JP4971398B2 - Resistance welding monitoring device and monitoring method - Google Patents

Description

In the resistance welding in which welding is performed by pressurizing a workpiece (workpiece to be welded) sandwiched between a pair of electrodes and causing a current to flow from the electrode to the workpiece to cause resistance heat generation in the workpiece, The present invention relates to a resistance welding monitoring device and a monitoring method for monitoring and determining the state of a workpiece sandwiched between electrodes.

For example, as shown in Patent Documents 1 and 2, a resistance welding system rectifies a commercial AC power source into a DC and converts it into an AC of a predetermined frequency using an inverter, a converted AC voltage A welding transformer for changing the shape, a pair of electrodes connected to the secondary side of the welding transformer, and, if necessary, a workpiece conveying device to be welded and a workpiece restraining device for restraining the workpiece at a predetermined position , And a welding robot that controls the position and posture of a welding gun having a pair of electrodes.

And in the automated welding system, because of troubles and failures of the above equipment and equipment, workpiece transfer errors, welding position shifts, etc. will occur, and if these are overlooked and welding is continued, defective products will be produced. Will do. Therefore, in order to prevent the occurrence of these defective products, 1) Method A for monitoring the presence or absence of workpiece loading using mechanical or laser light, and 2) Detecting the voltage between the electrodes to detect defective welding. The detection method B is generally performed (for example, Patent Document 3 and Patent Document 4).

Japanese Patent Laid-Open No. 2004-90027 JP 2006-188771 A Japanese Patent Laid-Open No. 06-106363 Japanese Patent No. 3421387

However, in the above-described method A, 1) it is necessary to change the device when the type of the workpiece changes as a dedicated device, 2) it becomes expensive overall, 3) when the steel plate (work) is thin, For example, when the thickness of one workpiece becomes 0.7 mm or less, there is a problem that the workpiece is warped or distorted during pressurization and the number of workpieces cannot be accurately detected.

In the method B described above, since the voltage between the electrodes reflects the resistance welding phenomenon, it is recognized that the detection and monitoring of the voltage between the electrodes is effective for the quality control of the resistance welding. Although it is used on a trial basis, it is often difficult to put into practical use and operation because of the following problems at production sites. That is, 1) A lead wire is required to detect the voltage between the electrodes, but the lead wire is easily damaged by scattering (high-temperature metal particles) scattered from the welded portion. 2) Since the welding gun for pressurizing the workpiece moves during welding, the lead wire also moves along with this, and there is a risk of fatigue and disconnection due to repeated stress.

The present invention has been made in view of such circumstances, and does not use a lead wire or the like that directly measures the voltage between electrodes, can be configured at low cost, and is held in a welded state, that is, sandwiched between a pair of electrodes. It is an object of the present invention to provide a resistance welding monitoring device and a monitoring method capable of accurately monitoring the state of a workpiece.

The resistance welding monitoring apparatus according to the first invention in accordance with the above-mentioned object performs a constant current control of a load current using a switching element that performs a PWM control using a commercial AC power supply as a direct current through a rectifier circuit. A resistance welding monitoring device that obtains a welding current from the secondary side of the welding transformer by applying an alternating current to the welding transformer by an inverter circuit,
The product (Tk) of the ratio Tk of the gate signal ON time (gate ON time) of the PWM control switching element corresponding to the primary or secondary voltage of the welding transformer during welding and the output voltage V2 of the rectifier circuit V2) is obtained, and a warning is issued when the product (Tk · V2) is outside the range between the upper limit level VH and the lower limit level VL set in advance.

Here, the gate on time ratio Tk [= on time / (on time + off time)] of the PWM control switching element and the output voltage V2 of the rectifier circuit are not limited to analog processing, but the ratio Tk and the output of the rectifier circuit. The present invention is also applied to a case where the voltage V2 is temporarily A / D converted and digitally processed (the same applies to the following method inventions).

In the resistance welding monitoring apparatus according to the first aspect of the invention, it is preferable that the upper limit level VH and the lower limit level VL can be set independently from the outside. Thereby, the welding state can be monitored and the upper limit level VH and the lower limit level VL can be set at appropriate positions.

In the resistance welding monitoring apparatus according to the first aspect of the present invention, the product (Tk · V2) may be displayed on a monitor during each welding. This can be used to monitor the welding state and set the upper limit level VH and the lower limit level VL at appropriate positions.

In the resistance welding monitoring device according to the first aspect of the present invention, it is preferable that the resistance welding monitoring device further includes means for transmitting any one or more of the ratio Tk, the output voltage V2, and the product (Tk · V2) to peripheral devices. . Thereby, the state of each resistance welding can be monitored from the outside.
In the resistance welding monitoring apparatus according to the first invention, the alternating current converted by the inverter circuit is, for example, between 100 and 20000 Hz. Thereby, the iron core amount of the welding transformer can be reduced.

A resistance welding monitoring method according to a second aspect of the present invention that meets the above-described object is an inverter that performs constant current control of a load current using a switching element that performs a PWM control using a commercial AC power supply as a direct current through a rectifier circuit. In the resistance welding monitoring method for obtaining a welding current from the secondary side of the welding transformer by applying an alternating current with a higher frequency by a circuit and applying the alternating current to the welding transformer,
The product (Tk) of the ratio Tk of the gate signal ON time (gate ON time) of the PWM control switching element corresponding to the primary or secondary voltage of the welding transformer during welding and the output voltage V2 of the rectifier circuit V2) is obtained, and a warning is issued when the product (Tk · V2) is outside the range between the upper limit level VH and the lower limit level VL set in advance.

In the resistance welding monitoring apparatus and monitoring method according to the present invention, the product (Tk · V2) of the gate-on time ratio Tk of the PWM-controlled switching element and the output voltage V2 of the rectifier circuit is obtained. Is proportional to the secondary voltage of the welding transformer. The secondary side voltage is the state of the workpiece sandwiched between the pair of electrodes connected to the secondary side of the welding transformer (that is, the number of workpieces stacked or increased, welding misalignment, and electrode wear status) ), The welding state can be grasped by constantly checking during welding that the product (Tk · V2) is within the range between the upper limit level VH and the lower limit level VL set in advance.

Here, there is a method of directly measuring the output voltage of the welding power source control device or the primary voltage of the welding transformer and monitoring the value at the upper limit level and the lower limit level. However, the inverter circuit supplies the primary voltage to the primary side of the welding transformer. When the primary current I1 is turned on or off by PWM control, noise is generated on the output side of the welding power source control device or the primary side of the welding transformer, and countermeasures for noise removal are necessary. On the other hand, there is no noise in the ratio Tk of the gate-on time of the switching element under PWM control and the output voltage V2 of the rectifier circuit, and the product (Tk · V2) corresponds to the secondary voltage, so the reliability is high.

1 is a block circuit diagram of a resistance welding apparatus to which a resistance welding monitoring apparatus according to an embodiment of the present invention is applied. It is operation | movement explanatory drawing of each part of the monitoring apparatus of the resistance welding. It is a flowchart of the monitoring apparatus of the resistance welding. It is explanatory drawing of Example 1 which applied the monitoring method of resistance welding which concerns on this invention. In Example 1, it is a graph which shows the example of a setting of the actual value of the output calculation voltage by the number of sheets of a workpiece | work, and the monitoring determination value. (A), (B), (C) is explanatory drawing of Example 2. FIG.

Next, with reference to the attached drawings, an embodiment of the present invention will be described for understanding of the present invention.
First, a resistance welding apparatus 10 to which a resistance welding monitoring apparatus according to an embodiment of the present invention is applied will be described with reference to FIG.

The resistance welding apparatus 10 receives a predetermined gate signal, a rectifier circuit 11 connected to a three-phase (or single-phase) commercial AC power supply, a smoothing capacitor 12 connected to the output side of the rectifier circuit 11, and An inverter circuit 13 that converts direct current converted by the rectifier circuit 11 into alternating current, a welding transformer 14 that becomes a load of the inverter circuit 13, a rectifier 15 that is connected to the secondary side of the welding transformer 14, and a secondary to the rectifier 15 A pair of electrodes 18 and 19 are connected via conductors 16 and 17. These will be described in detail below.

The rectifier circuit 11 is composed of a three-phase bridge circuit, and a smoothing capacitor 12 is connected in parallel to form a direct current having a substantially constant voltage. The inverter circuit 13 includes power transistors Q1 to Q4 which are examples of switching elements. The power transistors Q1 and Q4 and the power transistors Q2 and Q3 are alternately turned on by gate signals 1 and 2 (see b and c in FIG. 2). It is turned off, PWM control is performed, and an AC voltage (for example, 100 to 20000 Hz) is applied to the primary side of the welding transformer 14.

A CT (current transformer) 20 is provided on the primary side of the welding transformer 14, detects the primary current of the welding transformer 14, and the measurement current Is is input to the microcomputer 22 through the A / D conversion circuit 21. The microcomputer 22 receives a value Iw corresponding to a reference current determined from the outside in advance, compares the measured current Is with the reference current Iw, and the PWM control unit 23 so that the difference is always reduced during welding. The gate signals 1 and 2 are controlled via.

The PWM control using the gate signals of the power transistors Q1 to Q4 is well known, and is not affected by the welding load, that is, the state of the workpiece sandwiched between the electrodes and the fluctuation of the commercial power supply voltage, and is always constant during welding. The current (I1) is controlled to flow to the primary side of the welding transformer 14.
In the welding transformer 14, a welding current flows between the electrodes 18 and 19 through the secondary conductors 16 and 17 through the rectifier 15 on the secondary side. The welding current I2w is proportional to the primary current I1 of the welding transformer 14 and is always constant during welding.

The output voltage V2 of the rectifier circuit 11 is input to the microcomputer 22 via the output voltage measurement circuit 25 and the A / D conversion circuit 27. The microcomputer 22 has a CPU, RAM, and ROM therein, and incorporates a program for realizing the flow shown in FIG. 3 in addition to the program for making the primary current of the welding transformer 14 constant. .

Gate signals 1 and 2 sent to the power transistors Q1 to Q4 from the PWM control unit 23 that operates in response to a signal from the microcomputer 22 are output to the gate ON / OFF time measuring unit 28. Since the ON times of the gate signals 1 and 2 are the ON times of the power transistors Q1 to Q4, this is measured, and the output voltage calculation unit 29 averages the half cycle (or one cycle or a plurality of cycles). I do. The on-time of the power transistors Q1 to Q4 is Ton, the off-time is Toff, and the gate on-time ratio Tk = [Ton / (Ton + Toff)]. This ratio Tk corresponds to the average value of the ON times of the power transistors Q1 to Q4.

The product (Tk · V2) of the output voltage V2 of the rectifier circuit 11 obtained via the output voltage measurement circuit 25, the A / D conversion circuit 27, and the microcomputer 22 and the ratio Tk calculated by the output voltage calculation unit 29 ) Is output from the output voltage calculation unit 29 to the comparison circuit 30. Note that the output voltage V2 of the rectifier circuit 11 is also obtained by performing an averaging process in units of half cycles or one cycle corresponding to a certain period (that is, the time during which the ratio Tk is measured). The output voltage calculation average value Vw consisting of this product (Tk · V2) is displayed on the display 31 as a monitor.

In the comparison circuit 30, the upper limit level VH of the monitoring voltage and the lower limit level VL of the monitoring voltage are input, and when the output voltage calculation average value Vw is larger than the upper limit level VH, a signal is output to the alarm circuit 32 and an alarm output is generated. It is done. When the output voltage calculation average value Vw is smaller than the lower limit level VL, an alarm is output from the alarm circuit 32. The upper limit level VH and the lower limit level VL can be arbitrarily set from the outside using the switch means (or volume) of the monitoring voltage upper limit setting unit 33 and the monitoring voltage lower limit setting unit 34.

In this case, the upper limit level VH and the lower limit level VL can be set to an arbitrary number of times after the start of energization (for example, the fourth and eighth cycles).
A separate input / output circuit 35 is connected to the microcomputer 22 to communicate one or more of the gate on time ratio Tk, the output voltage V2, and the product (Tk · V2) to a peripheral device (for example, a personal computer 36). Transmit with.

Subsequently, the resistance welding monitoring device and the resistance welding monitoring method according to the embodiment of the present invention will be described in more detail with reference to FIGS. 2 and 3.
The resistance welding apparatus 10 controls the gate signals 1 and 2 of the power transistors Q1 to Q4 by a well-known technique so that the primary current I1 of the welding transformer 14 is always constant during welding.
In FIG. 2, a) shows the gate on / off times of the power transistors Q1 to Q4, b) the gate signal 1 and the switching operation of the power transistors Q1 and Q4, and c) the gate signal 2 and the power transistor Q2, The switch operation of Q3 is shown respectively. The output voltage Vo of the inverter circuit 13 (that is, the output voltage Vo of the welding power source control device 10a indicated by the broken line in FIG. 1) is shown in FIG. Tc is the cycle time of the inverter.

As is apparent from d) of FIG. 2, the output voltage Vo is proportional to the ON time of the power transistors Q1 to Q4, that is, the “ON time / (ON time + OFF time)” of the gate signals 1 and 2. Accordingly, the gate ON / OFF time measuring unit 28 measures the on time and the off time, and the output voltage calculating unit 29 measures the on time ratio Tk [= on time / (on time + off time)] of the gate signals 1 and 2. Is calculated (step S1).

On the other hand, the peak value of the output voltage Vo when the power transistors Q1 to Q4 of the welding power source control device 10a are on is clearly proportional to the output voltage V2 of the rectifier circuit 11 as shown in e) of FIG. Therefore, the average value of the output voltage of the rectifier circuit 11 is calculated (step S2). The product of the gate on-time ratio Tk and the output voltage V2 becomes the output voltage Vo corresponding to the welding power source control device 10a (primary side of the welding transformer 14). As shown in FIG. The calculated voltage Vw of (Tk · V2) is displayed on the display 31 (step S3).

On the other hand, since the resistance welding apparatus 10 performs constant current control, the voltage Vo on the primary side of the welding transformer 14 varies depending on the state of the workpiece (workpiece to be welded) 37 sandwiched between the electrodes 18 and 19. And since the relationship of voltage Vo (Tk * V2) = Vw is materialized, if the value of Vw is monitored, a welding state can be grasped | ascertained. That is, since the resistance (impedance) of the welding transformer 14, the secondary conductors 16 and 17, and the rectifier 15 are the same, the primary-side voltage Vo remains constant even when welding is performed a plurality of times in a fixed welding state. However, for example, when the resistance of the work 37 between the electrodes 18 and 19 decreases, the primary voltage Vo decreases, and when the resistance of the work 37 between the electrodes 18 and 19 increases. The primary-side voltage Vo increases.

Since the voltage Vo on the primary side is proportional to the value of Vw, the output voltage calculation average value Vw is monitored. When the output voltage calculation average value Vw is higher than the upper limit level (monitoring upper limit set value) VH (step) S4), an alarm output is issued from the alarm circuit 32 (step S6). When the output voltage calculation average value Vw is lower than the lower limit level (monitoring lower limit set value) VL (step S5), an alarm output is issued from the alarm circuit 32 (step S6).

Next, Embodiment 1 of the present invention will be described with reference to FIGS.
As shown in FIG. 4, 1) when no workpiece is inserted between the electrodes 18 and 19, 2) when one workpiece is inserted, 3) when two target workpieces are stacked, and 4) three workpiece stacks Welding experiments in the case of In this case, the workpiece was a 0.8 mm mild steel plate, the welding current was 7 kA, the energization time was 10 cycles, and the applied pressure was 2.45 kN.

FIG. 5 shows the relationship between the output voltage calculation average value Vw and time (energization cycle) in this case. Note that one cycle in this case is 1/60 second (one cycle at 60 Hz). The output voltage calculation average value Vw is displayed on the display 31 and can be obtained by the personal computer 36.

In FIG. 5, 116V in the fourth cycle is set to the lower limit level VL, 128V is set to the upper limit level VH, 112V in the eighth cycle is set to the lower limit level VL, and 123V is set to the upper limit level VH. This setting is performed by the monitoring voltage upper limit setting unit 33 and the monitoring voltage lower limit setting unit 34. With this setting, an alarm is issued when the fourth and eighth cycles are out of the upper limit level VH and lower limit level VL after the start of welding.

Using the upper limit level VH and the lower limit level VL set in the first embodiment, an experiment was conducted as a second embodiment in a state in which the workpiece is sandwiched as shown in FIGS. 6 (A) to 6 (C).
In the example shown in FIG. 6A, when the ends of the workpiece are welded, the end of the stacked upper workpiece is crushed and the resistance of the welded portion is reduced. Vw dropped and VL> Vw and an alarm was issued. In the example shown in FIG. 6 (B), the electrode was used excessively, the sectional area of the electrode tip increased, the contact resistance between the electrode tip and the workpiece decreased, and an alarm was issued with VL> Vw. Further, in the example shown in FIG. 6C, when the upper electrode is inclined, when the work is heated by the welding current, the electrode slides on the work, and the upper upper work is scraped by the electrode to reduce the thickness. As a result, the resistance of the welded portion decreased and VL> Vw and an alarm was issued.

In the above-described embodiment and examples, a rectifier is disposed on the secondary side of the welding transformer, the secondary current of the welding transformer is rectified, and the workpiece is welded using a DC welding current. Output from the inverter circuit when resistance welding is performed using an AC welding current (frequency may be higher or lower than commercial AC) converted by the inverter circuit without providing a rectifier on the secondary side The present invention is also applied when the load current is controlled at a constant current.

10: resistance welding device, 10a: welding power source control device, 11: rectifier circuit, 12: smoothing capacitor, 13: inverter circuit, 14: welding transformer, 15: rectifier, 16, 17: secondary conductor, 18, 19: Electrode, 20: CT, 21: A / D conversion circuit, 22: Microcomputer, 23: PWM control unit, 25: Output voltage measurement circuit, 27: A / D conversion circuit, 28: Gate ON / OFF time measurement unit, 29: output voltage calculation unit, 30: comparison circuit, 31: indicator, 32: alarm circuit, 33: monitoring voltage upper limit setting unit, 34: monitoring voltage lower limit setting unit, 35: input / output circuit, 36: personal computer, 37: work

Claims (6)

A commercial AC power supply is set to DC via a rectifier circuit, the DC is set to AC using a switching element that performs PWM control, and AC is applied to an inverter circuit that performs constant current control of a load current, and the AC is applied to a welding transformer, A resistance welding monitoring device for obtaining a welding current from a secondary side of a welding transformer,
The product (Tk · V2) of the on-time ratio Tk of the gate signal of the PWM control switching element corresponding to the primary or secondary voltage of the welding transformer at the time of welding and the output voltage V2 of the rectifier circuit A resistance welding monitoring apparatus characterized in that a warning is issued when the product (Tk · V2) is outside a range between a preset upper limit level VH and a lower limit level VL. The resistance welding monitoring apparatus according to claim 1, wherein the upper limit level VH and the lower limit level VL can be set independently from the outside. 3. The resistance welding monitoring apparatus according to claim 1, wherein the value of the product (Tk · V2) is displayed on a monitor during each welding. The resistance welding monitoring apparatus according to any one of claims 1 to 3, further comprising means for communicating the value of the product (Tk · V2) to a peripheral device. The resistance welding monitoring apparatus according to any one of claims 1 to 4, wherein the alternating current converted by the inverter circuit is between 100 and 20000 Hz. A commercial AC power supply is converted to a direct current via a rectifier circuit, and the direct current is converted to a higher frequency alternating current by an inverter circuit that performs constant current control of the load current using a switching element that performs PWM control, and the alternating current is applied to a welding transformer. In the resistance welding monitoring method for obtaining a welding current from the secondary side of the welding transformer,
The product (Tk · V2) of the on-time ratio Tk of the gate signal of the PWM control switching element corresponding to the primary or secondary voltage of the welding transformer at the time of welding and the output voltage V2 of the rectifier circuit A resistance welding monitoring method characterized in that a warning is issued when the product (Tk · V2) is outside a range between a preset upper limit level VH and a lower limit level VL.

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