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A heavy-duty industrial robot with a load of 3.5kn and a positioning accuracy of 0.05mm was combined with the end effector of friction stir welding and refitted into a robot friction stir welding system. In the friction stir welding machining test, it was found that although the structural strength of the robot met the use requirements, its stiffness under some working conditions didn’t meet the requirements, resulting in low positioning accuracy and unsatisfactory machining accuracy of the friction stir welding robot. To solve this problem, a closed-loop controlled heavy-duty friction stir welding robot system based on ultrasonic positioning technology was designed. The system was composed of dual ultrasonic transmitters, multiple ultrasonic receivers, positioning processor. Two ultrasonic transmitters with different transmission frequencies were arranged at two different positions of the end effector, and then two groups of receivers that could distinguish the ultrasonic frequency were used to receive the ultrasonic signal. The real-time coordinates of the two ultrasonic transmitters in the positioning space were calculated by the positioning system to determine the position and posture of the end effector. The end effector was controlled to track the predetermined trajectory in real time to realize the closed-loop control of the robot. Based on the three-point positioning principle, the relevant calculation and analysis were carried out, which proved the rationality and feasibility of the system in theory. The experimental results show that the positioning accuracy of the system is within 1mm and the posture error is within 0.01˚.
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