Welding quality assessment system, welding quality assessment method, weld nugget diameter estimation system and method for spot welding

Abstract

The present invention provides a welding quality assessment system for determining the quality of spot welding of a component to be welded by applying pressure and energizing a pair of electrodes. The welding quality assessment system comprises: a driving unit that drives at least one of the pair of electrodes by linearly moving in the same direction as the displacement direction of the electrodes; a detection unit that detects the amount of displacement between the electrodes; and a determination unit that determines the quality of the welding based on the amount of displacement between the electrodes during energization detected by the detection unit.

Description

TECHNICAL FIELD

[0001] The present application relates to a welding quality determination system, a welding quality determination method, a nugget diameter estimation system, and a nugget diameter estimation method for spot welding. BACKGROUND

[0002] In a production process of a vehicle, spot welding is performed in which a pair of electrodes is pressed against a workpiece such as a vehicle body or a component and current is passed between the pair of electrodes to melt the workpiece and thereby weld it. However, in spot welding, the welded portion does not generally appear on the outside, and it is difficult to determine the welding quality from the outside.

[0003] Therefore, for example, a technique is disclosed in Japanese Patent Application Publication No. 2001-300738 in which, in spot welding in which a workpiece is pressed by a pair of electrodes driven by the rotational motion of a servomotor and current is passed, the amount of displacement of the same electrode caused by thermal expansion of the workpiece is detected, and the welding quality is determined based on the value of the amount of displacement. SUMMARY

[0004] Here, regarding determination of the welding quality in such spot welding in which a workpiece is pressed by a pair of electrodes driven by the rotational motion of a servomotor and current is passed, the inventors have found the following problem.

[0005] With the aforementioned technique for determining the welding quality based on the amount of displacement of the electrode, the behavior (e.g., thermal expansion, etc.) that occurs at the processing point in welding that is believed to affect the welding quality is detected based on the amount of displacement of the electrode, and thus the welding quality is determined. Therefore, it is desirable to be able to detect the behavior that occurs at the processing point with high precision based on the amount of displacement of the electrode.

[0006] However, in spot welding, in the case of the mainstream electric servomotor method in which the electrode is driven by the rotational motion of a servomotor, since a ball screw that converts the rotational driving force of the servomotor into linear driving force in the up-down direction is provided, the ball screw slides when it rotates, and thus rotational-induced sliding resistance is generated. As a result, since this sliding resistance affects the amount of displacement of the electrode, the behavior that occurs at the processing point in welding that is believed to affect the welding quality cannot be detected with high precision based on the amount of displacement of the electrode. That is, there is a problem in that the rotational-induced sliding resistance becomes an interference, and the determination precision of the welding quality decreases.

[0007] The present application was made in view of the above circumstances, and provides a welding quality determination system and a welding quality determination method that can determine the quality of spot welding with high precision, and a nugget diameter estimation system and a nugget diameter estimation method that can estimate the nugget diameter of spot welding with high precision.

[0008] An aspect of the present application relates to a welding quality determination system that determines the quality of spot welding in which a pair of electrodes pressurizes and energizes a member to be welded, the welding quality determination system including:

[0009] a driving section that drives at least one of the pair of electrodes by linearly moving in the same direction as the displacement direction of the electrodes;

[0010] a detecting section that detects the electrode-to-electrode displacement amount of the pair of electrodes; and

[0011] a determining section that determines the quality of the welding based on the electrode-to-electrode displacement amount during the energization detected by the detecting section.

[0012] According to this configuration, by using the driving section that linearly moves in the same direction as the displacement direction of the electrodes and drives the electrodes, the sliding resistance generated during the energization is suppressed. Thus, the behavior at the processed spot during the welding can be detected with high precision according to the electrode-to-electrode displacement amount, and the quality of the spot welding can be determined with high precision.

[0013] In the welding quality determination system of the above aspect, the driving section can be a linear motor. According to this configuration, the pressurizing force of the electrodes can be controlled with higher precision.

[0014] In the welding quality determination system of the above aspect, the determining section can be configured to determine that the welding quality is bad when the electrode-to-electrode displacement amount of the pair of electrodes in the energization in the direction away is equal to or less than a predetermined value. According to this configuration, in a case where the member to be welded is not sufficiently melted during the energization, the quality of the spot welding can be determined with high precision.

[0015] In the welding quality determination system of the above aspect, the determining section can be configured to determine that the welding quality is bad when the electrode-to-electrode displacement amount of the pair of electrodes in the energization in the direction toward is equal to or more than a predetermined value. According to this configuration, in a case where a large amount of spatter is generated from the member to be welded during the energization, the quality of the spot welding can be determined with high precision.

[0016] In the welding quality determination system of the above aspect, the determining section can be configured to estimate the nugget diameter formed on the member to be welded based on the electrode-to-electrode displacement amount of the pair of electrodes in the energization in the direction toward, and determine the quality of the welding based on the nugget diameter. According to this configuration, by setting a desired nugget diameter, the quality of the spot welding can be determined with high precision based on the nugget diameter.

[0017] In the welding quality determination system of the above aspect, the determination section can be configured to estimate the nugget diameter when the electrode-to-electrode displacement amount is large as a value smaller than the nugget diameter when the electrode-to-electrode displacement amount is small. According to this configuration, by estimating the nugget diameter with higher precision and setting the desired nugget diameter, the quality of spot welding can be determined with high precision based on the nugget diameter.

[0018] In the welding quality determination system of the above aspect, the determination section can be configured to determine the good or bad of the welding quality before the driving section starts to retract the electrodes from the welded member. According to this configuration, by determining before the electrodes are retracted, in the case where the welding quality is bad, the electrodes can be directly re-energized without being retracted, and the process can be efficiently performed.

[0019] In the welding quality determination system of the above aspect, the determination section can be configured to, when the welding quality is determined to be bad by the determination section, re-energize the electrodes using the pair of electrodes before the driving section starts the retraction. According to this configuration, by determining before the electrodes are retracted, in the case where the welding quality is bad, the electrodes can be directly re-energized without being retracted, and the process can be efficiently performed.

[0020] Further, a welding quality determination method according to an aspect of the present application determines the good or bad of the welding quality of spot welding, the welding quality determination method including:

[0021] a driving step of driving at least one of the pair of electrodes by linearly moving in the same direction as the displacement direction of the electrodes;

[0022] a pressurized energization step of pressurizing and energizing the welded member using the pair of electrodes;

[0023] a detection step of detecting an electrode-to-electrode displacement amount of the pair of electrodes; and

[0024] a determination step of determining the good or bad of the welding quality based on the electrode-to-electrode displacement amount in the energization detected by the detection step.

[0025] Further, a nugget diameter estimation system according to an aspect of the present application estimates the nugget diameter of spot welding in which a pair of electrodes pressurizes and energizes a welded member, the nugget diameter estimation system including:

[0026] a driving section that drives at least one of the pair of electrodes by linearly moving in the same direction as the displacement direction of the electrodes;

[0027] a detection section that detects an electrode-to-electrode displacement amount of the pair of electrodes; and

[0028] estimating a nugget diameter formed on the welded member based on the electrode-to-electrode displacement amount of the pair of electrodes in the current-carrying in the approaching direction detected by the detecting section.

[0029] According to this configuration, by driving the electrodes by the driving section that linearly moves in the same direction as the displacement direction of the electrodes, the sliding resistance generated at the time of current-carrying is suppressed. Thus, the nugget diameter formed on the welded member can be estimated with high precision based on the electrode-to-electrode displacement amount of the electrodes in the approaching direction.

[0030] Further, a nugget diameter estimation method according to an aspect of the present application estimates a nugget diameter of spot welding, the nugget diameter estimation method including:

[0031] a driving step of driving at least one of the pair of electrodes by linearly moving in the same direction as the displacement direction of the electrodes;

[0032] a pressurized current-carrying step of pressurizing and current-carrying the welded member by the pair of electrodes;

[0033] a detecting step of detecting an electrode-to-electrode displacement amount of the pair of electrodes; and

[0034] an estimating step of estimating a nugget diameter formed on the welded member based on the electrode-to-electrode displacement amount of the pair of electrodes in the current-carrying in the approaching direction detected by the detecting step.

[0035] According to the present application, it is possible to provide a welding quality determination system and a welding quality determination method capable of determining the quality of spot welding with high precision, and a nugget diameter estimation system and a nugget diameter estimation method capable of estimating a nugget diameter of spot welding with high precision. BRIEF DESCRIPTION OF DRAWINGS

[0036] The features, advantages, and technical and industrial significance of exemplary embodiments of the application will be described in connection with the accompanying drawings, in which like numerals designate similar components throughout the figures.

[0037] Figure 1 is a diagram showing an outline of a hardware configuration of a welding quality determination system according to a first embodiment.

[0038] Figure 2 is a diagram showing a functional configuration of a welding quality determination system according to the first embodiment.

[0039] Figure 3A is a diagram for explaining the relationship between the electrode-to-electrode displacement amount and the nugget diameter.

[0040] Figure 3B is a diagram for explaining the relationship between the electrode-to-electrode displacement amount and the nugget diameter.

[0041] Figure 4 Fig. 1 is a flowchart showing a flow of a welding quality determination process in the welding quality determination system according to the first embodiment.

[0042] Figure 5 Fig. 2 is an explanatory diagram schematically showing a change in the electrode-to-electrode displacement amount in the flow of the welding quality determination process. DETAILED DESCRIPTION

[0043] Hereinafter, a specific embodiment to which the present application is applied will be described in detail with reference to the drawings. However, the present application is not limited to the following embodiment. In addition, the following description and drawings are appropriately simplified in order to make the description clear.

[0044] <First Embodiment>

[0045] First, the welding quality determination system 100 according to the first embodiment will be described. Figure 1 Fig. 1 is a diagram showing an outline of a hardware configuration of the welding quality determination system 100 according to the present embodiment.

[0046] As shown in Fig. 1, the welding quality determination system 100 according to the present embodiment is provided with a welding gun 200 that spot-welds a workpiece (welded member) 201, and a diagnosis device 300 that performs quality determination based on welding by the welding gun 200 or estimation of a nugget diameter. Figure 1

[0047] The welding gun 200 is provided with a linear motor (driving portion) 203 that drives at least one of a pair of electrodes 202 that pressurize and electrically conduct the workpiece 201 by linearly moving in the same direction as the displacement direction of the electrodes 202, and a linear encoder (detecting portion) 204 that detects the electrode-to-electrode displacement amount of the pair of electrodes 202. As a specific example, as shown in Fig. 2, the pair of electrodes 202 is held by a pair of welding gun arms 205, and the upper welding gun arm 205 moves up and down due to the linear movement of the linear motor 203, thereby being able to drive the upper electrode 202. In addition, the linear motor 203 is provided with the linear encoder 204 that detects the linear position thereof, and thus the electrode-to-electrode displacement amount of the pair of electrodes 202 can be detected. In this description, an example in which the upper electrode 202 is driven is described, but the lower electrode 202 can also be driven, and both of the electrodes 202 can also be driven. Figure 1

[0048] ​​The linear motor 203 is a motor without a rotating shaft, and the driving method is not limited, for example, the electrode 202 is driven to linearly move in the same direction as the displacement direction of the electrode 202 by repulsion and attraction of magnets. Therefore, compared with the existing method of converting the rotational driving force of a servo motor into linear driving force, it is possible to reduce the sliding resistance at the time of electrode driving, that is, it is possible to reduce the disturbance at the time of detecting the displacement amount between electrodes. Thus, it is possible to accurately detect the behavior occurring at the processing point in welding according to the displacement amount between electrodes in energization. Furthermore, by using a motor, it is possible to accurately control the pressing force compared with the case of using a cylinder or the like.

[0049] The diagnostic device 300 has computer resources possessed by general information processing devices such as personal computers. Specifically, it is provided with a CPU (Central Processing Unit) 301, a ROM (Read Only Memory) 302, a RAM (Random Access Memory) 303, an HDD (Hard Disk Drive) 304, and an input / output interface (I / F) 305. Note that, here, an example in which each part of the diagnostic device 300 is implemented by a general information processing device is described, and some or all of the functions can also be incorporated into a control device or the like that controls the welding gun 200, or can be implemented in an external device such as an edge, a cloud, or the like.

[0050] The CPU 301 is a microprocessor that comprehensively controls the diagnostic device 300. Specifically, it reads various control programs stored in the ROM 302, the HDD 304, which are executed in the present embodiment, and executes these programs expanded on the RAM 303. Here, as a storage device, an SSD (Solid State Drive) can be provided instead of the HDD 304, or in addition to the HDD 304.

[0051] The input / output interface 305 performs input / output of the diagnostic device 300 with the outside. For example, it can be provided with an output device such as a display that displays information on the welding quality, and an input device for an operator to use.

[0052] Next, the functional configuration of the welding quality determination system 100 related to the present embodiment will be described. Figure 2 is a diagram showing the functional configuration of the welding quality determination system 100 related to the present embodiment.

[0053] As Figure 2As shown, the welding quality determination system 100 has a determination section 401 and an estimation section 402 as functional configurations in the diagnosis device 300. These sections function by the CPU 301 executing various control programs stored in the ROM 302 or the like. In addition, part or all of these functions can be realized by hardware circuitry.

[0054] The determination section 401 determines the quality of the welding based on the electrode-to-electrode displacement amount of the pair of electrodes 202 in the electrification detected by the linear encoder 204. Here, in the electrification of spot welding, thermal expansion of the workpiece 201 or spatter that affects the welding quality is generated, and the generation of these causes the electrode-to-electrode displacement amount to change, so the quality of the welding can be determined based on the electrode-to-electrode displacement amount in the electrification, and the specific determination method will be described later. In addition, the determination section 401 can also have a function of determining the quality of the welding based on the nugget diameter formed on the workpiece 201 estimated by the estimation section 402 described later. In this case, for example, the nugget diameter of a prescribed range in which the welding quality is determined to be good is set in advance by the operator or the like, and in the case where the estimated nugget diameter falls within the set prescribed range, the welding quality is determined to be good, and in the case where it falls outside the range, the welding quality is determined to be bad, so the quality of the welding can be determined. In addition, the operator or the like can set the value of the desired nugget diameter in advance, and determine the quality of the welding depending on whether the estimated nugget diameter is within a prescribed range from the value.

[0055] The estimation section 402 estimates the nugget diameter formed on the workpiece 201 based on the electrode-to-electrode displacement amount of the pair of electrodes 202 in the electrification in the approaching direction detected by the linear encoder 204. Here, the inventors have found in the present invention that there is a correlation between the electrode-to-electrode displacement amount of the pair of electrodes 202 in the electrification in the approaching direction and the nugget diameter formed on the workpiece 201.

[0056] The above-described relationship will be specifically described using Figure 3A and 3B . Figure 3A is a schematic view showing the state of the nugget 501 in which a normal nugget diameter A is formed on the workpiece 201, Figure 3B is a schematic view showing the state of the nugget 501 in which a smaller nugget diameter B than normal is formed. If a large amount of spatter of the molten workpiece 201 is generated in the electrification of spot welding, as shown in Figure 3B , the plate thickness of the workpiece 201 is greatly reduced, and the nugget diameter B of the nugget 501 formed on the workpiece 201 is smaller than Figure 3AThe normal nugget diameter A is shown. At this time, as the plate thickness of the workpiece 201 decreases, the electrode-to-electrode displacement amount of the pair of electrodes 202 in the approaching direction becomes larger. Therefore, since there is a relationship in which the nugget diameter formed on the workpiece 201 becomes smaller if the electrode-to-electrode displacement amount becomes larger, by storing in advance in the diagnostic device 300 a relationship (for example, a relational expression regarding the ratio of these values, etc.) of the electrode-to-electrode displacement amount of the pair of electrodes 202 in the approaching direction during the power supply and the nugget diameter formed on the workpiece 201, the nugget diameter formed on the workpiece 201 can be estimated with high precision on the basis of the electrode-to-electrode displacement amount of the pair of electrodes 202 in the approaching direction during the power supply.

[0057] Further, here, as the functional configuration of the welding quality determination system 100, an example in which the estimation unit 402 that estimates the nugget diameter is provided is explained, but a functional configuration in which the estimation unit 402 is not provided can also be provided. That is, when the welding quality is determined on the basis of the electrode-to-electrode displacement amount during the power supply, the estimation unit 402 that estimates the nugget diameter formed on the workpiece 201 is not a necessary configuration. On the other hand, in the case where the present application is implemented as a nugget diameter estimation system, a functional configuration in which the determination unit 401 that determines the quality of the welding can not be provided. That is, in the case where the nugget diameter formed on the workpiece 201 is estimated on the basis of the electrode-to-electrode displacement amount of the pair of electrodes 202 in the approaching direction during the power supply, for example, since the welding quality can also be determined additionally by the operator or the like on the basis of the estimated nugget diameter, the determination unit 401 that determines the quality of the welding is not a necessary configuration.

[0058] Next, the flow of the welding quality determination processing in the welding quality determination system 100 according to the present embodiment, that is, the welding quality determination method, is explained. Figure 4 is a flowchart of the welding quality determination processing according to the present embodiment. This flow is started in a state in which it has been determined that the welding quality is to be determined at the welding spot.

[0059] First, the upper electrode 202 is lowered by the linear motor 203 to come into contact with the workpiece 201, and pressing is started (S11). Next, after the pressing is maintained for a prescribed pressing time (S12), the power supply is started via the electrodes 202 (S13). Then, after the power supply has continued for a prescribed time, the power supply is ended (S14). Here, the pressing force and the power supply current, etc. are appropriately determined and executed on the basis of a prescribed machining program, etc.

[0060] Next, in S15, the quality of the welding is determined by the determination section 401 based on the electrode-to-electrode displacement amount detected by the linear encoder 204 in the energization in S13-S14. In the case where the quality of the welding is determined to be good, the process proceeds to S16, the upper electrode 202 is retracted from the workpiece 201, and the present flow ends. On the other hand, in the case where the quality of the welding is determined to be bad, the process proceeds to S13, and the energization is performed again. In this way, by performing the determination of the quality of the welding before the retraction of the electrode 202 in S16, in the case where the quality of the welding is determined to be bad due to, for example, insufficient melting, etc., the correction is efficiently performed by the energization again without retraction and lowering of the electrode 202. On the other hand, even in the case where the quality of the welding is bad, the process can proceed to S16 without the energization again, and the present flow ends. Note that, although omitted in the present flow, a process of outputting the determination result of the quality of the welding to the input / output interface 305 can be added.

[0061] Further, here, as the flow of the welding quality determination process, the process of determining the quality of the welding in S15 is explained, but in the case where the present application is implemented as a nugget diameter estimation method, in S15, instead of or in addition to the determination of the quality of the welding, the process of estimating the nugget diameter formed on the workpiece 201 is performed by the estimation section 402. Then, the process proceeds to S16, the upper electrode 202 is retracted from the workpiece 201, and the present flow ends.

[0062] Next, a specific example of the determination of the quality of the welding is explained. Figure 5 is an explanatory diagram schematically showing the change of the electrode-to-electrode displacement amount in the flow of the aforementioned welding quality determination process. Here, as the determination example, the following four examples are explained: as shown in (a)-(d), (a) a welding process in which no spatter is generated and the melting is also appropriate should be determined to be good; (b) a welding process in which although no spatter is generated, the melting is shallow, should be determined to be bad; (c) a welding process in which although spatter is generated, it is within the allowable range, should be determined to be good; and (d) a welding process in which a large amount of spatter is generated, should be determined to be bad. Figure 5

[0063] At time tO, the lowering of the upper electrode 202 is started, the pressure is maintained from time tl, and the lowering of the upper electrode 202 is stopped. Next, at time t2, the energization is started, and the electrode-to-electrode displacement amount at this time is set to dO.

[0064] ​First, for the welding processing (a) in which the melting is appropriate, immediately after the time t2, the electrode-to-electrode displacement amount in the direction away from each other increases from d0 due to thermal expansion of the workpiece 201. On the other hand, for the welding processing (b) in which the melting is shallow, the electrode-to-electrode displacement amount in the direction away from each other decreases due to thermal expansion of the workpiece 201. Therefore, by previously setting a prescribed threshold value, in the case where the electrode-to-electrode displacement amount in the direction away from each other of the pair of electrodes 202 during the power supply is below the threshold value, the welding quality is determined to be bad, and the poor quality due to insufficient melting can be determined with high accuracy.

[0065] Next, for the welding processing (a) and (b) in which spatters are not generated, the electrode-to-electrode displacement amount in the direction away from each other is maintained until the time t3 at which the power supply ends, and, in contrast, for the welding processing (c) and (d) in which spatters are generated, the electrode-to-electrode displacement amount in the direction away from each other sharply increases from d0 due to melting and scattering of the workpiece 201 and a decrease in the plate thickness. In particular, since the electrode-to-electrode displacement amount in the direction away from each other depends on the amount of generation of spatters, by previously setting a prescribed threshold value, for example, in the case of the welding processing (c) in which spatters are generated but within an allowable range, the electrode-to-electrode displacement amount in the direction away from each other of the electrodes 202 during the power supply becomes below the threshold value, the welding quality can be determined to be good, and, in the case of the welding processing (d) in which a large amount of spatters is generated, the electrode-to-electrode displacement amount in the direction away from each other of the electrodes 202 during the power supply becomes above the threshold value, the welding quality is determined to be bad, and the poor quality due to generation of spatters can be determined with high accuracy. Furthermore, as described above, since there is a relationship between the electrode-to-electrode displacement amount in the direction away from each other of the pair of electrodes 202 during the power supply and the diameter of the nugget formed on the workpiece 201, in which the electrode-to-electrode displacement amount increases and the diameter of the nugget decreases, the diameter of the nugget formed on the workpiece 201 can be estimated based on the electrode-to-electrode displacement amount in the direction away from each other of the pair of electrodes 202 during the power supply. Furthermore, by previously setting a desired diameter of the nugget, the welding quality can be determined with high accuracy based on the estimated diameter of the nugget.

[0066] Next, at the time t3, if the power supply ends, the workpiece 201 that has been thermally expanded contracts, and, at the time t4, the upper electrode 202 is retracted from the workpiece 201.

[0067] As described above, according to the welding quality determination system 100 according to the present embodiment, by using the linear motor 203 that drives the electrodes 202 by linear movement, the sliding resistance generated at the time of the power supply is suppressed, and the behavior at the processing point in the welding can be detected with high accuracy according to the electrode-to-electrode displacement amount. As a result, the poor quality of the welding due to insufficient melting or generation of spatters can be determined with high accuracy according to the detected electrode-to-electrode displacement amount, and the diameter of the nugget formed on the workpiece 201 can be estimated with high accuracy.

[0068] <Other Embodiments>

[0069] In the first embodiment, the linear motor 203 is exemplified as the driving section, but as long as the electrode 202 is driven by linear movement, a cylinder, a hydraulic cylinder, or the like can be used as the driving section in addition to the linear motor 203. Further, in this case, by providing a strain gauge or the like in at least one of the pair of welding gun arms 205, the electrode-to-electrode displacement amount of the pair of electrodes 202 can be detected.

[0070] Note that the present application is not limited to the above-described embodiments, and can be appropriately changed within a scope that does not depart from the gist. For example, in addition to the welding quality determination system, the welding quality determination method, the nugget diameter estimation system, the nugget diameter estimation method, various modes such as a welding quality determination device, a nugget diameter estimation device, and a computer program that implements these methods can be implemented.

Claims

1. A welding quality determination system that determines whether or not a welding quality of spot welding in which a pair of electrodes press and electrically conduct a member to be welded, the welding quality determination system comprising: a drive unit that drives at least one of the pair of electrodes by linearly moving in the same direction as a displacement direction of the electrodes; a detection unit that detects an electrode-to-electrode displacement amount of the pair of electrodes; and a determination unit that determines whether or not the welding quality is good based on the electrode-to-electrode displacement amount in electrical conduction detected by the detection unit, wherein when the electrode-to-electrode displacement amount in a direction away from each other is below a first threshold value after the electrical conduction is started, the determination unit determines that the welding quality is bad, when the electrode-to-electrode displacement amount in the direction away from each other exceeds the first threshold value after the electrical conduction is started, the determination unit determines whether or not the electrode-to-electrode displacement amount in a direction close to each other of the pair of electrodes is above a second threshold value, when the electrode-to-electrode displacement amount in the direction close to each other is above the second threshold value, the determination unit determines that the welding quality is bad, and when the electrode-to-electrode displacement amount in the direction close to each other is below the second threshold value, the determination unit determines that the welding quality is good. the drive unit is a linear motor.

3. The welding quality determination system according to claim 1 or 2, wherein the determination unit estimates a nugget diameter formed on the member to be welded based on the electrode-to-electrode displacement amount in the direction close to each other in the electrical conduction, and determines whether or not the welding quality is good based on the nugget diameter.

4. The welding quality determination system according to claim 3, wherein the determination unit estimates the nugget diameter when the electrode-to-electrode displacement amount is large as a value smaller than the nugget diameter when the electrode-to-electrode displacement amount is small.

5. The welding quality determination system according to claim 1 or 2, wherein the determination unit determines whether or not the welding quality is good before the drive unit starts to retreat the electrodes from the member to be welded.

6. The welding quality determination system according to claim 5, wherein when the determination unit determines that the welding quality is bad, the pair of electrodes is electrically conducted again before the drive unit starts to retreat the electrodes.

7. A welding quality determination method that determines whether or not a welding quality of spot welding, the welding quality determination method comprising: a drive step of driving at least one of a pair of electrodes by linearly moving in the same direction as a displacement direction of the electrodes; a press and electrical conduction step of press and electrically conducting a member to be welded by the pair of electrodes; a detection step of detecting an electrode-to-electrode displacement amount of the pair of electrodes; and a determination step of determining whether or not the welding quality is good based on the electrode-to-electrode displacement amount in electrical conduction detected by the detection step, wherein in the determination step, when the electrode-to-electrode displacement amount in a direction away from each other is below a first threshold value after the electrical conduction is started, it is determined that the welding quality is bad, when the electrode-to-electrode displacement amount in the direction away from each other exceeds the first threshold value after the electrical conduction is started, it is determined whether or not the electrode-to-electrode displacement amount in a direction close to each other of the pair of electrodes is above a second threshold value, when the electrode-to-electrode displacement amount in the direction close to each other is above the second threshold value, it is determined that the welding quality is bad, and when the electrode-to-electrode displacement amount in the direction close to each other is below the second threshold value, it is determined that the welding quality is good. ​ 2. The welding quality determination system according to claim 1, wherein ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ a driving step, wherein, ​ ​ ​ ​ ​ ​ ​ When the electrode interval displacement amount of the pair of electrodes in the approaching direction is equal to or greater than the second threshold value, the welding quality is determined to be bad, and when the electrode interval displacement amount of the pair of electrodes in the approaching direction is less than the second threshold value, the welding quality is determined to be good.

8. A nugget diameter estimation system that estimates a nugget diameter of spot welding in which a pair of electrodes pressurize and electrify a member to be welded, the nugget diameter estimation system comprising: a driving section that drives at least one of the pair of electrodes by linearly moving in the same direction as a displacement direction of the electrodes; a detecting section that detects an electrode interval displacement amount of the pair of electrodes; a determining section that determines good or bad of a welding quality based on the electrode interval displacement amount in electrification detected by the detecting section; and an estimating section that estimates a nugget diameter formed on the member to be welded based on the electrode interval displacement amount of the pair of electrodes in an approaching direction in electrification detected by the detecting section, when the electrode interval displacement amount of the pair of electrodes in a separating direction is equal to or less than a first threshold value after electrification is started, the determining section determines the welding quality to be bad, when the electrode interval displacement amount of the pair of electrodes in the separating direction exceeds the first threshold value after electrification is started, the determining section determines whether the electrode interval displacement amount of the pair of electrodes in the approaching direction is equal to or greater than a second threshold value, when the electrode interval displacement amount of the pair of electrodes in the approaching direction is equal to or greater than the second threshold value, the determining section determines the welding quality to be bad, and when the electrode interval displacement amount of the pair of electrodes in the approaching direction is less than the second threshold value, the determining section determines the welding quality to be good.

9. A nugget diameter estimation method that estimates a nugget diameter of spot welding, the nugget diameter estimation method comprising: a driving step, wherein, driving at least one of a pair of electrodes by linearly moving in the same direction as a displacement direction of the electrodes; a pressurizing and electrifying step in which the pair of electrodes pressurize and electrify a member to be welded; a detecting step in which an electrode interval displacement amount of the pair of electrodes is detected; a determining step in which good or bad of a welding quality is determined based on the electrode interval displacement amount in electrification detected by the detecting step; and an estimating step in which a nugget diameter formed on the member to be welded is estimated based on the electrode interval displacement amount of the pair of electrodes in an approaching direction in electrification detected by the detecting step; in the determining step, when the electrode interval displacement amount of the pair of electrodes in a separating direction is equal to or less than a first threshold value after electrification is started, the welding quality is determined to be bad, when the electrode interval displacement amount of the pair of electrodes in the separating direction exceeds the first threshold value after electrification is started, it is determined whether the electrode interval displacement amount of the pair of electrodes in the approaching direction is equal to or greater than a second threshold value, when the electrode interval displacement amount of the pair of electrodes in the approaching direction is equal to or greater than the second threshold value, the welding quality is determined to be bad, and when the electrode interval displacement amount of the pair of electrodes in the approaching direction is less than the second threshold value, the welding quality is determined to be good.

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