New Volume Change Mechanism Using Metal Bellows for Buoyancy Control Device of Underwater Robots

We propose a new volume change mechanism using a metal bellows for a buoyancy control device of underwater robots and vehicles. Our proposed buoyancy control method utilizes the volume change caused by the phase-change of materials. We chose paraffin wax as a phase-change material because its volume change exceeds other candidates. Our proposed device consists of a metal bellows and an aluminum housing that contains paraffin wax and water. The paraffin wax is heated and cooled by a nichrome wire and a peltier device. We conducted two experiments and confirmed that the heat sink in the aluminum housing increases the speed of the buoyancy change and that the thickness of the air layer is crucial for efficient insulating. Then, we built a prototype robot with the four devices and confirmed that the robot can change its buoyancy up to its maximum value. 1. Introduction Buoyancy control is an essential ability for underwater robots and vehicles to submerge and surface. Currently, many underwater vehicles control their buoyancy by discarding or taking on ballast. However, this method is harmful to the environment because releasing materials into the water harms organisms. Also, discarding ballast water can introduce nonnative species to an area; this is the ballast water problem [1]. To solve this problem, a buoyancy control device has to be developed without material exchange. We focused on the hypothesis of a buoyancy control mechanism of sperm whales proposed by Clarke in 1978 [2–4]. He posited that sperm whales control their buoyancy by changing the volume of the spermaceti organ in their head by cooling and heating the sperm oil in it. This hypothesis is attractive because this buoyancy control method is achieved without material exchange, although it is not perfect [5]. The solid and liquid phases can also withstand high pressure in the deep sea because the volume change can be used as an actuator [6]. Therefore, we built a buoyancy control device based on Clarke’s hypothesis. There are other methods that use volume change, for example, an electrical motor that moves pistons [7, 8]. Recently, a thermal glider was developed that utilizes phase change of material to increase and decrease its buoyancy and pitch angle [9]. Also, it utilizes the temperature difference between the sea surface and the deep sea to achieve phase change of a material. However, its phase change mechanism and material remain unknown. To change the buoyancy, a buoyancy control device has to change its own volume; some volume change mechanisms exist. We tested two types. One is a