JPH09108843A - Seam welding method of tank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent defective welding due to the change in speed of electrode rings without changing the energizing pitch or the rotational speed of the electrode rings. SOLUTION: A fuel tank is supported in a freely rotatable condition by an arm of an articulated robot. In this condition, a joining flange part of the fuel tank is held between upper and lower electrode rings of a seam welding machine. The joining flange part is seam welded by intermittently flowing the DC current between the electrode rings while the electrode rings are rotated. The DC current is variable in frequency by the inverter control, and when the speed of the electrode rings is dropped at the inlet of a corner part of the joining flange part, the frequency is dropped to reduce the size of a nugget N. On the other hand, when the speed of the electrode rings is increased at the outlet of the corner part of the joining flange part, the frequency is increased to increase the size of the nugget N. The overlap amount between nuggets N is made constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tank seam welding method suitable for a fuel tank for automobiles, and more particularly to controlling a direct current flowing between upper and lower electrode wheels by inverter control in proportion to the welding speed of the upper and lower electrode wheels. The present invention relates to an increased seam welding method.

[0002]

2. Description of the Related Art In a fuel tank for automobiles, as shown in FIG. 2, an upper member a and a lower member b formed by a sheet metal press are vertically aligned, and a joining flange portion c at the peripheral edge of the divided surface is spot-welded. After being temporarily fixed at several points, the joint flange portion c is continuously welded. A seam welder having upper and lower electrode wheels is used for this welding.
Is rotatably supported by a lower jig or a robot arm. The tank supporting method using the lower jig has a drawback that it takes time to manufacture the jig and thus cannot quickly respond to the shape change of the tank. For this reason, a tank supporting method using a robot arm is becoming mainstream these days. This robot arm support method is shown in FIGS.
As described above, a pair of freely rotating upper and lower rotating shafts 6b and 6c are arranged on the C-shaped frame 6a of the fuel tank gripping means 6 attached to the tip of the arm 5a of the multi-axis robot 5, and the rotating shaft 6b is provided. , 6c press and support the upper and lower surfaces of the upper and lower members a and b, and the rotation angle detector 8 detects the rotation angle of the fuel tank d accompanying the rotation of the upper and lower electrode wheels 3 and 4 of the seam welding machine 2. The multi-axis robot 5 is controlled by the control device 10 on the basis of the detected rotation angle and the peripheral contour shape of the joint flange portion c so that the joint flange portion c is always fed tangentially to the electrode wheels 3 and 4. Rotating shafts 6b, 6
c is sequentially moved. In FIG. 3,
6d ... Vertical movement device, 7 ... Rotation imparting means, 9 ... Contact detector. The rotation imparting means 7 is operated before starting welding in order to move the welding start position of the joint flange portion c of the fuel tank d between the upper and lower electrode wheels 3 and 4. In addition, the pressing force detector 9
Detects the pressing force between the joint flange portion c of the fuel tank d and the lower electrode wheel 4 during the welding operation, and when the pressing force falls below a predetermined value, the gripping means 6 of the fuel tank d is moved downward. A signal is sent to the control device 10 of the multi-axis robot 5 so as to increase the pressing force to a predetermined value.

[0003]

In the conventional supporting method using the robot arm 5a, the rotating shafts 6b and 6c are moved with the rotation of the electrode wheels 3 and 4 or the movement of the welding portion. When welding a linear portion other than the corner portion of the fuel tank d, the rotating shafts 6b and 6c also linearly move and the fuel tank d moves substantially in parallel without changing its posture.
The feed load of the fuel tank d acting on the electrode wheels 3 and 4 is close to zero. However, when the electrode wheel 3 (4) welds the corner of the fuel tank d as shown in FIG. 5 (b), as shown in FIG. 5 (a), the electrode wheel 3 (4) moves the fuel tank d. Since it must be rotated, a relatively large feed load acts on the electrode wheels 3 and 4. Therefore, the electrode wheels 3, 4
The rotation speed of the is slightly reduced, and as shown in FIG.
The amount of overlap of the weld mass (nugget) N increases,
Welding defects may occur due to overheating. Immediately after the electrode wheels 3 and 4 pass through the corners, the fuel tank d shifts from the rotational movement to the parallel movement, and the feed load acting on the electrode wheels 3 and 4 is drastically reduced accordingly. There is a possibility that welding speed may be deteriorated due to a rapid increase in rotation speed and a defective overlap of the nugget N.

In order to eliminate the welding failure due to the variation of the feed load of the welding flange portion at the periphery of the fuel tank d and the accompanying change in the advancing speed of the electrode wheels 3 and 4, the time pitch of intermittent energization is changed. Alternatively, it is necessary to take measures such as changing the rotational speeds of the electrode wheels 3 and 4, but to change them, the control system needs to be greatly modified, which is too costly. An object of the present invention is to prevent welding defects due to changes in the feed speed of the electrode wheels without changing the energization pitch or the rotation speed of the electrode wheels.

[0005]

In order to solve the above-mentioned problems, the present invention increases the DC current flowing between the upper and lower electrode wheels by inverter control in proportion to the relative speed of the upper and lower electrode wheels with respect to the joint flange portion. did.

As a result, even if the energization pitch and the rotational speed of the electrode wheels are not controlled, when the traveling speed of the upper and lower electrode wheels decreases, the current flowing through the upper and lower electrode wheels becomes smaller and the nugget N becomes smaller. When the traveling speed of the upper and lower electrode wheels is increased, the current flowing through the upper and lower electrode wheels is increased and the nugget N is increased. Therefore, the overlap amount of the nugget N is always constant, and stable welding quality can be obtained.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 (b) shows a method of flowing a welding current according to the present invention. When the upper and lower electrode wheels 3 and 4 are at normal speed (the straight portion of the joint flange portion c is 700 Hz at the time of welding), 500 Hz at a slow speed (when welding of the corner portion of the joint flange portion c) starts, at a high speed (at the time of finishing welding of the corner portion of the joint flange portion c)
Is 1000 Hz, and a direct current is chopped to pass electricity between the upper and lower electrode wheels 3 and 4. Chopping is performed by inverter control, and one chopping switch O
Inverter control is performed such that a current of a predetermined value flows for N for a predetermined short time. Therefore, the higher the chopping frequency, the larger the amount of heat input to the joint flange portion c.

In the present invention, since the frequency of the welding current is increased in proportion to the moving speed of the electrode wheels 3, 4, the electrode wheels 3, 4 are
When the pitch of the nugget N is narrow at a low speed, a small nugget N is formed at a low frequency, and when the electrode wheels 3 and 4 are fast and the pitch of the nugget N is wide, a large nugget N is formed at a high frequency. Therefore, even if the intermittent energization pitch of the welding current is the same as the conventional one, even if the moving speed of the electrode wheels 3 and 4 changes, the overlap amount of the nugget N is made constant and welding defects can be prevented.

Since the speeds of the electrode wheels 3 and 4 are uniquely determined by the shape of the joint flange portion c or the radius of curvature of the corner portion, the angle detected by the rotation angle detector 8 in FIG. 3 and the joint flange portion c in advance. If the control device 10 stores the relationship with the specific positions before and after the corner portion, it is possible to form the nugget of the optimum size by the optimum inverter control without detecting the rotation speeds of the electrode wheels 3 and 4 one by one.
The overlap amount of the nugget N can always be optimized. The present invention is suitable for application to seam welding of fuel tanks for automobiles, but it can also be applied to other similar airtight or liquid-tight tank seam welding to obtain similar effects.

[0010]

According to the present invention, the welding current is increased in proportion to the moving speed of the electrode wheel. Therefore, even if the speed changes when the electrode wheel passes through the corner portion around the tank,
Even if the conventional energizing pitch remains the same, the size of the welding nugget is changed so that the amount of overlap with the adjacent nugget is always constant, so changes in the control system of the seam welder are minimized to prevent welding defects. can do.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a method of flowing a welding current and an overlapping state of a nugget in comparison with the present invention and a conventional one.

FIG. 2 is a sectional view of a fuel tank.

FIG. 3 is a side view of a conventional seam welder in which a fuel tank is supported by a robot arm.

FIG. 4 is a perspective view showing a joint flange portion of a fuel tank and an electrode wheel.

FIG. 5 is a plan view of the fuel tank when welding a corner portion.

[Explanation of symbols]

 d Fuel tank 2 Seam welder 3 Upper electrode wheel 4 Lower electrode wheel 5 Multi-axis robot 6 Fuel tank gripping means 8 Rotation angle detector 10 Control device

Claims (1)

[Claims] 1. A method of seam welding by sandwiching a joining flange portion at the peripheral edge of a tank between upper and lower electrode wheels in a state in which a vertically divided tank is rotatably supported by a tip end portion of a robot arm. A seam welding method for a tank in which a direct current flowing between the upper and lower electrode wheels is increased by inverter control in proportion to the relative speed of the upper and lower electrode wheels with respect to the section.