AUV based Precise Seabed Mapping with a Wave Energy-harvesting Surface Vehicle

Abstract

The AUVs have a wide range of applications and are being deployed for various purposes in an oceanographic survey, geoscience, military surveillance, and industrial areas. However, in most cases, the AUVs have preprogrammed a plan to follow a preset route of waypoints and there are few reasoning and adapting for changes against an unexpected situation even in case of the commercial AUVs. To reach a higher intelligence level for AUV technology, the AUVs must perceive the surroundings and infer their current states based on the perceived information. For underwater perception, vision-based sensors are widely used but have limits to use in water due to rapid wavelength-dependent attenuation of light by water. With consideration for the water turbidity, sonars are a generic solution for underwater sensing. Compared to vision-based sensors, the lack of information of sonar data is indisputable: the loss of elevation information, perceptual ambiguity, and a high proportion of outlier, which complicate sonar data processing and three-dimensional (3D) map building. Another issue on AUV exploration is about connectivity. The AUVs should be connected to a network for sharing the data obtained from onboard sensors and intervention for high-level work. Therefore the subsea data can be transmitted to the air only via a relay station on the surface such as relay buoys. To overcome the issues, we propose a sustainable connected AUV system that consists of an AUV and surface vehicle. The AUV is able to perceive the environment regardless of water turbidity. The surface vehicle has affordable electrical payload for long-range data communication and maneuvering for relocation. The two vehicles are interlinked via acoustic communication. For the perception, sonar-based mapping is proposed, and for the electrical payload of the surface vehicle, a novel wave energy harvesting device is developed. First, we present a three-dimensional (3D) mapping method in one-way rectilinear scanning with an autonomous underwater vehicle (AUV) equipped with a forward-looking sonar (FLS) and a profiling sonar (PS). Our approach is to use an additional sonar and fuse acoustic measurements provided by the two sonar sensors. The FLS has a high resolution in a horizontal scan but has an uncertainty in the vertical direction. On the other hand, the PS provides a reliable vertical profile but its beam width is extremely narrow. An initial map is generated by the FLS and refined by combining vertical scan data provided by the PS. Second, a novel surface vehicle was proposed to support a long-term survey of AUV by harvesting wave energy. We proposed a wave energy converter called the wave turbine system (WTS) and verified the feasibility of the proposed system. To verify the proposed mechanism and identify the system parameters, we developed a hydrodynamic model for the WTS and simulated its behavior and power generation capability. From the quantitative simulation, optimal system parameters were analyzed. To check the reliability of the simulation result, we carried out verification tests in a water tank, and the simulation result was verified. Finally, The hardware systems for an AUV named Cyclops and an energy-harvesting surface vehicle were developed. The proposed method is implemented in the developed system and to demonstrate the validity and effectiveness of the proposed method, we conducted a series of tests in a water tank and also at sea. The total system was integrated, and validity was demonstrated through the sea trial.