JPS5841952B2 - Adaptive control method in resistance welding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an adaptive control method for obtaining a predetermined quality of a weld in resistance welding.

Various monitoring methods such as a welding current method, an electrode tip resistance method, an electrode tip voltage method, and an ultrasonic method are considered as methods for checking the quality of a welded part in conventional resistance welding, for example, spot welding.

However, although the scope of application of these monitoring methods differs depending on the method, all of them can only roughly judge the quality of the welded part after welding is completed.
The quality of the welded parts is not positively guaranteed.

Therefore, in the conventional resistance welding method, quality defects occur in the welded part, which not only requires rework but also sometimes requires the product to be discarded.

The present invention has been made in view of the above points and provides an adaptive control method in resistance welding, which automatically guarantees a predetermined quality of the weld during the welding process.

In other words, the voltage between the electrodes (including the electrode tip in the case of spot welding) that sandwich the workpiece while welding current is being applied has a close relationship with the temperature of the weld zone and the nugget diameter, that is, the strength of the weld zone. A voltage curve showing a temporal change in the voltage between the electrodes is determined once the type, shape, plate thickness, etc. of the material to be welded are determined, and a voltage curve with good welding quality is determined among the voltage curves.

Therefore 1. It has already been confirmed through experiments that a certain quality of the weld (nugget diameter) can be obtained if the voltage between the electrodes that sandwich the material to be welded changes according to the voltage curve (reference voltage curve). The invention is based on this.

In order to suppress the voltage between the electrodes, it is effective to control the pressing force between the electrodes.

Furthermore, even when voltage fluctuations are relatively large, if the voltage integral value of the part where the interelectrode voltage exceeds a certain voltage effective for resistance welding changes according to a certain reference voltage integral value curve,
It was also confirmed that good weld quality was obtained.

Based on these experimental results, the present invention is based on the idea that the integral value of the differential voltage when the voltage between the electrodes sandwiching the welding material exceeds a predetermined level voltage during welding current application during the welding process is the reference voltage integral. The present invention relates to an adaptive control method for automatically guaranteeing the quality of a resistance weld during a welding process by controlling the pressure force between the electrodes so as to vary according to a value curve.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows a typical example of a one-hour electrode tip voltage curve (hereinafter simply referred to as "voltage curve") when spot welding a mild steel plate with a single plate thickness.

In the figure, when welding according to the voltage curve shown in a, the tensile shear strength of the welded part is approximately 700 kg, and the welding quality is good, but when welded according to the voltage curve shown in b, the welded part The tensile shear strength is very small, about 6Qkg, and the welding quality is poor.

Conversely, when the interelectrode voltage value changes over time as shown by curve a, a nugget having a tensile shear strength of about 7QOkg is formed in each welded part, but a curved line-like nugget is formed. When the weld changes over time, almost no nuggets are formed in the weld, and the weld quality is poor.

FIG. 2 shows an example of an hourly interelectrode voltage curve (voltage curve) when spot welding stainless steel plates having a thickness of 0.8 mm.

In this case, as in the case of mild steel plates, the quality of the weld is good if the interelectrode voltage value changes over time like curve a, but the quality of the weld is good if the interelectrode voltage value changes over time like curve a. In this case, the quality of the weld is poor and has almost no strength.

In this way, the quality of the welded part in resistance welding corresponds to the temporal change in the voltage between the electrodes that sandwich the material to be welded, so this can be determined by the first, If the welding current is controlled while being energized as shown by curve a in FIG. 2, the quality of the welded part can always be guaranteed.

This interelectrode voltage can be controlled by applying pressure.

Figure 3 shows the temporal change in the voltage between the electrodes when only the pressure force between the electrodes was varied from 18 ok to 340 kg (curves a to d) with the welding current value constant at 880 OA. .

As shown in the figure, as the pressurizing force increases, the interelectrode voltage decreases overall.

At this time, it has been confirmed that the nugget diameter also decreases as the pressing force increases.

This is because as the pressing force increases, the plate-to-plate contact area and the electrode-to-plate heat transfer coefficient increase.

In FIG. 3, when the pressurizing force is 180 kl; i+ (curve a), dust occurs in the fourth cycle and a sudden drop in voltage is observed.

However, when there are many voltage fluctuations, it is difficult to control the inter-electrode voltage so that it follows a desired voltage curve by controlling the electrode pressing force.

In other words, in general, when the power supply voltage fluctuates, the interelectrode voltage also changes in proportion to the amount of current-voltage fluctuation, but if this voltage fluctuation range is large and rapid, the temporal responsiveness of the electrode pressurization system becomes relatively low. As a result, it becomes difficult to control the electrode pressing force so that the inter-electrode voltage follows the waveform of a desired voltage curve, resulting in poor control accuracy.

However, even in such a case, the difference voltage when the voltage between the electrodes exceeds the level voltage effective for resistance welding is integrated from time to time, and
It is possible to control the integral value to follow a reference voltage integral value curve obtained by successively integrating the difference between the voltage on the voltage curve and the above-mentioned level voltage when good welding quality is obtained in advance. Since the integral value curve of the above-mentioned differential voltage is a relatively gentle curve, it is relatively easy to use even in an electrode pressurization system with poor response, and control accuracy can be greatly improved, thereby ensuring good weld quality. It was confirmed through experiments that this could be obtained.

FIG. 4 shows how to obtain the interelectrode voltage integral value for carrying out the present invention.

As shown in the figure, the voltage Vc between the electrodes sandwiching the welding current is detected, and the detected voltage Vc is a preset level voltage (2).

Only when the voltage difference (VcVo) exceeds , the voltage difference (VcVo) is integrated over the energization time.

→l of the voltage integral value curve thus obtained is shown in FIG.

The method of the present invention improves the quality of resistance welds by controlling this voltage integral value curve to follow a reference voltage integral value curve that has been set and stored in advance so that the quality of the weld is good. This is guaranteed.

Here, an example of the relationship between the current voltage, the interelectrode voltage curve, and the voltage integral value Σ(v-Vo) curve when the current voltage fluctuates is shown in FIG. 10.

As shown in Figure 10a, when the power supply voltage Vs changes rapidly and greatly, the interelectrode voltage V also changes proportionally as shown by the solid line in the figure, causing a delay in the response of the electrode pressurization system. Therefore, it is no longer possible to control the interelectrode voltage V so that it follows the reference voltage curve shown by the broken line, which is a desirable voltage curve.

However, since the above voltage integral value curve is a relatively gentle curve as shown by the solid line in Figure 10C, even if there is a response delay of the electrode pressurization system, it will almost match the reference voltage integral value curve shown by the broken line. It can be controlled to imitate.

FIG. 6 is a block diagram of an apparatus for implementing the invention.

In the figure, 1 is the damaged material, 2a is a movable electrode connected to the piston 3a of the cylinder 3, and 2b is a fixed electrode, which clamps and pressurizes the material 1 to be welded in the welding process, and the welding current is applied from the transformer 4. Power is applied.

5 is a voltage detection circuit that momentarily detects and rectifies the voltage between electrodes 2a and 2b (hereinafter referred to as "interelectrode voltage") while the welding current is being applied, and 6 is a half cycle of the maximum value of each half wave of the detected voltage Va. This is a waveform representative point holding circuit that holds the waveform for a long time.

7 is a constant level voltage V effective for resistance welding.

A level voltage setting circuit 8 outputs the output voltage Vc of the waveform representative point holding circuit 6 and the level voltage ■.

This is an integration/addition circuit that inputs the voltage and integrates and adds the difference voltage when V c > Vo.

Reference numeral 9 denotes a reference voltage generation circuit, which generates a reference voltage Vr based on a reference voltage integral value curve that has been set and stored in advance so that the quality of the welded part is good (similar to the integral value curve in FIG. 5, it increases with time).

) is output every time a timing signal Tp from an energization time control circuit 14, which will be described later, is input.

10 is a differential amplifier, which outputs the output voltage (integrated value) vi of the integrating/adding circuit 8 and the reference voltage V from the reference voltage generating circuit 9.
r and outputs a signal Vd according to the voltage difference.

11 controls the shilling 3 using this signal Vd to connect the electrode 2.
This is a pressurizing force control device that controls the pressurizing force between a and 2b, and a specific example thereof will be described later.

In addition, 12 is an AC power supply, 13 is a constant current control circuit,
Reference numeral 14 denotes an energization time control circuit which sends energization start and stop signals for making welding current flow for the required time to the constant current control circuit, and also sends a timing signal Tp, which synchronizes each circuit after energization is started, through a path shown by a broken line. is output and sent, for example, every half wave of the AC voltage from the AC power supply 12.

Incidentally, as the waveform representative point holding circuit 6, for example, a commercially available sample-hold amblifier can be used.

As shown in FIG. 7, a detection voltage Va obtained by detecting and full-wave rectifying the inter-electrode voltage output from the voltage detection circuit 5
The peak values of each half-wave of are measured at times t1, t2, . . . using the timing signal Tp as a sampling pulse. t3・・・・・・・・・
... is sampled, held for half a cycle, and outputs the held voltage Vc.

In this way, sampling of the representative point of the waveform at the peak of each half-wave is performed to calculate the time change ai / at of the welding current.
! This is because at this point in time when becomes zero, the induced voltage component L di / dt (L is the inductance of the inter-electrode voltage extraction portion) of the inter-electrode voltage is removed, and only the pure inter-electrode voltage can be detected.

According to this embodiment, the maximum value Vc is output from the waveform representative point holding circuit 6 for each half wave of the interelectrode voltage Va detected by the welding current voltage detection circuit 5, and the level voltage setting circuit 7 Level voltage set by■.

Only when the voltage exceeds , the differential voltage is integrated and added.

This integral value is indicated by a circle in FIG.

The integral value Vi as the output of the integrating/adding circuit 8 is compared from time to time with the reference voltage Vr based on the reference voltage integral value curve generated by the reference voltage generating circuit 7 by the differential amplifier 10, and the voltage difference is A signal Vd that has reached the peak is output.

The pressure control device 11 is activated by this signal Vd,
□By controlling the pressing force between the electrodes 2a and 2b via the cylinder 3, the differential voltage is controlled to be zero.

That is, when the applied pressure is increased, the inter-electrode voltage decreases, and when the applied pressure is reduced, the inter-electrode voltage increases.

Therefore, even if the above integral value fluctuates and deviates from the reference voltage integral value curve as shown by circle mark A in Figure 9, ○ will be returned in the next half wave.
It can be modified as shown by mark B to follow the reference voltage integral value curve.

Therefore, it is impossible to guarantee that the quality of the welded part is always good. □ Immediately after starting the welding process, an abnormally large voltage between the electrodes may be detected due to the contact resistance between the materials to be welded 1.
Since this is not related to the quality of the welded part, it is desirable to monitor the output signal of the differential amplifier 10 during this period to prevent malfunction.

Therefore, after the start of energization, the timing signal Tp sent from the energization time control circuit 14 prevents the differential amplifier 10 from outputting for the first few cycles of energization, and during that time,
The pressurizing force control device 11 may control the cylinder 3 so that the pressurizing force between the electrodes 2a and 2b becomes a predetermined pressure set in advance.

For temporal control of the pressurizing force, a frequency response of approximately 50 hertz can be easily obtained by using an electro-hydraulic servo valve.

FIG. 8 shows a specific example of the pressurizing force control device 11.
11 is a hydraulic pump driven by a motor 112;
13 is a directional control valve, 114 is an electric/hydraulic servo valve for adjusting pressurizing force, 115 and 116 are check valves, and 117 is a relief valve. Hydraulic pressure is controlled by piping connecting these and the upper and lower ends of the cylinder chamber of cylinder 3. A circuit is formed.

118 is a pressure detector that detects the pressure applied to the cylinder 3 based on the pressure difference between pressure gauge port P□ and P2, and 119 is a detection signal Vp from this pressure detector 118 and from the differential amplifier 10 described above. This is a servo amplifier that inputs the signal Vd and converts it into a current signal necessary to drive the electric/hydraulic servo valve 114 according to the sum thereof.

By the way, electric/hydraulic servo valve 1 used in this example
Reference numeral 14 is composed of a known torque motor and a nozzle flapper mechanism, which displaces the flapper by passing a drive current from a servo amplifier to the torque motor, and controls the amount of oil leaking from the nozzle, thereby adjusting the output hydraulic pressure. It is designed to change the

Therefore, when the signal Vd from the differential amplifier 10 becomes zero, the servo amplifier 119 uses the signal Vp corresponding to the pressure detected by the pressure detector 118 to generate the drive current necessary to maintain the pressure at that time. Operates to continue supplying (holding current) to the electric/hydraulic servo valve.

Then, when the input signal Vd is input again after once becoming zero, the signal V that is input from the pressure detector 11B at that time is
The pO value and the signal VdO value input from the differential amplifier 10 are added and converted into a current that drives the electric/hydraulic servo valve 114, and the above-mentioned holding current increases depending on the sign and magnitude of the input signal Vd.・Decreased.

As a result, for example, when the integral value Vi in FIG. 6 becomes larger than the reference voltage Vr and a positive signal Vd is input to the servo amplifier 119, the driving current of the electric/hydraulic servo valve 114 increases and the flapper closes the nozzle. As the displacement occurs in the direction, the output oil pressure increases, and as the electrode pressing force increases, the inter-electrode voltage Va decreases, and the integral value Vi becomes equal to the reference voltage Vr.
acts to match.

Conversely, when the integral value Vi becomes smaller than the reference voltage Vr and a negative signal Vd is input to the servo amplifier 119, the electrical
The driving current of the hydraulic servo valve 114 decreases, and the flapper moves in the direction of opening the nozzle, so the output oil pressure decreases, and the electrode pressing force decreases, so the interelectrode voltage Va increases, and the integral value Vi becomes equal to the reference voltage Vr. acts to match.

When the directional control valve 113 is in the position shown, the cylinder 3
Hydraulic pressure is supplied from the lower end side of the cylinder chamber, and the electrode 2a is raised. However, when the directional control valve 113 is switched in the opposite direction to that shown in the figure by a signal from a control panel (not shown), the hydraulic pressure from the hydraulic pump 111 is changed to electric pressure. It is supplied from the upper end of the cylinder chamber of the cylinder 3 via the hydraulic servo valve 114, and the electrode 2a descends together with the piston 3a as shown by the imaginary line.
It works together with the electrode 2b to pressurize the material to be welded 1.

As described above, during the welding current application period, the pressure between the electrodes is controlled so that the inter-electrode voltage (inter-tip voltage) changes according to the reference voltage integral value curve, and the quality of the welded part is guaranteed. Ru.

In this way, the range of control over changes in the conditions of the resistance welding part is widened, and accurate control can always be performed according to the conditions of the workpiece 1 to be welded.

Note that the explanation of the above embodiment is for welding using alternating current, and therefore sample value control is performed in which the maximum value of the half wave of the waveform is taken as the representative point and the sampling value is used for control. Continuous control is also possible when welding using direct current.

In addition, in both cases, the present invention is explained for the case where it is applied to spot welding, but it can be similarly applied to other resistance welding, such as projection welding, seam welding, flash welding, upset welding, etc. It is.

As described above, according to the applicable part side method of the present invention, the pressure force between the electrodes that sandwich the welded material is controlled so as to reliably obtain a predetermined quality of the resistance weld during the welding process. Rework due to defective parts and disposal of defective products are almost eliminated, the defective rate of products can be significantly reduced, and work efficiency can be improved.

Furthermore, controlling the pressure between the electrodes is particularly effective in preventing dust and flakes, thereby improving safety and preventing damage to appearance quality.

In addition, by controlling the integral value of the voltage difference between the electrodes that exceeds the standard voltage to follow the standard voltage integral value curve (the curve necessary to obtain good welding quality), voltage fluctuations are relatively reduced. Extremely good welding quality can be guaranteed even when the welding temperature is high.

[Brief explanation of the drawing]

Figures 1 and 2 are interelectrode voltage one-hour curve diagrams when spot welding mild steel plates and stainless steel plates, respectively;
FIG. 3 is a diagram showing a one-hour curve of the interelectrode voltage when the pressing force is changed. FIG. 4 is a curve diagram showing how to obtain the interelectrode voltage integral value for carrying out the present invention, and FIG. 5 is a curve diagram showing the -
It is a curve diagram showing ψ11. FIG. 6 is a block diagram of a device for carrying out the present invention, FIG. 7 is a waveform diagram for explaining the operation of the waveform representative point holding circuit, and FIG. 8 is a specific example of the pressurizing force control device. It is a block diagram which shows an example. FIG. 9 is a diagram for explaining the operation of the embodiment shown in FIG. 6. FIG. 10 is a diagram showing an example of the relationship between the power supply voltage, the interelectrode voltage curve, and the voltage integral value curve when the power supply voltage fluctuates. 1... Material to be welded, 2 a t 2 b...
... Electrode, 3 ... Cylinder, 4 ... Transformer, 5 ... Voltage detection circuit, 6 ... Waveform representative point holding circuit, 7 ... ...Level voltage setting circuit,
8... Integration/addition circuit, 9°°'...° reference voltage generation circuit, 10... Differential amplifier, 11...
... Pressure control device, 12 ... AC power supply, 1
3.--. Constant current control circuit, 14. . . . Current-carrying time control circuit.

Claims (1)

[Claims] 1. In resistance welding, the voltage between the electrodes sandwiching the welding current is detected moment by moment, and when this detected voltage exceeds a preset level voltage, the differential voltage is integrated moment by moment, and the voltage is calculated from moment to moment. The integral value of the differential voltage is compared from time to time with the integral value according to a reference voltage integral value curve set separately in advance, and the pressing force between the electrodes is controlled according to the difference. An adaptive control method characterized in that the quality of a resistance weld is guaranteed by changing the voltage according to a reference voltage integral curve.