JPS6284877A - Burn through avoiding method - Google Patents

Abstract

PURPOSE:To optimize the welding conditions by operating the ratio of the minority time mean value to the majority time mean value on each short circuit period of an arm, then by controlling the power source output current, etc. based on the comparison of the ratio thereof with the threshold value. CONSTITUTION:A gate signal is transmitted to a counter 2 while under arc generation by generating a short circuit detecting signal by inputting the arc voltage by providing a short circuit detecting part 1, and a latch signal is fed to a latch part 3 and an interrupt signal to CPU 4 as well. The CPU 4 operates the ratio of the minority time mean value to majority time mean value on each period from the short circuit period stored on the latch part 3. Based on the comparison of the meanvalue ratio thereof with the set threshold value, the burn through is forecast and the power source output current or welding speed is reduced in non-stepping state as well. After the prescribed time, then, the welding conditions thereof is returned upto the level prior to the forecast. Consequently the welding can be performed under the optimum conditions all the time.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for avoiding burn-through that may 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 will become a fused state and it will not be possible to restore normal welding. . Conventionally, there is no technology to predict the occurrence of this burn-through phenomenon, or to avoid or prevent the occurrence of the burn-through phenomenon by predicting it, and when the burn-through phenomenon occurs, welding is stopped, which reduces work efficiency. In addition, there was a problem in that workpieces had to be replaced and reworked in order to restart welding, which was 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, calculates the ratio of the short circuit period average value to the short circuit period average value for each short circuit period, and compares it with the acid ratio to predict low level burn-through. A threshold value and a high-level burn-through detection threshold are set, and when the ratio exceeds the low-level threshold and burn-through is predicted, the ratio exceeds the low-level threshold. The welding power source output current or welding speed is changed stepwise to the burn-through avoidance level at each short circuit cycle or at a certain set time until the output current or welding speed becomes smaller than the above value, and after a predetermined period of time, the output current or welding speed is changed to the burn-through avoidance level. The recovery is performed in a ramp-like or step-like manner, and the width of the stepwise change increases or decreases in response to the increase or decrease in the ratio.

[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.

Figures 1 (al to d) show representative waveform diagrams of measurements repeatedly carried out by the present inventors in order 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.The arc voltage waveform during the arc welding is shown in Figure 2.Figure 1 (a) ) is the output waveform of the optical sensor, 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.11th m In (b), the above output waveform is filtered by a low pass filter (LPF) (in this case, the cutoff frequency f
C = 5 Hz), and the variation in the short circuit period T is averaged. Figure 1 (e) shows f C = I
This is a waveform smoothed by a Hz LPF.

As is clear from the waveforms in FIG. 1 (bl and (C)), the short circuit period increases before the start of burn-through, and in the present invention, the arc voltage is monitored and if this increasing tendency appears, , determines that burn-through is a precursor to occurrence 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 measurement, the increasing tendency starts, for example, 200 to 300 msec before the burn-through occurs, so that the output signal of the comparator ((d) in the same figure)
is used as a predictive signal, and using this signal, welding conditions can be changed before burn-through occurs.

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. From the standpoint of versatility, there is a waistband that is indispensable.

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

That is, the average value Td of a large number N of short circuit cycles to be measured and the average value Tc of a decimal number n are determined, and the ratio of the two is 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 (1) is established, a prediction signal is generated to predict burn-through.

T c / T d > A... (1) Here, (1) The average value Td is a value calculated from the short circuit cycle data during normal welding that is taken in while welding, As shown in FIG. 3, 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) Since the threshold value A is for the average value ratio T c / T d, unlike VOlVH and VL, it has contents as a relative value, and there is no need to change it corresponding to each welding condition. One value can correspond to a wide range of welding conditions.

In the present invention, the threshold value is set in two levels, high and low, and when the low level threshold value ABL (burn-through prediction level) is exceeded, a prediction signal is generated to change the welding conditions,
When a high level threshold value ABH (burn-through detection level) is exceeded, a burn-through detection signal is 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 an arc generation gate signal to the counter 2, and supplies a latch signal to the latch section 3 and an interrupt signal to the CPU 4. 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, Tc, Tc/Td.
Calculate Tc/Td,! = Compare the size of ABL, T
If c/Td>ABL, it is determined that the state is in the burn-through transition state, and the burn-through prediction signal a is similarly set as Tc/Td>
In the case of ABH, it is determined that burn-through cannot be avoided and a burn-through detection signal is generated.

The present invention uses the prediction signal a and the detection signal S obtained as described above, and the flow of the burn-through avoidance method according to the present invention is shown in FIG. FIG. 9 is a block diagram of a conventional welding device, and 10 is a welding power source (high frequency inverter).
, 11 is a transformer, 12 is a rectifier, 13 is a 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 ninth
The output current I supplied from the welding power source 10 to the wire 15 in the figure is changed from the current level before prediction (this level of current is referred to as the main current Is) to the burn-through prevention level (the lowest level is the current level of the main current Is). For example, it is reduced toward about 80-70%). This lowest level is the current level that maintains airtightness and avoids weld bead discontinuity (burn-through); in this case, in the present invention,
In order to ensure bead continuity, the output current of the welding power source 10 is reduced in stages.

That is, when the prediction signal a is generated, Icn = Is-(Is-IO)X (Acn=1: -ABL) / (ABH-ABL
) ... (2) However, ID: predetermined current value (<l5) Acn: short circuit cycle average value ratio T c / T
d.ABL<Acn-H-≦-ABH n :1.2.3...k... is determined, and the output current I is lowered to the current Icn value.

As the output current ■ decreases, the value of Acn- also decreases, so
The reduction width at this time is smaller than the reduction width of I cn in the previous short circuit cycle. The level step reduction control as shown in FIG. 6, in which Icn is determined for each short-circuit period and the output current is controlled to the current level, is repeated until Acn becomes less than ABL. In the figure, time tl
, n-k), the current value I at n- during a predetermined time tf
After maintaining ck, the level of the main current Is (in the negative direction) is increased. When Acn<ABL, the second term of equation (2) becomes positive and Icn>Is all at once.
In this case, the current is increased at a certain increase rate ΔIup, and the main current Is is returned to i1 after a slow-up time tu, for example, 1 to 3 seconds. However, Δrup= (Is-10)/lu... (3) If burn-through is predicted during recovery to the main current Is (time ti in the figure, n=1), That is, Acn>AB
If it becomes L, from that point on, the output current of the welding power source 10 is controlled to be reduced step by step according to the following formula until Acn<ABL. Icn=Ic1- (Ici-
IO)X (A c -ABL)/ (ABH-ABL) ・
・ ・ ・(4) In this way, in this example, when the burn-through transition phenomenon occurs, the welding conditions are changed stepwise to the burn-through avoidance level side by predicting this phenomenon, and the width of the stepwise change is As the cycle average value ratio Ac decreases, it is reduced in proportion to this, so it is possible to not only avoid burn-through, but also ensure bead continuity, and change the welding conditions to normal welding conditions. Since the return is performed linearly in a non-step manner, if a burn-through transfer phenomenon occurs during the return, it is possible to prevent the situation that would result in burn-through even though it was predicted. The probability of occurrence of a drop can be significantly reduced compared to the conventional method.

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

Such a current pattern can be realized by having the CPU 4 execute calculations and determinations according to the flow shown in FIG. 5 above, and providing the results to the control device 19 as a current command.

Note that Td, which is the self-reference value of the short-circuit period, changes depending on the current, but if the change cannot be ignored, it may be fixed to the value at the time of burn-through prediction.

By the way, in the case of the method of the above embodiment, after the burn-through prediction, the output current temporarily decreases to a level below Is, but when the short-circuit period average value Acn no longer decreases, the current at that time decreases. level is maintained. ABL<Ac
As long as n<ABH holds, it is sufficient to have 100% reliability that burn-through will not occur, but in reality, the arc phenomenon of arc welding does not have good reproducibility, so
If the state of BL<Acn<ABH continues for a long time, there is a risk of burn-through.

The flowchart shown in FIG. 7 shows another embodiment of the present invention that is designed to solve the above problem. In this embodiment, as shown in FIG. 8, the output current is reduced to a current value according to the following formula at every predetermined time ta, not every new short-circuit cycle.

Icn −Icn−+ −(Icn−+ −10
) X (Acn-+ -ABL) / (ABH-A
BL)...(5) In this case, Icn: Next output current Icn-+: Previous output current Acn-1: Previous short circuit cycle average value ratio ABL< A Cn < ABH Note that if the time ta is set short, arc discontinuity occurs. On the other hand, if the length is too long, there will be a situation where burn-through occurs even though burn-through was predicted, so it is necessary to choose it appropriately. In experiments conducted by the present inventors, approximately 0.1 to 0.3 seconds was suitable.

In the case of this embodiment as well, when Acn-1<ABL as a result of the processing (time tl, n=k in the figure), the current value 1ck at n= is maintained for a predetermined time ta, and then , to the level of the main current Is. In this case, as described above, it may be increased linearly at a certain increase rate Δ■up, but in this embodiment, it is increased stepwise.

That is, in this embodiment, the output current Icn after the time ta is determined using the previous output current Icn-r as one of the parameters, and since the time ta is set larger than the short circuit period, ABL<Acn-+<ABH It is possible to prevent the situation from continuing to a dangerous extent.

Note that since the time ta is set in advance, the target value of the current ICN may be calculated using equation (3) and complemented so that the target value is reached after the time ta. This is effective in obtaining bead continuity when the process progresses to

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

〔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 the necessary minimum level to avoid burn-through, and promptly restores normal welding conditions while monitoring the possibility of bacteriolysis. This allows welding to be performed under optimal conditions at all times, and eliminates the need to interrupt welding due to burn-through and replace or repair workpieces, increasing the efficiency of arc welding of thin plates. In particular, when using a welding robot, the welding conditions can be kept to the minimum necessary, so the effect is particularly large.

[Brief explanation of drawings]

Figures 1 (a) to (d) are waveform diagrams for explaining the principle of the welding prediction technology of the present invention, Figure 2 is an arc voltage waveform diagram, and Figure 3 is for explaining an embodiment of the welding prediction technology described above. Fig. 4 is a block diagram of a burn-through prediction device that implements the above-mentioned bacteriolysis prediction technology, Fig. 5 is a flowchart showing an embodiment of the present invention, and Fig. 6 shows changes in welding conditions in the above embodiment. FIG. 7 is a flowchart showing another embodiment of the present invention; FIG. 8 is a waveform time chart explaining the welding condition change mode in the other embodiment; FIG. FIG. 9 is a block diagram of a conventional 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 base metal, is monitored from the arc voltage, and the ratio of the average value of the short circuit period to the average value of the short circuit period of many times is calculated for each short circuit period. A low-level burn-through prediction threshold and a high-level burn-through detection threshold are calculated and compared with the ratio, and burn-through is detected when the ratio exceeds the low-level threshold. is predicted, the welding power source output current or welding speed is changed to the burn-through avoidance level at each short-circuit period or every set time until the ratio becomes smaller than the low-level threshold. A method for avoiding burn-through, characterized in that the ratio is changed using a variation parameter, and after a predetermined period of time has passed, the ratio is restored to the level before the prediction in a non-step manner. (2) The stepwise change width is proportional to the ratio of the difference between the low threshold value and the short circuit period average value ratio to the difference between the high and low threshold values. How to avoid it.