JPS6284876A - Burn through avoiding method - Google Patents

Abstract

PURPOSE:To increase the efficiency in arc welding by previewing the burn through from the change in the short circuit period of an arc and by controlling the output current of a power source, then by restoring upto the level prior to the preview after the lapse of the prescribed time. CONSTITUTION:The short circuit detecting part 1 detecting the short circuit of an arc is provided and a counter 2, latch part 3 and CPU 4 are respectively arranged as well. The counter 3 counts the clock which an oscillating part 5 outputs via the gate signal of the detecting part 1 and the short circuit period is stored in the latch part 3. The CPU 4 operates the mean value of the minority time and majority time of the short circuit period and the ratio thereof and transmits the burn through signal by comparing with the threshold value and reduces the power source output current in non-stepping shape as well. Then, after the prescribed time the current value is returned upto the level prior to the preview in the non-stepping shape as well. Due to the trouble in the repair, etc. being saved by avoiding the burn through, the welding efficiency is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for avoiding burn-through that can occur during consumable electrode arc welding.

[Conventional technology]

During consumable electrode arc welding, especially when the base material is a thin plate, a so-called burn-through phenomenon in which holes are formed in the base material may occur. This occurs due to an increase in welding current, an increase in the gap between the base metal grooves, etc., but once this occurs, it becomes a fusion state and normal welding is restored. It is not possible. Conventionally, there is no technology to predict the occurrence of this burn-through phenomenon, or technology to avoid or prevent the occurrence of the burn-through phenomenon, and when the burn-through phenomenon occurs, welding is stopped, which reduces work efficiency. Moreover, in order to resume welding, the work must be replaced and reworked, which is uneconomical. Furthermore, in the case of robot welding, welding conditions are usually set with sufficient safety in mind for fear of the occurrence of this burn-through phenomenon, resulting in a problem of low efficiency.

[Purpose of the invention]

The present invention has been made in order to solve the above-mentioned conventional problems, and aims to provide a burn-through avoidance method that can prevent the occurrence of burn-through and realize arc welding with higher efficiency than conventional methods. purpose.

[Structure of the invention]

In order to achieve the above object, the present invention monitors the short-circuit period, predicts burn-through from an increase in the short-circuit period exceeding a predetermined level, and simultaneously adjusts the output current or welding speed of the welding power source by setting a preset value. The burn-through avoidance level is controlled in a non-step manner, and after a predetermined time, the level is restored to the level before the prediction in a non-step manner.

[Embodiments of the invention]

When so-called burn-through occurs, if welding is stopped at the point when the burn-through occurs, the hole in the base metal will close naturally due to the surface tension of the molten metal, and repair welding can be performed. If you do not want the appearance of the weld bead to be disturbed or if you want to continue welding, it is necessary to predict burn-through. If burn-through can be predicted, welding conditions can be changed (reducing current or increase welding speed)
By doing so, it is possible to prevent burn-through from occurring.

Figure 1 (al to (dl) shows representative waveform diagrams of measurements repeatedly conducted by the inventors to find factors closely correlated with the burn-through phenomenon. The welding point was traced from the back side of the base metal using the optical sensor during arc welding of a thin plate using the optical sensor.The arc voltage waveform during the arc welding is shown in Figure 2.Figure 1 (a) The waveform is the output waveform of the optical sensor, and its Y-axis direction indicates the magnitude of the short circuit period T at the arc voltage, and part A of the waveform indicates the output waveform at the burn-through part. In (b), the above output waveform is filtered by a low pass filter (LPF) (in this case, the cutoff frequency f c
Figure 1 (C) is a waveform obtained by averaging the variation in the short circuit period T by passing the frequency of fc = 5 Hz).
This is a waveform smoothed by PF.

As is clear from the waveforms Tb) and (C1 in FIG. 1), the short circuit period increases before the start of burn-through, and in the present invention, when the arc voltage is monitored and this increasing tendency appears, It is determined that there is a sign that burn-through will occur and predicts burn-through.

The increasing tendency of the short circuit period T can be electrically detected by setting a threshold level Vo (voltage) and comparing the output waveform of the LPF with the threshold level Vo using a comparator, According to the above measurements, the increasing tendency starts, for example, 200 to 300 m5ec before the burn-through occurs, so the output signal of the comparator (FIG. 2(d)) is used as a prediction signal, and using this signal, Welding conditions can be changed before burn-through occurs. The earlier we can predict burn-through, the easier it is to change the welding conditions, which is suitable for avoiding or preventing burn-through.However, if the threshold level Vo is set too low for this purpose, the variation in individual short-circuit cycles will be reduced. Therefore, the probability of false detection increases. As a way to deal with this, the threshold level can be set to a higher threshold level ■1
1 (burn-through detection level) and low threshold level VL
When the short circuit period T exceeds the low threshold level VL, the welding conditions are changed, and when the short circuit period T exceeds the high threshold level VH, welding is stopped. It is also possible to have a configuration in which it is stopped.

By the way, the above-mentioned short circuit period T cannot be controlled by the welding power source, but is controlled by the current, voltage, speed, wire protrusion length,
It is impossible to predict because it varies depending on the thickness of the base material, surface condition, etc., and the above absolute value comparison method, which directly compares the measured short circuit period with the threshold level, performs general-purpose prediction or detection. It's difficult. Also, since the individual short circuit periods T vary, it is necessary to average the short circuit periods using an LPF as described above, but when using an analog LPF, the optimal cutoff frequency fc must be selected for each welding condition. There are limits to its versatility.

Highly versatile burn-through prediction and detection that can be applied to a wide range of welding conditions can be achieved as follows.

First, find the average value Td of the many times N of the short-circuit period to be measured and the average value Tc of the decimal times n, and calculate the ratio of both T c / T
When d is compared with a certain threshold value A (prediction magnification) and the former exceeds the latter, that is, when the following formula is established, a prediction signal is generated to predict burn-through.

Tc/Td>A... (1) Here, the fil average value Td is a value calculated from the data of the short circuit period during normal welding that was taken in while welding, and is shown in Figure 3. As shown, the calculation is updated by taking in new data and discarding old data every short circuit cycle.

(2) The average value Tc is also updated by taking in new data and discarding old data every short circuit cycle, but since it is the average value of a small number of n times, it can be used to smooth out variations in individual short circuit cycles and It can be set to a value that sensitively follows an increase in the period.

(3) Threshold value A is relative to the average value ratio T c / T d, so unlike Vo, VH, and VL, it has contents as a relative value and needs to be changed in response to each welding condition. Therefore, one value can correspond to a wide range of welding conditions.

In the case of this burn-through prediction method, the threshold value A is set in two levels, high and low, and when the low level ABL (burn-through prediction level) is exceeded, a prediction signal is generated to change the welding conditions, and the welding conditions are changed to a higher level. If ABH (burn-through detection level) is exceeded, a burn-through detection signal may be generated to stop welding. The burn-through prediction level ABL is the level at which burn-through can be prevented by changing the welding conditions after prediction, and the burn-through detection level ABH is the level at which burn-through will still occur even if the welding conditions are changed after detection. .

FIG. 4 is a block diagram showing a specific device implementing the burn-through prediction method using the average value ratio of the short circuit period. In the figure, 1 is a short circuit detection section, 2 is a counter,
3 is a latch section, 4 is a CPU, and 5 is an oscillation section.

The short circuit detection section 1 generates a short circuit detection signal in response to the arc voltage, sends a gate signal to the counter 2 during the arc generation period, and sends a latch signal to the latch section 3, cpυ4.
provides an interrupt signal to

The counter 2 is gated by the gate signal and counts the clock output from the oscillation section 5, and the counted value (short circuit period T) is stored in the latch section 3.

When the CPU 4 receives the interrupt signal, it reads the short circuit period stored in the latch section 3, and calculates Td, TC1Tc/Td.
Calculate T c / T d and ABL.

The magnitude of ABH is compared, and when Tc/Td>ABL, a burn-through prediction signal a is generated, and when Tc/Td>ABH, a burn-through detection signal S is generated.

The present invention uses the prediction signal and the detection signal obtained as described above, and the flow of the burn-through avoidance method is shown in FIG. Fig. 7 shows a schematic block diagram of a conventional welding device, in which 10 is a welding power source (high frequency inverter);
1 is a transformer, 12 is a rectifier, 13 is a DC reactor, 1
4 is a power supply tip, 15 is a welding wire, 16 is a welding base material,
17 is a voltage detector, 18 is a current detector, and 19 is a control device.

In the present invention, at the same time that the prediction signal a is generated, the seventh
The output current I supplied from the welding power source 10 shown in the figure to the wire 15, which is an electrode, is adjusted from a pre-prediction current level (this level of current is referred to as main current Is) to a predetermined low level (this level of current is processed). (referred to as current 1c), for example, is limited to about 80 to 70% of the main current Is. This processing current 1c
is a current level that can maintain airtightness and avoid burn-through without causing discontinuity of the weld bead (burn-through avoidance level), and switching this current level is
If it is sudden, that is, in a step-like manner, welding may be interrupted due to the inertia of the motor of the wire feeding device, etc. Therefore, the output current command value is reduced at a certain reduction rate ΔIdown, as shown in Fig. 6. , the welding power source 1 is controlled so that the processing current decreases to Ic after a predetermined time td (for example, 0.2 to 0.5 sec) after burn-through is predicted. However, ΔIdown= (I 5-1c)/l'd
...12) After the output current of the welding power source has decreased to the processing current 1c, this current level is adjusted to the tf waiting time, e.g., 0 to 1
sec) to return to a state in which normal welding is possible, and when the time has elapsed, the current is increased to the level of the main current Is.

In this case, conditions such as the workpiece groove gap may be changing, and it is undesirable to raise the workpiece in a stepwise manner because there is a risk of burn-through.The workpiece is raised at a certain increase rate ΔIup, and the time tu, for example, l ~3sec (>td)
The current is then restored to the main current Is. However, Δru
p= (Is-1c)/lu... (3) If burn-through is predicted during recovery to the main current Is, the output current of the welding power source 1 is reduced from that point on. Rate ΔI
d o w n to reduce the processing current to the level of Ic,
After maintaining the level for the tf waiting time, the increase/decrease rate ΔIup
The current is restored to the main current Is.

Such a current pattern is calculated by the CPU 4, and the pattern output is sent to the control device 19 in FIG.
This can be realized by giving it as a current command.

As described above, in this embodiment, when the burn-through transition phenomenon occurs, it is predicted and the welding conditions are adjusted to the burn-through avoidance level in a non-step manner, thereby reducing bead discontinuity and avoiding burn-through. When the welding conditions are restored to normal welding conditions, the welding conditions are restored in a non-step manner, so if a burn-through migration phenomenon occurs during the restoration, a situation that could have been predicted but eventually results in burn-through can be avoided. can be prevented, and the probability of occurrence of burn-through can be significantly reduced compared to conventional methods.

If burn-through occurs despite such changes in welding conditions when predicting burn-through, and a burn-through detection signal is generated, this signal is used to temporarily suspend welding and generate an alarm. Make it.

In this example, burn-through is avoided by controlling the output current of the welding power source, but burn-through can also be avoided by adjusting the welding speed. If there is a prediction, the speed level will be increased.

Note that it goes without saying that output current control and welding speed control may be used together.

〔Effect of the invention〕

As explained above, when there is a sign of burn-through, the present invention stabilizes the welding conditions by controlling them to a burn-through avoidance level, and returns to normal welding conditions while monitoring whether there is a risk of burn-through. This makes it possible to almost eliminate the need to interrupt welding to replace or repair workpieces, and improve the efficiency of arc welding of thin plates.Especially, when using a welding robot, welding is on the safe side due to the fear of burn-through. There is no need to set conditions, and the effect is particularly great.

[Brief explanation of drawings]

Figures 1 (al to d) are waveform diagrams for explaining the principle of the welding prediction technology in the present invention, Figure 2 is an arc voltage waveform diagram, and Figure 3 is for explaining an embodiment of the above welding prediction technology. Figure 4 is a block diagram of a burn-through prediction device that implements the burn-through prediction technique described above, Figure 5 is a flowchart showing an embodiment of the present invention, and Figure 6 is a welding condition change mode in the above embodiment. FIG. 7 is a block diagram of a welding device.

Claims (2)

[Claims] (1) In consumable electrode arc welding, the short circuit period, which is the time from short circuit to short circuit between the wire and the base metal, is monitored from the arc voltage, and burn-through is predicted from an increase in the short circuit period that exceeds a predetermined level. The welding method is characterized in that the output current or welding speed of the welding power source is controlled in a non-step manner to a preset burn-through avoidance level at the same time as the prediction is made, and after a predetermined time, the output current or welding speed is restored to the level before the prediction in a non-step manner. How to avoid falling. (2) Burn-through prediction is performed by comparing the ratio of the average value of a small number of short circuit cycles to the average value of a large number of short circuit cycles for each short circuit cycle with a predetermined threshold level. How to avoid burn-through.