Abstract: A seam tracking system with contactless ultrasonic sensor is introduced. Fuzzy-P control theory is used in the seam tracking system. That is: the P control(proportional control) is adopted when the error of the seam position is great, and the self – tuning fuzzy control is adopted when the error of the seam position is small. Firstly, the threshold value of the Fuzzy-P control and the control rules of the proportional control are studied. Then, the principle of the self – tuning fuzzy control method is put on the seam tracking. And the fuzzy control rules are generated by checking the table. Based on the experiments, the values of factors are determined. Meanwhile, it is analyzed which the system’s dynamic characteristics are influenced by the scale factor. The scale factor value in the system is determined by the experiments. The experimental results show that the system has highly response speed precision and stability and the system can satisfy the requirements of the engineering application.

Key words: Control rule, Fuzzy-P control, Seam tracking