Development of underwater ultrasonic positioning system for construction machines

The construction and maintenance of port facilities are carried out by the human labor of divers. Mechanization of underwater work is necessary for safer and more efficient operation of large-scale port construction work. To satisfy the underwater construction standards by machine work, high accuracy positioning technology that has accuracy of ±10 cm or less is required in a rather narrow area for port construction work. However, conventional ultrasonic positioning devices such as LBLs and SBLs target at wide area positioning and have accuracy of 50-60 cm at least. The main factors of such accuracy of the conventional system are caused by the measurement error of the sound propagation time from responders to receivers and the incorrectness of the average sound velocity in the propagation path. Therefore, to achieve the positioning accuracy less than ±10 cm in the arrangement that receivers can be equipped on a work vessel, we have developed the new type underwater positioning system. This system consists of 3 responders on the underwater construction machine and 4 receivers on the work vessel. The 4 receiver system can give information about the mean sound velocity required for calibrating the distance between responders and receivers. To improve the measurement accuracy of propagation time, the system employs the ultrasonic signal modulated by M-sequence. To confirm the basic performance and potential advantages of this positioning system, numerical simulation has been performed. In this simulation, accuracy of the system and the required length of the base line have been calculated. The simulation has shown that the proposed system has higher accuracy than conventional system and base line longer than 5.9 m is required in order to achieve the measurement accuracy of ±10 cm or less when the underwater construction machine moves within 20 m in radius from work vessel. These results have been verified with several tank tests using one transmitter/receiver. In the next step, we develop prototype ultrasonic positioning system, and demonstrate the absolute positioning of the underwater machines, combined with GPS in the real sea.