JPH01241385A - Electrification control method for resistance welding machine - Google Patents

Abstract

PURPOSE:To surely stop electrification at the early stage of the occurrence of expulsion and surface flash by connecting electrodes to the secondary side of a welding transformer via a rectifier circuit and connecting the primary side of the transformer to an AC power source via an inverter and a rectifier circuit. CONSTITUTION:The secondary side of the welding transformer 1 is connected to a couple of electrodes 3 and 3 via the rectifier circuit 2. The primary side of the welding transformer 1 is connected to a three-phase AC power source terminal 8 via the inverter consisting of four power transistors 51, 52, 53 and 54, a filter circuit 6 and the rectifier circuit 7. The electrification pulse width at the primary side 1 is then changed according to the pulse width of a control pulse added to these power transistors 51-54 to control a welding current and the control pulse is allowed to rise when the occurrence of the expulsion and surface flash is detected and the electrification is stopped immediately.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a method for controlling energization in a resistance welding machine.

(Conventional technology) In resistance welding, a nugget is formed in the part of the work sandwiched between electrodes due to heat generation due to energization, but if the current is continued after that, the electrode pressure force cannot suppress the thermal expansion force of the nugget. The strength of the weld reaches its maximum when the current is stopped just before the weld metal ejects, but it is not possible to know in advance when the weld metal will occur. Therefore, conventionally, as known from Japanese Patent Publication No. 57-37430, energization is carried out at a constant energization time, and when expulsion occurs, the welding current is reduced by a predetermined amount when welding at the next welding point. , or JP-A-58-4757
As seen in Japanese Patent No. 9 and Japanese Patent Application Laid-Open No. 511-47580, the electricity supply is stopped at the point where scattering occurs.

In the above method, parameters such as the distance between the electrodes, the resistance value, the current value, and the electrode pressing force that change due to scattering are detected, and the occurrence of dust is detected from changes in the detected values.

(Problem 8 to be Solved by the Invention) The technique disclosed in Japanese Patent Publication No. 57-37430 is a measure taken after expulsion occurs, and cannot guarantee the welding strength of the welding point where expulsion occurs.

Also, JP-A-58-47579 and JP-A-58-47
The one described in Publication No. 580 is a normal AC welding machine that supplies power from a commercial power source to a welding transformer via a contactor, and the waveform of the primary current is as shown in Figure 9, where □ Since the contactor is generally composed of a thyristor, the current cannot be cut during energization, and
Even if dissipation occurs at time , energization continues until time t2, when half-cycle energization ends, and the time lag from the occurrence of dissipation until energization stops is at most 10 milliseconds when the power supply frequency is 50 fiz. Become.

By the way, the inventor of this application conducted an experiment using a high-speed camera, and found that the scattering speed of the particles was 200 b/h (5.5 cm/h).
”+) seconds) was confirmed.

As mentioned above, if there is a time lag of 10 milliseconds, the occurrence of expulsion cannot be suppressed in the initial stage, resulting in a decrease in welding strength.

In view of the above points, an object of the present invention is to provide an energization control method that reliably stops energization at the initial stage when expulsion occurs, thereby achieving good welding. do.

(Means for Solving the Problem) In order to achieve the above object, the present invention detects a parameter whose value changes due to the occurrence of scattering when a workpiece is sandwiched between a pair of electrodes and energized, and detects a parameter whose value changes due to the occurrence of scattering. In a device that detects the occurrence of dissipation from a change in value and stops energization, the electrode is connected to the secondary side of a welding transformer via a rectifier circuit, and the primary side of the transformer is connected to a power transistor. The welding current is connected to an AC power supply terminal through an inverter and a rectifier circuit, and the welding current is controlled by changing the energization pulse width on the primary side according to the pulse width of the control pulse applied to the inverter. The present invention is characterized in that the control pulse is raised and the energization is immediately stopped when the occurrence of is detected.

In this case, it is desirable to gradually increase the welding current after starting to apply the welding current.

(Function) The power transistor of the inverter is turned on and off at the rising and falling edges of the control pulse, thereby changing the energization pulse width on the primary side of the welding transformer in accordance with the control pulse. Then, when the control pulse falls due to the occurrence of expulsion, the power transistor is instantly turned off, and the current supply is reliably stopped at the initial stage of occurrence of expulsion.

By the way, the welding current value required to form a nugget changes depending on the type of workpiece, plate thickness, etc., so for example, when welding points with different plate thicknesses with a constant current, it is necessary to set the current value for each point. In contrast, if the welding current is gradually increased, there is no need to set the current value again. Since the energization is automatically stopped when scattering occurs, there is no need to set the energization time, making the setting work extremely simple.

(Example) Referring to FIG. 1, (1) shows a welding transformer, and the secondary side of the transformer (1) is connected to a pair of electrodes (3) through a rectifier circuit (2). Connect the controller (
The workpiece W is moved between the electrodes (3) by the pressurizing cylinder (4) operated by the pneumatic servo circuit
The workpiece W was spot welded by passing a direct current welding current between the image electrodes (3) (3) while holding and pressurizing the workpieces W.

The primary side of the transformer (1) includes an inverter (5) consisting of four power transistors (51), (52), (53), and (54), a filter circuit (6), and a rectifier circuit (7). These power transistors (51) to (54) are controlled to be turned on and off by a pulse width control circuit (9) via a base drive circuit (IO).

In the figure (l'D is the controller of the resistance welding machine consisting of a microcomputer, a is the current detection circuit that detects the effective current value by the signal from the current detection element (12a), 113 is the current setting device, (IO is the current A9 is a comparator that compares the outputs of the detection circuit (13 and current setter 03) and outputs a low level signal until the effective current value reaches the set current value, and outputs a high level signal when the two match. High frequency e.g. 16
00 indicates an oscillator that generates a clock pulse of I2,
These controllers (signals from Iv and comparator (IΦ
A signal from the oscillator (+51) and a clock pulse from the oscillator (+51) are input to a pulse width control circuit (9), and the circuit (9) outputs a control pulse as described later.

The details of the pulse width control circuit (9) are as shown in Fig. 2, and include an AND gate ('
AND gate a71 that inputs lG and clock pulse
and the output of both AND gates aea7) is connected to the O1i gate (
Flip-flop a! In addition, a flip-flop ■ that manually generates a clock pulse, a monostable multivibrator 0 whose output at the same terminal becomes low level for a short time due to the manual input of the clock pulse, and two AND gates ■■ on the output side are installed. Input the signals from the Q terminal of the flip-flop (+9), the same terminal of the monostable multivibrator ■, and the controller (+1) into both ND gates ■■, and input the signals from the Q terminal of the flip-flop ■ to one AND gate ■■. The signal and the signal at the same terminal of the flip-flop (2) were input to the other AND gate (2).

Thus, until the effective current value reaches the set current value, AN
A low level signal is input from the comparator (I41) to the D gate ae, and a high level signal is continuously output from the Q terminal of the flip-flop marked a, while the flip-flop ■
The Q terminal of
Since the output of the same terminal of I) becomes low level for a short time every time a clock pulse is input, high level control pulses are outputted alternately from the AND gate ■■ with a short pause period synchronized with the clock pulse. The control pulse from one AND gate ■ turns on the power transistor (5+) (52), and the control pulse from the other AND gate turns on the power transistor (53) (5A), which turns on the welding transformer (1). A rectangular alternating current pulse as shown in FIG. 8 is applied to the primary coil.

When the effective current value matches the set current value, a high level signal is input from the comparator (IΦ to the AND gate (161), and the output of the Q terminal of the flip-flop a9 becomes low level. The control pulse falls before the input is input, and the pulse width of the control pulse decreases. Accordingly, the energization pulse width T on the primary side of the welding transformer (1) also decreases, and the welding current reaches the set current value. controlled to be equal.

Then, a setting signal having the characteristics shown in FIG. 7 was outputted from the current setting device (13), and the welding current was gradually increased according to the characteristics after the welding current started being applied.

The controller (11) includes a signal from an opening detector (1) consisting of an optical distance sensor that detects the opening between the electrodes (3) (3) from the displacement of the piston of the pressurizing cylinder (4);
Electrode (3) (3) Voltage and current between electrodes (3)
(3) The signal from the resistance detection circuit ■ that detects the resistance value between electrodes (3) (3) is input manually, and the signal from the opening detector QΦ
) is confirmed to be closed to a predetermined pressurized opening degree, output a high-level energization command signal from the controller (l'D) to the AND gate ■■ to start energization, In addition, the amount of change ΔR of the resistance value detected by the resistance detection circuit ■ per unit time is calculated by the controller (IT CPU), and when it exceeds the specified value, the controller (
A low-level energization stop signal was output from 'l'D to the AND gate ■■, and the control pulse was lowered to stop the energization.

The a-line in Figure 4 shows the electric current W (3
) (3) The resistance value decreases due to the decrease in contact resistance at the initial stage of energization, then the resistance value gradually increases due to heat generation, and the resistance value increases due to the occurrence of dispersion after nuggets are formed. Declines rapidly.

In addition, if the workpiece is contaminated or the electrode (3) is unevenly touched, the electrode (3)
When the contact area between the electrode (3) and the workpiece becomes small, local heating of the workpiece causes scattering at the initial stage of energization, and the resistance value change characteristic becomes as shown by line b in Figure 4.Also, when the electrode (3) is excessively worn, As the current density decreases, nuggets are no longer formed well, and as a result, no scattering occurs, and the resistance value change characteristic becomes as shown by line a in FIG. 4.

Figure 3 shows a program for the controller (11). First, the resistance value is read and stored, and the amount of change ΔR per unit time is determined from the deviation between the resistance value read a predetermined unit time ago and the resistance value read this time. (■), then determine whether ΔR is greater than or equal to the set value α (■), and if ΔR<a, the elapsed time t from the start of energization is the predetermined maximum energization time t l1a
It is determined whether or not x has been reached (■), and if t<taax, the process returns to step (■) and the above determination process is repeated.

Note that the unit time is n of the cycle time of this discrimination process.
(integer) times the resistance value read n times before in step ■. A) Read from I and calculate ΔR. If the unit time is set to about 100 microseconds, ΔR will be large enough to be determined only when expulsion occurs, and ΔR will be almost zero when the resistance value changes other than when expulsion occurs, without causing malfunction. The occurrence of scattering can be detected with good responsiveness.

When ΔR≧α due to the occurrence of scattering, it is determined whether the elapsed time t exceeds a predetermined minimum energization time tllljn (■). When t>win and ΔR>α, it is when good welding is performed as shown by line a in FIG. 4. In this case, proceed to step (3) and output a energization stop signal. According to this, the control pulse output from the pulse width control circuit (9) falls and becomes ΔR>α at t in FIG.
Power is immediately cut off at this point.

In addition, initial dispersion occurs as shown by line b in Figure 4, and t<tIli
When ΔR>α at n, the process proceeds to step (2) to output a energization stop signal, and then proceeds to step (2) to activate an alarm.

Also, if ∆R≧α does not hold even after the maximum energization time ttnax as shown by line a in Fig. 4, proceed from step ① to ②.
Proceed to step (2) to output the energization stop signal, then proceed to step (2) to output the shaping command signal, move the resistance welding machine to the location where the shaping device is placed by the action of the robot to which it is attached, and attach the electrode ( 3) Format.

In addition, since the welding current is gradually increased as described above, even if the type of workpiece or plate thickness changes, a nugget is reliably formed as long as the electrode (3) is not excessively worn, and tstn and tma
If x is set with some leeway so that the nugget formation times of various workpieces are included, welding can be performed satisfactorily without setting the welding current and welding time for each workpiece.

Further, in the above embodiment, the resistance value is used as a parameter for detecting the occurrence of expulsion, but it is also possible to use the distance between the electrodes (3), that is, the opening degree of the electrodes, as a parameter.

The electrode opening degree gradually increases due to expansion due to heat generation of the workpiece, and rapidly decreases due to the occurrence of expulsion, and its change characteristics are as shown in Figure 6, line a, electrode (3) when welding is well performed. The line shown in the figure shows the line when initial scattering occurs due to poor contact, and the line C shows the line C in the figure when excessive wear of the electrode (3) occurs.

Fig. 5 shows a control program when the electrode opening degree θ is used as a parameter; in step (■), the electrode opening degree θ detected by the opening sensor Q@ is read and stored; Calculate the opening change amount △θ per unit time from the deviation between the electrode opening read previously and the electrode opening read this time, and check whether 80 becomes greater than the set value β in step ■. Determine. The determination processing from step (2) onward is the same as that in FIG.

Incidentally, in FIG. 5, a program up to the start of energization has been added, so this will be explained.

After the start of pressurization, read the electrode opening θ and determine whether θ has decreased to the normal pressurization opening θ, which corresponds to the thickness of the workpiece at the dot position.0, @), θ>θ, When 0), the pressurizing force is increased via the pneumatic servo circuit (2), and when θ≦θ, energization is started.

According to this, energization is performed only when the workpieces are brought into close contact with each other under pressure, and it is possible to prevent problems such as holes in the workpieces due to energization in a state of poor adhesion.

If the electrode opening degree is used as a scattering detection parameter as in this embodiment, a separate detection means such as the resistance detection circuit (2) becomes unnecessary, and costs can be reduced.

(Effects of the Invention) As is clear from the above description, according to the invention of claim 1, it is possible to reliably stop energization at the initial stage of occurrence of expulsion, and it is possible to automatically perform good welding. According to the second invention, it becomes easy to set welding conditions, which has the effect of further improving productivity.

[Brief explanation of the drawing]

Fig. 1 is a block circuit diagram of an example of an energization control device used to carry out the method of the present invention, Fig. 2 is a block circuit diagram of an example of a pulse width control circuit that generates control pulses, and Fig. 3 is a controller program. Figure 4 is a flowchart showing the change characteristics of the resistance value, Figure 5 is a flowchart showing the program when electrode opening is used as a parameter, and Figure 6 is a line showing the change characteristics of the electrode opening. Figure 7 is a diagram showing the setting characteristics of the welding current, Figure 8 is a diagram showing the waveform of the primary current of the welding transformer, and Figure 9 is the primary current of the welding transformer in the conventional technology for AC welding. FIG. W...Ware (1)...Welding transformer (2)...Rectifier circuit (3)...Electrode (
5)...Inverter (51) to (5a)...Power transistor (7)...
・Rectifier circuit (8)...AC power supply terminal (9)・
...Pulse width control circuit (11)...3 people outside the controller\-

Claims (1)

[Claims] 1. When a workpiece is held between a pair of electrodes and energized, a parameter whose value changes due to the occurrence of expulsion is detected, and the occurrence of expulsion is detected from a change in the parameter value, and then energization is applied. The electrode is connected to the secondary side of the welding transformer via a rectifier circuit, and the primary side of the transformer is connected to the alternating current via an inverter composed of power transistors and a rectifier circuit. The welding current is connected to a power supply terminal, and the welding current is controlled by changing the energization pulse width on the primary side according to the pulse width of the control pulse applied to the inverter, and when the occurrence of expulsion is detected, the control pulse is raised. A method for controlling energization in a resistance welding machine, characterized in that energization is immediately stopped. 2. The method of controlling current flow in a resistance welding machine according to claim 1, wherein the welding current is gradually increased after the start of current flow.