JP2015193020A - Control method and control device of resistance welder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To take countermeasures such as exactly setting weighting of PID control elements (proportion, integration, differentiation), etc. in a negative feedback circuit by predicting poor welding to be generated due to inappropriate setting of a negative feedback amount of PID control, or disturbance of welding conditions, etc.SOLUTION: In a control method of a resistance welder which performs welding by converting direct output into AC by an inverter 42 to be guided to a primary side of a welding transformer 28, and guiding secondary side AC output reduced by the welding transformer 28 to a welding electrode 24 via a full wave rectifier 30, PWM control is performed to the inverter 42 so as to set welding conditions including a current setting pattern of the welding electrode 24, and on the other hand, negatively feeding applied voltage and welding current of the welding electrode 24 back to the inverter 42 to fit to the welding conditions, and stability of control is discriminated by integrating frequencies N in which deviation D between the welding current and the current setting pattern performs zero cross and comparing the frequencies N with a setting value N.

Description

  The present invention relates to a control method and a control apparatus for an inverter type resistance welding machine that rectifies an AC power supply output and further guides the output to an welding transformer via an inverter.

  Conventionally, direct current is converted to high-frequency alternating current using an inverter and then guided to the primary side of the welding transformer. At this time, the AC low voltage induced on the secondary side of the welding transformer is rectified and guided to the welding electrode for welding. Inverter type resistance welding machines are known. According to this method, since a current having a higher frequency than the frequency of the commercial AC power supply is led to the welding transformer, the welding transformer is downsized compared to a single-phase AC resistance welding machine that leads the AC of the commercial power supply frequency to the welding transformer. There is a feature that can be done.

  In such an inverter type, a current flowing through the welding electrode is detected, and the inverter is controlled by a power control (hereinafter also referred to as PWM control) by a PWM (Pulse Width Modulation) method or the like so that the current (effective value) becomes constant. There is a constant current control method. In this method, the welding is stopped after the current flowing through the welding electrode reaches a time set in advance by a timer. There is also a constant voltage control method in which a voltage applied to the welding electrode is detected and the inverter is PWM controlled so that this voltage becomes constant. This method also sets the welding time with a timer. Furthermore, a constant power control method is also known in which the current and voltage of the welding electrode are detected at the same time, power is obtained from the product of these, and the inverter is controlled so that this power is constant (Patent Document 1).

  When the welding current, voltage, power, and the like are negatively fed back to the inverter as described above, it is also known that the inverter is PWM controlled so that the welding current has a predetermined setting pattern. In this case, in the negative feedback circuit, a proportional value, an integral value, and a differential value with respect to fluctuations in the welding current, welding voltage, welding power, etc. are obtained, and the sum total obtained by adding a predetermined weight coefficient to these values may be used as the negative feedback amount. Known (PID control; Patent Document 2).

Japanese Patent Laid-Open No. 2001-30084 JP 2002-160071 A

  However, PID control has the following problems. When the fluctuation amount, ie, deviation (including absolute value) of the welding current, etc., is small, it is considered that the control is smooth and the control well following the target setting pattern is performed. When a disturbance such as an abnormality occurs, the amount of variation of the deviation may increase. At this time, a state in which control is severely exposed (for example, a state in which oscillation or resonance is about to occur) may occur.

  In such a state, a proportional value (P value), an integral value (I value), a differential value (D value), etc. are set large (negative) in order to make the actual control pattern approach the target value (setting pattern) quickly. This may occur when the feedback amount is increased) or when the setting pattern is impossible (such as when the waveform of a control pattern exceeding the power tracking capability is set).

  Here, as disturbance, when there is a large variation in the ease of melting of the workpiece at the weld location and the resistance of the weld shows unexpected changes, changes in the oxide film, dirt, surface condition (roughness) of the workpiece, etc. This may occur when the contact resistance is unstable. The occurrence of such welding abnormalities can be noticed from the fact that, for example, the occurrence of splash (sparks and splashes) is high, and the occurrence of such splashes can be noticed. It is difficult to predict and prevent the occurrence of abnormalities, and there is a problem that high-precision welding cannot be handled.

  The present invention has been made in view of such circumstances, and predicts welding defects caused by improper setting of the negative feedback amount of PID control or disturbance of welding conditions, and the like in the negative feedback circuit. It is a first object of the present invention to provide a control method for a resistance welder that can take countermeasures such as appropriately setting weights of control elements (proportional, integral, differential). The second object is to provide this resistance welder.

According to the present invention, a first object is to convert a direct current output to an alternating current by an inverter and lead it to the primary side of the welding transformer, and to convert the secondary side alternating current output stepped down by the welding transformer to a welding electrode via a full-wave rectifier. In the control method of the resistance welding machine that guides and welds,
While setting the welding conditions including the current setting pattern of the welding electrode, the inverter is PWM-controlled so that the applied voltage and the welding current of the welding electrode are negatively fed back to the inverter so as to meet the welding condition, and this welding current The resistance welding machine control method is characterized in that the stability of control is determined by integrating the number N of times the deviation between the current setting pattern and the current setting pattern is zero-crossed and comparing it with the set value N ₀ .

The second purpose is to convert the direct current output to alternating current by an inverter and lead it to the primary side of the welding transformer. The secondary side alternating current output stepped down by this welding transformer is guided to the welding electrode via the full-wave rectifier and welded. In the resistance welding machine control device to perform,
A PWM control unit that controls the output of the inverter, an amplification multiplication unit that detects the applied voltage and current of the welding electrode and negatively feeds back to the PWM control unit, and a program selection for selecting a control program corresponding to the control mode An input unit, a welding condition setting unit that obtains a current setting pattern of a welding current based on the selected control program and sends it to the PWM control unit, a monitoring condition input unit that inputs a set value N _{0 of the} number of zero crosses, Comparing the welding current detected by the amplification multiplication unit with the current setting pattern and integrating the number of zero cross detections, and comparing the zero cross integration value N with the zero cross setting value N ₀ to control stability It is achieved by a resistance welding machine control device comprising a comparison / determination unit for determining whether or not.

According to the first invention, while setting the welding conditions including the current setting pattern of the welding electrode, the inverter is PWM-controlled so that the applied voltage and welding current of the welding electrode are negatively fed back to the inverter and adapted to the welding condition. and, since that determines the stability of the control by the deviation between the welding current and the current setting pattern is compared with a set value N ₀ by integrating the number of times N to the zero-crossing, for example, the control system when N> N ₀ is not It can be determined that the control system is stable, and the control system can be stabilized by changing the negative feedback amount of the PWM control. Further, when a set pattern such as a current includes a pattern that cannot be controlled, the control system can be stabilized by changing the pattern.
According to the second aspect of the present invention, a resistance welder that is directly used for carrying out this control method is obtained.

1 is an external view of a resistance welder that is an embodiment of the present invention. Its circuit configuration block diagram Flow chart showing the operation Diagram showing welding current fluctuation and current setting pattern

The quality of the welding can be configured to output a warning for prompting the change of the welding conditions when N> N ₀ (Claim 2). Since the operator of the welding machine can know the necessity of changing the welding conditions by this warning (alarm), it becomes possible to prevent the occurrence of subsequent welding defects in advance. In addition, by making it possible to set the monitoring range in which the number of zero crossings N is integrated and the set value N ₀ , for example, by excluding the current unstable state at the initial stage of welding or the end of welding from the monitoring range, the control accuracy can be improved. (Claim 3).

The stability D of the control may be determined by obtaining a deviation D of the welding current from the current setting pattern together with the integrated value N of the number of times of zero crossing and comparing the absolute value of this deviation D with the setting value D _0. 4). In this case, not only when the number of zero crossings N exceeds the set value N ₀ , but also when the fluctuation range of the deviation D becomes excessive (out of the set value ± D ₀ range), a warning is issued and welding is performed. Since the necessity of condition change can be notified, the detection accuracy of control instability is further improved. The inverter performs PID control of the negative feedback amount of the welding voltage and welding current, and the welding condition can be changed by changing the weighting coefficient of these PID controls.

  In FIG. 1, reference numeral 10 is a controller, 12 is a welding transformer case, and 14 is a welding head. The controller 10 has a power switch 16, a program selection input unit 18, and a display panel 20. The program selection input unit 18 selects one of control modes such as constant current control, low voltage control, constant power control, and fixed pulse width control.

  The welding head 14 has an upper clamp portion 22a that can move up and down, and a lower clamp portion 22b that is fixed to a predetermined height, and welding electrodes 24a and 24b are fixed to the clamp portions 22a and 22b, respectively. . Each electrode 24a, 24b is connected to the secondary side of the welding transformer 28 (FIG. 2) accommodated in the welding transformer case 12 by a weld cable 26 via diodes 30 and 30 (FIG. 2) as rectifiers. The primary side of the welding transformer 28 is connected to the controller 10 by a power cable 32 (FIG. 1).

  The movable clamp portion 22a of the welding head 14 is driven up and down by an air cylinder (both not shown) via a spring (not shown). A welded part (not shown) of the workpiece is placed between the electrodes 24a and 24b.

  An ON signal sent from a foot switch (not shown) is sent to the controller 10 via the actuator cable 34, and the air cylinder is operated by this signal (pressing signal). When the pressing force of the electrodes 24a, 24b against the welded portion becomes a constant value, a limit switch (not shown) is turned on, and this on signal is sent to the controller 10 via the actuator cable 34 (step S100 in FIG. 3). The controller 10 starts a welding operation based on this ON signal. That is, this ON signal becomes a start signal.

  In FIG. 1, reference numeral 36 denotes a weld sense cable, which guides a signal indicating the secondary side current of the welding transformer 28 to the controller 10. This signal is a signal indicating the welding current value. A voltage detection cable 38 is connected to the clamp portions 22a and 22b. The cable 38 detects the voltage between the electrodes 24 a and 24 b and guides it to the controller 10.

  Next, the main circuit of the controller 10 will be described with reference to FIG. In this figure, reference numeral 40 denotes a power source such as a three-phase AC power source. Three phases are led from the power source 40 to the inverter 42 via the power switch 16 (FIG. 1).

  The inverter 42 is constituted by a bridge composed of four NPN transistors. This inverter 42 rectifies the three-phase alternating current of the power supply 40 with a diode (not shown), and converts the direct current power supply smoothed with the diode (not shown) into a high-frequency current by PWM control. Supply to the primary side. The secondary output of the welding transformer 28 is full-wave rectified by the diodes 30 and 30 of the rectifier and guided to the welding electrodes 24a and 24b.

  A PWM control unit 44 controls the power of the inverter 42 by a PWM (Pulse Width Modulation) method. The PWM control unit 44 can select four different control methods such as constant current, constant voltage, constant power, and fixed pulse width. This selection is performed by the program selection input unit 18 (FIGS. 1 and 2). Since these control methods are described in detail in Patent Document 1, they will not be described here.

  The PWM control unit 44 performs on / off control of the transistor of the inverter 42 based on a welding condition set by a program corresponding to the selected method. This welding condition includes a current setting pattern A (indicated by a dotted line in FIG. 4) and is set by the welding condition setting unit 46. In FIG. 2, reference numeral 48 denotes an amplification multiplier, which amplifies the signals i and v indicating the welding current I flowing through the welding electrodes 24a and 24b and the welding voltage V between both electrodes, and calculates the power W by multiplying them. . These current I, voltage V, and power W are led to the PWM control unit 44, and according to the program selected here, the proportional component P, integral component I, and differential component D of PID control are weighted. A negative feedback amount is calculated.

Reference numeral 50 denotes a zero cross detection unit, which compares the current setting pattern A obtained by the welding condition setting unit 46 with the actual welding current waveform I obtained by the amplification multiplication unit 48 (solid line in FIG. 4). The deviation is taken, the zero crossing of this deviation is detected, and the number of crossings (zero crossing number) within the preset monitoring range a (FIG. 4) is integrated. The integrated value N is compared with the reference value N ₀ by the comparison / determination unit 52, and if N> N ₀ , it is determined that control is inappropriate and the result is output to the comparison / determination result output unit 54 and a warning display unit 56. To warn with a warning sound.

Operator receiving this warning modifies the weighting of the components of the PID control for negative feedback to the PWM controller 44 is set so as cumulative value N of the zero crossing count is not greater than the set value N _0. The set value N ₀ is input by the monitoring condition input unit 58 and stored in the set value memory 60.

The monitoring condition input unit 58 also sets a reference value ± D ₀ that determines an allowable range of variation in deviation, and this reference value D ₀ is sent to the zero cross detection unit 50, where the welding current deviation is determined. D detects whether within the width of the width or ± D ₀ of the reference value. In this case, the reference value D ₀ is a threshold value for determining whether or not the fluctuation range of the deviation D is excessive. If the absolute value | D | of the deviation D is larger than the width of the reference value ± D ₀ , the deviation D is excessive and unstable, and if it is within the width of the reference value ± D ₀ , it is determined that the control is stable.

  Next, the operation will be described with reference to FIGS. As described above, when the pressing force of the electrodes 24a and 24b against the welded portion becomes a constant value, a limit switch (not shown) is turned on, and this on signal is sent to the controller 10 via the actuator cable 34 (FIG. 3, step). S100). Based on this ON signal, the controller 10 starts energizing the power supply 40 (step S102) and starts the welding operation.

  The controller 10 sets welding conditions in the welding condition setting unit 46 in accordance with a control program corresponding to the control mode selected in advance by the program selection input unit 18. This welding condition differs depending on the control mode, and includes, for example, a current setting pattern A that is a control target in the case of constant current control (step S104).

  The PWM control unit 44 performs on / off control of each transistor of the inverter 42 according to the welding conditions, and guides a high-frequency predetermined voltage and current to the primary side of the welding transformer 28. The alternating current induced on the secondary side of the welding transformer 28 is full-wave rectified by the diodes 30 and 30 and guided to the electrodes 24a and 24b, and welding starts.

  The welding current i and the welding voltage v are detected and input to the amplification multiplier 48. Here, the signals of current i and voltage v are amplified to obtain current I and voltage V, and power W, which is the product of these, is calculated. The PWM control unit 44 uses these current I, voltage V, and power W to calculate the negative feedback amount of PID control, and performs PWM control of the inverter 42 (step S106).

  The current I obtained by the amplification multiplication unit 48 and the target control pattern set by the welding condition setting unit 46 (for example, current setting pattern A in the case of constant current control) are sent to the zero cross detection unit 50, where these Deviation D is detected (step S108). This deviation D is sampled at a period sufficiently shorter than the welding time. The deviation D is small when the control is smooth (stable) and appropriate as shown in FIG. 4A, but is positive and negative when the control is non-smooth (unstable) as shown in FIG. 4B. Changes drastically.

The zero cross detector 50 detects a zero cross where the deviation D changes positively or negatively within a predetermined monitoring range a (step S110), and integrates the number N of positive / negative changes, that is, the zero cross number N (step S112). The comparison determination unit 52 compares the number of zero crossings N with the set value N ₀ stored in the memory 60 (step S114). If N> N _{0, the} comparison determination result output unit 54 outputs a sound such as a buzzer and a warning display. An alarm is displayed on the unit 56 (step S116).

In this embodiment, the deviation D detected by the zero cross detector 50 is compared with the reference value D ₀ input from the memory 60, and the absolute value | D | of the deviation D is larger than the width of the reference value D _0. to detect and became possible (that becomes excessive than the positive and negative of the width of the reference value D ₀ fluctuation range of the deviation D) (step S118), a warning is in (step S116) as. Thus, not only the number of zero crossings N but also the fluctuation range (amplitude) of the deviation D is monitored, so that it is possible to detect with high accuracy that the control has become unstable.

  As described above, if the comparison / determination unit 52 does not determine that the control is unstable, it is determined whether or not energization of the welding current necessary for welding the welded portion is completed (step S120). Returning to step 106, the above operation is repeated. If the comparison determination unit 52 determines that the control is unstable in the middle, for example, the warning display unit 56 of the controller 10 indicates that the welding is defective, and if it determines that the control is stable during the welding, indicates that welding is good ( Step S122), the welding operation is terminated (Step S124).

DESCRIPTION OF SYMBOLS 10 Controller 18 Program selection input part 28 Welding transformer 24a, 24b Welding electrode 40 Three-phase alternating current power supply 42 Inverter 44 PWM control part 46 Welding condition setting part 48 Amplification multiplication part 50 Zero cross detection part 52 Comparison determination part 54 Comparison determination result output part 56 Warning display 58 Monitoring condition input A Current setting pattern (target value pattern)
I Welding current waveform D Deviation a Monitoring range

Claims (6)

A control method for a resistance welding machine that converts a direct current output to an alternating current by an inverter and leads it to a primary side of a welding transformer, and guides a secondary side alternating current output stepped down by the welding transformer to a welding electrode through a full-wave rectifier and performs welding. In
While setting the welding conditions including the current setting pattern of the welding electrode, the inverter is PWM-controlled so that the applied voltage and the welding current of the welding electrode are negatively fed back to the inverter so as to meet the welding condition, and this welding current A control method for a resistance welding machine, wherein the stability of control is determined by integrating the number N of times the deviation between the current setting pattern and the current setting pattern is zero-crossed and comparing it with a set value N ₀ . The resistance welding machine control method according to claim 1, wherein a warning for prompting the change of the welding condition is output when the welding quality is N> N ₀ . The resistance welding machine control method according to claim 1, wherein a monitoring range in which the number of zero crossings N is integrated and the set value N ₀ can be set. The deviation D from the current setting pattern of the welding current is obtained together with the integrated value N of the number of times of zero crossing, and the stability of the control is determined because the deviation D is within the range of the set value ± D _0. Item 1. A resistance welding machine control method according to Item 1.   2. The resistance welding machine control method according to claim 1, wherein the inverter performs PID control of a negative feedback amount of a welding voltage and a welding current, and the change of the welding condition is performed by changing a weight coefficient of the PID control. A control device for a resistance welding machine that converts DC output to AC by an inverter and leads it to the primary side of the welding transformer. The secondary AC output that is stepped down by the welding transformer is guided to the welding electrode via a full-wave rectifier and welded. In
A PWM controller for controlling the output of the inverter;
An amplification multiplier that detects the applied voltage and current of the welding electrode and negatively feeds back to the PWM controller;
A program selection input unit for selecting a control program corresponding to the control mode;
A welding condition setting unit for obtaining a current setting pattern of a welding current based on the selected control program and sending it to the PWM control unit;
A monitoring condition input unit for inputting a set value N _{0 of the} number of zero crossings;
A zero cross detection unit that integrates the number of zero cross detections by comparing the welding current detected by the amplification multiplication unit with the current setting pattern;
A comparison determination unit that compares the zero-cross integrated value N with the zero-cross set value N ₀ to determine control stability;
A resistance welding machine control device comprising:

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