Spot welding device and welding method

Abstract

To provide a spot welding device and a welding method capable of accurately determining welding quality even if the plate thickness changes. [Solution] A welding device that applies pressure and current to overlapping workpieces to be welded, which are placed between a pair of electrodes, is provided with a welding quality determination means, which has an information acquisition unit that acquires the resistance value between the electrodes and the electrode area, and a determination unit that determines the welding quality based on an evaluation value including the relationship between the resistance value and the electrode area and a set threshold value.

Description

[Technical Field]

[0001] The present invention relates to a spot welding apparatus and a welding method that can guarantee the quality of welding in spot welding. [Background technology]

[0002] Conventionally, in the field of resistance welding, typified by spot welding, a method for determining the quality of welding in real time (during welding) involves determining the properties of the welded portion (nugget) formed on the welded material.

[0003] For example, Patent Document 1 proposes a method for detecting the amount of movement between welding electrodes, which derives the amount of movement between welding electrodes by adding the amount of movement of the electrode drive part in the electrode movement direction due to expansion and contraction of the welded part (nugget) during welding and the amount of deflection of the welding gun due to the pressure applied from the electrode to the workpiece.

[0004] Furthermore, Patent Document 2 discloses a quality assessment system for spot welding, which compares the difference in decrease between the maximum resistance value during the current flow period, detected when the workpiece (steel plate) is sandwiched between the upper and lower electrodes, and the final resistance value at the end of current flow, with a preset threshold value, and continues current flow if the comparison result is lower than the threshold value, and stops current flow if the comparison result is higher. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3593981 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-55893 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the method of Patent Document 1, the expansion and contraction of the nugget is estimated based on the correlation between the inter-electrode movement amount and time, but this expansion and contraction of the nugget deforms the welding equipment, so it is not detected as the inter-electrode movement amount.

[0007] Specifically, when welding workpieces of different thicknesses, it is necessary to prepare multiple pieces of data correlating the inter-electrode movement amount and time in order to determine whether the nugget is good or bad. Since the current measurement value is compared and determined based on these multiple correlation data, it takes a long time to make the determination, which results in a slow welding speed. In addition, there are many variable and irregular factors, such as the material, plate thickness, and welding conditions of the parts to be welded, and welding speed is also required in mass production sites, so it is thought that this method is not suitable for detecting the quality of welding in-line.

[0008] On the other hand, in the invention described in Patent Document 2, the resistance value varies depending on the material and plate thickness, and therefore it is necessary to set a threshold value according to the plate combination. For example, as shown in Figure 11, even with normal welding, the difference in the decrease between the maximum and minimum resistance values ​​will be smaller when welding thin plates together than when welding thick plates together. Furthermore, the material must also be taken into consideration, and ultimately, setting a threshold value for determining pass / fail is difficult as long as it is based only on resistance values.

[0009] Therefore, a main object of the present invention is to provide a spot welding apparatus and a welding method that can accurately determine the welding quality even if the plate thickness changes. [Means for solving the problem]

[0010] The embodiment that solves the above problem is as follows. (First aspect) A spot welding device that welds overlapping workpieces placed between a pair of electrodes by applying pressure and current to the electrodes, A welding quality determination means is provided, The welding quality determination means an information acquisition unit that acquires the resistance value between the electrodes and the electrode area; a determination unit that determines welding quality based on an evaluation value including a correlation between the resistance value and the electrode area and a set threshold value, A spot welding device characterized by:

[0011] (Second aspect) A spot welding method using the spot welding device according to claim 1 or 2, dressing or replacing the electrode; A process of spot welding by applying pressure and current to obtain the resistance value. measuring the tip diameter of the electrode and calculating the electrode area; a step of passing current between the pair of electrodes without sandwiching the workpieces between them to perform blank welding; calculating an evaluation value including a relationship between the resistance value and the electrode area; and determining welding quality based on the evaluation value and a set threshold value. [Effects of the Invention]

[0012] According to the present invention, the welding quality can be accurately determined even if the plate thickness changes. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram of a spot welding device. [Figure 2] FIG. 2 is a signal processing flow diagram in the spot welding device. [Figure 3] FIG. 10 is a flowchart of a determination process. [Figure 4] FIG. 10 is a flowchart of another determination process. [Figure 5] FIG. [Figure 6] FIG. [Figure 7] FIG. 10 is a correlation diagram between the change in electrode tip diameter and the resistance value. [Figure 8] FIG. 10 is a correlation diagram between the change in electrode tip diameter and the resistance value. [Figure 9] FIG. 10 is a correlation diagram between the change in electrode tip diameter and the resistance value. [Figure 10] 1 is a graph showing changes in resistance value over time. [Figure 11] 10 is a graph showing the change in resistance value over time and the time course of current flow. DETAILED DESCRIPTION OF THE INVENTION

[0014] Next, embodiments of the present invention will be described in detail below with reference to the drawings.

[0015] (Welding equipment overview) 1 shows an overview of a spot welding device, in which the main body, welding device 10, has a pair of first and second electrode support parts 11A and 11B. First and second electrode support parts 11A and 11B hold first and second electrodes 12A and 12B at their tip parts, respectively, so that they can move toward and away from each other (open and close).

[0016] This welding device 10 is provided, for example, at the tip of a welding robot 14. The movement of the welding robot 14 is controlled by a control device 20. The control device 20 is equipped with a welding timer 30 that controls the welding current and current application time of the welding device 10.

[0017] When the first electrode 12A and the second electrode 12B are moved in a direction approaching each other, a predetermined pressure is applied to the target welded members (e.g., overlapping automotive steel plates) 2P via the tips at the ends of the first electrode 12A and the second electrode 12B.

[0018] A welding current flows between first electrode 12A and second electrode 12B from welding power source 16. Furthermore, first electrode support portion 11A and second electrode support portion 11B are provided with voltage detection terminals 18, 18 for use when clamping welded members 2P or when not clamping welded members 2P.

[0019] In this embodiment, a welding quality assurance determination means is provided, and a configuration example is shown in Fig. 2. The welding quality determination means 40 of this embodiment includes a control unit 41, a memory unit 42, a calculation unit 43, a resistance value change recording unit 43A, an operation unit 44, a determination unit 45, an output unit 46, an input unit 47, and a conversion unit 47A.

[0020] Input unit 47 of welding quality determination means 40 receives current and voltage values ​​from current value detection means 10A and voltage value detection means 10B provided in welding device 10.

[0021] On the other hand, the welding device 10 is provided with an electrode tip state confirmation means 50 for the electrodes 12 (first electrode 12A, second electrode 12B). The electrode tip state confirmation means 50 is, for example, an imaging device such as a camera, and is used to confirm the state of the electrode tip (for example, measuring the electrode tip diameter φ shown in FIG. 6) before welding begins or after welding (for a certain product) is completed. A specific example is the "Network Type Electrode Tip Monitoring Camera TMN-01" commercially available from Kyokuto Corporation. Incidentally, Japanese Patent Application Laid-Open No. 07-148581 discloses the placement of a line sensor camera near a tip dresser to measure the electrode tip diameter.

[0022] As described above, the current value and the voltage value are input to input unit 47 of welding quality determination means 40, and calculation unit 43 calculates the resistance value between the electrodes based on the voltage value and the current value. In addition to being stored in memory unit 42, control commands such as the current application time and the current value are sent to welding device 10 via control unit 41. On the other hand, electrode tip state information from electrode state confirmation means 50 is also taken in by input unit 47, and the electrode area is input to calculation unit 43 via conversion unit 47A, which calculates the electrode area from the electrode tip diameter. An evaluation value is calculated by calculation unit 44, which calculates an evaluation value using the calculated resistance value and electrode area, and the quality of the welding is judged in judgment unit 45. The result of the judgment of whether the welding quality is good or bad is displayed on a display device D1 near the welding device 10 via an output unit 46, and also on a display device D2 installed at a location where quality control is being performed.

[0023] The control unit 41 controls the current value based on the resistance value calculated by the calculation unit 43 so that the target heat generation amount is reached. Even if the electrodes wear out, the current value is adjusted to always generate the optimum amount of heat, preventing welding defects.

[0024] Before proceeding to a detailed explanation, it will be noted that the "resistance value between electrodes" referred to in the present invention includes various "resistance values", including the resistance values ​​shown in FIG. That is, the "resistance value with product" shown in (a) is the resistance value when the workpiece is sandwiched between a pair of electrodes. The "blank resistance value" (resistance value without product) shown in (b) is the resistance value between a pair of electrodes when there is no welded material (plate assembly product). The "product resistance value" shown in (c) is the resistance value that occurs only between the welded components (assembly products) when they are sandwiched between a pair of electrodes. In other words, "resistance value with product (a)" = "blank resistance value (b)" + "resistance value with product (c)".

[0025] [Table 1]

[0026] [Table 2]

[0027] [Table 3]

[0028] The "contact resistance value" shown in Table 2 along with the actual measured values ​​indicates how much the voltage drops (whether there is energy loss) when current passes through the contact surface between the electrode and the material to be welded, and is an index showing how easily electricity passes through the contact surface, and is calculated by multiplying the "bad resistance value" (resistance value without the product) by the "electrode area." In other words, "contact resistance value" = "bad resistance value (b)" (resistance value without the product) x "electrode area." In addition, the "resistance per electrode area" shown in Table 3 along with the actual measured values ​​is 2 This shows the electrical resistance of the welded material per area, and is calculated by dividing the "product resistance" by the "electrode area." In other words, "resistance per electrode area" = "product resistance (c)" ÷ "electrode area" In both cases, the element of "electrode area" is incorporated. "Contact resistance value" and "resistance value per electrode area" are evaluation values ​​and threshold values ​​for determining welding quality.

[0029] In the embodiment of the present invention, the welding quality determination means 40 includes an information acquisition unit (corresponding to the input unit 47 in the example shown in FIG. 2) that acquires the resistance value between the electrodes and the electrode area, and a determination unit 45 that determines the welding quality based on an evaluation value including the relationship between the electrode area and the resistance value and a set threshold value.

[0030] The electrode diameter gradually increases during the welding process (in the example shown in Table 1, from φ6 to φ7 to φ8.5), and when it reaches a certain electrode (tip) diameter (for example, φ8.5), the welding quality becomes poor. Therefore, it is conceivable to use the electrode diameter as an evaluation value, but it is not possible to determine whether there are impurities such as plating. It is also conceivable to set the resistance value detected when the workpiece (steel plate) is sandwiched between the upper and lower electrodes as the evaluation value and make a judgment, as in Patent Document 2. However, if the difference between the evaluation values ​​for pass and fail products is narrow, setting the threshold value becomes difficult. For example, as shown in Table 1 and Figure 7, the change in the resistance value with the product is small. In other words, the change in the resistance value with the product between φ7 (OK product) and φ8.5 (NG product) is 211.9 - 210.4 = 1.5, which is 1.5 / 211.9 * 100 = 0.7% (change rate), making it difficult to set the threshold value.

[0031] However, if, in accordance with an embodiment of the present invention, the evaluation value is determined not only based on the resistance value but also includes the relationship between the electrode area (chip area) and the resistance value, the area is related to the square of the radius, so the range of evaluation values ​​between good and bad products is expanded and it becomes easier to set the threshold value (the set threshold value increases the stability and reliability of distinguishing between good and bad products).

[0032] A first example of an "evaluation value including the relationship between resistance value and electrode area" according to the present invention is an evaluation value that uses the "contact resistance value" obtained by multiplying the "blank shot resistance value" and the "electrode area" (chip area), as shown in Table 2. As shown in the example in Figure 8, the electrode condition can be determined by setting the evaluation value based on this "contact resistance value" as the threshold. In other words, the change in contact resistance value (μΩ) between φ7 (OK product) and φ8.5 (NG product) is 7973 - 5424 = 2549, which is 2549 / 7973 × 100 = 31.9% (change rate), which is a very large change, making it easy to set the threshold.

[0033] A second example of the "evaluation value including the relationship between resistance value and electrode area" according to the present invention is an evaluation value that uses the "resistance value per electrode area" obtained by dividing the "product resistance value" (resistance value of the welded member) by the "electrode area" (tip area), as shown in Table 3. As shown in FIG. 9, the evaluation value based on this "resistance value per electrode area" can be set as the threshold value to determine the welding condition. In other words, the change in resistance value (μΩ) per electrode area between φ7 (OK product) and φ8.5 (NG product) is 1.84 - 1.23 = 0.61, which is 0.61 / 1.84 * 100 = 33% (change rate), which is a very large change, making it easy to set the threshold value.

[0034] If the evaluation value is the resistance value per electrode area, it is possible to determine the welding state, i.e., whether or not there is a possibility that the welding state will deteriorate during the current application process to each welding point, resulting in a defective product. In this case, it is preferable to use the "product resistance value" obtained by subtracting the "blank resistance value" when no welding material is sandwiched between the pair of electrodes from the "product resistance value" when the welding material is sandwiched between the pair of electrodes. That is, since "product resistance value" = "resistance value with product" - "blank hitting resistance value," the evaluation value is "resistance value per electrode area" = {"resistance value with product" - "blank hitting resistance value"} ÷ "electrode area." If the "product resistance value" is used, the resistance value generated in the fusion zone (nugget) is detected, so the load on the calculation unit 43 and the operation unit 44 is reduced.

[0035] Referring to FIG. 3, the "resistance value with product" is obtained at the workpiece welding current application stage, and the "blank welding resistance value" is obtained at the short circuit current application stage after the electrode area calculation.

[0036] On the other hand, the resistance value can be the "blank resistance value" when no workpiece is sandwiched between the pair of electrodes, and the evaluation value can be the "contact resistance value", which is the value obtained by multiplying the "blank resistance value" by the "electrode area". As shown in FIG. 4, the evaluation value can be obtained by calculating the electrode area and performing short-circuit current application (empty firing).

[0037] The main cause of defective welding in spot welding is the condition of the electrode. Therefore, if the condition of the electrode after welding is poor, it can be determined that the welded material welded with that electrode will also be defective. When determining the electrode condition, taking the electrode area into consideration makes the difference between good and bad products clearer (the rate of change becomes larger), making it easier to determine whether the electrode is good or bad relative to the threshold value. If the "contact resistance value," or "blank firing resistance value," is excessively large, it means that the heat that should be generated in the molten part (nugget) is being generated in the electrode, and it is easy to predict that the amount of heat generated in the molten part (nugget) will be less when actual welding current is applied, resulting in poor welding.

[0038] The calculation unit 43 has a resistance value change recording unit 43A that records the calculated change in resistance value between the electrodes over time, and when the contact resistance value is below a threshold value, it transitions to the welding stage, and it is preferable that the judgment unit 45 judges the quality of each welding point based on the average value of the change in resistance value over time or the magnitude of the change in resistance over time in the welding stage as shown in Figure 10(a) for thick plate / thick plate welding or (b) for thin plate / thin plate welding.

[0039] If the contact resistance value is below the threshold, the electrode condition is determined to be good, and then the change in resistance value over time during the welding stage is determined. This is a two-stage determination, and by monitoring all welding points, the reliability of quality assurance is increased.

[0040] Input unit 47 is a PLC (programmable logic controller), and conversion unit 47A is mounted on input unit 47. Information including electrode area information from conversion unit 47A of the PLC (programmable logic controller) and resistance value information output from calculation unit 43 can be input into welding timer 30, which includes a calculation unit (not shown) that controls welding robot 14, and calculated by the calculation unit. In the embodiment, the parameters are current, voltage, and electrode area. The current value is controlled and recorded by welding timer 30, so by collecting other parameters in welding timer 30, the processing speed of the entire control system is increased.

[0041] When the judgment result of the judging unit 45 on the welding quality is negative, the output unit 46 can selectively display the product identification number and the welding point identification number on the display device D1 and / or the display device D2. If a welding abnormality occurs, the manufacturer will be notified and the welding robot will be stopped, preventing the continuous production of defective products.It is also possible to distinguish between judging the quality of each workpiece and that of each welding point.

[0042] In the judgment process flow diagram (Figure 3), the tip diameter of the electrode is measured after spot welding, the electrode area is calculated, and current is passed (short-circuit current) between the pair of electrodes without the workpiece being sandwiched between them. This allows the product resistance value divided by the electrode area to be used as the evaluation value, and the evaluation value of each welding point is compared with a threshold value to judge the welding quality of all welding points.

[0043] In addition, in another judgment process flow diagram [Figure 4], the electrode tip diameter is measured, the electrode area is calculated, and spot welding is performed after current (short-circuit current) is passed between the pair of electrodes without the welded material sandwiched between them. The contact resistance value is used as the evaluation value, and the quality can be judged by comparing it with a threshold value for all spot welding points.

[0044] The blank welding step can be performed at selected welding points in the sequence of welding points.

[0045] In this embodiment, the evaluation value is calculated by multiplying the "blank shot resistance value" and the "electrode area", but the evaluation value may also be calculated by dividing the "blank shot resistance value" and the "electrode area". Furthermore, although the evaluation value is calculated by dividing the "product resistance value" by the "electrode area", the evaluation value may be calculated by multiplying the "product resistance value" by the "electrode area". [Explanation of symbols]

[0046] 10...Welding equipment 11A,11B...Electrode support part 12A,12B…electrode 14...Welding robot 20...Robot control device 30...Welding timer 40...Means for determining welding quality 50...Electrode state confirmation means 60...Display device (D1, D2)

Claims

1. A spot welding device that welds overlapping workpieces placed between a pair of electrodes by applying pressure and current to the electrodes, A welding quality determination means is provided, The welding quality determination means an information acquisition unit that acquires the resistance value between the electrodes and the electrode area; a determination unit that determines welding quality based on an evaluation value including a correlation between the resistance value and the electrode area and a set threshold value, A spot welding device characterized by:

2. The welding quality determination means an input unit that inputs a voltage value and a current value between the pair of electrodes; a storage unit that stores a voltage value and a current value when the current is applied; a calculation unit that calculates a resistance value between the electrodes based on the voltage value and the current value; a conversion unit that calculates an electrode area from the tip diameter of the electrode; a calculation unit that calculates the evaluation value using the resistance value and the electrode area, 2. The spot welding device according to claim 1.

3. 3. The spot welding device according to claim 2, wherein the evaluation value is the resistance value per area of ​​the electrode.

4. 4. The spot welding device according to claim 3, wherein the resistance value is a product resistance value obtained by subtracting a blank resistance value when the workpieces are not sandwiched between the pair of electrodes from a product resistance value when the workpieces are sandwiched between the pair of electrodes.

5. 3. The spot welding device according to claim 2, wherein the resistance value is a blank resistance value when the workpiece is not sandwiched between the pair of electrodes, and the evaluation value is a contact resistance value obtained by multiplying the blank resistance value by an area of ​​the electrodes.

6. The welding quality determination means an input unit that inputs a voltage value and a current value between the pair of electrodes; a storage unit that stores a voltage value and a current value when the current is applied; a calculation unit that calculates a resistance value between the electrodes based on the voltage value and the current value; having 2. The spot welding device according to claim 1.

7. the calculation unit has a resistance value change recording unit that records the calculated change in the resistance value between the electrodes over time, When the contact resistance value is equal to or less than the threshold value, the process proceeds to a welding stage, and the judgment unit judges whether each welding point is good or bad based on the average value of the change in the resistance value over time or the magnitude of the change in the resistance value over time in the welding stage.

7. The spot welding device according to claim 6.

8. 3. The spot welding device according to claim 2, wherein the input unit is a programmable logic controller, the conversion unit is mounted on the input unit, and information including the electrode area information from the conversion unit of the programmable logic controller and the resistance value information output from the calculation unit is input to a welding timer including the calculation unit that controls a welding robot, and is calculated by the calculation unit.

9. A spot welding apparatus as described in claim 2 or claim 6, wherein the welding quality judgment means is provided with an output unit, and the output unit selectively displays the product identification number and the welding point identification number on a result display device when the judgment result of the judgment unit is negative for the welding quality.

10. The welding quality determination means 7. The spot welding device according to claim 2, further comprising a control unit that controls a current value based on the calculated resistance value so as to reach a target heat generation amount.

11. A spot welding method using the spot welding device according to claim 1 or 2, dressing or replacing the electrode; A process of applying pressure and current to perform spot welding and obtaining a resistance value; measuring the tip diameter of the electrode and calculating the electrode area; a step of passing current between the pair of electrodes without sandwiching the workpieces between them to perform blank welding; calculating an evaluation value including a relationship between the resistance value and the electrode area; and determining welding quality based on the evaluation value and a set threshold value. A spot welding method characterized by:

12. The spot welding method according to claim 11, wherein the step of blank welding is performed at selected welding points in a series of welding points.

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