correlated with the welding point (the number of consecutive welding operations).

[0057] 9 is a flowchart showing a method for estimating the nugget diameter LN. Each process from step S1 to step S9 is executed by the estimation system 1. In the method for estimating the nugget diameter LN, an acquisition process (step S1), a difference calculation process (step S3), a correction formula creation process (step S5), and an estimation process (step S9) are executed in this order. In this embodiment, a case where a welding parameter A different from the expansion amount is corrected will be described as an example. That is, in this embodiment, at least a welding parameter A different from the expansion amount is included in the estimation formula 670 as an explanatory variable.

[0058] The acquisition step (step S1) is a step of acquiring time series data of welding parameters including data before the start of welding current flow, i.e., data in a pre-current flow section, for each welding point. In this embodiment, time series data of two or more welding parameters including an expansion amount calculated based on the displacement between the upper electrode 2 and the lower electrode 3 is acquired.

[0059] In the acquisition step (step S1), acquisition unit 210 acquires actual measured values ​​of welding parameters and generates time series data for each welding parameter as shown in Fig. 4. The generated time series data is displayed on display 40 as appropriate and stored in storage unit 60.

[0060] As shown in FIG. 9, after the acquisition step (step S1), a difference calculation step (step S3) is executed. FIG. 10 is a flowchart showing the details of the difference calculation step (step S3). The difference calculation step (step S3) is a step of calculating a first difference and a second difference. The first difference is a difference between an expansion amount at a first point in a section before the start of energization and an expansion amount at an Nth point in a section before the start of energization. The second difference is a difference between a welding parameter A at a first point after the start of energization T2 and a welding parameter A at an Nth point after the start of energization T2. ​​At this time, the welding parameter A, which is the original data for calculating the second difference, is a value under the same selection conditions as the welding parameter A included as an explanatory variable in the estimation formula 670 of the nugget diameter LN. The "selection conditions" referred to here are selection conditions for the actual measurement values, including which actual measurement value at which time or section is to be used among the multiple actual measurement values ​​of the welding parameter A after the start of energization T2, and whether or not some kind of numerical processing is performed on each actual measurement value. Numerical processing refers to, for example, processing such as calculating an integral value of a plurality of actual measured values ​​or calculating an average value from a plurality of actual measured values.

[0061] For example, when the welding parameter A included as an explanatory variable in the estimation formula 670 for the nugget diameter LN is an integral value of an actual measurement value (e.g., an amount of expansion) in the entire current-on section, the second difference is a difference between the first point and the Nth point of the integral value of the actual measurement value in the entire current-on section. For example, when the welding parameter A included as an explanatory variable in the estimation formula 670 for the nugget diameter LN is a maximum value of the actual measurement value in the current-on section, the second difference is a difference between the first point and the Nth point of the maximum value of the actual measurement value in the current-on section. For example, when the welding parameter A included in the estimation formula 670 for the nugget diameter LN is an average value of the actual measurement value within a predetermined selected time range in the current-on section, the second difference is a difference between the first point and the Nth point of the average value of the actual measurement value within the selected time range.

[0062] Here, the difference calculation unit 220 calculates the first difference and the second difference for at least two or more welding points from the 1st point to the Mth point. For example, when time series data exists for each welding point from the 1st point to the 25th point, the first difference and the second difference may be calculated for all welding points from the 1st point to the 25th point, or may be calculated for each welding point from the 1st point to an arbitrary welding point (for example, the 6th point). Fig. 10 shows a method of calculating the difference for each welding point from the 1st point to the Nth point among the welding points from the 1st point to the Mth point.

[0063] In step S310, difference calculation unit 220 refers to the time-series data of the expansion amount and welding parameter A, and obtains the expansion amount in the pre-energization section and welding parameter A after energization start time T2 for each welding point.

[0064] After step S310, a combination value calculation step (steps S321 to S335) is executed. The combination value calculation step (steps S321 to S335) is a step of calculating a first difference and a second difference for each welding point, and calculating a combination value determined by a combination of the first difference and the second difference. In the combination value calculation step (steps S321 to S335), the difference calculation unit 220 calculates a combination value for each welding point, and sets it as difference data. The calculated difference data is appropriately displayed on the display 40, and is also stored in the storage unit 60.

[0065] In the combination value calculation process (steps S321 to S335), first, steps S321 and S322 are executed. In step S321, difference calculation unit 220 calculates a first difference at a first point. As shown in Fig. 4, when actual measured values ​​of the amount of expansion at a plurality of time points in the pre-energization section are calculated at the same welding point, difference calculation unit 220 calculates the first difference by using, for example, an average value obtained by averaging the actual measured values ​​of the amount of expansion at each time point in the pre-energization section.

[0066] In step S322, difference calculation unit 220 calculates the second difference at the first point. At this time, both the first difference and the second difference represent the difference between the first point and the Nth point. Therefore, both the first difference of the first point and the second difference of the first point are zero. Note that, after step S310, step S321 and step S322 may be executed one after the other, or may be executed simultaneously in parallel.

[0067] After steps S321 and S322, step S325 is executed. In step S325, difference calculation unit 220 calculates a first combination value determined by a combination of a first difference at the first point and a second difference at the first point. In this embodiment, the combination value is a coordinate value in a coordinate system in which the first difference is an x-coordinate value and the second difference is a y-coordinate value. Note that the combination value may be defined by other indices.

[0068] After step S325, steps S331 and S332 are executed. In step S331, the difference calculation unit 220 calculates the first difference at the second point. Specifically, the difference calculation unit 220 calculates the first difference by, for example, subtracting the average value of the expansion amount at the second point in the pre-energization section from the average value of the expansion amount at the first point in the pre-energization section. In step S332, the difference calculation unit 220 calculates the second difference at the second point. Specifically, the difference calculation unit 220 calculates the second difference by, for example, subtracting the maximum value of the welding parameter at the second point in the energization section from the maximum value of the welding parameter at the first point in the energization section. Note that, after step S325, either step S331 or step S332 may be executed first, or may be executed simultaneously in parallel. The difference calculation unit 220 may also calculate the difference between the first point and the second point by a method other than the above.

[0069] Step S335 is executed after steps S331 and S332. In step S335, difference calculation section 220 calculates a second combination value determined by a combination of the first difference at the second point and the second difference at the second point.

[0070] The processing contents of each of the third to M-th welding points by the difference calculation unit 220 are similar to the processes from step S331 to step S335. The calculated first difference, second difference, and combination value are stored in the storage unit 60 as difference data.

[0071] 9, after the difference calculation step (step S3), a correction equation creation step (step S5) is executed. The correction equation creation step (step S5) is a step of creating a correction equation 650 indicating the correlation between the first difference and the second difference calculated in the difference calculation step (step S3). In other words, the correction equation 650 is a relational equation in which the first difference is an explanatory variable and the second difference is an objective variable.

[0072] Fig. 11 is a diagram showing an example of correction formula 650. The horizontal axis of Fig. 11 represents the first difference. The vertical axis of Fig. 11 represents the second difference. Fig. 11 shows an example of correction formula 650 for welding parameter A created using combination values ​​at each of the first to sixth welding points. Welding parameter A is a welding parameter included as an explanatory variable in estimation formula 670 for nugget diameter LN, and is a welding parameter to be corrected in this embodiment.

[0073] For example, when creating correction equation 650 for welding parameter A using combination values ​​(x1, y1) to (x6, y6) at each of the first to sixth welding points, correction equation creation unit 230 regresses combination values ​​(x1, y1) to (x6, y6) at each of the first to sixth welding points. In this way, correction equation 650 for welding parameter A is created.

[0074] In detail, the correction equation creation unit 230 linearly regresses the combination values ​​(x1, y1) to (x6, y6) at the first to sixth welding points, for example, as shown by the regression line GC in Fig. 11. As a result, the correction equation creation unit 230 calculates a linear equation shown in the following formula (1) as the correction equation 650. Note that a and b are constants determined by the combination value of the first difference and the second difference. y=ax+b Equation (1) The created correction formula 650 is displayed on the display 40 as appropriate and stored in the storage unit 60. Note that the calculation method of the correction formula 650 is not limited to this. The correction formula 650 may be created by a method other than the linear regression method of the combination value. Also, the constants a, b and each combination value of the correction formula 650 are not limited to this.

[0075] As shown in FIG. 9, after the correction equation creation step (step S5), an estimation step (step S9) is executed. FIG. 12 is a flowchart showing details of the estimation step (step S9). In the estimation step (step S9), a correction value calculation step (step S7), a parameter correction step (step S91), and an estimated value calculation step (step S95) are executed in this order. The correction value calculation step (step S7) is a step of calculating a correction value of welding parameter A using correction equation 650 of welding parameter A to be corrected. In the correction value calculation step (step S7), first, step S71 is executed.

[0076] In step S71, the correction value calculation unit 240 acquires time-series data during welding for estimating the nugget diameter LN, that is, time-series data of the expansion amount (inter-electrode displacement amount) and welding parameter A for correction.

[0077] After step S71, step S73 is executed. When estimating the nugget diameter LN at the time of welding the Pth point, the correction value calculation unit 240 acquires the expansion amount at the first point in the pre-energization section and the expansion amount at the Pth point in the pre-energization section in step S73. Then, the correction value calculation unit 240 calculates the difference between the expansion amount at the first point in the pre-energization section and the expansion amount at the Pth point in the pre-energization section (hereinafter, the third difference). After that, the correction value calculation unit 240 substitutes the third difference into the correction formula 650. Thereby, the correction value of the welding parameter A to be corrected is calculated (step S75). The correction value may be a positive value, a negative value, or zero.

[0078] After the correction value calculation step (step S7), a parameter correction step (step S91) is executed. In the parameter correction step (step S91), step S911 is executed first. The parameter correction step (step S91) is a step of correcting the calculated value of the welding parameter at the Pth point using the correlation between the first difference and the second difference and the expansion amount at the Pth point in the pre-energization section.

[0079] In step S911, parameter correction unit 291 adds the correction value to the calculated value of welding parameter A at the Pth point, which is welding parameter A after current application start time T2, i.e., the value before correction. This results in corrected welding parameter A (step S915). In other words, corrected welding parameter A is defined by the relational expression shown in the following equation (2). Corrected welding parameters = Uncorrected welding parameters + Correction values ​​Equation (2)

[0080] FIG. 13 is a diagram for comparing welding parameter A before and after correction using correction formula 650. The left diagram in FIG. 13 shows welding parameter A before correction, in the order of welding points. The right diagram in FIG. 13 shows welding parameter A after correction, in the order of welding points. The vertical axis in the left diagram and the right diagram in FIG. 13 represents the value of welding parameter A. The horizontal axis in the left diagram and the right diagram in FIG. 13 represents the welding points.

[0081] FIG. 13 plots data obtained when welding each of the first to 25th welding points under three different welding conditions. The welding under the reference condition is performed on each welding point with 2.0 liters of cooling water circulating inside the electrodes 2 and 3. At this time, the welding under the reference condition is performed with the influence of welding on the previous welding point largely eliminated. Specifically, for example, welding of the next welding point is started when a predetermined waiting time (for example, 10 seconds) has elapsed from the current stop time T3 for the previous welding point. The welding under the fifth condition is performed on each welding point with 2.0 liters of cooling water circulating inside the electrodes 2 and 3. The welding under the sixth condition is performed on each welding point with 1.3 liters of cooling water circulating inside the electrodes 2 and 3, which is less than that under the fifth condition. Under the fifth and sixth conditions, welding is performed continuously on each welding point without providing a waiting time between welding at each welding point. In other words, the fifth and sixth conditions are conditions that mimic a case where welding is performed in a state where the electrode temperature is elevated due to an increase in the number of consecutive welding operations. The set values ​​of the welding parameters are the same under each condition. Therefore, the data for the reference condition is a comparative example of the data for the fifth and sixth conditions.

[0082] As shown in the left diagram of Fig. 13, when welding the second point and thereafter, there is a significant difference between the plot for the reference condition and the plots for the fifth and sixth conditions for the welding parameter A before correction. In particular, the difference ΔE6 between the plot for the sixth condition and the reference condition is larger than the difference ΔE6 between the plot for the fifth condition and the reference condition. In other words, it is considered that in the welding under the sixth condition, the flow rate of the cooling water is reduced, so that the degree of rise in the electrode temperature is larger than that in the welding under the fifth condition, and the difference ΔE in the welding parameter A is significantly manifested.

[0083] In contrast, the corrected welding parameter A has a small difference ΔE between the plots related to the reference condition and the plots related to the fifth and sixth conditions at any of the second and subsequent welding points, regardless of the cooling water flow rate. This shows that, at least when the cooling water flow rate is within the range of 1.3 liters to 2.0 liters, the welding parameter A substituted into the estimation formula 670 for the nugget diameter LN can be well corrected regardless of the cooling water flow rate.

[0084] 12, after the parameter correction step (step S91), an estimate value calculation step (step S95) is executed. The estimate value calculation step (step S95) is a step of calculating an estimate value of the nugget diameter LN at the Pth point using the corrected welding parameter A.

[0085] In step S951, the estimated value calculation unit 292 substitutes the corrected welding parameter A into the estimation equation 670 for the nugget diameter LN. This allows the estimated value of the nugget diameter LN at the Pth point to be obtained (step S955).

[0086] In fact, as shown in Fig. 8, by correcting the welding parameter A substituted into the estimation equation 670 of the nugget diameter LN by the correction equation 650, the nugget diameter LN can be accurately estimated when welding is performed continuously at a plurality of welding points located at different positions of the welded member W. The method for estimating the nugget diameter LN in this embodiment is completed by performing each process (steps S1 to S9) up to step S955 shown in Fig. 9 and Fig. 12. The P-th welding point to be estimated for the nugget diameter LN may be a welding point located on a welded member different from the welded member W used to create the correction equation 650.

[0087] According to the first embodiment, as shown in FIG. 5, when resistance spot welding is performed continuously on a plurality of welding points at different positions of the workpiece W, the electrode temperature changes with an increase in the number of continuous weldings. As a result, as shown in FIG. 6 and FIG. 7, a difference may occur in the actual measurement value of the welding parameter at each welding point due to an increase in the number of continuous weldings. When a difference occurs in the value of the welding parameter, the welding parameter included in the estimation formula 670 as an explanatory variable is substituted into the estimation formula 670 of the nugget diameter LN without being corrected, as shown in FIG. 8, the estimation accuracy of the nugget diameter LN may be reduced. In contrast, according to the first embodiment, as shown in FIG. 11, the welding parameter A substituted into the estimation formula 670 of the nugget diameter LN is corrected by the correction formula 650 of the welding parameter A created using the expansion amount in the pre-energization section that is correlated with the electrode temperature. Then, the corrected welding parameter A is substituted into the estimation formula 670 of the nugget diameter LN to estimate the nugget diameter LN. Therefore, when resistance spot welding is performed successively on a plurality of weld points located at different positions on the welded workpiece W, even if a difference occurs in the welding parameter A for each weld point due to an increase in the number of successive welding operations, it is possible to suppress a decrease in the estimation accuracy of the nugget diameter LN. In other words, even when resistance spot welding is performed successively on a plurality of weld points located at different positions on the welded workpiece W, it is possible to maintain the estimation accuracy of the nugget diameter LN at the same level as when welding is performed only on the first weld point.

[0088] According to the first embodiment, as shown in Fig. 11, the correction formula 650 is a linear equation calculated by linear regression of a plurality of combination values ​​determined by a combination of the first difference and the second difference. In this way, the correlation between the first difference and the second difference can be uniquely expressed. Furthermore, the correction formula 650 can be easily created from a plurality of combination values.

[0089] According to the first embodiment, as shown in Fig. 4, when the expansion amounts at a plurality of time points in the pre-energization section are calculated for the same welding point, the first difference is calculated using an average value of the expansion amounts at the respective time points in the pre-energization section. In this way, even if an abnormal value is included in the time-series data of the welding parameters between the measurement start time T1 of the welding parameters and the current start time T2, it is possible to avoid calculating the first difference based only on the abnormal value. Therefore, when resistance spot welding is performed continuously on a plurality of welding points located at different positions on the workpiece W, it is possible to further suppress a decrease in the estimation accuracy of the nugget diameter LN.

[0090] Moreover, according to the first embodiment, the estimation system 1 can generate time-series data from the welding parameters measured by the measurement mechanism 9 and estimate the nugget diameter LN.

[0091] B. Second embodiment: In this embodiment, a method for estimating the P-th nugget diameter LN by correcting both the welding parameter A and the expansion amount when the estimation formula 670 of the nugget diameter LN includes the welding parameter A and the expansion amount as explanatory variables will be described. When there are a plurality of welding parameters included as explanatory variables in the estimation formula 670 of the nugget diameter LN and a plurality of welding parameters are to be corrected, a correction formula 650 is created for each welding parameter. The configuration of the estimation system 1 is the same as that of the first embodiment (FIGS. 1 and 3). The same steps and configurations as those in the first embodiment are denoted by the same reference numerals and description thereof will be omitted.

[0092] In this embodiment, the acquisition step (step S1), the difference calculation step (step S3), the correction equation creation step (step S5), and the estimation step (step S9) shown in Fig. 9 are executed in this order. The process content in the acquisition step (step S1) is the same as that in the first embodiment.

[0093] In the difference calculation process (step S3), the difference calculation unit 220 calculates the first difference, the second difference related to the welding parameter A, and the second difference related to the expansion amount. The calculation method of the first difference and the second difference related to the welding parameter A is the same as that of the first embodiment. The second difference related to the expansion amount is the difference between the expansion amount at the first point after the current supply start time T2 and the expansion amount at the Nth point after the current supply start time T2. Specifically, for example, when the expansion amount included as an explanatory variable in the estimation formula 670 of the nugget diameter LN is the maximum value of the actual measurement value in the current supply section, the second difference related to the expansion amount is the difference between the first point and the Nth point of the maximum value of the actual measurement value in the current supply section.

[0094] Next, in a correction equation creating step (step S5) shown in FIG. 12, the correction equation creating unit 230 calculates a first correction equation for calculating a correction value of the welding parameter A and a second correction equation for calculating a correction value of the expansion amount. The method of creating the first correction equation is the same as the correction equation 650 in the first embodiment (for example, FIG. 11). The second correction equation is the correction equation 650 indicating the correlation between the first difference and the second difference related to the expansion amount. The method of creating the second correction equation is the same as the method of creating the first correction equation.

[0095] Next, in a correction value calculation step (step S7), correction value calculation unit 240 calculates a correction value of welding parameter A at point P and a correction value of the expansion amount at point P. The method of calculating the correction value of welding parameter A is the same as that of the first embodiment shown in Fig. 12. The correction value of the expansion amount is calculated by substituting the third difference into the second correction formula.

[0096] Next, the estimation unit 290 executes a parameter correction step (step S91) and an estimated value calculation step (step S95) in this order.

[0097] Specifically, in a parameter correction step (step S91), the parameter correction unit 291 adds a correction value of the welding parameter A calculated by the first correction formula to the calculated welding parameter A at the Pth point, which is the welding parameter A at or after the current start time T2. This allows the corrected welding parameter A to be obtained. Then, the parameter correction unit 291 adds a correction value of the expansion amount calculated by the second correction formula to the calculated expansion amount at the Pth point, which is the expansion amount at or after the current start time T2. This allows the corrected expansion amount to be obtained. Then, in an estimated value calculation step (step S95), the estimated value calculation unit 292 substitutes the corrected welding parameter A and the corrected expansion amount into the estimation formula 670 of the nugget diameter LN. This allows the estimated value of the nugget diameter LN at the Pth point to be obtained.

[0098] According to the second embodiment, the estimation formula 670 of the nugget diameter LN includes the welding parameter A different from the expansion amount and the expansion amount as explanatory variables. At this time, according to the second embodiment, two correction formulas 650 are created, namely, the first correction formula which is the correction formula 650 of the welding parameter A and the second correction formula which is the correction formula 650 of the expansion amount. Then, both the welding parameter A and the expansion amount are corrected using the first correction formula and the second correction formula. As a result, when the estimation formula 670 of the nugget diameter LN includes the welding parameter and the expansion amount different from the expansion amount as explanatory variables, the nugget diameter LN can be estimated using the corrected welding parameter A and the corrected expansion amount. Therefore, even when the estimation formula 670 of the nugget diameter LN includes the welding parameter and the expansion amount different from the expansion amount as explanatory variables, the estimation accuracy of the nugget diameter LN can be improved.

[0099] C. Other embodiments: C-1. Alternative embodiment 1: In the above embodiment, as shown in FIG. 11, the correction formula 650 is a linear equation calculated by performing linear regression on a plurality of combined values ​​determined by a combination of the first difference and the second difference. However, the present disclosure is not limited to this. The correction formula 650 may be, for example, a higher-order equation calculated by performing nonlinear regression on a plurality of combined values ​​determined by a combination of the first difference and the second difference. In this form, it is possible to create a correction formula 650 that reflects the correlation between the first difference and the second difference in more detail than when the correction formula 650 is a linear equation. This can further improve the estimation accuracy of the nugget diameter LN when resistance spot welding is performed continuously on a plurality of welding points that are located at different positions on the welded member W.

[0100] C-2. Alternative embodiment 2: In the above embodiment, as shown in Fig. 1, the welded members W are two overlapping metal plates W1 and W2. However, the present disclosure is not limited to this. The welded members W may be three or more overlapping metal plates. Even in such a form, it is possible to suppress a decrease in the estimation accuracy of the nugget diameter LN.

[0101] C-3. Alternative embodiment 3: In the above embodiment, the first difference is calculated using an average value obtained by averaging the actual measured values ​​of the expansion amount at a plurality of points in time in the pre-energization section. However, the present disclosure is not limited to this. The first difference may be calculated using a welding parameter expressed by an index other than the actual measured value and the average value, such as an integral value obtained by integrating the actual measured value of the welding parameter in the pre-energization section. In this form, a correction formula 650 of the welding parameter expressed by an appropriate index can be created according to the type of welding parameter included as an explanatory variable in the estimation formula 670 of the nugget diameter LN, the number of metal plates W1 and W2 as the welded member W, the growth mode of the nugget N, and the like.

[0102] C-4. Alternative embodiment 4: In the above embodiment, the first difference is calculated using an average value obtained by averaging the actual expansion amount at a plurality of points in the pre-energization section. However, the present disclosure is not limited to this. The first difference may be calculated based on the actual expansion amount at any one time point in the pre-energization section without using the average value. Even in this form, it is possible to suppress a decrease in the estimation accuracy of the nugget diameter LN. Furthermore, in this form, even if the time series data of the welding parameters includes an abnormal value, it is possible to select any time point other than the time point showing the abnormal value and calculate the first difference.

[0103] C-5. Alternative embodiment 5: In the above embodiment, as shown in FIG. 9, the estimation device creates the correction formula 650 by executing each step up to the correction formula creation step (step S5) and then estimates the nugget diameter LN. However, the present disclosure is not limited to this. For example, the estimation device may estimate the nugget diameter LN using the correction formula 650 stored in advance in the storage unit 60. In such a form, the correction value of the welding parameter can be calculated using the correction formula 650 created in advance.

[0104] C-6. Alternative embodiment 6: In another embodiment, the estimation device may calculate a new correction equation 650 using two or more combination values ​​that were the basis for calculating the correction equation 650 and that were previously stored in the memory unit 60, and combination values ​​calculated based on time series data acquired when estimating the nugget diameter LN at the Pth point. That is, in another embodiment, the estimation device may update the coefficients of the correction formula 650 every time the nugget diameter LN is estimated. In this manner, the calculation accuracy of the correction value can be improved.

[0105] C-7. Alternative embodiment 7: In the above embodiment, the correction formula 650 indicating the correlation between the first difference and the second difference is created, and the correction value of the welding parameter is calculated using the correction formula 650. However, the present disclosure is not limited to this. The correction value of the welding parameter may be calculated, for example, by referring to a correlation table indicating the correlation between the first difference and the second difference. Even in such a form, the correction value of the welding parameter can be calculated. This makes it possible to suppress a decrease in the estimation accuracy of the nugget diameter LN.

[0106] C-8. Other embodiment 8: In the second embodiment, the estimation formula 670 of the nugget diameter LN includes the welding parameter A different from the expansion amount and the expansion amount as explanatory variables. Therefore, in the second embodiment, the welding parameter A and the expansion amount substituted into the estimation formula 670 of the nugget diameter LN are corrected respectively. However, the present disclosure is not limited to this. For example, when three or more welding parameters are included as explanatory variables of the estimation formula 670 of the nugget diameter LN, the estimation system 1 may create correction formulas 650 for all welding parameters, calculate corrected values, and estimate the nugget diameter LN. In this case, the correction formulas 650 for each welding parameter can be created in the same manner as the creation method of the correction formula 650 for the welding parameter A shown in the first embodiment. In addition, when three or more welding parameters are included as explanatory variables of the estimation formula 670 of the nugget diameter LN, an arbitrary welding parameter may be selected, and the correction formula 650 may be created only for the selected welding parameter. In this embodiment, when three or more welding parameters are included as explanatory variables of estimation formula 670 for nugget diameter LN, the estimation accuracy of nugget diameter LN can be improved by correcting any selected welding parameter. Also, correction formula 650 for each welding parameter can be created without limiting the type of welding parameter.

[0107] 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]

[0108] 1...estimation system, 2, 22...upper electrode, 2a, 22a...tip of upper electrode, 3, 33...lower electrode, 3a...tip of 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 part, 11b...lower part, 20...CPU, 30...communication unit, 40...display, 41...servo motor, 42...lifting member, 50...input operation unit, 60...storage unit, 91...current value measurement unit, 92...voltage value measurement unit, 93...resistance value calculation unit, 95...pressure measurement unit, 96...displacement measurement unit, 97...inter-electrode displacement calculation unit, 100...control device, 200...operation control unit, 210...acquisition unit, 220...difference calculation unit output unit, 230...correction formula creation unit, 240...correction value calculation unit, 290...estimation unit, 291...parameter correction unit, 292...estimated value calculation unit, 650...correction formula, 670...estimated formula, 961...first displacement amount measurement unit, 962...second displacement amount measurement unit, D1...pressure direction, D2...extension direction, D3...expansion direction, G...welding gun, GC...regression line, GS...straight line, L1, L2...separation distance, L10, L20...length, LN...nugget diameter, M...melted portion, N...nugget, O...center, RA...robot arm, T1...measurement start time, T2...current start time, T3...current stop time, T4...measurement end time, W...welded member, W1...first metal plate, W2...second metal plate, WS...joint interface

Claims

1. A method for estimating a nugget diameter in resistance spot welding of welded members sandwiched between an upper electrode and a lower electrode, comprising: The resistance spot welding is The welding is performed in order from a first welding point to an Mth welding point (M is an integer of 2 or more) that are present at different positions of the workpiece, The estimation method includes: an acquiring step of acquiring, for each of the welding points, time-series data including data before start of application of a welding current, for two or more welding parameters that contribute to the size of the nugget diameter and include an expansion amount calculated based on a displacement between the upper electrode and the lower electrode; (i) a difference calculation step of calculating a first difference which is a difference between the expansion amount at the first point before the start of energization and the expansion amount at an N-th point (N is an integer of 2 to M) before the start of energization, and (ii) a second difference which is a difference between the welding parameter at the first point after the start of energization and the welding parameter at the N-th point after the start of energization; and estimating the nugget diameter at a P-th point (P is an integer equal to or greater than 1 and equal to or less than M) by using the welding parameters, The estimation process includes correcting the calculated welding parameters for the Pth point using the expansion amount for the Pth point before the start of current flow and a correlation between the first difference and the second difference, and estimating the nugget diameter using the corrected welding parameters.

2. The estimation method according to claim 1, further comprising: A correction formula creating step of creating a correction formula showing the correlation, The estimation step includes: In the case of estimating the nugget diameter at the P-th point, a third difference, which is a difference between the expansion amount at the first point before the start of current application and the expansion amount at the P-th point before the start of current application, is substituted into the correction formula to calculate a correction value of the welding parameter. and calculating the estimated value of the nugget diameter by substituting the corrected welding parameters, which have been corrected by adding the correction value to the calculated welding parameters at the Pth point, into a predetermined estimation equation, which has the welding parameters as explanatory variables and the estimated value of the nugget diameter as a target variable.

3. The estimation method according to claim 2, The estimation method, wherein the correction equation is a linear equation calculated by performing linear regression on a plurality of combined values ​​determined by combinations of the first difference and the second difference.

4. The estimation method according to claim 2, The estimation method, wherein the correction equation is a higher-order equation calculated by performing nonlinear regression on a plurality of combined values ​​determined by combinations of the first difference and the second difference.

5. The estimation method according to claim 1 or 2, In the case where the expansion amount is calculated at a plurality of time points before the start of energization at the same welding point, The estimation method includes calculating the first difference using an average value of a plurality of the expansion amounts before the start of the energization.

6. A device for estimating a nugget diameter in resistance spot welding of a welded member sandwiched between an upper electrode and a lower electrode, comprising: The resistance spot welding is The welding is performed in order from a first welding point to an Mth welding point (M is an integer of 2 or more) that are present at different positions of the workpiece, The estimation device includes: an acquisition unit that acquires, for each of the welding points, time-series data including data before a welding current is started to be applied, for two or more of the welding parameters that contribute to the size of the nugget diameter and include an expansion amount calculated based on a displacement between the upper electrode and the lower electrode; a difference calculation unit that calculates (i) a first difference that is a difference between the expansion amount at the first point before the start of current flow and the expansion amount at an Nth point (N is an integer greater than or equal to 2 and less than or equal to M) before the start of current flow, and (ii) a second difference that is a difference between the welding parameter at the first point after the start of current flow and the welding parameter at the Nth point after the start of current flow; an estimation unit that estimates the nugget diameter at a P-th point (P is an integer equal to or greater than 1 and equal to or less than M) by using the welding parameters, The estimation unit corrects the calculated welding parameters for the Pth point using the expansion amount for the Pth point before the start of current flow and a correlation between the first difference and the second difference, and estimates the nugget diameter using the corrected welding parameters.

7. A system for estimating a nugget diameter in resistance spot welding of a welded member sandwiched between an upper electrode and a lower electrode, comprising: The resistance spot welding is The welding is performed in order from a first welding point to an Mth welding point (M is an integer of 2 or more) that are present at different positions of the workpiece, The estimation system comprises: The estimation device according to claim 6 ; and a measurement mechanism that generates time series data of the welding parameter.

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