Adaptive Control of Back Bead and Bead Height under Gap Variation in Narrow Gap Robotic Welding

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If the amount of the deposited metal becomes big in the narrow gap, the bead height becomes high. In this case, the arc heat is given on the weld pool and is not given to the root edge. Lack of fusion takes place and cause the problem concerning the mechanical strength. Therefore, in general, the amount of the deposited metal is determined so that the bead height is less than 4mm. It takes more than three times of the torch traveling from the beginning to the end of the welding line, when the thickness of the metal plates is over 12mm. Moreover, the arc behavior depends on the welding power source characteristic ; the electrode wire feed rate and the welding torch motion. The short arc length is necessary to get a high quality of the welding.In order to control the bead height and the back bead, authors propose a cooperative control method of the welding equipments including the power source, the electrode wire feeder and the welding robot by using the computer network.Authors try to n … More ot only oscillate the welding torch on the groove at 10 Hz, but also move along the weaving centre back and forth like switch back, i.e., authors propose the switch back welding method.First, the authors move the weaving centre forward, to pre-heat the root edges of the base metal by the arc. The stable back bead is formed without the lack of fusion. The mentioned welding process is called forward process. Next, the authors move weaving centre backward to deposit droplets to the root edges and to the groove surface, so that the desired bead height is obtained. This is called backward process. If the arc length becomes long, it is difficult to give the heat to the root edge and a lack of fusion in the joint part takes place. In order to observe the weld pool, it is useful to use a CCD camera. But, it is difficult to apply the observation method of the weld pool, which is proposed for thin plate welding, i.e., it is important that the shutter of the CCD camera is synchronized with torch oscillating. In this paper, the shutter of the CCD camera is opened when the torch passes on the centre of the weaving. In the switch back welding, there are many parameters of welding conditions, such as traveling speed of the torch, the current, the wire feed rate and so on. To find the optimum parameters of the welding conditions, the numerical model of the weld pool is described by using an equation of heat conduction. It takes long time to solve the numerical model by applying the finite difference in a numerical analysis. In stead of the numerical model, the fuzzy model of the weld pool is proposed to save the calculation time. Its model is adjusted by using the numerical results. The optimum stroke length is estimated from 4 to 6mm gap. In order to verify the validity of the fuzzy model and the proposed welding method, the experiment is carried out under the variation of the gap. A good result of the welding can be obtained, i.e., the smooth surface bead and wide back bead are obtained. Less