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In general, the teaching by showing or offline programming is used for path planning and motion programming for the manipulators. The actions preset are merely repeated in the working process. If the states of work piece varied, the manufacture quality would be influenced too intensely to satisfy the demand of production. In addition, the teaching by showing or offline programming costs much time, especially in the situations that much manufacture variety with little amount. The introduction of visual measurement in robot manufacture system could eliminate the teaching time and ensure the quality even if the state of the work piece were changed. Obviously, visual control can make the robot manufacture system have higher efficiency and better results (Bolmsjo et al., 2002; Wilson, 2002).There are many aspects concerned with the visual control for robotic welding such as vision sensor, image processing, and visual control method.As a kind of contactless seam detecting sensors, structured light vision sensor plays an important role in welding seam tracking. It has two categories. One uses structured light to form a stripe, and the other uses laser scanning. Structured light vision is regarded as one of the most promising methods because of its simplicity, higher accuracy and good performance in real-time (Wu & Chen, 2000). Many researchers pay their attention to it (Bakos et al., 1993; Zou et al., 1995; Haug & Pristrchow, 1998; Zhang & Djordjevich, 1999; Zhu & Qiang, 2000; Xu et al., 2004). For example, Bakos established a structured light measurement system, which measurement precision is 0.1mm when the distance is 500 mm. Meta Company provides many kinds of laser structured light sensors. In general, the sensor should be calibrated before putting into action. Camera calibration is an important classic topic, and a lot of literatures about it can be found (Faugeras & Toscani, 1986; Tsai, 1987; Ma, 1996; Zhang, 2000). But the procedure is complicated and tedious, especially that of the laser plane’s calibration (Zhang & Djordjevich, 1999). Another problem in structured light vision is the difficulty of image processing. The structured light image of welding seam is greatly affected by strong arc light, smog and splash in the process of arc welding (Wu & Chen, 2000). Not only the image is rough, but also its background is noisy. These give rise

Visual Control System for Robotic Welding