JP2001087867A - Control system and control method for resistance welding machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the development of the expulsion and surface flash by detecting indication of the development of the expulsion and surface flash based on time-varying rate of welding power and reducing the welding power before developing the expulsion and surface flash. SOLUTION: Voltage between electrodes is detected at a voltage detecting part 8 and the welding current is detected at a current detecting part 9. The instantaneous value of the welding power is calculated at each time interval sufficiently shorter than a PWM modulation period and also, the changing rate of the welding power is calculated, with a welding power changing rate calculating means 11. When the changing rate of the welding power exceeds the preset permissible increasing rate in a nugget growing period, this phenomenon is caught as the indication of the development of expulsion and surface flash, the supply of the welding power is immediatey stopped and also, the supplied power is reduced by reducing the duty of a PWM signal over few periods from the next PWM modulation period to prevent the development of expulsion and surface flash, with a welding power control means 13. When the changing rate of the voltage between the electrodes exceeds the permissible increasing rate, by monitoring the change of the voltage between the electrodes, the phenomenon is caught as the indication of the development of expulsion and surface flash, and the supplied power may be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control apparatus and a control method for a resistance welding machine, and more particularly, to detecting a sign of occurrence of spatter based on a rate of change of welding power with time and performing welding before spatter occurs. TECHNICAL FIELD The present invention relates to a control device and a control method for a resistance welding machine that reduce generation of dispersion by reducing electric power.

[0002]

2. Description of the Related Art In resistance welding, the occurrence of scattering has been a problem in terms of quality and safety. For this reason, various techniques for suppressing the occurrence of scattering and improving welding quality have been proposed as described below.

[0003] Japanese Patent Application Laid-Open No. 54-58655 (JP-B-5-58655)
Japanese Patent Application Laid-Open No. 7-37430) discloses a method in which a change in the thickness of a member at a welding point during an energizing time is detected, and the occurrence of spatter is detected when the change amount exceeds a reference value. A resistance point welding control method is described in which the current is decreased by a predetermined amount, and when no scattering occurs, the welding current is increased by a predetermined amount to control the welding current in accordance with the limit current value at which scattering occurs.

Japanese Unexamined Patent Publication No. Hei 6-344155 discloses that the occurrence of dispersion is predicted by comparing the voltage between electrodes with a reference value, and countermeasures are taken in accordance with the result of the prediction to suppress the occurrence of dispersion and to reduce the welding nugget diameter. A control device for a spot welder capable of performing uniform welding of the same is described. By reducing the amount of current only during a predetermined period during welding power supply, generation of scattering can be suppressed. It has been experimentally confirmed that scattering is likely to occur during this predetermined period. This predetermined period is a period in which the area of the current path between the workpieces is about to suddenly increase. By reducing the welding current during this period, the area of the welding current path is gently enlarged, thereby suppressing the occurrence of scattering. Because the presence or absence of scattering is predicted based on whether the voltage between the electrodes exceeds the reference value,
Scattering can be effectively suppressed.

Japanese Unexamined Patent Publication No. Hei 9-216070 discloses a method of estimating a risk of occurrence of scatter and an estimated time of occurrence of scatter from an energy distribution estimated using a welding current and a voltage between electrodes. A control device for a resistance welding machine is described in which the welding current is changed and controlled so as to be immediately after the passage, so that heat input is increased without generating dispersion and a nugget having the maximum strength is obtained. .

[0006] Japanese Patent Application Laid-Open No. Hei 10-94883 discloses that a welding is performed well by detecting a welding current and a voltage between chips, simulating a welded portion by heat conduction calculation, and estimating a nugget generation state. A method for controlling the welding conditions of the resistance welding machine is described.

[0007]

Problems to be Solved by the Invention Japanese Patent Application Laid-Open No. 54-5865
Japanese Patent Application Laid-Open No. 5 (1999) -1995 controls the welding current when the occurrence of scattering is detected. For this reason, when the scattering occurs, the welding current can be reduced or the supply of the welding current can be stopped, but the occurrence of the scattering cannot be prevented beforehand.

A control apparatus for a spot welding machine described in Japanese Patent Application Laid-Open No. Hei 6-344155 discloses a control method for a reference electrode which is obtained by conducting an experiment in advance to determine a voltage between electrodes which is generated when welding is performed with a current slightly smaller than a limit current for occurrence of scattering. The voltage is compared with the actually detected inter-electrode voltage, and the occurrence of dispersion is predicted based on whether the actually detected inter-electrode voltage exceeds the reference inter-electrode voltage. Since the reference electrode voltage is obtained by conducting an experiment beforehand on the electrode voltage generated when welding was performed with a current slightly smaller than the splattering limit current, the actually detected electrode voltage is the same as the reference electrode voltage. Even in the case where the occurrence of scattering does not lead to the occurrence of slight deviation, the occurrence of scattering may be predicted.

The control device for a resistance welding machine described in Japanese Patent Application Laid-Open No. 9-216070 requires a large number of arithmetic processes to estimate an energy distribution. For this reason, the configuration of the control device becomes complicated. The method of controlling welding conditions of a resistance welding machine described in Japanese Patent Application Laid-Open No. H10-94883 uses a heat conduction simulator, so that the configuration of a control device and the like is complicated.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problem, and accurately predicts the occurrence of scattering with a relatively simple configuration, and controls the amount of heat generated at a weld based on the prediction result. It is therefore an object of the present invention to provide a control device and a control method for a resistance welding machine, which can perform good welding.

[0011]

According to a first aspect of the present invention, there is provided a control device for a resistance welding machine, comprising: a current detector for detecting a welding current; a voltage detector for detecting a voltage between welding electrodes; Welding power change rate calculating means for calculating welding power based on the welding current and the voltage between welding electrodes and calculating a time rate of change of welding power, and welding power during a nugget growth period after the welding power reaches a maximum value. A standard range storage means for storing the standard range of the time change rate, and the time change rate calculated by the welding power change rate calculation means is within the standard range of the time change rate of the welding power stored in the standard range storage means. Welding power control means for controlling the welding power.

According to a first aspect of the present invention, there is provided a control device for a resistance welding machine.
The temporal change rate of the welding power during the nugget growth period is monitored, and the welding power is controlled so that the temporal change rate of the welding power is within a preset standard range. By monitoring the time rate of change of the welding power, a sign of a change in the welding power can be quickly detected. This makes it possible to reduce the welding power before the welding power becomes too large during the nugget growth period to cause the dispersion, thereby preventing the occurrence of the dispersion. In addition, it is possible to prevent the welding power during the nugget growth period from becoming too small and the welding quality from being lowered.

According to a second aspect of the present invention, there is provided a control device for a resistance welding machine.
A current detector for detecting a welding current, a voltage detector for detecting a voltage between welding electrodes, and welding power for calculating welding power based on the welding current and the voltage between welding electrodes and calculating a time rate of change of welding power. Rate-of-change calculation means, and when the time rate of change of the welding power exceeds a preset rate of increase during the nugget growth period after the welding power reaches the maximum value, the welding power is judged as a sign of occurrence and the welding power is determined. And welding power control means for reducing the welding power.

According to a second aspect of the present invention, there is provided a control device for a resistance welding machine.
The rate of change of the welding power over time during the nugget growth period is monitored, and when the rate of change of the welding power over time exceeds a preset rate of increase, it is judged as a sign of occurrence and the welding power is reduced. Therefore, occurrence of scattering can be prevented beforehand.

According to a third aspect of the present invention, there is provided a control device for a resistance welding machine.
A voltage detecting unit for detecting a voltage between welding electrodes, an inter-electrode voltage change rate calculating means for calculating a time change rate of the voltage between welding electrodes, and a time-dependent change rate of the voltage between welding electrodes during a nugget growth period, which is set in advance. And welding power control means for reducing the welding power by judging the occurrence as a sign of occurrence when exceeding.

A control device for a resistance welding machine according to claim 3 is
The time change rate of the voltage between the welding electrodes during the nugget growth period is monitored, and when the time change rate of the voltage between the welding electrodes exceeds a predetermined rate of increase, it is judged as a sign of occurrence and the welding power is reduced. . Therefore, occurrence of scattering can be prevented beforehand.

According to a fourth aspect of the present invention, there is provided a method for controlling a resistance welding machine.
During the nugget growth period, the time rate of change of the welding power is monitored, and if the time rate of change of the welding power exceeds a preset allowable increase rate, the supply of welding power is immediately stopped, and the subsequent supply of welding power is performed. The amount is reduced over a predetermined period.

According to a fourth aspect of the present invention, there is provided a method for controlling a resistance welding machine.
The temporal rate of change of the welding power during the nugget growth period is monitored, and when the temporal rate of change of the welding power exceeds a predetermined allowable increase rate, it is determined as a sign of occurrence of scattering and the welding power is reduced. Therefore, occurrence of scattering can be prevented beforehand.

According to a fifth aspect of the present invention, there is provided a method for controlling a resistance welding machine.
During the nugget growth period, the time rate of change of the voltage between the welding electrodes is monitored, and when the time rate of change of the voltage between the welding electrodes exceeds a preset allowable increase rate, the supply of welding power is immediately stopped, and thereafter, It is characterized in that the supply amount of welding power is reduced over a predetermined period.

According to a fifth aspect of the present invention, there is provided a method for controlling a resistance welding machine.
Monitors the rate of change of welding electrode voltage over time during the nugget growth period.If the rate of change of welding electrode voltage over time exceeds a preset allowable increase rate, it is judged as a sign of occurrence and the welding power is reduced. Let it. Therefore, occurrence of scattering can be prevented beforehand.

[0021]

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

FIG. 1 is a block diagram of a control device for a resistance welding machine according to the present invention. The control device 1 for a resistance welding machine according to the present invention includes a three-phase AC power source 2 and a three-phase full-wave rectifier 3.
, An inverter unit 4, a step-down transformer 5, and a rectifying unit 6
, A pair of welding electrodes 7 a and 7 b, a voltage detector 8, a current detector 9, and a controller 10. The control unit 10
Welding power change rate calculating means 11 and standard range storing means 12
Welding power control means 13 and inverter drive circuit 14
And Reference numeral 30 denotes a workpiece, and reference numeral 31 denotes a nugget (melting portion).

The three-phase full-wave rectifier 3 performs full-wave rectification on the three-phase AC supplied from the three-phase AC power supply 2 to generate a DC power supply. The generated DC power is supplied to the inverter unit 4. The inverter unit 4 converts a DC power supply into a high-frequency AC and drives a primary winding of the step-down transformer 5. The inverter unit 4 includes four H-bridge connected power semiconductor switching elements (for example, IGBTs and the like). The inverter unit 4 includes an inverter driving circuit 14 in the control unit 10.
The power semiconductor switching elements are switched in a predetermined order on the basis of a plurality of drive signals 10a supplied from the power supply, thereby generating a high-frequency AC. The welding power control means 13 in the control unit 10 controls the output power of the inverter 4 by changing the duty of the PWM signal 13a supplied to the inverter driving circuit 14, thereby controlling the step-down transformer 5, the rectifying unit 6, the welding power The welding power to be supplied to the welded portion 30 via 7a and 7b is controlled. A low-frequency high-frequency alternating current is induced in the secondary winding of the step-down transformer 5. The low-frequency high-frequency alternating current is converted to direct current by the rectifier 6. The rectifying unit 6 includes two rectifying diodes 6
a, 6b. The pulsating flow generated by the rectification unit 6 is supplied to the welded part 30 via the welding electrodes 7a and 7b.

The voltage detector 8 includes a pair of welding electrodes 7a, 7
It detects the voltage between b and outputs an electric signal (voltage detection signal) 8a corresponding to the detected voltage. This voltage detection signal 8
a is supplied to the welding power change rate calculating means 11 in the control unit 10.

The current detector 9 detects a welding current, and an electric signal (current detection signal) 9a corresponding to the detected welding current.
Is output. The current detection signal 9a is supplied to the welding power change rate calculating means 11 in the control unit 10.

The welding power change rate calculating means 11 calculates the welding power instantaneous value 11a at preset time intervals based on the voltage detection signal 8a and the current detection signal 9a, and calculates the welding power instantaneous value over time. The rate 11b is calculated.
The calculated instantaneous value 11a of the welding power and the time rate of change 11b of the instantaneous value of the welding power are supplied to the welding power control means 13.

The standard range storage means 12 stores a standard range of the time rate of change of the welding power during the nugget growth period after the welding power reaches the maximum value. The standard range of the time rate of change of the welding power is obtained by presetting an upper limit and a lower limit every time the nugget growth period elapses.

The welding power control means 13 generates a PWM signal 13a based on the command value and generates a PWM signal 13a.
Supply of welding power is started by supplying the welding power integrated value for each PWM modulation cycle or the maximum value of the welding power instantaneous value 11a for each PWM modulation cycle based on the welding power instantaneous value 11a. Ask.

The welding power control means 13 monitors a change in the integrated value of the welding power for each PWM modulation cycle or the maximum value of the instantaneous value 11a of the welding power for each PWM modulation cycle.
The maximum value of the welding power integral value for each modulation cycle or the instantaneous value 11a of the welding power for each PWM modulation cycle is the maximum value (maximum point)
, It is recognized that the temperature rising period shown in FIG. 2 has ended and a falling period (nugget growth period) has entered. The falling period (nugget growth period) is a heating suppression period due to the autonomous action of welding.

When the descent period (nugget growth period) starts, the welding power control means 13 sets the welding power integrated value in the previous PWM modulation cycle or the maximum value of the welding power instantaneous value 11a,
A change rate of the welding power integrated value or the maximum value of the welding power instantaneous value 11a in the current PWM modulation cycle is calculated. Then, the calculated time change rate of the welding power for each PWM modulation cycle is compared with the standard range. If the time change rate of the welding power for each PWM modulation cycle exceeds the upper limit of the standard range, the PWM signal is output. The welding power is reduced by reducing the duty of the welding power 13a, and when the time rate of change of the welding power for each PWM modulation cycle exceeds the lower limit of the standard range, the welding power is reduced.
The welding power is increased by increasing the duty of the WM signal 13a. Thereby, the control is performed such that the time rate of change of the welding power for each PWM modulation cycle is within a preset standard range.

Further, when the time change rate 11b of the instantaneous value of the welding power exceeds the preset detection value of the occurrence sign of the scattering, the welding power control means 13 outputs the current P value.
The output of the WM signal 13a is immediately stopped, and the PWM signal 1a is output for several periods from the next PWM modulation period.
The duty of 3a is changed to an extremely small value. By detecting the sign of the occurrence of the scattering and reducing the supply power, the occurrence of the scattering can be prevented beforehand.

FIG. 2 is a graph showing the change characteristics of the voltage between electrodes, the change rate of welding power, and the change characteristics of power consumption during welding. In FIG. 2, a time from time 0 to time t0 is a preliminary energization period. In the pre-energization period, the workpiece is heated, the contact resistance disappears, and the voltage between the electrodes drops. The length of the pre-energization period can be statistically obtained. Time t0 to time tp is a temperature rise period. During this temperature rise period, the workpiece is heated and the temperature rises. The peak time tp can be obtained based on the maximum point of the welding power. The period from time tp to time te is a falling period. After the peak time tp, the voltage between the electrodes decreases due to the autonomous action of welding. This falling period is a period during which the nugget grows.
Time te is an energization end time. This energization end time te
Thereafter, as a termination process, the energizing current is gradually reduced. FIG.
The area indicated by hatching in (b) is the standard range of the welding power change rate.

FIG. 3 is an explanatory diagram showing an operation of detecting a sign of occurrence of scattering to reduce welding power. 3A shows a change characteristic of the welding power, FIG. 3B shows a welding power change rate (welding power differential value), and FIG. 3C shows a switching pulse (supply power). Scattering is known to occur during the falling period (nugget growth period). FIG. 3 (a)
As shown in (1), the welding power increases at the time of occurrence of scattering. This increase in welding power is detected as a change in the welding power change rate. That is, as shown in FIG. 3 (b), the welding power change rate changes abruptly before the occurrence of scattering. This rapid change in the welding power change rate is regarded as a sign of occurrence of scattering, and the supplied power is suppressed as shown in FIG. Thereby, the occurrence of scattering can be prevented beforehand.

FIG. 4 is a welding power waveform diagram showing an example of a power waveform at the time of welding when the supply of welding power is controlled by an AC resistance welding machine. In FIG. 4, the horizontal axis represents time, and the vertical axis represents welding power. Here, an example is shown in which scattering occurs at two points (a) and (b) during the falling period (nugget growth period). 5 and 6 are welding power waveform diagrams in which the time axis of the welding power waveform shown in FIG. 4 is enlarged. 7 and 8 are power waveform diagrams showing power waveforms before and after the occurrence of scattering.

The welding power change rate calculating means 11 shown in FIG.
Has a D / A converter for D / A conversion of the inter-electrode voltage 8a, and a D / A converter for D / A conversion of the welding current 9a. The welding power change rate calculating means 11 calculates P
The inter-electrode voltage 8a and the welding current 9a are sampled at the same timing using the above two D / A converters in a short cycle of, for example, 1/100 or more of the WM modulation cycle, and the respective D / A conversion data Is sequentially repeated. Then, the instantaneous value 11a of the welding power is calculated by multiplying the inter-electrode voltage and the welding current based on the inter-electrode voltage data and the welding current data.
a is sequentially supplied to the welding power control means 13. The welding power change rate calculating means 11 calculates the change rate between the instantaneous value of the welding power calculated based on the previous sampling and the instantaneous value of the welding power calculated based on the current sampling, and calculates the calculated instantaneous change rate 11b. Are sequentially supplied to the welding power control means 13.

The welding power control means 13 integrates the instantaneous values 11a of the welding power sequentially supplied in a short cycle of, for example, 1/100 or more of the PWM modulation cycle over one PWM modulation cycle, thereby obtaining each PWM. The welding power for each modulation cycle is determined.

As shown in FIG. 2C, during the temperature rise period, the welding power continues to increase, and from the time when the welding power exceeds the maximum value (maximum value), the falling period (nugget growth period) starts. The welding power control means 13 recognizes that it is in the temperature rise period based on the fact that the welding power for each PWM modulation cycle continues to increase. Welding power control means 13
Is the current PW with respect to the welding power of the previous PWM modulation cycle.
It is recognized that the falling period (nugget growth period) has entered based on the fact that the welding power in the M modulation cycle is equal or decreased.

When the welding power control means 13 recognizes that the descent period (nugget growth period) has entered, the previous PWM control
The calculation of the rate of change between the welding power in the modulation cycle and the welding power in the current PWM modulation cycle is repeated for each PWM modulation cycle. The welding power control means 13 compares the calculated change rate of the welding power with the change rate set as a standard range.
If the calculated change rate of the welding power exceeds the upper limit of the standard range, the welding power control means 13 reduces the duty of the PWM signal 13a generated in the next PWM modulation cycle by a predetermined value. I do. Thereby, the welding power in the next PWM modulation cycle is reduced. If the calculated change rate of the welding power exceeds the lower limit of the standard range, the welding power control means 13 increases the duty of the PWM signal 13a generated in the next PWM modulation cycle by a predetermined value. I do. As a result, the next PWM
The modulation period welding power is increased.

The welding power control means 13 calculates the maximum value of the instantaneous value of the welding power in each PWM modulation cycle without obtaining the welding power for each PWM modulation cycle, and based on the change in the maximum value, the falling period ( (Nugget growth period) may be recognized. The welding power control means 13 includes:
The welding power may be controlled by calculating the change rate of the maximum value of the instantaneous value of the welding power in each PWM modulation cycle, and comparing the calculated change rate with a standard range.

When recognizing that the descent period (nugget growth period) has entered, the welding power control means 13 recognizes the instantaneous value of the welding power calculated by the previous sampling and the instantaneous value of the welding power calculated by the current sampling. If the rate of change (instantaneous rate of change) exceeds a preset rate of increase (permissible rate of increase), it is regarded as a sign of occurrence of scattering and the output of the PWM signal 13a that is currently output is immediately output. Stop. The welding power control means 13 outputs the PWM signal 1 over a period of several cycles from the PWM modulation cycle following the PWM modulation cycle in which the output of the PWM signal 13a is forcibly stopped.
The duty of 3a is changed to an extremely small value. By reducing the supplied power over a predetermined period after the detection of the sign of the occurrence of the scattering, the occurrence of the scattering is prevented.

As shown in FIGS. 7 and 8, when the scatter occurs, a sharp increase in the welding power occurs as a sign of the scatter. Therefore, by monitoring the rapid increase in welding power, the sign (A) shown in FIG. 7 and the sign (B) shown in FIG.
The welding power supply is immediately stopped when the signs (A) and (B) are detected, so that the sign (A) and the signs (B) can be detected for about 1 to 2 milliseconds. It is possible to prevent the occurrence of scattering that occurs later.

FIG. 9 is a flowchart showing the operation of the control device 1 of the resistance welding machine shown in FIG. Step S1
After the welding conditions are set in accordance with the material, plate thickness, etc. of the workpiece, energization is started in step S2. In step S3, it is determined whether or not the elapsed time t from the start of energization has reached the preliminary energization time (preliminary energization end time) t0.

When the pre-energization period ends, the temperature rise period starts. During this temperature rise period, in step S4, each PWM
The welding power for each modulation cycle is calculated, and it is determined in step S5 whether the welding power has increased. If the welding power is increasing, it is checked in step S6 whether or not the power supply end time te has been reached. If the welding power continues to increase even after the power supply end time te has been reached, it is determined in step S7 that the nugget generation is defective, and step S8 is performed.
, And abnormal processing such as displaying that an abnormality has occurred is performed.

When it is detected in step S5 that the temperature rise period has ended, the processing in the nugget growth period after step S9 is started. In step S9, it is determined whether or not the instantaneous change rate of the welding power exceeds the allowable increase rate. If the instantaneous change rate of the welding power exceeds the allowable increase rate, a scattering prevention process is performed in step S10. In this scattering prevention processing, the output of the currently output PWM signal 13a is immediately stopped, and the PWM signal 13a is output for several cycles from the next PWM modulation cycle.
By setting the duty of the welding power to an extremely small value, the supply amount of the welding power is significantly reduced over a predetermined time from the time when the instantaneous change rate of the welding power exceeds the allowable increase rate. This prevents the occurrence of scattering.

If the instantaneous change rate of the welding power does not exceed the allowable increase rate, the welding power is integrated in step S11. Then, when it is detected in step S12 that one cycle of the PWM modulation is completed, in step S13, the previous PWM
It is determined whether the rate of change between the welding power in the M modulation cycle and the welding power in the current PWM modulation cycle exceeds the upper limit of the standard range, and in step S14, the welding power in the previous PWM modulation cycle and the current PWM are determined. It is determined whether or not the rate of change from the welding power in the modulation cycle exceeds the lower limit of the standard range.

If the rate of change from the welding power exceeds the upper limit of the standard range, the supply power is reduced in step S15. In this supply power reduction processing, the duty of the PWM signal 13a in the next PWM modulation cycle is set to a value smaller than the previous duty by a predetermined amount. Thereby, the welding power supplied in the next PWM modulation cycle is reduced.

If the rate of change from the welding power exceeds the lower limit of the standard range, the supply power is increased in step S16. In the supply power increasing process, the duty of the PWM signal 13a in the next PWM modulation cycle is set to a value larger than the previous duty by a predetermined amount. Thus, the welding power supplied in the next PWM modulation cycle is increased.

At step S17, it is checked whether or not the power supply end time te has been reached, and the processing from step S9 is repeated until the power supply end time te is reached. When the power supply end time te is reached, a normal end process of stopping the supply of the welding power while gradually reducing the welding power is performed in step S18, and a series of welding controls is ended.

FIG. 10 is a block diagram of a control device of another resistance welding machine according to the present invention. The control device 50 of the resistance welding machine shown in FIG. 10 differs from that shown in FIG. The control unit 60 shown in FIG. 10 includes a welding power integrating unit 61 that calculates and outputs a welding power integrated value 61a for each PWM modulation cycle based on the signal 8a related to the inter-electrode voltage and the signal 9a related to the welding current. , The instantaneous change rate of the inter-electrode voltage is calculated in a cycle sufficiently shorter than the PWM modulation cycle, for example, about 1/100 of the PWM modulation cycle, and the calculated inter-electrode voltage change rate 62a is output. It includes a rate calculating means 62, a welding power control means 63, and the inverter drive circuit 14.

When the welding power control means 63 recognizes that the nugget growth period has started by monitoring the change of the welding power integrated value 61a for each PWM modulation cycle, it monitors the instantaneous change rate 62a of the voltage between the electrodes. If the instantaneous change rate 62a of the inter-electrode voltage exceeds the preset allowable increase rate, the output of the PWM signal 13a is immediately stopped to stop the supply of the welding power, and the number of pulses from the next PWM modulation cycle is reduced. By setting the duty of the PWM signal 13a to an extremely small value over the period, the instantaneous change rate
The supply amount of welding power is greatly reduced until a predetermined time elapses from the time when 2a exceeds a preset allowable increase rate.

FIG. 11 is a waveform diagram showing a change in inter-electrode voltage when scattering occurs. As shown in FIG. 11, the voltage between the electrodes rapidly increases immediately before the occurrence of the scattering. By catching the sudden increase in the inter-electrode voltage as a sign of occurrence, the supply of welding power is immediately stopped when the inter-electrode voltage suddenly increases.
Scattering can be prevented from occurring.

[0052]

As described above, the control device of the resistance welding machine according to the first aspect monitors the time change rate of the welding power during the nugget growth period, and the time change rate of the welding power falls within a predetermined standard range. Therefore, the welding power can be reduced before the welding power becomes excessive during the nugget growth period and the scattering occurs, and the occurrence of the scattering can be prevented beforehand. In addition, it is possible to prevent the welding power during the nugget growth period from becoming too small and the welding quality from being lowered.

A control device for a resistance welding machine according to claim 2 is
Since the time rate of change of the welding power during the nugget growth period is monitored, and when the time rate of change of the welding power exceeds a preset rate of increase, it is determined to be a sign of occurrence and the welding power is reduced. The occurrence of scattering can be prevented beforehand.

A control device for a resistance welding machine according to claim 3 is
The time change rate of the voltage between the welding electrodes during the nugget growth period is monitored, and when the time change rate of the voltage between the welding electrodes exceeds a predetermined rate of increase, it is judged as a sign of occurrence and the welding power is reduced. With the configuration, it is possible to prevent the occurrence of scattering.

According to a fourth aspect of the present invention, there is provided a method for controlling a resistance welding machine.
The time rate of change of the welding power during the nugget growth period is monitored, and when the time rate of change of the welding power exceeds a preset allowable increase rate, it is judged as a sign of occurrence of spatter and the welding power is reduced. Can be prevented from occurring.

According to a fifth aspect of the present invention, there is provided a method for controlling a resistance welding machine.
Monitors the rate of change of welding electrode voltage over time during the nugget growth period.If the rate of change of welding electrode voltage over time exceeds a preset allowable increase rate, it is judged as a sign of occurrence and the welding power is reduced. Therefore, occurrence of scattering can be prevented beforehand.

[Brief description of the drawings]

FIG. 1 is a block diagram of a control device for a resistance welding machine according to the present invention.

FIG. 2 is a graph showing a change characteristic of a voltage between electrodes during welding, a change rate of welding power, and a change characteristic of power consumption.

FIG. 3 is an explanatory diagram showing an operation of detecting a sign of occurrence of scattering to reduce welding power.

FIG. 4 is a welding power waveform diagram showing an example of a power waveform at the time of welding when the supply of welding power is controlled by an AC resistance welding machine.

5 is a welding power waveform diagram in which the time axis of the welding power waveform shown in FIG. 4 is enlarged.

6 is a welding power waveform diagram in which the time axis of the welding power waveform shown in FIG. 4 is enlarged.

FIG. 7 is a power waveform diagram showing power waveforms before and after the occurrence of scattering.

FIG. 8 is a power waveform diagram showing power waveforms before and after the occurrence of scattering.

FIG. 9 is a flowchart showing an operation of the control device of the resistance welding machine shown in FIG. 1;

FIG. 10 is a block diagram of a control device of another resistance welding machine according to the present invention.

FIG. 11 is a waveform diagram showing a change in inter-electrode voltage when scattering occurs.

[Explanation of symbols]

 Reference Signs List 1,50 Resistance welding machine control device 4 Inverter unit 5 Step-down transformer 6 Rectifying unit 7a, 7b Welding electrode 8 Voltage detection unit 9 Current detection unit 10, 60 Control unit 11 Welding power change rate calculation unit 12 Standard range storage unit 13, 63 Welding power control means 14 Inverter drive circuit 30 Workpiece 31 Nugget (melting part) 61 Welding power integrating means 62 Electrode voltage change rate calculating means

Claims (5)

[Claims] 1. A current detector for detecting a welding current, a voltage detector for detecting a voltage between welding electrodes, a welding power is calculated based on the welding current and the voltage between the welding electrodes, and a time of the welding power is calculated. Welding power change rate calculating means for calculating the change rate; standard range storage means for storing a standard range of the time change rate of welding power during the nugget growth period; and the time change rate calculated by the welding power change rate calculating means is A control device for a resistance welding machine, comprising: welding power control means for controlling welding power so as to fall within a standard range of a time rate of change of welding power stored in a standard range storage means. 2. A current detector for detecting a welding current, a voltage detector for detecting a voltage between welding electrodes, a welding power is calculated based on the welding current and the voltage between welding electrodes, and a time of the welding power is calculated. A welding power change rate calculating means for calculating a change rate, and when the time change rate of the welding power exceeds a preset increase rate during the nugget growth period, the welding power is reduced as a sign of occurrence and the welding power is reduced. A control device for a resistance welding machine, comprising: welding power control means. A voltage detecting unit for detecting a voltage between the welding electrodes; a voltage change rate calculating means for calculating a time change rate of the voltage between the welding electrodes; a time change of the voltage between the welding electrodes during a nugget growth period. A control device for a resistance welding machine, comprising: welding power control means for reducing the welding power by judging the occurrence as a sign of occurrence when the rate exceeds a preset increase rate. 4. A time change rate of the welding power during the nugget growth period is monitored, and when the time change rate of the welding power exceeds a preset allowable increase rate, the supply of the welding power is immediately stopped, A method for controlling a resistance welding machine, characterized in that a supply amount of a subsequent welding power is reduced over a predetermined period. 5. A method for monitoring a time change rate of a voltage between welding electrodes during a nugget growth period, and when the time change rate of a voltage between welding electrodes exceeds a predetermined allowable increase rate, immediately supplies welding power. A method for controlling a resistance welding machine, comprising: stopping and reducing a supply amount of welding power thereafter over a predetermined period.