JP2010000528A - Spot welding method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spot welding method by which a workpiece is clamped by proper pressurizing force and welded even when the secular change is generated in a mechanism part or the like. <P>SOLUTION: In the spot welding method by which the spot welding of the workpiece is performed by flowing electric current between electrode tips in such a state that the workpiece is held between the electrode tips and pressurized by driving the pair of electrode tips with a servomotor, in a step S1, a disturbance torque value is determined by operating the servomotor and a determined disturbance torque value is recorded as the reference value d<SB>0</SB>. After elapse of optional time, in a step S2, a disturbance torque value d is determined by operating the servomotor and, in a step S3, the correction value of the pressurizing force: α=A(d-d<SB>0</SB>) is determined from the difference between the disturbance torque value d and the reference value d<SB>0</SB>. In a step S4, the set value of the command value of the pressurizing force to the workpiece with the electrode tips upon spot welding is shifted by the correction value α of the pressurizing force and is corrected to p'=p+α. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a spot welding method, and more particularly, to a spot welding method in which an electrode tip constituting a spot welding gun is driven by a servo motor and a workpiece (workpiece) is welded while being pressed between the electrode tips.

  In spot welding, if the workpiece pressure by the spot welding gun is insufficient, poor welding and spatter are caused. In order to prevent this, various measures for maintaining the applied pressure during spot welding within a predetermined range have been proposed.

  In the method disclosed in Patent Document 1, the current value of a servo motor that drives a spot welding gun to pressurize a workpiece is monitored during welding. When the current value of the servo motor falls below the reference value, it is determined that the applied pressure has decreased, the current command value of the servo motor is corrected, and the applied pressure is adjusted.

  In reality, however, the relationship between the servo motor current command value, that is, the force command value and the actual gripping force value, changes due to factors such as increased friction in the mechanism due to aging of the spot welding gun. Occurs. For this reason, the grip welding force of the spot welding gun is periodically measured manually when the spot welding system is stopped, and based on the measurement result, the force command value and the actually generated gripping force value are A readjustment operation for correcting the relational expression is generally performed.

  On the other hand, in the method disclosed in Patent Document 2, the pressure detector is disposed within the operation range of the articulated robot that moves the spot welding gun. Then, the pressure force actually generated by the spot welding gun is periodically measured by this pressure detector, and from the relationship with the current command value of the servo motor at that time, the current command value of the servo motor, that is, the pressure command value and The relational expression with the actual applied pressure is automatically corrected.

  In the method disclosed in Patent Document 3, the value of the current flowing through the servo motor when the spot welding gun is moved at a slow speed is measured. The measured current value is used to suppress a change in gripping pressure due to a change in the position of the spot welding gun.

Japanese Patent No. 3180530 Japanese Patent No. 3959302 Patent No. 3580330

  An object of the present invention is to provide a novel spot welding method capable of gripping and welding a workpiece with an appropriate pressurizing force even when a mechanical part or the like undergoes secular change.

  In order to achieve the above-described object, the spot welding method of the present invention uses a servo motor to drive a pair of electrode tips in accordance with a pressure command value corresponding to a set value, thereby sandwiching a workpiece between the electrode tips and applying pressure. In the spot welding method in which the workpiece is spot-welded by energizing between the electrode tips in a state of being performed, a first step of obtaining a disturbance torque value by operating a servo motor and recording the obtained disturbance torque value as a reference value Then, after an arbitrary time has elapsed from the first step, the second step of calculating the disturbance torque value by operating the servo motor and the correction of the applied pressure from the difference between the disturbance torque value obtained in the second step and the reference value The third step of obtaining the value and the command value of the workpiece pressing force by the electrode tip at the time of spot welding, not only the pressing force correction value from the set value, A fourth step of correcting the value obtained, characterized by having a.

  According to this configuration, the change in the disturbance torque value is used to evaluate the secular change such as an increase in the friction of the mechanical part, and the pressurization force is reduced so as to reduce the fluctuation due to the secular change of the work force during spot welding. The command value can be corrected. At this time, the disturbance torque value can be obtained by using a detection unit usually provided in the servo motor, so that it is not necessary to newly install a dedicated detection device.

  In the spot welding method of the present invention, following the first step, the servomotor is driven in accordance with a predetermined pressure command value, and the electrode tips are pressed against each other without interposing a workpiece therebetween. After the fifth step of detecting the position of the servo motor when the value is satisfied and recording it as the reference position, and the third step, the pressure command value in the fifth step is changed to the third step. The servo motor is driven according to the new pressure command value obtained by adding the pressure correction value obtained in the step, and the pressure command value is satisfied when the electrode tips are pressed without interposing a workpiece between them. When the difference between the servo motor position detected in the sixth step and the reference position exceeds a predetermined threshold, a warning is issued. Preferably further comprising the steps of the seventh, the.

  If there is a difference in the pressure applied to the electrode tip between the fifth step and the sixth step, there will be a difference in the amount of deflection of the holding member of the electrode tip, and the difference between the fifth step and the sixth step. A difference occurs in the position of the detected servo motor. Therefore, the validity of the pressure correction value can be evaluated by evaluating this difference.

  If the difference between the position of the servo motor detected in the sixth step and the reference position exceeds a predetermined threshold value, the spot welding process may not be executed. Thereby, it is possible to avoid the spot welding process being executed in a state where the pressure correction value is not valid.

  The torque characteristics of the servo motor change to some extent depending on the motor temperature. Therefore, the accuracy of the applied pressure correction value can be increased by correcting the disturbance torque value of the servo motor based on the temperature value detected by the temperature detector attached to the servo motor.

  In calculating the pressure correction value, a change in the motor current value of the servo motor may be used instead of the change in the disturbance torque value.

  According to the spot welding method of the present invention, it is possible to reduce the fluctuation caused by the secular change of the workpiece pressing force during spot welding by correcting the pressurization command value with the pressing force correction value. As a result, a spot welding method is provided in which a workpiece can be gripped and welded with an appropriate applied pressure even when a mechanical part or the like undergoes secular change.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of a spot welding gun system according to an embodiment of the present invention.

  The spot welding gun system is provided with a spot welding gun 10. The spot welding gun 10 has an arm 13 with a fixed electrode tip 11 attached to the tip and an arm 14 with a movable electrode tip 12 attached to the tip. The arm 14 can move relative to the arm 13 so that the distance between the fixed electrode tip 11 and the movable electrode tip 12 can be changed while keeping the fixed electrode tip 11 and the movable electrode tip 12 facing each other. It is connected to. A servo motor 20 that generates a driving force for moving the arm 14 relative to the arm 13 is connected to the arm 14 via a mechanism unit such as a gear mechanism (not shown). The fixed-side electrode tip 11 and the movable-side electrode tip 12 are configured so that a predetermined power for welding a workpiece (not shown) gripped therebetween can be energized therebetween.

  Well-known devices can be used for the mechanism for driving the movable electrode tip 12, the energization device between the fixed electrode tip 11 and the movable electrode tip 12, and the like. Since it is not directly related, detailed description is omitted. The spot welding gun system normally includes a transport mechanism for positioning the workpiece between the fixed electrode tip 11 and the movable electrode tip 12, a mechanism for controlling the position and posture of the spot welding gun 10, and the like. However, detailed description of these will also be omitted. In addition, a plurality of pairs of electrode chips may be provided, and they may be driven by a common servo motor or by separate servo motors. A configuration using the servo motor 20 and a pair of electrode chips will be described.

  A control device 30 is connected to the servo motor 20. In addition to the servo motor 20, the control device 30 may control the above-described energization device between the electrode chips, a workpiece transport mechanism, an arm mechanism attached to the spot welding gun 10, and the like. Further, the spot welding gun system shown in FIG. 1 may be a component of a production line that performs various processes other than spot welding on a workpiece, and the control device 30 performs other processes other than spot welding. It may have a control function of a mechanism for executing

  A teaching operation panel 40 is connected to the control device 30, and an operation and setting of the image display unit 41 for the operator to browse information in the control device 30 and the control device 30 are performed on the teaching operation panel 40. And an input unit 42 for performing the above. Furthermore, a peripheral device 50 that performs communication with the control device 30 via a communication interface provided in the control device 30 and performs dedicated processing may be provided. The peripheral device 50 includes, for example, a line control panel that manages a control device group for controlling various mechanisms of the production line as described above, and activates the control device and monitors the state of the control device. Alternatively, a workpiece position detection / monitoring mechanism may be included.

  FIG. 2 is a block diagram of a part mainly related to the present invention of the control system of the spot welding gun system of the present embodiment. As shown in the figure, the control device 30 has a servo CPU 31 for controlling the servo motor 20. In addition to the rotation mechanism unit 21, the servo motor 20 is provided with a detection unit for detecting and outputting the position, speed, etc., and FIG. 2 shows the position detection unit 22 in particular. Yes. The servo CPU 31 controls the power supplied to the rotation mechanism unit 21 via the servo amplifier 32 in accordance with the received signal from the detection unit. Thereby, the servo CPU 31 can execute position control and torque control of the servo motor 20 and can execute speed control. Such control of the servo motor 20 by the servo CPU 31 is realized by control using a signal from the detection unit of the servo motor 20 or a current value of the servo motor 20 as a feedback signal. May be used. In particular, in the present embodiment, the servo CPU 31 predicts disturbance torque generated by friction of various mechanism units by comparing the torque command value with an actual speed value obtained from a signal from the detection unit, etc. It has a function to detect.

  The control device 30 further includes a main CPU 35 that performs various overall controls including information exchange with the teaching operation panel 40 and peripheral devices 50. The main CPU 35 includes a calculation unit 36 that performs various calculation processes, and a storage unit 37 that stores information such as various settings, a program for executing a spot welding process, and the like. Although not shown, the servo CPU 31 may also have a storage unit or the like as necessary. Further, the servo CPU 31 and the main CPU 35 may not be configured by separate hardware, and may be realized by respective software executed by the same hardware.

  As described above, the servo CPU 31 can execute position control and torque control of the servo motor 20, and these control commands are given from the main CPU 35. The main CPU 35 can move the rotating mechanism 21 of the servo motor 20 and thus the movable electrode tip 12 to a predetermined position by a position command. Further, the generated torque of the servo motor 20 and thus the applied pressure by the fixed electrode tip 11 and the movable electrode tip 12 can be set to a predetermined value by the applied pressure command.

  Next, the operation will be described. The spot welding process is executed by the main CPU 35 of the control device 30 operating in accordance with a program stored in the storage unit 37. At this time, the control device 30 may cooperate with other control devices that control the workpiece conveyance mechanism or the like, or may respond to information from the workpiece position detection mechanism or the like. Since the operation is not directly related to the present invention, detailed description is omitted.

  First, the workpiece is positioned between the movable electrode tip 12 and the fixed electrode tip 11 in a state where the movable electrode tip 12 is disposed at a predetermined position sufficiently away from the fixed electrode tip 11. . From this state, the servomotor 20 moves the movable electrode tip 12 to a predetermined position where the workpiece is sandwiched between the fixed electrode tip 11 and the servomotor 20 so that a predetermined pressure is applied to the workpiece. Torque is generated by the motor 20. In this manner, the workpiece is gripped between the fixed electrode tip 11 and the movable electrode tip 12 and a predetermined pressure is applied between the fixed electrode tip 11 and the movable electrode tip 12. A predetermined energization is performed on the workpiece, and the workpiece is spot welded.

  In order to perform desired spot welding satisfactorily as described above, the pressure command value p when energizing the electrode tip is appropriately set in advance. This pressure command value p can be set appropriately in accordance with the type of workpiece when designing a spot welding gun system, and is also adjusted appropriately through tests after each spot welding gun system is installed. May be.

  However, in a spot welding gun system, generally, the pressure applied to the workpiece when it is operated with the same pressure command value p changes over time due to aging such as increased friction of the mechanism. Therefore, in the present embodiment, a correction operation for the influence of such aging is performed.

A flowchart of this correction operation is shown in FIG. First, in step S1 (first step), a reference value d ₀ of disturbance torque applied to the servomotor 20 is acquired. That is, as described above, the servo CPU 31 has a function of predicting and detecting disturbance torque, and using this, obtains a disturbance torque value and sets it as a reference value d ₀ . More specifically, the servo CPU 31 executes a control program prepared for measuring disturbance torque to operate the servo motor 20 in a predetermined pattern, and compares the torque command value and the actual speed value at that time. The disturbance torque value is calculated from the above. Any known configuration may be used for calculating the disturbance torque value. The reference value d ₀ calculated by the servo CPU 31 is transmitted to the main CPU 36 and stored in the recording unit 37.

  At this time, when the movable electrode tip 12 comes into contact with the fixed electrode tip 11, the torque generated by the contact becomes dominant, and it becomes difficult to accurately measure a smaller disturbance torque due to friction of the mechanism portion or the like. For this reason, it is preferable that the disturbance torque is measured by causing the movable electrode tip 12 to perform a certain operation within a range where the movable electrode tip 12 does not contact the fixed electrode tip 11.

  Next, after an arbitrary time has elapsed, in step S2 (second step), the disturbance torque value d is acquired again. In this case, it is preferable to obtain the disturbance torque value d under the same conditions by executing the same control program as that executed in step S1.

Next, in step S3 (third step), the difference between the disturbance torque value d ₁ obtained in the reference value d ₀ and Step S2 obtained in step S1, pressure correction value by multiplying the proportional constant A thereto α = A (d−d ₀ ) is calculated. In step S4 (fourth step), the pressure command value at the time of spot welding is corrected by the pressure correction value α, and p ′ = p + α is set again.

  Thus, at the time of subsequent spot welding, by using the corrected pressure command value p ′, it is possible to suppress the workpiece pressure from fluctuating due to secular change and to apply an appropriate pressure to the workpiece. Thus, welding can be performed satisfactorily. That is, the pressure of the workpiece when it is operated with the same pressure command value that occurs due to a secular change such as an increase in friction of the mechanism during the time elapsed from the time of step S1 to the time of step S2. The change is substantially proportional to the difference between the disturbance torque values measured in step S1 and step S2. Therefore, according to this difference, the pressurization command value is corrected by the change in the pressurizing force caused by the secular change, thereby canceling the influence of the secular change and reducing the fluctuation due to the secular change of the workpiece. be able to.

  In addition, it is preferable to perform step S1 at an early stage after the spot welding gun system is constructed. Alternatively, when the pressure command value p is adjusted by a test or the like after the construction, the step S1 is performed at a time as close as possible thereafter. It is preferable to execute S1. Accordingly, an appropriate welding process can be realized by performing correction so as to reproduce the state based on the state in which the pressurizing force of the workpiece is most appropriately adjusted.

  Steps S2 and S3 can be configured to be executed by the operator via the teaching operation panel 40 at an appropriate time, and may be repeated periodically. In particular, the control device 30 is preferably configured to automatically acquire time information and execute it at an appropriate time. As a result, correction can be performed at appropriate intervals without delaying the correction time, and thereby, fluctuation due to secular change in gripping pressure can be suppressed.

  As described above, according to the present embodiment, even if the mechanical unit or the like undergoes secular change, the pressurization command value is corrected accordingly to suppress the change in the pressurizing force of the workpiece. Thereby making the welding process feasible. At this time, in the configuration of the present embodiment, the correction process of the pressurization command value can be automatically performed by the control device 30. Therefore, compared with the case where the readjustment work is performed manually, the work man-hour is reduced, System operating costs can be reduced. Moreover, since the correction can be performed without completely shutting down the system, the correction can be performed periodically at an appropriate interval, thereby increasing the fluctuation due to the secular change of the gripping pressure. Can be suppressed. Furthermore, the correction value is obtained using the detection unit originally provided in the servo motor 20 without requiring a dedicated detection mechanism for the correction process, so that the system is not complicated and the cost is low. It is feasible.

  The above-described embodiments are examples of the present invention, and various modifications can be made within the scope of the present invention defined in the claims. For example, in the above-described embodiment, the example in which the difference in the disturbance torque value is used to evaluate the influence of the aging of the pressurizing force due to the passage of time from the time of step S1 to the time of step S2 is shown. A difference in motor current value of the motor 20 may be used. That is, normally, the motor current value is proportional to the torque generated by the servo motor 20. Therefore, if the difference of the motor current value when the servo motor 20 is driven under the same conditions at the time of step S1 and the time of step S2, the difference value is a fluctuation caused by the secular change value. Proportional to quantity. Therefore, even if the motor current value is used instead of the disturbance torque value, it is possible to realize correction of the aging change of the applied pressure.

  Further, the validity of the correction of the pressing operation may be further evaluated using the detection signal of the position detection unit 22 of the servo motor 20. A flowchart of such processing is shown in FIG. In FIG. 4, the same parts as those in FIG. 3 are denoted by the same reference numerals, and detailed description thereof is omitted.

In the process shown in FIG. 4, following step S1, in step S11 (fifth step), a predetermined pressure command value _{pc 0} is output from the main CPU 36 to the servo CPU 31. As a result, the movable electrode tip 12 is pressed against the fixed electrode tip 11 with a pressure corresponding to the pressure command value. This operation is performed without sandwiching the workpiece between the movable electrode tip 12 and the fixed electrode tip 11. Then, the position of the servo motor 20 when the servo CPU 31 determines that the pressure command value _{pc 0} is satisfied is detected by the position detector 22 and recorded as the reference position f ₀ .

Next, following steps S2 and S3 performed after an arbitrary time has elapsed, in step S12 (sixth step), the pressure command value p _c0 in step S11 is corrected by the pressure correction value α. In accordance with the value p _c = p _c0 + α, pressurization is performed as in step S11. The position detector 22 detects the position f of the servo motor 20 when it is determined in the servo CPU 31 that the pressure command value _pc is satisfied.

At this time, if the actual pressurizing force is different between step S11 and step S12, the stress applied to the arms 13, 14 and the like of the spot welding gun 10 will be different, and the amount of deflection will be different. Occurs. As a result, a difference occurs between the positions f ₀ and f. Therefore, in step S13, it determines whether the difference between the position f ₀ and f has exceeded a predetermined threshold value f _b.

If it is determined in step S13 that the difference between the positions f ₀ and f is smaller than the predetermined threshold f _b , the process proceeds to step S4. That is, it is determined that the pressurizing force correction value α is appropriate, and the pressurizing force is corrected by the pressurizing force correction value α as in the case described with reference to FIG.

On the other hand, if the difference between the positions f ₀ and f exceeds the predetermined threshold value f _b , the pressure correction value α may not be valid. A warning is issued in step (1). The warning can be performed by being displayed on the screen display unit 41 of the teaching operation panel 40, or may be performed by using an alarm device that is provided as the peripheral device 50 and issues an alarm by sound or light.

Subsequent to step S14, the pressure correction value α is corrected to 0 in step S15, and then the process proceeds to step S4. That is, the use of the pressure correction value α is excluded. The pressing force correction value α may be corrected to another predetermined value so as not to cause a problem. Alternatively, when the difference between the positions f ₀ and f exceeds a predetermined threshold value f _b , the spot welding process may not be performed thereafter.

  Further, as shown in FIG. 5, a temperature detector 25 for measuring the temperature is attached to the servo motor 20, and the detected value is taken into the main CPU 35 and used to calculate the pressure correction value α. Also good. That is, the grease viscosity of the movable part of the spot welding gun 10 or the torque characteristics of the servo motor is usually affected to some extent by the temperature. Can do.

Therefore, other than the motor temperature by calculating the disturbance torque at the same conditions, as shown in FIG. 6, we obtain the correlation between the motor temperature T and the disturbance torque value d _T in advance. Further, the motor temperature T ₀ of the servo motor 20 when the spot welding gun 10 is normally used, that is, when spot welding is executed, is obtained in advance. From these, the disturbance torque value d _T0 when the motor temperature T _0, the disturbance torque value d _T when the motor temperature T, in general, by using the function _{f (T), d T =} d T0 × It can be expressed as f (T).

Since the applied pressure correction value α is applied to the applied pressure when spot welding is performed, the disturbance torque value d and its reference value d ₀ used to calculate it are determined when spot welding is performed. It is preferable to use the one calculated in the state of the motor temperature T ₀ . On the other hand, since the disturbance torque value d and its reference value d ₀ are not calculated at the time of spot welding, the motor temperature T at the time of calculation usually does not coincide with the motor temperature T ₀ . Therefore, a correction value d _T0 = dT / corresponding to the disturbance torque value in the motor temperature T ₀ state from the disturbance torque value d _T obtained in the motor temperature T state using the function f (T) described above. f (T) is obtained and used to calculate the pressure correction value α. Thereby, the accuracy of the pressure correction value α can be increased.

In this case, the same applies to the pressure-force command value p _c0, p _c of the foregoing for evaluating the validity of the pressure correction value alpha, it may be considered the influence of motor temperature. That is, the pressure correction value α is set assuming the state of the motor temperature T ₀ when spot welding is performed as described above. Therefore, even pressure-force command value p _c0, p _c for assessing pressure correction value alpha, should be treated as a value in the state of the motor temperature T _0. On the other hand, the motor temperature when the pressurization is performed actually in accordance with pressure-force command value p _c0, p _c does not match the T _0. Therefore, when performing the pressurization for the evaluation of pressure correction value α is a pressure-force command value p _c0, p _c, is preferable to use a command value corrected to a value of the motor temperature at that time.

For this reason, as shown in FIG. 7, by obtaining the pressure command value p at which the pressure force actually generated is the same under the same conditions except for the motor temperature, the motor temperature T and the pressure command value p _T The correlation is obtained in advance. Then, the pressure-force command value p _T is general, for pressure-force command value p _T0 when the motor temperature T _0, using the function _{g (T), p T =} p T0 × g (T) It can be expressed as. In this manner, the pressure command value converted into the value at the motor temperature T when pressurizing can be obtained using the function g (T), and by using the pressure command value, the pressure command value can be obtained. The reliability of the evaluation of the pressure correction value α can be improved.

It is a typical lineblock diagram of the spot welding gun system of one embodiment of the present invention. It is a block diagram of the part mainly related to this invention of the control system of the spot welding gun system of FIG. It is a flowchart of the correction | amendment operation | movement with respect to the influence by a secular change of the applied pressure setting at the time of spot welding. It is a flowchart of the modification of FIG. It is a block diagram of the modification of FIG. It is a graph which shows the correlation of motor temperature T and disturbance torque value d. It is a graph which shows the correlation of motor temperature T and the applied pressure command value p.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Spot welding gun 11 Fixed side electrode tip 12 Movable side electrode tip 20 Servo motor 30 Control apparatus

Claims (5)

By driving the pair of electrode chips by a servo motor in accordance with a pressure command value corresponding to the set value, the work piece is energized between the electrode chips in a state where the work piece is sandwiched and pressed between the electrode chips. In the spot welding method of spot welding,
A first step of operating the servo motor to obtain a disturbance torque value, and recording the obtained disturbance torque value as a reference value;
A second step of determining a disturbance torque value by operating the servo motor after an arbitrary time has elapsed from the first step;
A third step of obtaining a pressure correction value from a difference between the disturbance torque value obtained in the second step and the reference value;
A fourth step of correcting a command value of the pressing force of the workpiece by the electrode tip at the time of spot welding to a value shifted from the set value by the pressing force correction value;
A spot welding method characterized by comprising: Following the first step, the servo motor is driven in accordance with a predetermined pressure command value, and the servo motor is in a state where the electrode chips are pressed against each other to satisfy the pressure command value. A fifth step of detecting the position of and recording as a reference position;
Subsequent to the third step, the servo motor according to a new pressure command value obtained by adding the pressure correction value obtained in the third step to the pressure command value in the fifth step. A sixth step of detecting the position of the servo motor when the electrode tips are pressed against each other and the pressure command value is satisfied;
A seventh step of issuing a warning when a difference between the position of the servo motor detected in the sixth step and the reference position exceeds a predetermined threshold;
The spot welding method according to claim 1, further comprising:   3. The spot welding method according to claim 2, wherein the spot welding process is not executed when a difference between the position of the servo motor detected in the sixth step and the reference position exceeds a predetermined threshold.   The spot welding method according to any one of claims 1 to 3, wherein a correction is made to a disturbance torque value of the servo motor based on a temperature value detected by a temperature detector attached to the servo motor. By driving the pair of electrode chips by a servo motor in accordance with a pressure command value corresponding to the set value, the work piece is energized between the electrode chips in a state where the work piece is sandwiched and pressed between the electrode chips. In the spot welding method of spot welding,
A first step of operating the servo motor to measure a motor current value and recording the measured motor current value as a reference value;
A second step of obtaining a motor current value by operating the servo motor under the same condition as the first step after an arbitrary time has elapsed from the first step;
A third step of obtaining a pressure correction value from a difference between the motor current value obtained in the second step and the reference value;
A fourth step of correcting a command value of the pressing force of the workpiece by the electrode tip at the time of spot welding to a value shifted from the set value by the pressing force correction value;
A spot welding method characterized by comprising:

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