Method for estimating nugget diameter of resistance spot welding

Abstract

A kind of estimation method is disclosed, using the welding parameter that the size of the nugget diameter formed in resistance spot welding is acted on as explanatory variable and the estimation value of nugget diameter as the purpose variable, to estimate nugget diameter using the predetermined estimation formula, wherein the estimation method has: resistance value obtaining procedure, as welding parameter, obtains the latter half resistance value, the latter half resistance value is the resistance value of the latter half of the current period during welding and is the resistance value of the whole current circuit including the welding object;And nugget diameter calculating procedure, the obtained latter half resistance value is substituted into estimation formula to calculate nugget diameter.

Description

Technical Field

[0001] This disclosure relates to a method for estimating the diameter of the weld nugget in resistance spot welding. Background Technology

[0002] Traditionally, resistance spot welding has been used in the manufacture of vehicle body frames and the like to join multiple metal plates together. This method involves clamping the workpieces (multiple metal plates) with a pair of electrodes while applying an electric current. The Joule heat generated by the resistance of the workpieces themselves melts the metal plates together. Furthermore, to improve the weld strength of the metal plates, it is necessary to ensure the diameter of the weld nugget obtained through resistance spot welding. Therefore, a method for estimating whether the weld nugget diameter has been successfully ensured is required.

[0003] For example, in the method for estimating the weld nugget diameter in a resistance spot welding apparatus described in Japanese Patent Application Publication No. 2020-171942, a preparatory energizing process for registering the main mode is performed under pre-set welding conditions. Then, when performing the actual welding under the pre-set welding conditions, the weld nugget diameter obtained through the actual welding is estimated based on the deviation between the resistance value in the main mode and the resistance value during the actual welding. Furthermore, in such an estimation method, the resistance value uses, for example, information from the start of energizing to the stop of energizing. Summary of the Invention

[0004] However, in the presence of various interferences at the welding site, such as gaps between steel plates, tilting of steel plates, and wear on the tip of the electrode pressed against the steel plate, the correlation between the resistance value and the diameter of the weld nugget is not high throughout the entire energizing period, resulting in poor estimation accuracy of the weld nugget diameter.

[0005] This disclosure may be implemented in the following ways.

[0006] (1) According to one aspect of this disclosure, a method for estimating the diameter of a weld nugget is provided. This method for estimating the diameter of a weld nugget is a predetermined estimation formula that uses welding parameters that affect the size of the weld nugget diameter formed in resistance spot welding as explanatory variables and the estimated value of the weld nugget diameter as the objective variable. The estimation method comprises: a resistance value acquisition step, in which a second half resistance value is acquired as the aforementioned welding parameter, which is the resistance value of the second half of the energizing period during welding and is the resistance value of the entire energizing circuit including the object being welded; and a weld nugget diameter calculation step, in which the acquired second half resistance value is substituted into the aforementioned estimation formula to calculate the weld nugget diameter. According to this method, the resistance value of the second half of the energized phase (i.e., the latter half resistance value) is used as a welding parameter to estimate the weld nugget diameter. Research by the inventors of this application shows that the latter half resistance value has a higher correlation with the weld nugget diameter compared to the resistance value of the first half of the energized phase. By using the latter half resistance value, which has a high correlation with the weld nugget diameter, as a welding parameter for estimation, the accuracy of the weld nugget diameter estimation can be improved. (2) In the above method, the aforementioned energizing period during welding may include the initial energizing period and the formal energizing period performed at intervals after the initial energizing period, and the aforementioned second half resistance value is the resistance value of the second half of the formal energizing period. According to this method, when the energizing period includes the initial energizing and the formal energizing period, by using the resistance value of the second half of the formal energizing period, which has a high correlation, as the welding parameter, the estimation accuracy of the weld nugget diameter can be further improved. (3) In the above method, the aforementioned presumption can also be a linear equation calculated by performing linear regression on the plotting points corresponding to the aforementioned second-half resistance value and the aforementioned molten core diameter. According to this method, the correlation between the second-half resistance value and the molten core diameter can be uniquely expressed. In addition, by using the plotting points corresponding to the second-half resistance value and the molten core diameter, the presumption can be easily made. Attached Figure Description

[0007] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which the same reference numerals denote the same elements. Figure 1 This is a schematic diagram showing the configuration of the melting core diameter estimation system as a first embodiment of the present disclosure. Figure 2 This is a block diagram showing the general structure of the control device. Figure 3 This is a flowchart illustrating the steps in the method for estimating the diameter of the melt core. Figure 4 It is a graph that shows the change of resistance value over time. Figure 5 yes Figure 4 An enlarged view of the portion shown in the chart, enclosed by a double-dotted line. Figure 6 It is a graph showing the relationship between the resistance value of the half after energization and the diameter of the molten core. Figure 7 This diagram illustrates the effect of using the half-resistance value as a welding parameter to estimate the weld nugget diameter. Figure 8 This diagram illustrates the effect of using the half-resistance value as a welding parameter to estimate the weld nugget diameter. Figure 9This is a graph showing the variation of welding current in the second embodiment of this disclosure. Detailed Implementation

[0008] A. First implementation method: A1. Overall structure of the melting nugget diameter estimation system 1: When describing the method for estimating the diameter of the melt core in the first embodiment of this disclosure, the configuration of the melt core diameter estimation system 1 (hereinafter also simply referred to as "estimation system 1") will first be described. Figure 1 This is a schematic configuration diagram showing the weld nugget diameter estimation system 1 as a first embodiment of the present disclosure. The estimation system 1 is a system for estimating the diameter of the weld nugget formed on the welded part W by resistance spot welding performed by the resistance spot welding apparatus 10. The estimation system 1 includes the resistance spot welding apparatus 10, the control device 100, and the measuring mechanism 9.

[0009] The resistance spot welding apparatus 10 is a device that melts and joins multiple overlapping metal plates W1, W2 into a weldable component W. The resistance spot welding apparatus 10 includes a welding torch G and a robotic arm RA.

[0010] The welding torch G comprises a torch body 11, an upper electrode 2 and a lower electrode 3 as a pair of electrodes, an electrode lifting device 4, a pressurizing device 5, and a current adjusting device 6. The torch body 11 is held in place by a robotic arm RA. The lower electrode 3 is a fixed electrode disposed in a state fixed to the lower part 11b of the torch body 11. The upper electrode 2 is a movable electrode that can move in the direction relative to the lower electrode 3. The upper electrode 2 is mounted to the upper part 11a of the torch body 11 via the electrode lifting device 4. The upper electrode 2 and the lower electrode 3 each have internal flow paths (not shown) for the flow of cooling water.

[0011] The electrode lifting device 4 is an electrically powered device that holds and raises the upper electrode 2. The electrode lifting device 4 is mounted on the top of the upper part 11a of the gun body 11. The electrode lifting device 4 includes a servo motor 41 and a lifting component 42 coupled to the drive shaft of the servo motor 41. The electrode lifting device 4 raises and lowers the lifting component 42 by operating the servo motor 41 according to a lifting command from the control device 100.

[0012] The pressurizing device 5 presses the upper electrode 2 and the lower electrode 3 towards each other. Specifically, the pressurizing device 5 is a device that applies force to the upper electrode 2 and the lower electrode 3 respectively to press the welded part W with a predetermined pressure while the welded part W is held by the upper electrode 2 and the lower electrode 3. The pressurizing device 5 applies a predetermined force to the upper electrode 2 and the lower electrode 3 respectively according to the pressurizing command from the control device 100.

[0013] The current adjusting device 6 adjusts the value (hereinafter, current value) of the welding current flowing between the upper electrode 2 and the lower electrode 3 according to the current command sent 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] Figure 2 This is a block diagram showing a general configuration of the control device 100. In this embodiment, in addition to controlling the operation of the resistance spot welding apparatus 10, the control device 100 also functions as a estimation device for estimating the diameter of the weld nugget in resistance spot welding. Furthermore, the estimation device may also be configured to be separate from the control device 100 and to communicate with it via wired or wireless means.

[0015] 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 the components 20 to 60. The communication unit 30 communicatively connects the resistance spot welding apparatus 10 and the measuring mechanism 9 to the control device 100. The display 40 is, for example, a liquid crystal display (LCD), which displays information according to instructions from the CPU 20. The input operation unit 50 includes, for example, a keyboard and a mouse, and receives instructions from the user.

[0016] The storage unit 60 stores various programs and estimation formulas 670, including various data such as RAM, ROM, and rewritable non-volatile memory. The estimation formula 670 is a pre-prepared formula for calculating the estimated value of the weld nugget diameter. Specifically, the estimation formula 670 is a formula that uses welding parameters that affect the size of the weld nugget diameter as explanatory variables and the estimated value of the weld nugget diameter as the target variable. In the first embodiment, as welding parameters, the estimation formula 670 uses the resistance value of the second half of the energizing period during welding, and is the resistance value of the entire energizing circuit including the object being welded, i.e., the second half resistance value, as shown by equation (1) of the following linear equation. Furthermore, details regarding the "second half resistance value" will be described later. The diameter of the molten core = C1 × [position of the second half of the resistor] + C2 (C1 and C2 are specified constants) ... Equation (1)

[0017] The CPU 20 functions as the motion control unit 200, the acquisition unit 210, and the estimation unit 290 by expanding various programs stored in the storage unit 60.

[0018] The motion control unit 200 controls the operation of the resistance spot welding apparatus 10. The operation of the resistance spot welding apparatus 10 is controlled during welding based on preset parameters (hereinafter, welding parameters). That is, the welding parameters are welding conditions preset during welding. Welding parameters include, for example, current value, voltage value, resistance value, inter-electrode displacement, expansion amount, and pressure applied to electrodes 2 and 3. The motion control unit 200 comprehensively controls their operation. For example, the motion control unit 200 controls the operation of the resistance spot welding apparatus 10 based on the preset values ​​of the welding parameters set by the user via the input operation unit 50.

[0019] The acquisition unit 210 acquires the half-resistance value during the aforementioned welding process. The estimation unit 290 calculates the estimated value of the weld nugget diameter by substituting the half-resistance value into the estimation formula (equation (1) above). Details of each function and the estimation method of the weld nugget diameter will be described later. In addition, at least some of the functions of the CPU 20 can also be implemented by hardware circuitry.

[0020] The measuring mechanism 9 is a mechanism that measures the actual values ​​of welding parameters as physical quantities required for welding using the resistance spot welding device 10. The "actual values ​​of welding parameters" referred to here are the values ​​of the welding parameters actually observed when welding is performed according to the welding condition instructions sent from the motion control unit 200, that is, when welding is performed using preset welding parameter settings. The actual values ​​of the welding parameters measured by the measuring mechanism 9 are sent to the CPU 20 via the communication unit 30.

[0021] In this embodiment, the measuring mechanism 9 includes a current measuring unit 91, a voltage measuring unit 92, a resistance calculating unit 93, a pressure measuring unit 95, an inter-electrode displacement measuring unit 96, and an inter-electrode displacement calculating unit 97. Furthermore, the configuration and function of the measuring mechanism 9 are not limited to this. For example, the measuring mechanism 9 may also include other mechanisms for measuring welding parameters. Additionally, at least a portion of the functions of the measuring mechanism 9 can be implemented as a function of the CPU 20.

[0022] Each component 91 to 97 of the measuring mechanism 9 measures the actual values ​​of welding parameters at predetermined measurement times during welding. The current measuring unit 91 measures the current flowing between the upper electrode 2 and the lower electrode 3. The current measuring unit 91 is, for example, a current sensor. The voltage measuring unit 92 measures the voltage (potential difference) between the upper electrode 2 and the lower electrode 3. The voltage measuring unit 92 is, for example, a voltage sensor.

[0023] The resistance calculation unit 93 calculates the resistance value (hereinafter, resistance value) using the measured values ​​of the current and voltage measured when the power is applied. Specifically, the resistance calculation unit 93 calculates the resistance value by dividing the voltage value by the current value. In this case, the current value and voltage value at the same measurement time point are used in the calculation of the resistance value.

[0024] The pressure measuring unit 95 measures the pressure applied by each electrode 2, 3 to the welded component W. The pressure measuring unit 95 is, for example, a force sensor housed inside the electrode lifting device 4. The inter-electrode displacement measuring unit 96 includes a first displacement measuring unit 961 that measures the displacement of the upper electrode 2 (hereinafter, the first displacement) and a second displacement measuring unit 962 that measures the displacement of the lower electrode 3 (hereinafter, the second displacement).

[0025] The first displacement measuring unit 961 measures, for example, the lifting position of the upper electrode 2 at a first measuring time point and the lifting position of the upper electrode 2 at a second measuring time point after a predetermined measuring time has elapsed from the first measuring time point, and calculates the first displacement based on the difference between the measured values. The first displacement measuring unit 961 is, for example, an encoder housed inside the electrode lifting device 4 that measures the lifting position of the upper electrode 2 by detecting the rotational angle position of the output shaft of the servo motor 41.

[0026] The second displacement measuring unit 962 measures a second displacement. 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 deforms without moving. Thus, the second displacement measuring unit 962 measures the force (deformation value) applied to the lower electrode 3 as the second displacement. The second displacement measuring unit 962 is, for example, a deformation sensor that measures the deformation value of the lower electrode 3.

[0027] The inter-electrode displacement calculation unit 97 uses the first displacement measured and calculated by the first displacement measuring unit 961 and the second displacement measured by the second displacement measuring unit 962 to calculate the displacement between the upper electrode 2 and the lower electrode 3 (hereinafter, the inter-electrode displacement). In this case, the value at the same measurement time point is used in the calculation of the inter-electrode displacement. For example, the inter-electrode displacement calculation unit 97 calculates the inter-electrode displacement during energization by adding the first and second displacements during the energization period. However, the method for calculating the inter-electrode displacement is not limited to this.

[0028] A2. Method for estimating the diameter of the molten core: Next, the method for estimating the diameter of the melt core in the first embodiment of this disclosure, which uses the estimation system 1 described in detail above, will be explained. Figure 3 This is a flowchart illustrating the steps in the method for estimating the diameter of the melt nugget. For example... Figure 3As shown, the method for estimating the diameter of the melt core includes a resistance value acquisition step (S101) and a melt core diameter calculation step (S102).

[0029] In the resistance value acquisition process (S101), the acquisition unit 210 acquires the second half resistance value. In the resistance value estimation process (S101), the estimation unit 290 substitutes the second half resistance value into equation (1), which is an estimation formula, to calculate the estimated value of the molten core diameter.

[0030] Next, in the first embodiment, the second half resistance value used as a welding parameter will be explained in more detail. Figure 4 It is a graph that shows the change of resistance value over time. Figure 5 yes Figure 4 A magnified view of the portion shown in the chart, enclosed by a double-dotted line. Figure 5 The diagram shows the change in resistance over time at the end of the energizing period. Figure 4 , Figure 5 In the diagram, the current value is shown by a thick solid line, and the resistance values ​​of multiple (four in this embodiment) test modes 1 to 4 in the test welding are shown by thin solid lines, dashed lines, single-dot dashed lines, and double-dot dashed lines, respectively.

[0031] like Figure 4 As shown, the energizing start time T1 is 100ms, and the energizing stop time T4, where energizing to the welded component W ceases, is 300ms. The energizing midpoint T2, which is 200ms, is the midpoint between energizing start time T1 and energizing stop time T4. Here, "the latter half of the energizing period" means from the energizing midpoint T2 to the energizing stop time T4. Furthermore, "the latter half of the energizing period" will be simply referred to as "the latter half of the energizing period" below. The energizing end time T3 lies between the energizing midpoint T2 and the energizing stop time T4, and is set to be, for example, tens of milliseconds before the energizing stop time T4. This energizing end time T3 corresponds, for example, to a time point representing 10-20% of the energizing period backward from the energizing stop time T4. The period from the energizing end time T3 to the energizing stop time T4 is called the "energizing end period S." In other words, the energizing end period S is included in the latter half of the energizing period.

[0032] Through the inventors' research, this application identified characteristic quantities that correlate with the weld nugget diameter in the resistance value. The results showed that the correlation between the resistance value and the weld nugget diameter was higher in the latter half of the energizing period. A larger weld nugget diameter resulted in a lower resistance value. In particular, the correlation between the resistance value and the weld nugget diameter was significant during the final period (S) of energizing. This was found to be because various interferences can occur at the welding site, but their effects are less noticeable in the first half of the energizing process, while they become more significant in the final period (S). Examples of interference include gaps between steel plates, tilting of the steel plates, and wear on the tip of the electrode pressed against the steel plate. Figure 5 As shown, during the final period S of the power-on period, the change in resistance value caused by the interference can be significantly read compared to other periods during the power-on period.

[0033] The inventors of this application believe that by using the resistance value during a period with greater correlation, the estimation accuracy of the weld nugget diameter can be improved. Furthermore, in the latter half of the energizing period, a molten pool is formed through welding, and the molten pool reaches a size close to the target weld nugget diameter, while the molten state becomes a stable state compared to the initial energizing state.

[0034] Figure 6 This is a graph showing the relationship between the resistance value and the diameter of the molten core after energization. When creating the above estimation formula, a first current value was set to produce a target molten core diameter, and a second current value was set to be smaller than the first current value, intentionally reducing the current value to produce an excessively small molten core diameter. At each current value, the resistance value and the molten core diameter were measured, obtaining multiple data points. Figure 6 In the figure, multiple data points were plotted. Additionally, the average resistance value during the final energizing period S was used as the latter half-resistance value. Based on these data, a single regression analysis was performed to derive the aforementioned presumption. The presumption is a linear equation calculated by performing a linear regression on the plotted points corresponding to the latter half-resistance value and the molten core diameter, as shown below. Figure 6 The straight line in the middle.

[0035] Figure 7 , Figure 8 This diagram illustrates the effect of using the half-resistance value as a welding parameter to estimate the weld nugget diameter. Figure 7 In the graph, the horizontal axis represents the measured diameter of the molten core (hereinafter referred to as "measured diameter"), and the vertical axis represents the estimated diameter of the molten core (hereinafter referred to as "estimated diameter"). The results are plotted. Figure 8 In the figure, the horizontal axis represents the estimated accuracy, and the vertical axis represents the measured diameter. The results were then plotted. Figure 7 , Figure 8 In this case, where the estimated diameter matches the measured diameter, the plotting point is located on the ideal line L1 shown as a single-dotted line in the diagram. Figure 7 , 8In the diagram, all plotting points are located on or near the ideal line L1. Furthermore, all plotting points are within the range of ±20% of the estimated error, indicated by dashed lines L2 and L3 in each figure. This confirms that the estimated melt core diameter has achieved high accuracy.

[0036] According to the method for estimating the weld nugget diameter in the first embodiment described above, the resistance value in the latter half of the energizing period (i.e., the second half resistance value) is used as a welding parameter to estimate the weld nugget diameter. The second half resistance value has a higher correlation with the weld nugget diameter compared to the resistance value in the first half of the energizing period. By using the second half resistance value, which has a high correlation with the weld nugget diameter, as a welding parameter for estimation, the accuracy of the weld nugget diameter estimation can be improved compared to, for example, using the resistance value during the entire energizing period.

[0037] Furthermore, the estimation formula used in the method for estimating the weld nugget diameter in the first embodiment described above is a linear equation calculated by performing linear regression on plotting points corresponding to the second half resistance value and the weld nugget diameter. Therefore, it can uniquely express the correlation between the second half resistance value and the weld nugget diameter. In addition, by using plotting points corresponding to the second half resistance value and the weld nugget diameter, the estimation formula can be easily created.

[0038] B. Second implementation method: Next, refer to Figure 9 The second embodiment will now be described. In the second embodiment, only the power supply method differs from the first embodiment described above; the other components and presumed methods are the same, so detailed descriptions are omitted. Figure 9 This is a graph illustrating the variation of the welding current I in the second embodiment of this disclosure. (See figure) Figure 9 As shown, in the resistance spot welding of the second embodiment, after initial energization for a predetermined time, energization is stopped for a predetermined period, and then formal energization is performed. That is, the energization period includes the period of initial energization and the period of formal energization performed at intervals after the initial energization.

[0039] In the second embodiment, the resistance value in the latter half is the average resistance value during the latter half of the formal energization period (times T5 to T6). The second embodiment also achieves the same effect as the first embodiment described above.

[0040] C. Other implementation methods: (C1) In the above embodiments, the presumed formula is set to include only the second half resistance value as a welding parameter as shown in formula (1), but it may also include other welding parameters such as expansion amount.

[0041] (C2) In the first embodiment described above, the second half resistance value uses the average resistance value during the last period S of energization, but is not limited to the last period S of energization; it can be the average resistance value of any period in the second half of energization after the intermediate time point T2. Alternatively, it may not be an average value; the integral value of the overall resistance value during any period in the second half of energization can be used. These are all referred to as the "second half resistance value".

[0042] This disclosure is not limited to the embodiments described above, and can be implemented in various configurations without departing from its spirit. For example, the technical features in each embodiment corresponding to the technical features in the various methods described in the Summary of the Invention section 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. In addition, any technical feature that is not described as an essential technical feature in this specification can be appropriately deleted.

Claims

1. A method for estimating the diameter of a weld nugget, comprising using a predetermined estimating formula that takes welding parameters that affect the size of the weld nugget diameter formed in resistance spot welding as explanatory variables and the estimated value of the weld nugget diameter as the objective variable, wherein, The estimation method comprises: The resistance value acquisition process, as the welding parameter, acquires the second half of the resistance value, which is the resistance value during the latter half of the energizing period during welding and is the overall resistance value of the energizing circuit including the object being welded; and The process of calculating the weld nugget diameter involves substituting the obtained second-half resistance value into the estimated formula to calculate the weld nugget diameter. The energizing period during the welding process includes the initial energizing period and the formal energizing period performed at intervals after the initial energizing. The latter half of the resistance value refers to the resistance value during the latter half of the formal energization period. The estimated formula is: fused core diameter = C1 × [second half resistance value] + C2, where C1 and C2 are specified constants.

2. The method for estimating the diameter of the melt nugget according to claim 1, wherein, Regarding the aforementioned presumption. It is a linear equation calculated by performing linear regression on plotting points corresponding to the second half resistance value and the diameter of the molten core.

Images