KR900002481B1 - Control method of spot welding - Google Patents

Abstract

The method is for extending a period to replace a tip and improve the productivity by detecting an actual abrasion state of an electrode tip from the minimum value of an inter-electrode resistance in a welding process. After the initial electric conduction of spot welding, the welding is executed in accordance with a selected welding condition, and an output of an average value operating circuit is compared with a boundary value set in advance by a comparing and operating circuit at every welding peak point.

Description

Control method of spot welding

1 is a waveform diagram showing a change in resistance between the tips of electrodes during a welding operation.

2 is a block diagram showing an embodiment of realizing a control method according to the present invention.

3 is a graph showing the relationship between the change in the average value of the minimum resistance between the electrode tips calculated according to the degree of wear of the electrode tip and the series of limit values corresponding to the wear class for correcting the welding quality.

4 is a diagram showing a method of calculating the average value of the lowest resistances corresponding to a preset number of welding points.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to spot welding, and more particularly, to a welding control method for controlling various welding conditions during spot welding, such as welding current and resistance welding time, depending on the degree of wear of the electrode tip.

In the spot welding machine, when the welding is performed for a long time, the tip tip of the electrode gradually expands, and the shape of the end portion is gradually expanded and deformed due to the strong pressing force and heating during welding, which directly affects the welding performance. Considering this situation, in order to perform a good welding, it is generally necessary to periodically trim the tip tip of the electrode or replace it with a new electrode. As a countermeasure to prolong the period of such maintenance work, in the prior art, a so-called current stepper control system or welding current that estimates the wear level of the electrode tip in advance and increases the welding current by several steps for each welding number. The correction corresponding to the change in the shape of the tip of the electrode tip was performed by using a so-called linear current stepper control system in which is continuously increased every few times of the welding spot.

However, in the current multi-line mixed production line that handles galvanized steel sheets or polished mild steel sheets using several welding robots, these various materials are mixed in complex proportions, especially the degree of wear of the electrode tip. When spot welding a soft iron plate plated under different conditions, a large amount of current is required, and contamination and wear of the electrode tip is more severe than welding an uncoated steel plate. Therefore, the period in which the electrodes are sharpened or the electrodes need to be replaced becomes shorter. In terms of production at a high working rate, this time consuming side task cannot be ignored, and a current stepper control system or a linear current stepper control system cannot be satisfactory in this situation. This leads to problems like the one described below. This led to the search for a different kind of solution.

To describe in detail the prior art in this field, it is not easy to calculate the proper rate of increase in current, whether it is a current stepper control system or a linear current stepper control system. In some cases, no reasonable welding power could be supplied, and the weld recovery (weld number) was used only as a parameter for power compensation to provide an estimate of the weld quality. In other words, this conventional system does not define the limit of current increase to the extent that it does not cause deterioration in welding quality depending on the actual degree of wear of the electrode tip. And the current rate of increase of current did not match or correspond to the actual wear of the electrode tip.

Therefore, in the prior art, the amount of current performed slightly higher current for the safety of welding quality. The inconvenience caused by this was that welding under such unfavorable conditions greatly facilitated the wear of the electrode tip, sharpened the tip of the electrode and increased the frequency of exchange of the electrode.

Experiment with hundreds to thousands of weld specimens for the purpose of conducting an appropriate rate of current increase, to determine how much wear occurs on the tip of the electrode when several welds are performed continuously with one electrode. This was very difficult in the prior art.

It is an object of the present invention to provide a method for controlling spot welding, and the present invention does not rely on the method of forming welding conditions corresponding to the degree of wear of the electrode tip through experiments as in the prior art, and the present invention does not depend on the welding operation every time. In a multi-product mixed production line with different wear rates at each electrode tip, each welding point performed during the spot welding process detects a change in the wear level of the electrode tip at each point and the most appropriate current ratio based on the detected change state. Alternatively, it can supply current through the energization time, thereby extending the period of trimming or replacing the electrode tip, and improving the productivity even in a multi-product mixed production line.

Now, a control method to which the present invention is applied will be described in detail with reference to the accompanying drawings.

As is already known in the art, when the amount of welding current is normal, the resistance between the electrode tips changes during welding, as representatively shown in FIG. Section (a) shown in this figure is an unstable area immediately after the start of energization or immediately after welding. The type of change in resistance between the electrodes depends on the fit state of the weldment and the surface contamination of the weldment such as oil or rust. Depends. Within 1 to 2 cycles after the start of energization, the surface contact resistance disappears (refer to the horizontal axis which is the "welding time"), and the resistance between the electrode tips also decreases drastically. In the section (b), the following two processes proceed together. That is, the resistivity of the welded material due to the temperature rise of the welded part increases, and the area of the energization path is enlarged by softening and sinking the welded part.

In this process, the increase in resistance due to the increase in temperature between the electrodes exceeds the decrease in resistance due to the extension of the current path through the current, and consequently, the total resistance between the electrodes increases, nearing the end of this process or this section. Is the highest in resistance. A nugget appears in this section (b) and corresponds to the early stage of its growth.

In the section (c), the conduction path continues to expand, and the nugget also grows. However, the resistance between the electrode tips decreases as the temperature rise of the weld reaches saturation and this condition is almost constant. Since the tip of each electrode is used for continuous spot welding, the corresponding portion of the electrode tip gradually expands to decrease the contact resistance. On the other hand, the diameter of the conduction path in the weldment also expands, reducing the resistance between the electrode tips.

Therefore, the lowest resistance value between the electrode tips within 4 to 5 cycles after the start of energization includes a state of instability after the start of energization, that is, a state that does not depend on the contact of the weld extinguished within 1 to 2 cycles after the start of energization or the state of surface contamination. In addition, it can be said that it reflects the diameter value of the pure electrode tip which has not yet been affected by the increase in the resistivity due to the temperature rise of the weld and the enlargement of the conduction path area. Therefore, it can be said that this minimum resistance value is generally proportional to the degree of enlargement of the diameter of the electrode tip tip, that is, the wear degree of the electrode tip.

The spot welding control method according to the present invention detects the minimum value of the resistance varying between the electrode tips during welding and continuously stores the minimum value of each detected value. The average value of the minimum value is obtained by a set of welding points composed of welding points, and the average value corresponds to each threshold value when the average value reaches the threshold value of the welding current change set in several steps in response to the wear of the electrode tip. It is configured to supply the welding current according to the conditions.

In implementing the control method of the present invention, the limiter for determining the increase rate and timing of the welding current in several steps in accordance with the wear degree of the electrode tip 10 by the apparatus shown in the block diagram of FIG. ).

For example, prepare a worn electrode of about 2 to 10 steps in advance, and determine the electrode tip as the number of steps of welding conditions necessary to maintain the welding quality from the new state to the expected maximum wear state. ). In other words, according to the wear degree of the electrode tip, various data such as welding current and welding time are obtained by experiments for each wear electrode (1-Ⅲ), and the obtained data is input to the setter (1). (See Figure 3).

In practice, the following conditions were obtained through the experiment and set in the setter 1.

(1) Current value and welding time before initial welding time.

The initial welding time is preferably relatively short beyond the initial unstable region.

(2) Current value and welding time after initial welding time.

A set of welding conditions should be established considering the effect of the initial welding time state on the welding operation.

(3) Limits by class, which are the basis for changing some conditions such as welding current.

As described above, the degree of wear from the non-abrasion state of the electrode tip to the anticipated maximum wear state is classified into the grade grades necessary to maintain the welding quality sufficiently.

(4) Standard number of welding points (standard number of welding number) to calculate average minimum resistance value.

The standard number of welding points is determined by considering the welding gun, the mechanical properties of the welded material, and the material according to the actual condition of the production line. In other words, when the resistance value of only one welding point is used as a control criterion, the welding system becomes unstable when unexpected disturbance occurs. Therefore, baseline recovery should be chosen to eliminate this instability.

Referring to FIG. 2, in addition to the voltage between the electrode tips, first, the welding current is half-cycled by an annular coil 11, CT (current transformer) 12, etc. The resistance between electrodes is calculated by the lowest resistance calculation circuit 2 which detects each signal and receives this output signal based on the following formula.

Only the minimum value of the various resistance values between the electrode tips obtained from the calculation circuit 2 is continuously stored in the memory holding circuit 3.

Next, when the system is adjusted to perform 1 to 100 welding times, for example, if the welding average number is set to 10, the number of times is set in the average calculation circuit 4 through the setter 1. Assuming that this average calculation circuit 4 draws ten minimum values from the memory resistor circuit 3 for each welding point, the average calculation circuit 4 calculates the average value of the minimum resistance values between the electrode tips. The process of calculating the average value is as follows. That is, in FIG. 4, the number of set points (the number of welding points in a group of welding spots) is set to 10, the use contacts are called "1" to "n", and the number of welding points just performed is "11". The lower surface group (a) excludes the welding point "1" which is the oldest working point and includes the welding point "11" which was last worked. The lowest resistance mean value corresponding to the weld points "2" to "11" for the reconstructed group (a) is calculated according to the following formula.

The average resistance value thus calculated is treated as the minimum resistance value of the welding point "11". Then, if the welding point "12" is performed, the next oldest welding point "2" is excluded and the last welding point "12" is included (see group (b)). Next, the lowest average value corresponding to the welding points "3" to "12" is calculated and used as the lowest resistance value of the welding point "12". In this way, as the welding process proceeds, the oldest welding point is excluded from the group, and the weld point just performed is included in the group, and the average of the reconstructed group is calculated. Use as the lowest resistance value. The above described work methods can protect the welding system from the effects of unforeseen disturbances. Examples of such disturbances include incorrect contact or fitting of the weldment.

After the initial energization phase of the spot welding operation, the welding process at each welding point is carried out according to the selected welding conditions described below. That is, as each welding point progresses, the comparison calculation circuit 5 compares the output of the average value calculating circuit 4 and the preset threshold point for every welding point. When the average value exceeds the threshold, the grade of the threshold is advanced one step by the instruction of the comparison operation circuit 5, and the thyristor (3) is passed through the phase control ignition circuit 6 so as to faithfully meet all the conditions preset in the grade. 13) Proper welding is performed by controlling.

In FIG. 2, reference numeral 14 denotes a welding transformer, 15 is a welding power source, and 16 is a switch for selecting the annular coil 1 or the CT 12. In FIG.

In the case of welding the plated iron sheet, as the welding operation is continued, an alloy layer made of copper and zinc is formed at the electrode tip. The alloy layer thus formed acts in a direction that negates the reduction in resistance caused by wear of the electrode tip as described above. That is to say, even though the wear of the electrode tip actually proceeds, judging only on the basis of the detected resistance between the electrode tips, the degree of wear of the electrode appears to be very light. This uncomfortable problem can be easily overcome by considering the above-described effects when determining each grade standard and corresponding welding condition.

When unexpected extreme wear occurs on the electrode tip or when the lowest resistance decreases above the preset final limit, it is completed, for example, as provided in conventional linear current stepper control systems and current stepper control systems. It is of course possible to give the signal the ability to emit a signal.

As is clear from the above description, the control method of the present invention, which increases the welding current in response to the change of the shape of the electrode tip, functions regardless of the material of the welded material, that is, a plated iron plate, an uncoated iron plate, or the like. You can work with any combination of these. In other words, the method according to the present invention detects the actual wear state of the electrode tip by using the minimum resistance value between the electrode tips during welding, and supplies the power corresponding to the welding condition corresponding to the actual state. Thus, the method of the present invention deviates from the unreasonable method of checking how much wear occurs between the electrode tips when performing several welding through experiments and setting the welding conditions based on the results. It must be carried out under the same conditions as those of the line, and this method is necessary in the conventional method.

In the method according to the present invention, suitable conditions corresponding to the wear state of the electrode tip can be obtained by experiment only, and the welding conditions can be set relatively easily. In the present invention, it is not necessary to set a slightly higher level of current increase, whereas in the conventional method, it is necessary to set a slightly higher current value in order to correct an expected wave in electrode tip wear. Therefore, in the present invention, welding can be performed under more suitable welding conditions. In the present invention, suitable welding conditions can be used to prolong the period of trimming or replacing the electrode tip, thereby reducing the additional work such as changing the tip and improving the productivity of the mixed production line.

Claims (1)

In controlling welding conditions such as welding current and energization time according to the shape of the tip of the welding electrode in spot welding, the electric resistance between welding electrodes that changes every hour for each welding operation is motorized, so Detecting the lowest resistance between the welding electrode tips; Storing the continuous lowest resistance value thus detected; Maintaining said successive set of lowest resistance values applied to a fixed number of consecutively already detected resistance values, the last detected resistance value; Calculating an average value of the consecutive lowest resistance values of the set; Comparing the average value with a plurality of threshold values; When the average value reaches one of the threshold values, the welding condition is modified to compensate for the change in the average value, thereby keeping the welding condition constant so that the average value is between successive threshold values, and the threshold value is the welding electrode. Spot welding control method comprising the step of selecting corresponding to the wear.

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