JP7598019B2 - Method for measuring thrust force in resistance spot welding, method for manufacturing resistance spot welded joint, and resistance spot welding device - Google Patents

Description

The present invention relates to a method for measuring the force of a stroke in resistance spot welding, a method for manufacturing a resistance spot welded joint, and a resistance spot welding device.

For example, resistance spot welding, which has excellent mass productivity, is widely used in the welding assembly process of automobile parts. Generally, a resistance spot welding robot is used as a resistance spot welding machine, which combines an industrial robot with a welding gun having a pair of electrodes for resistance spot welding.

One of the most common disturbances that occurs in an actual welding assembly process is the electrode impact angle. An impact angle is when the axis of the electrode is inclined relative to the normal of the contact surface between the electrode and the workpiece. If spot welding is performed with an impact angle, there is a risk of adversely affecting the weld quality. In particular, when galvanized steel sheets are used as the welding workpiece, liquid metal embrittlement cracking (hereinafter referred to as LME cracking; LME: Liquid Metal Embrittlement) caused by molten zinc occurs, which can be problematic.

For example, Patent Documents 1 to 6 describe a technique for detecting the impact angle using an impact angle detection means. Patent Document 7 describes a technique for attaching an optical strain gauge to an electrode and detecting the direction and magnitude of the load applied to the electrode based on a signal output from the strain gauge after the start of current flow.

JP 2002-35952 A JP 2002-45976 A Japanese Patent Application Publication No. 10-143212 Patent No. 3359012 Patent No. 5101466 Patent No. 3982603 JP 2015-155103 A JP 2011-156564 A JP 2018-39019 A International Publication No. 2017/033455 Japanese Unexamined Patent Publication No. 52-149242 Japanese Patent Application Publication No. 4-274885

The inventors believed that if the direction and magnitude of the impact angle could be detected, it would be possible to, for example, determine whether welding is possible for each spot welding point and change the welding conditions, thereby stabilizing the welding quality. The inventors also believed that if the electrode pressure could be detected, it would be possible to detect the vertical positional deviation of the welding gun, and that by correcting the vertical positional deviation of the welding gun, the welding quality could be further stabilized. Furthermore, the inventors realized that changes in the electrode temperature could reduce the detection accuracy of the impact angle and pressure, and believed that by eliminating the effects of thermal strain caused by changes in the electrode temperature, the welding quality could be further stabilized.

The first object of the present invention is to provide a method for measuring the stroke and pressure in resistance spot welding that can detect the stroke and pressure with high accuracy without being affected by thermal strain when the electrode temperature changes.

The second object of the invention is to provide a method for manufacturing resistance spot welded joints that can stabilize the welding quality.

Furthermore, the third object of the invention is to provide a resistance spot welding device that can stabilize welding quality.

A method for measuring a stroke force in resistance spot welding according to a first aspect of the present invention includes: sandwiching a welding target having a plurality of overlapping metal materials between a first electrode and a second electrode arranged opposite to each other; measuring a plurality of strain measurement values with a plurality of strain gauges attached to a plurality of positions spaced apart in a circumferential direction on a surface of the first electrode; measuring one or more surface temperatures of the first electrode with a temperature measurement means; calculating a plurality of strain correction values _εe by subtracting a thermal strain _εT calculated based on the one or more surface temperatures T from the plurality of strain measurement values _ε ; and calculating a direction θ and a magnitude φ of a stroke angle, which is an inclination of an axis of the first electrode with respect to a normal to a contact surface of the welding target with the first electrode, and/or a stroke force between the first electrode and the welding target, based on the plurality of strain correction values εe.

The method for manufacturing a resistance spot welded joint according to the second aspect of the present invention includes calculating the direction θ and magnitude φ of the stroke and/or the applied force by the stroke force measurement method for resistance spot welding according to the first aspect of the present invention, and then passing current through the first electrode and the second electrode to spot weld the welded object, thereby obtaining a resistance spot welded joint from the welded object.

a control unit that performs control to apply current to the first electrode and the second electrode to spot weld the objects to be welded, the control unit being configured to: calculate a plurality of strain correction values ε e by subtracting thermal strain ε T from the plurality of strain measurement values ε based on a plurality of strain measurement values ε measured by the plurality of strain gauges and a surface temperature _T measured by the temperature measurement means, with a welding object having a plurality of overlapping metal materials sandwiched between the first electrode and the second electrode; calculate a direction θ and a magnitude _φ of an impact _angle , which is an inclination of an axis of the first electrode with respect to a normal to a contact surface of the welding object with the first electrode, and/or a pressure between the first electrode and the welding object, based on the plurality of strain correction values ε e; and

According to the method for measuring the tap angle and pressure in resistance spot welding according to the first aspect of the present invention, when the electrode temperature changes, the tap angle and pressure can be detected with high accuracy without being affected by thermal strain.

The manufacturing method for resistance spot welded joints according to the second aspect of the present invention makes it possible to stabilize the welding quality.

The resistance spot welding device according to the third aspect of the present invention can stabilize the welding quality.

1 is a diagram showing a resistance spot welding device according to a first embodiment. 2 is a diagram for explaining the direction θ and the magnitude φ of the impact angle of the upper electrode in the resistance spot welding device of FIG. 1 . FIG. 2 is a diagram showing a time chart of the welding pressure of the upper electrode and the lower electrode and the current supplied to the upper electrode and the lower electrode in the resistance spot welding device of FIG. 1 . FIG. 4 is a graph showing strain measurement values ε measured by four strain gauges in the pressing step of FIG. 3 for each impact angle φ. 5 is a graph showing the deviation strain ε' calculated based on the graph of FIG. 4 for each magnitude φ of the impact angle. 6 is a graph showing a deviation strain change amount Δε′ calculated based on the graph of FIG. 5 for each magnitude φ of the impact angle. As an example, this is a graph showing the deviation strain change amount Δε′ for each hit angle magnitude φ when the hit angle direction θ=120°. FIG. 11 is a plan view for explaining a schematic diagram of a resultant deviation strain change amount Δε _N ′. 10 is a graph showing the relationship between a resultant deviation strain change amount Δε _N ′ and a strike angle magnitude φ for each of a plurality of conditions. 11A and 11B are diagrams illustrating an example of deformation of a hitting angle direction θ. 2 is a flowchart showing a process flow in a control unit in FIG. 1 . 13 is a graph showing measurement results of the direction θ of the hitting angle in an experimental example. 13 is a graph showing the measurement results of the magnitude φ of the hit angle in an experimental example. FIG. 2 is a diagram showing a first modified example of the resistance spot welding device according to the first embodiment. 15 is a diagram for explaining the direction θ and the magnitude φ of the impact angle of the lower electrode in the resistance spot welding device of FIG. 14 . FIG. FIG. 13 is a diagram showing a second modified example of the resistance spot welding device according to the first embodiment. 17A and 17B are diagrams for explaining the direction θ1 and magnitude φ1 of the impact angle of the upper electrode and the direction θ2 and magnitude φ2 of the impact angle of the lower electrode in the resistance spot welding device of FIG. 16 . 11 is a graph showing an example of the relationship between the pressures applied by the upper and lower electrodes and the average value ε _m of strain measurements of four strain gauges attached to each of the upper and lower electrodes. FIG. 13 is a diagram showing an example of the relationship between the strike angle direction θ and the average value ε _m of the strain measurement values when the number of strain gauges is one, two, and four. FIG. 13 is a diagram showing an example of the relationship between the pressure applied by the upper electrode and the lower electrode and the average value ε _m of the strain measurement values measured under a condition without an impact angle. 10 is a graph showing an example of the relationship between the actual welding pressure of each of the upper electrode and the lower electrode and the position in the height direction of the welding gun. FIG. 11 is a diagram showing an example of detection items and calculation methods for impact angle and pressure force when the number of strain gauges is two, three, and four. FIG. 13 is a diagram showing an example of data obtained when a deviation strain change amount Δε′ is calculated based on strain measurement values ε of four strain gauges arranged at equal intervals in the circumferential direction on an upper electrode. 10 is a flowchart showing an example of the flow of a method for manufacturing a resistance spot welded joint according to a second embodiment. 1A and 1B are diagrams illustrating an example of a welding gun before and after impact angle correction is performed; 1 is a diagram showing a resistance spot welding device according to an embodiment of the present invention; 3 is a flowchart showing an example of a flow of a method for manufacturing a resistance spot welded joint according to an embodiment of the present invention. 4 is a time chart showing the timing of strain measurement and temperature measurement in an embodiment of the present invention.

First, we will explain the first embodiment that is the premise of the present invention.

[Description of Resistance Spot Welding Apparatus 10]
Fig. 1 is a diagram showing a resistance spot welding apparatus 10 according to a first embodiment. In Fig. 1, (A) is a side view of the resistance spot welding apparatus 10 including a block diagram, and (B) is a cross-sectional view taken along line F1-F1. As shown in Fig. 1, the resistance spot welding apparatus 10 includes a welding gun 12, a plurality of strain gauges 14, a power source 16, an actuator 18, a robot 20, and a control unit 22. This resistance spot welding apparatus 10 spot welds a welding object 32 having a plurality of overlapping steel plates 30.

The number of the multiple steel plates 30 constituting the welding object 32 is not limited to two, and may be, for example, three. The welding object 32 is not limited to multiple steel plates 30, and may have multiple metal materials. The material of the metal materials is not limited to steel, and may be metals other than steel. Furthermore, the shape of the metal materials is not limited to plate-like, and may be other than plate-like. In the first embodiment, as an example, a case where the number of the multiple steel plates 30 is two will be described. The multiple steel plates 30 are an example of "multiple metal materials".

The welding gun 12 has an upper electrode 40 and a lower electrode 42 arranged opposite each other. The upper electrode 40 and the lower electrode 42 each have a shank 44 and an electrode tip 46. The electrode tip 46 is attached to the tip of the shank 44. The shape of the welding gun 12 is not particularly limited, but it is possible to use, for example, a C-gun that is C-shaped when viewed from the side or an X-gun that is X-shaped when viewed from the side.

The number of the multiple strain gauges 14 may be any number greater than or equal to three, but in the first embodiment, as an example, a case where four strain gauges 14 are used will be described. As an example, the four strain gauges 14 are attached to the upper electrode 40 of the upper electrode 40 and lower electrode 42 at equal intervals in the circumferential direction. In other words, the four strain gauges 14 are attached to the upper electrode 40 at positions spaced apart at 90° intervals in the circumferential direction. The intervals between the four strain gauges 14 are preferably within 90°±5°, and more preferably within 90°±3°.

In addition, the mounting positions of the four strain gauges 14 are, as an example, positions on the electrode tip 46 side of the shank 44. The upper electrode 40 to which the four strain gauges 14 are attached is an example of a "first electrode," and the lower electrode 42 is an example of a "second electrode."

The four strain gauges 14 have the same configuration. For these four strain gauges 14, for example, non-electrical configurations such as optical type, or electrical configurations such as resistive type can be used. In the first embodiment, as an example, the four strain gauges 14 are electrically configured. The four strain gauges 14 are each attached to the upper electrode 40 with the measurement direction (expansion direction) of the strain gauge 14 aligned with the axial direction of the upper electrode 40.

The power supply 16 is electrically connected to each of the electrode tips 46 of the upper electrode 40 and the lower electrode 42. When the upper electrode 40 and the lower electrode 42 sandwich the welding object 32 and apply a pressure to the upper electrode 40 and the lower electrode 42, and a voltage is applied between the electrode tips 46 of the upper electrode 40 and the lower electrode 42 by the power supply 16, a current flows between the electrode tips 46. When a current flows between the electrode tips 46, a nugget (molten part) is formed at the overlapping part of the welding object 32 by Joule heat due to contact resistance, and this nugget cools and solidifies, thereby welding and joining the overlapping part.

The actuator 18 is, for example, an electric linear actuator, a hydraulic linear actuator, or a pneumatic linear actuator. This actuator 18 is mechanically connected to one or both of the upper electrode 40 and the lower electrode 42, and operates to move one or both of the upper electrode 40 and the lower electrode 42 in a direction toward or away from each other. When the actuator 18 presses one or both of the upper electrode 40 and the lower electrode 42 toward each other while the upper electrode 40 and the lower electrode 42 are sandwiching the welding object 32, a pressure force is applied to the upper electrode 40 and the lower electrode 42.

The robot 20 is, for example, a six-axis vertical articulated robot. The robot 20 moves the welding gun 12 in the horizontal and vertical directions, and operates to rotate the welding gun 12 around an arbitrary rotation axis. By operating the robot 20 in this manner, even if the welding object 32 is tilted in an arbitrary direction, the welding gun 12 can follow the tilt of the welding object 32, and the welding object 32 can be sandwiched between the upper electrode 40 and the lower electrode 42.

The control unit 22 controls the power supply 16, the actuator 18, and the robot 20, and is electrically connected to the power supply 16, the actuator 18, and the robot 20. The control unit 22 is realized, for example, by a computer having a processor 50 and a memory 52. The processor 50 is, for example, configured by a CPU (Central Processing Unit), and the memory 52 is, for example, configured by a ROM (Read Only Memory), a RAM (Random Access Memory), and storage.

The memory 52 has a non-volatile storage unit, and this non-volatile storage unit stores a program 54. The processor 50 executes this program 54 to perform various functions described below. These functions include the function of executing steps S1 to S8 (see FIG. 11) in the manufacturing method of a resistance spot welded joint described below.

[Explanation of the direction and size of the hitting angle]
Fig. 2 is a diagram illustrating the direction θ and magnitude φ of the impact angle of the upper electrode 40 in the resistance spot welding apparatus 10 of Fig. 1. In Fig. 2, (A) is a side view of the welding gun 12 illustrating the magnitude φ of the impact angle of the upper electrode 40, and (B) is a plan view diagrammatically illustrating the direction θ of the impact angle of the upper electrode 40.

In an actual welding assembly process, the object to be welded 32 may be tilted in any direction depending on the shape of the product. If the welding gun 12 can perfectly follow the tilt of the object to be welded 32, the normal to the contact surface between the upper electrode 40 and the lower electrode 42 on the object to be welded 32 can be aligned with the axis B of the upper electrode 40 and the lower electrode 42.

However, in reality, for example, an error in the inclination angle of the welding object 32 or an error in the operation of the robot 20 may cause the axes of the upper electrode 40 and the lower electrode 42 to be inclined relative to the normal of the welding object 32. In addition, for example, if a gap occurs between the multiple steel plates 30 that constitute the welding object 32 or if a processing error occurs in the multiple steel plates 30, the inclination angle of the axis of the upper electrode 40 and the inclination angle of the axis B of the lower electrode 42 may differ. Although the inclination angles of the axis of the upper electrode 40 and the axis of the lower electrode 42 may differ, in the first embodiment, as an example, a case will be described in which the inclination angle of the axis of the upper electrode 40 and the inclination angle of the axis of the lower electrode 42 are the same.

In the first embodiment, the upper electrode 40 is the target, and the inclination of the axis B of the upper electrode 40 with respect to the normal A of the contact surface 32A with the upper electrode 40 on the welding target 32 is called the strike angle. Furthermore, if the cross section of the upper electrode 40 when cut by a plane perpendicular to the axis B of the upper electrode 40 is taken as cross section 40A as shown in FIG. 2(B), a line that is set in advance on this cross section 40A and extends from the center of this cross section 40A to the radially outer side of the upper electrode 40 is defined as a reference line C. The four strain gauges 14 are respectively arranged at a position on the reference line C, a position 90° away from the reference line C in the circumferential direction of the upper electrode 40, a position 180° away from the reference line C in the circumferential direction of the upper electrode 40, and a position 270° away from the reference line C in the circumferential direction of the upper electrode 40.

The angle that the perpendicular line D, which is dropped from the normal line A toward the axis line B, and the reference line C make on the cross section 40A of the upper electrode 40 is defined as the direction of the strike angle θ [°] (0°≦θ<360°). Furthermore, the angle that the normal line A of the contact surface 32A of the welding object 32 makes on the same plane with the axis line B of the upper electrode 40 is defined as the magnitude of the strike angle φ [°] (0°≦φ<90°).

[About the idea behind the first form]
The inventors thought that if it were possible to detect the direction θ and magnitude φ of the impact angle, it would be possible to determine whether or not welding is possible for each spot welding point and to change the welding conditions, thereby stabilizing the welding quality.

First, four strain gauges 14 were attached at 90° intervals to four locations around the circumference of the upper electrode 40, of the upper electrode 40 and lower electrode 42, which were arranged opposite each other. The object to be welded 32 was then sandwiched between the upper electrode 40 and the lower electrode 42, and the strain of the upper electrode 40 was measured with the four strain gauges 14 when pressure was applied to the object to be welded 32 by the upper electrode 40 and the lower electrode 42. As a result, it was found that the device has the characteristic of being able to obtain strain measurement values that correspond to the direction θ and magnitude φ of the strike angle. The idea was then conceived of utilizing this characteristic to detect the direction θ and magnitude φ of the strike angle.

[Key points and mechanisms of the first form]
The inventors have found that the direction θ and magnitude φ of the hit angle can be quantitatively calculated in the following manner when four strain gauges 14 are used. Below, the details of the investigation that led to the quantitative calculation of the direction θ and magnitude φ of the hit angle when four strain gauges 14 are used, as well as the key points and mechanisms for solving the problem will be specifically explained.

Figure 3 is a diagram showing a time chart of the pressure applied to the upper electrode 40 and the lower electrode 42 in the resistance spot welding device 10 of Figure 1 and the current supplied to the upper electrode 40 and the lower electrode 42. Note that in the first embodiment, the pressure mainly means the set pressure set in a conventional resistance spot welding device.

Resistance spot welding is generally performed according to a time chart as shown in FIG. 3. That is, with the upper electrode 40 and the lower electrode 42 sandwiching the welding object 32, pressure is applied to the upper electrode 40 and the lower electrode 42, and then current supply to the upper electrode 40 and the lower electrode 42 is started. After the current supply to the upper electrode 40 and the lower electrode 42 is stopped, pressure is applied to the upper electrode 40 and the lower electrode 42. The pressure application process in FIG. 3 corresponds to the period from when pressure is applied to the upper electrode 40 and the lower electrode 42 begins to when current supply to the upper electrode 40 and the lower electrode 42 begins, and the constant pressure application period in FIG. 3 corresponds to the period during which the pressure applied to the upper electrode 40 and the lower electrode 42 increases after pressure is applied to the upper electrode 40 and the lower electrode 42, and this pressure is maintained constant.

The inventors measured the strain of the upper electrode 40 using four strain gauges 14 during the constant pressurization period shown in Fig. 3. Fig. 4 is a graph showing the strain measurements ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε ₂₇₀ measured by the four strain gauges 14 during the pressurization process shown in Fig. 3 for each strike angle φ (φ = 0°, 3°, 6°, and 9°). As an example, this graph is a graph for a strike angle direction θ = 90°.

The vertical axis of FIG. 4 indicates the strain measurement value, and the horizontal axis of FIG. 4 indicates the mounting positions [°] of the four strain gauges 14. In FIG. 4, graph G1 shows the case where the impact angle is φ=0°, graph G2 shows the case where the impact angle is φ=3°, graph G3 shows the case where the impact angle is φ=6°, and graph G4 shows the case where the impact angle is φ=9°.

The strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε ₂₇₀ are values measured by the four strain gauges 14 attached to the upper electrode 40 at 90° intervals in the circumferential direction. That is, the strain measurement value ε ₀ is a value measured by the strain gauge 14 located on the reference line C, the strain measurement value ε ₉₀ is a value measured by the strain gauge 14 located at a position 90° away from the reference line C in the circumferential direction of the upper electrode 40, the strain measurement value ε ₁₈₀ is a value measured by the strain gauge 14 located at a position 180° away from the reference line C in the circumferential direction of the upper electrode 40, and the strain measurement value ε ₂₇₀ is a value measured by the strain gauge 14 located at a position 270° away from the reference line C in the circumferential direction of the upper electrode 40.

As shown in Fig. 4, when there is a hit angle, the inventors considered that the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε ₂₇₀ would show different values. Therefore, in order to extract the amount of change in the strain measurement value due to the hit angle, the inventors calculated the average value ε _m of the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε ₂₇₀ according to formula (1), and calculated the deviation strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', and ε ₂₇₀ ' by subtracting the average value ε _m from the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε ₂₇₀ according to formulas (2) to (5).

ε _m = (ε ₀ + ε ₉₀ + ε ₁₈₀ + ε ₂₇₀ )/4...(1)
ε ₀ '= ε ₀ - ε _m ...(2)
ε ₉₀ '= ε ₉₀ - ε _m ...(3)
ε ₁₈₀ ' = ε ₁₈₀ - ε _m ...(4)
ε ₂₇₀ '= ε ₂₇₀ - ε _m ...(5)

Fig. 5 is a graph showing the deviatoric strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', and ε ₂₇₀ ' calculated based on the graph in Fig. 4 for each hit angle φ (φ = 0°, 3°, 6°, 9°). As an example, this graph is a graph for a hit angle direction θ = 90°.

The vertical axis of FIG. 5 indicates the deviation strain, and the horizontal axis of FIG. 5 indicates the mounting positions [°] of the four strain gauges 14. In FIG. 5, graph G1 indicates the case where the impact angle is φ=0°, graph G2 indicates the case where the impact angle is φ=3°, graph G3 indicates the case where the impact angle is φ=6°, and graph G4 indicates the case where the impact angle is φ=9°.

Here, the inventors considered that the deviatoric strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', and ε ₂₇₀ ' include, in addition to the deviatoric component caused by the strike angle, a deviatoric component caused by non-uniformity in the circumferential direction of the rigidity of the upper electrode 40. The inventors also noticed that when there is no strike angle, that is, when the magnitude of the strike angle φ = 0, the deviatoric strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', and ε ₂₇₀ ' do not become 0, as shown in Fig. 5 .

The inventors then considered that in order to extract only the deviation components caused by the impact angle, it would be preferable to remove the influence of the rigidity of the upper electrode 40 from the deviation strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', and ε ₂₇₀ '.

Therefore, the inventors defined the deviatoric strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', and ε ₂₇₀ ' when there is no strike angle as reference deviatoric strains ε _0,φ=0 ', ε _90,φ=0 ', ε _180,φ=0 ', and ε _270,φ=0 '.

The inventors then calculated the deviation strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', and Δε ₂₇₀ ' by subtracting the reference deviation strains ε _0,φ=0 ', ε _90,φ=0 ', ε _180,φ=0 ', and ε _270,φ=0 ' from the deviation strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', and ε ₂₇₀ ', respectively, according to equations (6) to (9).

The deviation strain change amount Δε ₀ ' is the deviation strain change amount detected at a position on the reference line C, the deviation strain change amount Δε ₉₀ ' is the deviation strain change amount detected at a position 90° away from the reference line C in the circumferential direction of the upper electrode 40, the deviation strain change amount Δε ₁₈₀ ' is the deviation strain change amount detected at a position 180° away from the reference line C in the circumferential direction of the upper electrode 40, and the deviation strain change amount Δε ₂₇₀ ' is the deviation strain change amount detected at a position 270° away from the reference line C in the circumferential direction of the upper electrode 40.

Δε₀'＝ε₀'-ε_{0, φ=0}' ...(6)
Δε₉₀'＝ε₉₀'-ε_{90, φ=0}' ...(7)
Δε₁₈₀'＝ε₁₈₀'-ε_{180, φ=0}' ...(8)
Δε₂₇₀'＝ε₂₇₀'-ε_{270, φ=0}' ... (9)

Fig. 6 is a graph showing the deviation strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', and Δε ₂₇₀ ' calculated based on the graph of Fig. 5 for each hit angle φ (φ = 0°, 3°, 6°, 9°). As an example, this graph is a graph for a hit angle direction θ = 90°.

The vertical axis of FIG. 6 indicates the amount of change in deviation strain, and the horizontal axis of FIG. 6 indicates the mounting positions [°] of the four strain gauges 14. In FIG. 6, graph G1 indicates the case where the impact angle is φ=0°, graph G2 indicates the case where the impact angle is φ=3°, graph G3 indicates the case where the impact angle is φ=6°, and graph G4 indicates the case where the impact angle is φ=9°.

As shown in FIG. 6, when there is no hit angle, that is, when the hit angle is φ=0°, the deviation strain changes Δε ₀ ′, Δε ₉₀ ′, Δε ₁₈₀ ′, and Δε ₂₇₀ ′ are all zero.

On the other hand, when there is a hit angle, among the deviation strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', and Δε ₂₇₀ ', the deviation strain change amount in the direction of the hit angle was a negative value, the deviation strain change amount in the direction opposite to the hit angle was a positive value, and the deviation strain change amount in the direction perpendicular to the hit angle was approximately zero.

In the example shown in Figure 6, for example, when the direction of the hit angle θ = 90° and the magnitude of the hit angle φ = 3°, 6°, or 9°, the deviation strain change amount Δε ₉₀ ' in the hit angle direction is a negative value, the deviation strain change amount Δε ₂₇₀ ' in the direction opposite to the hit angle direction is a positive value, and the deviation strain change amount Δε ₁₈₀ ' in the direction perpendicular to the hit angle direction is approximately 0. Note that when the direction of the hit angle is other than θ = 90°, a similar trend was found.

In this manner, the inventors have found that the hit angle direction θ can be detected by calculating the deviation strain changes Δε ₀ ′, Δε ₉₀ ′, Δε ₁₈₀ ′, and Δε ₂₇₀ ′.

6 also reveals that the absolute values of the deviation strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', and Δε ₂₇₀ ' increase with an increase in the hit angle magnitude φ. The inventors then discovered that the hit angle magnitude φ can be detected by calculating the deviation strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', and Δε ₂₇₀ '.

Here, the inventors considered that depending on the direction θ of the hitting angle, the direction θ of the hitting angle and the mounting position of the strain gauge 14 may not coincide. Therefore, as an example, the inventors considered the case where the direction θ of the hitting angle is 120°.

FIG. 7 is a graph showing the deviation strain changes Δε ₀ ′, Δε ₉₀ ′, Δε ₁₈₀ ′, and Δε ₂₇₀ ′ for each strike angle φ (φ=0°, 3°, 6°, 9°) when the strike angle direction θ=120°.

The vertical axis of FIG. 7 indicates the amount of change in deviation strain, and the horizontal axis of FIG. 7 indicates the mounting positions [°] of the four strain gauges 14. In FIG. 7, graph G1 shows the case where the impact angle is φ=0°, graph G2 shows the case where the impact angle is φ=3°, graph G3 shows the case where the impact angle is φ=6°, and graph G4 shows the case where the impact angle is φ=9°.

In this way, when the direction of the hit angle θ=120°, the strain gauges 14 located on both sides of the hit angle direction θ=120°, i.e., the strain gauges 14 at the 90° position and the strain gauges 14 at the 180° position, had negative deviation strain changes Δε ₉₀ ' and Δε ₁₈₀ '. Furthermore, as the magnitude φ of the hit angle increased, the absolute values of the deviation strain changes Δε ₉₀ ' and Δε ₁₈₀ ' increased.

From the results shown in Figure 7, the inventors concluded that even if the direction θ of the hit angle does not coincide with the mounting position of the strain gauge 14, it is possible to estimate the direction θ and magnitude φ of any hit angle by detecting the deviation strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', and Δε ₂₇₀ '.

The inventors have come up with a method for estimating the direction θ and magnitude φ of an arbitrary hit angle even when the direction θ of the hit angle does not match the mounting position of the strain gauge 14. Below, we will explain the method for estimating the direction θ and magnitude φ of an arbitrary hit angle when the direction θ of the hit angle does not match the mounting position of the strain gauge 14.

First, the composite deviatoric strain change amount Δε _N ′ is defined by equation (10).
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(10)

8 is a plan view illustrating a composite deviation strain change amount Δε _N '. The first deviation strain change amount Δε1' is the deviation strain change amount whose value is the largest on the negative side among the deviation strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', and Δε ₂₇₀ '. The second deviation strain change amount Δε2' is the second deviation strain change amount whose value is the largest on the negative side among the deviation strain changes detected at two positions separated by ±90° in the circumferential direction of the upper electrode 40 from the position where the first deviation strain change amount Δε1' is detected.

8 shows an example in which the strike angle direction θ=120°. In the example shown in FIG. 8, the first deviation strain change amount Δε1' is the deviation strain change amount _Δε90 ' that has the largest negative value among the deviation strain changes Δε0', _Δε90 ', _Δε180 ', and _Δε270 '. The second deviation strain change amount Δε2' is the deviation strain change amount _Δε180 ' that has the largest negative value among the deviation strain changes _Δε0 ' and _Δε180 ' detected at two positions (the 0° position and the 180° position) that are separated by ±90° in the circumferential direction of the upper electrode ₄₀ from the position where the deviation strain change amount _Δε90 ' was detected.

In this method, the direction θ of the hit angle is calculated from the direction of the resultant deviation strain change amount Δε _N ′ when it is regarded as a vector, and the magnitude φ of the hit angle is calculated from the magnitude of the resultant deviation strain change amount Δε _N ′.

Fig. 9 is a graph showing the relationship between the amount of change in composite deviation strain ε _N ' and the magnitude of the impact angle φ for each of a number of conditions. The vertical axis of Fig. 9 shows the amount of change in composite deviation strain ε _N ', and the horizontal axis of Fig. 9 shows the magnitude of the impact angle φ. In Fig. 9, graph G1 shows the case under the reference condition, graph G2 shows the case where the pressure forces of the upper electrode 40 and the lower electrode 42 are increased relative to the reference condition, and graph G3 shows the case where the rigidity of the upper electrode 40 and the lower electrode 42 is increased relative to the reference condition.

As shown in Fig. 9, a linear relationship is established between the amount of change in composite deviatoric strain ε _N ' and the magnitude of the impact angle φ. For this reason, it is considered possible to quantitatively predict the magnitude of the impact angle φ by experimentally determining the proportionality coefficient α from the amount of change in composite deviatoric strain ε _N ' and the magnitude of the impact angle φ. Note that since the proportionality coefficient α is mainly affected by the pressure forces of the upper electrode 40 and the lower electrode 42 and the rigidity of the upper electrode 40 and the lower electrode 42, it is necessary to experimentally determine the proportionality coefficient α under the conditions of the welding assembly process before the start of the actual welding assembly process.

In this method, the direction θ and magnitude φ of the hit angle are calculated as follows: Figure 10 shows an example of deformation of the hit angle direction θ.

10A, for example, when the direction θ of the hit angle is 0°<θ<45°, Δε1'= _Δε0 '(<0) and Δε2'= _Δε90 '(<0). In this way, when Δε1'= _Δε0 '(<0) and Δε2'= _Δε90 '(<0), the direction θ and magnitude φ of the hit angle are calculated using the following equations (A-1) to (A-3).
θ=arctan(|Δε2'|/|Δε1'|)×180/π...(A-1)
φ=α×Δε _N '...(A-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(A-3)

10B, for example, when the direction θ of the hit angle is 45°<θ<90°, Δε1'= _Δε90 '(<0) and Δε2'= _Δε0 '(<0). In this way, when Δε1'= _Δε90 '(<0) and Δε2'= _Δε0 '(<0), the direction θ and magnitude φ of the hit angle are calculated using the following equations (B-1) to (B-3).
θ=arctan(|Δε1'|/|Δε2'|)×180/π...(B-1)
φ=α×Δε _N '...(B-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(B-3)

10C, for example, when the direction θ of the hit angle is 90°<θ<135°, Δε1'= _Δε90 '(<0) and Δε2'= _Δε180 '(<0). In this way, when Δε1'= _Δε90 '(<0) and Δε2'= _Δε180 '(<0), the direction θ and magnitude φ of the hit angle are calculated using the following formulas (C-1) to (C-3).
θ={π/2+arctan(|Δε2'|/|Δε1'|)}×180/π...(C-1)
φ=α×Δε _N '...(C-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(C-3)

10(D), for example, when the direction θ of the hit angle is 135°<θ<180°, Δε1'= _Δε180 '(<0) and εΔ2'= _Δε90 '(<0). In this way, when Δε1'= _Δε180 '(<0) and εΔ2'= _Δε90 '(<0), the direction θ and magnitude φ of the hit angle are calculated using the following equations (D-1) to (D-3).
θ={π/2+arctan(|Δε1'|/|Δε2'|)}×180/π...(D-1)
φ=α×Δε _N '...(D-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(D-3)

10(E), for example, when the direction θ of the hit angle is 180°<θ<225°, Δε1'= _Δε180 '(<0) and Δε2'= _Δε270 '(<0). In this way, when Δε1'= _Δε180 '(<0) and Δε2'= _Δε270 '(<0), the direction θ and magnitude φ of the hit angle are calculated using the following equations (E-1) to (E-3).
θ={π+arctan(|Δε2|'/|Δε1'|)}×180/π...(E-1)
φ=α×Δε _N '...(E-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(E-3)

10(F), for example, when the direction θ of the hit angle is 225°<θ<270°, Δε1'= _Δε270 '(<0) and Δε2'= _Δε180 '(<0). In this way, when Δε1'= _Δε270 '(<0) and Δε2'= _Δε180 '(<0), the direction θ and magnitude φ of the hit angle are calculated using the following equations (F-1) to (F-3).
θ={π+arctan(|Δε1|'/|Δε2'|)}×180/π...(F-1)
φ=α×Δε _N '...(F-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(F-3)

10(G), for example, when the direction θ of the hit angle is 270°<θ<315°, Δε1'= _Δε270 '(<0) and Δε2'= _Δε0 '(<0). In this way, when Δε1'= _Δε270 '(<0) and Δε2'= _Δε0 '(<0), the direction θ and magnitude φ of the hit angle are calculated using the following equations (G-1) to (G-3).
θ={3π/2+arctan(|Δε2'|/|Δε1'|)×180/π...(G-1)
φ=α×Δε _N '...(G-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(G-3)

10(H), for example, when the direction θ of the hit angle is 315°<θ<360°, Δε1'= _Δε0 '(<0) and Δε2'= _Δε270 '(<0). In this way, when Δε1'= _Δε0 '(<0) and Δε2'= _Δε270 '(<0), the direction θ and magnitude φ of the hit angle are calculated using the following equations (H-1) to (H-3).
θ={3π/2+arctan(|Δε1'|/|Δε2'|)×180/π...(H-1)
φ=α×Δε _N '...(H-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(H-3)

Depending on the direction θ of the strike angle, there may be two deviation strain changes whose values are the largest on the negative side. In this case, one of the two deviation strain changes can be selected as the first deviation strain change Δε1'.

For example, when the direction of the hit angle θ=45°, the deviation strain change amount having the largest negative value may be the deviation strain change amount Δε ₀ ' and the deviation strain change amount Δε ₉₀ '. In this case, one of the deviation strain change amount Δε ₀ ' and the deviation strain change amount Δε ₉₀ ' may be selected as the first deviation strain change amount Δε 1 '.

Furthermore, when the direction of the hit angle θ=135°, the deviation strain change amount having the largest negative value may be the deviation strain change amount Δε ₉₀ ' and the deviation strain change amount Δε ₁₈₀ '. In this case, one of the deviation strain change amount Δε ₉₀ ' and the deviation strain change amount Δε ₁₈₀ ' may be selected and set as the first deviation strain change amount Δε 1 '.

Furthermore, when the direction of the hit angle θ=225°, the deviation strain change amount having the largest negative value may be the deviation strain change amount Δε ₁₈₀ ′ and the deviation strain change amount Δε ₂₇₀ ′. In this case, one of the deviation strain change amount Δε ₁₈₀ ′ and the deviation strain change amount Δε ₂₇₀ ′ may be selected and set as the first deviation strain change amount Δε 1 ′.

Furthermore, when the direction of the hit angle θ=315°, the deviation strain change amount having the largest negative value may be the deviation strain change amount Δε ₂₇₀ ′ and the deviation strain change amount Δε ₀ ′. In this case, one of the deviation strain change amount Δε ₂₇₀ ′ and the deviation strain change amount Δε ₀ ′ may be selected and set as the first deviation strain change amount Δε 1 ′.

Similarly, depending on the direction θ of the strike angle, there may be two deviation strain changes that are candidates for the second deviation strain change amount Δε2'. In this case, one of the two deviation strain changes can be selected as the second deviation strain change amount Δε2'.

For example, if the deviation strain change amounts Δε ₀ ′ and Δε ₉₀ ′ are the deviation strain change amounts that are candidates for the second deviation strain change amount Δε2′, one of the deviation strain change amount Δε ₀ ′ and the deviation strain change amount Δε ₉₀ ′ may be selected as the second deviation strain change amount Δε2′.

Furthermore, when the deviation strain change amounts that are candidates for the second deviation strain change amount Δε2' are the deviation strain change amount _Δε90 ' and the deviation strain change amount _Δε180 ', one of the deviation strain change amount _Δε90 ' and the deviation strain change amount _Δε180 ' may be selected as the second deviation strain change amount Δε2'.

Furthermore, when the deviation strain change amounts Δε ₁₈₀ ′ and Δε ₂₇₀ ′ are the candidates for the second deviation strain change amount Δε2′, one of the deviation strain change amount Δε ₁₈₀ ′ and the deviation strain change amount Δε ₂₇₀ ′ may be selected as the second deviation strain change amount Δε2′.

Furthermore, when the deviation strain change amounts that are candidates for the second deviation strain change amount Δε2' are the deviation strain change amount _Δε270 ' and the deviation strain change amount _Δε0 ', one of the deviation strain change amount _Δε270 ' and the deviation strain change amount _Δε0 ' is selected to be the second deviation strain change amount Δε2'.

In particular, when either or both of Δε1' and Δε2' are 0, the following formula can be used.

For example, when the direction θ of the hit angle is 0°, Δε1' = _Δε0 '(<0) and Δε2' = 0. In this way, when Δε1' = _Δε0 '(<0) and Δε2' = 0, the direction θ and magnitude φ of the hit angle are calculated using the following equations (I-1) to (I-3).
θ=0°...(I-1)
φ=α×Δε _N '...(I-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(I-3)

Furthermore, for example, when the direction θ of the hit angle is 90°, Δε1' = _Δε90 '(<0) and Δε2' = 0. In this way, when Δε1' = _Δε90 '(<0) and Δε2' = 0, the direction θ and magnitude φ of the hit angle are calculated using the following equations (J-1) to (J-3).
θ=90°...(J-1)
φ=α×Δε _N '...(J-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(J-3)

Furthermore, for example, when the direction θ of the hit angle is 180°, Δε1' = _Δε180 '(<0) and Δε2' = 0. In this way, when Δε1' = _Δε180 '(<0) and Δε2' = 0, the direction θ and magnitude φ of the hit angle are calculated using the following equations (K-1) to (K-3).
θ=180°...(K-1)
φ=α×Δε _N '...(K-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(K-3)

Furthermore, for example, when the direction θ of the hit angle is 270°, Δε1' = _Δε270 '(<0) and Δε2' = 0. In this way, when Δε1' = _Δε270 '(<0) and Δε2' = 0, the direction θ and magnitude φ of the hit angle are calculated using the following equations (L-1) to (L-3).
θ=270°...(L-1)
φ=α×Δε _N '...(L-2)
Δε _N '=√{(Δε1') ² + (Δε2') ² }...(L-3)

Also, for example, when the magnitude φ of the hit angle is 0°, Δε1' = 0 and Δε2' = 0. In this way, when Δε1' = 0 and Δε2' = 0, the magnitude φ of the hit angle is calculated by (M-1). The direction θ of the hit angle cannot be defined, so it is not calculated.
φ=0...(M-1)

By using the above method, the direction θ and magnitude φ of the impact angle can be quantitatively determined.

[Method of manufacturing resistance spot welded joint]
Next, a case where the method for manufacturing a resistance spot welded joint according to the first embodiment is applied to an actual welding and assembly process will be described.

The method for manufacturing a resistance spot welded joint according to the first embodiment is executed by the resistance spot welding apparatus 10 shown in Fig. 1. The program 54 of the control unit 22 includes a number of processes for calculating the direction θ and magnitude φ of the impact angle of the upper electrode 40 from the strain measurements ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ measured by the four strain gauges 14 described above.

Before the actual welding and assembly process is started, the control unit 22 stores in the memory 52 a proportionality coefficient α calculated in advance from the relationship between the composite deviation strain change amount Δε _N ' defined by the above formula (10) and the magnitude φ of the strike angle. This proportionality coefficient α is experimentally calculated under the conditions of the welding and assembly process before the actual welding and assembly process is started. In addition, the control unit 22 stores in the memory 52 the standard deviation strains ε _0,φ=0 ', ε _90,φ=0 ', ε _180,φ=0 ', and ε _270,φ=0 ' calculated under conditions when there is no strike angle.

Figure 11 is a flowchart showing the flow of processing in the control unit 22 of Figure 1. As shown in Figure 11, the control unit 22 executes steps S1 to S8. Of these steps S1 to S8, steps S1 to S5 are processes for calculating the direction θ and magnitude φ of the impact angle of the upper electrode 40, and correspond to the impact angle measurement method in resistance spot welding according to the first embodiment.

First, in step S1, the control unit 22 controls the robot 20 and the actuator 18, and the welding target 32 is sandwiched between the upper electrode 40 and the lower electrode 42. At this time, the pressures of the upper electrode 40 and the lower electrode 42 are adjusted so that the welding target 32 is pressurized by the upper electrode 40 and the lower electrode 42. Then, the strain of the upper electrode 40 when the welding target 32 is pressurized by the upper electrode 40 and the lower electrode 42 is measured by the four strain gauges 14. The strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ measured by the four strain gauges 14 are output to the control unit 22. The strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ are an example of "plurality of strain measurement values ε."

Next, in step S2, the control unit 22 acquires the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ output from the four strain gauges 14, and calculates the average value ε _m of the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ according to the above formula (1). The control unit 22 also calculates the deviation strains ε ₀ ', ε 90 ', ε ₁₈₀ ', ε ₂₇₀ ' by subtracting the average value ε _m from the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ according to the above formulas (2) to ( ₅ ). These deviation strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', ε ₂₇₀ ' are examples of "plurality of deviation strains ε'".

Subsequently, in step S3, the control unit 22 calculates the deviation strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', Δε ₂₇₀ ' by subtracting the reference deviation strains ε _0,φ=0 ', ε _{90,φ=0 ', ε 180,φ= 0 ', ε 270, φ=0} ' previously stored in the memory 52 from the deviation strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', ε ₂₇₀ ', respectively, according to equations (6) to (9). The standard deviational strains ε _0,φ=0 ', ε _90,φ=0 ', ε _180,φ=0 ', and ε _270,φ=0 ' are examples of "multiple standard deviational strains ε _φ=0 '", and the deviational strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', and Δε ₂₇₀ ' are examples of "multiple deviational strain changes Δε'".

Next, in step S4, the control unit 22 identifies the first deviational strain change amount Δε1' whose value is the largest on the negative side among the deviational strain change amounts _Δε0 ', _Δε90 ', _Δε180 ', and _Δε270 ', and also identifies the second deviational strain change amount Δε2' whose value is the largest on the negative side among the deviational strain change amounts detected at two positions located ±90° away in the circumferential direction of the upper electrode 40 from the position where the first deviational strain change amount Δε1' was detected.

Then, in step S5, the control unit 22 calculates the direction θ and magnitude φ of the impact angle of the upper electrode 40 based on the first deviation strain change amount Δε1' and the second deviation strain change amount Δε2'.

10A, for example, when the direction θ of the hit angle is 0°<θ<45°, the control unit 22 specifies that Δε1'= _Δε0 '(<0) and Δε2'= _Δε90 '(<0). Then, the control unit 22 calculates the direction θ and magnitude φ of the hit angle using the above formulas (A-1) to (A-3).

10(B), for example, when the direction θ of the hit angle is 45°<θ<90°, the control unit 22 specifies that Δε1'= _Δε90 '(<0) and Δε2'= _Δε0 '(<0). Then, the control unit 22 calculates the direction θ and magnitude φ of the hit angle using the above formulas (B-1) to (B-3).

10C, for example, when the direction θ of the hit angle is 90°<θ<135°, the control unit 22 specifies that Δε1'= _Δε90 '(<0) and Δε2'= _Δε180 '(<0). Then, the control unit 22 calculates the direction θ and magnitude φ of the hit angle using the above formulas (C-1) to (C-3).

10(D), for example, when the direction θ of the hit angle is 135°<θ<180°, the control unit 22 specifies that Δε1'= _Δε180 '(<0) and εΔ2'= _Δε90 '(<0). Then, the control unit 22 calculates the direction θ and magnitude φ of the hit angle using the above formulas (D-1) to (D-3).

10(E), for example, when the direction θ of the hit angle is 180°<θ<225°, the control unit 22 specifies that Δε1'= _Δε180 '(<0) and Δε2'= _Δε270 '(<0). Then, the control unit 22 calculates the direction θ and magnitude φ of the hit angle using the above formulas (E-1) to (E-3).

10(F), for example, when the direction θ of the hit angle is 225°<θ<270°, the control unit 22 specifies that Δε1'= _Δε270 '(<0) and Δε2'= _Δε180 '(<0). Then, the control unit 22 calculates the direction θ and magnitude φ of the hit angle using the above formulas (F-1) to (F-3).

10(G), for example, when the direction θ of the hit angle is 270°<θ<315°, the control unit 22 specifies that Δε1'= _Δε270 '(<0) and Δε2'= _Δε0 '(<0). Then, the control unit 22 calculates the direction θ and magnitude φ of the hit angle using the above formulas (G-1) to (G-3).

10(H), for example, when the direction θ of the hit angle is 315°<θ<360°, the control unit 22 specifies that Δε1'= _Δε0 '(<0) and Δε2'= _Δε270 '(<0). Then, the control unit 22 calculates the direction θ and magnitude φ of the hit angle using the above formulas (H-1) to (H-3).

As mentioned above, depending on the direction θ of the strike angle, there may be two deviational strain changes whose values are the largest on the negative side. In this case, the control unit 22 selects one of the two deviational strain changes and sets it as the first deviational strain change Δε1'.

Similarly, depending on the direction θ of the strike angle, there may be two deviational strain changes that are candidates for the second deviational strain change amount Δε2'. In this case, the control unit 22 selects one of the two deviational strain changes as the second deviational strain change amount Δε2'.

In particular, when either or both of Δε1' and Δε2' are 0, the following formula is used for the calculation.

For example, when Δε1′=Δε ₀ ′ (<0) and Δε2′=0, the calculation is performed according to the above formulas (I-1) to (I-3).

Furthermore, for example, when Δε1′=Δε ₉₀ ′ (<0) and Δε2′=0, the calculation is performed using the above formulas (J-1) to (J-3).

Furthermore, for example, when Δε1′=Δε ₁₈₀ ′ (<0) and Δε2′=0, the calculation is performed using the above formulas (K-1) to (K-3).

Furthermore, for example, when Δε1′=Δε ₂₇₀ ′ (<0) and Δε2′=0, the calculation is performed using the above formulas (L-1) to (L-3).

Furthermore, when Δε1' = 0 and Δε2' = 0, the magnitude of the hit angle φ is calculated using the above formula (M-1). The direction θ of the hit angle cannot be defined, so it is not calculated.

In this way, the control unit 22 quantitatively determines the direction θ and magnitude φ of the impact angle of the upper electrode 40 through the above steps S1 to S5.

Next, in step S6, the control unit 22 determines whether the magnitude φ of the strike angle is equal to or greater than a predetermined value. The value is set to an arbitrary value. For example, the value is 1°.

If the control unit 22 determines that the magnitude φ of the hit angle is less than the specified value, the process proceeds to step S8, which will be described later. On the other hand, if the control unit 22 determines that the magnitude φ of the hit angle is equal to or greater than the specified value, the process proceeds to step S7. In step S7, the control unit 22 controls the robot 20 to change the positions of the upper electrode 40 and the lower electrode 42 so that the magnitude φ of the hit angle is corrected to less than the specified value. The control unit 22 then proceeds to step S8.

Next, in step S8, the control unit 22 controls the power source 16 to pass current through the upper electrode 40 and the lower electrode 42. When current is passed through the upper electrode 40 and the lower electrode 42 in this manner, the object to be welded 32 is resistance spot welded, thereby obtaining a resistance spot welded joint from the object to be welded 32.

When the multiple steel sheets 30 include one or more steel sheets whose overlapping surfaces are coated with zinc-based plating, the control unit 22 may do the following. That is, when the total sheet thickness of the welded parts of the multiple overlapped steel sheets 30 is t [mm], the control unit 22 may set the current application time to the upper electrode 40 and the lower electrode 42 to (5t + 2.8φ + 2)/50 [sec] or more. The use of this formula to calculate the current application time is described in JP 2018-39019 A.

Furthermore, in the case where at least one of the plurality of steel plates 30 is a surface-treated steel plate having a metal plating layer, the control unit 22 may perform the following operation. In other words, if the time for which the pressure is held after current is passed to the upper electrode 40 and the lower electrode 42 is terminated is H [msec], the thickness of the steel plate with the greatest thickness among the multiple steel plates 30 is t [mm], the tensile strength of the steel plate with the greatest tensile strength among the multiple steel plates 30 is T [MPa], and the respective pressure forces of the upper electrode 40 and the lower electrode 42 are F [N], the control unit 22 may control the time for which the ^pressure is held after current is passed to the upper electrode 40 and the lower electrode 42 and the respective pressure forces of the upper electrode 40 and the lower electrode 42 so as to satisfy 2·φ·(t·T/F) ^1/2 ≦H when 0≦φ<1, satisfy (3·φ-1)·(t·T/F) 1/2 ≦H when 1≦φ<10, and satisfy (φ+19)·(t·T/F) ^1/2 ≦H when 10≦φ<20. The calculation of the pressure holding time and pressure using these formulas is described in WO 2017/033455.

[Action and Effects]
Next, the operation and effect of the first embodiment will be described.

As described above in detail, according to the first embodiment, when calculating the direction θ and magnitude φ of the hit angle, the deviation strains ε _{0 '} , ε ₉₀ ', ε _{180 '} , ε _{270 '} are calculated by subtracting the average value ε _m of the strain measurements ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ from the strain measurements ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ , respectively. This makes it possible to extract the amount of change in the strain measurement due to the hit angle, and therefore the direction θ and magnitude φ of the hit angle can be detected with higher accuracy than, for example, when the strain measurements ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ are used as is.

Furthermore, according to the first form, when calculating the direction θ and magnitude φ of the hit angle, the deviation strain changes Δε ₀ ′, Δε ₉₀ ′, Δε ₁₈₀ ′, Δε ₂₇₀ ′ are calculated by subtracting the reference deviation strains ε ₀ ,φ=0 ′, ε ₉₀ , _φ=0 ′, ε _{180,φ=0 ′, ε 270,φ=0} ′ _, which are the deviation strains ε 0 ′, ε 90 ′, ε ₁₈₀ ′, ε ₂₇₀ ′ in the absence of a hit angle, from the deviation strains ε ₀ ′, ε ₉₀ ′, ε ₁₈₀ ′, ε ₂₇₀ ′, respectively. This makes it possible to remove the deviation components caused by the non-uniform rigidity of the upper electrode 40 in the circumferential direction and extract only the deviation components caused by the impact angle, thereby improving the detection accuracy of the direction θ and magnitude φ of the impact angle compared to, for example, using the deviation strains ε ₀ ', ε ₉₀ ', ε ₁₈₀ ', and ε ₂₇₀ ' as they are.

According to the first embodiment, the first deviational strain change amount Δε1' having the largest negative value among the deviational strain changes _Δε0 ', _Δε90 ', _Δε180 ', and _Δε270 ' is identified, and the second deviational strain change amount Δε2' having the largest negative value among the deviational strain changes detected at two positions separated by ±90° in the circumferential direction of the upper electrode 40 from the position where the first deviational strain change amount Δε1' is detected is identified, and the direction θ and magnitude φ of the hit angle are calculated based on the first deviational strain change amount Δε1' and the second deviational strain change amount Δε2'. Therefore, the direction θ and magnitude φ of the hit angle are calculated based on the first deviational strain change amount Δε1' and the second deviational strain change amount Δε2' which sensitively increase and decrease according to the direction θ and magnitude φ of the hit angle, thereby improving the detection accuracy of the direction θ and magnitude φ of the hit angle.

According to the first embodiment, the proportionality coefficient α is calculated in advance from the relationship between the composite deviation strain change amount Δε _N ', which is the composite value of the first deviation strain change amount Δε1' and the second deviation strain change amount Δε2', and the magnitude φ of the impact angle, and in measuring the actual impact angle, the magnitude φ of the impact angle is calculated by multiplying the composite deviation strain change amount Δε _N ' by the proportionality coefficient α, as in the above formula (A-2) etc. This makes it possible to detect the magnitude φ of the impact angle while eliminating the effects of the pressure forces of the upper electrode 40 and the lower electrode 42 and the rigidity of the upper electrode 40 and the lower electrode 42.

Furthermore, according to the first embodiment, the direction θ and magnitude φ of the hit angle are calculated using the above formulas (A-1) to (H-3) that are predetermined according to the identified first deviational strain change amount Δε1' and second deviational strain change amount Δε2'. Therefore, since the direction θ and magnitude φ of the hit angle are calculated geometrically, the detection accuracy of the direction θ and magnitude φ of the hit angle can be improved.

According to the first embodiment, as an example, the direction θ and magnitude φ of the impact angle are calculated, and then a process proceeds to spot welding the welding object 32 by passing current through the upper electrode 40 and the lower electrode 42. Therefore, for example, before the process of spot welding the welding object 32, the magnitude φ of the impact angle can be corrected or the transition to the spot welding process can be canceled.

According to the first embodiment, the direction θ and magnitude φ of the impact angle are calculated, and if the magnitude φ of the impact angle is equal to or greater than a predetermined value, the magnitude φ of the impact angle is corrected to less than the specified value, and then current is passed through the upper electrode 40 and the lower electrode 42 to spot weld the welding object 32. This makes it possible to prevent the welding object 32 from being resistance spot welded in a state in which the magnitude φ of the impact angle is equal to or greater than the specified value, thereby stabilizing the welding quality. As a result, for example, when a galvanized steel sheet is used as the welding object 32, the occurrence of LME cracking can be suppressed.

In addition, according to the first embodiment, an inexpensive and small strain gauge 14 is used to calculate the direction θ and magnitude φ of the impact angle, which saves space by not interfering with the surrounding components of the upper electrode 40 and the lower electrode 42, and solves economic issues such as initial costs and running costs.

In addition, according to the first embodiment, the four strain gauges 14 are attached to the upper electrode 40, and current is passed through the upper electrode 40 and the lower electrode 42 to weld the welding object 32. Therefore, compared to removing the impact angle detection means for detecting the impact angle and then passing current through the upper electrode 40 and the lower electrode 42 to weld the welding object 32, the number of steps can be reduced, and the welding costs can be reduced.

[Differences from the techniques described in patent documents]
Next, the first embodiment will be described with respect to differences from the techniques described in the patent documents.

(1) Regarding Patent Document 1 Patent Document 1 describes a surface straightness detection jig that can be attached to and detached from an electrode. This surface straightness detection jig has three or more detection rods that come into contact with the steel plate, and detects the strain of a leaf spring connected to the detection rods using a strain gauge 14.

In Patent Document 1, the strain of the leaf spring connected to the contact rod is measured by the strain gauge 14, whereas in the first embodiment, the strain of the upper electrode 40 itself is directly measured by the strain gauge 14. In Patent Document 1, the surface perpendicularity detection jig needs to be removed during welding, but in the first embodiment, it is not necessary to remove the strain gauge 14 during welding.

(2) Regarding Patent Document 2 Patent Document 2 describes a method of attaching a multi-axis vibration sensor to an electrode, calculating a three-dimensional displacement of the electrode from the vibration (acceleration) detected by the multi-axis vibration sensor, and detecting the impact angle.

Patent Document 2 differs in that it uses electrode displacement information calculated from vibration (acceleration), whereas the first embodiment detects the impact angle by measuring the strain of the upper electrode 40 with a strain gauge 14.

(3) Regarding Patent Document 3 Patent Document 3 describes a welding robot that determines the distance between the electrodes when pressure is applied to the welding material and detects the impact angle from this value.

In Patent Document 3, in order to utilize the basic functions of the welding robot, no sensor is installed on the electrode, whereas the first embodiment differs in that a strain gauge 14 is attached to the electrode as a sensor.

(4) Regarding Patent Document 4 Patent Document 4 describes a welding robot that moves an electrode to a plurality of positions in the vicinity of a welding point and detects an impact angle from the magnitude of the reaction force at that time.

In Patent Document 4, in order to utilize the basic functions of the welding robot, no sensor is installed on the electrode, whereas the first embodiment differs in that a strain gauge 14 is attached to the electrode as a sensor.

(5) Regarding Patent Document 5 Patent Document 5 describes a method for measuring the inclination angle of an electrode by using a visual indicator.

The difference between Patent Document 5 and the first embodiment is that a strain gauge 14 attached to an electrode is used as a sensor, whereas Patent Document 5 uses a visual indicator.

(6) Regarding Patent Document 6 Patent Document 6 describes a method of measuring the surface perpendicularity of an electrode in a pseudo manner by attaching a pressure-sensitive element to a head that is attached to an electrode holder instead of the electrode.

The present invention differs from Patent Document 6 in that a pressure sensor is used, whereas the present invention uses a strain gauge 14. Also, Patent Document 6 differs in that a head is attached to the electrode holder instead of an electrode, and therefore welding cannot be performed as is, whereas the first embodiment differs in that a strain gauge 14 is attached to the upper electrode 40, and therefore welding can be performed as is.

(7) Regarding Patent Document 7 Patent Document 7 describes a technique in which a non-electrical strain gauge 14 is attached to an electrode to detect the direction and magnitude of a load applied to the electrode.

Patent Document 7 differs in that the direction and magnitude of the load applied to the electrode are detected, whereas the first embodiment detects the direction and magnitude of the strike angle. Also, Patent Document 7 differs in that the electrode distortion is detected after current is passed through the electrode, whereas the first embodiment detects the distortion of the upper electrode 40 before current is passed through the upper electrode 40 and the lower electrode 42.

[Experimental Example]
Next, an experimental example of the first embodiment will be described.

Table 1 shows the measurement conditions in the experimental example of the first form. Figure 12 is a graph showing the measurement results of the direction θ of the hitting angle in the experimental example. The vertical axis of Figure 12 shows the direction θ of the hitting angle calculated by the control unit 22 described above, and the horizontal axis of Figure 12 shows the direction θ of the hitting angle that was actually set. In Figure 12, the ◇ mark shows the case where the hitting angle is φ = 3°, the □ mark shows the case where the hitting angle is φ = 6°, and the △ mark shows the case where the hitting angle is φ = 9°.

Figure 13 is a graph showing the measurement results of the magnitude φ of the hitting angle in the experimental example. The vertical axis of Figure 13 shows the magnitude φ of the hitting angle calculated by the control unit 22 described above, and the horizontal axis of Figure 13 shows the magnitude φ of the hitting angle that was actually set. In Figure 13, the ◇ mark indicates the case where there is no hitting angle, the □ mark indicates the case where the hitting angle direction θ = 90°, the △ mark indicates the case where the hitting angle direction θ = 105°, the × mark indicates the case where the hitting angle direction θ = 120°, the * mark indicates the case where the hitting angle direction θ = 135°, the O mark indicates the case where the hitting angle direction θ = 150°, the + mark indicates the case where the hitting angle direction θ = 165°, and the ▽ mark indicates the case where the hitting angle direction θ = 180°.

From the measurement results shown in Figures 12 and 13, it was confirmed that the direction θ and magnitude φ of the impact angle can be predicted with sufficient accuracy for practical use.

In addition, a pair of overlapping galvannealed steel sheets (GA steel sheets) was used as the welding object 32, and resistance spot welding was performed using the first mode. The center of the welded portion of the welding object 32 was observed to check for the presence or absence of LME cracks. The results are shown in Table 2.

Test No. 1 is an example in which the current flow time to the upper electrode 40 and the lower electrode 42 was (5t + 2.8φ + 2)/50 [sec] or more, assuming that the total thickness of the welded portion of a pair of overlapping steel plates is t [mm].

In addition, test number 2 is an example in which (3·φ-1)·(t·T/F) 1/2 ≦H is satisfied when the pressure holding time after the current is stopped being applied to the upper electrode 40 and the lower electrode 42 is H [msec], the thickness of each of the pair of steel plates is t [mm], the tensile strength of each of the pair of steel plates is T [MPa], the pressure forces of the upper electrode 40 and the lower ^{electrode 42} are F [N], and the magnitude of the impact angle is φ = 6°.

Test No. 3 is an example in which the impact angle φ was set to 6° when pressure was applied, and was then corrected to φ = 0° before welding began.

As shown in Table 2, in test number 1, the first mode was applied, and further, a current flow time according to the impact angle size φ was applied, and as a result, no LME cracking was observed. In addition, in test number 2, a holding time according to the impact angle size φ was applied, and as a result, no LME cracking was observed. Furthermore, in test number 3, the impact angle size φ = 6° was detected when pressure was applied, and then it was corrected to φ = 0° before welding began, and as a result, no LME cracking was observed. In this way, it was confirmed from the experimental examples that the first mode is effective in suppressing LME cracking.

[Modification]
Next, a modification of the first embodiment will be described.

(First Modification)
Fig. 14 is a diagram showing a first modified example of the resistance spot welding apparatus 10 according to the first embodiment. In Fig. 14, (A) is a side view of the resistance spot welding apparatus 10 including a block diagram, and (B) is a cross-sectional view taken along the line F14-F14. Also, Fig. 15 is a diagram explaining the direction θ and magnitude φ of the impact angle of the lower electrode 42 in the resistance spot welding apparatus 10 of Fig. 14. In Fig. 15, (A) is a side view of the welding gun 12 explaining the magnitude φ of the impact angle of the lower electrode 42, and (B) is a bottom view diagrammatically explaining the direction θ of the impact angle of the lower electrode 42.

In the first embodiment, four strain gauges 14 are attached to the upper electrode 40, but in the first modified example shown in Figures 14 and 15, four strain gauges 14 are attached to the lower electrode 42. The lower electrode 42 to which the four strain gauges 14 are attached is an example of a "first electrode," and the upper electrode 40 is an example of a "second electrode."

As shown in FIG. 15B, in this first modified example, the reference line C is defined as follows. That is, if the cross section of the lower electrode 42 when cut by a plane perpendicular to the axis B of the lower electrode 42 is defined as cross section 42A as shown in FIG. 15B, a line that is set in advance on this cross section 42A and extends from the center of this cross section 42A to the radially outer side of the lower electrode 42 is defined as the reference line C. The four strain gauges 14 are respectively arranged at a position on the reference line C, a position 90° away from the reference line C in the circumferential direction of the upper electrode 40, a position 180° away from the reference line C in the circumferential direction of the upper electrode 40, and a position 270° away from the reference line C in the circumferential direction of the upper electrode 40.

The inclination of the axis B of the lower electrode 42 relative to the normal A of the contact surface 32B of the welding object 32 is the strike angle of the lower electrode 42. In the first modified example, the angle between the perpendicular line D dropped from the normal A toward the axis B and the reference line C on the cross section 42A of the lower electrode 42 is defined as the strike angle direction θ [°] (0°≦θ<360°). Furthermore, as shown in FIG. 15(A), the angle between the normal A of the contact surface 32B of the welding object 32 and the axis B of the lower electrode 42 on the same plane is defined as the strike angle magnitude φ [°] (0°≦φ<90°).

In this first modified example, the direction θ and magnitude φ of the impact angle of the lower electrode 42 are calculated by the same method as in the first embodiment. That is, the direction θ and magnitude φ of the impact angle of the lower electrode 42 are calculated based on the four strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε ₂₇₀ measured by the four strain gauges 14 attached to the lower electrode 42.

This first variant can achieve the same effects and advantages as the first variant.

(Second Modification)
16 is a diagram showing a second modified example of the resistance spot welding apparatus 10 according to the first embodiment. In Fig. 16, (A) is a side view of the resistance spot welding apparatus 10 including a block diagram, (B) is a cross-sectional view taken along line F16A-F16A, and (C) is a cross-sectional view taken along line F16B-F16B.

Figure 17 is a diagram for explaining the direction θ1 and magnitude φ1 of the impact angle of the upper electrode 40 and the direction θ2 and magnitude φ2 of the impact angle of the lower electrode 42 in the resistance spot welding device 10 of Figure 16. In Figure 17, (A) is a side view of the welding gun 12 for explaining the magnitude φ1 of the impact angle of the upper electrode 40 and the magnitude φ2 of the impact angle of the lower electrode 42, (B) is a plan view for typically explaining the direction θ1 of the impact angle of the upper electrode 40, and (C) is a bottom view for typically explaining the direction θ2 of the impact angle of the lower electrode 42.

This second modified example is a combination of the first embodiment and the first modified example. That is, in the second modified example, four strain gauges 14A are attached to the upper electrode 40, and four strain gauges 14B are attached to the lower electrode 42. The four strain gauges 14A attached to the upper electrode 40 correspond to the four strain gauges 14 in the first embodiment (see Figures 1 and 2), and the four strain gauges 14B attached to the lower electrode 42 correspond to the four strain gauges 14 in the first modified example (see Figures 14 and 15).

In this second modified example, the upper electrode 40 is an example of a "first electrode," and the lower electrode 42 is an example of a "second electrode." The four strain gauges 14A attached to the upper electrode 40 are an example of a "plurality of first strain gauges," and the four strain gauges 14B attached to the lower electrode 42 are an example of a "plurality of second strain gauges."

In this second modified example, the direction θ1 and magnitude φ1 of the impact angle of the upper electrode 40 are calculated using the same method as in the first modified example. Similarly, the direction θ2 and magnitude φ2 of the impact angle of the lower electrode 42 are calculated using the same method as in the first modified example.

According to this second modified example, the direction θ1 and magnitude φ1 of the impact angle of the upper electrode 40, and the direction θ2 and magnitude φ2 of the impact angle of the lower electrode 42 can be calculated individually. This makes it possible to deal with cases where an impact angle exists only on one of the upper electrode 40 and the lower electrode 42 due to the influence of gaps between the multiple steel plates 30 that make up the welding object 32, for example.

The impact angle direction θ2' calculated by the following formula (11) may be the impact angle direction of the lower electrode 42. In this way, the impact angle direction θ2' of the lower electrode 42 and the impact angle direction θ1 of the upper electrode 40 can be compared with each other while aligning the impact angle direction θ2' of the lower electrode 42 with the impact angle direction θ1 of the upper electrode 40.
θ2'=θ2-180[°]...(11)

(Other Modifications)
In the first form, as an example of the multiple strain gauges 14, four strain gauges 14 are attached to the upper electrode 40, but the number of multiple strain gauges 14 attached to the upper electrode 40 at equal intervals in the circumferential direction may be any number as long as it is three or more.

In addition, in the first modified example, three or more strain gauges 14 may be attached to the lower electrode 42 at equal intervals in the circumferential direction, and similarly, in the second modified example, three or more strain gauges 14A may be attached to the upper electrode 40 at equal intervals in the circumferential direction, and three or more strain gauges 14B may be attached to the lower electrode 42 at equal intervals in the circumferential direction.

In addition, in the first embodiment, the first modified example, and the second modified example, an example is described using a diagram in which the inclination angle of the axis of the upper electrode 40 and the inclination angle of the axis of the lower electrode 42 are the same. However, in the first embodiment, the first modified example, and the second modified example, the impact angle measurement method in resistance spot welding may be applied when the inclination angle of the axis of the upper electrode 40 and the inclination angle of the axis of the lower electrode 42 are different, when the axis of the upper electrode 40 is perpendicular to the steel sheet and the axis of the lower electrode 42 is inclined relative to the normal to the steel sheet, and when the axis of the upper electrode 40 is inclined relative to the normal to the steel sheet and the axis of the lower electrode 42 is perpendicular to the steel sheet.

Next, we will explain the second form that is the premise of the embodiment of the present invention.

[About the idea behind the second form (changes from the first form)]
In the second embodiment, the number of the multiple strain gauges 14 attached at equal intervals in the circumferential direction to at least one of the upper electrode 40 and the lower electrode 42 does not have to be three or more. Also, in the second embodiment, even if the multiple strain gauges 14 are arranged at uneven intervals, the direction θ and magnitude φ of the impact angle can be detected.

Furthermore, in the second embodiment, the pressure applied to the upper electrode 40 and the lower electrode 42 can be detected, which allows the positional deviation of the welding gun 12 in the height direction to be detected, and the positional deviation of the welding gun 12 in the height direction can be corrected to further stabilize the welding quality. Note that in the second embodiment, the pressure applied mainly means the actual pressure calculated by the resistance spot welding device according to this embodiment.

As an example, four strain gauges 14 are attached at 90° intervals to four locations on the circumference of the upper electrode 40 and the lower electrode 42, the welding object 32 is sandwiched between the upper electrode 40 and the lower electrode 42, and the strain of the upper electrode 40 and the strain of the lower electrode 42 are measured by the four strain gauges 14 when the welding object 32 is pressed by the upper electrode 40 and the lower electrode 42. As a result, it was found that there is a characteristic that a strain measurement value corresponding to each pressure applied by the upper electrode 40 and the lower electrode 42 is obtained. In the second embodiment, this characteristic is utilized to detect each pressure applied by the upper electrode 40 and the lower electrode 42.

[Key points and mechanisms of the second form]
Below, we will specifically explain the considerations that led to the quantitative calculation of each pressure force of the upper electrode 40 and the lower electrode 42, the considerations as to why the number of multiple strain gauges 14 attached at equal intervals around the circumference of at least one of the upper electrode 40 and the lower electrode 42 does not have to be three or more, and a method for detecting the direction θ and magnitude φ of the impact angle even when multiple strain gauges 14 are arranged at uneven intervals.

The inventors assume a disturbance condition in which the positional deviation of the welding gun 12 and the impact angle are combined. Therefore, even when there is an impact angle, it is necessary to detect the pressure applied to the upper electrode 40 and the lower electrode 42 with high accuracy. Therefore, in the second embodiment, four strain gauges 14 are attached at four locations around the circumference of the upper electrode 40 and the lower electrode 42 at 90° intervals, and the strain measurement values of each of the four strain gauges 14 are measured by changing the pressure applied to the upper electrode 40 and the lower electrode 42.

Fig. 18 is a graph showing the relationship between the pressure applied to the upper electrode 40 and the lower electrode 42 obtained by the above measurement and the average value ε _m (four-point average value) of the strain measurement values of the four strain gauges 14 attached to the upper electrode 40 and the lower electrode 42. The vertical axis of Fig. 18 represents the pressure [N], and the horizontal axis of Fig. 18 represents the average value ε _{m of the strain measurement values of the four strain gauges 14 attached to the upper electrode 40. Furthermore, the ◇ mark represents the average value ε m of} the strain measurement values of the four strain gauges 14 attached to the upper electrode 40, and the □ mark represents the average value ε _m of the strain measurement values of the four strain gauges 14 attached to the lower electrode 42.

18, it was found that the average value ε _m of the strain measurement values of the four strain gauges 14 attached to the upper electrode 40 and the average value ε _m of the strain measurement values of the four strain gauges 14 attached to the lower electrode 42 are both proportional to the respective applied pressures of the upper electrode 40 and the lower electrode 42. It was also found that the average value ε _m of the strain measurement values of the four strain gauges 14 attached to the upper electrode 40 and the average value ε _m of the strain measurement values of the four strain gauges 14 attached to the lower electrode 42 are similar values even if the respective applied pressures of the upper electrode 40 and the lower electrode 42 change.

From the above measurement results, the inventors considered that the average value _εm of the strain measurements obtained from the multiple strain gauges 14 arranged at equal intervals is not easily affected by the impact angle and is correlated with the applied pressure. The following provides further data on which such a method for measuring applied pressure is based. Note that the multiple strain gauges 14 arranged at equal intervals are an example of "equally spaced strain gauges".

FIG. 19 shows the relationship between the direction of the impact angle θ [°] and the average value ε _m of the strain measurement value when the number of strain gauges 14 is one, two, and four. In the measurement shown in FIG. 19, the pressure of the upper electrode 40 and the lower electrode 42 were set to a constant condition. FIG. 19(A) shows the measurement result when the number of strain gauges 14 is one, FIG. 19(B) shows the measurement result when the number of strain gauges 14 is two, and FIG. 19(C) shows the measurement result when the number of strain gauges 14 is four. In FIGS. 19(A) to (C), the vertical axis shows the average value ε _m of the strain measurement value, and the horizontal axis shows the direction of the impact angle θ [°]. In the measurement shown in FIG. 19, the pressure was set to 4000 N, and the magnitude φ of the impact angle was set to 3°.

19(A), the square marks represent the measurement results when the strain gauge 14 is placed at a position on the reference line C (0° position), the triangle marks represent the measurement results when the strain gauge 14 is placed at a position 90° away from the reference line C, the cross marks represent the measurement results when the strain gauge 14 is placed at a position 180° away from the reference line C, and the asterisk marks represent the measurement results when the strain gauge 14 is placed at a position 270° away from the reference line C. In the measurement shown in FIG. 19(A), since there is one strain gauge 14, the average strain measurement value _εm is the strain measurement value of one strain gauge 14 itself.

In FIG. 19(B), graph G1 shows the strain measurement value of the strain gauge 14 placed at a position on the reference line C (0° position) and the average value ε m of the strain measurement values of the strain gauge 14 placed at a position 180° away from the reference line C, and graph G2 shows the average value ε _m of the strain measurement values of the strain gauge 14 placed at a position on the reference line C (0° position) and the strain measurement values of the strain gauge 14 placed at a position ₉₀ ° away from the reference line C.

In Figure 19 (C), graph G1 shows the average value ε m (four-point average value) of the strain measurement value of a strain gauge 14 placed at a position on reference line C, the strain measurement value of a strain gauge 14 placed at a position 90° away from reference line C, the strain measurement value of a strain gauge 14 placed at a position 180° away from reference line C, and the strain measurement value of a strain gauge ₁₄ placed at a position 270° away from reference line C.

Moreover, Fig. 20 shows the relationship between the pressure [N] of the upper electrode 40 and the lower electrode 42 measured under the condition of no impact angle and the average value _εm of the strain measurement values. Fig. 20(A) shows the measurement results when the number of strain gauges 14 is two, and Fig. 20(B) shows the measurement results when the number of strain gauges 14 is four. In Figs. 20(A) and (B), the vertical axis shows the average value _εm of the strain measurement values, and the horizontal axis shows the pressure [N].

In FIG. 20A , a △ mark indicates the average value ε m of the strain measurement value of the strain gauge 14 arranged at a position on the reference line C of the upper electrode 40 (position 0°) and the strain measurement value of the strain gauge 14 arranged at a position 180° away from the reference line C, a × mark indicates the average value ε _m of the strain measurement value of the strain gauge 14 arranged at a position on the reference line C of the lower electrode 42 (position 0°) and the strain measurement value of the strain gauge 14 arranged at a position 180° away from the reference line C, a ◇ mark indicates the average value ε _m of the strain measurement value of the strain gauge 14 arranged at a position 90° away from the reference line C of the upper electrode 40 and the strain measurement value of the strain gauge 14 arranged at a position 270° away from the reference line C, and a □ mark indicates the average value ε _m of the strain measurement value of the strain gauge 14 arranged at a position 90° away from the reference line C of the lower electrode 42 and the strain measurement value of the strain gauge 14 arranged at a position ₂₇₀ ° away from the reference line C.

In Figure 20 (B), the ◇ mark indicates the average value ε _m (four-point average value) of the strain measurement values of the four strain gauges 14 attached to the upper electrode 40, and the square mark indicates the average value ε _m (four-point average value) of the strain measurement values of the four strain gauges 14 attached to the lower electrode 42.

From the measurement results shown in FIG. 19(A), it was found that when there was one strain gauge, the average value _εm of the strain measurement value changed depending on the direction θ of the impact angle, and the strain gauge was not suitable as a pressing force detection means.

From the measurement results shown in Fig. 19(B), when the number of strain gauges is two, as shown in graph G2, the average value ε _m of the strain measurement values of two strain gauges 14 arranged at positions 90° apart from each other changes depending on the direction θ of the strike angle, and it was found that the strain gauges are not suitable as a pressing force detection means. On the other hand, as shown in graph G1, the average value ε _m of the strain measurement values of two strain gauges 14 arranged at positions 180° apart from each other hardly changes depending on the direction θ of the strike angle. Also, as shown in Fig. 20(A), it was found that the average value ε _m of the strain measurement values of two strain gauges 14 arranged at positions 180° apart from each other shows a correlation with the pressing forces of the upper electrode 40 and the lower electrode 42.

19(C), when the number of strain gauges is four, as shown in graph G1, the average value _εm of the strain measurement values of the four strain gauges 14 hardly changes depending on the direction θ of the impact angle. Also, as shown in FIG 20(B), it was found that the average value _εm of the strain measurement values of the four strain gauges 14 shows a correlation with each of the pressure forces of the upper electrode 40 and the lower electrode 42.

From the above measurement results, it was found that the average value ε _m calculated from the strain measurement values of multiple strain gauges 14 arranged at equal intervals is a detection value that enables high-precision detection of the pressing force even in a situation where an impact angle is present.

The inventors also measured the actual pressure applied by the upper electrode 40 and the lower electrode 42 by changing the height position of the welding gun 12.

Figure 21 is a graph showing the relationship between the actual pressure of each of the upper electrode 40 and the lower electrode 42 and the height position of the welding gun 12. The vertical axis of Figure 21 shows the actual pressure [N], and the horizontal axis of Figure 21 shows the height position of the welding gun 12 [mm]. Graph G1 shows the actual pressure of the upper electrode 40, and graph G2 shows the actual pressure of the lower electrode 42.

21, when the position of the welding gun 12 is shifted, the actual pressure force of the lower electrode 42 changes while the actual pressure force of the upper electrode 40 remains constant. For example, when the position of the welding gun 12 is shifted toward the lower electrode 42 from the positional relationship in which the lower electrode 42 and the welding object 32 are just touching, a clearance (i.e., a gap) is generated between the lower electrode 42 and the welding object 32, so that the actual pressure force of the lower electrode 42 decreases compared to the actual pressure force of the upper electrode 40. Conversely, when the position of the welding gun 12 is shifted toward the upper electrode 40 from the positional relationship in which the lower electrode 42 and the welding object 32 are just touching, the lower electrode 42 pushes up the welding object 32, so that the actual pressure force of the lower electrode 42 increases compared to the actual pressure force of the upper electrode 40. Therefore, the inventors thought that it would be possible to detect the positional shift of the welding gun 12 by utilizing the difference in pressure force between the upper electrode 40 and the lower electrode 42.

These changes are due to the relative positional relationship between the welding gun 12 and the object to be welded 32, and it is believed that the same changes will occur even if the position of the welding gun 12 does not change but the position of the object to be welded 32 does.

21, the actual pressure of the upper electrode 40 is constant regardless of the position of the welding gun 12 because the torque of the motor provided in the robot 20 is controlled so that the actual pressure of the upper electrode 40 is constant. Also, the fact that the actual pressure of the lower electrode 42 changes depending on the position of the welding gun 12 can be interpreted from the balance of forces acting on the welding gun 12. When the upper electrode 40 and the lower electrode 42 pressurize the welding object 32, the forces acting on the welding gun 12 are (1) a reaction force that the upper electrode 40 receives from the contact surface of the welding object 32, (2) a force acting from the arm of the robot 20 that holds the welding gun 12, and (3) a reaction force that the lower electrode 42 receives from the contact surface with the welding object 32. Of these, the reaction force (1) is constant regardless of the positional deviation of the welding gun 12, as described above. On the other hand, the force (2) is considered to change depending on the positional deviation of the welding gun 12. For example, if there is no clearance between the upper electrode 40 and the lower electrode 42 and the welding object 32, the upper electrode 40 and the lower electrode 42 will come into contact with the welding object 32 almost simultaneously, but if there is clearance, the lower electrode 42 will come into contact with the welding object 32 first, and then the upper electrode 40 will come into contact with the welding object 32. As a result, an external load acts on the joint of the arm of the robot 20, but the position of the arm of the robot 20 is controlled to maintain its posture, so the motor torque of the joint is changed by the amount of the external load. That is, the force (2) acting on the welding gun 12 will change. To summarize the above, it is considered that the reaction force (1) does not change due to the positional deviation of the welding gun 12, but the force (2) changes. In addition, since the forces acting on the welding gun 12 when the upper electrode 40 and the lower electrode 42 pressurize the welding object 32 must be balanced, it is considered that the reaction force (3) acting on the lower electrode 42 changes by the amount of the change in the force (2).

In this way, when the robot 20 is controlled to adjust the vertical position of the welding gun 12 so that the actual pressure force of the upper electrode 40 remains approximately constant even if the vertical position of the welding gun 12 changes, it was found that the actual pressure force of the lower electrode 42 is proportional to the vertical position of the welding gun 12.

From the above, the inventors have considered that, for example, when control is performed on robot 20 to adjust the vertical position of welding gun 12 so that the actual pressure of upper electrode 40 is kept substantially constant, the vertical position of welding gun 12 can be estimated by attaching a plurality of strain gauges 14 at equal intervals to lower electrode 42 and calculating the average value ε _m of the strain measurement values of these plurality of strain gauges 14.

The inventors also considered application to C-guns or X-guns. First, we will explain C-guns. C-guns are generally one-sided drive, with one of the upper and lower electrodes being a movable electrode and the other being a fixed electrode. For the movable electrode, the pressure can be calculated from the torque of the motor that drives the movable electrode, but the fixed electrode does not have a means for detecting the pressure. Therefore, it is necessary to place a strain gauge on at least the fixed electrode. On the other hand, the impact angle can be measured by attaching a strain gauge to at least one of the movable electrode and the fixed electrode. Therefore, for a C-gun, it is considered that patterns 1 to 6 in Table 3 should be used to measure the impact angle and detect the respective pressure forces of the upper and lower electrodes.

That is, pattern 1 is a pattern in which the pressure is calculated from the torque of the motor driving the movable electrode, and both the pressure and the impact angle are detected by a strain gauge attached to the fixed electrode. Pattern 2 is a pattern in which the pressure is detected by a strain gauge attached to both the movable electrode and the fixed electrode, and the impact angle is measured by the strain gauge attached to the fixed electrode. Pattern 3 is a pattern in which the pressure is calculated from the torque of the motor driving the movable electrode, the impact angle is measured by a strain gauge attached to the movable electrode, and the pressure is detected by a strain gauge attached to the fixed electrode. Pattern 4 is a pattern in which the pressure is detected by a strain gauge attached to both the movable electrode and the fixed electrode, and the impact angle is measured by the strain gauge attached to the movable electrode. Pattern 5 is a pattern in which the pressure is calculated from the torque of the motor driving the movable electrode, the impact angle is measured by a strain gauge attached to both the movable electrode and the fixed electrode, and the pressure is detected by a strain gauge attached to the fixed electrode. Pattern 6 is a pattern in which strain gauges attached to both the movable electrode and the fixed electrode are used to detect both the pressure force and measure the impact angle.

Because the pressure force is a parameter that affects the welding quality, it is believed that there are no welding guns that do not control the pressure force to a constant value. However, even if the pressure force is not controlled to a constant value, it is believed that patterns 1 to 6 can be used for the arrangement pattern, just as with "constant pressure force control."

Next, the X-shaped gun will be described. For a one-sided drive X-shaped gun in which one of the upper and lower electrodes is a movable electrode and the other is a fixed electrode, it is considered that the arrangement pattern can adopt patterns 1 to 6, similarly to the C-shaped gun. For a two-sided drive X-shaped gun in which both the upper and lower electrodes are movable electrodes and the upper and lower electrodes can be driven individually by two motors, it is considered that the arrangement pattern can adopt a pattern in which the pressure is calculated from the torque of the motor driving each movable electrode and the impact angle is measured by a strain gauge attached to either the movable electrode or the fixed electrode, in addition to patterns 1 to 6. For a two-sided drive X-shaped gun in which the upper and lower electrodes can be driven by one motor and the pressure force of the upper electrode and the pressure force of the lower electrode can be calculated from the one motor, it is considered that the arrangement pattern can be adopted similarly to the two-sided drive X-shaped gun in which the upper and lower electrodes can be driven individually by the above-mentioned two motors.
In addition, for a double-sided driven X-gun in which a single motor can drive both the upper and lower electrodes but the pressure force of the upper electrode and the pressure force of the lower electrode cannot be calculated from a single motor, it is considered appropriate to adopt a pattern in which the pressure force is detected by a strain gauge attached to both the upper and lower electrodes and the impact angle is measured by a strain gauge attached to either the movable electrode or the fixed electrode.

The inventors first devised a specific method for calculating the impact angle when four strain gauges 14 are arranged at equal intervals as described above, but discovered that the impact angle can also be calculated from the strain measurement values of two adjacent strain gauges that are 90° apart, as shown in Figure 22. Figure 22 shows the impact angle, pressure, and the number and arrangement of strain gauges suitable for detecting the impact angle and pressure. The black dots in Figure 22 represent strain gauges.

As an example, Fig. 23 shows data obtained by calculating a deviatoric strain ε'(ε' = ε - ε _m ) from strain measurement values ε of four strain gauges 14 arranged at equal intervals (90° intervals) on the shank portion of the upper electrode 40, and then calculating a deviatoric strain change amount Δε'(Δε' = ε' - ε _{φ = 0} '). Figs. 23(A) to (C) correspond to the above-mentioned Figs. 4 to 6, and Fig. 23(D) corresponds to the above-mentioned Fig. 7.
As shown in the formula in FIG. 23, assuming that ε _m ≒ ε _{m, φ = 0} , the relationship Δε' ≒ Δε holds. In other words, by utilizing the fact that the average value ε _m of the strain measurement values is approximately equal to the reference deviation strain ε _{m, φ = 0} , the deviation strain change amount Δε'(Δε' = ε' - ε _{φ = 0} ') can be replaced with the strain change amount Δε (Δε = ε - ε _{φ = 0} ). Even if there are no multiple strain gauges (equally spaced strain gauges) arranged at equal intervals in the circumferential direction, such as when only the strain measurement values of two adjacent strain gauges separated by 90 degrees can be obtained, and the average value ε _m of the strain measurement values cannot be calculated, it is possible to estimate the magnitude φ of the hit angle and the direction θ of the hit angle by using the strain change amount Δε instead of the deviation strain change amount Δε' without using the deviation strain ε'.

Although not shown in the figure, the magnitude φ of the hit angle and the direction θ of the hit angle can be estimated by selecting the first strain change Δε1 and the second strain change Δε2 from the calculation results of the strain change Δε and calculating the resultant strain change Δε _N from equation (12).
Δε _N =√{(Δε1) ² + (Δε2) ² }...(12)

The first strain change amount Δε1 is the strain change amount Δε₀, Δε₉₀, Δε₁₈₀, Δε₂₇₀The second strain change amount Δε2 is the second strain change amount having the larger negative value among the strain changes detected at two positions separated by ±90° in the circumferential direction of the upper electrode 40 from the position where the first strain change amount Δε1 is detected.₀, Δε₉₀, Δε₁₈₀, Δε₂₇₀is calculated using equations (13) to (16).
Δε₀= ε₀-ε_0,φ=0...(13)
Δε₉₀= ε₉₀-ε_90,φ=0...(14)
Δε₁₈₀= ε₁₈₀-ε_180,φ=0...(15)
Δε₂₇₀= ε₂₇₀-ε_270,φ=0 ... (16)

Here, looking at the graph of deviation strain change Δε' in Figure 23(C) (almost the same as the graph of strain change Δε), the strain measurement value at the mounting position 90° and the strain measurement value at the mounting position 270°, which is 180° away from the mounting position 90°, are almost the same in magnitude but have opposite signs. Also, from the value of deviation strain change Δε' when the striking angle direction is 120° shown in Figure 23(D) (almost the same as the value of strain change Δε), the relationship between the strain measurement value at the mounting position 0° and the strain measurement value at the mounting position 180°, and the relationship between the strain measurement value at the mounting position 90° and the strain measurement value at the mounting position 270° are the same in magnitude but have opposite signs. Therefore, it is possible to estimate the distribution of the deviation strain change amount Δε' at four circumferential locations from the deviation strain change amount Δε' at two adjacent mounting points spaced 90° apart, and it is possible to estimate the distribution of the deviation strain change amount Δε' at four circumferential locations from the strain change amount Δε at two adjacent mounting points spaced 90° apart. In other words, the magnitude φ and direction θ of the strike angle can be estimated from the strain change amount Δε at two adjacent mounting points spaced 90° apart.

To summarize the above, as shown in FIG. 22, the number of strain gauges is not limited to four, but may be two or more. When the number of strain gauges is three, they may be arranged at equal intervals of 120°. In this case, the pressure is calculated from the average value ε _m of the three strain measurement values, and the impact angle may be calculated from the deviation strain change amount Δε' or the strain change amount Δε. In addition, the three strain gauges may be arranged at unequal intervals of 90°, 90°, and 180°. In this case, the pressure is calculated from the average value ε _m of the strain measurement values obtained from two strain gauges (two facing each other) spaced 180° apart, and the impact angle may be calculated from the strain change amount Δε. In this case, the deviation strain change amount Δε' cannot be used. When the number of strain gauges is four or more, the strain gauges may be arranged at equal intervals (i.e., the interval obtained by 360°/number of strain gauges). In this case, the pressure is calculated from the average value _εm of the strain measurement values, and the impact angle is calculated from the deviation strain change amount Δε' or the strain change amount Δε.

[Method of manufacturing resistance spot welded joint]
Next, a case where the method for manufacturing a resistance spot welded joint according to the second embodiment is applied to an actual welding and assembly process will be described.

In the manufacturing method of the resistance spot welded joint according to the second embodiment, as an example, a resistance spot welding device 10 shown in FIG. 16 is used. When there is no distinction between the multiple strain gauges 14A attached to the upper electrode 40 and the multiple strain gauges 14B attached to the lower electrode 42, the multiple strain gauges 14A and the multiple strain gauges 14B are each referred to as multiple strain gauges 14. Also, in the second embodiment, as an example, the upper electrode 40 is described as a movable electrode, and the lower electrode 42 is described as a fixed electrode.

The manufacturing method for a resistance spot welded joint according to the second embodiment includes steps S10 to S12 shown in FIG. 24. Step S10 is a pressure force and impact angle detection step, step S11 is a pressure force and impact angle correction step, and step S12 is a current application step.

In the pressing force detection process of step S10, an average value ε _m of strain measurements obtained by the multiple strain gauges 14A attached to the upper electrode 40 and an average value ε _m of strain measurements obtained by the multiple strain gauges 14B attached to the lower electrode 42 are calculated and converted into the pressing forces of the upper electrode 40 and the lower electrode 42 from the relationship between the previously obtained average value ε _m and the magnitude of the pressing force (see FIG. 18 ). The positional deviation of the welding gun 12, including the clearance from the welding object 32, is estimated from the magnitude relationship between the pressing force of the lower electrode 42 and the pressing force of the upper electrode 40. In other words, if the value of the pressure force of the lower electrode 42 is equal to the value of the pressure force of the upper electrode 40, it is determined that there is no misalignment of the welding gun 12; if the value of the pressure force of the lower electrode 42 is smaller than the value of the pressure force of the upper electrode 40, it is determined that there is a clearance; and if the value of the pressure force of the lower electrode 42 is greater than the value of the pressure force of the upper electrode 40, it is determined that the welding gun 12 is misaligned from the ideal contact position toward the upper electrode 40 (i.e., misaligned in the direction of pushing up the welding object 32 by the lower electrode 42).

In addition, in the impact angle measurement process of step S10, the deviation strain change amount Δε' is calculated using at least one of the multiple strain gauges 14A attached to the upper electrode 40 and the multiple strain gauges 14B attached to the lower electrode 42, and the impact angle is calculated from the distribution of this value around the shank. Note that, as shown in FIG. 22, the strain change amount Δε may be used instead of the deviation strain change amount Δε'. The detailed method of calculating the impact angle is the same as in the first embodiment.

Next, in step S11, the position of the welding gun 12 is changed in the height direction (i.e., in the plate thickness direction of the welding object 32) so that the pressure of the upper electrode 40 and the pressure of the lower electrode 42 become equal. Specifically, if the value of the pressure of the lower electrode 42 is smaller than the value of the pressure of the upper electrode 40, the pressure position of the welding gun 12 is moved from the initial pressure position toward the upper electrode 40. Conversely, if the value of the pressure of the lower electrode 42 is larger than the value of the pressure of the upper electrode 40, the pressure position of the welding gun 12 is moved from the initial pressure position toward the lower electrode 42. In addition, regarding the impact angle, the attitude of the welding gun 12 with respect to the welding object 32 is changed to correct the magnitude φ of the impact angle to be less than a specified value.

Next, in step S12, electricity is passed through the upper electrode 40 and the lower electrode 42 and maintained for a certain period of time. This causes the welding object 32 to be resistance spot welded, thereby obtaining a resistance spot welded joint from the welding object 32.

In the correction process of step S11, the correction of the pressure force and the correction of the angle of impact can be performed in this order, or the correction of the angle of impact and the correction of the pressure force can be performed in this order, but the correction of the angle of impact and the correction of the pressure force is more preferable. Details are described below. That is, FIG. 25 shows the welding gun 12 (C-shaped gun) before and after the correction of the angle of impact. If there is a slight positional deviation between the control point of the robot shown by the control point P in the figure and the tip position of the lower electrode 42, it is predicted that after the correction of the angle of impact, due to a mismatch of the rotation center, a positional deviation in the plate thickness direction of the welding target 32 will occur at the same time, and as a result, an imbalance will occur in the pressure forces of the upper electrode 40 and the lower electrode 42. Therefore, it is desirable to perform the correction of the angle of impact first, and then the correction of the pressure force. In general, the posture control of the robot is performed with the control point P as the center. The position of the control point P can be set arbitrarily by the user, and in the welding gun 12, it is usually set at the tip of the fixed electrode (in this case, the lower electrode 42).

[Action and Effects]
As described above in detail, according to the second embodiment, it is possible to simultaneously detect the positional deviation of the welding gun 12 and measure the impact angle. Therefore, for example, in resistance spot welding of galvanized steel sheets, the welding method utilizing the second embodiment can reduce LME cracks that occur under conditions in which the positional deviation of the welding gun 12 and the impact angle are combined. That is, LME cracks tend to occur easily when there are disturbances such as a clearance, which is a gap between the welding object 32 and the lower electrode 42, and an impact angle, which is an inclination of the upper electrode 40 and the lower electrode 42 with respect to the steel sheet surface of the welding object 32, and are particularly likely to occur when the clearance and the impact angle act simultaneously. However, even when the positional deviation of the welding gun 12 including the clearance is combined with the impact angle, according to the second embodiment, these disturbances can be quantitatively detected and the position and attitude of the welding gun 12 can be corrected based on the detected disturbance to reduce LME cracks.

[Differences from the techniques described in patent documents]
Next, the second embodiment will be described with respect to differences from the techniques described in Patent Documents 8. Patent Document 8 describes a welding method in which strain gauges are attached to the upper and lower electrodes of a welding gun to detect the pressure forces of the upper and lower electrodes, and the position of the welding gun is corrected so that the pressure forces are uniform.

In the welding method described in Patent Document 8, the number and arrangement of strain gauges are not specified. Also, the second embodiment differs in that in addition to detecting the welding force, the impact angle is measured at the same time.

[Experimental Example]
An experimental example based on the second embodiment will be described. Two galvanized steel sheets with a thickness of 1.6 mm and a tensile strength of 980 MPa were used as the steel sheets, and the plate assembly to be welded was constructed by stacking two of them. A C-shaped gun was used as the welding gun. Four strain gauges were arranged around the shank of the upper electrode and the shank of the lower electrode, at equal intervals of 90°. The pressure of the upper electrode and the lower electrode was set to 4000 N.

The following description will be given with reference to Table 4. In the present invention example, which is an experimental example of the present invention, the initial impact angle and clearance were set to 4° and 0.3 mm, respectively. In the pressing force and impact angle detection process, a pressing force of 3500 N was detected by the lower electrode with respect to a pressing force of 4000 N by the upper electrode, and an impact angle of 3.8° was detected. In the pressing force and impact angle correction process, the position of the welding gun was changed in the thickness direction of the steel plate so that the pressing forces of the upper electrode and the lower electrode both indicated 4000 N. The movement amount of the welding gun at this time was ultimately 0.28 mm. In addition, based on the detection result of the impact angle, the attitude of the welding gun was changed by 3.8° in the direction in which the impact angle approached 0. In the current application conditions, the above-mentioned plate assembly was applied with the conditions shown in the table. After the plate assembly after welding was polished and corroded, the cross section of the welded part was observed using an optical microscope, and as a result, no LME cracks were observed in the welded part of the present invention example.
On the other hand, in the comparative example, the same disturbance conditions as those in the example of the present invention were set, and the pressing force and impact angle detection process and the pressing force and impact angle correction process were not performed. In the comparative example, the current conditions were the same as those in the example of the present invention. As a result of observing the cross section of the weld, LME cracks were observed in the weld of the comparative example.

[Modification]
Next, a modification of the second embodiment will be described.

As explained in "Key points and mechanism of the second embodiment" (see Figure 22), in the second embodiment, the number of strain gauges 14 attached to the upper electrode 40 and the lower electrode 42 may be two or more.

As explained in "Key points and mechanisms of the second embodiment" (see Table 3), the welding gun 12 may be an X-gun instead of a C-gun. Furthermore, the X-gun may be one-sided or two-sided.

The detection form of the strain gauge 14 may be any one of the multiple patterns shown in Table 3.

Next, an embodiment of the present invention will be described.

[New ideas for solving problems]
The inventors have realised that further improvements are possible with respect to the first and second aspects.
In other words, when the temperature of the electrode changes, the measured strain value measured by the strain gauge includes not only the strain due to pressure but also the thermal strain (apparent strain) due to the temperature change of the electrode. Therefore, the detection accuracy of the impact angle and pressure force decreases due to the temperature change of the electrode, and as a result, the effect of disturbance correction decreases, which may reduce the effect of suppressing LME cracking.

Furthermore, factors that cause changes in electrode temperature include the outside air temperature and changes in the temperature of the cooling water inside the electrodes, but the inventors believed that in addition to these, the influence of current flow was also significant.
That is, when focusing on the temperature change of the electrode due to the current passing, the electrode temperature rises mainly due to heat conduction from the object to be welded (e.g., steel plate) during the current passing, and after the current passing, the temperature gradually drops due to heat transfer to the cooling water passing through the inside of the electrode and heat conduction to the surroundings of the electrode. In particular, in the welding and assembly process of the automobile body, it is common for one robot to be in charge of welding at multiple points, and the time interval between welding at each point is set as short as possible from the viewpoint of production efficiency. Therefore, in the welding and assembly process of the automobile body, the electrode temperature that has risen due to the current passing does not completely drop before the next welding point is pressed, and the electrode temperature rises when pressing with each increase in the number of welding points (see FIG. 28).
In Fig. 28, before the first current supply, there is no rise in electrode temperature due to current supply, but the electrode temperature changes by ΔT1 compared to a certain reference temperature due to the influence of the outside air temperature and the cooling water temperature. After the second current supply, the electrode temperature changes by ΔT2 compared to a certain reference temperature due to a further temperature rise caused by current supply.

The inventors therefore believed that if it were possible to remove the effects of thermal strain from the strain measurements taken with a strain gauge, it would be possible to prevent a decrease in the detection accuracy of the impact angle and pressure force, and as a result, it would be possible to maintain the effect of disturbance correction and the effect of suppressing LME cracking.

[New problem-solving points and mechanisms]
As a result of extensive investigation, the inventors have found that the above problems can be solved by the following method.
That is, the surface temperature T of the electrode is measured by a temperature measuring means such as a thermocouple, and the thermal strain ε _{T calculated from the measured surface temperature T} is subtracted from the measured strain value ε measured by a strain gauge to calculate the strain correction value ε _e . The strain correction value ε _e is then used in place of the measured strain value ε in the first and second forms. This makes it possible to detect the impact angle and applied force with high accuracy without being affected by the temperature change of the electrode. A specific description will be given below.

The corrected strain value ε _e is calculated by subtracting the thermal strain ε _T from the measured strain value ε according to equation (17).

ε _e = ε−ε _T ...(17)

The thermal strain ε _T can be calculated, for example, as a function of the electrode temperature T and the reference temperature T′ according to equation (18).

ε _T =f(T, T')...(18)

Here, the reference temperature T' is a constant value that serves as a reference for the temperature change of the electrode, and is preferably the following temperature.
In detecting the impact angle, the temperature when the standard deviation strain ε _φ=0 ' or the standard strain ε _φ=0 is obtained. In detecting the applied force, the temperature when the relationship between the average strain ε _m and the applied force (see FIG. 18) is obtained. The standard temperature T' may simply be the electrode temperature before the current is passed at the first impact point shown in FIG. 28. This makes it possible to take into account at least the thermal strain caused by the rise in the electrode temperature due to the current passing.

The specific calculation formula for the thermal strain ε _T =f(T, T′) is not particularly limited, but for example, formula (19) can be used.

f(T, T')=α×(T-T')=α×ΔT...(19)

Here, the thermal strain ε _T to be subtracted from each of the multiple strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ may be (A) a common thermal strain ε _T calculated from a temperature T measured at one location, or (B) multiple thermal strains ε T, ₀ , ε T, ₉₀ , ε T, ₁₈₀ , ε _{T, 270} calculated from temperatures T ₀ , T ₉₀ , T ₁₈₀ , T ₂₇₀ measured at multiple locations corresponding to the multiple strain measurement values _{ε 0} , _{ε 90} , ε ₁₈₀ , ε 270.

(A: Method using a common thermal strain calculated from the electrode temperature measured at one location)
A method of using a common thermal strain ε _T calculated from a temperature T measured at one location will be specifically described. As an example, a case in which four strain gauges are attached to an electrode at 90° intervals in the circumferential direction will be described.

According to this method, the strain correction values ε _e,0 , ε _e,90 , ε _e,180 , and ε _{e,270 corresponding} to the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε 270 can be calculated by the following equations (20) to (23).

ε _{e, 0} = ε ₀ - ε _T = ε ₀ - f(T, T') ...(20)
ε _e,90 = ε ₉₀ - ε _T = ε ₉₀ - f(T, T') ...(21)
ε _{e, 180} = ε ₁₈₀ - ε _T = ε ₁₈₀ - f(T, T') ...(22)
ε _{e, 270} = ε ₂₇₀ - ε _T = ε ₂₇₀ - f(T, T') ... (23)

The surface temperature T is the temperature measured at one point on the surface of the electrode.

In this way, the method of subtracting the thermal strain ε _T calculated from the temperature T measured at one location for multiple strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ can simplify the device configuration compared to a method of measuring temperatures at multiple locations. This is particularly effective when the shank cross-sectional shape is circular or when external disturbances are expected to be small, as this tends to result in a uniform temperature distribution in the shank circumferential direction.

(B: Method using multiple thermal strains calculated from electrode temperatures at multiple locations corresponding to multiple strain measurements)
Next, a method using a plurality of thermal strains ε _T calculated from electrode temperatures at a plurality of locations corresponding to a plurality of strain measurement values will be described.

According to this method, the strain correction values ε _e,0 , ε _e,90 , ε _e,180 , and ε _{e,270 corresponding} to the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε 270 can be calculated by the following equations (24) to (27).

ε _e,0 = ε ₀ -ε _T,0 = ε ₀ -f(T ₀ ,T' ₀ )...(24)
ε _e,90 = ε ₉₀ - _{ε T,90} = ε ₉₀ - f(T ₉₀ , T' ₉₀ ) ...(25)
ε _e,180 = ε ₁₈₀ - ε _{T, 180} = ε ₁₈₀ - f(T ₁₈₀ , T' ₁₈₀ ) ...(26)
ε _{e, 270} = ε ₂₇₀ - ε _{T, 270} = ε ₂₇₀ - f (T ₂₇₀ , T' ₂₇₀ ) ... (27)

The measured temperatures _T0 , _T90 , _T180 , and _T270 are values measured at four locations corresponding to the four strain gauges arranged on the electrode at 90° intervals in the circumferential direction. That is, the measured temperature _T0 is a value measured by the thermocouple closest to the strain gauge located on the reference line C, the measured temperature _T90 is a value measured by the thermocouple closest to the strain gauge located 90° away from the reference line C in the circumferential direction of the upper electrode 40, the measured temperature _T180 is a value measured by the thermocouple closest to the strain gauge located 180° away from the reference line C in the circumferential direction of the upper electrode 40, and the measured temperature _T270 is a value measured by the thermocouple closest to the strain gauge located 270° away from the reference line C in the circumferential direction of the upper electrode 40.

Reference temperatures T' ₀ , T' ₉₀ , T' ₁₈₀ , and T' ₂₇₀ are reference temperatures T' corresponding to the measured temperatures T ₀ , T ₉₀ , T ₁₈₀ , and T ₂₇₀ , respectively.

In this way, according to the method using the multiple thermal strains εT _,0 , εT, ₉₀ , εT _{,180 ,} εT, ₂₇₀ calculated from the temperatures at multiple locations corresponding to the multiple strain measurements _ε0 , ε90, _ε180 , _ε270 , it is possible to take into account the temperature non-uniformity in the circumferential direction of the electrode, and therefore it is possible to calculate the thermal strain _εT with higher accuracy than when the temperature is measured at one location. Note that it is considered that the non-uniformity in the temperature distribution in the circumferential direction is likely to occur, for example, when the shank cross-sectional shape is not circular or when the electrode does not contact the steel sheet uniformly due to disturbances such as an impact angle, and this method is particularly effective in such cases.

When providing the same number of thermocouples corresponding to multiple strain gauges, it is preferable to place the thermocouples close to the corresponding strain gauges. In addition, it is preferable that the position at which the temperature is measured by the thermocouple is the same position as the corresponding strain gauge in the axial direction of the electrode. This is because a temperature distribution occurs in the axial direction of the electrode where the temperature is high at the tip and decreases with distance from the tip of the electrode.

As shown in Fig. 28, the timing for measuring the electrode temperature T is after the steel sheet is pressed and before the start of current flow. Also, as shown in Fig. 28, the timing for measuring the electrode temperature T is preferably the same as the timing for measuring the strain measurements ε ₀ , ε ₉₀ , ε ₁₈₀ , and ε ₂₇₀ .

[Description of Resistance Spot Welding Apparatus 10]
26 is a diagram showing the resistance spot welding apparatus 10 according to this embodiment, in which (A) is a side view including a block diagram of the resistance spot welding apparatus 10, (B) is a cross-sectional view taken along line F26A-F26A, and (C) is a cross-sectional view taken along line F26B-F26B.

The resistance spot welding apparatus 10 according to this embodiment differs from the resistance spot welding apparatus 10 according to the second modified example of the first embodiment (see FIG. 16) in the following points.
(1) The inclusion of thermocouples 15A and 15B as "temperature measuring means" (2) The control unit 22 corrects the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ using temperatures T ₀ , T ₉₀ , T ₁₈₀ , T ₂₇₀ measured by thermocouples 15A and 15B. Below, the differences will be explained, and the same symbols will be used in the drawings to denote common points, and explanations of these common points will be omitted.

As shown in FIG. 26, the resistance spot welding device 10 according to this embodiment is equipped with multiple thermocouples 15A, 15B. Four thermocouples 15A, 15B are attached to the shank of the upper electrode 40, and four thermocouples 15A, 15B are attached to the shank of the lower electrode 42. The thermocouples 15A, 15B are connected to the control unit 22, which is configured to be able to obtain the temperature T of the electrodes 40, 42 measured by the thermocouples 15A, 15B.

The four thermocouples 15A attached to the upper electrode 40 correspond to the four strain gauges 14A, respectively. As shown in FIG. 26(B), the four thermocouples 15A are configured to be able to measure the temperature at a position circumferentially close to the corresponding strain gauge 14A. The distance between the strain gauge 14A and the temperature measurement position (thermocouple 15A) corresponding to the strain gauge 14A is preferably within 10° in the circumferential direction, and more preferably within 5°.

The four thermocouples 15B attached to the lower electrode 42 correspond to the four strain gauges 14B, respectively. As shown in FIG. 26(C), the four thermocouples 15B are configured to be able to measure the temperature at a position circumferentially close to the corresponding strain gauge 14B. The distance between the strain gauge 14B and the temperature measurement position (thermocouple 15B) corresponding to the strain gauge 14B is preferably within 10° in the circumferential direction, and more preferably within 5°.

The control unit 22 executes steps S9 to S12 (see FIG. 27) in the manufacturing method for resistance spot welded joints, which will be described later, by the processor 50 executing the program 54 stored in the memory 52.

[Method of manufacturing resistance spot welded joint]
Next, a case where the method for manufacturing a resistance spot welded joint according to this embodiment is applied to an actual welding and assembly process will be described.

In the manufacturing method of the resistance spot welded joint according to this embodiment, as an example, a resistance spot welding device 10 shown in FIG. 26 is used. When there is no distinction between the multiple strain gauges 14A attached to the upper electrode 40 and the multiple strain gauges 14B attached to the lower electrode 42, the multiple strain gauges 14A and the multiple strain gauges 14B are each referred to as multiple strain gauges 14. When there is no distinction between the thermocouples 15A and 15B, they are referred to as thermocouples 15. In this embodiment, as an example, the upper electrode 40 is described as a movable electrode, and the lower electrode 42 is described as a fixed electrode.

The manufacturing method for a resistance spot welded joint according to this embodiment includes steps S9 to S12 shown in FIG. 27. Step S9 is a thermal strain removal process, step S10 is a pressure force and impact angle detection process, step S11 is a pressure force and impact angle correction process, and step S12 is a current application process.

In the thermal strain removal process of step S9, the control unit 22 removes the effects of thermal strain by subtracting the thermal strains ε _T,0 , ε _T,90 , ε T, ₁₈₀ , ε _{T,270 from} the multiple strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε 270 measured by the multiple strain gauges 14A.
That is, the control unit 22 acquires the strain measurement values ε ₀ , ε ₉₀ , ε ₁₈₀ , ε ₂₇₀ output from the four strain gauges 14, and also acquires the electrode temperatures T ₀ , T ₉₀ , T ₁₈₀ , T ₂₇₀ output from the four thermocouples 15, and calculates the strain correction values ε _e,0 , ε _e,90 , ε _e,180 , ε _e,270 according to the above formulas (24) to (27). When the reference temperature T' for pressure detection and the reference temperature T' for impact angle detection are different, the control unit 22 calculates the strain correction values ε _e,0 , ε _e,90 , ε _e,180 , ε _e,270 for each.

In the pressing force detection step of step S10, the strain corrected value obtained in the thermal strain removal step is used instead of the strain measured value in the manufacturing method for a resistance spot welded joint according to the second embodiment.
That is, the control unit 22 calculates the average value _εm of the distortion correction values εe _,0 , _εe,90 , _εe,180 , and _εe,270 , and converts it into the electrode pressure force from the relationship between the previously obtained average value _εm and the magnitude of the pressure force (see FIG. 18).

In the strike angle measuring step of step S10, the strain measured value obtained in the thermal strain removing step is used instead of the strain measured value in the manufacturing method for a resistance spot welded joint according to the second embodiment.
That is, the control unit 22 calculates an average value ε _m of the multiple distortion correction values ε _e,0 , ε _{e,90 ,} ε _{e, 180 , ε e,270, calculates multiple deviational strains ε 0 ', ε 90 ', ε 180 ', ε 270 ' by subtracting the average value ε m from the multiple distortion correction values ε e,0 , ε e ,90 , ε e, 180 , ε e , 270 , calculates multiple} deviational strains _{ε 0 '} , ε ₉₀ ', ε ₁₈₀ ', ε _{270 ',} and calculates multiple deviational strain changes Δε ₀ ', Δε _{90 '} A first deviational strain change amount Δε1' and a second deviational strain change amount Δε2' _are specified from the multiple _deviational strain changes Δε ₀ ', Δε ₉₀ ', Δε ₁₈₀ ', Δε ₂₇₀ ', and the direction θ and magnitude φ of the hit angle are calculated.
The control unit 22 may calculate the magnitude φ of the hit angle and the direction θ of the hit angle by using the strain change amount Δε instead of the deviation strain change amount Δε' without using the deviation strain ε'. This point is as described in the second embodiment.

Next, in step S11, the position of the welding gun 12 is changed in the height direction (i.e., in the thickness direction of the plate to be welded 32) so that the pressure of the upper electrode 40 and the pressure of the lower electrode 42 become equal. In addition, regarding the impact angle, the attitude of the welding gun 12 relative to the plate to be welded 32 is changed to correct the size of the impact angle φ so that it is less than a specified value.

Next, in step S12, electricity is passed through the upper electrode 40 and the lower electrode 42 and maintained for a certain period of time. This causes the welding object 32 to be resistance spot welded, thereby obtaining a resistance spot welded joint from the welding object 32.

In the correction process of step S11, the corrections can be made in the order of pressure force correction and impact angle correction, or the corrections can be made in the order of impact angle correction and pressure force correction, but it is more preferable to make the corrections in the order of impact angle correction and pressure force correction. The same applies to the second embodiment.

[Action and Effects]
As described above in detail, according to this embodiment, when the electrode temperature changes, particularly when the electrode temperature rises during continuous spot welding at short intervals, the impact angle and application force can be detected with high accuracy without being affected by thermal strain.

In order to avoid the influence of the temperature change of the electrode, for example, it is effective to place the strain gauge away from the electrode tip. This is because the temperature rise caused by the current flow is mainly due to the heat conduction from the steel plate to be welded. However, from the viewpoint of the strain detection accuracy, it is desirable to place the strain gauge near the electrode tip, and there is a limit to the method of placing the strain gauge away from the electrode tip.
In contrast, in the present embodiment, the effect of thermal strain is eliminated based on the temperature of the electrode, making it possible to perform highly accurate detection, and therefore it is not necessary to place the strain gauge far from the tip of the electrode. Therefore, when attaching the strain gauge 14A to the shank, the strain gauge 14A can be disposed near the electrode tip, improving the accuracy of strain detection.

For example, a method using a self-temperature-compensating strain gauge can be considered to avoid the effects of temperature changes in the electrode. However, even with this method, it is difficult to completely eliminate the effects of thermal strain, and it is expected that the detection accuracy may be insufficient for the purpose of detecting the impact angle and pressure with high accuracy.

Another possible method is to predict the electrode temperature rise caused by current flow and predict the thermal strain. However, the electrode temperature during current flow is affected by various factors such as the current value, current flow time, plate thickness, presence or absence of plating, and presence or absence of disturbance, so it is practically difficult to predict the electrode temperature taking all of these factors into account.
In contrast, in this embodiment, the electrode temperature is actually measured, so that the thermal strain can be calculated with high accuracy.

In addition, in this embodiment, the strain and electrode temperature are measured before current is applied, so disturbances in the state before current is applied can be detected and the impact angle and electrode position can be corrected before current is applied.

In addition, in this embodiment, the temperature is measured at multiple locations on the electrode that are spaced apart in the circumferential direction, and the effects of thermal strain are eliminated using the temperatures at multiple locations, making it possible to detect the impact angle and pressure force with high accuracy, taking into account the temperature distribution in the circumferential direction of the electrode.

[Differences from the techniques described in patent documents]
Next, the differences between this embodiment and the techniques described in the patent documents will be described.

Patent document 11 discloses a technique in which a thermocouple is installed on the electrode tip to measure the temperature near the tip of the electrode tip and use this temperature for welding monitoring. Consumption of the thermocouple due to electrode tip dressing is acceptable.

Patent Document 11 measures the temperature during current flow for the purpose of welding monitoring, whereas this embodiment measures the temperature before current flow. Also, Patent Document 11 measures the temperature at the tip of the electrode tip, whereas this embodiment measures the temperature at the shank. Furthermore, this embodiment differs from the technology of Patent Document 11 in that it is equipped with a strain gauge.

Patent document 12 discloses a spot welding device that spot welds a laminated conductive film with an insulating sheet to a conductor. In this device, a thermocouple is placed at the tip of the electrode and is used to control the temperature of the electrode.

Patent Document 12 measures the temperature while current is being applied for the purpose of controlling the temperature of the electrode, whereas this embodiment measures the temperature before current is applied. Also, Patent Document 12 measures the temperature of the electrode tip, whereas this embodiment measures the temperature of the shank. Furthermore, this embodiment differs from the technology of Patent Document 12 in that it is equipped with a strain gauge.

[Example]
An example carried out based on this embodiment will be described.

The impact angle was set to 6° and the clearance to 1 mm, and 20 continuous welding was performed three times to investigate the effect of this embodiment in detecting disturbances and reducing LME cracks.

The steel plates used were zinc-plated steel plates with a thickness of 1.6 mm and a tensile strength of 980 MPa, and two of these were stacked together to form a plate assembly. A C-shaped gun was used for the welding gun. Four strain gauges were placed around the upper and lower shanks, at equal intervals of 90°. A strain gauge for steel was used for the strain gauge. One thermocouple was placed on each of the upper and lower shanks. The distance from the tip of the electrode tip of the strain gauge and thermocouple was 50 mm for both. The electrode pressure of the welding gun was set to 400 kgf. After continuous welding of 20 points, the cross section of the weld at the 20th point was polished and corroded, and LME cracking was observed using an optical microscope.

Table 5 shows the results of the present invention and comparative examples at the 20th impact point.

In the present invention, the electrode temperature was measured to calculate the thermal strain in the thermal strain removal process, and the thermal strain was removed from the strain measurement value. The reference temperature for calculating the thermal strain was set to 20°C. In the pressing force and impact angle detection process, the pressing force and impact angle of the upper and lower electrodes were detected using the strain value after the thermal strain was removed. The impact angle was calculated using the strain value of the upper electrode. Next, in the pressing force and impact angle correction process, the welding gun position was corrected based on the detection results of the pressing force and impact angle so that the pressing forces of the upper and lower electrodes were equal, and the inclination of the welding gun was corrected. In the 20th weld, no LME cracks were observed in any of the three cases.

On the other hand, in the comparative example, the thermal strain removal process was not performed. The values detected in the pressure force and impact angle detection process were different from those detected in the present invention example. The correction amounts in the pressure force and impact angle correction process were also different from those in the present invention. In the 20th weld, LME cracks were observed in all three examples.

From the above, it was found that the present invention enables highly accurate disturbance detection when the electrode temperature rises due to continuous striking, and furthermore, it is effective in suppressing LME cracking by compensating for disturbances. It was also found that highly accurate thermal strain compensation is possible even when using strain gauges for steel materials.

[Modification]
Next, a modification of the embodiment will be described.

In the above embodiment, an example was described in which both the pressure force and the impact angle were detected, but it is also possible to detect only one of the pressure force and the impact angle.

In the above embodiment, an example was described in which multiple strain gauges 14A, 14B are attached to both the first electrode 40 and the second electrode 42, but multiple strain gauges may be attached to only one of the first electrode 40 and the second electrode 42.

In the above embodiment, an example has been described in which the thermocouples 15A, 15B are attached to both the first electrode 40 and the second electrode 42, but a thermocouple may be attached to only one of the first electrode 40 and the second electrode 42. In addition, in the above embodiment, an example has been described in which the temperature measuring means is the thermocouple 15, but the temperature measuring means of the present invention is not limited to this and may be, for example, a resistance temperature detector or a non-contact measuring means using infrared rays.
In the above embodiment, when multiple surface temperatures are measured, the number of positions at which the multiple surface temperatures are measured is the same as the number of multiple strain gauges, but the present invention is not limited to this. Even when multiple surface temperatures are measured, the number of positions at which the multiple surface temperatures are measured does not have to be the same as the number of multiple strain gauges. In the above embodiment, the number of measurement temperatures is the same as the number of reference temperatures, but the number of measurement temperatures is not the same as the number of reference temperatures.

In the above embodiment, an example was described in which the number of strain gauges 14 attached to one electrode is four, but as explained in the first and second forms, the number of strain gauges 14 is not limited to four.
Furthermore, in the above embodiment, the thermal strain is calculated based on the temperature change ΔT obtained by subtracting the reference temperature T' from the surface temperature T, but the calculation formula for the thermal strain is not limited to the above formula (19), i.e., ε _T =f(ΔT). As another example, the calculation formula for the thermal strain ε _T =f(T, T') can be formula (28).
f(T,T')=g(T)-h(T')...(28)

The above describes an embodiment of the present invention, but the present invention is not limited to the above, and can of course be modified in various ways without departing from the spirit of the invention.

10 Resistance spot welding device 12 Welding gun 14 Strain gauge 14A Strain gauge (an example of a first strain gauge)
14B Strain gauge (an example of a second strain gauge)
15A Thermocouple (temperature measuring means)
15B Thermocouple (temperature measuring means)
22 Control unit 30 Steel plate 32 Welding object 40 Upper electrode (an example of a first electrode or a second electrode)
42 Lower electrode (an example of a second electrode or a first electrode)
44 Shank 46 Electrode tip

Claims (18)

A welding target having a plurality of overlapping metal materials is sandwiched between a first electrode and a second electrode arranged opposite to each other,
measuring a plurality of strain measurement values ε using a plurality of strain gauges attached to a plurality of positions spaced apart in a circumferential direction on a surface of the first electrode, and measuring one or a plurality of surface temperatures T of the first electrode using a temperature measuring means;
Calculating a plurality of strain correction values ε _e by subtracting thermal strain ε _T calculated based on the one or more surface temperatures T from the plurality of strain measurement values ε;
Calculating a direction θ and a magnitude φ of an impact angle _, which is an inclination of an axis of the first electrode with respect to a normal line of a contact surface between the first electrode and the welding object, and/or a pressing force between the first electrode and the welding object, based on the plurality of distortion correction values ε e.
A method for measuring a tapping force in resistance spot welding, comprising:
calculating a plurality of deviation strains ε' by subtracting an average value εm of _the plurality of distortion correction values εe from the plurality of _distortion correction values _εe , and calculating a direction θ and a magnitude φ of the strike angle based on the plurality of deviation strains ε';
A method for measuring the impact force in resistance spot welding. A plurality of standard deviation strains ε _φ=0 ′, which are the plurality of deviation strains ε′ in the absence of the strike angle, are subtracted from the plurality of deviation strains ε′ to calculate a plurality of deviation strain changes Δε′, and a direction θ and a magnitude φ of the strike angle are calculated based on the plurality of deviation strain changes Δε′.
The method for measuring stroke pressure in resistance spot welding according to claim 1. The thermal strain is calculated using the surface temperature T and a reference temperature T′;
The reference temperature T′ is a temperature at which the plurality of reference deviation strains ε _φ=0 ′ are acquired .
The method for measuring tap pressure in resistance spot welding according to claim 2 . A welding target having a plurality of overlapping metal materials is sandwiched between a first electrode and a second electrode arranged opposite to each other,
measuring a plurality of strain measurement values ε using a plurality of strain gauges attached to a plurality of positions spaced apart in a circumferential direction on a surface of the first electrode, and measuring one or a plurality of surface temperatures T of the first electrode using a temperature measuring means;
Calculating a plurality of strain correction values ε _e by subtracting thermal strain ε _T calculated based on the one or more surface temperatures T from the plurality of strain measurement values ε;
Calculating a direction θ and a magnitude φ of an impact angle _, which is an inclination of an axis of the first electrode with respect to a normal line of a contact surface between the first electrode and the welding object, and/or a pressing force between the first electrode and the welding object, based on the plurality of distortion correction values ε e.
A method for measuring a stroke force in resistance spot welding, comprising:
A plurality of reference strains ε _φ=0 , which are the plurality of strain measurement values ε when there is no hit angle, are subtracted from the plurality of strain correction values ε _e to calculate a plurality of strain change amounts Δε, and a direction θ and a magnitude φ of the hit angle are calculated based on the plurality of strain change amounts Δε.
A method for measuring the impact force in resistance spot welding. The thermal strain is calculated using the surface temperature T and a reference temperature T′;
The reference temperature T′ is a temperature when the plurality of reference strains ε _φ=0 are acquired .
The method for measuring stroke pressure in resistance spot welding according to claim 4 . A welding target having a plurality of overlapping metal materials is sandwiched between a first electrode and a second electrode arranged opposite to each other,
measuring a plurality of strain measurement values ε using a plurality of strain gauges attached to a plurality of positions spaced apart in a circumferential direction on a surface of the first electrode, and measuring one or a plurality of surface temperatures T of the first electrode using a temperature measuring means;
Calculating a plurality of strain correction values ε _e by subtracting thermal strain ε _T calculated based on the one or more surface temperatures T from the plurality of strain measurement values ε;
Calculating a direction θ and a magnitude φ of an impact angle _, which is an inclination of an axis of the first electrode with respect to a normal line of a contact surface between the first electrode and the welding object, and/or a pressing force between the first electrode and the welding object, based on the plurality of distortion correction values ε e.
A method for measuring a tapping force in resistance spot welding, comprising:
Using a relationship between an average strain and a pressing force obtained in advance, a pressing force between the first electrode and the welding object is calculated based on an average strain that is an average value of the plurality of strain correction values εe.
A method for measuring the impact force in resistance spot welding. The thermal strain is calculated using the surface temperature T and a reference temperature T′;
The reference temperature T′ is the temperature at which the relationship between the average strain and the applied pressure is obtained .
The method for measuring a tap pressure in resistance spot welding according to claim 6 . the one or more surface temperatures are a plurality of surface temperatures;
The thermal strain ε _T to be subtracted from the plurality of strain measurement values ε is a thermal strain calculated based on the surface temperature T at a position closest to the strain gauge corresponding to each of the plurality of strain measurement values ε.
The method for measuring a tap pressure in resistance spot welding according to any one of claims 1 to 7 . The number of positions at which the surface temperatures are measured is the same as the number of the strain gauges.
The method for measuring a tap pressure in resistance spot welding according to claim 8 . The position at which the one or more surface temperatures are measured coincides with the positions of the plurality of strain gauges in the axial direction of the first electrode.
The method for measuring a tap pressure in resistance spot welding according to any one of claims 1 to 9 . The temperature measuring means is a thermocouple.
The method for measuring a tap pressure in resistance spot welding according to any one of claims 1 to 10 . calculating a pressing force between the first electrode and the welding object based on an average strain that is an average value of the plurality of strain correction values ε _e by using a relationship between an average strain and a pressing force obtained in advance;
The method for measuring a tapping force in resistance spot welding according to any one of claims 1 to 11 . Attaching a plurality of first strain gauges to the first electrode as the plurality of strain gauges;
a plurality of second strain gauges are attached to a surface of the second electrode at a plurality of circumferentially spaced positions;
Calculating a direction θ and a magnitude φ of an impact angle, which is an inclination of an axis of the second electrode with respect to a normal line of a contact surface of the welding object with the second electrode, and/or a pressing force between the second electrode and the welding object, using a plurality of strain measurement values ε measured by the plurality of second strain gauges and one or a plurality of surface temperatures T of the second electrode measured by the temperature measuring means.
The method for measuring a tapping force in resistance spot welding according to any one of claims 1 to 12 . A welding target having a plurality of overlapping metal materials is sandwiched between a first electrode and a second electrode arranged opposite to each other,
measuring a plurality of strain measurement values ε using a plurality of strain gauges attached to a plurality of positions spaced apart in a circumferential direction on a surface of the first electrode, and measuring one or a plurality of surface temperatures T of the first electrode using a temperature measuring means;
Calculating a plurality of strain correction values ε _e by subtracting thermal strain ε _T calculated based on the one or more surface temperatures T from the plurality of strain measurement values ε ;
Calculating a direction θ and a magnitude φ of an impact angle _, which is an inclination of an axis of the first electrode with respect to a normal line of a contact surface between the first electrode and the welding object, and/or a pressing force between the first electrode and the welding object, based on the plurality of distortion correction values ε e.
Calculating the direction θ and magnitude φ of the stroke and the pressing force by a method for measuring stroke pressure in resistance spot welding, the method comprising the steps of :
Then, current is passed through the first electrode and the second electrode to spot weld the objects to be welded, thereby obtaining a resistance spot welded joint from the objects to be welded.
A method for manufacturing a resistance spot welded joint , comprising:
Adjusting the position of the welding gun based on the applied pressure;
Thereafter, current is passed through the first electrode and the second electrode to spot weld the objects to be welded.
A method for manufacturing resistance spot welded joints. Calculating a direction θ and a magnitude φ of the hit angle, and if the magnitude φ of the hit angle is equal to or greater than a predetermined value, correcting the magnitude φ of the hit angle to be less than the predetermined value;
Thereafter, current is passed through the first electrode and the second electrode to spot weld the objects to be welded.
The method for producing a resistance spot welded joint according to claim 14 . The resistance spot welded joint is obtained by continuously welding two or more locations.
A method for manufacturing a resistance spot welded joint according to claim 14 or 15 . A first electrode and a second electrode disposed opposite each other;
a plurality of strain gauges attached at circumferentially spaced positions on a surface of the first electrode;
A temperature measuring means for measuring a surface temperature of the first electrode;
a control unit that, in a state in which a welding target having a plurality of overlapping metal materials is sandwiched between the first electrode and the second electrode, calculates a plurality of strain correction values ε e by subtracting a thermal strain ε _T from the plurality of strain measurement values ε based on a plurality of strain measurement values _ε measured by the plurality of strain gauges and a surface temperature _T measured by the temperature measuring means, calculates a direction θ and a magnitude φ of an impact angle that is an inclination of an axis of the first electrode with respect to a normal to a contact surface of the welding target with the first electrode and/or a pressure between the first electrode and the welding target based on the plurality of strain correction values ε e, and then controls current to be passed through the first electrode and the second electrode to spot weld the welding target;
A resistance spot welding apparatus comprising:
The control unit calculates the pressing force, adjusts a position of a welding gun based on the pressing force, and then controls the welding object to be spot-welded by passing current through the first electrode and the second electrode.
Resistance spot welding equipment. The control unit calculates a direction θ and a magnitude φ of the impact angle, and when the magnitude φ of the impact angle is equal to or greater than a predetermined value, corrects the magnitude φ of the impact angle to be less than the predetermined value, and then controls current to be passed through the first electrode and the second electrode to spot weld the welding object.
18. The resistance spot welding apparatus of claim 17 .