JP2006110554A - Method for judging quality of resistance spot welding and monitoring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To quicken judgment of welding quality and to increase the accuracy of the judgment by monitoring a welded nugget production process under energizing utilizing a change in electric power difference between electrodes per half-cycle. <P>SOLUTION: When welding current and voltage between electrodes changing every moment under energizing are detected at least per half-cycle and the difference in electric power is time sequentially monitored, the difference in electric power being obtained by subtracting, from the electric power applied to a work in the reducing process of the electric power between electrodes corresponding to the respective same current values in the increasing process and reducing process of welding current per half-cycle, the electric power applied to the work in the increasing process, a curve showing the behavior of the difference in electric power is expressed on a cycle diagram with the difference in electric power as the axis of ordinate and the cycle number as the axis of abscissa, and, from the change in the difference in electric power at the measurement reference point of the optional designated cycle number in the axis of abscissa, the evaluation of welding quality is performed under a welded nugget growing process. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention is intended to determine the quality of a welded part by observing the nugget generation process to determine whether a weld nugget of an appropriate size adapted to the strength required for the work piece (workpiece) is formed during spot welding. In more detail, the non-destructive monitoring method and device of the above are described. More specifically, the welding current and the interelectrode voltage (or interelectrode resistance) that change every moment during energization are detected, and the power difference between the electrodes is obtained every half cycle. The present invention relates to a resistance pot welding quality determination method and a monitoring device that observe a generation process of a weld nugget in real time from a change in difference and determine quality of a welded portion.

In resistance spot welding, a work piece is sandwiched between a pair of electrode tips, a pressure is applied to this work, a welding current is applied intensively, and melting is caused by resistance heat generation to form a welding nugget with the necessary strength for the work. To do. Destructive inspection methods and non-destructive inspection methods are known for checking whether or not proper welding that sufficiently satisfies the welding strength is performed.

Destructive inspection methods include macro tests, chisel tests, and torsion tests. This type of destructive inspection method literally breaks and inspects the welded part off-line and inspects it, and it affects the productivity because the welding sample is extracted and the tensile strength of the welded part is checked manually. In recent years, instead of this, a nondestructive inspection method has been developed to monitor a welding nugget generation process during spot welding so that rapid and appropriate inspection can be performed online.

Conventional nondestructive inspection methods include measuring the amount of displacement between electrodes using the thermal expansion of the weld nugget, and measuring the size of the weld using ultrasonic waves. Judgment of welding quality (and / or welding current) using the method (see Patent Document 1) and the behavior of the transmission curve of the ultrasonic wave applied to the weld during the welding nugget generation process in comparison with the transition of the model transmission The workpiece temperature is calculated based on a heat conduction model based on a method of adaptively controlling welding condition elements such as welding pressure, energizing time, etc. (see Patent Document 2), or a detected value of welding current and inter-chip voltage. A method is known in which an estimated nugget diameter is calculated from the distribution and compared with a model nugget diameter necessary for ensuring appropriate welding strength to determine a welding result (see Patent Document 3).

There are several other types of these nondestructive inspection methods. However, in the case of using ultrasonic waves, an ultrasonic transmitter / receiver and a complicated measurement and processing system are required, resulting in expensive equipment. In addition, the amount of ultrasonic reflection varies depending on the workpiece material and workpiece surface treatment (for example, coating, plating layer, etc.). The echo back may be disturbed on the surface and the measurement accuracy may be distorted. Also, in the case of one temperature distribution, the nugget diameter calculation method is based on estimation from the temperature distribution, and the measurement calculation program is complicated, and there are still problems to be improved in terms of speeding up the inspection and inspection accuracy. .
JP-A-6-138100 Special table 2004-507365 gazette JP-A-6-170552 JP 2004-58153 A Hiroshi Abe, 1 other "Research on Scattering Prevention Control of Spot Welding" MP-313-2002, Light Structure Joining Research Committee, Japan Welding Society, Published June 6, 2002

  The problems to be solved by the present invention are that the conventional non-destructive inspection method has a complicated and expensive measuring system, whether it is an ultrasonic or a temperature sensor, and further, on-line inspection speed and inspection accuracy judgment by mixed flow production. The evaluation was not sufficient. The present invention calculates the power difference obtained by subtracting the inter-electrode power applied to the workpiece in the welding current increasing process from the inter-electrode power applied to the workpiece in the welding current decreasing process at the same current point in each half cycle, and the behavior of the power difference. A welding quality judgment method capable of creating a curve and monitoring the welding nugget generation process during spot welding using the behavior curve of the power difference and performing a high-precision welding quality inspection on the entire number of hit points online in real time, and A monitoring device will be provided to improve quality judgment accuracy and speed up non-destructive inspection.

  According to a first aspect of the present invention, in a resistance spot welding method in which a workpiece is sandwiched between a pair of electrode tips and a welding current is applied to the workpiece while applying a pressure to the workpiece to generate a nugget by resistance heat generation, a suitable welding condition is achieved. The welding current and the interelectrode voltage, which change from moment to moment during energization, are detected at least every half cycle, and correspond to the same predetermined current value for each of the increasing and decreasing processes of the welding current for each half cycle. When using the inter-electrode power applied to the workpiece and monitoring the power difference obtained by subtracting the inter-electrode power in the welding current increasing process from the inter-electrode power in the welding current decreasing process, the power difference is plotted on the vertical axis, The behavior curve of the power difference is represented on a cycle diagram with the number of cycles on the horizontal axis, and the behavior curve of the power difference is substantially horizontal at the measurement reference point of the arbitrarily specified number of cycles on the horizontal axis. When it reaches the wherein the nugget growth is determined to be normal.

A second aspect of the present invention is characterized in that it is determined that nugget growth is insufficient when the behavior curve of the power difference falls at the measurement reference point having an arbitrarily specified number of cycles.

According to a third aspect of the present invention, when the behavior curve of the power difference continues to rise at the measurement reference point having an arbitrarily specified number of cycles, or has risen for a certain time on the way to the measurement reference point, it changes to a substantially horizontal state. It is characterized in that it is determined that it is at the scattering occurrence limit point.

    According to a fourth aspect of the present invention, when the behavior curve of the power difference rises immediately after falling or falls immediately after rising at a steep angle, it is determined that scattering occurs at that time.

    According to claim 5 of the present invention, in the resistance spot welding method, an inter-electrode power ratio corresponding to a predetermined same current value in each of the increasing and decreasing processes of the welding current for each half cycle, or an inter-electrode voltage difference, or The time difference between the time when the interelectrode voltage ratio, the interelectrode resistance difference, or the interelectrode resistance ratio, or when the maximum value of the welding current in each half cycle is reached and when the interelectrode voltage reaches the maximum value Even in the case of monitoring in the same manner, a graph similar to the behavior curve of the power difference is displayed, and the power ratio between the electrodes, the voltage difference between the electrodes, or the electrode The change in the voltage ratio between the electrodes, or the difference in resistance between the electrodes, or the ratio in resistance between the electrodes, or the difference in maximum arrival time is monitored and determined by the method according to any one of claims 1 to 4. .

    According to a sixth aspect of the present invention, at least one of the pair of electrode tips is driven opposite to the other electrode tip using fluid pressure, electricity, or other power source, and the workpiece is moved by the electrode tip. Detection by detecting the welding current and the voltage between electrodes (or resistance between electrodes) at least every half cycle in a resistance spot welder that presses and pressurizes and melts and joins with resistance heating by flowing welding current A voltage extracting means for extracting the voltage between the electrodes at the same current point in the increasing and decreasing processes of the welding current every half cycle, and a predetermined same current in each of the increasing and decreasing processes of the welding current. The inter-electrode power applied to the workpiece corresponding to the value is calculated, and the power applied to the workpiece during the welding current increasing process from the inter-electrode power applied to the welding current decreasing process corresponding to the same current value Power difference calculating means for calculating power difference obtained by subtracting power between power sources in time series, and a power difference calculating unit for calculating a power difference change value at a measurement reference point of an arbitrarily specified number of cycles from the power difference obtained by the calculation. On a cycle diagram in which a change value calculation means, an arithmetic unit having a judgment means for judging the quality of nugget growth from the change in the power difference, and the power difference on the vertical axis and the number of cycles on the horizontal axis A power difference behavior curve display unit for displaying the power difference behavior curve in a graph, and a result of determining whether the nugget growth is good or not by a change in the power difference at the measurement reference point of the power difference behavior curve displayed in the graph There is provided a resistance spot welding quality monitoring device provided with a display device having a determination result display unit to perform.

    Claim 7 of the present invention takes the inter-electrode voltage of the welding current increasing process and the welding current decreasing process at several current points in each half cycle, and the inter-electrode power applied to the workpiece calculated from the inter-electrode voltage A power difference is calculated by subtracting the inter-electrode power in the welding current increasing process from the inter-electrode power in the welding current decreasing process at each current point, and each power difference is calculated every half cycle to reduce detection error. The power difference is averaged, the average value of the power difference is taken on the vertical axis, and the behavior curve of the power difference is plotted on the horizontal axis.

According to the method of claims 1 to 5 of the present invention, the welding current and the interelectrode voltage that change every moment during energization are detected at least every half cycle, and the welding current increasing process for each half cycle is performed. Using the inter-electrode power applied to the workpiece corresponding to a predetermined current value in each decrease process, the power difference obtained by subtracting the inter-electrode power in the welding current increasing process from the inter-electrode power in the welding current decreasing process is time-series. The power difference is plotted on the vertical axis, the number of cycles is plotted on the horizontal axis, the power difference is displayed as a behavior curve on the graph, and the measurement of the arbitrarily specified number of cycles provided on the horizontal axis The nugget growth status can be determined instantaneously from the change in the power difference at the reference point. In addition, the inter-electrode power ratio, the inter-electrode voltage difference, or the inter-electrode voltage ratio, or the inter-electrode resistance difference, or the electrode corresponding to a predetermined same current value in each of the welding current increasing and decreasing processes for each half cycle Even when the time difference between the time when the maximum resistance value of the welding current in each half cycle and the time when the interelectrode voltage reaches the maximum value is monitored in time series, the same graph as the behavior curve of the power difference Display, the inter-electrode power ratio, or the inter-electrode voltage difference, or the inter-electrode voltage ratio, or the inter-electrode resistance difference at the measurement reference point of the arbitrarily specified number of cycles provided on the horizontal axis of the diagram, or A change in the resistance ratio between the electrodes or the difference in the maximum value arrival time can be monitored to instantly determine the nugget growth state. Of course, if the welding conditions are extremely different, this is not the case, but it is the range that can be empirically understood from the publicly available welding conditions table, and such insane settings are not considered as excluded.

In contrast to the above-described conventional inspection method using ultrasonic reflection, the present invention eliminates instability in measurement accuracy due to disturbance of echo back at the nugget boundary surface by workpiece material and workpiece surface treatment. Further, in contrast to the conventional inspection method based on temperature distribution, the present invention does not require a complicated program or expensive temperature measurement equipment for measuring and calculating the nugget diameter by estimating the temperature distribution. Therefore, the present invention can always carry out high-precision welding quality inspection over the entire number of hit points in real time on the same multi-mix mixed-flow production line in real time, thereby improving quality judgment accuracy and speeding up nondestructive inspection. it can.

According to the apparatus of claims 6 to 7 of the present invention, a detection device for detecting the welding current and the interelectrode voltage (or interelectrode resistance) during energization at least every half cycle, and several points in each half cycle A voltage extracting means for extracting the inter-electrode voltage at the same current point from the inter-electrode voltage, and extracting the inter-electrode voltage at the same current point from the inter-electrode voltage; From the inter-electrode power applied to the workpiece calculated from the above, the power difference obtained by subtracting the inter-electrode power in the welding current increasing process from the inter-electrode power in the welding current decreasing process at each current point is calculated to reduce the detection error. Therefore, a power difference calculating means for averaging each power difference every half cycle and calculating a power difference in time series, and a measurement reference point for an arbitrarily specified number of cycles from the average value of the power differences A power difference change value calculating means for calculating a change value of the average value of the power difference, and the power on the cycle diagram in which the average value of the power difference is taken on the vertical axis and the number of cycles is taken on the horizontal axis. A power difference behavior curve display unit that graphically displays a difference behavior curve, and a determination unit that determines whether nugget growth is good or not from a change in an average value of the power difference in an arbitrarily specified number of cycles of the power difference behavior curve displayed in the graph And a display device having a determination result display unit for displaying the determined result.

This eliminates the need for ultrasonic transceivers and complex measurement and processing systems compared to conventional ultrasonic nondestructive inspection equipment. Compared to conventional inspection equipment based on temperature distribution, there is no need for complex measurement programs and equipment based on estimation from temperature distribution, high-precision welding quality inspection can be performed in real time, quality judgment accuracy is improved, and non-destructive inspection is accelerated. It is possible to provide a resistance spot welding quality monitoring device that is simple and inexpensive.

    FIG. 5 shows waveforms of the welding current, the electrode voltage, and the electrode power in a half cycle. FIG. 6 shows the transition of the power difference (Ps) at the same current value in the embodiment of the present invention.

The present invention is an application of the basic technique shown in FIGS. 5 and 6, and the basic principle is described in detail in the above-mentioned Patent Document 4 and Non-Patent Document 1 already published and announced by the present applicant.

The interelectrode power (P), which is the product of the welding current (I) and the interelectrode voltage (V), is P = V · I. In other words, if the interelectrode resistance is R, then P = I ² R. , P is proportional to the interelectrode resistance. As shown in FIG. 4, the inter-electrode resistance (1) includes the inter-plate contact resistance (2) of the work as a welded part, the specific resistance (3) of the work itself, the electrode tip (6) and the work (7). Of the contact resistance (4) and the specific resistance (5) of the electrode tip itself and other conductors. The resistance between the electrodes decreases when the workpiece is softened by energization, and the contact state between the workpiece's plate-to-plate contact and the contact resistance between the electrode tip and the workpiece becomes good. Varies depending on the temperature coefficient characteristics of the material resistivity.

    In the present invention, the power difference obtained by subtracting the inter-electrode power related to the increasing work from the inter-electrode power related to the decreasing work of the same current point is monitored in time series every half cycle. That is, as shown in Fig. 5, the inter-electrode voltage corresponding to each of the increasing and decreasing processes of the half-cycle of the welding current that changes from moment to moment is detected under the same current value (I). Then, the difference between the interelectrode power (Pup = Eup × I and Pdown = Edown × I) (Pdown−Pup), which is the product of the interelectrode voltage (Eup and Edown) and the predetermined current value (I) ) As Ps.

    At this time, when the value of Ps is taken on the vertical axis and the number of cycles is taken on the horizontal axis, the Ps-cycle diagram has a waveform as shown in the graph of FIG. 6 taking the behavior curve of the power difference as an example. From this power difference behavior curve, it can be determined that the measurement reference point is almost horizontal.

According to the prior art described in Patent Document 4 and Non-Patent Document 1, it is detected at which point the change in the power difference shown in the behavior curve of the power difference in FIG. 6 has reached a predetermined value or less. Thus, it is possible to prevent the occurrence of scattering while generating an appropriate welding nugget by changing the welding current or the applied pressure thereafter, or by stopping the energization at that time.

    The present invention uses the power difference behavior curve shown in FIG. 6 to graphically monitor the change in the power difference between the electrodes every half cycle in the welding nugget generation process, and from the change in the power difference at the measurement reference point. We have developed a nondestructive inspection method and device for quality evaluation of whether or not proper welding that sufficiently satisfies the welding strength is performed.

    In Example 2 of the present invention, FIG. 7 shows the welding current and the interelectrode voltage in a half cycle. The welding current and the interelectrode voltage during energization of the welding current that changes from moment to moment are detected momentarily, and when the maximum value of the welding current in each half cycle is reached (Ti-peak) and the interelectrode voltage is the maximum value. The time difference (Ts) until the point of arrival (Te-peak) reaches the current interelectrode voltage (Eup and Edown) in each of the current increasing process and current decreasing process under the same current value (I). If the difference (Pdown−Pup) between the electrode power (Pup = Eup × I and Pdown = Edown × I), which is the product of the predetermined current value (I), is large (or small), the time difference is large ( (Or smaller).

    When the maximum value arrival time difference (Ts) is plotted on the vertical axis and the number of cycles is plotted on the horizontal axis, the behavior curve of the maximum value arrival time difference is taken as an example in the waveform shown in the graph of FIG. It becomes. In the behavior curve of the maximum value arrival time difference, it can be determined that the measurement reference point is almost horizontal.

According to the prior art described in Patent Literature 4 and Non-Patent Literature 1, at which point the change in the maximum value arrival time difference shown in the behavior curve of the maximum value arrival time difference in FIG. It is possible to prevent the occurrence of scattering while generating an appropriate welding nugget by changing the welding current or the applied pressure thereafter, or by stopping the energization at that time.

    The present invention uses the behavior curve of the maximum value arrival time difference shown in FIG. 8 to graphically monitor the change in the maximum value arrival time difference every half cycle in the welding nugget generation process, and the measurement reference point at the measurement reference point. We have developed a nondestructive inspection method and device for quality evaluation of whether or not proper welding that sufficiently satisfies the welding strength is performed based on the change in the maximum value arrival time difference.

    Next, a third embodiment of the present invention will be described. Example 3 shows an apparatus and operation for carrying out the method of the present invention based on the electric block circuit diagram of FIG. 2 and the flowchart of FIG. The resistance welder uses at least one of the paired electrodes on the secondary side of the welding transformer T as a movable side electrode 1 and is driven by an actuator using fluid pressure or electricity as a drive source. The other electrode is supported by the arm as the fixed electrode 2.

    When starting the welding machine, the work 3 to be welded is sandwiched between the two electrodes 1 and 2, and when a predetermined pressurizing force rises, the thyristor is driven by the energization signal from the timer device to control the main current of the transformer. A welding current is passed between the electrodes, the work 3 is melted by resistance heat generation, and a nugget is generated and joined.

    Only the voltage waveform c is extracted using the induced voltage b obtained from the toroidal 4 by the induction cancel circuit 6 of the detecting device 6 and the voltage a generated in the energized electrodes 1 and 2 is made into an appropriate waveform by the interface circuit 7 of the detecting device. The waveform is captured by the waveform capture circuit 8 of the detection device. The voltage extraction means 11 at the same current point of the arithmetic unit voltage and increase of the decrease process at the same current point of the welding current obtained from the CT 5 through the interface circuit 10 of the detection device and the waveform acquisition circuit 9 of the detection device every half cycle. The voltage of the process is obtained, the power difference calculating means 12 of the arithmetic unit calculates the power in the decreasing process and the power in the increasing process at the same current point, and the power difference at that point is obtained.

    The change of the power difference calculated by the power difference change calculation means 14 of the arithmetic device is monitored by the determination means 13 of the arithmetic device in time series, the power difference is taken on the vertical axis, and the number of cycles on the horizontal axis. A graph showing the behavior of the power difference is displayed on the behavior curve display unit 15 of the power difference of the display device on the cycle diagram taken.

    In this case, the horizontal axis is provided with a measurement reference point having an arbitrarily specified number of cycles, and the behavior curve of the power difference is substantially horizontal at the measurement reference point as shown in FIG. When the transition to, the appropriate thermal energy is given to the weld and the power applied to the work in the process of decreasing the inter-electrode power corresponding to the same predetermined current value in each of the increasing and decreasing processes of the welding current every half cycle. It is determined that the nugget growth is normal because the change in the power difference obtained by subtracting the power applied to the workpiece in the increasing process is lost at the measurement reference point and the thermal change is considered to be stable. The determination result is displayed on the determination result display unit 16 of the display device.

    Similarly, according to the determination of claim 2, as shown in FIG. 1 (B), when the descent continues at the measurement reference point, the thermal energy applied to the welded portion is low, so that each half cycle is The change in the power difference gradually falls at the measurement reference point, and it is considered that the thermal change of the welded part is continued, and it is determined that the nugget growth is insufficient. The determination result is displayed on the result display unit 16 of the display device. In addition, in the determination operation of claim 3, when it continues to rise at the measurement reference point, as shown in FIG. The power difference for each half cycle is gradually increasing due to the high thermal energy applied to the slab, or after the gradual increase, there is no change in the power difference, and the weld has excessive thermal energy. It is judged that it is at the limit point of occurrence of scattering because it seems that the thermal change is gradually stabilized from the state just before the occurrence of scattering and no scattering occurs. The determination result is displayed on the result display unit 16 of the display device.

    According to the determination operation of claim 4, when the behavior curve of the power difference rises immediately after descending or descends immediately after ascending at a steep angle as shown in FIG. 1 (D), the thermal energy applied to the welded portion. Therefore, it is determined that a part of the molten welded material is scattered to the outside and the power difference for each half cycle is sharply changed. The determination result is displayed on the result display unit 16 of the display device. In this way, the quality of the weld quality is judged quickly and accurately from several evaluation patterns during the nugget generation process.

  The present invention provides a power difference obtained by subtracting the power applied to the workpiece in the increasing process from the power applied to the workpiece in the decreasing process of the inter-electrode power corresponding to the predetermined same current value in each of the welding current increasing process and the decreasing process in each half cycle. However, when the inter-electrode power ratio, the inter-electrode voltage difference, the inter-electrode voltage ratio, the inter-electrode resistance difference, the inter-electrode resistance ratio, or the maximum welding current in each half cycle is reached. It can be determined in the same manner from the behavior curve of the time difference until the time when the interelectrode voltage reaches the maximum value.

  The present invention is not limited to the field of resistance spot projection welders and seam welders, but can also be applied to energized heat caulking joints that use resistance heat generation or electrical upsetting.

In the Example of this invention, it is a cycle diagram which shows some welding quality evaluation patterns at the time of welding nugget production | generation from transition of the behavior curve of the power difference between chips | tips in the same electric current value. It is an electric block circuit diagram which shows the Example of the welding spot quality monitoring apparatus for enforcing the method of this invention. It is a flowchart figure which shows operation | movement of this invention apparatus. It is a figure which shows the component of the resistance between electrodes which is an Example of this invention. In the Example of this invention, the difference of the electric power between chips in the same electric current value is shown. In the Example of this invention, transition of the difference (Ps) of the electric power between chips | tips in the same electric current value is shown. In the Example of this invention, the time difference (Ts) of the maximum value of the welding current and the maximum value of a chip voltage in a half cycle is shown. FIG. 5 shows the transition of the time difference (Ts) between the maximum value of the welding current and the maximum value of the tip voltage in the example of the present invention.

Explanation of symbols

1 One electrode (movable side electrode) 2 The other electrode (fixed side electrode)
3 Work 4 Toroidal 5 CT 6 Induction Cancel Circuit 7 Interface Circuit 8 Waveform Capture Circuit 9 Waveform Capture Circuit 10 Interface Circuit 11 Voltage Extraction Unit 12 at Same Current Point 12 Power Difference Calculation Unit 13 Determination Unit
14 power difference change calculation means 15 power difference behavior curve display section 16 determination result display section

Claims (7)

In a resistance spot welding method in which a workpiece is sandwiched between a pair of electrode tips and a welding current is applied to the workpiece to generate a nugget by resistance heating to generate a nugget and joined. The welding current and the voltage between the electrodes that change every moment are detected at least every half cycle, and between the electrodes applied to the work piece corresponding to the predetermined same current value in each of the increasing and decreasing processes of the welding current in each half cycle. When calculating the power and monitoring the power difference obtained by subtracting the inter-electrode power in the welding current increasing process from the inter-electrode power in the welding current decreasing process, the power difference is plotted on the vertical axis, The power difference behavior curve is represented on a cycle diagram with the number of cycles on the axis, and the power difference behavior curve is substantially horizontal at the measurement reference point of the arbitrarily specified number of cycles on the horizontal axis. Resistance spot welding quality evaluation method characterized by determining the nugget growth is normal when it reaches the state. The resistance spot welding quality determination method according to claim 1, wherein nugget growth is determined to be insufficient when the behavior curve of the power difference falls at the measurement reference point having an arbitrarily specified number of cycles. . When the behavior curve of the power difference continues to rise at the measurement reference point for an arbitrarily specified number of cycles, or when it rises for a certain period of time on the way to the measurement reference point and changes to a substantially horizontal state, it is at the scattering occurrence limit point The resistance spot welding quality determination method according to claim 1, wherein:   2. The resistance spot welding according to claim 1, wherein when the behavior curve of the power difference rises immediately after being lowered at a steep angle or falls immediately after the rise, it is determined that scattering occurs at that time. Quality judgment method.     In the resistance spot welding method, an inter-electrode power ratio, an inter-electrode voltage difference, an inter-electrode voltage ratio, or an inter-electrode corresponding to a predetermined same current value in each of the increasing and decreasing processes of the welding current in each half cycle. Even when the time difference between the time when the resistance difference or the resistance ratio between the electrodes reaches the maximum value of the welding current in each half cycle and the time when the voltage between the electrodes reaches the maximum value is monitored in time series, the power difference The graph is displayed in the same manner as the behavior curve, and the power ratio between the electrodes, or the voltage difference between the electrodes, or the voltage ratio between the electrodes, in time series at the measurement reference point of the arbitrarily specified number of cycles provided on the horizontal axis of the diagram, The resistance spot welded product according to claim 1, wherein the resistance spot welded product is determined by monitoring a change in the interelectrode resistance difference, the interelectrode resistance ratio, or the change in the maximum value arrival time difference. Determination method. At least one electrode tip of the pair of electrode tips is driven opposite to the other electrode tip using fluid pressure, electricity, or other power source, and the workpiece is sandwiched and pressurized by the electrode tip, In a resistance spot welder that melts and joins with resistance heating by flowing a welding current, the welding current and the voltage between electrodes (or resistance between electrodes) are detected at least every half cycle under appropriate welding conditions. A detecting device, a voltage extracting means for extracting the voltage between the electrodes at the same current point in each of the increasing and decreasing processes of the welding current every half cycle, and a predetermined method for each of the increasing and decreasing processes of the welding current. Calculate the inter-electrode power applied to the workpiece corresponding to the same current value, and increase the welding current from the inter-electrode power applied to the workpiece in the welding current decreasing process corresponding to the same current value. A power difference calculation means for calculating a power difference obtained by subtracting the inter-electrode power applied in a time series, and a change value of the power difference at a measurement reference point of an arbitrarily specified number of cycles from the calculated power difference. A power difference change value calculation means, a power difference behavior curve display unit for displaying the power difference behavior curve on a cycle diagram having the power difference on the vertical axis and the number of cycles on the horizontal axis; A display having a calculation device having a determination means for determining the quality of nugget growth from a change in the power difference at the measurement reference point of the behavior curve of the power difference displayed in the graph, and a determination result display section for displaying the determined result Resistance spot welding quality monitoring device.     Take the voltage between the electrodes in the welding current increasing process and welding current decreasing process at several current points in each half cycle, and the welding power at each current point is calculated by the inter-electrode power applied to the work piece calculated from the inter-electrode voltage. A power difference obtained by subtracting the inter-electrode power in the welding current increasing process from the inter-electrode power in the current decreasing process is calculated, and each power difference is averaged every half cycle in order to reduce a detection error. The resistance spot welding quality monitoring apparatus according to claim 6, wherein a behavior curve of the power difference is plotted on a vertical axis, and the horizontal axis represents the number of cycles.

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