Control method of arc welding and arc welder

Details Control method of arc welding and arc welder Control method of arc welding and arc welder

2002-06-27

2004-06-01

Shaw, Clifford C. (Department: 1725)

Electric heating

Metal heating

By arc

C219S124020

Reexamination Certificate

active

06744012

ABSTRACT:

TECHNICAL FIELD
This invention relates to an effective technique for automating an arc welding apparatus.
BACKGROUND ART
With the progress of automation of welding apparatuses, various techniques have been proposed, as disclosed in Japanese Patent Laid-Open Publications Nos. HEI-7-148576 entitled “Non-Consumable Electrode Automated Arc Welding Process” and HEI-10-249526 entitled “TIG Welding Apparatus for Rotating Body”.
The automated arc welding process of Japanese Patent Laid-Open Publication No. HEI-7-148576 shows a distance measuring method during welding process in which a welding torch driven under constant arc length control or constant voltage control is provided with a laser sensor for measuring the distance from a base metal.
As is well known, an electric arc formed across the electrode and the base metal emits very bright light, which may cause an operation error of the laser sensor. To deal with this problem, in the aforementioned automated arc welding process sensing of the base metal by the laser sensor is performed at a position remote from the electrode. However, since the distance between a tip end of the electrode and a weld metal (molten pool) is important for welding, such a control, which is based on the distance between the electrode and the base metal, hardly achieves desired control accuracy. An attempt to bring the laser sensor close to the electrode for increasing the control accuracy will result in an operation error of the laser sensor because light from the electric arc (hereinafter referred to for brevity as “arc light”) projects in the sensing field of the laser sensor. Thus, the foregoing automated arc welding process still has a problem of insufficient welding control accuracy.
The welding apparatus disclosed in Japanese Patent Laid-open Publication No. HEI-10-249526 includes a slit light source and a television camera. The slit light source illuminates a bevel or groove, and an image of the groove is picked up by the television camera and in turn is subjected to image processing so that control in a direction of the width of the groove is performed according to a result of the image processing. The vertical axis of the welding apparatus is controlled in a direction of the depth of the groove on the basis of weld voltage. Due to the arc light and weld spatters having high illumination intensities, which are present in the field of the television camera together with the groove illuminated with the slit light source, a special technique is required to selectively take up an image of the groove alone. This requirement increases cost of the welding apparatus. Furthermore, height control of the electrode achieved in reliance on the control of the arc voltage gives rise to a problem, as discussed below.
The arc voltage control relies on a physical phenomenon in which the length of an electric arc (hereinafter referred to for brevity as “arc length”) is nearly in proportion to the voltage between the electrode and the base metal (hereinafter referred to for brevity as “electrode to base metal voltage”). In the arc voltage control, the electrode to base metal voltage is measured to estimate an arc length based on a measured value, and vertical movement of the electrode is controlled so that the estimated arc length is in equal to a desired value.
However, an experiment made by the present inventors have proved that under a certain condition, the arc voltage control is difficult to achieve regardless of whether the arc is a pulsed arc or a continuous arc.
FIGS. 8A and 8B
, respectively, show an experimental apparatus used for carrying out the conventional pulsed arc voltage control, and a graph showing the experimental result obtained using the experimental apparatus.
The experimental apparatus shown in
FIG. 8A
was arranged such that a welding source
101
produces an electric arc between a base metal
102
and an electrode
103
and, at the same time, a recorder
105
via a filter
104
records the arc voltage. Welding conditions utilized a reference current set to 4 A, a peak current set to 20 A, a pulse frequency set to 2 Hz, and a duty cycle set to 40%.
For purposes of illustration, the electrode
103
is shown as taking three different operation modes designated by reference characters “A”, “B” and “C”, respectively. The number of the electrode
103
used was only one. From “A” point to “B” point, welding was carried out by moving a torch (not shown) rightward in
FIG. 8A
over a distance of about 20 mm while moving the torch upward at a constant speed such that the arc length LA at “A” point is 0.3 mm and the arc length at “B” point is 1.0 mm. From “B” point to “C” point, the welding further continued by moving the torch rightward over a distance of about 20 mm while moving the torch downward at the constant speed so that the arc length LC at “C” point is 0.3 mm.
In
FIG. 8B
, the horizontal axis represents welding modes or conditions “A”, “B” and “C”, and the vertical axis represents the measured voltage. Under the welding condition “A” where the arc length is 0.3 mm, the voltage is in a range of 10.2 to 10.8 V. Similarly, under the welding condition “B” where the arc length is 1.0 mm, the voltage is in a range of 10.0 to 10.6 V. Under the welding condition “C” where the arc length is 0.3 mm, the voltage is in a range of 10.2 to 10.8 V. Although there is a little variation, the measured voltage is nearly constant even through the arc length varies from 0.3 to 1.0 mm. It may be safely said that the proportional relationship between the arc length and the voltage is not established.
FIGS. 9A and 9B
are graphs showing correlations between the arc length and the voltage that were obtained through an experiment using continuous arcs. In the experiment, welding of a copper plate was performed using a continuous arc produced by a TIG arc welding machine having a 1.6-mm-diameter tungsten electrode.
FIG. 9A
shows the experimental result obtained when the continuous arc welding was achieved at a current of 105 A. In this figure, the horizontal axis represents the arc length and the vertical axis represents the voltage. It can be seen from
FIG. 9A
that the arc length and the voltage are proportional to each other and they substantially assume a linear function.
FIG. 9B
shows the experimental result obtained from the continuous arc welding achieved at a current of 14 A. As seen from this figure, the arc length and the voltage are proportional to each other as long as the arc length indicated by the horizontal axis is not less than 1.0 mm. When the arc length is below 0.5 mm, the voltage shows an increase. As a consequence, there is no proportional relationship established between the arc length and the voltage when the arc length is less than 1.0 mm. This confirmed that the arc voltage control is not suitable for a continuous arc welding process using a small current.
The present inventors have reached a view that the known proportional relationship between the arc length and the voltage can be used when the welding is achieved with a relatively long arc length (larger than 1.0 mm and in a range of 2.0 mm to 5.0 mm) and at a relatively high current value (larger than 100 A), but the same relationship cannot apply when the arc length is less than 1.0 mm.
On the other hand, from the energy saving and environmental preservation point of view, very thin sheet metals have found an increased use in the manufacture of vehicle bodies and exhaust systems. For the manufacture of such vehicle bodies and exhaust systems, a welding process that can be achieved at a relatively small current not much exceeding 20 A and with an arc length of about 0.5 mm have become absolutely necessary.
DISCLOSUREE OF THE INVENTION
The present invention provides a novel welding technique which can realize welding of very thin sheet metals that cannot be achieved by the conventional arc voltage control.
In groping a weld control technique replacing the conventional arc length control and arc voltage control, the present inventors found a fact that the arc light changes nicely as th

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