JP2012071331A - Double electrode arc welding method and double electrode arc welding system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a double electrode arc welding method and a double electrode arc welding system, capable of suppressing generation of spatters that can be generated over the entire welding period, and efficiently executing the welding.SOLUTION: The double electrode arc welding method includes: steps ((s-1)-(s-3)) of generating a MIG (metal inert gas) arc a1 between a base material W and a consumable electrode 15; a step (s-4) of generating a plasma arc a2 between the base material W and a non-consumable electrode surrounding the consumable electrode 15 with the MIG arc a1 being the pilot arc; a step (s-5) of continuing a state in which both the MIG arc a1 and the plasma arm a2 are generated while flowing MIG current running between the consumable electrode 15 and the base material W at a MIG preheating value after generating the plasma arc a2; and a step (s-6) of increasing the value of the MIG current from the MIG preheating value, and starting the droplet transfer from the consumable electrode 15 to the base material W. According to this constitution, generation of the spatter that can be generated over the entire welding period can be suppressed, and the welding can be efficiently executed.

Description

  The present invention relates to a two-electrode arc welding method and a two-electrode arc welding system.

  Conventionally, a two-electrode arc welding method has been proposed in which welding is performed while generating a consumable electrode arc and a non-consumable electrode arc by using a welding torch that holds the consumable electrode and the non-consumable electrode (for example, Patent Documents). 1). In the conventional method, first, a mig arc is generated between the consumable electrode and the base material. Next, using this MIG arc as a pilot arc, a plasma arc is generated between the non-consumable electrode and the base material. Next, the mig arc is extinguished and the state where only the plasma arc is generated is continued. Thereby, the base material is heated by the plasma arc. Next, after continuing the state in which only the plasma arc is generated for a certain period, the MIG arc is regenerated between the consumable electrode and the base material. When the MIG arc is generated, the droplet is transferred from the consumable electrode to the base material while continuing the state in which both the MIG arc and the plasma arc are generated.

  In such a welding method, when the MIG arc is regenerated, the consumable electrode is relatively soft because it is heated by the plasma arc. If the consumable electrode is soft, the contact area between the consumable electrode and the base material becomes large because the tip of the consumable electrode is bent when the consumable electrode comes into contact with the base material. When the contact area increases, the consumable electrode that is in contact may be sufficiently melted so that the arc cannot be regenerated. If it does so, the probability that it will fail in the reoccurrence | production of a mig arc becomes high. Further, even when the mig arc can be regenerated, there is a possibility that spatter may occur when the mig arc is regenerated.

JP 2009-72802 A

  The present invention has been conceived under the circumstances described above, and is a two-electrode arc welding method capable of suppressing the occurrence of spatter that can occur over the entire welding period and performing welding efficiently. It is an object of the present invention to provide a two-electrode arc welding system.

  The two-electrode arc welding method provided by the first aspect of the present invention includes a step of generating a MIG arc between a base material and a consumable electrode, and surrounding the base material and the consumable electrode using the MIG arc as a pilot arc. A step of generating a plasma arc between the non-consumable electrode; and after the plasma arc is generated, while the MIG current flowing between the consumable electrode and the base material is caused to flow at a MIG preheating value, the MIG arc and the above A step of continuing the state in which any of the plasma arcs are generated, and a step of increasing the value of the MIG current from the MIG preheating value and starting the droplet transfer from the consumable electrode to the base material. Prepare.

  In a preferred embodiment of the present invention, the method further includes a step of measuring a time elapsed from the time when the plasma arc is generated, and the step of starting the droplet transfer is performed when the elapsed time exceeds a set time. To do.

  In a preferred embodiment of the present invention, the method further comprises a step of detecting a value of a Mig voltage between the consumable electrode and the base material, and the step of starting the droplet transfer is performed after the plasma arc is generated. When the Mig voltage value becomes smaller than the set voltage value.

  In a preferred embodiment of the present invention, the method further includes a step of measuring a time elapsed from a time when the plasma arc is generated, and a step of detecting a value of a Mig voltage between the consumable electrode and the base material. The step of starting the droplet transfer is performed when the elapsed time exceeds the set time and when the Mig voltage value becomes smaller than the set voltage value, whichever comes first .

  In a preferred embodiment of the present invention, after the step of starting the droplet transfer, the plasma current flowing from the non-consumable electrode to the base material is caused to flow at a steady plasma value, and then from the consumable electrode to the base material. The method further includes the step of transferring the droplets, and in the step of continuing the generated state, the plasma current is caused to flow at a plasma preheating value larger than the plasma steady value.

  In a preferred embodiment of the present invention, the method further comprises the step of starting to move the consumable electrode relative to the base material along the base material after the step of continuing the generated state.

  A two-electrode arc welding system provided by the second aspect of the present invention includes a plasma output circuit for passing a plasma current between a non-consumable electrode and a base material, and a plasma between the non-consumable electrode and the base material. A detection circuit that sends a detection signal when the occurrence of an arc is detected, a MIG preheating value storage circuit that stores a MIG preheating value, and a droplet that receives the detection signal and sends a droplet transfer start instruction signal after receiving the detection signal When the transition start instruction circuit and the detection signal are received, a preheating period is started in which the MIG current flowing between the consumable electrode and the base material flows at the MIG preheating value, and the droplet transition start instruction signal is received. And a MIG output circuit for starting a droplet transfer period for transferring droplets from the consumable electrode to the base material.

  In a preferred embodiment of the present invention, the droplet transfer start instruction circuit includes a time measurement circuit that measures a time elapsed from the time when the detection signal is received, a set time storage circuit that stores a set time, and the above And a comparison circuit that sends the droplet transfer start instruction signal when the elapsed time exceeds the set time.

  In a preferred embodiment of the present invention, the droplet transfer start instruction circuit includes a voltage detection circuit that detects a Mig voltage applied between the consumable electrode and the base material, and a set voltage that stores a set voltage. And a storage circuit and a comparison circuit that sends the droplet transfer start instruction signal when the MIG voltage falls below the set voltage after receiving the detection signal.

  In a preferred embodiment of the present invention, the droplet transfer start instruction circuit includes a time measurement circuit that measures a time elapsed from the time when the detection signal is received, a set time storage circuit that stores a set time, and the above A first comparison circuit that sends a time lapse signal when the elapsed time exceeds the set time, a voltage detection circuit that detects a Mig voltage applied between the consumable electrode and the base material, and a set voltage A set voltage storage circuit for storing, a second comparison circuit for sending a voltage drop signal when the MIG voltage falls below the set voltage after receiving the detection signal, and receiving the time lapse signal or the voltage drop signal And an OR circuit for sending the droplet transfer start instruction signal.

  In a preferred embodiment of the present invention, the apparatus further comprises a plasma preheating value storage circuit for storing a plasma preheating value and a plasma steady value storage circuit for storing a plasma steady value, wherein the plasma output circuit starts the droplet transfer start. When the instruction signal is received, the plasma current is supplied at the plasma steady value, and the plasma current is supplied at the plasma preheating value before the droplet transfer start instruction signal is received.

  In a preferred embodiment of the present invention, the robot further comprises a welding robot including a welding torch for holding the consumable electrode, and an operation control circuit for sending an operation control signal to the welding robot, the operation control circuit including the droplet When a transition start instruction signal is received, a signal for moving the consumable electrode relative to the base material along the base material is sent as the operation control signal.

  According to such a structure, generation | occurrence | production of the spatter which can arise over the whole welding period can be suppressed, and welding can be performed efficiently.

  Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.

It is a figure which shows the structure of an example of the 2 electrode arc welding system concerning 1st Embodiment of this invention. It is principal part sectional drawing which shows the detail of the welding torch of FIG. It is a block diagram which mainly shows the detail of the MIG power supply device of FIG. It is a block diagram which mainly shows the detail of the plasma power supply device of FIG. It is a timing chart of each signal etc. in the welding method concerning a 1st embodiment of the present invention. It is a front view which shows the state of the MIG arc and the plasma arc in the welding method concerning 1st Embodiment of this invention. It is a block diagram which mainly shows the detail of the MIG power supply device of the 2 electrode arc welding system concerning 2nd Embodiment of this invention. It is a block diagram which mainly shows the detail of the MIG power supply device of the 2 electrode arc welding system concerning 3rd Embodiment of this invention. It is a block diagram which mainly shows the detail of the plasma power supply device of the 2 electrode arc welding system concerning 4th Embodiment of this invention. It is a timing chart of each signal etc. in the welding method concerning this embodiment.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. In the following, even if the current value or voltage value is a time average value of absolute values, it is simply called a current value or voltage value.

  1st Embodiment of this invention is described using FIGS. FIG. 1 is a diagram illustrating a configuration of an example of a two-electrode arc welding system according to the present embodiment.

  A two-electrode arc welding system A1 shown in the figure includes a welding robot 1, a robot control device 2, a MIG power supply device 3, and a plasma power supply device 4. The welding robot 1 automatically performs, for example, arc welding on the base material W. The welding robot 1 includes a base member 11, a plurality of arms 12, a plurality of motors 13, a welding torch 14, a wire feeding device 16, and a coil liner 19.

  The base member 11 is fixed to an appropriate location such as a floor. Each arm 12 is connected to the base member 11 via a shaft.

  FIG. 2 is a cross-sectional view of the main part showing details of the welding torch 14 in the two-electrode arc welding system shown in FIG.

  The welding torch 14 has a contact tip 141, a non-consumable electrode 142, a plasma nozzle 143, and a shield gas nozzle 144. The contact tip 141, the non-consumable electrode 142, and the plasma nozzle 143 are accommodated in the shield gas nozzle 144 and arranged on a concentric axis. From the gap between the shield gas nozzle 144 and the plasma nozzle 143, for example, a shield gas Gs such as Ar is supplied. A plasma gas Gp such as Ar is supplied into the gap between the plasma nozzle 143 and the non-consumable electrode 142. A center gas Gc such as Ar is supplied between the non-consumable electrode 142 and the contact tip 141.

  The consumable electrode 15 is fed from the through hole provided in the contact chip 141. The contact chip 141 is electrically connected to the consumable electrode 15. Non-consumable electrode 142 is made of, for example, Cu or Cu alloy. The non-consumable electrode 142 is indirectly water-cooled by cooling water passing through a path outside the figure. The non-consumable electrode 142 has a shape surrounding the consumable electrode 15 fed during welding. The plasma nozzle 143 is made of, for example, Cu or a Cu alloy. The plasma nozzle 143 is directly water-cooled by forming a channel through which cooling water passes.

  The motor 13 is a moving mechanism and is rotationally driven by the robot control device 2. By this rotational drive, the movement of each arm 12 is controlled, and the welding torch 14 holding the consumable electrode 15 and the non-consumable electrode 142 can freely move up and down, front and rear, and right and left. The motor 13 is provided with an encoder (not shown). The output of the encoder is sent to the robot control device 2.

  The wire feeding device 16 is provided in the upper part of the welding robot 1. The wire feeding device 16 is for feeding the consumable electrode 15 to the welding torch 14. The wire feeding device 16 includes a feeding mechanism 161 (motor), a wire reel (not shown), and a wire push device (not shown). Using the feeding mechanism 161 as a drive source, the wire push device feeds the consumable electrode 15 wound around the wire reel to the welding torch 14.

  One end of the coil liner 19 is connected to the wire feeder 16 and the other end is connected to the welding torch 14. The coil liner 19 has a tube shape, and a consumable electrode 15 is inserted through the coil liner 19. The coil liner 19 guides the consumable electrode 15 delivered from the wire feeder 16 to the welding torch 14. The fed consumable electrode 15 protrudes from the welding torch 14.

  FIG. 3 is a block diagram mainly showing details of the MIG power supply device 3 of FIG. FIG. 4 is a block diagram mainly showing details of the plasma power supply device 4 of FIG.

  The robot control device 2 includes an operation control circuit 21 and a teach pendant 23. The robot control device 2 is for controlling the operation of the welding robot 1.

  The operation control circuit 21 has a microcomputer and a memory (not shown). The memory stores a work program in which various operations of the welding robot 1 are set. Further, the operation control circuit 21 sets the robot moving speed Vr. The operation control circuit 21 sends an operation control signal Ms to the welding robot 1 based on the work program, the coordinate information from the encoder, the robot moving speed Vr, and the like. The welding robot 1 receives the operation control signal Ms and rotates each motor 13. As a result, the welding torch 14 moves to a predetermined welding start position in the base material W or moves along the in-plane direction of the base material W. Further, the operation control circuit 21 receives the droplet transfer start instruction signal Ss.

  The teach pendant 23 is connected to the operation control circuit 21. The teach pendant 23 is for the user of the arc welding system A1 to set parameters for executing welding.

  The MIG power supply device 3 includes a MIG output circuit 31, a droplet transfer start instruction circuit 32, a MIG start value storage circuit 34, a MIG preheat value storage circuit 35, and a feed control circuit 38. The MIG power supply device 3 is a device for applying a MIG voltage Vwa between the consumable electrode 15 and the base material W and flowing a MIG current Iwa. The MIG power supply device 3 is a device for feeding the consumable electrode 15.

  The mig start value storage circuit 34 stores the mig start value ir1. The value of the mig start value ir1 is input from the teach pendant 23 before starting welding, for example, and stored in the mig start value storage circuit 34 via the operation control circuit 21. The MIG preheat value storage circuit 35 stores the MIG preheat value ir2. The MIG preheat value ir2 is input from the teach pendant 23 before starting welding, for example, and stored in the MIG preheat value storage circuit 35 via the operation control circuit 21.

  The MIG output circuit 31 includes a power generation circuit MCa, a power supply characteristic switching circuit SW, a MIG current detection circuit IDa, a MIG current control circuit IRa, a current error calculation circuit EIa, a voltage detection circuit VDa, and a MIG voltage control circuit. VRa and voltage error calculation circuit EVa are included. The MIG output circuit 31 causes the MIG current Iwa to flow between the consumable electrode 15 and the base material W based on the values stored in the MIG start value storage circuit 34 and the MIG preheat value storage circuit 35.

  The power generation circuit MCa receives, for example, a commercial power supply such as three-phase 200 V, performs output control such as inverter control and thyristor phase control according to an error signal Ea described later, and outputs a Mig voltage Vwa and a Mig current Iwa.

  The MIG current detection circuit IDa is for detecting the value of the MIG current Iwa flowing between the consumable electrode 15 and the base material W. The Mig current detection circuit IDa sends a Mig current detection signal Ida corresponding to the value of the actually flowing Mig current Iwa to a current error calculation circuit EIa described later.

  The MIG current control circuit IRa is for setting the value of the MIG current Iwa flowing between the consumable electrode 15 and the base material W. The MIG current control circuit IRa is a MIG current setting signal for indicating the value of the MIG current Iwa based on at least one of the MIG start value ir1 and the MIG preheat value ir2 stored in each storage circuit. Ira is generated. Then, the MIG current control circuit IRa sends the generated MIG current setting signal Ira to a current error calculation circuit EIa described later. In the present embodiment, the MIG current control circuit IRa receives a welding start signal St and a detection signal Sd described later. When receiving the welding start signal St, the MIG current control circuit IRa sends, to the current error calculation circuit EIa, the MIG current setting signal Ira for causing the MIG current Iwa to flow at the MIG start value ir1. When receiving the detection signal Sd, the MIG current control circuit IRa sends to the current error calculation circuit EIa the MIG current setting signal Ira for causing the MIG current Iwa to flow with the MIG preheating value ir2.

  The current error calculation circuit EIa is for calculating the difference ΔIwa between the value of the Mig current Iwa that is actually flowing and the value of the set Mig current. Specifically, the current error calculation circuit EIa receives the Mig current detection signal Ida and the Mig current setting signal Ira corresponding to the set Mig current value, and sends a current error signal Eia corresponding to the difference ΔIwa. The current error calculation circuit EIa may send a current error signal Eia corresponding to a value obtained by amplifying the difference ΔIwa.

  The voltage detection circuit VDa is for detecting the value of the Mig voltage Vwa applied between the consumable electrode 15 and the base material W. The voltage detection circuit VDa sends a Mig voltage detection signal Vda corresponding to the Mig voltage Vwa to a voltage error calculation circuit EVa described later.

  The Mig voltage control circuit VRa is for setting the value of the Mig voltage Vwa applied between the consumable electrode 15 and the base material W. The Mig voltage control circuit VRa sends a Mig voltage setting signal Vra for instructing the value of the Mig voltage Vwa to the voltage error calculation circuit EVa, which will be described later, based on the voltage setting value stored in a memory circuit (not shown).

  The voltage error calculation circuit EVa is for calculating the difference ΔVwa between the value of the Mig voltage Vwa actually applied and the value of the set welding voltage. Specifically, the voltage error calculation circuit EVa receives the Mig voltage detection signal Vda and the Mig voltage setting signal Vra corresponding to the set welding voltage value, and sends a voltage error signal Eva corresponding to the difference ΔVwa. The voltage error calculation circuit EVa may send a voltage error signal Eva corresponding to a value obtained by amplifying the difference ΔVwa.

  The power supply characteristic switching circuit SW is for switching the power supply characteristic (constant current characteristic or constant voltage characteristic) of the MIG output circuit 31. When the power supply characteristic of the MIG output circuit 31 is a constant current characteristic, the output is controlled in the MIG output circuit 31 so that the value of the MIG current Iwa becomes a set value. On the other hand, when the power supply characteristic of the MIG output circuit 31 is a constant voltage characteristic, the output is controlled in the MIG output circuit 31 so that the value of the MIG voltage Vwa becomes a set value. More specifically, the power supply characteristic switching circuit SW receives a droplet transfer start instruction signal Ss, a current error signal Eia, and a voltage error signal Eva, which will be described later. When the switch in the power supply characteristic switching circuit SW is connected to the b side in FIG. 3, the power supply characteristic of the MIG output circuit 31 is a constant current characteristic, and the power supply characteristic switching circuit SW uses the current error signal Eia as the error signal Ea. This is sent to the power generation circuit MCa. At this time, the power generation circuit MCa performs control so that the value of the Mig current Iwa becomes a set value (that is, the above-described difference ΔIwa becomes zero). On the other hand, when the switch in the power supply characteristic switching circuit SW is connected to the a side in FIG. 2, the power supply characteristic of the MIG output circuit 31 is a constant voltage characteristic, and the power supply characteristic switching circuit SW converts the voltage error signal Eva into an error signal. Ea is sent to the power generation circuit MCa. At this time, the power generation circuit MCa performs control so that the value of the Mig voltage Vwa becomes a set value (that is, the above-described difference ΔVwa becomes zero). When receiving the droplet transfer start instruction signal Ss, the switch in the power supply characteristic switching circuit SW changes from the state connected to the b side in FIG. 2 to the state connected to the a side.

  The droplet transfer start instruction circuit 32 outputs an output for transferring the droplet to the MIG output circuit 31 when it is determined that the base material W has been heated by the plasma arc a2 to a temperature suitable for starting the droplet transfer. Let it begin. The droplet transfer start instruction circuit 32 receives a detection signal Sd described later, and sends a droplet transfer start instruction signal Ss after receiving the detection signal Sd. In the present embodiment, the droplet transfer start instruction circuit 32 sends a droplet transfer start instruction signal Ss when a certain set time has elapsed from the time when the detection signal Sd is received. More specifically, the droplet transfer start instruction circuit 32 includes a time measurement circuit 321, a set time storage circuit 322, and a comparison circuit 323.

  The time measurement circuit 321 measures a time T1 that has elapsed since the time when the detection signal Sd was received. The set time storage circuit 322 stores the set time T2. The set time T2 is input from the teach pendant 23 before starting welding, for example, and is stored in the set time storage circuit 322 via the operation control circuit 21. The comparison circuit 323 sends a droplet transfer start instruction signal Ss when the time T1 exceeds the set time T2.

  The feed control circuit 38 is for controlling the speed at which the consumable electrode 15 is fed from the welding torch 14 (feed speed Fw). The feed control circuit 38 sends a feed speed control signal Fc for instructing the feed speed Fw to the feed mechanism 161.

  As shown in FIG. 4, the plasma power supply device 4 includes a plasma output circuit 41, a plasma steady value storage circuit 45, and a detection circuit 47. The plasma power supply device 4 is a device for applying a plasma voltage Vwb and causing a plasma current Iwb to flow between the non-consumable electrode 142 and the base material W.

  The plasma steady value storage circuit 45 stores the value of the plasma steady value ir5. The value of the plasma steady value ir5 is input from the teach pendant 23 before starting welding, for example, and stored in the plasma steady value storage circuit 45 via the operation control circuit 21.

  Plasma output circuit 41 includes a power generation circuit MCb, a plasma current detection circuit IDb, a plasma current control circuit IRb, and a current error calculation circuit EIb. The plasma output circuit 41 causes the plasma current Iwb to flow between the non-consumable electrode 142 and the base material W based on the value stored in the plasma steady value storage circuit 45.

  The power generation circuit MCb receives, for example, a commercial power supply such as three-phase 200 V, performs output control such as inverter control and thyristor phase control according to a current error signal Eib described later, and outputs a plasma voltage Vwb and a plasma current Iwb.

  The plasma current detection circuit IDb is for detecting the value of the plasma current Iwb flowing between the non-consumable electrode 142 and the base material W. The plasma current detection circuit IDb sends a plasma current detection signal Idb corresponding to the value of the plasma current Iwb that is actually flowing to a current error calculation circuit EIb and a detection circuit 47 described later.

  The plasma current control circuit IRb is for setting the value of the plasma current Iwb flowing between the non-consumable electrode 142 and the base material W. The plasma current control circuit IRb generates a plasma current setting signal Irb for indicating the value of the plasma current value Iwb based on the plasma steady value ir5 stored in the plasma steady value storage circuit 45. Then, the plasma current control circuit IRb sends the generated plasma current setting signal Irb to a later-described current error calculation circuit EIb. In the present embodiment, the plasma current control circuit IRb receives the welding start signal St. When the plasma current control circuit IRb receives the welding start signal St, the plasma current control circuit IRb sends a plasma current setting signal Irb for flowing the plasma current Iwb at the plasma steady value ir5 to the current error calculation circuit EIb.

  The current error calculation circuit EIb is for calculating the difference ΔIwb between the value of the plasma current Iwb that is actually flowing and the value of the set plasma current. Specifically, the current error calculation circuit EIb receives the plasma current detection signal Idb and the plasma current setting signal Irb corresponding to the set plasma current value, and sends a current error signal Eib corresponding to the difference ΔIwb. The current error calculation circuit EIb may send a current error signal Eib corresponding to a value obtained by amplifying the difference ΔIwb. The power supply characteristic of the plasma output circuit 41 shown in FIG. 4 is a constant current characteristic. At this time, the power generation circuit MCb performs control so that the value of the plasma current Iwb becomes a set value (that is, the above-described difference ΔIwb becomes zero).

  The detection circuit 47 is for detecting the generation of the plasma arc a2 between the non-consumable electrode 142 and the base material W. When detecting the generation of the plasma arc a2, the detection circuit 47 sends a detection signal Sd to the MIG current control circuit IRa and the droplet transfer start instruction circuit 32. In the present embodiment, the detection circuit 47 receives the plasma current detection signal Idb. The detection circuit 47 determines whether or not the plasma arc a <b> 2 has occurred by detecting a change in the plasma current Iwb. For example, the detection circuit 47 determines that the plasma arc a2 has occurred when the value of the plasma current Iwb exceeds a certain threshold value. Although different from the present embodiment, the detection circuit 47 may determine whether or not the plasma arc a2 is generated by detecting a change in the plasma voltage Vwb. In this case, the detection circuit 47, for example, when the value of the plasma voltage Vwb decreases below a certain threshold after the generation of the mig arc a1, or the amount of decrease in the value of the plasma voltage Vwb exceeds a certain threshold. When it becomes larger, it may be determined that the plasma arc a2 has occurred.

  Next, an arc welding method using the two-electrode arc welding system A1 will be described with reference to FIGS. FIG. 5 is a timing chart of signals and the like in the welding method according to the present embodiment. FIG. 6 is a front view showing a state of a MIG arc and a plasma arc in the welding method according to the present embodiment. 5A is a power supply characteristic switching circuit SW, FIG. 5B is a feeding speed Fw, FIG. 5C is a MIG voltage Vwa, FIG. 5D is a MIG current setting signal Ira, FIG. 5E is a MIG current Iwa, and FIG. Indicates a change state of the plasma voltage Vwb, (g) indicates the plasma current setting signal Irb, (h) indicates the plasma current Iwb, (i) indicates the detection signal Sd, and (j) indicates the droplet transfer start instruction signal Ss. Note that the high level in FIG. 5A represents a state in which the switch of the power supply characteristic switching circuit SW is connected to the b side, and the low level is connected to the a side. Further, the states of the MIG arc a1 and the plasma arc a2 at the time of (s-1) to (s-6) in FIG. 5 correspond to those shown in (s-1) to (s-6) in FIG. To do.

(1) Generation period of mig arc a1 and plasma arc a2 (time t1 to time t4)
<Time t1 to Time t2>
In this embodiment, in order to generate the mig arc a1, a normal retract start is performed. First, at time t1, for example, a welding start signal St from the teach pendant 23 is sent to the operation control circuit 21, and then the power generation circuit MCa, the Mig current control circuit IRa, the power generation circuit MCb, and the plasma current control. It is sent to the circuit IRb (partially not shown). As shown in FIG. 5A, at time t1, the switch in the power supply characteristic switching circuit Sw is connected to the b side. Therefore, at time t1, the MIG output circuit 31 has a constant current characteristic. As shown in FIG. 5B, at time t1, the feed control circuit 38 sets the feed speed Fw as a feed speed control signal Fc to a positive value fw1 (slow-down feed speed). Is sent to the wire feeding device 16 of the welding robot 1. As a result, the consumable electrode 15 is fed forward from the welding torch 14 with the feeding speed Fw as the value fw1. The value fw1 is, for example, 100 to 300 cm / min. Further, as shown in FIG. 4D, when the MIG current control circuit IRa receives the welding start signal St, the MIG current setting signal Ira is a current error calculation circuit that causes the MIG current Iwa to flow at the MIG start value ir1. Send to EIa. At time t1, since the consumable electrode 15 and the base material W are separated from each other, as shown in FIG. 5E, during a certain period (time t1 to time t2 in this embodiment) from time t1. The Mig current Iwa does not flow between the consumable electrode 15 and the base material W. Then, as shown in FIG. 5C, during the period from time t1 to time t2, a no-load voltage of about 80 V, for example, is applied as the Mig voltage Vwa between the consumable electrode 15 and the base material W. .

  Similarly, as shown in FIG. 5G, when the plasma current control circuit IRb receives the welding start signal St, the plasma current setting signal Irb that causes the plasma current Iwb to flow at the plasma steady value ir5 is calculated as a current error. Send to circuit EIb. The plasma steady value ir5 is, for example, 50 to 200A. At time t1, as shown in FIG. 6 (h), the plasma current Iwb is generated between the non-consumable electrode 142 and the base material W during a certain period from time t1 (time t1 to time t4 in this embodiment). Does not flow. As shown in FIG. 5F, a non-load voltage of about 80 V, for example, is applied as a plasma voltage Vwb between the non-consumable electrode 142 and the base material W from time t1 to time t4. The

<Time t2 to time t3>
As shown in FIG. 6 (s-2), the consumable electrode 15 is fed and contacts the base material W at time t2. As a result, as shown in FIG. 5E, at time t2, the Mig current Iwa starts flowing between the consumable electrode 15 and the base material W at the Mig start value ir1. The mig start value ir1 is a small value of about 30 to 70A. Therefore, even when energization of the Mig current Iwa from the consumable electrode 15 to the base material W is started, the consumable electrode 15 is hardly heated by Joule heat. Further, as shown in FIG. 4C, the Mig voltage Vwa becomes almost 0 at time t2. As shown in FIG. 4B, at time t2, the feed control circuit 38 uses a wire feed rate of the welding robot 1 as a feed rate control signal Fc, with the feed rate Fw being a negative value fw2. Send to feeding device 16. Thereby, the consumable electrode 15 is fed backward.

<Time t3 to Time t4>
As shown in FIG. 6 (s-3), the consumable electrode 15 is separated from the base material W by being fed backward at time t3. Then, a mig arc a1 is generated between the consumable electrode 15 and the base material W. The generation of the MIG arc a1 at time t3 occurs not by blowing the consumable electrode 15 but by pulling up the consumable electrode 15. Further, as described above, the value of the Mig current Iwa at the time t3 is the Mig start value ir1, and thus is relatively small. Therefore, sputtering is very unlikely to occur at time t3. After the mig arc a1 is generated at time t3, the consumable electrode 15 continues to be fed backward until the plasma arc a2 is generated between the non-consumable electrode 142 and the base material W (time t4).

  Then, as shown in FIG. 6 (s-4), a plasma arc a2 is generated at time t4. The generation of the plasma arc a <b> 2 is promoted by the growth of the MIG arc a <b> 1, whereby the plasma atmosphere is immediately below the consumable electrode 15 and the non-consumable electrode 142. That is, the plasma arc a2 is generated using the mig arc a1 as a pilot arc. Due to the generation of the plasma arc a2, as shown in FIG. 5 (h), the plasma current Iwb starts to flow at the plasma steady value ir5 between the non-consumable electrode 142 and the base material W at time t4. Further, as shown in FIG. 5F, the plasma voltage Vwb decreases at time t4. When the plasma arc a2 is generated, the detection circuit 47 determines that the plasma arc a2 is generated based on the change in the plasma current Iwb. As shown in FIG. 6I, when the detection circuit 47 determines that the plasma arc a2 has occurred, the detection signal Sd is sent to the Mig current control circuit IRa and the droplet transfer start instruction circuit 32 (in this embodiment, the time measurement). Circuit 321).

(2) Preheating period of base material W (time t4 to time t5)
As shown in FIG. 5 (b), at time t4, the feed control circuit 38 sets the feed speed Fw as a feed speed control signal Fc to a value fw3 which is a positive value. Send to feeding device 16. As a result, the consumable electrode 15 is fed forward from the welding torch 14 with the feed speed Fw as the value fw3. The value fw3 is, for example, 50 to 150 cm / min. As shown in FIG. 4D, when the MIG current control circuit IRa receives the detection signal Sd, the MIG current setting signal Ira that causes the MIG current Iwa to flow at the MIG preheat value ir2 is sent to the current error calculation circuit EIa. . As a result, as shown in FIG. 5E, at time t4, the MIG current Iwa starts to flow at the MIG preheat value ir2. That is, at time t4, the MIG output circuit 31 starts a preheating period in which the MIG current Iwa flows at the MIG preheating value ir2. The MIG preheating value ir2 is, for example, 15 to 30A. Preferably, the MIG preheat value ir2 is smaller than the MIG start value ir1. Thus, the state where the mig arc a1 is generated continues from time t4 to time t5. On the other hand, from time t4 to time t5, the state in which the plasma arc a2 is generated continues. From time t4 to time t5, the welding start portion of the base material W is heated by the plasma arc a2. Thereby, sufficient heat input is performed at the welding start portion of the base material W. Therefore, the penetration depth of the weld bead can be made uniform from the welding start part to the steady welding part.

  On the other hand, when the droplet transfer start instruction circuit 32 receives the detection signal Sd, the droplet transfer start instruction circuit 32 measures a time T1 that has elapsed since the time when the detection signal Sd was received. When the comparison circuit 323 determines that the time T1 has exceeded the set time T2 stored in the set time storage circuit 322, as shown in FIG. This is sent to the power supply characteristic switching circuit SW in the output circuit 31 and the operation control circuit 21. The set time T2 may be set to a time that is considered suitable for heating the base material W. The set time T2 is, for example, 0.1 to 1.0 seconds.

(3) Droplet transfer period (time t5)
As shown in FIG. 5 (a), when the power supply characteristic switching circuit SW receives the droplet transfer start instruction signal Ss at time t5, the state in which the switch of the power supply characteristic switching circuit SW is connected to the b side is It changes to the state connected to the side. Therefore, at time t5, the power supply characteristic of the MIG output circuit 31 changes to a constant voltage characteristic. Also, as shown in FIG. 5B, at time t5, the feed control circuit 38 replaces the welding robot 1 with a feed speed Fw that is a positive value fw4 as the feed speed control signal Fc. To the wire feeder 16. As a result, the consumable electrode 15 is fed forward from the welding torch 14 with the feeding speed Fw as the value fw4. Then, as shown in FIG. 5E, the value of the MIG current Iwa increases from the MIG preheat value ir2 to a relatively large MIG steady value ir3. That is, the MIG output circuit 31 starts a droplet transfer period during which droplet transfer is transferred from the consumable electrode 15 to the base material W. The Mig steady value ir3 is larger than the Mig start value ir1 and the Mig preheat value ir2, and is, for example, 70 to 250A. On the other hand, the state in which the plasma arc a2 is generated continues between the non-consumable electrode 142 and the base material W after time t5. The value of the plasma current Iwb after time t5 is the plasma steady value ir5. Further, when the operation control circuit 21 receives the droplet transfer start instruction signal Ss, the operation control circuit 21 moves the welding torch 14 holding the consumable electrode 15 and the non-consumable electrode 142 relative to the base material W as the operation control signal Ms. A signal is sent to the welding robot 1. Thereby, the welding robot 1 starts moving the welding torch 14 relative to the base material W along the base material W. In this way, at time t5, a droplet transfer period for transferring the droplets from the consumable electrode 15 to the base material W is started, and two-electrode arc welding is performed. In the droplet transfer period, the MIG arc a1 is in a state of being surrounded by the plasma arc a2. Thereby, plasma arc a2 has the effect | action which restrains that mig arc a1 spreads.

  Next, the effect of this embodiment is demonstrated.

  In the present embodiment, from time t4 to time t5, the state in which both the MIG arc a1 and the plasma arc a2 are generated is continued while the MIG current Iwa flows at the MIG preheat value ir2. Further, at time t5, the value of the MIG current Iwa is increased from the MIG preheat value ir2, and the droplet transfer from the consumable electrode 15 to the base material W is started. Therefore, the droplet transfer period (time t5) can be started without regenerating the mig arc a1 as in the prior art. Therefore, the spatter that may occur when the MIG arc a1 is regenerated as in the prior art does not occur in the first place. Therefore, generation | occurrence | production of the spatter which can arise over the whole welding period can be suppressed. Further, in the present embodiment, after the mig arc a1 is once generated, the droplet transfer period is started without extinguishing the mig arc a1. Therefore, the reoccurrence of mig arc a1 does not fail. Therefore, welding can be performed efficiently. As mentioned above, according to this embodiment, generation | occurrence | production of the spatter which can arise over the whole welding period can be suppressed, and welding can be performed efficiently.

  Preferably, the MIG preheat value ir2 is smaller than the MIG start value ir1. If the MIG preheating value ir2 is smaller than the MIG start value ir1, the consumable electrode 15 is less likely to be heated than when the MIG preheating value ir2 and the MIG start value ir1 are the same. Therefore, droplet transfer from the consumable electrode 15 to the base material W during the preheating period (time t4 to time t5) can be suppressed. Further, during the preheating period (time t4 to time t5), the plasma arc a2 is generated. Therefore, even if the MIG preheating value ir2 is smaller than the MIG start value ir1, the MIG arc a1 is difficult to extinguish and it is easy to maintain the state where the MIG arc a1 is generated.

  In the present embodiment, the plasma arc a2 is generated using the MIG arc a1 as a pilot arc. Therefore, it is not necessary for the plasma power supply device 4 to include a high-frequency circuit or the like for generating the plasma arc a2.

  Hereinafter, other embodiments of the present invention will be described. In the following description and the drawings to be referred to, the same reference numerals are given to the same or similar components as those in the above-described embodiment, and the description thereof is omitted.

  Next, a second embodiment of the present invention will be described with reference to FIG.

  FIG. 7 is a block diagram mainly showing details of the MIG power supply device of the two-electrode arc welding system according to the present embodiment.

  In the two-electrode arc welding system A2 according to the present embodiment, unlike the first embodiment, the droplet transfer start instruction circuit 32 does not include the time measurement circuit 321, the set time storage circuit 322, and the comparison circuit 323. Instead, in the two-electrode arc welding system A2, the droplet transfer start instruction circuit 32 includes a voltage detection circuit 324, a set voltage value storage circuit 325, and a comparison circuit 326. In this respect, the two-electrode arc welding system A2 according to the present embodiment is different from the two-electrode arc welding system A1 according to the first embodiment.

  The voltage detection circuit 324 detects the Mig voltage Vwa applied between the consumable electrode 15 and the base material W. The set voltage value storage circuit 325 stores the set voltage Vth. The set voltage Vth is input from the teach pendant 23 before starting welding, for example, and stored in the set voltage value storage circuit 325 via the operation control circuit 21. The comparison circuit 326 sends a droplet transfer start instruction signal Ss when the Mig voltage Vwa falls below the set voltage Vth after receiving the detection signal Sd.

  As shown in FIG. 5, when welding is performed using the two-electrode arc welding system A <b> 2, almost the same process as described above is performed. In the present embodiment, the step of determining in the droplet transfer start instruction circuit 32 whether or not to start the droplet transfer period (time t5) is different from the first embodiment.

  During the period from time t4 to time t5, the state in which the mig arc a1 and the plasma arc a2 are generated is continued in substantially the same manner as in the first embodiment. During the period from time t4 to time t5, the consumable electrode 15 hardly transfers droplets to the base material W. Further, the consumable electrode 15 is fed forward. Therefore, as shown in FIG. 5C, the Mig voltage Vwa gradually decreases. In the present embodiment, the comparison circuit 326 determines that the base material W has been sufficiently heated when the Mig voltage Vwa falls below the set voltage Vth after receiving the detection signal Sd. At this time, the comparison circuit 326 sends a droplet transfer start instruction signal Ss to the power supply characteristic switching circuit SW in the MIG output circuit 31 and the operation control circuit 21. Thereby, the droplet transfer period (time t5) which transfers a droplet from the consumable electrode 15 to the base material W is started.

  According to this embodiment, for the same reason as described in the first embodiment, it is possible to suppress the generation of spatter that may occur over the entire welding period, and to perform welding efficiently.

  The value of the mig voltage Vwa is proportional to the arc length of the mig arc a1. Therefore, in this embodiment in which droplet transfer starts when the MIG voltage Vwa falls below the set voltage Vth, it can be said that droplet transfer has started when the MIG arc a1 falls below a certain arc length. Therefore, after time t4, the problem that the consumable electrode 15 comes into contact with the base material W before starting the droplet transfer due to the consumable electrode 15 approaching the base material W more than expected is avoided. Can do.

  Next, a third embodiment of the present invention will be described with reference to FIG.

  FIG. 8 is a block diagram mainly showing details of the MIG power supply device of the two-electrode arc welding system according to the present embodiment.

  In the two-electrode arc welding system A3 according to the present embodiment, unlike the above-described embodiment, the droplet transfer start instruction circuit 32 includes a time measurement circuit 321, a set time storage circuit 322, and a first comparison circuit 323 ′. , A voltage detection circuit 324, a set voltage value storage circuit 325, a second comparison circuit 326 ′, and an OR circuit 329.

  Since the time measurement circuit 321, the set time storage circuit 322, the voltage detection circuit 324, and the set voltage value storage circuit 325 in the present embodiment are the same as those in the above-described embodiment, description thereof is omitted. The first comparison circuit 323 ′ according to the present embodiment is different in that the comparison circuit 323 according to the first embodiment sends a droplet transfer start instruction signal Ss while sending a time lapse signal Ss1, and the other points are the same. It is. The second comparison circuit 326 ′ in the present embodiment is different in that the comparison circuit 326 according to the second embodiment sends a droplet transfer start instruction signal Ss, whereas the second comparison circuit 326 ′ sends the voltage drop signal Ss2, and the other points are the same. It is. The OR circuit 329 receives the time lapse signal Ss1 and the voltage drop signal Ss2. When the logical sum circuit 329 receives either the time lapse signal Ss1 or the voltage drop signal Ss2, it sends a droplet transfer start instruction signal Ss. That is, when the time T1 elapsed from the time when the plasma arc a1 occurs exceeds the set time T2 and when the value of the Mig voltage Vwa becomes smaller than the set voltage value Vth, whichever comes first In addition, the logical sum circuit 329 sends a droplet transfer start instruction signal Ss. Thereby, the droplet transfer period (time t5) is started.

  In the two-electrode arc welding method according to the present embodiment, the process of sending the droplet transfer start instruction signal Ss is only different from that of the above-described embodiment, and thus the description thereof is omitted.

  According to this embodiment, for the same reason as described in the first embodiment, it is possible to suppress the generation of spatter that may occur over the entire welding period, and to perform welding efficiently.

  As in the second embodiment, droplet transfer starts when the Mig voltage Vwa falls below the set voltage Vth. When the Mig arc a1 falls below a certain arc length, droplet transfer starts. It can be said that. Therefore, after time t4, the problem that the consumable electrode 15 comes into contact with the base material W before starting the droplet transfer due to the consumable electrode 15 approaching the base material W more than expected is avoided. Can do.

  Further, according to the present embodiment, whether or not to start droplet transfer is also determined based on the time T1 that has elapsed since time 4 when the plasma arc a2 occurred. Therefore, even after the time t4, even if the consumable electrode 15 has not approached the base material W as expected (that is, even if the Mig voltage Vwa is not lower than the set voltage Vth), the time T1 is equal to the set time T2. When the value exceeds the value, droplet transfer starts. Therefore, even after the time t4, even if the consumable electrode 15 is not as close as expected to the base material W, the droplet transfer can be started without preheating the base material W excessively.

  Next, a fourth embodiment of the present invention will be described using FIG. 9 and FIG.

  FIG. 9 is a block diagram mainly showing details of the plasma power supply device of the two-electrode arc welding system according to the present embodiment. FIG. 10 is a timing chart of signals and the like in the welding method according to the present embodiment.

  The two-electrode arc welding system A4 according to the present embodiment is different from the two-electrode arc welding system A1 according to the first embodiment in that the plasma power supply device 4 further includes a plasma preheat value storage circuit 44. The plasma preheating value storage circuit 44 stores the plasma preheating value ir4. For example, the plasma preheat value ir4 is input from the teach pendant 23 before starting welding, and is stored in the plasma preheat value storage circuit 44 via the operation control circuit 21. Further, the plasma current control circuit IRb in the plasma output circuit 41 receives the droplet transfer start instruction signal Ss.

  Next, the difference from the point described in the first embodiment in the two-electrode arc welding method according to the present embodiment will be described.

  In this embodiment, as shown in FIG. 10 (g), when the plasma current control circuit IRb receives the welding start signal St at time t1, the plasma current Iwb is set to the plasma preheating value ir4 as the plasma current setting signal Irb. The flow is sent to the current error calculation circuit EIb. Then, as shown in FIG. 9H, at time t4, the plasma current Iwb starts to flow at the plasma preheat value ir4 between the non-consumable electrode 142 and the base material W due to the generation of the plasma arc a2 (that is, The plasma output circuit 41 starts flowing the plasma current Iwb at the plasma preheat value ir4). The plasma preheating value ir4 is preferably set to a value larger than the plasma steady value ir5. The plasma preheating value ir4 is, for example, 150 to 250A.

  As shown in FIG. 11H, when the plasma current control circuit IRb receives the droplet transfer start instruction signal Ss at time t5, the plasma current setting signal Irb is used to cause the plasma current Iwb to flow at the plasma steady value ir5. To the current error calculation circuit EIb. Thereby, in the droplet transfer period after time t5, the plasma current Iwb flows at the plasma steady value ir5 (that is, the plasma output circuit 41 flows the plasma current Iwb at the plasma steady value ir5).

  In the present embodiment, in the preheating period (time t4 to time t5), the plasma current Iwb is caused to flow at a plasma preheating value ir4 that is larger than the plasma steady value ir5. Such a configuration is suitable for increasing the amount of heat that the plasma arc a2 gives to the base material W during the preheating period (time t4 to time t5). Therefore, according to this embodiment, the penetration depth of the weld bead at the welding start portion of the base material W can be made deeper. Thereby, a flat bead can be formed.

  Note that the configuration further including the plasma preheat value storage circuit 44 described in the present embodiment may be combined with the configuration described in the second or third embodiment. Further, the method described in this embodiment in which the plasma current Iwb is caused to flow at a plasma preheating value ir4 that is larger than the plasma steady value ir5 in the preheating period (time t4 to time t5) is described in the second or third embodiment. You may combine with a method.

  The scope of the present invention is not limited to the embodiment described above. The specific configuration of each part of the present invention can be changed in various ways. For example, in the above-described embodiment, the value of the Mig voltage Vwa does not change at time t5, but the present invention is not limited to the case where the Mig voltage Vwa does not change at time t5.

  The Mig voltage Vwa and Mig current Iwa after time t5 and the plasma voltage Vwb and plasma current Iwb after time t4 may take a constant DC value, a DC pulse, or an AC pulse.

A1, A2, A3, A4 Two-electrode arc welding system 1 Welding robot 11 Base member 12 Arm 13 Motor 14 Welding torch 141 Contact tip 142 Non-consumable electrode 143 Plasma nozzle 144 Shield gas nozzle 15 Consumable electrode 16 Wire feeding device 161 Feeding mechanism 19 Coil liner 2 Robot control device 21 Operation control circuit 23 Teach pendant 3 Mig power supply device 31 Mig output circuit 32 Droplet transfer start instruction circuit 321 Time measurement circuit 322 Set time storage circuit 323 Comparison circuit 323 ′ First comparison circuit 324 Voltage detection Circuit 325 Set voltage value storage circuit 326 Comparison circuit 326 ′ Second comparison circuit 329 OR circuit 34 Mig start value storage circuit 35 Mig preheat value storage circuit 38 Feed control circuit 4 Plasma power supply device 41 Plasma output circuit 44 Plasma preheat value Memory circuit 45 Plasma steady-state memory circuit 47 Detection circuit Ea Error signal EIa Current error calculation circuit Eia Current error signal EIb Current error calculation circuit Eib Current error signal EVa Voltage error calculation circuit Eva Voltage error signal Fc Feed rate control signal Fw Feed Speed IDa Mig current detection circuit Ida Mig current detection signal IDb Plasma current detection circuit Idb Plasma current detection signal ir1 Mig start value ir2 Mig preheating value ir3 Mig steady value ir4 Plasma preheating value ir5 Plasma steady value IRa Mig current control circuit Ira Mig current setting Signal IRb Plasma current control circuit Irb Plasma current setting signal Iwa Mig current Iwb Plasma current MCa Power generation circuit MCb Power generation circuit Ms Operation control signal Sd Detection signal Ss Droplet transfer start instruction signal St Welding start signal SW Power supply characteristic cut off Circuit T1 time T2 set time VDa voltage detecting circuit Vda MIG voltage detection signal Vr robot moving speed VRa MIG voltage control circuit Vra MIG voltage setting signal Vwa MIG voltage Vwb plasma voltage

Claims (12)

Generating a mig arc between the base material and the consumable electrode;
Using the MIG arc as a pilot arc, generating a plasma arc between the base material and the non-consumable electrode surrounding the consumable electrode;
A step of continuing the state in which both the MIG arc and the plasma arc are generated while the MIG current flowing between the consumable electrode and the base material is caused to flow at the MIG preheating value after the plasma arc is generated. When,
A step of increasing the value of the MIG current from the MIG preheating value and starting droplet transfer from the consumable electrode to the base material. A step of measuring a time elapsed from the time when the plasma arc is generated;
The arc welding method according to claim 1, wherein the step of starting the droplet transfer is performed when the elapsed time exceeds a set time. Further comprising the step of detecting the value of the Mig voltage between the consumable electrode and the base material,
The arc welding method according to claim 1, wherein the step of starting the droplet transfer is performed when the value of the Mig voltage becomes smaller than a set voltage value after the plasma arc is generated. Measuring the time elapsed from the time when the plasma arc is generated;
Detecting the value of the Mig voltage between the consumable electrode and the base material,
The step of starting the droplet transfer is performed when the elapsed time exceeds the set time, and when the value of the Mig voltage becomes smaller than the set voltage value, whichever comes first, The arc welding method according to claim 1. After the step of starting the droplet transfer, further comprising a step of transferring the droplet from the consumable electrode to the base material while flowing a plasma current flowing from the non-consumable electrode to the base material at a plasma steady value,
5. The two-electrode arc welding method according to claim 1, wherein, in the step of continuing the generated state, the plasma current is caused to flow at a plasma preheating value larger than the plasma steady value.   The two-electrode according to claim 1, further comprising a step of starting to move the consumable electrode relative to the base material along the base material after the step of continuing the generated state. Arc welding method. A plasma output circuit for passing a plasma current between the non-consumable electrode and the base material;
A detection circuit that sends a detection signal when detecting the occurrence of a plasma arc between the non-consumable electrode and the base material;
A mig preheat value storage circuit for storing the mig preheat value;
A droplet transfer start instruction circuit that receives the detection signal and sends a droplet transfer start instruction signal after receiving the detection signal;
When the detection signal is received, a preheating period in which the MIG current flowing between the consumable electrode and the base material flows at the MIG preheating value is started, and when the droplet transfer start instruction signal is received, the consumable electrode A two-electrode arc welding system comprising: a MIG output circuit that starts a droplet transfer period for transferring droplets to the base material.   The droplet transfer start instruction circuit includes a time measurement circuit that measures a time elapsed from the time when the detection signal is received, a set time storage circuit that stores a set time, and the elapsed time exceeds the set time. The two-electrode arc welding system according to claim 7, further comprising a comparison circuit that sometimes sends the droplet transfer start instruction signal.   The droplet transfer start instruction circuit receives a voltage detection circuit for detecting a Mig voltage applied between the consumable electrode and the base material, a set voltage storage circuit for storing a set voltage, and the detection signal. The two-electrode arc welding system according to claim 7, further comprising a comparison circuit that sends the droplet transfer start instruction signal when the MIG voltage falls below the set voltage.   The droplet transfer start instruction circuit includes a time measurement circuit that measures a time elapsed from the time when the detection signal is received, a set time storage circuit that stores a set time, and the elapsed time exceeds the set time. A first comparison circuit that sometimes sends a time lapse signal, a voltage detection circuit that detects a Mig voltage applied between the consumable electrode and the base material, a set voltage storage circuit that stores a set voltage, and the detection A second comparison circuit that sends a voltage drop signal when the MIG voltage falls below the set voltage after receiving the signal, and the droplet transfer start instruction signal when receiving the time lapse signal or the voltage drop signal. The two-electrode arc welding system according to claim 7, further comprising a sending OR circuit. A plasma preheat value storage circuit for storing the plasma preheat value;
A plasma steady value storage circuit for storing the plasma steady value;
When the plasma output circuit receives the droplet transfer start instruction signal, the plasma current flows at the plasma steady value, and before receiving the droplet transfer start instruction signal, the plasma current flows at the plasma preheating value. A two-electrode arc welding system according to any one of claims 7 to 10. A welding robot including a welding torch for holding the consumable electrode;
An operation control circuit for sending an operation control signal to the welding robot;
12. The operation control circuit, when receiving the droplet transfer start instruction signal, sends a signal for moving the consumable electrode relative to the base material along the base material as the operation control signal. The two-electrode arc welding system according to any one of the above.

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