JPS6159835B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a resistance welding control method, and an object of the present invention is to improve and make uniform the quality of resistance welding, particularly spot welding.

Conventionally, measures to improve the welding quality of spot welding include (1) stabilization control of welding conditions; (2)
Detect the formation of weld nuggets using some physical quantity,
Two methods have been considered for performing in-process control so that the nugget diameter becomes uniform. Method (1) is a method of stably controlling the three major conditions of resistance welding, such as welding current, energization time, and pressurizing force, so that there are few fluctuations.
The embodiment of this can be seen in the synchronous timer, which incorporates a circuit that compensates for input voltage fluctuations and a constant current control circuit that controls the current to always flow at a constant level even when the load power factor varies. . However, these problems include pressure fluctuations, shunt flow, chip deformation, the shape of the workpiece (object to be welded), surface condition, etc.
It has no function to compensate for variations in contact, etc., and it is difficult to maintain constant welding quality.

On the other hand, method (2) more directly detects the welding quality, that is, the state of nugget formation, using sound or voltage, and attempts to stabilize the quality by cutting off the current when the nuggets have grown sufficiently. It is. For example, the voltage between welding electrodes during welding (hereinafter referred to as
As _shown in FIG _.
By cutting off the current when the inter-chip voltage decreases by a preset percentage (when it reaches the V _c value), welding quality can be maintained even when the above-mentioned pressure fluctuations occur. The purpose is to achieve uniformity during welding, that is, in-process (Japanese Patent Application Laid-open No. 114541/1983).

However, the disadvantage of this method is that it only controls the welding current when it is energized, so if the welding current drops considerably for some reason, the energization time will be longer, but sufficient nugget growth cannot be expected, and the strength will be low. It turns out. Here, () in FIG. 2 shows the envelope of the peak value of the inter-chip voltage when sufficient nuggets are obtained, and () shows the envelope when nuggets are insufficiently formed.

Furthermore, this method has the disadvantage that only materials with high resistivity, such as mild steel or stainless steel, can be used as the workpiece. This is because materials with low resistivity such as aluminum do not necessarily have a maximum value in the envelope of the wave height values.

On the other hand, the acoustic emission (abbreviated as AE) signal generated from the welding part during welding is monitored, and a certain threshold hold level (V _TH ) is detected.
A method has also been attempted in which the energization is cut off when the above AE signal is detected. This detects the large AE signal associated with the generation of dust, especially during welding, and
It is based on the idea that sufficient nuggets have been formed at the time when medium dust occurs, and the current is cut off at that point.

However, like the chip-to-chip voltage method, this method only restricts the energization time and does not actively control the welding current, so if nugget formation is insufficient due to a drop in current, etc.
The AE signal is also small, and only the energization time period becomes longer, making it impossible to obtain a uniform nugget diameter.

The present invention focuses on the fact that the inter-chip voltage is closely related to nugget growth, and monitors the inter-chip voltage during welding to adjust the inter-chip voltage corresponding to the energization for half a cycle of the power supply to the energization for half a cycle. This is an energization control method in which energization is cut off when the cumulative increase in the rectified value of the integral value integrated over a range exceeds a preset value, and the most important point is that from the start of energization The method is adopted in which the standard welding current for the workpiece is applied for a certain period of time, and then the welding current is sequentially increased at a substantially constant rate for each cycle.

First, a method for monitoring inter-chip voltage will be explained. When detecting the chip-to-chip voltage, there are two methods: simply detecting the peak value, as shown in Figure 3a, and integrating the chip-to-chip voltage corresponding to half a cycle of energization, as shown in Figure 3b, for that half cycle. There are cases where it is detected as a value. Method b can greatly eliminate the superimposed voltage induced by the welding current, which is especially noticeable when the inter-chip voltage detection probe is placed along the edge of the spot welding machine, thereby improving detection accuracy. This has the advantage of improving performance. Now, the temporal change in the inter-chip voltage after the start of energization is the second
The result will be as shown in the figure. Here, the vicinity of the peak value of the envelope of the inter-chip voltage in Fig. 2 is said to be the peak period of nugget growth, and the reason why the envelope curve declines after that is because the resistance value decreases due to the expansion of the nugget, and therefore the inter-chip voltage increases. This is because the voltage drop decreases. This tendency is substantially the same even in the case of a method in which the welding current is increased after the start of energization or after a certain period of time, as in the method according to the present invention. Therefore, by sequentially rectifying and further accumulating the values obtained by integrating the inter-chip voltage for a half-cycle energization range over the energization range for the half-cycle, the change in the cumulative value increases. is monitored, and if the change (increase) becomes less than the preset value,
The aim is to obtain a proper nugget by cutting off the current. This also makes it possible to prevent dust.

Incidentally, the basis of the waveform shown in FIG. 3a will be explained here.

The inter-chip voltage e is usually measured by detecting the voltage between the upper and lower tips using a detection probe, but at this time, the inter-chip voltage e is the resistance due to the welding current flowing between the chips. It is detected as the sum of the voltage drop (Ri) and the induced voltage (Kdi/dt) induced in the closed loop of the detection probe by the welding current.

That is, e=iR+Kdi/dt (K: induced voltage coefficient, i: welding current, R: resistance between chips) This is illustrated in figures 4a, b, and c.

Next, the setting of welding current will be explained.

The welding current is set to standard welding current conditions at the start of energization. For a certain period of time after energization is started, the temperature of the nugget forming part of the work increases, and finally reaches the melting point and the nugget begins to grow. During this time, it seems best to keep the welding current approximately constant or to slightly increase the slope.

Now, after this section, the welding current should be increased to 2 to 20
It is increased sequentially within the range of 1 cycle.
Specifically, the welding current setting voltage value of the constant current timer and input voltage fluctuation compensation timer is individually preset and programmed for each cycle, and as welding progresses, the preset value is selected with a switch. For example,
As shown in Fig. 5, there are a plurality of variable resistors VR ₁ , VR ₂ , VR ₃ , . . . VR _o giving preset values, and analog switches AS ₁ , AS ₂ , AS ₃ , . ...AS _o and a shift register SR that has a clock input that is a pulse synchronized with the power supply frequency to control the analog switch.
It can be easily configured by And VR ₁ above is VR ₂ for setting the welding current for the first cycle.
is for setting the welding current for the second cycle,
VR _o is a variable resistance for setting the welding current of the nth cycle. AND is an AND gate in the previous stage of the shift register SR, and the power supply frequency pulse and the energization time signal are both applied from the timer to its input side. The energization time limit is limited by the energization cut signal, but if it does not exceed the upper limit energization cycle value set by the timer, it is cut by the upper limit energization cycle value.

Next, an example of a circuit for carrying out the method of the present invention will be described with reference to the drawing of FIG. In the figure, 1 is a welding electrode (electrode tip), 2 is a workpiece (workpiece), 3 is a filter circuit, 4 is an AC integration circuit, 5 is a rectifier circuit, 6 is an A/D conversion circuit, and 7 is a current cut setting. value circuit, 8 is a microcomputer, 9 is an input voltage fluctuation compensation timer, 10 is a fifth
This is the welding current setting circuit shown in the figure.

The inter-chip voltage is then input to an AC integrating circuit 4 via a filter circuit 3, subjected to integration processing, and then input to a rectifier circuit 5. A/D conversion is performed by creating a conversion start pulse at the time when the welding current for each half cycle has finished flowing. Next, this integrating circuit 4 is reset by the A/D conversion end signal. In this way, the inter-chip voltage for each half cycle is stored and accumulated in the memory of the microcomputer 8.
The circuit 7 is composed of a digital switch, and the increase in the cumulative value of the inter-chip voltage from the start of energization, which is taken into the microcomputer 8, and the set value circuit 7 are configured.
are compared with the respective values.

Here, if the increase in the cumulative value of the inter-chip voltage accumulated in the microcomputer 8 is smaller than the set value of the set value circuit 7, the microcomputer 8 sends a signal to the timer 9 to cut off the current. Output a signal. When the timer 9 receives this signal, it immediately cuts off the current. Further, the welding current setting of the timer 9 is performed using a welding current setting circuit 10.

Now, an example of a concrete implementation is as follows.

Workpiece: Mild steel plate (thickness 1mm) Welding machine: 35KVA single-phase spot welding machine Control device: The device shown in Figure 6 Welding conditions: Initial welding current 8100A Upper limit current cycle value 15 cycles Current increase start cycle 5 cycles Current increase value 300A /1 cycle energized cut Accumulated voltage value between chips 4.5V Chip detection voltage value between chips 0.25V or more, welding is performed by changing the pressure force from 170 to 260 kg and the diameter of the chip (CF chip) from 5.8 to 7.0 mm. Ivy. As a result, the current application time was within 7 to 15 cycles, and the welding results (strength) were also very good.

As described above, according to the resistance welding control method of the present invention, after a certain period of time has elapsed from the start of energization, the welding current is gradually increased within the range of 2 to 20% compared to the previous value, and the Effective combination with control that guarantees the growth of weld nuggets by monitoring the increase in the cumulative value of the integral value every half cycle of the voltage, prevents chip deformation, pressure fluctuations, current fluctuations, workpiece surface conditions, and contact. Even when welding conditions vary, it is possible to always guarantee good and uniform welding quality, and its industrial potential is great.

[Brief explanation of the drawing]

Figure 1 is a typical waveform diagram of inter-chip voltage, Figure 2
The figure shows an example of energization time control using inter-chip voltage, Figure 3 a and b are waveform diagrams of an example of integrating the inter-chip voltage every half cycle, and Figure 4 a, b and c are inter-chip voltage waveforms. FIG. 5 is a circuit diagram of a welding current setting circuit for sequentially switching the welding current, and FIG. 6 is a block diagram of an apparatus for carrying out the method of the present invention. DESCRIPTION OF SYMBOLS 1... Welding electrode, 2... Workpiece to be welded, 3... Filter circuit, 4... AC integration circuit, 5... Rectifier circuit, 6... A/D conversion circuit, 7... Current cut setting value circuit, 8...Microcomputer, 9...
Input voltage fluctuation compensation timer, 10...Welding current setting circuit.

Claims (1)

[Claims] 1 After a preset time period from the start of resistance welding, the welding current is sequentially increased at a preset rate,
The voltage between the welding electrodes is monitored during welding, and the increase in the cumulative value of the integrated value obtained by integrating the voltage between the welding electrodes for half a cycle of energization over the energization range for the half cycle becomes less than or equal to a preset value. A resistance welding control method characterized by cutting off energization at a point in time.