ABSTRACT

The authors developed an autonomous surface vehicle (ASV)/ remotely operated vehicle (ROV) joint mobility vehicle for ocean investigations. The vehicle was selected as the next-generation sea mobility vehicle by the Ministry of Land, Infrastructure, Transport, and Tourism in Japan. The ASV, a catamaran type autonomous ship, has onshore and underwater cameras. The ROV, which holds an underwater camera, detaches and attaches automatically from the ASV to take 3D images of underwater objects. Its mobility was shown by sea-litter monitoring tests conducted around Tsushima, Japan. Great capabilities of the ASV/ROV joint mobility vehicle for exploration of undersea ruins and underwater structures were demonstrated by offshore trial test results.

INTRODUCTION

Nagasaki prefecture, located in western Japan has a long coast seashore and remote islands. They present several social challenges to be solved. Tsushima is an island city grouped in Nagasaki Prefecture, Japan. Tsushima Island is an island of the Japanese archipelago situated in-between the Tsushima Strait and Korea Strait, approximately halfway between Kyushu and the Korean Peninsula in shown Figure 1.

(https://en.wikipedia.org/wiki/Tsushima_Island)

Tsushima island of Nagasaki prefecture at the border of Japan has persistent difficulties ocean drifting litter in coastal areas, as shown in Figure 2. However, the drifting ocean litter is only partly manageable in many areas because of labor shortages and difficulty of access by land routes. Furthermore, underwater monitoring and seabed litter must not influence living environments or damage fisheries. Therefore, the authors developed the autonomous surface vehicle (ASV)/ remotely operated vehicle (ROV) joint mobility vehicle for monitoring ocean drifting litter, as shown in Figure 3. Mobility vehicles are useful not only for monitoring ocean drifting litter, but also for general ocean investigations to examine undersea ruins and underwater structures.

ASV/ROV JOINT MOBILITY VEHICLE

The authors developed the ASV called "Kenbot" and its system shown in Figure 4. Kenbot has four thrusters located in a rhombus arrangement, which enable omnidirectional movement. Table 1 presents the Kenbot specifications: its size is 1400 × 1100 × 650 mm; its weight is 30 kg. Kenbot is equipped with GNSS, with such functions as dynamic positioning and automatic navigation by setting up route coordinate in advance. Additionally, it can be operated remotely by an operator on land. By a camera and a transmission device equipped with Kenbot, images can be transmitted in real time to an operator on land. Moreover, it has a cable control device for linkage with an ROV. A winding drum with a DC motor and locking by electromagnetic braking are possible. A cable wound by drum is connected with a PC on the ASV through a slipring. In addition, a cage for ROV storage is set up between floating units of the ASV. The cage height is adjustable for the ROV to limit the hydrodynamic resistance during ASV cruising. Moreover, the ROV, called "Caibot III", with size of 470 × 450 × 200 mm and weight of less than 10 kg, is connected by a cable to the ASV. It can take 3D underwater pictures and transmit images to land in real time. Figure 5 and Figure 7 show the ROV and underwater camera unit equipped with the ASV. Table 2 and Figure 6 present specifications of the ROV and its system structure. The system configuration, including the ROV, ASV, and acoustic positioning device, is shown diagrammatically in Figure 6. In this system, the ROV is operated by the operator, and the ASV moves autonomously according to the position and heading of the ROV. The ASV determines its movement from the relative position and azimuth of the ROV, as well as the cable length. The relative position of the ROV to the ASV is determined by an acoustic positioning system, which has a sampling rate of approximately 1 Hz. Figure 8 shows the developed ASV/ROV joint system. The ROV has Pixhawk and RaspberryPi for its controller. The developed ASV/ROV joint mobility vehicle has important unique features not shared with ROV-related mobility vehicles described in earlier reports from throughout the world (Fletcher et al., 2008; Meinecke et al., 2011; Morinaga et al., 2023; Roman et al., 2000; Sherman et al., 2001; Shibata et al., 2010; Wasserman et al., 2003; Yamamoto et al., 2013; Yamamoto, 2016; Yamamoto et al., 2019).

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