JP3120933B2 - Resistance welding control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance welding control method, and more particularly to a resistance welding control method for appropriately controlling a welding current in resistance point welding.

[0002]

2. Description of the Related Art Resistance spot welding in which two members are point-joined by utilizing resistance heat generated when current is applied to a contact portion of lap-joined members is well known, and is used for thin metal products such as automobiles, aircrafts, and railway vehicles. It has been applied to bonding and has shown extremely high productivity.

[0003] However, this resistance welding has the drawback that the welding result tends to vary due to various conditions such as the surface condition of the joint, the welding spot position, the number of spots, and other problems, and welding defects are likely to occur. Such poor welding results from the inability to obtain an optimum welding current due to the various factors described above. The normal optimum welding current is selected as a current value immediately before the so-called scattering occurs when the joining member is melted and scattered by the welding current (hereinafter, this current is referred to as a current lower limit).

At a current value lower than the lower limit of the current, spattering is prevented. However, poor welding is likely to occur . On the other hand, at an excessive current higher than the lower limit of the current (hereinafter, this current is referred to as an upper limit current), the spattering is prevented. A phenomenon in which the mechanical strength is reduced by occurrence is seen. Normally, in order to obtain a desired mechanical strength, a state of occurrence of slight scattering is also allowed, whereas welding failure due to an undercurrent must be avoided.

In a resistance spot welding operation in an automobile production process or the like, a large number of welding processes must be continuously performed. In order to stabilize the overall welding quality, the actual welding current is set to 10 times the optimum welding current. The welding current is set to about 30% larger. This setting takes into account a decrease in the welding current density due to wear of the welding electrode, a fluctuation in the power supply voltage, and the like, and the work can be performed under the same welding conditions until the electrode is replaced.

However, according to the above-described method, in the initial use state of the electrode, an excessive current flows through the joining member, resulting in large scattering or burrs, welding of the electrode and the joining member, and further significant consumption of the electrode. Various disadvantages such as acceleration are caused. In particular, when welding a rust-proof surface-treated steel sheet such as a zinc alloy plating, there is a problem that the consumption of the electrode due to an excessive current increases, and the welding workability is remarkably reduced.

Therefore, as a conventional resistance spot welding method, attention is paid to the fact that the optimum welding current changes along the current value immediately before the occurrence of spattering, and at the same time, when the spattering occurs, the thickness of the joining member at the welding point greatly changes. The change in the wall thickness of the welding point is detected, and the amount of change is greater than or equal to the reference value, the scatter generated during resistance point welding is detected, and the welding current is scattered. (Japanese Patent Publication No. 57-37430).

[0008]

In the resistance spot welding method disclosed in the above publication, the presence or absence of occurrence of scattering is detected, and the welding current is reduced if the amount of scattering is large, and the welding current is reduced if the amount of scattering is small. By increasing the current, it is intended to obtain an optimal welding current corresponding to the wear of the electrodes (hereinafter referred to as a scattering limit current).

However, the limit current of the generation of the splatter varies depending on not only the state of the splay but also the surface condition of the tip of the electrode. In other words, in the process of consuming the electrode, the surface condition of the tip of the electrode is not only blunted, but also as shown in FIG.
As shown in FIG. 8A, an island-shaped portion 101a is formed in the center of the electrode 101, and as shown in FIG. 8B, a recess 102a in which the center of the electrode 102 is recessed in a crater shape is formed. . The island-shaped portion 101a is formed of a mild steel plate,
2a is a phenomenon often observed in galvanized steel sheets.

The problem here is the crater-shaped recess 102a often found in the latter galvanized steel plate. When the galvanized steel sheet is welded, an alloy of copper and zinc is formed at the tip of the electrode when the galvanized steel plate is welded. However, as the wear of the electrode progresses, heat concentrates on the central portion. This is caused by melting of the alloy in the part.

In a state where the crater-shaped recess 102a is generated, as shown in FIG.
The contact area between the electrode and the electrode 102 is reduced, and the nucleus of the formed nugget 105 is formed at a position deviated from the center of the electrode (indicated by a dashed line in the figure) or at a plurality of positions. In this case, since the contact area between the members to be welded 103 and 104 and the electrode 102 is small, the nugget 10
Insufficient pressing force at the formation position of No. 5 causes easy scattering. Further, the nugget 105 to be formed is
This is smaller than a normal case where a does not occur.

When the welding process is performed using the electrode 102 having the concave portion 102a as described above and employing the above-described resistance spot welding method, the conventional resistance point welding method simply determines the state of occurrence of scattering. Since the increase / decrease control of the spill occurrence limit current was performed, the spill occurrence limit current was reduced even in the state where spillage occurred due to a small pressing force, so that the formed nugget 105 became small and sufficient welding strength was obtained. There was a problem that it was not possible.

In order to remove the concave portion 102a and obtain a normal electrode shape, a high current is applied to the electrode 102 to heat the electrode 102, and the protrusion 102b formed around the concave portion 102a is heated.
(See FIG. 8B).
However, since the heat generation at the tip of the electrode is suppressed when the scatter generation limit current is reduced as described above, the electrode 1
In No. 02, the state in which the concave portion 102a is formed is kept forever, and therefore, the state in which the limit current for generating the scatter is reduced continues forever. As a result, the scattering generation limit current value is greatly reduced, and the nugget 105 cannot be sufficiently generated even by this current reduction.
There is a problem that welding strength is reduced.

This problem is caused when the contact area between the electrode and the workpiece is small, for example, as shown in FIG.
6 has unevenness 106a on the surface, and FIG.
As shown in FIG. 3B, this is a problem that also occurs when the ends of the workpieces 103 and 104 are welded using the electrodes 107 (when the electrodes 107 partially contact the workpieces 103 and 104). .

The present invention has been made in view of the above points, and sets the welding current value to the current lower limit value when the welding current value set based on the occurrence of scattering becomes smaller than a predetermined current lower limit value. Accordingly, an object of the present invention is to provide a resistance welding control method capable of performing reliable resistance welding.

[0016]

In order to solve the above-mentioned problems, in the method of the present invention, a plurality of members to be welded are sandwiched between two opposing electrodes while being pressurized at a constant pressure, and a predetermined distance is provided between the electrodes. By applying a welding current for a certain period of time, in the resistance welding control method of spot welding a plurality of members to be welded using the resistance heat generated at the welding point between the members to be welded, the presence or absence of occurrence of scattering is detected. , The state of occurrence of this scattering
Next welding current value which is the next actual welding current value
Dispersion that changes according to the state of dispersal as well as setting
The current lower limit value, which is the current value immediately before the occurrence, is obtained, and the next hit point current value set above is excessively reduced, and the next hit point current value becomes the current value.
When the current value becomes smaller than the lower limit value , the next hit point current value is reset to the current lower limit value.

Further, when the number of times that the welding current value becomes smaller than the predetermined current lower limit value continues for a predetermined number of times or more, an abnormality may be output.

[0018]

According to the above-mentioned method, the following method is used according to the state of occurrence of scattering.
While controlling the hitting point current value , when the welding current value is excessively reduced and becomes equal to or less than the current lower limit value, by performing welding at the current lower limit value, a concave portion is formed at the electrode tip portion as described above. Even in the case where the welding current is reduced due to the occurrence of dispersion due to the above-mentioned reasons, the next strike point current value does not become lower than the current lower limit value, and a relatively high current value is maintained.

Therefore, a large decrease in the welding current can be prevented by guarding a decrease in the current caused by the shape of the tip of the electrode or the like with a predetermined current value (current lower limit value). Nugget can be generated enough to perform resistance welding, and the reliability of resistance welding can be improved.

As described above, since the welding current value maintains a relatively high current value, the temperature of the electrode tip rises, and even if a deformed portion occurs at the electrode tip, the deformed portion is melted at the electrode tip. By doing so, the electrode is crushed and returns to a normal electrode shape, so that an appropriate welding process can be performed.

[0021]

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a flowchart showing the basic processing of the resistance welding control method according to one embodiment of the present invention. As shown in the figure, in the resistance welding control method according to the present embodiment, first, in step 10, the secondary resistance value is measured from the primary current value and voltage of the welding transformer. Next step 1
In step 2, detection processing is performed to determine whether or not dispersion has occurred based on the resistance value of the welding transformer on the secondary side determined in step 10. In step 14 according to the state of occurrence of dispersion determined in step 12, The calculated welding current value is calculated. In the next step 16, a comparison process is performed between the welding current value obtained in step 14 and the current lower limit value and the upper limit current value, which will be described later. If the state where the value exceeds the upper limit current value continues for a long time, an abnormality is notified. Hereinafter, each of the above-described processes will be described in detail.

First, a method of obtaining the resistance value on the secondary side of the welding transformer will be described.

In a RL AC circuit generally used for resistance point welding, if the voltage value is V, the resistance value is R, the inductance is L, and the current is i, the following relationship is established between them. It has been known.

V = R ・ i + L ・ (di / dt) Therefore, at the moment when di / dt = 0 (that is, at the moment of welding), the formula becomes: R = V / i ... The formula does not include the component of L. That is, the amount of change in current (di
By sampling the voltage V and the current i at the moment when / dt) is zero, the resistance value R of the RL AC circuit used for resistance point welding can be easily obtained from the equation. Also, assuming that the winding number ratio between the primary side and the secondary side of the welding transformer is m,
The relationship shown in the following equation is established between the primary side and the secondary side.

V2 = V1 / m I2 = I1 · m where V1: primary-side voltage V2: secondary-side voltage I1: primary-side current I2: secondary-side current From the relationship of the above formulas, the secondary-side resistance value R2 can be obtained by the following formula.

R2 = V2 / I2 = V1 / (I1 · m ² ) Therefore, from the equation, the primary voltage V1 and the primary current I1 of the welding transformer are sampled at the timing of di / dt = 0, and the primary voltage is sampled. The secondary resistance value R2 of the welding transformer can be calculated by dividing the side voltage value V1 by a value obtained by multiplying the primary side current value I1 by the square of the winding number ratio m and multiplying the result.

FIG. 2 is a block diagram showing a resistance value calculation circuit incorporated in the resistance spot welding apparatus 1 according to the present embodiment.
In the figure, reference numeral 2 denotes a power supply, which is connected to the adjuster 8. The adjuster 8 is connected to the microcomputer 32, and performs current adjustment and energization and de-energization of the primary coil 10 of the welding transformer 3 based on an instruction of the microcomputer 32. A high current is generated in the secondary coil 16 by the timing of the energization and the stop of the energization, and the electrodes 12, 14 disposed with the workpiece W interposed therebetween.
Is configured to weld the workpiece W.

Also, between the power source 2 and the adjuster 8,
A current detector 4 for detecting a primary current value and a voltage detector 6 for detecting a primary voltage value are provided. As a current obtained by the current detection unit 4, a differential signal of an actual current waveform is input. For this reason, the current detection unit 4 is provided with the integration circuit 18
The integration circuit 18 performs integration processing to form an actual current waveform. The voltage detector 6 functions as a transformer, and the level is adjusted by the amplifier circuit 26 after the actual welding power supply voltage is reduced.

The signal generated by the current detector 4 (differential waveform of the actual current waveform; hereinafter, referred to as di / dt signal) is also supplied to the di / dtｄ0 detection circuit 24. . The di / dt ≒ 0 detection circuit 24 detects that the signal di / dt signal supplied from the current detection unit 4 is substantially zero (d
At the timing when i / dt ≒ 0), the signal is supplied to the sample and hold circuit 20 disposed on the current detection side, the sample and hold circuit 28 disposed on the voltage detection side, and the microcomputer 32. The signal di / dt supplied from the current detector 4 causes the microcomputer 32 to generate an interrupt. The microcomputer 32 converts the current signal and the voltage signal sampled by the sample / hold circuit 28 into the analog / digital-to-analog 3
Take in via 0.

When the primary current signal (I1) and the voltage signal (V1) are fetched in this way, the microcomputer 32 calculates the secondary-side resistance value R2 based on the above equation. As described above, the arithmetic processing of the resistance value R2 on the secondary side can be obtained by a relatively easy arithmetic processing, and the circuit configuration can be a relatively simple configuration as shown in FIG.

Next, a process for detecting occurrence of scattering based on the secondary-side resistance value R2 obtained by the above process will be described.

FIG. 3 shows an example of a change in the resistance value when no scattering occurs, and FIG. 4 shows an example of a change in the resistance value when the scattering occurs. As can be seen from each figure, when the scattering occurs, the resistance value between the electrodes sharply decreases. Therefore, it is possible to determine whether the scattering has occurred by capturing this phenomenon.

Therefore, as shown in FIG. 5, the total amount of the drop in the resistance value from the time point (Rpk) at which the resistance value becomes the largest in a series of resistance value changes to the last cycle is represented by ΔR, and R
In the amount of descent between each cycle from pk to the last cycle, the value with the largest amount of descent is Rmax. Usually, a change in the resistance of 25% to 100% of ΔR is seen at the moment when the scattering occurs. Therefore, in this embodiment, (R
When the value of (max / ΔR) · 100 is 25 or more, it is determined that scattering has occurred.

Next, the welding current calculation processing and the abnormality detection processing will be described together with reference to FIGS. 6 and 7. FIG. FIG. 6 is a flowchart showing the welding current calculation process and the abnormality detection process. FIG. 7 shows the transition of the actual welding current, the transition of the current lower limit value, and the upper limit current value when the method of this embodiment is employed. Is shown. It should be noted that the welding current calculation processing and the abnormality detection processing shown in FIG.

When the welding current calculation processing and the abnormality detection processing shown in FIG. 6 are started, first, in step 20, a known welding sequence is executed to start welding to the workpiece W. In the subsequent step 22, the current value of the next hit point (hereinafter, this current value is referred to as the next hit point current value) is determined based on the scattering occurrence state obtained by the resistance value calculation processing and the scattering detection processing (see FIG. 1). calculate.

Specifically, when it is determined that the value of (Rmax / ΔR) · 100 calculated in the scattering detection process is 25 or more, the microcomputer 32 determines that scattering has occurred. When the value of (Rmax / .DELTA.R) .multidot.100 is less than 25, it is determined that no scattering has occurred, and the next hit point current value is increased. I do.

The next strike point current value is a current value just before the so-called scattering occurs when the joining member is melted and scattered by the welding current (hereinafter, referred to as “the current value”).
It is desirable to set this current as the current lower limit value). At a current value lower than the current lower limit value, scattering is prevented, but welding failure is likely to occur. A phenomenon in which the mechanical strength of welding decreases is observed.

Further, in the resistance spot welding operation in the automobile production process or the like, the welding current is set to a value larger than a lower limit value of the current by a predetermined value (this is referred to as an allowable drop current value) in order to stabilize welding quality. I have. This allowable drop current value is selected to be a value of about 10 to 30% of the lower limit value of the current.

FIG. 7 shows the transition of the actual welding current, the transition of the lower limit of the current, and the upper limit of the current. As described above, the actual welding current increases or decreases in accordance with the presence or absence of scattering, and tends to increase due to an increase in the contact area with the workpiece W due to wear of the electrodes 12 and 14. Further, the actual welding current is set to a current value larger than the current lower limit value by the allowable drop current value.

Returning to FIG. 6, the description will be continued. Step 2
In 4, it is determined whether the next hit point current value obtained in step 22 is smaller than the lower limit current value. As described above, the lower limit current value is a current value immediately before the occurrence of scattering.
Therefore, if a positive determination is made in step 24,
The process proceeds to step 26, in which the lower limit current value is used as a new next hit point current value instead of the next hit point current value calculated in step 22.

In FIG. 7, between the hitting points N1 and N2, the next hitting point current value (shown by a broken line) is smaller than the lower limit current value. However, by performing the processing in step 26, the next strike point current value (actual welding current value) does not become lower than the lower limit current value as a limit, so that a relatively high current value is maintained.

Therefore, a large decrease in the welding current can be prevented by guarding a decrease in the current caused by the shape of the tip of the electrode or the like with a predetermined current value (current lower limit value). As a result, a nugget sufficient to perform strong welding can be generated, and the reliability of resistance welding can be improved.

As described above, since the welding current value maintains a relatively high current value, the temperature of the electrode tip rises, and even if a deformed portion (for example, a concave portion) is formed at the electrode tip, this deformed portion is not removed. The electrode tip is crushed by melting to return to a normal electrode shape. By this self-maintenance action, it is possible to always perform an appropriate welding process.

When a new next hit point current value is set by the processing of step 26, the next step 28 is step 2.
The number of rotations in which the next hit point current value calculated in step 2 is smaller than the lower limit current value is counted, and it is determined whether or not the count has continued for a predetermined number of times.

Then, the next hit point current value falls below the lower limit current value.
The number of hits is equal to or greater than a predetermined
1 to N3 ), there is a possibility that some abnormality has occurred in the resistance point welding apparatus. Therefore, in this case, the process proceeds to step S30, and an output of occurrence of abnormality is made to the operator. If a negative determination is made in step 28, the process ends without outputting an abnormality.

On the other hand, if a negative determination is made in step 24, the process proceeds to step 32,
It is determined whether or not the next hit point current value obtained in the step is larger than the upper limit current value. When an affirmative determination is made in step 32, the process proceeds to step 34,
The upper limit current value is used as a new next hit point current value instead of the next hit point current value calculated in step 2.

In FIG. 7, after the number N3 of hit points, the next hit point current value is larger than the upper limit current value (the next hit point current is indicated by a broken line). However, by performing the processing in step 34, the next hitting point current value (actual welding current value) does not become larger than the upper limit current value (14.4 KA in this embodiment). For this reason, it is possible to prevent the occurrence of poor welding due to supplying a high current, and to improve the reliability of resistance point welding.

When a new next hit point current value is set by the processing of step 34, the next step 36 is step 2
The number of rotations in which the next hit point current value calculated in Step 2 exceeds the upper limit current value is counted, and it is determined whether or not the counted number has continued for a predetermined number of times or more. If the number of times that the next hit point current value exceeds the upper limit current value is equal to or more than a predetermined number of times, there is a possibility that some abnormality has occurred in the resistance point welding apparatus. Therefore, in this case, the processing is performed at step 38.
To output to the operator when an abnormality has occurred.
If a negative determination is made in step 36, the process ends without outputting an abnormality occurrence.

If a negative determination is made in both the steps 24 and 32, that is, the next hit point current value calculated in the step 22 is the difference between the upper limit current value and the current lower limit value .
If the values are between , the process proceeds to step 40,
It is determined whether or not the next hit point current value calculated in step 22 is the largest current value in the past.

If an affirmative determination is made in step 40, that is, if it is determined that the next hit current value calculated in step 22 is the largest current value in the past, the process proceeds to step 42, in which the current lower limit is set. The value is updated. As described above, in general, the electrodes 12, 14 (see FIG. 2) are deformed by performing the welding process, and the contact area with the workpiece W increases with time, and the current value of the next hit point also tends to increase accordingly. It is in. Therefore, the value of the current lower limit value indicating the generation limit of the scattering also increases.

In this embodiment, when the next hit point current value reaches the past maximum current value, a process for updating and increasing the current lower limit value is performed. Specifically, step 4
When it is determined that the current value of the next hit point is the largest current value in the past at 0, the allowable drop current value is subtracted from the current value of the next hit point at the time of the determination, and the value obtained by this subtraction processing is used as a new value. The current lower limit. On the other hand, if a negative determination is made in step 40, the process ends without performing the process of step 42.

Therefore, as shown in FIG. 7, the transition of the current lower limit value is maintained at the current lower limit value calculated on the basis of the previous maximum next hit current value until the past maximum next hit current value is calculated. Is done. Therefore, after the current lower limit value is updated, even if the next hit current value smaller than the maximum next hit current value is calculated in step 22, the current lower limit value is not changed. Maintain a very high current value. As a result , the electrode tip can be maintained at a high temperature, and even if a deformed portion occurs at the electrode tip, the deformed portion is returned to a normal electrode shape by melting the electrode tip, and an appropriate welding process is performed. It can be carried out.

[0054]

Effects of the Invention According to the present invention as described above, controls the next RBI current value according to occurrence of expulsion, the next RBI electrostatic
By performing welding at a current lower limit value when the current values becomes lowered than the current limit value excessively, even when the scattering due to the electrode tip shape occurs the welding current is lowered, the next The hit point current value does not fall below the current lower limit value, and a relatively high current value can be maintained.

Therefore, a large decrease in the welding current can be prevented by guarding a decrease in the current caused by the shape of the tip of the electrode or the like with a predetermined current value (current lower limit value). Nugget can be generated enough to perform resistance welding, and the reliability of resistance welding can be improved.

As described above, since the welding current value maintains a relatively high current value, the temperature of the electrode tip rises, and even if a deformed portion occurs at the electrode tip, the deformed portion is melted at the electrode tip. By doing so, the electrode is crushed and returns to a normal electrode shape, so that an appropriate welding process can be performed.

[Brief description of the drawings]

FIG. 1 is a flowchart showing basic processing of a resistance welding control method according to one embodiment of the present invention.

FIG. 2 is a block diagram showing a resistance value calculation circuit incorporated in the resistance point welding apparatus 1 according to the embodiment.

FIG. 3 is a diagram illustrating an example of a change in resistance value when no scattering occurs.

FIG. 4 is a diagram illustrating an example of a change in resistance value when scattering occurs.

FIG. 5 is a diagram for explaining a process of detecting occurrence of scattering.

FIG. 6 is a flowchart showing welding current calculation processing and abnormality detection processing.

FIG. 7 is a diagram showing a transition of an actual welding current, a transition of a current lower limit value, and an upper limit current value when the method of the present embodiment is adopted.

FIG. 8 is a diagram for explaining deformation occurring at an electrode tip.

FIG. 9 is a view for explaining a problem of conventional resistance point welding.

FIG. 10 is a view for explaining a problem of conventional resistance point welding.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 resistance point welding device 2 power supply 8 regulator 10 primary coil 16 secondary coil 12, 14 electrode 24 di / dt ≒ 0 detection circuit 32 microcomputer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-180384 (JP, A) JP-A-4-300078 (JP, A) JP-A-2-220785 (JP, A) JP-A-3-3 8585 (JP, A) Hikaru 2-123385 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) B23K 11/24 B23K 11/25

Claims (2)

(57) [Claims] 1. Two electrodes facing a plurality of members to be welded
Between the electrodes while pressing at a constant pressure
The welding current is supplied for a certain period of time,
Utilizing the resistance heat generated at the welding point between the materials,
In a resistance welding control method for spot welding a member to be welded, the presence or absence of scattering is detected,Depending on the state of the scattering
Set the next spot current value, which is the next actual welding current value
In addition, the scattering occurrence time varies according to the state of occurrence of the scattering.
Find the current lower limit value that is the previous current value,  Set aboveNext point current valueExcessively decreased,Next RBI
The current value is the current lower limit valueIf it becomes smaller,Next shot
Point current valueIs reset to the above current lower limit value.
Resistance welding control method. 2. The resistance welding control method according to claim 1, wherein when the number of times the welding current value becomes smaller than a predetermined current lower limit value continues for a predetermined number of times or more, an abnormality occurrence is output.