Control apparatus for resistance welding machine

Abstract: A control apparatus for a resistance welding machine comprises a welding current detecting device (5; 205) for detecting a welding current, interelectrode voltage detecting device (6; 206) for detecting an interelectrode voltage, a calculating section (13; 213) for predicting a nugget growth or estimating an expulsion & surface-flash generation probability, and control section (16, 17, 18; 212, 215) for controlling the welding current so that the nugget growth or the expulsion & surface-flash generation probability can be adjusted to a predetermined value.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a control apparatus for a resistance welding machine, especially to a control apparatus for a spot welding machine. 2. Prior Art In a resistance welding machine used for la welding of steel plates or the like, three principal parameters to stabilize the welding quality are a welding current, an energizing time, and an electrode pressurizing force. It is generally known that these parameters are feedback controlled instead of dial setting recommendable conditions or each base metal based on expertise and experiences. Enhancement of the welding quality has been recently request severely. To satisfy such a requirement, Unexamined Japanese Patent Application No. 4-178275, published in 1992, discloses a technology for controlling a welding operation by directly monitoring a nugget, since the nugget growth reflects the result of the welding operation. Hereinafter, the control apparatus of a conventional resistance welding machine disclosed in the Unexamined Japanese Patent Application No. 4-178275 will be explained with reference to the accompanied drawing. In Fig. 1, reference numeral 21 represents a power source unit for a resistance welding machine, and reference numeral 22 represents a controller for the welding power source unit 21. Reference numeral 23 represents a welding current detecting device. Reference numeral 24 represents a secondary conductor. Reference numeral 25 represents a lower arm. Reference numeral 26 represents a base metal to be welded. Reference numeral 27 represents a pair of electrodes. Reference numeral 28 represents a pressurizing cylinder. Reference numeral 29 represents an upper arm. Reference numeral 30 represents an electropneumatic proportional valve. Reference numeral 31 represents a pressure sensor. Reference numeral 32 represents a controller for the electropneumatic proportional valve 30. Reference numeral 33 represents a cable which detects an interelectrode voltage (i.e. a voltage between electrodes 27). Reference numeral 34 represents a distance detector which detects a shift amount of electrodes 27. Reference numeral 35 represents a signal processing section which acts as a hardware circuit for processing signals representing the electrode pressurizing force, the electrode shift amount, the interelectrode voltage and the welding current. Reference numeral 36 represents an estimating section which estimates a nugget size and a heat input density. And, reference numeral 37 represents a control signal generating section which generates control signals for the welding current and the electrode pressurizing force. The above-described control apparatus of the conventional resistance welding machine will be explained with reference to a flow chart shown in Fig.2. First, the plate thickness of base metal 26, the number of metal plates to be lapped or stacked, and the material information of base metal 26 are respectively entered (step 101). Next, a welding operation is started (step 102). Prior to an energizing operation, electrodes 27 are pressurized to confirm the total thickness of metal plates. Then, a relationship between the actual pressurizing force and the shift amount of electrodes 27 is measured. Based on the measuring result, an electrode pressurizing force value is set to an appropriate value to make the metal plates fit to each other sufficiently (step 103). Referring to the lapping number of metal plates, a standard heat input density pattern and a standard energizing diameter increase pattern are selected respectively (step 104). The standard heat input density pattern and the standard energizing diameter increase pattern are both determined in advance in accordance with numerical calculations and experiments. Selection of these two standard patterns is essential for realizing an effective control of the variation of the heat input density and the increase of the energizing diameter during the actual welding operation. More specifically, both the heat input density and the energizing diameter are controlled to be equalized to the values designated by the selected standard patterns. Next, the energizing operation is started (step 105). Then, a momentary change of a nugget diameter is monitored through a simulation of numerical calculations while performing a heat input density control later described (step 112). When the estimated nugget diameter becomes larger than a required nugget diameter (step 113), the energizing operation is terminated (step 114). Through this operation, it becomes possible to realize an excellent welding portion with reliability. Next, the simulation of numerical calculations for obtaining the nugget diameter and the heat input density control will be explained. An interelectrode voltage and a welding current value are detected during the welding operation after starting the energizing operation (step 106). The detected values of the interelectrode voltage and the welding current are substituted into the following equation (1) to calculate the energizing diameter (step 107). dc= 4 π · ρm· A· ∑hi Rtip - R0 where "dc" is a representative energizing diameter of a welding portion, "ρm" is a mean specific resistance of the welding portion, "A" is a correction coefficient relating to current spread, "Σhi" is a total plate thickness, "Rtip" is an interelectrode resistance (=Vtip/I, when "Vtip" is an interelectrode voltage and "I" is a welding current), and "R0" is an electrode resistance. In the above-described equation (1), the mean specific resistance "ρm" of the welding portion is determined based on a mean temperature in the welding portion. When the energizing operation is started (t=0), the mean specific resistance "pm" is equal to a specific resistance "ρm0" at the room temperature. Temperature change during a tiny time interval Δt can be regarded as negligible. Next, the energizing diameter thus calculated and the detected welding current value are substituted into the following equation (2) to estimate a temperature distribution to be obtained after a time elapse of Δt. c σ ∂ T ∂ t = ∂ ∂ x (K ∂ T ∂ x )+ρ δ 2 where "c" is a specific heat, "σ" is a density, "K" is a heat conductivity, "δ" is a current density (≡AJ/(π·dc 2 /4)), "T" is a temperature, "t" is a time, "x" is a distance in the direction of plate thickness, and "∂" is a partial derivative symbol. From this temperature distribution, "ρm1" is obtained as a mean specific resistance to be obtained after a time elapse of Δt. By substituting "ρm1" into the equation (1), an energizing diameter to be obtained after a time elapse of Δt can be calculated. In this manner, momentary values of the energizing diameter, temperature distribution, and heat input density are successively obtained during a duration from start of the energizing operation to a certain time (step 110). Furthermore, a correct nugget diameter can be estimated by taking into account a heating start-up delay time at each radial position. Then, the welding current and the electrode pressurizing force are controlled to equalize the energizing diameter and heat input density obtained here to the preferable standard heat input pattern and preferable standard energizing diameter increase pattern selected in step 104 (step 111). A detection of an electrode shift amount (step 108) is necessary to confirm and correct (step 109) an error of the energizing diameter calculated in step 107, which is likely to be caused in the initial stage of the energizing operation. The detected electrode shift amount is substituted into the following equation (3) to calculate a mean temperature of the welding portion. Then, the mean temperature thus obtained is used to correct the mean temperature distribution at present time which is previously obtained. Tm = B· Δ 1 3· αm· ∑hi where "Tm" is a mean temperature of the welding portion, "Δ1" is an electrode shift amount, "am" is a mean value of linear expansion coefficient, and "B" is a proportional constant. However, according to the above-described conventional arrangement, it is necessary to successively identify the mean temperature and the energizing diameter of the welding portion during the welding operation, and it is complicated to determine the standard energizing diameter increase pattern and the standard heat input density pattern to compare them with the identified energizing diameter and the heat input density obtained from this energizing diameter. Furthermore, it is necessary to perform an adaptive control in real time for adjusting the welding current and the electrode pressurizing force based on the comparison result of the energizing diameter and the heat input density in connection with the standard energizing diameter increase pattern and the standard heat input density pattern. Therefore, the control apparatus becomes complicated and expensive. Furthermore, in the resistance welding machine used for lap welding of steel plates or the like, increasing a heat input amount to a welding portion is important to increase the welding strength. The factors determining the heat input amount are a welding current, an energized portion resistance, and an energizing time. Especially, when the number of welding points is increased, the tip of an electrode is worn and deformed. This leads to deterioration of the welding strength. Thus, it becomes necessary to further increase the heat input amount to the welding portion. However, increasing the heat input amount provokes generation of expulsion & surface-flash. It is well known that, when the expulsion & surface-flash is once generated, the welding strength is extremely reduced and the appearance or configuration of the welding portion is terribly worsened. From the enhancement of the welding quality, the recent development of the welding technology makes it possible to obtain a sufficient size of nugget while suppressing the generation of the expulsion & surface-flash at a minimum level. For example, it is effective to intentionally reduce the welding current during a start-up period of a welding operation and also during the second-half period of the welding operation, because the expulsion & surface-flash tends to be caused in these specific durations. Meanwhile, it is allowed to increase the welding current largely during an intermediate period of the welding operation, because the expulsion & surface-flash seldom occurs in this intermediate duration. Patent Abstracts of Japan of unexamined Japanese Patent Application JP-A-05 337 657, which represents the closest prior art, discloses a resistance welding controller which detects at least one physical quantity on a part to be welded and carries out numerical analysis of the energizing diameter and the nugget diameter by using that physical quality. That result is compared to a model waveform and at least one of the welding current, the welding time and the welding force is varied depending on the result of the comparison and the numerical analysis. In view of the foregoing, a pattern welding current control and a constant-power welding method are already proposed, as disclosed in Unexamined Japanese Patent Application No. 63-180304, published in 1988. Furthermore, there is a method of stopping the energizing operation as soon as the generation of the expulsion & surface-flash is detected by the sudden change of a pressurizing force or a welding current as disclosed in Unexamined Japanese Patent Application No. 1-241385, published in 1989. Moreover, for a plurality of welding points which are continuously welded, there is a method of varying a welding current for the next welding point when an expulsion & surface-flash is detected during a welding operation for a certain welding point, as disclosed in Unexamined Japanese Patent Application No. 5-337655, published in 1993. However, according to the above-described conventional technologies, the following problems arise. According to the pattern welding current control or the constant-power welding method, the generation rate of the expulsion & surface-flash can be reduced to a certain degree compared with the constant-current welding method. However, it is inherently impossible to obtain a nugget having a maximum strength without causing any expulsion & surface-flash. Meanwhile, according to the method of stopping the energizing operation immediately upon detecting the generation of expulsion & surface-flash, it necessarily permits to generate some of expulsion & surface-flash. Furthermore, according to the method of varying the welding current for the next welding point when any generation of the expulsion & surface-flash is detected in a present welding operation, it is impossible to prevent the generation of the expulsion & surface-flash during the present welding operation. In short, there was no method capable of perfectly preventing the generation of the expulsion & surface-flash. SUMMARY OF THE INVENTION Accordingly, in view of the above-described problems encountered in the prior art, a principal object of the present invention is to provide a simple and cheap control apparatus for a resistance welding machine capable of surely obtaining a stable and preferable nugget. To accomplish this and related objects, the present invention provides a control apparatus for a resistance welding machine, comprising: welding current detecting means for detecting a welding current; interelectrode voltage detecting means for detecting an interelectrode voltage; calculating means for estimating an expulsion and surface-flash generation probability by estimating the energy distribution based on the welding current and the inter-electrode voltage and comparing the estimated energy distribution to preset expulsion and surface-flash generation probability data; and control means for controlling said welding current so that said expulsion and surface-flash generation probability can be adjusted to a predetermined value. Furthermore, the present invention has an object to provide a control apparatus for a resistance welding machine which precisely controls the heat input amount to prevent the heat input amount from exceeding a critical point in view of the generation of expulsion & surface-flash. In a preferred embodiment, the present invention provides a control apparatus for a resistance welding machine, comprising welding current detecting means for detecting a welding current, interelectrode voltage detecting means for detecting an interelectrode voltage, energy distribution estimating means for estimating an energy distribution in an energized portion based on time variations of the welding current and the interelectrode voltage detected by the welding current detecting means and the interelectrode voltage detecting means, and expulsion & surface-flash generation probability estimating means for estimating an expulsion & surface-flash generation probability based on the energy distribution obtained by the energy distribution estimating means. According to this arrangement, the energy distribution estimating means estimates an energy distribution in an energized portion based on time variations of the welding current and the interelectrode voltage detected by the welding current detecting means and the interelectrode voltage detecting means. The expulsion & surface-flash generation probability estimating means estimates an expulsion & surface-flash generation probability based on the energy distribution obtained by the energy distribution estimating means. According to preferred embodiment of the present invention, in addition to the above-described arrangement, there is heat input control means for controlling a welding heat input amount so as to maintain the expulsion & surface-flash generation probability at a predetermined value. With this arrangement, the heat input amount can be increased to an allowable upper limit while preventing the generating any expulsion & surface-flash. Alternatively, it is preferable that the expulsion & surface-flash generation probability estimating means comprises expulsion & surface-flash generation time predicting means for predicting an expulsion & surface-flash generation time. And, the heat input control means controls a welding heat input amount so that the expulsion & surface-flash generation time is in a predetermined period of time after finishing a welding operation. With this arrangement, the heat input amount can be increased to an allowable upper limit while preventing the generating any expulsion & surface-flash. Moreover, it is preferable that the heat input control means comprises optimum welding current estimating means for generating a control signal representing an optimum welding current value and welding current control means for controlling the welding current in accordance with the control signal produced from the optimum welding current estimating means. Still further, it is preferable that there is display means for displaying at least one of the expulsion & surface-flash generation probability estimated by the expulsion & surface-flash generation probability estimating means, the expulsion & surface-flash generation time predicted by the expulsion & surface-flash generation time predicting means, and a processing result of the data relating to expulsion & surface-flash generation probability and the expulsion & surface-flash generation time. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description which is to be read in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing an arrangement of a conventional control apparatus for a resistance welding machine; Fig.2 is a flow chart showing an operation of the conventional control apparatus for a resistance welding machine; Fig.3 is a block diagram showing an arrangement of a control apparatus for a resistance welding machine in accordance with an embodiment of the present invention; Fig.4 is a flow chart showing an operation of the control apparatus of the embodiment of the present invention; Fig. 5 is a flow chart showing a procedure for estimating an energy distribution, performed by the control apparatus of the embodiment of the present invention; Fig.6A is a view illustrating micro sections dissected along a cross section of a welding portion in accordance with the embodiment of the present invention; Fig. 6B is a view illustrating an annular 3-D section in accordance with the embodiment of the present invention; Fig. 7 is a graph showing a time variation of the expulsion & surface-flash generation probability in accordance with a conventional welding operation; and Fig.8 is a graph showing a time variation of the expulsion & surface-flash generation probability in accordance with the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be explained in greater detail hereinafter with reference to the accompanying drawings. Identical parts are denoted by the same reference numerals throughout the views. In Fig.3, reference numeral 201 represents base metals to be welded, and reference numeral 202 represents a pair of welding electrodes which holds or clamps the base metals 201 and is pressurized by an appropriate pressurizing mechanism (not-shown) to supply welding current to the welding portion. Reference numeral 203 represents a welding power source unit which generates the welding current. Reference numeral 204 represents a secondary conductor connecting the welding power source unit 203 and electrodes 202. Reference numeral 205 represents a welding current detecting device which is, for example, constituted by a toroidal coil or a current shunt and a detecting wire. Reference numeral 206 represents an interelectrode voltage detecting device which may comprise a detecting wire connected to a predetermined position of the electrode 202 or an electrode holder (not shown). Reference numeral 207 represents a control apparatus. Control apparatus 207 comprises the following components. Reference numeral 208 represents a signal conversion section which converts the output signals of the welding current detecting device 205 and the interelectrode voltage detecting device 206 into data signals processible in the control apparatus 207. Reference numeral 209 represents an input key. A calculating section 213 consists of an energy distribution estimating section 210, an expulsion & surface-flash generation probability estimating section 211 and an optimum welding current estimating section 212. Reference numeral 214 represents an expulsion & surface-flash generation probability table obtained based on the experiments conducted by the inventors. Reference numeral 215 represents a welding current control section. And, reference numeral 216 represents a display section 216. The optimum welding current estimating section 212 and the welding current control section 215 cooperatively constitute heat input control section. Furthermore, the control apparatus 207 comprises other components, such as a timer used for setting a welding timer and controlling the welding operation, a pattern generating section for executing a pattern control for the welding current, and a pressurizing force control section. These component are not directly related to the present invention but conventionally well-known and, therefore, are not shown in the drawing and not explained in the following description. An operation of the above-described control apparatus for a resistance welding machine in accordance with the embodiment will be explained with reference to Fig. 4. First, information (e.g. material, plate thickness, lapping number, etc) relating to base metal 201 given by a design package or specification are entered through input key 209. Furthermore, information (material, tip configuration etc.) relating to the used welding electrodes 202 are entered through input key 209, too. Subsequently, welding conditions (welding current, pressurizing force, welding time. etc.) are entered (Steps 21 and 22). After finishing the above-described preparation, base metals 201 are sandwiched or clamped between a pair of upper and lower electrodes 202 to start the welding operation. A predetermined pressurizing force is applied between electrodes 202. Then, a welding current is supplied. The welding current value is determined in accordance with the welding conditions previously set (step 23). After starting the energizing operation, the welding current detecting device 205 detects a momentary value of the welding current and the interelectrode voltage detecting device 206 detects a momentary value of the interelectrode voltage (step 24). Then, the signal conversion section 208 converts these detected values into data signals processible in the control apparatus 207. These data signals are supplied to the calculating section 213. In the calculating section 213, the energy distribution estimating section 210 estimates an energy distribution based on the detected momentary data representing the welding current and the interelectrode voltage (step 25). A calculating means for estimating an expulsion & surface-flash generation probability estimates the energy distribution based on the welding current and the interelectrode voltage and with reference to the expulsion & surface-flash generation probability table 214 estimates an expulsion & surface-flash generation probability as herein-after described. Estimation of the energy distribution (step 25) is performed in the following manner. As shown in Fig.6A, the cross section of the welding portion is dissected into a plurality of micro lattice sections Aij (i=1, 2, ---; j=1, 2,--- ) each having a size of Δr·Δz. Fig. 6A shows an annular 3-D section Mij obtained by rotating the above micro lattice section Aij by an angle of 360° about the central axis of the electrode 202. Fig.5 is a flow chart used for obtaining numerical values representing the specific resistance, electrical potential, temperature, and energy in each annular 3-D section Mij. More specifically, the specific resistance of each annular 3-D section is determined based on the temperature distribution in the welding portion (step 201). At the moment the energizing operation is just started, the specific resistance is equal to a specific resistance at the room temperature in every annular 3-D section. In the calculation, it is assumed that there is no significant changes in the values of the temperature and the specific resistance during a small time interval Δt. Next, a mean specific resistance in the energizing section is obtained as a composite specific resistance of all annular 3-D sections. All of data representing the mean specific resistance thus obtained, the welding current and the interelectrode voltage detected previously are entered into the following equation (4) to calculate an energizing diameter (step 202). The obtained energizing diameter determines the region of succeeding calculations which are executed within this energizing diameter. Dc= 4 π · ρm· A· ∑hi Rtip - R0 where "Dc" is a representative energizing diameter of a welding portion (m), "ρm" is a mean specific resistance of the welding portion (Ω·m), "A" is a correction coefficient relating to current spread, "Σhi" is a total plate thickness (m), "Rtip" is an interelectrode resistance (=Vtip/I when "Vtip" is an interelectrode voltage and "I" is a welding current) (Ω), and "R0" is an electrode resistance (Ω). Next, the electrical potential in each annular 3-D section is calculated in accordance with the following equation (5). Then, using the electrical potential thus obtained, the current density in each annular 3-D section is calculated in accordance with the following equation (6) (step 203). Furthermore, using the calculated current density and the specific resistance, the temperature distribution in each annular 3-D section is calculated in accordance with the following equation (7) (step 204). Then, referring to this temperature distribution, the energy distribution in each annular 3-D section is calculated (step 205). These calculations are repetitively performed by the S.O.R method, until the resultant data are converged to specific values. The energy distribution estimating section 210 sends this energy distribution to the expulsion & surface-flash generation probability estimating section 211, and determines a specific resistance in each annular 3-D section based on the newly obtained temperature distribution (step 201). Then, using the welding current and the interelectrode voltage newly detected, the above-described calculations are repeated. ▿ · ( k ▿ V) = 0 where "i" is a lattice number in the radial direction, "j" is a lattice number in the axial direction, "k(i.j)" is an electric conductivity in an annular micro section Mi,j (Ω/m), and "V(i,j)" is a voltage in the micro section Mi,j (V). δ 2 = 1 (ρ×l) 2 · |grad V | 2 where δ is a current density (A/m 2 ), ρ is a specific resistance (m), l is a lattice clearance (m), and V is a potential difference (V/m). c σ ∂ T ∂ t = ▿ (▿ k T)+ρ δ 2 where "c" is a specific heat (cal/kg), "σ" is a density (kg/m 3 ), "k" is a heat conductivity, "k" is a heat conductivity, "δ" is a current density (≡AI/(π·Dc 2 /4)), "A" is a correction coefficient relating to current spread, "I" is a welding current (A), "T" is a temperature, "t" is a time, and "▿" is a partial derivative symbol. The expulsion & surface-flash estimating section 211 estimates an expulsion & surface-flash generation probability corresponding to the energy distribution calculated by the energy distribution estimating section 210 with reference to the expulsion & surface-flash generation probability table 214 (step 26). The expulsion & surface-flash generation probability table 214 was obtained experimentally by the inventors based on the fact that the expulsion & surface-flash generation probability is substantially determined by an amount of unavailable energy stored in a contact area in the periphery of a nugget. The unavailable energy amount is equal to a difference between the total energy supplied from the welding power unit 203 to the welding portion and the sum of an effective energy amount contributing the growth of the nugget, a heat transfer loss in the base metals 201, and other energy lost by electrode cooling water or radiated into the air. More specifically, according to the expulsion & surface-flash generation probability table 214, the expulsion & surface-flash generation probability is given as a function of the unavailable energy in various combinations of base metals 201 and electrodes 202, so as to be somewhere in a range from 0 to 100 (%). Ordinarily, the expulsion & surface-flash generation probability increases in accordance with the progress of the welding operation. It is believed that the expulsion & surface-flash is generated when the probability reaches 100 (%). The expulsion & surface-flash generation probability estimating section 211 extrapolates the curve representing the expulsion & surface-flash generation probability which increases in accordance with the progress of the welding operation, and predicts the expulsion & surface-flash generation time (i.e. a time an expulsion & surface-flash is generated for the first time) in a case where the welding operation is continuously performed under the welding conditions initially set (step 27). It is desirable that the expulsion & surface-flash generation time comes immediately after the setting welding time has elapsed. When the expulsion & surface-flash generation time comes earlier than the setting welding time, it is considered that the welding current is excessively large and there is a possibility that any expulsion & surface-flash may be generated during the present welding operation. On the other hand, when the expulsion & surface-flash generation time comes too much lately than the setting welding time, the welding current is excessively small and a nugget having a satisfactory size cannot be obtained. In view of the foregoing, the optimum welding current estimating section 212 compares the predicted expulsion & surface-flash generation time with the welding time initially set at a predetermined time, and estimates an optimum value for the welding current to be supplied during the succeeding welding operation (step 28). The estimated optimum welding current value is sent to the welding current control section 215. The welding current control section 215 changes the welding current value in accordance with the optimum welding current value (step 29). Meanwhile, the estimation of the optimum welding current by the optimum welding current estimating section 212 is forcibly started by considering the remaining time for the welding operation whenever the expulsion & surface-flash generation probability reaches 80%. Then, the command signal for changing the welding current is generated at a time corresponding to two cycles after the moment the expulsion & surface-flash generation probability reached 80%. The welding current control section 215 stops the energizing operation when the total energizing time reaches the welding time initially set (step 30). After stopping the energizing operation, the display section 216 displays the finally predicted expulsion & surface-flash generation time of the welding operation completed (step 31). This value is referred in the setting of the next welding conditions. As explained in the foregoing description, the above-described second embodiment comprises the welding current detecting device 205 for detecting the welding current, the interelectrode voltage detecting device 206, the signal conversion section 208 which converts the outputs of welding current detecting device 205 and interelectrode voltage detecting device 206 into data signals processible in the control apparatus 207, the input key 209, the calculating section 213 consisting of energy distribution estimating section 10, expulsion & surface-flash generation probability estimating section 211 and optimum welding current estimating section 212, the expulsion & surface-flash generation probability table 214, and the welding current control section 15. The energy distribution is estimated based on the momentary values of the welding current and the interelectrode voltage. Then, both the expulsion & surface-flash generation probability and the expulsion & surface-flash generation time are estimated from the energy distribution thus obtained. Then, the welding current is controlled to accomplish an adjustment such that the predicted expulsion & surface-flash generation time comes immediately after the termination of the setting welding time. Thus, the heat input amount can be increased up to the maximum level without causing any expulsion & surface-flash, thereby obtaining a desirable nugget having the maximum strength. According to the above-described second embodiment, the welding current control is performed at the predetermined time or at the time corresponding to two cycles after the moment the expulsion & surface-flash generation probability reaches 80%. However, the timing for the welding control can be flexibly changed. Hence, there is no necessity of limiting the welding current control timing to a specific time, such as the time corresponding to two cycles after the 80% reaching moment. Furthermore, instead of varying the welding conditions during the concerned welding operation, it will be possible to change the welding conditions for the next welding operation with reference to the predicted expulsion & surface-flash generation tine displayed on the display sectinn 216. Still further, according to the above-described second embodiment, the display section 216 displays the finally predicted expulsion & surface-flash time when the concerned welding operation is completed. Needless to say, it is preferable to display an expulsion & surface-flash generation probability at a time immediately before stopping the energizing operation or at any arbitrary tine, or a value resultant from statistic or comparison processing on these data, or a hysteresis curve representing an increase of the expulsion & surface-flash probability. It is also preferable to use these data as reference values. Yet further, in Fig.3, the welding current detecting device 205 is constituted by a toroidal coil or a current shunt. However, it is possible to measure the primary current of a transformer of the welding power source unit 203 to detect the welding current. Hereinafter, a demonstration result of the present invention will be explained in more detail. According to an experiment conducted by the inventors, galvanized sheet irons of 1.2 mmt is used as the lapped base metals 201. A total energizing time was set to 20 cycles. The pressurizing force was set to 320 kgf. The welding current was first set at 8 kA for a duration of 6 cycles and then increased to 14 kA after the passage of this duration. When the expulsion & surface-flash generation probability reaches 80%, the welding current is reduced at a time corresponding to 2 cycles after the 80% reaching moment. Fig.8 shows a curve representing a time variation of the expulsion & surface-flash generation probability in this experiment, in comparison with curves representing the welding current and the interelectrode resistance. As shown in Fig. 11, the expulsion & surface-flash generation probability reached 80% at the time corresponding to 14 cycles. The welding current is reduced to 10 kA at the time corresponding to 2 cycles after the 80% reaching moment (i.e. the time corresponding to 14 cycles). As a result of this precise welding current control, generation of the expulsion & surface-flash was surely prevented. Fig.7 shows one example of a welding operation not relying on the precise welding current control of the present invention, but was performed under the same welding conditions as those of the welding operation of Fig.8. As shown in Fig. 7, the expulsion & surface-flash generation probability has increased with elapsing energizing time. At the time corresponding to 17 cycles, an expulsion & surface-flash was generated. Occurrence of this expulsion & surface-flash was recognized by the steep drop of the interelectrode resistance. Furthermore, according to the above-described welding operation not relying on the welding current control of the present invention, a reduction of the welding strength by an amount of 90% to 70% was found at the welding point which caused an expulsion & surface-flash, compared with the welding strength of a normal welding point free from generation of the expulsion & surface-flash. On the contrary, according to the welding operation incorporating the welding current control of the present invention, dispersion of the welding strength at each welding point was small and the welding strength value was maintained at a higher level of 95% or above. As explained in the foregoing description, the present invention makes it possible to predict the generation of any expulsion & surface-flash in advance. Thus, the heat input amount can be increased up to the maximum level without causing any expulsion & surface-flash, thereby obtaining an excellent nugget having the maximum strength. As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiments described are therefore intended to be only illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the metes and bounds of the claims, or equivalents of such metes and bounds, are therefore intended to be embraced by the claims.

Claims: A control apparatus for a resistance welding machine, comprising: welding current detecting means (205) for detecting a welding current; interelectrode voltage detecting means (206) for detecting an interelectrode voltage; calculating means (213) for estimating an expulsion and surface-flash generation probability by estimating the energy distribution based on the welding current and the inter-electrode voltage and comparing the estimated energy distribution to preset expulsion and surfaceflash generation probability data; and control means (212, 215) for controlling said welding current so that said expulsion and surface-flash generation probability can be adjusted to a predetermined value. The control apparatus for a resistance welding machine in accordance with claim 1, further comprising heat input control means (212, 215) for controlling a welding heat input amount so as to maintain said expulsion and surface-flash generation probability at a predetermined value. The control apparatus for a resistance welding machine in accordance with claim 1, wherein said expulsion and surface-flash generation probability estimating means (211) comprises expulsion and surface-flash generation time predicting means for predicting an expulsion and surface-flash generation time, and there is heat input control means (212, 215) for controlling a welding heat input amount so that said expulsion and surface-flash generation time is in a predetermined period of time after finishing a welding operation. The control apparatus for a resistance welding machine in accordance with any one of claims 2 or 3, wherein said heat input control means comprises: optimum welding current estimating means (212) for generating a control signal representing an optimum welding current value, and welding current control means (215) for controlling the welding current in accordance with said control signal produced from said optimum welding current estimating means. The control apparatus for a resistance welding machine in accordance with any one of claims 1, 2, 3 and 4, further comprising display means (216) for displaying at least one of said expulsion and surface-flash generation probability estimated by said expulsion and surface-flash generation probability estimating means, said expulsion and surface-flash generation time predicted by said expulsion and surface-flash generation time predicting means, and a processing result of data relating to said expulsion and surface-flash generation probability and said expulsion and surface-flash generation time. The control apparatus for a resistance welding machine in accordance with any one of the preceding claims, further comprising condition input means (10) for inputting information relating to a base metal, recommendable condition memory means (11), and welding condition setting means (9) for automatically setting initial welding conditions suitable for said base material.

Inventors: FUJII KOJI, GOTO YASUHIRO, RYUDO MAKOTO, FUJII, KOJI, GOTO, YASUHIRO, RYUDO, MAKOTO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: B23K11
IPC codes: B23K11