KR100322345B1 - Method and apparatus for deciding welding current of resistance spot welder - Google Patents

Abstract

The present invention relates to an apparatus for controlling a welding current of a spot welding machine, comprising: a welder having a cycle number corresponding to a cycle control signal and performing welding with a current corresponding to the welding current control signal; A copper resistance detector for detecting a copper resistance value generated during welding of the welder; An expulsion detector for detecting whether an expulsion has occurred using the copper resistance value detected by the copper resistance detector; A welding quality assurance cycle (QAC) detector for providing the cycle control signal using the copper resistance value detected by the copper resistance detector; And a welding current setting circuit for providing a welding current control signal according to the number of occurrences of the pulsation of the explosion detector and the number of welding cycles corresponding to the cycle control signal.

That is, in the present invention, the welding current of the welding machine is controlled by using the number of occurrences of expulsion and QAC information acquired during a certain RBI, and thus welding can be performed under optimum welding conditions without occurrence of an expansion.

Description

METHOD AND APPARATUS FOR DETERMINING THE WELDING CURRENT OF A SPOT WELDER {METHOD AND APPARATUS FOR DECIDING WELDING CURRENT OF RESISTANCE SPOT WELDER}

The present invention relates to a welding current control device of a spot welder, and more particularly, the welding current determination method of determining the welding current by using the quality assurance cycle (QAC) information and the occurrence of explosion information. And to an apparatus.

Resistance spot welding (hereinafter referred to as spot welding) is a process in which a large current (8,000A to 30,000A) is supplied between base electrodes by pressing two or more thin sheets between the electrode tips with air pressure. It is a welding method that can obtain a simple and clean joint by melting and joining the contact part of the base material by Joule heat using contact and compression resistance. There are many factors that determine welding quality in this spot welding, but among them, the welding current, energization time (or energization cycle), and electrode pressing force are three basic factors.

1 is a view showing a typical dynamic resistance curve generated during spot welding, Figures 2a to 2b is a view showing a change in the dynamic resistance characteristics generated when the welding current is changed while the pressing force is kept constant, As a base material, the results obtained using two layers of cold rolled steel sheets having a thickness of 1.0 mm which are commonly used in automobile production are shown.

Welding quality is generally a series of melting start (time a), nugget growth (time b), and indentation (time c) as the number of energizing cycles increases as shown in FIG. It is stabilized through the process of. At time point d, an expulsion occurs. If the welding time is longer than necessary or the welding current is too large, the melt spreads, and the weld specimens around the melt cannot sustain the electrode force. In this case, an expulsion occurs and molten metal is ejected between the specimens, and at this time, the dynamic resistance is rapidly reduced. In this case, the expulsion phenomenon is used as a guarantee for welding quality because it ensures a sufficient nugget size, but such an expulsion gives an operator anxiety about a disaster and risks damage to welding equipment. Therefore, the optimum welding condition that can produce the highest welding strength with the least energy is considered to be the welding condition immediately before the expulsion occurs.

Therefore, the optimum welding quality can be ensured that the welding is performed only until the dynamic resistance value increases to a certain degree of slope and then falls to a value immediately after the expulsion occurs after having the maximum value R _max . If the heat input (determined by the welding current and the number of energizing cycles, but once the number of energizing cycles is fixed. Therefore, the heat input corresponds to the welding current value) is insufficient, the dynamic resistance value as shown in FIG. As it is not reduced, the maximum dynamic resistance value R _max does not drop to an appropriate dynamic resistance value and thus the welding quality cannot be guaranteed. On the contrary, in the case where the heat input is excessive, the copper resistance value rapidly changes to the maximum dynamic resistance value Rmax as shown in FIG. 2B, which is not preferable.

However, the user cannot set the welding current value while confirming the change of the dynamic resistance value one by one, and especially when the electrode tip wears, the current value decreases as the electrode tip wears, and thus the current value needs to be increased. That is, when performing welding, it is necessary to automatically provide a desirable welding current value according to the characteristics of the welding machine and the base metal, and in response to this need, Patent Application Nos. 94-3146 and 96-29481 provide an automatic stepper. Suggested a function. In the automatic stepper function, the user sets the welding reference RBI number and the reference explosion occurrence number. The welding criterion RBI is the number of times the welding is performed, and the reference explosion occurrence number is the number of times the expulsion should be generated within the set welding criterion RBI. In the state where these reference values are set, welding is performed a number of times corresponding to the welding reference RBI number, and the number of occurrences of expulsion and the number of reference occurrences during welding are compared. As a result of the comparison, the welding current is excessive when the number of occurrence of the pulverization is greater than or equal to the reference occurrence, and the welding current is lowered to a preset ratio. However, when the number of occurrences of the pulsation is less than or equal to the reference occurrence, the welding current is preset. Increase the ratio so that the welding current is near the threshold of occurrence of the explosion.

However, in the conventional automatic stepper function, since the welding current is added or subtracted at a predetermined rate corresponding to the number of occurrences of the impulse after the number of weldings corresponding to the welding reference RBI number specified by the user, an appropriate welding current can be detected immediately. There is no problem.

Meanwhile, in order to perform the conventional automatic stepper function, it is necessary to detect the occurrence of the explosion. For this purpose, conventionally, the probability of occurrence of expulsion was used. The probability of occurrence of expulsion Epro detects the probability of occurrence of expulsion during welding by using a change amount of the dynamic resistance value, and is detected as Equation 1 below.

Here, R _max means the maximum value of the copper resistance in the change curve of the copper resistance value, as shown in Figure 3, ΔR _max means the maximum value of the change in copper resistance sampled every half cycle in each welding cycle, R _end means the dynamic resistance value sampled at the last energization cycle, and R _min is the minimum value of the dynamic resistance values sampled every half cycle. In other words, the probability of occurrence of expulsion Epro in Equation 1 is the difference between the maximum value R _max of the dynamic resistance value and the dynamic resistance value R _end of the last energizing cycle, and the change amount ΔR _max of the maximum dynamic resistance. Set to the ratio of.

Since the occurrence probability (Epro) literally means only the probability of occurrence of an explosion, the final occurrence probability (Epro) must be greater than or equal to the predetermined value in order to determine that the occurrence has occurred. Therefore, conventionally, the test welding is carried out with a real material, and the judgment reference value which can determine that an expulsion generate | occur | produced is set, and it determines finally that an expulsion generate | occur | produced only when the expulsion generation probability Epro is more than a judgment reference value.

However, such a conventional occurrence probability detection method has a problem in the reliability of the amount of change in the dynamic resistance per half cycle under a noisy environment, which causes a lot of problems in applying it alone in a real environment. In addition, the applied welding machine also has a variable change in dynamic resistance (R _max -R _end ) depending on the type of sheet, and when the change width (R _max -R _end ) is very small, a small amount of noise causes a change in the maximum dynamic resistance ( ΔR _max ) has a big influence, causing significant distortion in the occurrence probability (Epro) value. In other words, when the change resistance (R _max -R _end ) of the dynamic resistance is large, the influence by noise is small, but when the change resistance (R _max -R _end ) of the dynamic resistance is small, the influence by noise is large and it is determined whether or not an explosion occurs. There is a problem that cannot be judged accurately.

SUMMARY OF THE INVENTION The present invention has been made to solve this problem, and an object of the present invention is to utilize an expulsion using the number of welding cycles and the number of occurrences of an expulsion optionally additionally provided within a predetermined spot for optimal welding when performing welding. The present invention provides a method and apparatus for determining welding current of a spot welder capable of setting an optimum welding current value close to a limit.

1 is a view showing the change in the dynamic resistance of the spot welder,

2 is a view for explaining the relationship between the welding current and the dynamic resistance in the spot welder,

3 is a diagram illustrating a dynamic resistance change characteristic for explaining a conventional formula for detecting probability of occurrence of pulverization;

4 is a block diagram of an apparatus for determining welding current of a spot welder according to the present invention;

5 is a view showing a state in which the expulsion information amount is fuzzy according to the present invention;

6 is a view showing a state in which welding quality assurance cycle information is fuzzy according to the present invention;

7 shows the state of a function required to purge a welding quality assurance cycle in accordance with the present invention;

8 is a view illustrating a process of setting a welding current for the fuzzy inference method of the welding current determination apparatus of the spot welder according to the present invention;

9 is a diagram illustrating a copper resistance change characteristic for explaining a formula for detecting an occurrence probability of pulsation according to the present invention;

FIG. 10 is a diagram illustrating a copper resistance change characteristic for explaining a formula for detecting an occurrence probability of expulsion according to the present invention. FIG.

<Description of the symbols for the main parts of the drawings>

10: welding machine

20: copper resistance detector

30: expulsion detector

40: QAC detector

50: welding current setting circuit

In order to achieve the above object, the present invention comprises the steps of measuring the occurrence frequency of the explosion within the predetermined spot during spot welding; Calculating a frequency at which a welding quality cycle is applied within a predetermined RBI and an increase rate of an energization cycle according to the application of the welding quality cycle; Calculating a correction amount of the welding current by using an occurrence frequency of expulsion, an application frequency of the welding quality cycle, and an increase rate of an energization cycle.

The present invention also provides an apparatus for controlling an amount of current in a spot welder, comprising: a welder having a cycle number corresponding to a cycle control signal and performing welding with a current corresponding to the welding current control signal; A copper resistance detector for detecting a copper resistance value generated during welding of the welder; An expulsion detector for detecting whether an expulsion has occurred using the copper resistance value detected by the copper resistance detector; A welding quality assurance cycle (QAC) detector for providing a cycle control signal using the copper resistance value detected at the copper resistance detector; And a welding current setting circuit for providing the welding current control signal according to the number of occurrences of the pulsation of the explosion detector and the number of welding cycles corresponding to the cycle control signal.

In the present invention, in order to control the current of the welding machine, welding quality assurance cycle (QAC) information at the time of welding is used in addition to whether or not an explosion occurs. The QAC information is presented in the welding quality control method (Application No. 99-1174) of a spot welder, which is filed by the applicant, and will be briefly described below.

The QAC continuously detects the dynamic resistance in real time, so that the fluctuation range between the dynamic resistance value and the maximum dynamic resistance value in each spot at the completion of welding is maintained at a certain level or more. Increase the welding cycle). This was conceived in the experiment that even in the absence of expulsion, ideal welding quality can be obtained when the fluctuation of copper resistance is higher than a certain level. In other words, the energization time is controlled to sufficiently stabilize the nugget generated during welding. To this end, QAC uses a decrease in dynamic resistance (% ΔR _m ). The dynamic resistance reduction rate (% ΔR _m ) is detected using Equation 2.

Here, R _min means the minimum value of the dynamic resistance per half cycle, as shown in FIG.

QAC determines that the heat input is insufficient when the dynamic resistance reduction rate (% ΔR _m ) detected by Equation 2 is less than or equal to the predetermined reference dynamic resistance reduction rate (% ΔR _ref ). Detect.

As described above, when QAC is used, an optimal number of welding cycles for optimum welding conditions can be detected. On the other hand, as can be seen in Equation 2, it can be seen that the decrease in dynamic resistance (% ΔR _m ) expresses the change state of the copper resistance in a different manner from the probability of occurrence of explosion (Epro) in Equation (1). Therefore, the use of the dynamic resistance reduction rate (% ΔR _m ) and the probability of occurrence of impulse generation (Epro) will enable more accurate detection of an appropriate welding current.

Hereinafter, with reference to the accompanying drawings, an embodiment of the present invention for detecting the appropriate welding current using the dynamic resistance detection rate (% ΔR _m ) and the probability of occurrence of explosion (Epro).

4 is a schematic block diagram of the present invention. In the block diagram shown, reference numeral 10 denotes a general welding machine. The welding current of the welding machine 10 is controlled by a welding current control signal of the welding current setting circuit 50 described later, and the number of welding cycles is detected by QAC. Controlled by the cycle control signal of the circuit 40. The copper resistance value generated at the time of welding by the welder 10 is detected by the copper resistance detector 20, and the detected copper resistance value is provided to the impulse detector 30 and the QAC detector 40.

The expulsion detector 30 provides the welding current setting circuit 50 with an expulsion occurrence using Equation 1 or the like, and the QAC detector 40 supplies proper current using Equation 2 or the like. The number of cycles is detected and this information is provided to the welder 10 and the welding current setting circuit 50 as a cycle control signal.

The welding current setting circuit 50 sets an appropriate welding current using the information of the pulsation detector 30 and the QAC detector 40 and provides it to the welder 10. That is, the welding current setting circuit 50 detects an appropriate welding current value and fluctuation range according to the number of occurrences of expulsion, the number of QACs applied, and the number of additional energization cycles within a predetermined score set by the user, and the welding current is determined. The control signal is provided to the welder 10.

The welding current setting circuit 50 may calculate various welding currents according to the information of the implantation detector 30 and the QAC detector 40. For example, a method of detecting the variation of the welding current value according to the number of occurrences of expulsion, the number of QACs applied, and the number of cycles of energization by the QAC within the predetermined RBI is applied. It will be readily apparent to those skilled in the art that formulating and detecting may be used. However, the inventors have devised a way to more accurately set the fluctuation range of the welding current value, and found that it is most appropriate to use the fuzzy logic method. Hereinafter, a method in which the welding current setting circuit 50 sets the fluctuation range of the welding current value using the fuzzy logic method will be described.

First, the present inventor detects a ratio (R _expulsion ) between a set _expulsion generation ratio set to be generated by a user within a preset RBI and a real _expulsion generation ratio that actually occurs within a preset RBI, using the equation (3). The value was fuzzy as shown in FIG. 5.

In FIG. 5, the fuzzy function VS is very small (Very Small), SM is small (Small), NO is normal, LA is large and VL is very large.

In addition, the QAC information was also detected by performing equation (4) using the frequency with which QAC was applied and the actual number of energization cycles added within the preset RBI, and the weighted value Q was fuzzy as shown in FIG. .

Here, Q ₁ denotes a frequency at which QAC is applied, and Q ₂ denotes a ratio between the maximum number of cycles that can be energized per RBI and the number of cycles of energization actually applied. That is, Q ₂ is detected by Equation 5, and the function f (Q_2) may be a piece-linear function as shown in FIG. 7.

Here, AATC is Actual Applied Total Cycles, and TAC is Total Applicable Cycles.

When the value R _expulsion and the weight values Q are detected in the case of purging as shown in FIGS. 5 and 6, the fuzzy functions VS, SM, NO, LA, and VL corresponding to the values are shown in FIG. 8. As shown in FIG. 4, the detection is performed through the fuzzy blocks 51 and 52. That is, if the value R _expulsion corresponds to 83 as shown in Fig. 8, the fuzzy functions for this value are (LA) and (VL), and the function value of the fuzzy function LA at this time is 0.2 and the fuzzy function. The function value of (VL) is 0.6. When the weight value Q also corresponds to 1.0, the function value of the fuzzy function NO corresponds to 0.7 and the function value of the fuzzy function SM corresponds to 0.3.

Information about the detected fuzzy function is performed through the inference engine 53, and the fuzzy inference function block 54 determines the current control signal value of the welder 10 by using the result of the inference. In FIG. 8, the inference engine 53 is illustrated as a rule-based state space display method. In other words, the inference engine is very small (Negatave Big: NB), small (Nagatibe Small: NS), current state (Keep: KE), large (Positive Small: PS), and very large (Positive Big: PB). And the fuzzy variables (NB, NS, KE, PS, PB) corresponding to the fuzzy functions (VS, SM, NO, LA, VL) of the value (R _expulsion ) and the weight value (Q), and these The function value of is detected by the maximum-minimum inference method and provided to the non-fuzzy function block 54.

Specifically, in the example of FIG. 8, since the fuzzy functions of the values R _expulsion are (NO) and (SM) and the QAC fuzzy functions are (LA) and (VL), the inference engine 53 is a small block. As shown, three fuzzy variables PB and one PS corresponding to each of them are selected. The inference engine 53 compares the function values detected in each of the three selected fuzzy variables PB and one PS with rows and columns, respectively, and compares the small function values with fuzzy variables PB and PS. To each of them. In FIG. 8, the function values (hereinafter, referred to as grant function values) assigned to three fuzzy variables PB and one PS are added.

In the non-fuzzy function block 54, variable functions for fuzzy variables NB, NS, KE, PS, and PB are formed, respectively, and fuzzy variables PB and PS detected by the inference engine 53. And their assigned function values are finally outputted using the center of gravity method. That is, the non-fuzzy function block 54 compares the assigned function values assigned to the fuzzy variables PB and PS detected by the inference engine 53 with fuzzy variables PB and PS, The grant function values are set to the final grant function values PB: 0.6 and PS: 0.2 of the fuzzy variables PB and (PS), respectively. In addition, the non-fuzzy function block 54 detects the center of gravity for the variable functions PB and PS up to the final grant function values PB: 0.6 and PS: 0.2, and converts the center of gravity to the final output value. It sets and provides it to the welder 10 as a welding current control signal.

On the other hand, in the present invention as described above, the occurrence probability (Epro) of the expulsion is used, and the case where the occurrence probability (Epro) of the expulsion is detected by the equation (1) has been described as an example. However, as described in the prior art, the probability of occurrence of expansion (Epro) according to Equation 1 has a problem in the reliability of the amount of change in dynamic resistance per half cycle under a noisy environment. The present inventors have found that the occurrence probability of expulsion (Epro) can be detected as shown in Equation (6).

Here, R _max means the maximum value of the dynamic resistance in the curve of the dynamic resistance value detected every half cycle, as shown in Figure 9, ΔR _max means the maximum value of the change in the dynamic resistance of the adjacent cycle, R _end means dynamic resistance in the last energization cycle, and LD means the standard deviation of dynamic resistance values from the next cycle (t + 1) of the cycle where ΔR _max is sensed to the last energization cycle.

The physical meaning in Equation 6 is as follows. First, as a result of analyzing the dynamic resistance waveform, it can be seen that when an explosion occurs, the standard deviation of the dynamic resistance value from the next cycle (t + 1) at the time of occurrence of the explosion to R _end is extremely small. Therefore, when the explosion occurs in Equation 6, the denominator decreases according to the degree, and since the ΔR _max is relatively large, the total amount has a large value. On the contrary, when no pulsation occurs, the amount of deviation since the cycle in which ΔR _max is sensed is large, so that the value of the overall occurrence is reduced by that amount, thereby overcoming the ΔR _max reliability problem due to noise.

Another way of detecting the probability of occurrence of expulsion is to detect as equation (7).

In the above formula, ST denotes resistance values on the straight line when connected in a straight line from the time point t at which ΔR _max appears to the time of the last welding cycle t _end as shown in FIGS. 10A and B, and R is ΔR _max. It means the dynamic resistance value of the moments sampled from the time point (t) to the last welding cycle time (t _end ). In addition, since Dev means a command to obtain a standard deviation amount, Equation 7 represents a standard deviation amount of deviation amounts between the virtual dynamic resistance value ST and the sampled dynamic resistance value R.

The reason why the occurrence probability Epro can be detected by Equation 7 can be clearly seen from FIGS. 10A and 10B. 10A illustrates an occurrence of an explosion, and FIG. 10B illustrates a case in which an explosion does not occur. As shown in FIG. 10A, an occurrence probability Epro of Equation 6 is decreased as shown in FIG. 10A. If the explosion is not generated, as shown in FIG. 10B, the probability of occurrence of the explosion (Epro) by Equation 6 is low. This is because the maximum dynamic resistance value (R _max ) when the explosion is not generated. This is because the copper resistance is stable, i.e., gradually decreases, since it enters the cooling step after is generated.

Therefore, it will be preferable to use Equation 5 or Equation 6, rather than Equation 1, to determine whether occurrence of an expulsion is performed.

As described above, according to the present invention, the welding current of the welding machine is controlled by using the number of occurrences of expulsion and QAC information acquired during a certain RBI, so that welding can be performed under optimum welding conditions without occurrence of an expansion.

Claims (9)

Measuring the occurrence frequency of the explosion within the predetermined spot during spot welding; Calculating a frequency at which a welding quality assurance cycle (QAC) is applied within a predetermined RBI and an increase rate of an energization cycle according to the application of the welding quality cycle; And calculating a correction amount of the welding current by using the expulsion occurrence frequency, the application frequency of the welding quality cycle, and the increase rate of the energization cycle. The method of claim 1, wherein the occurrence of expulsion, (A) measuring the dynamic resistance at each cycle of the RBI, and obtaining the maximum value of the change amount between the copper resistances; (B) obtaining a standard deviation amount of the copper resistance measured for each cycle from immediately after the maximum value detection to the last energization cycle; (C) The welding current control method of the spot welder which judges the ratio between the standard deviation amount of (b) and the maximum value of (a) as a result of comparing with the reference value of the said ratio. The method of claim 1, wherein the occurrence of expulsion (A) measuring the dynamic resistance at each cycle of the RBI, and obtaining the maximum value of the change amount between the copper resistances; (B) connecting the dynamic resistance value of the cycle in which the maximum value appears on the dynamic resistance curve and the dynamic resistance value of the last energizing cycle in a straight line; (C) The standard deviation between the copper resistance value on the dynamic resistance curve of (B) and the copper resistance value on the linear dynamic resistance line is calculated for each cycle from the detection of the maximum value of (a) until the last energization. The welding current control method of the spot welder judging as a result of comparing with a predetermined reference value. With the current amount control device in the spot welding machine, A welder having a cycle number corresponding to the cycle control signal and performing welding with a current corresponding to the welding current control signal; A copper resistance detector for detecting a copper resistance value generated during welding of the welder; An expulsion detector for detecting whether an impulse is generated using the copper resistance value detected by the copper resistance detector; A welding quality assurance cycle (QAC) detector for providing the cycle control signal using the copper resistance value detected by the copper resistance detector; And a welding current setting circuit for providing the welding current control signal in accordance with the number of occurrences of the pulsation of the explosion detector and the number of welding cycles corresponding to the cycle control signal. The method of claim 4, wherein The expulsion detector detects an occurrence occurrence probability (Epro) of the following equation, Determining whether the occurrence of the explosion is determined by determining whether the occurrence occurrence probability Rpro is equal to or greater than a predetermined value; In the above formula, R _max means the maximum value of the dynamic resistance detected every half cycle of the energization cycle, ΔR _max means the maximum value of the change in the dynamic resistance, R _end means the dynamic resistance in the last energization cycle Device for setting the welding current of a spot welder. The method of claim 4, wherein The expulsion detector is The occurrence probability (Epro) of the explosion is detected by the following equation, Determining whether the occurrence of the explosion is determined by determining whether the occurrence occurrence probability Rpro is equal to or greater than a predetermined value; In the above formula, ΔR _max means the maximum value of the change in dynamic resistance detected every half cycle of each energizing cycle, and LD means the standard deviation of dynamic resistance values from the cycle after the cycle in which ΔR _max is sensed to the last energizing cycle. Device for determining the welding current of spot welders. The method of claim 4, wherein The expulsion detector, The occurrence probability (Epro) of the explosion is detected by the following equation, Determine whether the occurrence of the explosion is determined by determining whether the occurrence occurrence probability Epro is equal to or greater than a predetermined value; In the above formula, ST means the resistance values of the straight line when the ΔR _max is displayed from the time of the final welding time (t _end ) in a straight line, and R is the time from the time of the ΔR _max to the time of the last welding cycle (t _end ). Determination of the dynamic resistance value of the sampled moments, Dev means the welding current determination device of the point welder, which means the standard deviation amount. The method according to any one of claims 4 to 6, The welding current setting circuit Fuzzy function of the ratio (R _expulsion ) between the set _expulsion generation ratio set to be generated by the user within the preset RBI and the actual expiration occurrence ratio actually generated within the predetermined RBI, Fuzzy function of the detected QAC weighting value (Q) using the frequency with which QAC is applied and the number of actual energization cycles within the preset RBI, And a welding current control device for setting the welding current control signal using an inference engine having fuzzy variables for the fuzzy functions. The method of claim 8, The weight value Q is represented by the following equation Q = Q ₁ + Q ₁ × f (Q ₂ ) Detected by Where Q ₁ is the frequency at which the welding quality assurance cycle is applied, and Q ₂ is the ratio of the maximum energized cycles per RBI to the actual applied cycles.