SPATTER DETECTION METHOD

A spatter detection method is a method for detecting an occurrence of spatter at the time of joining a workpiece using a spot welding apparatus including a pair of electrode chips, a welding power circuit connected to the pair of electrode chips, and a current sensor configured to detect a current flowing through the pair of electrode chips. A spot welding method using the spot welding apparatus includes supplying a pulse-shaped welding current to the workpiece, the pulse-shaped welding current being generated when the welding power circuit alternately repeats power distribution control and a power distribution pause over a plurality of cycles while the workpiece is sandwiched and pressurized by the pair of electrode chips. The spatter detection method includes determining, based on a current detection value detected by the current sensor in a power distribution pause section for each cycle, whether or not the spatter occurs.

This application is based on and claims the benefit of priority from Chinese Patent Application No. 202210346150.2, filed on 31 Mar. 2022, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a spatter detection method. More specifically, the present invention relates to a spatter detection method for detecting an occurrence of spatter in spot welding for joining a workpiece by supplying a pulse-shaped welding current.

Related 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 as workpieces are 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 spot welding method proposed by the applicant disclosed in PCT International Publication No. WO2020/050011, a welding current having a pulse-shaped waveform is supplied to a plurality of metal plates over a plurality of cycles 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.Patent Document 1: PCT International Publication No. WO2020/050011Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2010-149144

SUMMARY OF THE INVENTION

In spot welding, while a welding current is supplied to a metal plate, a phenomenon called “spatter” occurs in which part of the metal plate is molten and scattered. When the spatter occurs, welding strength may decrease, and thus it is preferable to detect the spatter immediately when the spatter occurs.

Japanese Unexamined Patent Application, Publication No. 2010-149144 discloses a technique for detecting an occurrence of spatter based on a change in a resistance value during welding, using the fact that the resistance value of a workpiece decreases when spatter occurs. However, it is necessary to detect a voltage between a pair of electrode chips in order to monitor the resistance value of the workpiece. Therefore, in the technique disclosed in Japanese Unexamined Patent Application, Publication No. 2010-149144, it is necessary to provide a voltage detection line for detecting a voltage in the vicinity of the electrode chips. However, since the vicinity of the electrode chips is exposed to a high temperature during welding, the voltage detection line needs to be replaced periodically, which may increase costs and cycle time.

An object of the present invention is to provide a spatter detection method capable of detecting an occurrence of spatter using an existing sensor without newly providing a voltage detection line in the vicinity of electrode chips.(1) A spatter detection method according to the present invention is a method for detecting an occurrence of spatter at the time of joining a workpiece (e.g., a workpiece W to be described later), which is a multilayer body of a plurality of plates, using a welding apparatus (e.g., a spot welding apparatus1to be described later) including a pair of electrodes (e.g., an upper electrode chip21and a lower electrode chip26to be described later), a welding power circuit (e.g., a welding power circuit3to be described later) connected to the pair of electrodes, and a current sensor (e.g., a current sensor3dto be described later) configured to detect a current flowing through the pair of electrodes. A spot welding method using the welding apparatus includes supplying a pulse-shaped welding current to the workpiece, the pulse-shaped welding current being generated when the welding power circuit alternately repeats power distribution control and a power distribution pause over a plurality of cycles while the workpiece is sandwiched and pressurized by the pair of electrodes. The spatter detection method includes determining, based on a current detection value detected by the current sensor in a power distribution pause section for each cycle, whether or not the spatter occurs.(2) In this case, preferably, the spatter detection method further includes determining whether or not the spatter occurs, based on comparison of the current detection value in the power distribution pause section for an N-th cycle (N being an integer equal to or greater than 2) with the current detection value in the power distribution pause section for an M-th cycle (M being an integer smaller than N).(3) In this case, preferably, the M-th cycle is a cycle immediately before the N-th cycle.(4) In this case, preferably, the spatter detection method further includes determining that the spatter occurs when a difference value (e.g., a maximum difference value dImax(2), . . . , dImax(Nset) to be described later) is larger than a first threshold value (e.g., a first threshold value dIth to be described later), the difference value being obtained by subtracting the current detection value in the power distribution pause section for the M-th cycle from the current detection value in the power distribution pause section for the N-th cycle.(5) In this case, preferably, the spatter detection method further includes determining that the spatter occurs when a difference value (e.g., dS(2), . . . , dS(Nset) to be described later) is larger than a second threshold value (e.g., a second threshold value dSth to be described later), the difference value being obtained by subtracting an integral value of the current detection value over the power distribution pause section for the M-th cycle from an integral value of the current detection value over the power distribution pause section for the N-th cycle.(6) In this case, the spot welding method includes: executing the power distribution control so as to maintain the welding current within a set peak current range at an end of a power distribution control section. Further, the spatter detection method preferably includes determining that the spatter occurs when a difference value (e.g., time constant difference value dτ(2), . . . , dτ(Nset) to be described later) is larger than a third threshold value (e.g., a third threshold value dτth to be described later), the difference value being obtained by subtracting a time constant when the current detection value in the power distribution pause section for the M-th cycle falls from the peak current range from a time constant when the current detection value in the power distribution pause section for the N-th cycle falls from the peak current range.(1) The circuit formed by a combination of the workpiece, the pair of electrodes that sandwich the workpiece, and the welding power circuit connected to the pair of electrodes can be regarded as an RL series circuit in which a resistance element corresponding to the workpiece and an inductance element corresponding to the welding power circuit are connected in series. For this reason, when the pulse-shaped welding current generated by alternately repeating the power distribution control and the power distribution pause by the welding power circuit over a plurality of cycles is supplied to the workpiece, the time constant at the time of falling of the welding current in the power distribution pause section for each cycle is inversely proportional to the resistance element corresponding to the workpiece. For this reason, when the spatter occurs while the pulse-shaped welding current as described above is being supplied, falling of the welding current in the power distribution pause section becomes gentler. In the present invention, using such a phenomenon and the current sensor already installed in the welding apparatus, it is possible to determine the presence or absence of an occurrence of spatter based on the current detection value detected by the current sensor in the power distribution pause section for each cycle.(2) The falling characteristics of the welding current in the power distribution pause section as described above vary according to the material and the plate thickness of the workpiece. Therefore, in the present invention, the presence or absence of the occurrence of spatter is determined, based on comparison of the current detection value in the power distribution pause section for the N-th cycle with the current detection value in the power distribution pause section for the M-th cycle (M being an integer smaller than N) before the N-th cycle. In other words, since the current detection value in the power distribution pause section for the M-th cycle before the N-th cycle is used as a comparison target for the current detection value in the power distribution pause section for the N-th cycle, there is no need to re-set the threshold value each time the type of the workpiece changes, and thus convenience is high.(3) When the pulse-shaped welding current as described above continues to be supplied over a plurality of cycles, the resistance value of the workpiece will gradually change even when the spatter does not occur. Therefore, in the present invention, the current detection value in the power distribution pause section for the M-th cycle (i.e., M=N−1) immediately before the N-th cycle is used as a comparison target for the current detection value in the power distribution pause section for the N-th cycle, whereby it is possible to determine the presence or absence of the occurrence of spatter and the cycle of the occurrence of spatter with high accuracy.(4) In the present invention, when the difference value obtained by subtracting the current detection value in the power distribution pause section for the M-th cycle from the current detection value in the power distribution pause section for the N-th cycle is larger than the first threshold value, it is determined that the spatter has occurred. Thus, it is possible to detect the occurrence of spatter with a simple calculation.(5) In the present invention, the difference value obtained by subtracting the integral value of the current detection value over the power distribution pause section for the M-th cycle from the integral value of the current detection value over the power distribution pause section for the N-th cycle is larger than the second threshold value, it is determined that the spatter has occurred. Thus, it is possible to detect the occurrence of spatter with good accuracy in consideration of the change in the falling characteristics of the welding current in the entire power distribution pause section.(6) In the spot welding method according to the present invention, the power distribution control is performed so as to maintain the welding current within the set peak current range at the end of the power distribution control section, that is, immediately before the power distribution pause section starts. Thus, since the welding current at the time of starting the power distribution pause can be aligned within the peak current range for each cycle, it is possible to accurately extract the change in the falling characteristics of the welding current in the power distribution pause section for each cycle. In the present invention, when the difference value obtained by subtracting the time constant when the current detection value in the power distribution pause section for the M-th cycle falls from the peak current range from the time constant when the current detection value in the power distribution pause section for the N-th cycle falls from the peak current range is larger than the third threshold value, it is determined that the spatter has occurred. Thus, it is possible to detect the occurrence of spatter with good accuracy in consideration of the change in the falling characteristics of the welding current in the entire power distribution pause section.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.FIG.1is a view showing a configuration of a welding system S to which a spot welding method and a spatter detection method according to the present embodiment is applied.

The welding system S includes a spot welding apparatus1as a welding gun, a workpiece W as a multilayer body of metal plates joined to each other by the spot welding apparatus1, and a robot6supporting the spot welding apparatus1.

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 W1, a second metal plate W2, and a third metal plate W3as 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. A case will be described below in which a thickness of the first metal plate W1is thinner than a thickness of each of the second metal plate W2and the third metal plate W3and the first metal plate W1, the second metal plate W2, and the third metal plate W3are made of the same metal, but the present invention is not limited thereto. At least one metal plate of these metal plates W1to W3may have a rigidity different from those of the other metal plates.

The robot6includes a robot body60attached to a floor surface, an articulated arm61pivotally supported on the robot body60, and a robot control apparatus62that controls the robot6. The articulated arm61includes a first arm portion611pivotally supported on a base end side by the robot body60, a second arm portion612pivotally supported on a base end side by the first arm portion611, a third arm portion613pivotally supported on a base end side by the second arm portion612, and a fourth arm portion614pivotally supported on a base end side by the third arm portion613and attached to the spot welding apparatus1on a tip end side.

The robot control apparatus62drives a plurality of motors provided at the robot body60and the articulated arm61to drive each of the arm portions611to614, thereby controlling the position and orientation of the spot welding apparatus1attached to the fourth arm portion614and moving later-described electrode chips21,26provided at the spot welding apparatus1to a joint portion of the workpiece W.

The spot welding apparatus1includes a welding power circuit3as a welding current supply source, a gun body2on which a later-described upper electrode chip movement mechanism4and part of the welding power circuit3are mounted, the upper electrode chip21and the lower electrode chip26as a pair of electrodes, an upper electrode chip support portion22, an upper adaptor body23, a gun arm25, a lower electrode chip support portion27, and a lower adaptor body28.

The upper electrode chip support portion22is in the shape of a rod extending along the vertical direction, and the upper electrode chip21is attached to a tip end portion of the upper electrode chip support portion22. The upper adaptor body23is in a columnar shape, and connects the gun body2and the upper electrode chip support portion22to each other. The upper adaptor body23is, relative to the gun body2, provided slidably along a sliding direction parallel with the axis of the upper electrode chip support portion22.

The gun arm25extends to curve from the gun body2to below the upper electrode chip21in the vertical direction. The lower electrode chip support portion27is in the shape of a rod coaxial with the upper electrode chip support portion22, and the lower electrode chip26is attached to a tip end portion of the lower electrode chip support portion27. The lower adaptor body28is in a columnar shape, and connects a tip end portion of the gun arm25and the lower electrode chip support portion27to each other. As shown inFIG.1, the lower electrode chip26is supported by the lower electrode chip support portion27to face the upper electrode chip21with a predetermined clearance along the axes of the chip support portions22,27.

The upper electrode chip movement mechanism4includes a cylinder, a control apparatus therefor, etc., and together with the upper electrode chip support portion22and the upper electrode chip21, moves the upper adaptor body23back and forth along the sliding direction. With this configuration, the upper electrode chip21can contact an upper surface of the workpiece W with the lower electrode chip26contacting a lower surface of the workpiece W, and the workpiece W can be further sandwiched and pressurized by these electrode chips21,26.

FIG.2is a diagram showing a circuit configuration of the welding power circuit3. The welding power circuit3includes a welding control circuit3a, a DC welding transformer3b, power cables3c, a current sensor3d, and a voltage sensor3e. The welding power circuit3is connected to the upper electrode chip21and the lower electrode chip26through power lines L1, L2. As shown inFIG.1, the DC welding transformer3band the current sensor3dof the welding power circuit3configured as described above are mounted on the gun body2. Moreover, the welding control circuit3aof the welding power circuit3is mounted on a base separated from the gun body2, and is connected to the DC welding transformer3bthrough the power cables3c. With this configuration, the weight of the gun body2can be reduced.

The welding control circuit3aincludes a converter circuit31, an inverter circuit32, and a control apparatus33. The DC welding transformer3bincludes a transformer34and a rectification circuit35.

The converter circuit31performs full-wave rectification for a three-phase power input from a three-phase power source30, thereby converting the three-phase power into a DC power and supplying the DC power to the inverter circuit32.

The inverter circuit32converts the DC power input from the converter circuit31into a single-phase AC power, thereby outputting the single-phase AC power to the transformer34through the power cables3c. More specifically, the inverter circuit32includes four bridge-connected switching elements. The inverter circuit32turns on or off these switching elements according to a gate drive signal transmitted from a gate drive circuit mounted on the control apparatus33, thereby converting the DC power into the single-phase AC power.

The transformer34transforms the AC power input from the inverter circuit32, thereby outputting the transformed AC power to the rectification circuit35. The rectification circuit35rectifies the AC power input from the transformer34, thereby outputting a DC power to between the electrode chips21,26each connected to the power lines L1, L2. For example, a known full-wave rectification circuit including a combination of two rectification diodes351,352and a center tap353is used as the rectification circuit35.

The current sensor3ddetects a welding current supplied from the welding power circuit3to the chips21,26. The current sensor3dis, for example, provided on the power line L1connecting the rectification circuit35and the upper electrode chip21to each other, and to the control apparatus33, transmits a current detection signal according to the level of the welding current flowing in the power line L1.

The voltage sensor3edetects voltages on secondary sides (i.e., on sides of the chips21and26) of the DC welding transformer3b. The voltage sensor3eis connected to the power lines L1and L2connecting the DC welding transformer3band the chips21and26, and transmits, to the control apparatus33, a voltage detection signal according to the level of the secondary side-voltage V2between these power lines L1and L2.

The control apparatus33includes, for example, a microcomputer that executes later-described welding current control and spatter detection processing by means of the current detection signal transmitted from the current sensor3dand the voltage detection signal transmitted from the voltage sensor3eand 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 circuit32.

FIG.3is a graph showing a relationship between an AC voltage Vt input from the inverter circuit32to the transformer34and the welding current Iw applied to the electrode chips21,26in the welding power circuit3as described above.

When the inverter circuit32is driven, the AC voltage Vt in the shape of a square wave as shown inFIG.3is outputted from the inverter circuit32. The AC voltage output from the inverter circuit32is transformed in the transformer34, and is further rectified in the rectification circuit35. Then, the DC welding current Iw is applied to the workpiece W through the electrode chips21,26.

As shown inFIG.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 toFIGS.5and6, the control apparatus33determines the pulse width PW according to a known feedback control rule such as PI control such that the output current of the welding power circuit3detected by the current sensor3dreaches a target current set by not-shown processing, and performs ON/OFF drive of the plurality of switching elements in the inverter circuit32by 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 inFIG.1, the robot control apparatus62drives the robot body60and the articulated arm61, thereby controlling the position and posture of the spot welding apparatus1such that the workpiece W is arranged between the upper electrode chip21and the lower electrode chip26. At this point, the robot control apparatus62controls the position and posture of the spot welding apparatus1such that the lower electrode chip26contacts a lower surface of the third metal plate W3of the workpiece W.

Next, as shown inFIG.4, the upper adaptor body23is slid using the upper electrode chip movement mechanism4such that the upper electrode chip21approaches the lower electrode chip26. When the upper electrode chip21approaches the lower electrode chip26and comes into contact with an upper surface of the first metal plate W1, the workpiece W is sandwiched and pressurized by the upper electrode chip21and the lower electrode chip26.

Next, the control apparatus33of the welding power circuit3executes the welding current control by the steps described with reference toFIG.5while maintaining a state in which the workpiece W is pressurized from both sides by the electrode chips21,26, and applies the pulse-shaped welding current to between the upper electrode chip21and the lower electrode chip26. In this manner, as shown inFIG.4, a first nugget N1is formed between the first metal plate W1and the second metal plate W2, and a second nugget N2is formed between the second metal plate W2and the third metal plate W3. Thus, the first to third metal plates W1to W3are welded to each other.

FIG.5is a flowchart showing the specific steps of the welding current control in the control apparatus33.FIG.6is a graph showing the waveform of the welding current achieved by the welding current control ofFIG.5. As shown inFIG.6, the welding current generated by the welding current control ofFIG.5has a pulse-shaped waveform that peak holding sections in which the welding current is maintained within a set peak current range and non-peak sections in which the welding current increases to the peak current range again after decreases to a bottom current (e.g., zero) from the peak current range are alternately achieved. In other words, under the welding current control ofFIG.5, after the welding current increases from the bottom current toward the peak current range, power distribution control to maintain the welding current within the peak current range and a power distribution pause to decrease the welding current from the peak current range toward the bottom current are alternately executed for a plurality of cycles (at least, two or more cycles), whereby a welding current having a pulse-shaped waveform as shown inFIG.6is generated and supplied to the workpiece W. Hereinafter, a section in which the power distribution control is performed is referred to as a power distribution control section, and a section in which the power distribution pause is performed is referred to as a power distribution pause section.

First, at S1, the control apparatus33sets a value of a counter N for counting the number of pulse cycles (number of pulses) of the welding current supplied to the workpiece W to an initial value of 0, and proceeds to S2.

Next, at S2, the control apparatus33counts up the counter N by 1 (N=N+1), and proceeds to S4.

Next, at S4, the control apparatus33executes power distribution control processing, and proceeds to S2. As described later with reference toFIG.7, in the power distribution control processing, the control apparatus33increases the welding current from the bottom current toward the peak current range, and then maintains the welding current within the peak current range for a predetermined time.

Next, at S5, the control apparatus33determines whether or not a predetermined slope time has elapsed. As shown inFIG.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 fixed at a preset time. In other words, the slope time is fixed for all cycles of the welding current pulse. The control apparatus33returns to S4to continuously execute the power distribution control processing in a case where a determination result at S5is NO, and proceeds to S6in a case where the determination result at S5is YES.

Next, at S6, the control apparatus33executes power distribution pause processing, and then proceeds to S8. As described later in detail with reference toFIG.8, in this power distribution pause processing, the control apparatus33waits for execution of the power distribution control processing for a predetermined power distribution pause time (seeFIG.6), and records a change in a current detection value detected by the current sensor3dduring the power distribution pause section.

Next, at S8, the control apparatus33determines whether or not the counter N has reached a predetermined prescribed number of cycles Nset. The prescribed number of cycles Nset corresponds to the number of cycles of the welding current pulse required to join one spot of the workpiece W by the spot welding apparatus1, and is set in advance according to the thickness and material characteristics of the workpiece W. When the determination result at S8is NO, the control apparatus33returns to S2, and starts power distribution control processing for the next cycle. When the determination result at S8is YES, the control apparatus33proceeds to S9. In the present embodiment, the case has been described in which the welding current is continuously supplied until the number of cycles of the welding current pulse reaches the predetermined prescribed number of cycles Nset, but the present invention is not limited thereto. For example, the welding current may be continuously supplied until a predetermined power distribution time elapses after the start of the welding current control for the first cycle.

Next, at S9, the control apparatus33ends the processing ofFIG.5to start joining a next spot of the workpiece W or another workpiece W after executing spatter detection processing. As described later in detail with reference toFIG.10, in such spatter detection processing, based on the current detection value detected by the current sensor3din the power distribution pause section for each cycle and stored in a storage apparatus, the presence or absence of the occurrence of spatter while the welding current is being supplied, the timing of the occurrence of spatter, and the quality of the product due to the occurrence of spatter are determined.

As described above, in the welding current control, the control apparatus33repeatedly executes the power distribution control processing (see S4) and the power distribution pause processing (see S6) across the power distribution time, thereby applying the welding current with the pulse-shaped waveform as shown inFIG.6between the electrode chips21and26.

FIG.7is a flowchart showing the specific steps of the power distribution control processing. First, at S11, the control apparatus33acquires, using the current detection signal transmitted from the current sensor3d, a present current value Ipv as a present welding current value, and proceeds to S12. At S12, the control apparatus33sets a target current value Isp equivalent to a target welding current value, and proceeds to S14. As shown inFIG.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 S14, the control apparatus33calculates a current deviation Idev by subtracting the present current value Ipv acquired at S11from the target current value Isp set at S12, and proceeds to S15.

At S15, the control apparatus33calculates 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 S14such that the current deviation Idev reaches zero, and proceeds to S16. More specifically, the control apparatus33adds 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 S16, the control apparatus33starts a PW counter, and proceeds to S17. At S17, the control apparatus33turns on the switching elements provided in the inverter circuit32, and proceeds to S18. At S18, the control apparatus33determines 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 S16. The control apparatus33returns to S17to keep the switching elements ON in a case where a determination result at S18is NO, and proceeds to S19in a case where the determination result at S18is YES.

At S19, the control apparatus33turns off the switching elements provided in the inverter circuit32, and proceeds to S20. At S20, the control apparatus33determines whether or not the set carrier cycle has elapsed after the switching elements have been turned on at S17. The control apparatus33returns to S19to keep the switching elements OFF in a case where a determination result at S20is NO, and proceeds to S2ofFIG.5in a case where the determination result at S20is YES.

FIG.8is a flowchart showing the specific steps of the power distribution pause processing. At S31, the control apparatus33turns on a pause time timer for measuring a time that has elapsed after the start of the power distribution pause (hereinafter, referred to as pause time), and proceeds to S32. At S32, the control apparatus33obtains a current detection value Ipv using the current detection signal transmitted from the current sensor3d, and proceeds to S33. At S33, the control apparatus33stores the current detection value Ipv acquired at S32together with the counter N indicating the present number of cycles and a measurement value t of the pause time timer indicating the pause time in the storage apparatus (not shown), and proceeds to S34. In the following description, a current detection value at a pause time t of an N-th cycle is written as Ipv(N, t).

At S34, the control apparatus33determines whether or not the measurement value t of the pause time timer has reached a predetermined set pause time tset. When the determination result at S34is YES, the control apparatus33proceeds to S7ofFIG.5and starts power distribution control processing in the next cycle ((N+1)-th cycle). The control apparatus33returns to S32when the determination result at S34is No, and continues the power distribution pause until the set pause time tset elapses. As described above, the control apparatus33acquires, for each cycle, the current detection value from the current sensor in the power distribution pause section. In the present embodiment, the case has been described in which the power distribution pause is executed for a fixed set pause time, but the present invention is not limited thereto. The set pause time may be set according to the effective value of the detection value of the welding current, for example.

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 toFIG.6.

First, the control apparatus33repeatedly executes, between time points t1to t3, the power distribution control processing shown inFIG.7until a lapse of the preset slope time. As described with reference toFIG.7, in this power distribution 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 sensor3dreaches the target current value Isp. The inverter circuit32is driven by the PWM control with the pulse width PW. Accordingly, as shown inFIG.6, the welding current increases from the bottom current to the peak current range after the time point t1, and reaches the upper limit of the peak current range at the time point t2. Thereafter, at the end of the power distribution control section after the time point t2, the welding current is maintained within the peak current range by the PI control in the control apparatus33. Thereafter, at the time point t3, the control apparatus33ends the power distribution control processing (see S4) according to the fact that the predetermined slope time has elapsed after the start of the current control processing at the time point t1(see S5), and starts the power distribution pause processing (see S6).

By execution of the power distribution control processing as described above, the welding current maintained within the peak current range is applied to the workpiece W. Thus, as shown inFIG.4, growth of the nuggets N1, N2is accelerated between the first metal plate W1and the second metal plate W2and between the second metal plate W2and the third metal plate W3. Here, as shown inFIG.4, since the thickness of the first metal plate W1is smaller than each of the thicknesses of the second metal plate W2and the third metal plate W3, the first metal plate W1is easily deformed by pressurization. Thus, a contact area between the first metal plate W1and the second metal plate W2is larger than a contact area between the second metal plate W2and the third metal plate W3. Thus, a contact resistance between the first metal plate W1and the second metal plate W2is smaller than a contact resistance between the second metal plate W2and the third metal plate W3. 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 W2and the third metal plate W3than at a portion between the first metal plate W1and the second metal plate W2. Thus, in the peak state, the growth rate of the nugget N2generated between the second metal plate W2and the third metal plate W3is higher than the growth rate of the nugget N1between the first metal plate W1and the second metal plate W2.

Returning toFIG.6, the control apparatus33executes, between the time points t3to t5, the effective value control processing described with reference toFIG.8. In this effective value control processing, the control apparatus33calculates the effective value Irms of the welding current (see S33), and stops driving the inverter circuit32until the effective value Irms reaches within the target effective value range. Thus, after the time point t3, the welding current quickly decreases to the bottom current, and reaches the bottom current at the time point t4. Thereafter, at the time point t5, according to the fact that the predetermined set pause time has elapsed after the start of the power distribution pause at the time point t3, the control apparatus33ends the power distribution pause processing and starts the power distribution control processing for the next cycle. Thus, after the time point t5, the welding current increases from the bottom current to the peak current range again.

By execution of the power distribution pause processing as described above, the driving of the inverter circuit32is stopped until the set pause time elapses. 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 power distribution pause processing, and therefore, each of the nuggets N1and N2generated between the metal plates is cooled by heat dissipation. As described above, the thickness of the first metal plate W1is smaller than each of the thicknesses of the second metal plate W2and the third metal plate W3. Thus, heat dissipation between the second metal plate W2and the third metal plate W3is smaller than heat dissipation between the first metal plate W1and the second metal plate W2. 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 N2by heat dissipation is greater than the amount of cooling of the nugget N1by heat dissipation. Since the growth rate of the nugget N2is higher than the growth rate of the nugget N1in the peak state as described above, the state in which the welding current is limited to equal to or lower than the peak current range is maintained for the set pause time in this manner, and cooling of the nugget N2is accelerated, whereby the spatter can be prevented from occurring between the second metal plate W2and the third metal plate W3.

FIG.9is a graph showing a case where waveforms of a welding current are stacked in a power distribution pause section for two cycles before and after the occurrence of spatter.

As described above, in the welding current control shown inFIG.5, the welding current is maintained within the peak current range at the end of the power distribution control section in which the power distribution control is performed, that is, immediately before the power distribution pause is started. For this reason, as shown inFIG.9, levels of the welding current at the time of starting the power distribution pause substantially coincide with each other regardless of the presence or absence of the occurrence of spatter and the number of cycles. Further, while the welding current is supplied to the workpiece W from the spot welding apparatus1, a circuit formed by a combination of the workpiece W and the spot welding apparatus1including the chips21and26and the welding power circuit3can be regarded as an RL series circuit in which a resistance element corresponding to the workpiece W and an inductance element corresponding to the welding power circuit3and the gun arm25are connected in series. For this reason, as shown inFIG.9, the welding current in the power distribution pause section exponentially is reduced from the peak current range toward the bottom current. In the RL series circuit, a time constant at the time of falling of the welding current in the power distribution pause section is inversely proportional to the resistance element corresponding to the workpiece W. For this reason, as shown inFIG.9, when the waveforms of the welding current in the power distribution pause section compare with each other at cycles before and after the occurrence of spatter, the falling of the welding current becomes gentler after the occurrence of spatter.

Therefore, the occurrence of spatter is detected in the spatter detection processing, using the change in the falling characteristics of the welding current in the power distribution pause section in the cycles before and after the occurrence of spatter. More specifically, the presence or absence of the occurrence of spatter is determined in the spatter detection processing, based on comparison of current detection values (Ipv(N, t1), Ipv(N, t2), . . . ) in the power distribution pause section for the N-th cycle (N being an integer equal to or greater than 2) with current detection values (Ipv(M, t1), Ipv(M, t2), . . . ) in the power distribution pause section for the M-th cycle (M being an integer smaller than N).

Further, when the welding current control as described above continues to be executed over a plurality of cycles, the resistance of the workpiece W will gradually change even when the spatter does not occur. Therefore, in order to underscore the change in the falling characteristics of the welding current due to the occurrence of spatter in the present embodiment, the presence or absence of the occurrence of spatter is determined, based on the comparison of the current detection values (Ipv(N, t1), Ipv(N, t2), . . . ) in the power distribution pause section for the N-th cycle with the current detection values (Ipv(M, t1), Ipv(M, t2), . . . ) in the power distribution pause section for the M-th cycle (i.e., M=N−1) immediately before the N-th cycle.

FIG.10is a flowchart showing the specific steps in the spatter detection processing of a first example.

First, at S51, the control apparatus33sets the counter N for counting the number of cycles to an initial value of 1, and proceeds to S52.

Next, at S52, the control apparatus33counts up the counter N by 1 (N=N+1), and proceeds to S53.

Next, at S53, the control apparatus33sets the N-th cycle as a target cycle, and reads current detection values (Ipv(N, t1), (Ipv(N, t2), . . . ) at respective time points in a power distribution pause section for the target cycle and current detection values (Ipv(N−1, t1), Ipv(N−1, t2), . . . ) at respective time points in a power distribution pause section for an (N−1)-th cycle immediately before the target cycle from the storage apparatus (not shown), and proceeds to S54.

Next, at S54, the control apparatus33calculates a difference value obtained by subtracting the current detection value for the (N−1)-th cycle from the current detection value for the N-th cycle, for each of the time points (t1, t2, . . . ) in the power distribution pause section, and proceeds to S55. Hereinafter, a difference value at a time point t for the N-th cycle is expressed as dI (N, t) (=Ipv(N, t)−Ipv(N−1, t)). The difference value calculated in this manner becomes a large value when the spatter occurs as shown inFIG.9.

Next, at S55, the control apparatus33extracts the largest value from the difference values dI(N, t1), dI(N, t2), . . . calculated at the respective time point at S54, and stores the largest value as the maximum difference value dImax(N) in the storage apparatus (not shown), and proceeds to S56.

Next, at S56, the control apparatus33determines whether or not the counter N has reached the prescribed number of cycles Nset. The control apparatus33returns to S52when the determination result at S56is NO, and proceeds to S57when the determination result at S56is YES.

Next, at S57, the control apparatus33determines in order to determine the presence or absence of the occurrence of spatter whether or not there is any one of the maximum difference values dImax(2), . . . , dImax(Nset) for Nset−1 cycles that exceeds a predetermined first threshold value dIth.

When the determination result at S57is NO, that is, when all of the maximum difference values dImax(2), . . . , dImax(Nset) for Nset−1 cycles are less than the first threshold value dIth, the control apparatus33proceeds to S58, determines that no spatter occurs, and proceeds to S59. At S59, the control apparatus33determines that the quality of the product manufactured by joining the workpiece W is good, and ends the processing ofFIG.9.

Further, when the determination result at S57is YES, that is, when at least one of the maximum difference values dImax(2), . . . , dImax(Nset) for Nset−1 cycles exceeds the first threshold value dIth, the control apparatus33proceeds to S60, determines that spatter occurs, and proceeds to S61.

At S61, the control apparatus33calculates a spatter occurrence cycle P equivalent to a timing at which the spatter occurs, and proceeds to S62. More specifically, the control apparatus33acquires a cycle at which any of the maximum difference values dImax(2), . . . , dImax(Nset) for Nset−1 cycles exceeds the first threshold value dIth, and sets such a cycle as the spatter occurrence cycle P. Here, when there are a plurality of cycles at which any of the maximum difference values dImax(2), . . . , dImax(Nset) for Nset−1 cycles exceeds the first threshold value dIth, it is preferable to set a final cycle, at which the maximum difference value exceeds the first threshold value dIth at last, as the spatter occurrence cycle P.

At S62, the control apparatus33determines in order to determine the quality of the product based on the timing of the occurrence of spatter whether or not the spatter occurrence cycle P is less than a cycle threshold value Pth set from2to Nset.

FIG.11is a graph showing a correlation between a welding diameter (i.e., welding strength) formed when the welding current is continuously supplied over the prescribed cycle Nset and the timing of the occurrence of spatter. As shown inFIG.11, the slower the timing of the occurrence of spatter, the smaller the welding diameter formed finally by welding, and accordingly the lower the welding strength. It is considered that this is because when the timing of the occurrence of spatter is at a former period of the welding, the welding diameter can be expected to grow with subsequent power distribution, whereas when the timing of the occurrence of spatter is at a latter period of the welding, the welding diameter does not grow sufficiently due to subsequent power distribution.

Returning toFIG.10, when the determination result at S62is YES, that is, when the spatter occurrence cycle P is less than the cycle threshold value Pth, the control apparatus33proceeds to S59, determines that the quality of the product is good, and ends the processing ofFIG.10. Further, when the determination result at S62is NO, that is, when the spatter occurrence cycle P is equal to or greater than the cycle threshold value Pth, the control apparatus33proceeds to S63, determines that the quality of the product is defective, and ends the processing ofFIG.10.

In addition, it is preferable to check the quality again by visual inspection when it is determined that the quality of the product is defective by the above-described spatter detection processing.

FIG.12is a flowchart showing the specific steps in the spatter detection processing of a second example. In the processing shown inFIG.12, steps S71to S73, S76, and S78to S83are the same as steps S51to S53, S56, and S58to S63in the processing shown inFIG.10, respectively, and thus will not be described.

At S74, the control apparatus33calculates an integral value of the current detection value over the power distribution pause section for the N-th cycle and an integral value of the current detection value over the power distribution pause section for the (N−1)-th cycle, and proceeds to S75.

At S75, the control apparatus33calculates an integral difference value dS(N) by subtracting the integral value of the current detection value for the (N−1)-th cycle from the integral value of the current detection value for the N-th cycle which are calculated at S74, and proceeds to S76.

At S77, the control apparatus33determines in order to determine the presence or absence of the occurrence of spatter whether or not there is any one of integral difference values dS(2), . . . , dS(Nset) for Nset−1 cycles that exceeds a predetermined second threshold value dSth.

When the determination result at S77is NO, that is, when all of the integral difference values dS(2), . . . , dS(Nset) for Nset−1 cycles are less than the second threshold value dSth, the control apparatus33proceeds to S78. In addition, when the determination result at S77is YES, that is, when at least one of the integral difference values dS(2), . . . , dS(Nset) for Nset−1 cycles exceeds the second threshold value dSth, the control apparatus33proceeds to S80.

FIG.13is a flowchart showing the specific steps in the spatter detection processing of a third example. In the processing shown inFIG.13, steps S91to S93, S96, and S98to S103are the same as steps S51to S53, S56, and S58to S63in the processing shown inFIG.10, respectively, and thus will not be described.

At S94, based on time-series data (Ipv(N, t1), Ipv(N, t2), . . . ) of the current detection value for the N-th cycle acquired at S93and time-series data (Ipv(N−1, t1), Ipv(N−1, t2), . . . ) of the current detection value for the (N−1)-th cycle, the control apparatus33calculates a time constant τ(N) when the current detection value in the power distribution pause section for the N-th cycle decreases from the initial value Ipv(N, t1) within the peak current range toward the bottom current and a time constant τ(N−1) when the current detection value in the power distribution pause section for the (N−1)-th cycle decreases from the initial value Ipv(N−1, t1) within the peak current range toward the bottom current, and proceeds to S95. Note that the time constants τ(N) and τ(N−1) for each cycle can be calculated by a known method based on the time-series data acquired at S93. More specifically, for example, the time taken for the current detection value for each cycle to fall below a threshold value (Ipv(N, t1)×0.632, Ipv(N−1, t1)×0.632) obtained by multiplying each initial value by 0.632 after the start of power distribution pause can be regarded as the time constant τ(N) or τ(N−1) for each cycle.

At S95, the control apparatus33calculates a time constant difference value dτ(N) by subtracting the time constant τ(N−1) of the current detection value in the power distribution pause section for the (N−1)-th cycle from the time constant τ(N) of the current detection value in the power distribution pause section for the N-th cycle, and proceeds to S96.

At S97, the control apparatus33determines in order to determine the presence or absence of the occurrence of spatter whether or not there is any one of time constant difference values dτ(2), . . . , dτ(Nset) for Nset−1 cycles that exceeds a predetermined third threshold value dτth.

When the determination result at S97is NO, that is, when all of the time constant difference values dτ(2), . . . , dτ(Nset) for Nset−1 cycles are less than the third threshold value dτth, the control apparatus33proceeds to S98. In addition, when the determination result at S97is YES, that is, when at least one of the time constant difference values dτ(2), . . . , dτ(Nset) for Nset−1 cycles exceeds the third threshold value dτth, the control apparatus33proceeds to S100.

According to the spatter detection method related to the present embodiment, the following effects are achieved.(1) The circuit formed by a combination of the workpiece W, the pair of electrode chips21and26that sandwich the workpiece W, and the welding power circuit3connected to the pair of electrode chips21and26can be regarded as an RL series circuit in which the resistance element corresponding to the workpiece W and the inductance element corresponding to the welding power circuit3are connected in series. For this reason, when the pulse-shaped welding current generated by alternately repeating the power distribution control and the power distribution pause by the welding power circuit3over a plurality of cycles is supplied to the workpiece W, the time constant at the time of falling of the welding current in the power distribution pause section for each cycle is inversely proportional to the resistance element corresponding to the workpiece W. For this reason, when the spatter occurs while the pulse-shaped welding current as described above is being supplied, the falling of the welding current in the power distribution pause section becomes gentler. In the present embodiment, using such a phenomenon and the current sensor3dalready installed in the spot welding apparatus1, it is possible to determine the presence or absence of the occurrence of spatter based on the current detection value by the current sensor3din the power distribution pause section for each cycle.(2) The falling characteristics of the welding current in the power distribution pause section as described above vary according to the material and the plate thickness of the workpiece W. Therefore, in the present embodiment, the presence or absence of the occurrence of spatter is determined, based on comparison of the current detection value in the power distribution pause section for the N-th cycle with the current detection value in the power distribution pause section for the M-th cycle (M being an integer smaller than N) before the N-th cycle. In other words, since the current detection value in the power distribution pause section for the M-th cycle before the N-th cycle is used as a comparison target for the current detection value in the power distribution pause section for the N-th cycle, there is no need to re-set the threshold value each time the type of the workpiece W changes, and thus convenience is high.(3) When the pulse-shaped welding current as described above continues to be supplied over a plurality of cycles, the resistance value of the workpiece W will gradually change even when the spatter does not occur. Therefore, in the present embodiment, the current detection value in the power distribution pause section for the (N−1)-th cycle immediately before the N-th cycle is used as a comparison target for the current detection value in the power distribution pause section for the N-th cycle, whereby it is possible to determine the presence or absence of the occurrence of spatter and the cycle of the occurrence of spatter with high accuracy.(4) In the present embodiment, when the difference value dI(N, t) obtained by subtracting the current detection value in the power distribution pause section for the (N−1)-th cycle from the current detection value in the power distribution pause section for the N-th cycle is larger than the first threshold value dIth, it is determined that the spatter has occurred. Thus, it is possible to detect the occurrence of spatter with a simple calculation.(5) In the present embodiment, the integral difference value dS(N) obtained by subtracting the integral value of the current detection value over the power distribution pause section for the (N−1)-th cycle from the integral value of the current detection value over the power distribution pause section for the N-th cycle is larger than the second threshold value dSth, it is determined that the spatter has occurred. Thus, it is possible to detect the occurrence of spatter with good accuracy in consideration of the change in the falling characteristics of the welding current in the entire power distribution pause section.(6) In the spot welding method according to the present embodiment, the power distribution control is performed so as to maintain the welding current within the set peak current range at the end of the power distribution control section, that is, immediately before the power distribution pause section starts. Thus, since the welding current at the time of starting the power distribution pause can be aligned within the peak current range for each cycle, it is possible to accurately extract the change in the falling characteristics of the welding current in the power distribution pause section for each cycle. In the present embodiment, when the time constant difference value τ(N) obtained by subtracting the time constant when the current detection value in the power distribution pause section for the (N−1)-th cycle falls from the peak current range from the time constant when the current detection value in the power distribution pause section for the N-th cycle falls from the peak current range is larger than the third threshold value dτth, it is determined that the spatter has occurred. Thus, it is possible to detect the occurrence of spatter with good accuracy in consideration of the change in the falling characteristics of the welding current in the entire power distribution pause section.

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.

In the present embodiment, the case has been described in which the spatter detection processing is executed after the welding current is supplied over the prescribed cycle Nset to detect the occurrence of spatter, but the execution timing for the spatter detection processing is not limited thereto. In the spatter detection processing according to the present invention as described above, the occurrence of spatter is detected, based on the comparison of the welding current in the power distribution pause section for the target cycle with the welding current in the power distribution pause section for the cycle immediately before the target cycle, and thus the spatter detection processing may be executed while the welding current is supplied.