JP4486407B2 - Resistance welding control method - Google Patents

Description

  The present invention relates to resistance welding used for spot welding, and in particular, estimates the temperature of a welded part during resistance welding by heat conduction calculation so that the estimated temperature of the welded part substantially follows a predetermined temperature change target pattern. The present invention relates to improvement of a resistance welding control method for performing temperature pattern tracking current control for increasing or decreasing a welding current value.

  In resistance spot welding, in which a plurality of stacked materials to be welded are pressed and energized by a pair of electrodes, in order to obtain good quality of the welded portion, the nugget diameter must be formed within an appropriate range; It is important that no dust is generated during welding. In particular, in resistance welding of galvanized steel sheets, aluminum plated steel sheets, and high-tensile steel sheets, which are widely used recently, the nugget diameter is likely to fluctuate and dust is likely to occur. Proposed.

  In order to optimize the nugget diameter and prevent the occurrence of dust, the temperature change of the welded part during resistance welding is performed by determining the material and shape of the material to be welded, the shape and consumption of the electrode, the space between the materials to be welded and the material to be welded. It is necessary to optimize in accordance with the contact state between the electrodes. That is, by controlling the welding current so that the temperature change of the welded portion from the welding start time point to the welding end time point at a single spot position follows a predetermined temperature change target pattern according to the various welding conditions described above. In addition, it is possible to obtain good welding quality that does not generate dust with an appropriate nugget diameter. In such temperature pattern tracking current control, it is necessary to measure the temperature change of the weld during resistance welding, but it is difficult to directly measure the temperature change because the weld is not visible from the outside. For this reason, many methods have been proposed in which the temperature change of the weld is calculated in real time by heat conduction calculation to obtain the estimated temperature of the weld.

  The heat conduction calculation method includes a method using a differential equation and a finite element analysis method, a method of calculating an average temperature estimated value based on an energy balance model of a welded part, and the like. In these heat conduction calculation methods, the welding current temperature estimated value is calculated by inputting the welding current value, the welding voltage value, and the physical constants of the electrode and the workpiece to be welded.

  FIG. 5 is a time change diagram of the weld temperature estimation value Tc and the welding current Iw showing an example of the temperature pattern following current control described above. In the figure, the horizontal axis represents the elapsed time t from the start of welding as one cycle of commercial power supply (reciprocal of 50 Hz or 60 Hz) = 1 cycle (cyc). Further, the left vertical axis represents the weld temperature estimated value Tc [° C.] and the temperature change target pattern Tp [° C.], and the right vertical axis represents the welding current Iw [kA], which is the control operation amount, as an effective value. Is. In temperature pattern tracking current control, the weld temperature estimated value Tc at each elapsed time is calculated by heat conduction calculation, and an error amplification value ΔT = Gain · (Tp−Tc) with respect to the temperature change target pattern Tp is calculated. The welding current Iw is increased / decreased in accordance with the error amplification value ΔT so that the weld temperature estimated value Tc substantially follows the temperature change target pattern Tp. Thereby, the temperature change of a welding part is optimized and the favorable weld quality which does not generate | occur | produce the dust with appropriate nugget diameter can be obtained.

  As shown in the figure, the temperature change target pattern Tp is set to increase with time. For this reason, the welding part temperature estimated value Tc rises following this and reaches the maximum value Tm. Usually, the welding current Iw also increases with time. However, since the welding current Iw is changed by feedback control, the change pattern varies depending on the state of the welded portion, and sometimes does not increase and may be a substantially constant value. The temperature change target pattern Tp is set by a test in advance according to the material and shape of the electrode and the material to be welded. However, the reference temperature change target pattern is set in advance by determining the material and shape of the reference electrode and the workpiece, and if the plate thickness of the workpiece is different, the reference temperature change target pattern is used as the basis. There is also a method of newly generating a temperature change target pattern Tp by calculation (see, for example, Patent Document 1, Japanese Patent Application No. 2003-371663, etc. as the above-described conventional technology).

Japanese Patent No. 3212296

  As described above, in resistance welding to plated steel sheets, high-tensile steel sheets, etc., where it is difficult to optimize the nugget diameter and suppress the generation of dust, the degree of electrode wear, the electrode surface state, the contact state between the electrode and the workpiece, multiple Even if the state of the welded portion such as the contact state between the workpieces fluctuates, a high-quality welding result can be obtained by performing the above-described temperature pattern tracking current control. However, although it is a rare frequency, the weld state may abnormally vary beyond the normal variation range. As such a case, there is a case where a large granular dust (hereinafter referred to as spatter) generated immediately after the start of resistance welding is sandwiched between the electrode and the material to be welded, resulting in a very poor contact state. In some cases, the shape of the electrode is deformed due to wear and the contact state between the electrode and the material to be welded becomes very poor. In addition, there is a case where a gap exists between the materials to be welded and the contact state becomes very bad. When such abnormal fluctuations in the welded portion state occur, there may be a case where proper welding quality cannot be obtained even if temperature pattern tracking current control is performed. Hereinafter, temperature pattern tracking current control when this abnormal variation state occurs will be described with reference to the drawings.

  FIG. 6 is a diagram showing temporal changes in the weld temperature estimated value Tc and the welding current Iw when the weld state changes abnormally during temperature pattern tracking current control. This figure shows a case where large-sized spatter occurs immediately after the start of welding and is sandwiched between the electrode and the workpiece and the contact state becomes abnormal. In this state, the contact resistance value between the electrode and the material to be welded becomes an abnormal value, so that the estimated temperature Tc of the welded part by the heat conduction calculation includes a large error. However, since the welding portion temperature estimated value Tc including this error is controlled as a feedback value, as shown in the figure, the welding portion temperature estimated value Tc has a welding current Iw along the temperature change target pattern Tp. Increase or decrease. However, since this weld temperature estimated value Tc is not a value that accurately estimates the temperature of the weld, the nugget diameter is smaller than the appropriate value. In this case, since the weld temperature estimated value Tc is calculated to be higher than the true value, the value of the welding current Iw is considerably lower than the L1 curve (Iw in FIG. 5) during normal fluctuation. As a result, the nugget diameter becomes smaller than the appropriate range, resulting in a poor welding result.

  FIG. 7 is a diagram showing temporal changes in the weld temperature estimated value Tc and the welding current Iw when the weld state changes abnormally during temperature pattern tracking current control. This figure shows a case where large spatter is generated immediately after the start of welding and is sandwiched between the electrode and the workpiece to be welded, as in the case of FIG. 6 described above. However, unlike FIG. 6, when 5 cyc has elapsed, this spatter is melted and removed to return to the normal fluctuation state. Therefore, up to 5 cyc, since the weld temperature estimated value Tc is higher than the true value as in FIG. 6, the welding current Iw is lower than the normal fluctuation time L1. Since the normal fluctuation state is restored after 5 cyc has elapsed, the weld temperature estimated value Tc matches the true value. For this reason, the weld temperature estimated value Tc at the time when 5 cyc has elapsed is lower than the temperature change target pattern Tp. In order to reduce this deviation, the welding current Iw increases rapidly. However, if the welding current Iw is increased during welding, the welding is terminated while the weld temperature estimated value Tc cannot be matched with the temperature change target pattern Tp. As a result, since the welding current value Iw is low and the weld temperature estimation value Tc is also low compared to the normal fluctuation, the nugget diameter is smaller than the appropriate range, resulting in poor welding quality.

  Therefore, the present invention provides a resistance welding control method capable of obtaining good welding quality even when the welded portion state abnormally varies.

In order to solve the above-described problem, the first invention estimates the temperature of a welded part during resistance welding by heat conduction calculation so that the estimated temperature of the welded part substantially follows a predetermined temperature change target pattern. In the resistance welding control method for performing temperature pattern tracking current control to increase or decrease the welding current value,
When welding at one spot position is finished, the maximum value of the weld temperature estimated value is less than a predetermined reference maximum value and / or the average value of the energized welding current value is a predetermined reference In the resistance welding control method, if the average value is less than the average value, the resistance welding is performed again so that the nugget diameter is increased by the temperature pattern follow-up current control at the spot position so as to fall within an appropriate range. .

  According to the first aspect, even if an abnormal fluctuation state occurs in the welded part state during resistance welding, the abnormal fluctuation state is automatically determined and resistance welding is performed again. However, an appropriate nugget diameter can be formed.

  Embodiments of the present invention will be described below with reference to the drawings.

[Embodiment 1]
FIG. 1 is a flowchart showing a resistance welding control method according to Embodiment 1 of the present invention. Hereinafter, a description will be given with reference to FIG.

  In step ST1, resistance welding is performed by temperature pattern tracking current control at one spot position. In step ST2, a maximum value (estimated weld temperature) Tm of the weld temperature estimate Tc during welding at one spot and an average value Ia of the weld current Iw are calculated, and the weld temperature estimate maximum value Tm is calculated in advance. It is determined whether the welding current average value Ia is less than a predetermined reference maximum value It and is less than a predetermined reference average value It. If the determination result is YES, the process proceeds to step ST3, and if NO, the welding at this spot position is terminated. In step ST <b> 2 here, it is determined whether or not the welded part state is in an abnormal fluctuation state. This determination method is as follows.

  In FIG. 6 described above, when the welded part state changes abnormally, the average value Ia of the welding current Iw becomes smaller than the average value of the welding current L1 during normal fluctuation. Accordingly, the average value of the welding current L1 at the time of normal fluctuation or a value obtained by multiplying the average value by the coefficient is set in advance as a reference average value It. Determine.

  Further, in FIG. 7 described above, when the welded portion state is in an abnormal fluctuation state, the average value Ia of the welding current Iw becomes smaller than the average value of the welding current L1 during the normal fluctuation. Further, the maximum value Tm of the weld temperature estimation value Tc is lower than the maximum value of the temperature change target pattern Tp. Accordingly, the average value of the welding current L1 at the time of normal fluctuation or a value obtained by multiplying the average value by the coefficient is set in advance as a reference average value It. Determine. Alternatively, the maximum value of the temperature change target pattern Tp or a value obtained by multiplying the maximum value by the coefficient is set in advance as a reference maximum value Tt. When the maximum value Tm of the weld temperature estimation value Tc is less than the reference maximum value Tt, an abnormal variation state It is determined that it was. That is, when Tm <Tt and / or Ia <It, it can be determined that the welded portion state is an abnormal variation state. When an abnormal variation state occurs, the nugget diameter becomes smaller than the appropriate range as described above, resulting in a failure.

  In step ST3, resistance welding is performed again by temperature pattern tracking current control at the same spot position. By this resistance welding again, the nugget diameter is increased so as to fall within an appropriate range. As described above, even if the welded portion is in an abnormal fluctuation state, it is possible to ensure good welding quality by automatically distinguishing it and performing welding again. The reason why the temperature pattern follow-up current control is performed also during the re-welding is to increase the nugget diameter to be within an appropriate range and suppress the generation of dust even a little.

  FIG. 2 is a block diagram of a welding apparatus for carrying out the resistance welding control method according to Embodiment 1 described above. Hereinafter, each block will be described with reference to FIG.

  The thyristor SCR receives the commercial AC power supply AC as an input and performs phase control so that the effective value of the welding current Iw becomes a predetermined value according to a drive signal Dv described later. The transformer TR steps down to a voltage value suitable for resistance welding. The pair of electrodes 1a and 1b pressurize a plurality of materials to be welded 2, and a welding current Iw is passed through the electrodes and a welding voltage Vw is applied.

  The current detection circuit ID detects the welding current Iw and outputs a current detection signal Id. The voltage detection circuit VD detects the welding voltage Vw and outputs a voltage detection signal Vd. The weld temperature estimated value calculation circuit TC receives the current detection signal Id and the voltage detection signal Vd as input, and calculates the weld temperature by using a predetermined physical constant of the material to be welded and the electrode by heat conduction calculation. An estimated value signal Tc is output. The temperature change target pattern setting circuit TP outputs a predetermined temperature change target pattern signal Tp. The temperature error amplification circuit ET amplifies an error between the temperature change target pattern signal Tp and the welded portion temperature estimated value signal Tc, and outputs a temperature error amplification signal ΔT = Gain · (Tp−Tc). The temperature error integration circuit SDT receives the appropriate initial value Isi and the temperature error amplification signal ΔT, and outputs a current control setting signal Isc = Isi + ∫ΔT · dt. Temperature pattern tracking current control is performed by the temperature error amplification circuit ET and the temperature error integration circuit SDT.

  The current effective value calculation circuit IRM calculates the effective value of the current detection signal Id as an input, and outputs the current effective value signal Irm. The current error amplification circuit EI amplifies an error between the current control setting signal Isc and the current effective value signal Irm and outputs a current error amplification signal Ei. The drive circuit DV outputs a drive signal Dv for controlling the phase of the thyristor SCR according to the current error amplification signal Ei when a later-described activation signal On is at a high level.

  The weld zone temperature estimated maximum value detection circuit TM detects the maximum value of the weld zone temperature estimate value signal Tc and outputs a weld zone temperature estimate maximum value signal Tm. The welding current average value arithmetic circuit IA averages the current effective value signal Irm over one welding and outputs a welding current average value signal Ia. The welding control circuit CC outputs the start signal On for executing each step of FIG. 1 described above, with the above-described weld temperature estimation maximum value signal Tm and the welding current average value signal Ia as inputs. That is, this welding control circuit CC sets the activation signal On to the High level in order to execute step ST1, and performs resistance welding by the first temperature pattern following current control. Next, the discrimination in step ST2 is executed based on the above-mentioned weld zone temperature estimation maximum value signal Tm and welding current average value signal Ia. Then, step ST3 is executed based on the determination result.

[Embodiment 2]
FIG. 3 is a flowchart showing a resistance welding control method according to Embodiment 2 of the present invention. In the figure, the same steps as those in FIG. 1 described above are denoted by the same reference numerals, and description thereof will be omitted. Hereinafter, step ST31 indicated by a dotted line different from FIG. 1 will be described.

  If it is determined in step ST2 that the welded part state is in an abnormal fluctuation state, resistance welding is performed again with a predetermined welding current value in step ST31. In FIG. 1 described above, the resistance welding is performed again by the temperature pattern tracking current control. However, in the same figure, the resistance welding is performed again by stopping the temperature pattern tracking current control and supplying a predetermined constant welding current. Do it. The reason for this is to ensure that a sufficient nugget diameter is formed by energizing a sufficiently large welding current at the time of resistance welding again. This is because even if dust is generated at this time, priority is given to forming an appropriate nugget diameter to ensure strength. In the first place, as described above, since the abnormal fluctuation state occurs and the frequency of performing the welding again is rare, in such a case, the idea is to give priority to forming an appropriate nugget diameter.

  FIG. 4 is a block diagram of a welding apparatus for carrying out the resistance welding control method according to the second embodiment described above. In the figure, the same blocks as those in FIG. 2 described above are denoted by the same reference numerals, and description thereof is omitted. Hereinafter, the second welding control circuit CC2, the current setting circuit IS, and the current setting switching circuit SW indicated by dotted lines different from those in FIG. 2 will be described with reference to FIG.

  The second welding control circuit CC2 receives the welding portion temperature estimated maximum value signal Tm and the welding current average value signal Ia as inputs, and outputs a start signal On and a switching signal Sw for executing each step of FIG. That is, the second welding control circuit CC2 sets the start signal On and the switching signal Sw to High level in order to execute Step ST1, and performs resistance welding by the first temperature pattern tracking current control. Next, the discrimination in step ST2 is executed based on the above-mentioned weld zone temperature estimation maximum value signal Tm and welding current average value signal Ia. Then, in order to execute step ST31 based on the determination result, the activation signal On is set to the high level again, the switching signal Sw is set to the low level, and resistance welding is again performed with a predetermined welding current value determined by the current setting signal Is described later. . The current setting circuit IS outputs a predetermined current setting signal Is. The current setting switching circuit SW switches to the a side when the switching signal Sw is at the high level and outputs the current control setting signal Isc as the current switching setting signal Isw, and when the switching signal Sw is at the low level, the current setting signal Is. Is output as the current switching setting signal Isw. By this circuit, resistance welding is performed by temperature pattern following current control for the first time, and resistance welding is performed for the second time by a predetermined welding current value.

It is a flowchart which shows the resistance welding control method which concerns on Embodiment 1 of this invention. It is a block diagram of the welding apparatus which concerns on Embodiment 1 of this invention. It is a flowchart which shows the resistance welding control method which concerns on Embodiment 2 of this invention. It is a block diagram of the welding apparatus which concerns on Embodiment 2 of this invention. It is a figure which shows the time change of the welding part temperature estimated value Tc and welding current Iw when resistance welding by the temperature pattern tracking current control of a prior art is performed. It is a figure which shows the time change of the welding part temperature estimated value Tc and the welding current Iw in the abnormal fluctuation state for demonstrating the subject of a prior art. It is a figure which shows the time change different from FIG. 6 of the welding part temperature estimated value Tc and the welding current Iw in the abnormal fluctuation state for demonstrating the subject of a prior art.

Explanation of symbols

1a, 1b Electrode 2 Workpiece material CC Welding control circuit CC2 Second welding control circuit DV Drive circuit Dv Drive signal EI Current error amplification circuit Ei Current error amplification signal ET Temperature error amplification circuit IA Welding current average value calculation circuit Ia Welding current average Value (signal)
ID Current detection circuit Id Current detection signal IRM Current effective value calculation circuit Irm Current effective value signal IS Current setting circuit Is Current setting signal Isc Current control setting signal Isi Initial value Isw Current switching setting signal It Reference average value Iw Welding current On Start signal SCR Thyristor SDT Temperature error integration circuit SW Current setting switching circuit Sw Switching signal TC Weld temperature estimation value calculation circuit Tc Weld temperature estimation (signal)
TM Weld temperature estimation maximum value detection circuit Tm Weld temperature estimation maximum value (signal)
TP Temperature change target pattern setting circuit Tp Temperature change target pattern (signal)
TR transformer Tt reference maximum value VD voltage detection circuit Vd voltage detection signal Vw welding voltage ΔT temperature error amplification (value / signal)

Claims (1)

Resistance welding that performs temperature pattern follow-up current control that estimates the temperature of the weld during resistance welding by heat conduction calculation and increases or decreases the welding current value so that the estimated temperature of the weld is approximately in line with a predetermined temperature change target pattern In the control method,
When welding at one spot position is finished, the maximum value of the weld temperature estimated value is less than a predetermined reference maximum value and / or the average value of the energized welding current value is a predetermined reference A resistance welding control method comprising: performing resistance welding again so that the nugget diameter is increased by the temperature pattern tracking current control at a hit point position so that the nugget diameter is within an appropriate range when the average value is less than the average value.

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