JPS6213277A - Monitoring method for abnormality in work plate thickness in spot welding - Google Patents

Abstract

PURPOSE:To detect surely the abnormality due to the variation, etc. in the component plate thickness and to alert by finding the maximum, minimum and mean values of the resistance between electrode tips and by comparing those values with the reference value after welding in a sheet material spot welding. CONSTITUTION:The voltage between the welding current and electrode which varies hourly in spot welding is detected, the resistance between electrodes is calculated by an arithmetic circuit 6 and the value thereof is displayed on a display 7. On the other hand the resistance value thereof is fed to the large and small comparing circuit 8 and in case of the corresponding detected value being outside the reference value range by comparing it with the reference maximum value, minimum value and mean value of a setter 9 the abnormality generation in the welding place of a misplate thickness, etc. is alerted by warning. The conventional sensors numerously arranged are made disused by preventing in advance the generation of a defective welding item and the maintenance inspection is easily performed.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for monitoring abnormalities in the thickness of a workpiece during spot welding, and more specifically, when an abnormality occurs in the thickness of a welded plate at a spot welding location, The present invention relates to a workpiece thickness monitoring method in spot welding, which detects an abnormality from the change in resistance value between electrode tips during welding energization time and issues an alarm to prevent the occurrence of defective products.

(Future technology) Automotive body assembly lines automatically feed small parts such as welding pads, bolts, and washers, as well as pressed thin sheets with complex shapes to predetermined welding positions, using spot welding robots and Large quantities are welded using other automatic spot welding machines and produced on line. In such line production, parts may be incorrectly fed or pressurized and energized without being fed, resulting in defective welding products.

This defective product immediately stops the smooth operation of subsequent processing and assembly operations, and must be avoided at all costs.

In order to prevent the above defective products, air sensors,
Generally, various types of detectors, such as limit switches, proximity switches, and phototubes, are used to monitor feeding errors that occur during the component supply stage.

(Problems to be Solved by the Invention) However, the conventional method has the following drawbacks.

(]) In the case of an air sensor, it is necessary to make a small hole in the electrode tip of the spot welding machine or in the jig near the welding position to blow out the air. , their small holes are clogged by oil, etc., making maintenance difficult.

(2) For limit switches and proximity switches:
When the shape of the workpiece (object to be welded) is complex, the position and mounting of the workpiece not only makes the structural design of the jig complicated and large, but also, especially for proximity switches, when welding energization or external Easily affected by electromagnetic force from equipment, leading to malfunction.

(3) The same is true for phototubes; in addition to causing malfunctions due to the influence of surrounding dust and the influence of arc light generated during welding, the mounting position and jig structural design depend on the shape and size of the workpiece. It is restricted and maintenance is not easy.

(Means for Solving the Problem) Therefore, the present invention is completely different in concept from the abnormality monitoring method that includes mechanical elements using various sensors as mentioned above.
We have planned a new method for detecting abnormalities in welding locations based on welding phenomena.The specific method is to detect the resistance between electrode tips that changes moment by moment each time welding energization is applied during spot welding, and to After the end of the time, the minimum value, maximum value, average value, etc., or all of these values of the resistance between the electrodes 1 are determined, and the various values determined above and the various values set in advance are determined. It is characterized in that the value is compared with a corresponding reference value, and an alarm signal is issued when the comparison result is lower or higher than the reference value.

(Effect) The resistance typically shows a transition as shown in R4 in FIG.

Section a shown in the figure shows the anxiety immediately after starting energization.
The behavior of the resistance between the electrode tips during this period depends on the degree of contact with the workpiece and the contamination of the workpiece surface such as oil and rust. This surface contact resistance disappears in 1 to 2 cycles after the start of current supply, and the resistance between the electrode tips rapidly decreases.

Next, in section b, the resistivity of the workpiece increases due to the temperature rise of the weld, and the current carrying path area increases simultaneously due to softening and crushing of the weld. During this period, the increase in weld resistance due to the upper limit of specific resistance is more than the decrease in resistance due to the expansion of the current carrying path area, and as a result, the resistance between the electrode tips increases.
It reaches a maximum value near the end of this process. During this period, nuggets begin to form and grow.

Next, in section C, the current carrying path area continues to expand with the growth of the nugget, but the weld zone temperature reaches a saturation value and becomes approximately constant, so the resistance between the electrode tips decreases.

Based on the transition of the inter-electrode tip resistance R5 during the welding process as described above, the operation of the present invention will be explained with reference to FIGS. 1 to 3.

Now, as shown in Figure 2, for some reason the thickness of one or both of the workpieces has become thinner than the normal two-ply welding state shown in Figure 3 (the figure shows both When a thin plate) occurs, the nudging between the workpieces becomes better,
In addition, since the resistance between the electrode tips in the direction of the workpiece plate thickness also decreases, the resistance between the electrode tips is equal to R2 in FIG.
The total value is smaller than the value of 1. However, I? , which show fairly similar changes in the inter-electrode tip resistance.

Furthermore, when the abnormality in the combination of workpieces becomes severe, a state where there is no mating plate to overlap, that is, a single plate state, and furthermore, a state where the workpiece is not fully heated, that is, a so-called blank striking state, etc. occur. .

These can be caused by errors in feeding or falling off of the workpiece, or in the case of spot welding robots, poor positioning control of the welding point, and the like.

The apparatus of the present invention operates effectively even under such abnormal conditions. In other words, in the case of a single plate, there is no process of nugget generation between the plates as described above, so
As shown in Figure 3, the value of the resistance between the electrode tips is even smaller than when two thin plates are stacked, and there is less heat generation, so the change in resistance due to the temperature rise of the workpiece is also small, resulting in a gentle curve. In addition, if there is no workpiece between the electrode tips, the water-cooled electrode tips made of copper-based materials will come into direct contact with each other.
There is only a slight resistance between the electrode tips due to dirt on the contact surface, and as shown in FIG. 1 R4, a flat resistance change is exhibited with almost no change even when welding current is applied. Also, since the nugget in the figure is generated,
The electrolytic surface abrasion curve is R upward (high resistance side) from R1 in Figure 1.
Needless to say, it draws a curve that is quite similar to 1. Therefore, from such welding phenomena, the minimum value or maximum value of the resistance between the electrode tips, or the resistance value determined every half cycle, is added over the entire energization time, and the average value is calculated by dividing by the number of times, etc., and set in advance. By comparing the various values obtained with the corresponding reference values, it can be determined whether the welding position is normal or abnormal, such as blank welding due to part feeding errors or other reasons. can do.

It is also possible to make a judgment based on any one of the three detected values, but in order to ensure the accuracy of the judgment, it is necessary to make a judgment using at least two or more values, or all of them. By detecting (3 types) and issuing an alarm if even one of the values is abnormal, more advanced product quality control is possible. If the reliability of detecting the inter-electrode tip resistance value immediately after the start of energization cannot be obtained, the number of initial energization cycles to be excluded is set in advance, and, for example, the detected value up to the initial 3rd cycle is excluded. It is also possible.

(Embodiment) Hereinafter, an embodiment of an apparatus for carrying out the method of the present invention will be described based on the electrical block diagram shown in FIG.

1 is an electrode tip of a spot welding machine, 2 is a welding transformer, and 3 is a toroidal coil, which detects the current flowing in the secondary circuit of the welding transformer and the welding machine.

Reference numeral 4 denotes an electronic switching element such as a cyrisk installed between the welding transformer and the welding power source, and 5 a current detector CT inserted between the cyrisk and the welding transformer, which detects the current flowing in the primary circuit.

6 is an arithmetic circuit which divides the voltage between the electrode tips by the welding current detected by the toroidal coil 3 or the current detector CT5, etc., thereby obtaining the resistance between the electrode tips which changes momentarily every half cycle; After energizing,
Any or all of the minimum value, maximum value, and average value are calculated, and the results are output to the display 7 and the magnitude comparison circuit 8. Furthermore, this arithmetic circuit 6 obtains waveform data of the resistance between the electrode tips in the case of two-ply welding with normal workpiece plate thickness and in the case of abnormal one-piece welding from welding experiments conducted prior to the main welding. Based on the lowest value, highest value, average value, etc.
Find the value of any one, two or more, or all of the types of data. The obtained value can be confirmed on the display 7. Reference numeral 9 denotes a setting device, which sets the upper and lower limits of various values in the case of two-ply welding of normal workpiece plate thicknesses determined by the arithmetic circuit 6 as reference values. The magnitude comparison circuit 8 compares any or all of the minimum value, maximum value, and average value of the interelectrode resistance output from the arithmetic circuit 6 every time welding current is applied from the arithmetic circuit 6, and the respective values. Compare the size with the reference value corresponding to . And the comparison result is that one or more calculated values of the above comparison targets are lower than the reference value.)
When the temperature is high or high, an alarm is output to the outside,
Informs that there is an abnormality in the welded area. The operation of the monitoring device with the above configuration will be explained. First, in test welding, when the thyristor 4 is ignited and a welding current flows through the welding transformer 2 to the electrode tip l, this is taken into the arithmetic circuit 6 via the Cr2 or toroidal coil 3, and at the same time, the welding current flows between the electrode tips l. The inter-electrode tip voltage generated is also taken into the arithmetic circuit 6.

The arithmetic circuit 6 calculates the inter-electrode tip resistance every half cycle using the following equation. Repeat this for the entire energization time.

After the energization ends, the lowest value, highest value, and average value of all detected values for each half cycle are output to the display 7. This is done for normal two-layer stacks, different thicknesses, or abnormal cases with only one plate, and the upper and lower limits of the normal value range are determined from the three normal values and abnormal values. The setting device 9 is used to set the reference value. In this way, the actual welding follows the same process as the previous test welding, but this time the detected value is determined by the magnitude comparison circuit 8.
The magnitude comparison circuit 8 compares this with the reference values (3 types) output from the setting device 9, and issues an alarm when the corresponding detected value is outside the reference value range. Outputs and warns of abnormalities at welding locations such as plate thickness differences.

In addition to the various detected values shown in the embodiments of the present invention, the integral value of the inter-electrode tip resistance curve during the energization time also has the same effect.

(Effects of the Invention) As described above, according to the present invention, based on the waveform data of the inter-electrode tip resistance that changes moment by moment during the entire energization time for each spot welding point, parts feeding errors at the spot welding position can be detected. Abnormalities caused by changes in plate thickness, etc. can be reliably detected and alarmed, making it possible to prevent the occurrence of defective welding products, improving the efficiency of line production and ensuring product quality. Compared to the previous monitoring method, various sensors that were required in large numbers are no longer required, and sensor installation is not only free from troublesome problems such as location, jig design, and maintenance, but also has a simple structure, is economical, and is reliable. A high spot? An fJ contact plate thickness abnormality monitoring device can be realized.

[Brief explanation of the drawing]

FIG. 1 is a waveform diagram comparing the resistance between the electrode tips during the welding process in the case of welding a normal two-ply plate (8) and an abnormal plate thickness. FIG. 2 is an explanatory diagram showing an abnormal thin plate welding state. FIG. 3 is an explanatory diagram showing a normal two-layer temporary welding state. FIG. 4 is an electrical block circuit diagram showing an example of an apparatus for carrying out the method of the present invention. I...Electrode tip 7 6...Arithmetic circuit 7...Display device Fig. 1 Fig. 2 Fig. 3 Fig. 4 1st li

Claims (1)

[Claims] In the spot welding method, the resistance between the electrode tips, which changes moment by moment, is detected every time welding current is applied, and after the welding current is over, any one of various values, such as the minimum value, maximum value, average value, etc., of the resistance between the electrode tips is detected. Or, find all of these values, compare the various values found above with standard values corresponding to the various values set in advance, and when the comparison result is lower or higher than the standard value, A method for monitoring abnormalities in workpiece plate thickness during spot welding, characterized by emitting an alarm signal.