Prototype of positioning system for automatic motion control of underwater robot

Recently, the needs of underwater robot used for many kinds of underwater work become higher and higher. In order to control the motion of underwater robot automatically, it is an indispensable to measure its position correctly in real time. Conventional systems are based on time difference or phase lag. However, they must use expensive components, such as transponders or atomic clock, and its system becomes complex. Additionally they require a lot of signal-processing time, so they cannot be used for motion control. As a result, there exist no system which can be used for automatic motion control of underwater robot. Since 2008, we have proposed a new positioning system based on sound propagation loss and sensor network. In this system, we set many buoys that install sound reception unit, GPS receiving equipment and sensor network system on the surface of water. We use sound propagation loss to calculate the distance between robot and buoys, and with combining distance and position data for each buoy, we estimate robot position. In former study, we found that we can get higher SN ratio of signal to measure distance with using the sound of multiple frequency, we proposed a new distance measurement method M_SPL. It was introduced on the paper of “Underwater Acoustics Positioning System Based on Propagation Loss and Sensor Network” (OCEANS 2012 KOREA). In this paper, we introduce the design and construction of prototype system for this positioning system as the results of latest study. It contains transmission unit, reception unit and center unit. Transmission unit follows with principle of M_SPL, is set into underwater robot used as sound source. We also complete reception unit to be used to receive sound signal and calculate distance automatically. Finally, center unit receive distance and each buoy position data what are sent from reception unit, position can be calculated by using these data. We confirmed the performance of transmission unit and receiving unit in water-tank, and it showed that both units can be used effectively. Now we are on a final test of this system in open sea for the actual use of surveying sea floor with the underwater robot made by NAGASAKI UNIVERSITY in Japan.