KR950003260B1 - Measuring method and detector of shunt current in resistanse spot welding process - Google Patents

Abstract

The method for measuring the shunt current by using the difference of the electromotive forces induced from the both coils, has two opposite coils symmetrically arranged at the center of the welding electrode. The apparatus comprises a plate material having a through hole for inserting the electrode; a pair of coils each having the same diameter and the same winding number, symmetrically arranged at the insert hole; electrically connecting section for connecting the coils.

Description

Shunt Current Measurement Method and Shunt Current Detection Device in Resistance Spot Welding Process

1 is a coil arrangement for shunt current measurement according to the present invention.

2 is a view showing the structure of a shunt current detecting device according to the present invention.

3 is an exemplary installation diagram of a shunt current detection device according to the present invention.

Figure 4 is an exploded perspective view of the main portion of the test specimen for shunt current measurement used in the embodiment of the present invention.

FIG. 5 shows the flow of shunt current during welding of the specimen shown in FIG. 4. FIG.

6 is a graph showing the relationship between the actual welding current and the average induced electromotive force measured by the detection apparatus of the present invention for a specimen without a shunt weld in the embodiment of the present invention.

7 is a graph showing the relationship between the actual welding current and the average induced electromotive force measured by the detection apparatus of the present invention for commercially available shunt welding in the embodiment of the present invention.

8 is a view showing an example of measuring the welding current using a conventional toroidal coil.

9 is a view showing that when there is already a welded part (shunt welded part), a part of the welding current is classified as shunt welding.

10 is a view showing a state of suppressing a shunt current classified into a shunt welding portion by using a permanent magnet in a conventional method.

The present invention relates to a method of measuring a shunt current in a resistance spot welding process. More particularly, the present invention is already welded around a welding part (hereinafter referred to as "current welding part") that is currently being welded in a resistance spot welding process for welding two sheet metals. The present invention relates to a method for measuring some welding current (hereinafter referred to as "shunt current") that exits from the initial welding current to the shunt weld when there is a weld (hereinafter referred to as "shunt welding").

In general, resistance spot welding refers to two metal plates overlapping each other to apply a large amount of current while applying pressure to electrodes on both sides to generate Joule heat by the resistance of the contact surface of the material, and to melt the material as the heat. It says how to bond.

This resistance spot welding process is very fast, which is advantageous for mass production process, and the heating range is narrow, resulting in only local deterioration of materials, less residual stress and deformation than other welding methods, and also no additives such as welding rods. Since the weight of the material does not change significantly even after welding, it has been widely used as a sheet metal welding method such as an airplane body, an automobile body, a combustor, and the like. However, since the weld is formed inside the material, it is difficult to identify before the weld grows to the outside, and there is no reliable way to know the weld quality of the weld except for the fracture test. It is also very difficult to obtain a uniform weld quality because it can have a big impact on quality.

Resistance spot welding was first patented by Elihi Thomson in 1877, and in the early days, trial-and-error welding was the mainstream, with many attempts at welding, relying on the experience of almost skilled practitioners, and later delayed using cams or pneumatics. Following the device, electronic devices using RC constants and voltages were developed, leading to the method of counting input frequencies and connecting microprocessors. In addition, by mathematically modeling the resistance spot welding process, numerically, the spot welding process was numerically analyzed and controlled based on the heat transfer equation, and research on welding quality control using the electrode separation signal continued until recently.

Until now, the size of the weld is controlled by the above methods in the resistance spot welding process. If other welds already exist near the current weld in progress, part of the welding current is transferred to the existing weld (shunt weld) around the current weld. By flowing in, the current weld portion is not sufficiently melted, resulting in deterioration of the weld quality. This phenomenon is referred to as the "shunt effect", and the control signals described above do not sufficiently reflect the state of the welding part that changes during welding, unlike at the time of single point welding, so that the value as the control signal is lowered. In fact, when resistance spot welding is used, welding is often carried out at various places of the base metal, but even though the welding quality is severely degraded due to the shunt effect, effective countermeasures have been insufficient.

Until now, the existing production site has been using a method that the welding part is sufficiently melted regardless of the presence or absence of the shunt welding part by simply energizing the welding part more than an appropriate amount in order to prevent the welding quality deterioration due to the shunt effect. Not only is it a severely unproductive method, it also violates the intent of weld quality control to achieve uniform weld quality.

As a general welding current measurement method, as shown in FIG. 8, a pair of welded plate metal 50 is placed between the upper electrode 20a and the lower electrode 20b of the resistance spot welding machine, and the earth is disposed on the lower electrode 20b side. By placing the Lloyd coil 30, the electromotive force induced in the toroidal coil 30 by the change of the magnetic field generated when the welding current A flows has been measured. However, if there is a shunt welding portion 52 around the current welding portion 51 only by this method, as shown in FIG. 9, a part of the welding current A is classified as the shunt current A1 and flows to the shunt welding portion 52. The amount of the net welding current A2 for melting the current welding part 51 is reduced, resulting in a decrease in weld quality.

Patent related to the method to compensate for the shunt effect is Japanese Patent Laid-Open No. 59-223180. This method aims to prevent the loss of the shunt current and at the same time reduce the quality of the weld by installing the permanent magnet in the direction of the shunt welding part in the resistance spot welding process, and as shown in FIG. 10. Likewise, the upper and lower permanent magnets 40a and 40b and the permanent magnet insulators 41a and 41b in the form of a rectangular parallelepiped are disposed in the direction in which the shunt welding portion 52 is located with respect to the electrode, thereby preventing the flow of the shunt current A1. By trying to reduce the shunt effect. However, since the shunt current flowing through the inside of the welded sheet metal 50 cannot be completely blocked, welding quality deterioration due to a certain shunt effect is inevitable, and in this method, it is difficult to measure the shunt current. There were problems such as the inability to fundamentally compensate for the deterioration of the weld quality.

Accordingly, the present invention, in consideration of the problems according to the prior art in the art, indirect measurement of the shunt current flowing to the shunt welding portion and immediately compensate for the weld quality deteriorated due to the shunt effect during welding to reduce power consumption during actual welding work more In order to achieve the above object, the present invention provides a coil having the same diameter and winding number so as to be symmetrical with respect to the electrode center of the welder in the resistance point welding process of sheet metal. By using the difference in the electromotive force induced in both coils by the shunt current provides a method for measuring the shunt current.

In addition, the present invention provides a plate member having a center electrode detachment hole set to be fitted to an electrode of a resistance spot welder for the purpose of detecting a shunt current during resistance spot welding of a sheet metal, and the left and right sides of the center electrode detachment hole of the plate member. Provided is a shunt current detection device comprising a pair of coils having the same diameter and the number of turns arranged symmetrically, and an electrical connection portion for electrically connecting the coils.

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention.

1 is a flow diagram of the present invention when the welding current A flows in a state in which left and right coils 11a and 11b having the same number of windings and diameters are symmetrically disposed with respect to the centers of the electrodes 20a and 20b. It is a state diagram which shows the flow of the shunt current A1 from the current welding part 5 to the shunt welding part 52. As shown in FIG. As shown, if there is a shunt welding portion 52 around the current welding portion 51, the shunt current A1 flows to the shunt welding portion 52, and the left and right coils 11a are caused by the shunt current A1. At 11b, magnetic fields of different sizes are generated. At this time, the welding current A should be a current controlled by a phase control method with a constant frequency and no fluctuation of the maximum amplitude of the waveform. In this case, the magnetic field changes with time, and the coils 11a and 11b ), The induction current generated in the circuit by the electromagnetic induction action always occurs in the direction of interfering with the increase and decrease of the magnetic field generated by the welding current, and according to Lentz's law, in the negative direction of the rate of change of the magnetic field with respect to time. Proportional electromotive force is generated. Under these conditions, the electromotive force induced in the left hemisphere and the right hemisphere of the coil during one-point welding is the same in magnitude and opposite in direction, and the induced electromotive force generated in the coil by the magnetic field parallel to the coil is always zero. When the two coils are electrically connected to each other, the induced electromotive force of the two coils is always the same and the opposite direction of the induced electromotive force is obtained when the circuit is ideally symmetric, so the induced electromotive force obtained in the entire circuit becomes zero (0). On the other hand, if the shunt welding portion 52 is present, the shunt current flows with respect to the shunt welding portion, which is asymmetric to the center line of the detection device, and thus the magnitude of the electromotive force induced in the two coils is changed. The flow of the shunt current is assumed to be a wire and the Biot-Savart law gives the strength of the magnetic field generated at a point in space by the line on the wire when the current flows in the wire. When applied to, the approximate shunt current value can be obtained from the sum of the electromotive force induced at each center point of two coils installed on both sides of the electrode. By comparing the induced electromotive force obtained from the two coils, the direction of the shunt weld with respect to the electrode can be estimated. Will be.

2 shows a shunt current detection device 10 according to the present invention. As shown, the shunt current detection device 10 of the present invention has the same plate member 12 having the electrode detachment hole 14 formed in the center and the same symmetrically arranged on both the left and right sides of the center electrode detachment hole 14. It consists of a pair of coils 11a and 11b having a diameter and a winding number, and an electrical connection portion 13 for electrically connecting the coils. Electromotive force induced in both coils 11a and 11b is transmitted to the electrical contact 13 along the lead. The detection apparatus 10 of such a structure is inserted in the resistance spot welding machine upper electrode 20a as shown in FIG.

Hereinafter, the present invention will be illustrated with reference to the following examples.

Example 1

In order to measure the shunt current, an experiment was conducted by inserting a shunt current detection device as shown in FIG. 2 into the upper welding electrode of the resistance spot welding machine as shown in FIG.

The shunt current detection device used is a transparent acrylic plate member with a thickness of 1mm that forms a center electrode detachment hole to be inserted into the upper welding electrode of the resistance spot welding machine. By installing the coils symmetrically with respect to the center electrode center, the sum of the induced electromotive force from each coil was zero for symmetrical currents between the two coil centers. At this time, the coil was a coil having an inner diameter of 8 mm using an enamel wire of 0.08 mm thickness, and the distance between the electrode and the coil center was 15 mm. In addition, the shunt current was measured by winding the number of windings (CTN) of both coils of the shunt current detector at 150, 250, and 350 circuits, respectively.

The specimen used for the shunt current measurement is a cold-rolled steel sheet having a thickness of 1.5 mm, and an insulating paper 60 is inserted between the contact surfaces of the two sheet metals 50a and 50b, which corresponds to the current welding portion on the insulating paper 60. The heat- and pressure-resistant insulator 61 was attached at the position, and a hole 62 was drilled at a predetermined distance from the center of the specimen to overlap the two specimens, and these portions were welded to act as shunt welds (see FIG. 4). .

In addition, using a specimen in which two cold-rolled steel sheets having a thickness of 1.5 mm having the same material as the above steel sheets were directly stacked, a current was measured at one point.

[Other equipment used in the experiment]

Resistance spot welding machine: Resistance spot welding machine equipped with a pneumatic controller that can adjust the pressing force of the electrode, and a controller that can control the initial pressurization time, welding start time, holding time, and down time.

Weld Checker: Weld Checker with LED indicator for welding time in cycles and LED indicator for welding voltage or welding current.

Electrodes: copper-chromium (Cu-Cr) electrodes for resistance spot welding such as mild steel, low alloy stainless steel, brass and nickel alloys;

Data acquisition device: A data acquisition device that converts analog signals generated from detection devices into digital devices.

[Experiment result]

When welding only one point using a specimen without insulator between the sheet metals, the average value of the induced electromotive force (Y value) detected from the detection device and the induced electromotive force obtained from the toroidal coil are input to the weld checker and the actual value is displayed. It is shown graphically as the current (X value). The results are shown in FIG.

When the welding current flows to the siphon (FIG. 4) which has the shunt welding part with the current welding part insulated, weld checks the average value (Y value) of the induction electromotive force detected by the detection apparatus and the induction electromotive force obtained from the toroid coil. The value displayed by the weld checker when inputted in the graph is shown as the actual welding current (X value). The results are shown in FIG. FIG. 5 shows the welding current flowing from the electrode to the shunt weld in the specimen with the current weld insulated and the shunt weld (FIG. 4).

As can be seen from the graphs of Figs. 6 and 7, the point of electromotive force induced in the coil of the detection device becomes almost zero at one point welding, so that the average value of the induced electromotive force is near zero for the increase of the actual welding current. In the presence of a shunt weld, an induced electromotive force is generated in the detection device by the shunt current, which shows a good proportional relationship on both sides. In other words, it was possible to monitor the shunt current change indirectly. In addition, the larger the coil number (CTN) of the coil, the larger the difference in induced electromotive force, and as a result, it is easier to monitor the shunt current change.

As described above, according to the present invention, the actual shunt current can be measured by obtaining the induced electromotive force from the coil during actual welding, and comparing this value with the total welding current, thereby measuring the net welding current of the current welding part in progress. Indirect monitoring of current changes not only compensates for weld deterioration due to shunt effects, but also minimizes waste of power.

Claims (2)

In the resistance spot welding process of sheet metal, coils with the same diameter and number of windings are installed on the left and right sides of the welder's electrode center to measure the shunt current using the difference in induced electromotive force induced by both coils by the shunt current. Shunt current measurement method characterized in that. A device for detecting shunt current during resistance spot welding of sheet metal, comprising: a plate member having a center electrode detachment hole set to be inserted into an electrode of a resistance spot welder, and the same diameter disposed symmetrically on both left and right sides of the center electrode detachment hole of the plate member; And a pair of coils having a number of turns, and an electrical connection portion for electrically connecting the coils.