Patent Publication Number: US-2021187654-A1

Title: Spot welding method

Description:
BACKGROUND ART 
     In the case of welding a plurality of metal plates to each other, spot welding using a spot welding apparatus is performed. In spot welding, power is distributed between a pair of electrode chips in a state in which the plurality of metal plates is sandwiched between the pair of electrode chips, and in this manner, a nugget is generated between the plurality of metal plates to weld the plurality of metal plates. 
     In a technique described in Patent Document 1, a plurality of DC micropulses is applied to a plurality of metal plates in a state in which the plurality of metal plates is sandwiched by a pair of electrodes, and in this manner, the plurality of metal plates is welded to each other. 
     In spot welding, in a case where the time of power distribution to between the pair of electrode chips is short, the nugget cannot be grown to a size necessary for welding and welding cannot be performed in some cases. On the other hand, in a case where the time of power distribution between the pair of electrode chips is long, the nugget grows too much and protrudes from a corona bond (an unmelted pressure-contact portion formed outside the nugget) formed between the plurality of metal plates in some cases. As a result, the nugget is exposed, and spatter might be caused. Due to such a situation, it has been demanded in spot welding for welding to be reliably performed while occurrence of the spatter is reduced. 
     Patent Document 1: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2013-501628 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     At the step of manufacturing a vehicle body of an automobile, it is necessary to weld three or more metal plates with different thicknesses in some cases. In this case, a contact resistance between thick plates is greater than a contact resistance between a thin plate and a thick plate, and therefore, a nugget formed between the thick plates is grown faster than a nugget formed between the thin plate and the thick plate. Thus, when power distribution is continued until the nugget between the thin plate and the thick plate is sufficiently grown, the nugget between the thick plates grows too much, and for this reason, spatter might be caused. 
     An object of the present invention is to provide a spot welding method configured so that a workpiece can be reliably joined while occurrence of spatter is reduced. 
     Means for Solving the Problems 
     (1) The spot welding method according to the present invention is the method for joining a workpiece (e.g., a later-described workpiece W) by a supply of welding current. The welding current has such a pulse-shaped waveform that a peak state in which the welding current reaches or is maintained within a set peak current range and a non-peak state in which the welding current increases to the peak current range again after having decreased from the peak current range to a bottom current are alternately achieved. In the non-peak state, the current control of increasing the welding current to the peak current range is started in a case where an effective value (Irms) of the welding current reaches within a predetermined target range (e.g., a later-described target effective value range). 
     (2) In this case, if the current control is not started within a predetermined time, the supply of welding current is preferably stopped and occurrence of an abnormality is preferably informed. 
     (3) In this case, the workpiece is preferably a multilayer body configured such that three or more metal plates (e.g., later-described metal plates W 1 , W 2 , W 3 ) are stacked on each other, and at least one metal plate of the three or more metal plates is preferably formed to have a thickness different from the thicknesses of the other metal plates. 
     Effects of the Invention 
     (2) In the spot welding method of the present invention, the welding current having such a pulse-shaped waveform that the peak state in which the welding current reaches or is maintained within the peak current range and the non-peak state in which the welding current increases to the peak current range again after having decreased from the peak current range to the bottom current are alternately achieved is supplied to join the workpiece. In the present invention, in the non-peak state, the current control of increasing the welding current to the peak current range is started in a case where the effective value of the welding current reaches within the predetermined target range. In other words, in the present invention, the start of the current control for a subsequent cycle is put on hold until the effective value of the welding current reaches within the target range. With this configuration, the current control can be started at such proper timing that occurrence of spatter is reduced while energy for forming a proper size of a nugget by the welding current is supplied to the workpiece. Thus, according to the present invention, the workpiece can be reliably joined while occurrence of the spatter is reduced. 
     (2) In the present invention, in a case where the current control of increasing the welding current to the peak current range is not started within the predetermined time, the supply of welding current is stopped and occurrence of the abnormality is informed. With this configuration, continuation of welding in a case where some types of abnormalities are caused and manufacture of a low-quality product can be prevented. 
     (3) In the spot welding method of the present invention, the workpiece is the multilayer body configured such that the three or more metal plates are stacked on each other, and at least one metal plate has the thickness different from those of the other metal plates. In the peak state, the welding current reaches or is maintained within the peak current range, and therefore, growth of a nugget is accelerated between the metal plates. While the start of the current control for the subsequent cycle is put on hold until the effective value of the welding current reaches within the target range in the non-peak state as described above, the nugget is cooled by heat dissipation. As described above, the contact resistance between the thick plates is greater than that between the thin plate and the thick plate, and growth of the nugget between the thick plates is faster than that between the thin plate and the thick plate. Moreover, heat dissipation between the thick plates is greater than that between the thin plate and the thick plate. For this reason, in the present invention, the workpiece is the multilayer body of the metal plates as described above, and the start of the current control is put on hold until the effective value of the welding current reaches within the target range in the non-peak state. Thus, while a proper level of energy is supplied such that the nuggets are sufficiently grown between the thin plate and the thick plate and between the thick plates, the time for cooling the nugget between the thick plates can be ensured such that occurrence of the spatter between the thick plates is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a configuration of a welding system to which a spot welding method according to one embodiment of the present invention is applied; 
         FIG. 2  is a diagram showing a circuit configuration of a welding power circuit; 
         FIG. 3  is a graph showing a relationship between an AC voltage input from an inverter circuit to a transformer and a welding current applied to a pair of electrode chips in the welding power circuit; 
         FIG. 4  is a view schematically showing the section of a workpiece during welding, 
         FIG. 4  showing the view in a state in which the welding current is applied to the workpiece while the workpiece is sandwiched and pressurized by the upper electrode chip and the lower electrode chip; 
         FIG. 5  is a flowchart showing the specific steps of welding current control in a control apparatus; 
         FIG. 6  is a graph showing the waveform of the welding current achieved by the welding current control of  FIG. 5 ; 
         FIG. 7  is a flowchart showing the specific steps of current control processing; and 
         FIG. 8  is a flowchart showing the specific steps of effective value control processing. 
     
    
    
     PREFERRED MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, one embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is a view showing a configuration of a welding system S to which a spot welding method according to the present embodiment is applied. 
     The welding system S includes a spot welding apparatus  1  as a welding gun, a workpiece W as a multilayer body of metal plates joined to each other by the spot welding apparatus  1 , and a robot  6  supporting the spot welding apparatus  1 . 
     The workpiece W is a multilayer body configured such that a plurality of metal plates is stacked on each other. In the present embodiment, a case where a first metal plate W 1 , a second metal plate W 2 , and a third metal plate W 3  as three metal plates are stacked on each other in this order from the top to the bottom to form a multilayer body as the workpiece W will be described, but the present invention is not limited to such a case. The number of metal plates forming the workpiece W may be two or four or more. Hereinafter, a case where the thickness of the first metal plate W 1  is smaller than each of the thicknesses of the second metal plate W 2  and the third metal plate W 3  will be described, but the present invention is not limited to such a case. It may only be required that at least one metal plate of these metal plates W 1  to W 3  has a thickness different from those of the other metal plates. 
     The robot  6  includes a robot body  60  attached to a floor surface, an articulated arm  61  pivotally supported on the robot body  60 , and a robot control apparatus  62  that controls the robot  6 . The articulated arm  61  includes a first arm portion  611  pivotally supported on a base end side by the robot body  60 , a second arm portion  612  pivotally supported on a base end side by the first arm portion  611 , a third arm portion  613  pivotally supported on a base end side by the second arm portion  612 , and a fourth arm portion  614  pivotally supported on a base end side by the third arm portion  613  and attached to the spot welding apparatus  1  on a tip end side. 
     The robot control apparatus  62  drives a plurality of motors provided at the robot body  60  and the articulated arm  61  to drive each of the arm portions  611  to  614 , thereby controlling the position and orientation of the spot welding apparatus  1  attached to the fourth arm portion  614  and moving later-described electrode chips  21 ,  26  provided at the spot welding apparatus  1  to a joint portion of the workpiece W. 
     The spot welding apparatus  1  includes a welding power circuit  3  as a welding current supply source, a gun body  2  on which a later-described upper electrode chip movement mechanism  4  and part of the welding power circuit  3  are mounted, the upper electrode chip  21  and the lower electrode chip  26  as a pair of electrodes, an upper electrode chip support portion  22 , an upper adaptor body  23 , a gun arm  25 , a lower electrode chip support portion  27 , and a lower adaptor body  28 . 
     The upper electrode chip support portion  22  is in the shape of a rod extending along the vertical direction, and the upper electrode chip  21  is attached to a tip end portion of the upper electrode chip support portion  22 . The upper adaptor body  23  is in a columnar shape, and connects the gun body  2  and the upper electrode chip support portion  22  to each other. The upper adaptor body  23  is, relative to the gun body  2 , provided slidably along a sliding direction parallel with the axis of the upper electrode chip support portion  22 . 
     The gun arm  25  extends to curve from the gun body  2  to below the upper electrode chip  21  in the vertical direction. The lower electrode chip support portion  27  is in the shape of a rod coaxial with the upper electrode chip support portion  22 , and the lower electrode chip  26  is attached to a tip end portion of the lower electrode chip support portion  27 . The lower adaptor body  28  is in a columnar shape, and connects a tip end portion of the gun arm  25  and the lower electrode chip support portion  27  to each other. As shown in  FIG. 1 , the lower electrode chip  26  is supported by the lower electrode chip support portion  27  to face the upper electrode chip  21  with a predetermined clearance along the axes of the chip support portions  22 ,  27 . 
     The upper electrode chip movement mechanism  4  includes a cylinder, a control apparatus therefor, etc., and together with the upper electrode chip support portion  22  and the upper electrode chip  21 , moves the upper adaptor body  23  back and forth along the sliding direction. With this configuration, the upper electrode chip  21  can contact an upper surface of the workpiece W with the lower electrode chip  26  contacting a lower surface of the workpiece W, and the workpiece W can be further sandwiched and pressurized by these electrode chips  21 ,  26 . 
       FIG. 2  is a diagram showing a circuit configuration of the welding power circuit  3 . The welding power circuit  3  includes a welding control circuit  3   a,  a DC welding transformer  3   b,  power cables  3   c,  and a current sensor  3   d.  The welding power circuit  3  is connected to the upper electrode chip  21  and the lower electrode chip  26  through power lines L 1 , L 2 . As shown in  FIG. 1 , the DC welding transformer  3   b  and the current sensor  3   d  of the welding power circuit  3  configured as described above are mounted on the gun body  2 . Moreover, the welding control circuit  3   a  of the welding power circuit  3  is mounted on a base separated from the gun body  2 , and is connected to the DC welding transformer  3   b  through the power cables  3   c.  With this configuration, the weight of the gun body  2  can be reduced. 
     The welding control circuit  3   a  includes a converter circuit  31 , an inverter circuit  32 , and a control apparatus  33 . The DC welding transformer  3   b  includes a transformer  34  and a rectification circuit  35 . 
     The converter circuit  31  performs full-wave rectification for a three-phase power input from a three-phase power source  30 , thereby converting the three-phase power into a DC power and supplying the DC power to the inverter circuit  32 . 
     The inverter circuit  32  converts the DC power input from the inverter circuit  32  into a single-phase AC power, thereby outputting the single-phase AC power to the transformer  34  through the power cables  3   c.  More specifically, the inverter circuit  32  includes four bridge-connected switching elements. The inverter circuit  32  turns on or off these switching elements according to a gate drive signal transmitted from a gate drive circuit mounted on the control apparatus  33 , thereby converting the DC power into the single-phase AC power. 
     The transformer  34  transforms the AC power input from the inverter circuit  32 , thereby outputting the transformed AC power to the rectification circuit  35 . The rectification circuit  35  rectifies the AC power input, from the transformer  34 , thereby outputting a DC power to between the electrode chips  21 ,  26  each connected to the power lines L 1 , L 2 . For example, a known full-wave rectification circuit including a combination of two rectification diodes  351 ,  352  and a center tap  353  is used as the rectification circuit  35 . 
     The current sensor  3   d  detects a welding current supplied from the welding power circuit  3  to the chips  21 ,  26 . The current sensor  3   d  is, for example, provided on the power line L 1  connecting the rectification circuit  35  and the upper electrode chip  21  to each other, and to the control apparatus  33 , transmits a current detection signal according to the level of the welding current flowing in the power line L 1 . 
     The control apparatus  33  includes, for example, a microcomputer that executes later-described welding current control by means of the current detection signal transmitted from the current sensor  3   d  and the gate drive circuit that generates the gate drive signal according to an arithmetic processing result of the microcomputer to transmit the gate drive signal to the inverter circuit  32 . 
       FIG. 3  is a graph showing a relationship between an AC voltage Vt input from the inverter circuit  32  to the transformer  34  and the welding current Iw applied to the electrode chips  21 ,  26  in the welding power circuit  3  as described above. 
     When the inverter circuit  32  is driven, the AC voltage Vt in the shape of a square wave as shown in  FIG. 3  is outputted from the inverter circuit  32 . The AC voltage output from the inverter circuit  32  is transformed in the transformer  34 , and is further rectified in the rectification circuit  35 . Then, the DC welding current Iw is applied to the workpiece W through the electrode chips  21 ,  26 . 
     As shown in  FIG. 3 , the welding current Iw increases as a duty cycle increases, the duty cycle being the ratio of a pulse width PW as a period in which the AC voltage Vt is Hi or Lo to a predetermined carrier cycle T. As described later with reference to  FIGS. 5 and 6 , the control apparatus  33  determines the pulse width PW according to a known feedback control rule such as PI control such that the output current of the welding power circuit  3  detected by the current sensor  3   d  reaches a target current set by not-shown processing, and performs ON/OFF drive of the plurality of switching elements in the inverter circuit  32  by PWM control with the duty cycle set according to the pulse width PW. 
     Next, the steps of the spot welding method for joining the workpiece W by the welding system S as described above will be described. 
     First, as shown in  FIG. 1 , the robot control apparatus  62  drives the robot body  60  and the articulated arm  61 , thereby controlling the position and posture of the spot welding apparatus  1  such that the workpiece W is arranged between the upper electrode chip  21  and the lower electrode chip  26 . At this point, the robot control apparatus  62  controls the position and posture of the spot welding apparatus  1  such that the lower electrode chip  26  contacts a lower surface of the third metal plate W 3  of the workpiece W. 
     Next, as shown in  FIG. 4 , the upper adaptor body  23  is slid using the upper electrode chip movement mechanism  4  such that the upper electrode chip  21  approaches the lower electrode chip  26 . When the upper electrode chip  21  approaches the lower electrode chip  26  and comes into contact with an upper surface of the first metal plate W 1 , the workpiece W is sandwiched and pressurized by the upper electrode chip  21  and the lower electrode chip  26 . 
     Next, the control apparatus  33  of the welding power circuit  3  executes the welding current control by the steps described with reference to  FIG. 5  while maintaining a state in which the workpiece W is pressurized from both sides by the electrode chips  23 ,  26 , and applies the pulse-shaped welding current to between the upper electrode chip  21  and the lower electrode chip  26 . In this manner, as shown in  FIG. 4 , a first nugget N 1  is formed between the first metal plate W 1  and the second metal plate W 2 , and a second nugget N 2  is formed between the second metal plate W 2  and the third metal plate W 3 . Thus, the first to third metal plates W 1  to W 3  are welded to each other. 
       FIG. 5  is a flowchart showing the specific steps of the welding current control in the control apparatus  33 .  FIG. 6  is a graph showing the waveform of the welding current achieved by the welding current control of  FIG. 5 . As shown in  FIG. 6 , the welding current generated by the welding current control of  FIG. 5  has such a pulse-shaped waveform that a peak state in which the welding current reaches or is maintained within a set peak current range and a non-peak state in which the welding current increases to the peak current range again after having decreased to a bottom current (e.g., zero) from the peak current range are alternately achieved. 
     First, at S 1 , the control apparatus  33  executes current control processing, and proceeds to S 2 . As described later in detail with reference to  FIG. 7 , in this current control processing, the control apparatus  33  maintains the peak state for a predetermined time after having increased the welding current from the bottom current to the peak current range. 
     At S 2 , the control apparatus  33  determines whether or not a predetermined slope time has elapsed. As shown in  FIG. 5 , this slope time is time obtained in such a manner that current rise time which is time until the welding current reaches the upper limit, of the peak current range from the bottom current and peak holding time which is time for which the welding current is maintained within the peak current range are added up, and is set in advance. The control apparatus  33  returns to S 1  to continuously execute the current control processing in a case where a determination result at S 2  is NO, and proceeds to S 3  in a case where the determination result at S 2  is YES. 
     At S 3 , the control apparatus  33  executes effective value control processing, and proceeds to S 4 . As described later in detail with reference to  FIG. 3 , in this effective value control processing, the control apparatus  33  puts execution of the current control processing on hold across a standby time (see  FIG. 6 ) set based on an effective value of the welding current. At S 4 , the control apparatus  33  determines whether or not a set power distribution time has elapsed after the start of the welding current control of  FIG. 5 . This power distribution time is equivalent to time taken to join a single spot of the workpiece W by the spot welding apparatus  1 , and is set in advance. The control apparatus  33  returns to S 1  to execute the current control processing again in a case where a determination result at S 4  is NO. The control apparatus  33  ends the processing of  FIG. 5  to start joining a next spot of the workpiece W in a case where the determination result at S 4  is YES. 
     As described above, in the welding current control, the control apparatus  33  repeatedly executes the current control processing (see S 1 ) and the effective value control processing (see S 3 ) across the power distribution time, thereby applying the welding current with the pulse-shaped waveform as shown in  FIG. 6  to between the electrode chips  21 ,  26 . 
       FIG. 7  is a flowchart showing the specific steps of the current control processing. First, at S 11 , the control apparatus  33  acquires, using the current detection signal transmitted from the current sensor  3   d,  a present current value Ipv as a present welding current value, and proceeds to S 12 . At S 12 , the control apparatus  33  sets a target current value Isp equivalent to a target welding current value, and proceeds to S 13 . As shown in  FIG. 6 , the target current value Isp is set between predetermined current rise slopes or between the upper limit and the lower limit of the peak current range. 
     At S 13 , the control apparatus  33  calculates an effective value Irms of the welding current by using the present current value Ipv acquired at S 11 , and proceeds to S 14 . More specifically, the control apparatus  33  calculates the root-mean-square of the present current value Ipv across time elapsed from the start of the welding current control of  FIG. 5  to a present point of time, thereby calculating the effective value Irms. 
     At S 14 , the control apparatus  33  calculates a current deviation Idev by subtracting the present current value Ipv acquired at S 11  from the target current value Isp set at S 12 , and proceeds to S 15 . 
     At S 15 , the control apparatus  33  calculates the pulse width PW according to the feedback control rule (specifically, e.g., a PI control rule) based on the current deviation Idev calculated at S 14  such that the current deviation Idev reaches zero, and proceeds to S 16 . More specifically, the control apparatus  33  adds up the result of multiplication of the current deviation Idev by a predetermined proportional gain Kp and the result of multiplication of an integral value of the current deviation Idev by a predetermined integral gain Ki, thereby calculating the pulse width PW. 
     At S 16 , the control apparatus  33  starts a PW counter, and proceeds to S 17 . At S 17 , the control apparatus  33  turns on the switching elements provided in the inverter circuit  32 , and proceeds to S 13 . At S 18 , the control apparatus  33  determines whether or not the value of the PW counter reaches zero, i.e., whether or not time equivalent to the pulse width PW has elapsed after the start of the PW counter at S 16 . The control apparatus  33  returns to S 17  to keep the switching elements ON in a case where a determination result at S 18  is NO, and proceeds to S 19  in a case where the determination result at S 18  is YES. 
     At S 19 , the control apparatus  33  turns off the switching elements provided in the inverter circuit  32 , and proceeds to S 20 . At S 20 , the control apparatus  33  determines whether or not the set carrier cycle has elapsed after the switching elements have been turned on at S 17 . The control apparatus  33  returns to S 19  to keep the switching elements OFF in a case where a determination result at S 20  is NO, and proceeds to S 2  of  FIG. 5  in a case where the determination result at S 20  is YES. 
       FIG. 8  is a flowchart showing the specific steps of the effective value control processing. At S 31 , the control apparatus  33  starts a standby time counter to measure a power distribution standby time (see  FIG. 6 ) equivalent to an execution time of the effective value control processing, and proceeds to S 32 . At S 32 , the control apparatus  33  acquires the present current value Ipv by using the current detection signal transmitted from the current sensor  3   d,  and proceeds to S 33 . At S 33 , the control apparatus  33  calculates, using the present current value Ipv acquired at S 32 , the effective value Irms of the welding current by the same step as in S 13 , and proceeds to S 34 . 
     At S 34 , the control apparatus  33  determines whether or not the effective value Irms calculated at S 33  reaches within a target effective value range set between a predetermined lower limit Itrg_min and a predetermined upper limit Itrg_max (Itrg_min≤Irms≤Itrg_max). In a case where a determination result at S 34  is YES, the control apparatus  33  starts a subsequent cycle of the current control processing, and proceeds to S 4  of  FIG. 5  to increase the welding current to the peak current range again. 
     In a case where the determination result at S 34  is NO, the control apparatus  33  proceeds to S 35 , and determines whether or not the value of the standby time counter started at S 31  is zero, i.e., whether or not the power distribution standby time has exceeded a predetermined time. In a case where a determination result at S 35  is NO, the control apparatus  33  returns to S 32 , and waits to start the subsequent cycle of the current control processing until the effective value Irms decreases and reaches within the target effective value range. In a case where the determination result at S 35  is YES, i.e., a case where the effective value Irms does not reach within the target effective value range within the predetermined time, the control apparatus  33  proceeds to S 36  to inform an operator of occurrence of some types of abnormalities by, e.g., illumination of a warning light and end the welding current control. 
     Next, the waveform of the welding current generated by execution of the welding current control as described above will be described in detail with reference to  FIG. 6 . 
     First, the control apparatus  33  repeatedly executes, between time points t 1  to t 3 , the current control processing shown in  FIG. 7  until a lapse of the preset slope time. As described with reference to  FIG. 7 , in this current control processing, the target current value Isp is set, and the pulse width PW is determined by the PI control such that the present current value Ipv acquired through the current sensor  3   d  reaches the target current value Isp. The inverter circuit  32  is driven by the PWM control with the pulse width PW. Accordingly, as shown in  FIG. 6 , the welding current increases from the bottom current to the peak current range after the time point t 1 , and reaches the upper limit of the peak current range at the time point t 2 . After the time point t 2 , the welding current is maintained within the peak current range by the PI control in the control apparatus  33 . Thereafter, at the time point t 3 , the control apparatus  33  ends the current control processing (see S 1 ) according to whether or not the slope time has elapsed after the start of the current control processing at the time point t 1  (see S 2 ), and starts the effective value control processing (see S 3 ). 
     By execution of the current control processing as described above, the welding current maintained within the peak current range is applied to the workpiece W. Thus, as shown in  FIG. 4 , growth of the nuggets N 1 , N 2  is accelerated between the first metal plate W 1  and the second metal plate W 2  and between the second metal plate W 2  and the third metal plate W 3 . As shown in  FIG. 4 , the thickness of the first metal plate W 1  is smaller than each of the thicknesses of the second metal plate W 2  and the third metal plate W 3 . Thus, a contact resistance between the first metal plate W 1  and the second metal plate W 2  is smaller than a contact resistance between the second metal plate W 2  and the third metal plate W 3 . Thus, Joule heat generated due to the contact resistance caused by the flow of welding current is greater at a portion between the second metal plate W 2  and the third metal plate W 3  than at a portion between the first metal plate W 1  and the second metal plate W 2 . Thus, in the peak state, the growth rate of the nugget N 2  generated between the second metal plate W 2  and the third metal plate W 3  is higher than the growth rate of the nugget N 1  between the first metal plate W 1  and the second metal plate W 2 . 
     Returning to  FIG. 6 , the control apparatus  33  executes, between the time points t 3  to t 5 , the effective value control processing described with reference to  FIG. 3 . In this effective value control processing, the control apparatus  33  calculates the effective value Irms of the welding current (see S 33 ), and stops driving the inverter circuit  32  until the effective value Irms reaches within the target effective value range. Thus, after the time point t 3 , the welding current quickly decreases to the bottom current, and reaches the bottom current at the time point t 4 . Thereafter, at the time point t 5 , the control apparatus  33  ends the effective value control processing according to whether or not the effective value Irms has decreased and reached within the target effective value range, and starts the subsequent cycle of the current control processing. Thus, after the time point t 5 , the welding current increases from the bottom current to the peak current range again. 
     By execution of the effective value control processing as described above, drive of the inverter circuit  32  is stopped across the power distribution standby time until the effective value Irms of the welding current reaches within the target effective value range. Thus, a state in which the welding current is limited to equal to or lower than the lower limit of the peak current range is maintained during the effective value control processing, and therefore, each of the nuggets N 1 , N 2  generated between the metal plates is cooled by heat dissipation. As described above, the thickness of the first metal plate W 1  is smaller than each of the thicknesses of the second metal plate W 2  and the third metal plate W 3 . Thus, heat dissipation between the second metal plate W 2  and the third metal plate W 3  is smaller than heat dissipation between the first metal plate W 1  and the second metal plate W 2 . While the state in which the welding current is limited to equal to or lower than the peak current range is maintained, the amount of cooling of the nugget N 2  by heat dissipation is greater than the amount of cooling of the nugget N 1  by heat dissipation. The growth rate of the nugget N 2  is higher than the growth rate of the nugget N 1  in the peak state as described above. Thus, as described above, the state in which the welding current is limited to equal to or lower than the peak current range is maintained across the power distribution standby time, and cooling of the nugget N 2  is accelerated. Consequently, occurrence of spatter between the second metal plate W 2  and the third metal plate W 3  can be reduced. In the effective value control processing, the subsequent cycle of the current control processing is started according to whether or not the effective value Irms of the welding current has reached within the target effective value range, and the welding current is increased again. Thus, energy necessary for growing each of the nuggets N 1 , N 2  between adjacent ones of the metal plates W 1  to W 3  can be controlled, and therefore, the workpiece can be reliably joined while occurrence of the spatter is reduced as described above. 
     One embodiment of the present invention has been described above, but the present invention is not limited to above. Detailed configurations may be changed as necessary within the scope of the gist of the present invention. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
         
           
             S Welding System 
             W Workpiece 
             W 1  First Metal Plate 
             W 2  Second Metal Plate 
             W 3  Third Metal Plate 
               1  Spot Welding Apparatus (Welding Apparatus) 
               2  Gun Body 
               21  Upper Electrode Chip (Electrode) 
               26  Lower Electrode Chip (Electrode) 
               3  Welding Power Circuit 
               3   a  Welding Control Circuit 
               3   b  DC: Welding Transformer 
               3   d  Current Sensor 
               31  Converter Circuit 
               32  Inverter Circuit 
               33  Control Apparatus 
               34  Transformer 
               35  Rectification Circuit 
             L 1 , L 2  Power Line