RESISTANCE WELDING CONTROL METHOD

Abstract: A resistance welding control system is dis­ closed which comprises a resistance detection unit (5) for detecting an interelectrode resistance by measuring welding current (I) and voltage (V) applied between electrodes; a compensation section (3) for comparing a resistance value as detected with a predetermined model resistance value and determining an amount of compensa­ tion of the welding current in accordance with the difference therebetween; and a current control section (8) for controlling the welding current on the basis of the amount of compensation of the compensation section.

Description: The present invention relates to a resistance welding control method for securing a sound joint condition in the resistance welding process, according to the preamble of claim 1 (see US-A-4 700 042). Document US-A 4 634 828 discloses a control method of resistance welding for controlling welding conditions, such as welding current and welding time, in response to a change in material of weldments. To this end, the local lowest value of resistance between the electrode tips appearing in an initial welding period is compared with a plurality of reference resistance values. These stored reference resistance values are preset correspondingly to materials of weldments of different sorts and divided into plural classes to determine the material of a weldment under process. The varying welding conditions due to various weldments of different materials are detected during the initial welding period and the new welding conditions are provided accordingly after the initial welding period. Furthermore, document US-A 4 700 042 discloses an electrical resistance welding apparatus which comprises two welding electrodes connected through respective supply cables to an electrical power supply which can generate a varying welding current. The quality of the welding electrodes is detected by means of a control device which measures the resistance between the electrodes clamped on parts to be welded and the resistance between the end of the supply cables. A detected change in the resistance between the electrodes with time indicates the gradual wear of the welding electrodes. On the basis of this detection, the electrical power supply is controlled to permanently increase the rms value of the welding current supplied to the electrodes in order to compensate the wear of the electrodes. The wear of the power supply cables is monitored accordingly, and when the possibility of correction has once been exhausted, the control device indicates to the exterior the need to replace the electrodes and/or the supply cables. Generally, a study of the behavior of an interelectrode resistance R i.e. the resistance between two electrodes for securing a sound joint condition in the resistance welding process shows that the interelectrode resistance R assumes a waveform having a peak value (See Fig. 13) about when a nugget (molten part) begins to form at the joint after the temperature thereof reaches a level higher than the melting point of the base metal. It is thus possible to secure a proper nugget area for producing a satisfactory joint condition at a peak value of the interelectrode resistance R which is equal to a maximum resistance value Rmp less than a resistance drop ΔR. In view of this, unlike in the conventional control systems in which the welding conditions including the welding current and energization time are fixed in an open loop control, there have recently been suggested control systems aimed at securing a sound welding quality by stopping the energization at a resistance drop of ΔR from a maximum resistance value Rmp in accordance with the waveform of the above-mentioned interelectrode resistance R. (Japanese Patent Publication JP-A-57-109581). In the aforementioned art of stopping energization with a resistance drop of ΔR from the interelectrode resistance R, the temperature rise of the joint varies due to a change in the surface conditions of the base metal or the electrode conditions so that the time at which the resistance drop value ΔR appears undesirably changes. The result is a variation in energization time. If the value ΔR is reached within a short time, for instance, a welding fault such as a welding spark would be generated, while if it takes a considerable time before ΔR is reached, on the other hand, the recess of the weld zone grows to such an extent that the welding strength would be deteriorated, thus making it impossible to secure a satisfactory joint condition in steady fashion. SUMMARY OF THE INVENTION The object of the present invention, which has been developed in view of the above-mentioned situation, is to provide a resistance welding control method by which a reliable welding quality is obtained by securing a sound joint condition in a stable manner. In order to achieve the above-mentioned object, there is provided according to the present invention a method for controlling a resistance welding apparatus, wherein electrodes are applied to opposite faces of a material to be welded for welding, and after completion of the welding of the material in the present time, the electrodes are applied to opposite faces of another material to be welded in the next time, comprising the following steps: detecting an interelectrode resistance from a voltage and a current between the electrodes at each cycle of the welding period of one time of welding by a resistance detection means, storing a predetermined ideal resistance value by an ideal resistance value storing means, comparing each of the detected interelectrode resistance values with said predetermined ideal resistance value by a compensation means and determining a compensation value, controlling a welding current in accordance with the compensation value, said method being characterized by the following steps: storing a ratio Rml/Rmp by a Rml/Rmp memory means where Rmp is a peak value and Rml is a value a predetermined amount lower than the peak value Rmp based on a waveform of the interelectrode resistance involving a sound joint condition, and stopping the welding current when a ratio Rel/Rep reaches said stored ratio Rml/Rmp, where Rep is a peak value and Rel is a detection resistance value lower than Rep. Alternatively, according to the present invention, the above-mentioned object is achieved by a method for controlling a resistance welding apparatus, wherein electrodes are applied to opposite faces of a material to be welded for welding, and after completion of the welding of the material in the present time, the electrodes are applied to opposite faces of another material to be welded in the next time, comprising the following steps: storing a predetermined ideal resistance value by an ideal resistance value storing means, detecting an interelectrode resistance from a voltage and a current between the electrodes at each cycle of the welding period of one time of welding by a resistance detection means, comparing each of the detected interelectrode resistance values with said predetermined ideal resistance value by a compensation means and determining a compensation value, controlling a welding current in accordance with the compensation value, said method being characterized by the following steps: storing a first proper value corresponding to the ratio Rmc/Rma by a Rmc/Rma memory means, where Rma is an amount from an initial value to a peak value and Rmc is an amount from said peak value to a lower value of the interelectrode resistance at the time of deenergization based on a wave form of the interelectrode resistance associated with a satisfactory joint condition, detecting the initial value of the detection resistance value and the amount Rea to a substantial peak value thereof by a peak detection means, calculating a value corresponding to the ratio Rec/Rea by a Rec/Rea calculation means, where Rec is a difference between the peak value and the resistance value detected for each cycle, and stopping energization by said energization control means when the calculated ratio Rec/Rea reaches the first proper value Rmc/Rma. According to another aspect of the invention, in the case where the detection resistance value is lower than the ideal resistance value, the welding current is preferably increased at a rate lower than the rate at which the welding current would be reduced if the detection resistance value is higher than the ideal resistance value. According to still another aspect of the invention, the detection resistance value for the first cycle is rendered to coincide with a predetermined cycle of an ideal resistance value approximate to the resistance value, after which an amount of compensation of the welding current is determined by the compensation means. According to a further aspect of the invention, when the ratio Rec/Rea between Rec which is a resistance lower by a predetermined value than a peak value and Rea from an initial value to the peak value, reaches a first proper level Rmc/Rma, the welding current is stopped. According to still further aspect of the present invention, when the detection resistance value is decreased by a first predetermined value from a substantial peak value of the detection resistance value, the welding current is reduced by a predetermined amount. The control system of the resistance welding control method according to the present invention feeds back a detection resistance value detected by resistance detection means and compares it with the model or ideal resistance value set in advance. Subsequently, in accordance with the difference between the detection resistance value and the ideal resistance value, a compensation amount of the welding current is determined, and the welding current is controlled by feed back in such a manner that the detection resistance value follows the ideal resistance value. As a result, when the plate thickness or the electrodes undergo a change, the feedback control of the welding current enables the detection resistance value detected by the resistance detection means to approximate to an ideal resistance value, thereby stabilizing and optimizing the temperature increase of the joint. Further, the rate at which the current is increased is kept low, and therefore a sharp temperature increase is prevented, thereby preventing an overheat. Furthermore, an ideal starting point is selected from the resistance value for the first cycle and compared with a proper ideal condition thereby to prevent an increase in heat quantity. As another advantage, the welding current is stopped on the basis of the ratio Rmc/Rma, and therefore the reliability of the joint condition is improved while at the same time preventing the joint condition from being dependent on the error of the base metal or plate thickness. A further advantage is that the welding current is limited when the detection resistance value drops from a peak value by a predetermined amount, so that the resistance is decreased along gentle curve thereby to prevent an overheat. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a general configuration of a resistance welding control system useful for performing the method according to a first embodiment of the present invention. Fig. 2 is a graph showing a behavior of the detection resistance value. Fig. 3 is a sectional view of a joint. Fig. 4 is a graph showing a behavior of the interelectrode resistance. Fig. 5 is a diagram showing the result of an experiment on the joint efficiency with Rel/Rep calculated and energization stopped. Fig. 6 shows the result of an experiment of the joint efficiency with the energization stopped by a resistance drop value ΔR. Fig. 7 is a diagram showing a general configuration of a system useful for performing the method according to the second embodiment of the invention. Fig. 8 is a characteristic diagram showing the condition of the detection resistance value and the welding current. Fig. 9 is a characteristic diagram showing a proper ideal resistance value and a standard resistance value associated with a fault. Fig. 10 is a characteristic diagram showing the condition in which the initial values of the resistance values in Fig. 9 are rendered to coincide with each other. Fig. 11 is a characteristic diagram showing the condition in which the initial values of the resistance value associated with a fault in Fig. 9 is rendered to coincide with M₃. Fig. 12 is a characteristic diagram showing an ideal resistance value. Fig. 13 is a graph showing a behavior of the interelectrode resistance for explaining the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, a method for controlling a resistance welding apparatus according to the present invention will be explained with reference to an embodiment shown in the accompanying drawings. Fig. 1 is a diagram showing a general configuration of a resistance welding control system 1 for controlling the spot welding process, and Fig. 2 a graph showing the behavior of a detection resistance value. The resistance welding control system 1 (hereinafter called "the control system") according to this embodiment comprises a resistance pattern tracking control section 2 for detecting an interelectrode resistance R in welding operation at each energization cycle (1/60 second) and causing a detection resistance value Re to follow a predetermined curve of a model or ideal resistance value Rm (described later), and an energization control section 3 for determining a de-energization time in order to control the energization time. One energization cycle period as selected, for example, 1/60 sec when the welding current is an AC current of 60 Hz. However, the one cycle period is not limited to this value, but may be any selected value. The resistance pattern tracking control section 2 includes resistance detection means 5 for detecting an interelectrode resistance R by measuring a voltage V applied between electrodes 4 and a welding current I, an ideal resistance value memory section or ideal resistance value storing means 6 for storing a predetermined ideal resistance value Rm, a compensation means or compensation control section 7 for determining a compensation amount of the welding current I, and a current control section 8 for controlling the welding current I in accordance with the compensation amount thus determined. The compensation control section 7, on the other hand, includes a difference calculation circuit 9 for calculating the difference between the resistance value Re detected by the resistance detection means 5 and the ideal resistance value Rm for each cycle, a proportional control section 10 for calculating a compensation amount in accordance with the difference determined by the difference calculation circuit 9 and a differential control section 11. The energization control section 3 includes a peak hold section 12 for storing a peak value Rep of the detection resistance value Re, an Rel/Rep calculation circuit 13 for calculating the value Rel/Rep for each cycle based on the peak value Rep and a detection resistance value Rel detected after appearance of the peak value Rep, an Rml/Rmp memory section 14 for storing a first proper value Rml/Rmp (described later) set in advance, and a comparator circuit 15 for comparing the value Rel/Rep determined by the Rel/Rep calculation circuit 13 with the proper value Rml/Rmp and applying a de-energization signal to the current control section 8 for de-energizing the operation when Rel/Rep reaches Rml/Rmp. The ideal resistance value Rm will be explained below with reference to Figs. 3 and 4. The interelectrode resistance R under a constant current supplied between the electrodes 4 is expressed as R = ρ·L/Sn [µΩ] where ρ·L/Sn [µΩcm] is a specific resistance of a material to be welded or a base metal 16, Sn[cm²] a nugget area, and L[cm] the thickness of the weld zone. In the case where the constant current is supplied between the electrodes 4, therefore, the temperature ϑ of a joint 17 increases with time, and the specific resistance ρ increases accordingly thereby to increase the interelectrode resistance R. With a further increase in the temperature ϑ of the joint 17, assume that the base metal 16 reaches a temperature higher than a melting point. A nugget 18 is formed on the joint 17, and about that time, the interelectrode resistance R indicates a peak value Rmp as shown in Fig. 4. With the growth of the nugget 18 with time, the nugget area Sn increases and the interelectrode resistance R gradually decreases from the peak value Rmp. By following the processes mentioned above, the behavior of the interelectrode resistance R takes a waveform as shown in Fig. 4 with the temperature increase of the joint 17. It would thus be possible to produce a sound joint condition in stable manner if this waveform is realized in actual resistance welding operation. In view of this, according to the present embodiment, the initial rise portion from a base interelectrode resistance A at the first cycle to the peak value Rmp is used as a model pattern or ideal pattern, which is determined based on the behavior of the interelectrode resistance R producing a sound joint condition shown in Fig. 4. Such an interelectrode resistance value is set at every cycle of the ideal pattern as an ideal resistance value Rm. Fig. 3 is a sectional view of the joint 17, and Fig. 4 a graph showing the behavior of the interelectrode resistance R with time. Now, a proper value Rml/Rmp set in advance for determining the time of de-enerigization will be explained. In the case where the energization is stopped when the interelectrode resistance R decreases by a value ΔR from the maximum resistance value Rmp the interelectrode resistance value is extremely low or high at the first cycle, for example, due to the effect of the surface condition of the base metal 16, the heat input may become excessively large or small, resulting in generating a welding spark or causing a shortage of strength. Specifically, external disturbances such as the surface conditions of the base metal 16 and the electrode conditions gives an effect on the rising characteristics of the interelectrode resistance R (variation of the interelectrode resistance R from its value detected at the first cycle of energization to a peak value) so that the above-mentioned welding failure may be caused if the energization is always interrupted with a predetermined resistance drop of ΔR. In order to obviate this problem, according to the present embodiment, a proper value of Rml/Rmp is set in advance on the basis of a waveform of the interelectrode resistance R involving a sound joint condition, and this value Rml/Rmp is stored in a memory 14, where Rmp designates a peak value of the interelectrode resistance R and Rml an interelectrode resistance at the time of de-energization. The peak value Rep of the interelectrode resistance R during welding is measured. Using the peak value Rep and a resistance value Rel detected subsequently to appearance of the peak value Rep, the Rel/Rep value is calculated at every cycle, and when the value of Rel/Rep reaches Rml/Rmp, energization is stopped. The resistance value measured at this time point is designated as Rel. The de-energization is determined based on the calculation of the value of Rel/Rep this way, and therefore, in the case where the heat input is excessively large as mentioned above, the energization is stopped earlier (with a small ΔR) and in the case where the heat input is excessively small, the energization is stopped later (with a larger ΔR) than the case where the energization is stopped when the resistance drop ΔR reaches a predetermined value. As a result, when the time of de-energization is determined by calculating Rel/Rep, the reliability of the joint condition is improved as compared with the case where the energization is stopped at a predetermined value of the resistance drop value ΔR. Figs. 5 and 6 show results of experiments conducted in which the tensile shearing strength at the welded part is represented by the joint efficiency. A result of experiment in the case where the de-energization is determined based on the calculated Rel/Rep is shown in Fig. 5, where the abscissa represents (Rep - Rel)/Rep x 100 , and that in the case where the de-energization is determined based on the detected value of the resistance drop value ΔR is shown in Fig. 6. Also, the joint efficiency a is determined from a formula α = (measured value/maximum value of the tensile shearing strength in each experiment) x 100% . These results of experiments show that the variations in joint efficiency α are smaller with a stabler effect when the de-energization is determined based on calculation of Rel/Rep. In view of the fact that a welding spark is undesirably generated if the joint efficiency α is increased as shown by black circle and triangle as in Figs. 5 and 6, and also taking into consideration that the joint efficiency α is to be set to more than 90% according to the present embodiment, the value (Rep - Rel)/Rep x 100 is set to 63%. Now, the operation of the control system 1 will be explained. First, an interelectrode resistance R is detected by measuring the welding current I and the voltage V applied between the electrodes 4 by resistance detection means 5. The difference between the detection resistance value Re and a predetermined ideal resistance value Rm stored in the ideal resistance value memory 6 is calculated at each cycle. Subsequently, a compensation amount is calculated by the proportional control section 10 and the differential control section 11 in accordance with the difference determined by the difference calculation circuit 9. The current control section 8 adjusts the welding current I on the basis of the compensation amount, thus optimizing the temperature increase of the joint 17. The initial behavior of the interelectrode resistance R under welding operation is thus caused to follow the predetermined ideal or model pattern. After that, in order to secure a proper nugget area, the energization control section 3 is operated to stop the energization appropriately. First, the peak value Rep of the detection resistance value Re is stored by a peak hold section 12, after which the Rel/Rep calculation circuit 13 determines the value of Rel/Rep from the peak value Rep and a resistance value Rel detected subsequently to the detection of the peak value Rep. Further, the value Rel/Rep thus calculated is compared with the value Rml/Rmp stored in the Rml/Rmp memory 14 at the comparator circuit 15, and when Rel/Rep reaches Rml/Rmp, a de-energization signal for stopping energization is applied to the current control section 8. The current control section 8 receives this de-energization signal to stop the energization, thereby completing the control process. The aforementioned control operation stabilizes the temperature rise of the joint 17 and secure the proper nugget area, thus providing a sound welding quality in a stable manner. Now, a second embodiment of the control system according to the present invention is shown in Fig. 7. According to this second embodiment, first, preheat means 20 supplies current of 1000 to 1500 (A) during 5 to 10 cycles as shown in Fig. 8 in the case where the thickness of the base metal 16 is 1 mm. As a result, a portion between the base metals 16 is heated for stable contact resistance. After that, the interelectrode resistance R is detected by the resistance detection means 5. In the process, in the case of Fig. 8, a cooling time is provided to stop the energization during the period from preheating to the resistance detection by the resistance detection means 5. This cooling time is provided for preventing the base metals 16 which may be circular in section with a small contact area therebetween from being increased too much in temperature by an excessive current flow during the preheating. After this, an ideal starting point is determined by initial cycle determining means 21. The reason why this ideal starting point is determined will be explained with reference to Figs. 9 to 11. According to the first embodiment mentioned above, in order to stabilize the temperature rise of the weld zone, the initial resistance pattern between the electrodes 4 for proper welding conditions is controlled to follow an ideal pattern M₁ to M₇ by current control, as shown in Fig. 9. In the case where the deformation and surface condition of the base metal 16 or the pressure tracking ability of the electrodes 4 undergo a considerable change, however, the interelectrode resistance R at the first cycle may be increased as shown by D₁ in Fig. 9. The pattern D₁ to D₇ is shown as a typical standard obtained when a predetermined current is supplied without any current control unlike in the first embodiment. If the current is controlled as in the first embodiment, by contrast, it causes the interelectrode resistance R to follow the ideal pattern M₁ to M₇ with its starting point M₁ coinciding with D₁ as shown in Fig. 10, resulting in a substantial difference between the pattern D₁ to D₇ and the ideal pattern M₁ to M₇, which in turn causes the current to be increased considerably. With the increase in current, the heat quantity is also increased and the resultant overheating (or a welding spark) would reduce the strength. In order to prevent this inconvenience, the resistance D₁ measured at the first cycle is compared with the absolute or ideal value M₁ of the predetermined model of ideal resistance value at the first cycle, and select an ideal starting point in a manner mentioned below. Ideal starting point = α + 1 where α = {(Measurement D₁/ideal value M₁) - 1} x 10 (decimals omitted). If D₁ is 100 µΩ and M₁ is 80 µΩ, for example, the ideal starting point is set to the third cycle. Specifically, as shown in Fig. 11, the resistance value D₁ measured at the first cycle is rendered to coincide with the ideal resistance value M₃ at the third cycle. As a result, the control is carried out so that the measured resistance values D₁ to D₅ substantially coincide with the ideal values M₃ to M₇ of the model or ideal pattern, thereby eliminating the problems of overheating, etc. After determining an ideal starting point, the difference between the detection resistance value and the ideal resistance value is calculated by a difference calculation circuit 9 as in the first embodiment. As the ideal resistance value, an upper limit ideal value Mh and a lower limit ideal value Mℓ and set with ±5% allowances from an optimum ideal value M. (See Fig. 12) In the case where a detection resistance value is higher than the upper limit ideal value Mh, the welding current I is reduced according to Table 1 below. R D Current reduction 1 to 10 (µΩ) 5.0% down 11 to 20 (µΩ) 7.5% down 21 to 40 (µΩ) 10.0% down 41 (µΩ) or more 12.5% down When a detection resistance value is lower than the lower limit ideal value Mℓ, on the other hand, the welding current I is increased according to Table 2 below. R D Current reduction 1 to 10 (µΩ) 2.5% down 11 to 20 (µΩ) 5.0% down 21 to 40 (µΩ) 7.5% down Current is controlled in different amounts in Tables 1 and 2 above by reason of the fact that an overheat (or welding spatter) forms a serious welding defect in the resistance welding process. The amount of current increase, therefore, is limited in such a manner as to prevent a sharp temperature increase, while the amount of current reduction is increased due to a low thermal response of the weld zone thereby to prevent an overheated condition (or welding spatter). According to the present method of control, the energization time tends to lengthen. Nevertheless, this poses no problem as experiments show that a satisfactory result is obtained if the energization time is 10 to 25 cycles for the metal thickness of about 1.2 mm. The welding current I is controlled in such a way that the detection resistance takes a value between the upper limit ideal value Mh and the lower limit ideal value Mℓ. The energization control section 3 will be explained. According to the second embodiment under consideration, the peak hold section 12 detects a peak value Rep of the detection resistance value, followed by the difference Rea between the value Rep and the detection resistance value Res at the first cycle shown in Fig. 8. The Rec/Rea calculation circuit 24 fetches the value Rec which is the peak value Rep less a predetermined value at each cycle thereby to calculate the value Rec/Rea at each cycle. The Rmc/Rma memory section 25, on the other hand, stores a first proper value Rmc/Rma according to an ideal pattern shown in Fig. 2, where Rma is the difference between the initial value Rms and the peak value Rmp, and Rmc the difference between the peak value Rmp and the interelectrode resistance value Rml with energization stopped. When the value Rec/Rea calculated at each cycle reaches the first proper value Rmc/Rma, the energization is stopped. Specifically, unlike the first embodiment in which the energization is stopped by measuring an absolute value (peak value Rep and the value Rel for deenergization), the second embodiment is such that relative values (differences Rea and Rec) are calculated to stop energization, so that even if the thickness of the base metal 16 undergoes a small variation leading to a changing detection resistance value, the energization is always accurately stopped, thereby producing a satisfactory joint condition. According to the present embodiment, the resistance value is detected at each cycle, and therefore, even if energization is stopped when the calculated value Rec/Rea reaches the first proper value Rmc/Rma, a sharp power reduction (in two to four cycles) may form excessive nuggets before complete de-energization, often causing an overheat (or welding spark) due to the surface conditions of the base metal 16 or the pressure tracking ability or other disturbances. In order to obviate this problem, according to this embodiment, a predetermined second proper value Rmb/Rma is stored in the memory section 26, and when the value Rec/Rea reaches the second proper value Rmb/Rma as shown in Fig. 8, the welding current I is limited by a predetermined amount to realize a moderate resistance drop as shown by solid line. As a consequence, a sharp resistance drop is prevented. It has also been confirmed by our experiments that the earlier α time T R appears when the resistance value is maximum in Fig. 8, the more sharply the resistance drops (that is, the more an overheated condition tends to be caused). To meet this situation, the number of energization cycles is detected by an energization cycle detector 22 in response to a signal from the peak hold section 12, and in accordance with the number of cycles before detection of a peak value Rep, the welding current I is controlled in the manner shown in Table 3 below. Number of energization cycles Welding current 2 to 4 15% down 5 to 8 10% down 9 or more cycles 5% down According to the second embodiment, the energization cycle detector 22 detects the number of cycles, and if this number is more than a predetermined number (say, 30 cycles) before an energization stop, a fault is indicated. This is due to the fact that with the increase in energization cycles, the electrodes 4 would be embedded in the base metals 16 under pressure, with the result that the strength between the base metals 16 is decreased. In the above-mentioned embodiments, energization is stopped when the ratio Rel/Rep calculated reaches a predetermined value. Unless the behavior of the interelectrode resistance R in the welding operation deviates extremely from a model or ideal pattern, however, the energization may be stopped with the resistance drop ΔR as in the prior art with the same effect. Also, according to the embodiments mentioned above, the welding current I is limited when the calculated ratio Rec/Rea reaches the second proper value Rmb/Rma. Instead, the welding current I may be reduced by a predetermined value of resistance drop. As an alternative way, an energization time from energization start to energization stop or the time length from a peak value Rep to an energization stop may be set. As another alternative, a displacement value of the electrodes 4 which is said to have a correlationship with the nugget formation, the value of change in transmission of ultrasonic wave or the value of change in acoustic emission may be set appropriately in advance, and the energization may be stopped in accordance with the set value. Instead of setting the ideal starting point by the initial cycle determining means 21 as in the aforementioned second embodiment, several ideal or model patterns may be stored out of which an ideal pattern most proximate to the measured resistance value for the first cycle may be selected. According to the embodiment under consideration, the current control section 8 employs a thyristor control by an AC waveform. It is, however, of course possible to realize the same effect by a DC waveform using a DC power supply or by transistor control. In such a case, a predetermined time sufficiently short to assure the controllability is secured as a cycle of interelectrode resistance detection. Unlike in the present embodiment using the resistance welding control system 1 for controlling the spot welding process, it may be applied with equal effect to the projection welding, butt welding or other resistance welding processes as well.

Claims: A method for controlling a resistance welding apparatus, wherein electrodes (4) are applied to opposite faces of a material to be welded for welding, and after completion of the welding of the material in the present time, the electrodes (4) are applied to opposite faces of another material to be welded in the next time, comprising the following steps: storing a predetermined ideal resistance value by an ideal resistance value storing means (6), detecting an interelectrode resistance from a voltage and a current between the electrodes (4) at each cycle of the welding period of one time of welding by a resistance detection means (5), comparing each of the detected interelectrode resistance values with said predetermined ideal resistance value by a compensation means (7) and determining a compensation value, controlling a welding current (I) in accordance with the compensation value, said method being characterized by the following steps: storing a ratio Rml/Rmp by a Rml/Rmp memory means (14) where Rmp is a peak value and Rml is a value of the interelectrode resistance (R) at the time of deenergization a predetermined amount lower than the peak value Rmp based on a waveform of the interelectrode resistance (R) involving a sound joint condition, and stopping the welding current (I) when a ratio Rel/Rep reaches said stored ratio Rml/Rmp, where Rep is a peak value and Rel is a detection resistance value lower than Rep. A method according to claim 1, being characterized in that said compensation means (7) operates such that a first rate, at which the welding current (I) is reduced when the detection resistance value is higher than the ideal resistance value, is higher than a second rate at which the welding current (I) is increased when the detection resistance value is lower than the ideal resistance value. A method according to claim 1, being characterized by comprising the step of detecting the substantial peak value Rep of the detection resistance value by a peak hold means (12), being included in an energization control means (3). A method according to claim 3, being characterized in that the ratio Rml/Rmp is stored in said Rml/Rmp memory means (14) which is included in said energization control means (3). A method according to claim 1, being characterized by comprising the step of using an alternating current whose direction of current is reversed for predetermined cycles in one second. A method according to claim 1, being characterized by comprising the step of preheating the base metals (16) by a preheat means (20), wherein a predetermined current is supplied between the electrodes (4) to heat the base metals (16) and stabilize the interelectrode resistance (R) before detecting the interelectrode resistance (R). A method according to claim 1, being characterized by comprising the step of changing the ideal resistance value of the compensation means (7) by a means (2) in response to the magnitude of the interelectrode resistance (R) detected in a first cycle by the resistance detection means (5). A method for controlling a resistance welding apparatus, wherein electrodes (4) are applied to opposite faces of a material to be welded for welding, and after completion of the welding of the material in the present time, the electrodes (4) are applied to opposite faces of another material to be welded in the next time, comprising the following steps: storing a predetermined ideal resistance value by an ideal resistance value storing means (6), detecting an interelectrode resistance from a voltage and a current between the electrodes (4) at each cycle of the welding period of one time of welding by a resistance detection means (5), comparing each of the detected interelectrode resistance values with said predetermined ideal resistance value by a compensation means (7) and determining a compensation value, controlling a welding current (I) in accordance with the compensation value, said method being characterized by the following steps: storing a first proper value corresponding to the ratio Rmc/Rma by a Rmc/Rma memory means (25), where Rma is an amount from an initial value to a peak value and Rmc is an amount from said peak value to a lower value of the interelectrode resistance (R) at the time of deenergization based on a wave form of the interelectrode resistance (R) associated with a satisfactory joint condition, detecting the initial value of the detection resistance value and the amount Rea to a substantial peak value thereof by a peak detection means (12), calculating a value corresponding to the ratio Rec/Rea by a Rec/Rea calculation means (24), where Rec is a difference between the peak value and the resistance value detected for each cycle, and stopping energization by said energization control means (3) when the calculated ratio Rec/Rea reaches the first proper value Rmc/Rma. A method according to claim 5, being characterized in that said energization control means (3) reduces the welding current (I) by a predetermined amount when the detection resistance value decreases by a first predetermined amount from the substantial peak value of the detection resistance value and stops the supply of the welding current (I) when the detection resistance value decreases by a second predetermined amount greater than the first predetermined amount. A method according to claim 9, being characterized in that a number-of-cycles detection means (22) is provided for detecting the number of cycles before the detection resistance value reaches a peak value, the welding current (I) being reduced in an amount progressively greater with the decrease in number of cycles. A method according to claim 5, being characterized in that an alarm is issued when the number of cycles from the first cycle to a cycle associated with the energization stop is larger than a predetermined value. A method according to claim 5, being characterized in that said ideal resistance value includes an ideal upper limit (Mh) and an ideal lower limit (Mℓ) with a predetermined margin from a proper ideal resistance value therebetween, said compensation means (7) being such that the rate at which the welding current (I) is reduced when the detection resistance value is higher than said ideal upper limit (Mh) is higher than the rate at which the welding current (I) is increased when said detection resistance value is lower than said ideal lower limit (Mℓ), said rates being progressively greater with the increase in the difference between the detection resistance value and a selected one of the ideal lower limit (Mℓ) and the ideal upper limit (Mh). A method according to claim 6, being characterized in that a means is provided for cooling for a predetermined period after the heating of said preheat means (20). A method according to claim 7, being characterized in that said means (2) includes means for detecting a cycle in which an ideal resistance value was changed approximate to the resistance value detected in the first cycle by the resistance detection means (5) and for changing the resistance value detected in the first cycle to be substantially in consistance with the ideal resistance value in the detected cycle.

Inventors: ITO, SHINZO, SUGIYAMA, TSUYOSHI, ZYOKOU, KEIZOU
Applicants: NIPPONDENSO CO., LTD.
Application year: 1989
Main classification: B23K11
IPC codes: B23K11