# Performance Evaluation of a Novel Propulsion System for the Spherical Underwater Robot (SURIII)
## 1 Introduction  
With the unceasing underwater exploration of the ocean, autonomous underwater vehicles (AUVs) have also been improved and developed for underwater detection. If AUVs in the unknown underwater environment can still maintain adequate flexibility and sensitivity, they can be suitable for a variety of complex underwater tasks. The IFREMER L'Equalard AUV was probably the first AUV which deployed for marine geoscience [1; 2]. [PERSON] et al. developed a ball shaped underwater robot which was completely smooth and used jets to propel and maneuver through water-filled environments such as the inside of nuclear power plants [3]. MIT researchers also unveiled an oval-shaped submersible robot smaller than a football, flattened on one side and able to slide along an underwater surface to perform ultrasound scans [4]. But these two kinds of robots are both designedfor special environments such as pipelines or confined spaces, with relatively simple forward and backward motion. [PERSON] and [PERSON] proposed a swimming pattern generator mimicking locomotion of diving beetles with the viewpoint of biomimetics for legged underwater robots [5]. In our previous study, firstly a spherical underwater robot with multi vectored water-jet thrusters was developed [6; 7]. Then a Father-son Underwater Intervention Robotic System (FUIRS) was proposed [8]. In this system, the SURII plays the role of the father robot and the micro robot plays the role of the son robot [9]. The father robot has a spherical shape and a vectored water-jet-based propulsion system, so it can work with high stability and low noise. The FUIRS system is shown in Figure 1. With the development of the previous research, the SURIII which has four vectored water-jet thrusters for the propulsion system is proposed [10; 11; 12]. AUVs face different working conditions, requiring different configurations and size, thus many new underwater technologies are being explored and applied to AUVs. Because of the excellent diving capability of underwater creatures, some underwater robots are designed to mimic their characteristics. [PERSON] et al. developed a turtle-like swimming robot with biomimetic flippers. This robot uses a smart soft composite structure to realize smooth and soft flapping motion [13]. [PERSON] lab in Kagawa University developed an octopus-inspired micro robot using ionic polymer metal composite (IPMC). It can cooperate with other robot to execute underwater intervention tasks [14]. These above robots imitate the appearance or movement of aquatic creatures to better adapt to the underwater environment. [PERSON] et al. developed a micro-spherical underwater robot which used six propellers to generate propulsive force and this robot was designed for monitoring the nuclear storage pond and preventing leakage in wastewater treatment facilities [15; 16]. However, its propeller thruster cannot adjust direction. MIT researchers designed a robot called \"Omni-Egg\", which is smooth, spheroidal and completely appendage-free [17]. It used a novel pump-jet system built into the streamlined shell. Researchers at Harbin Engineering University also developed a spherical underwater robot with three water-jet thrusters [18; 19]. Spherical Underwater Robot (SUR) is a kind of special structural underwater robot which has better compression performance. Due to its overall symmetry, there are no coupling items in fluid dynamics calculation and the hydrodynamic parameters are also equal. Therefore, it has more extensive application. In this paper, a novel spherical underwater robot (SURIII) which has four vectored water-jet thrusters was developed. A novel annular symmetrical structure was proposed to ensure the stability of the propulsion system. The static analysis proved that this new structure is more stable than before. The improvement of structure increases the utilization of space. Therefore, the rotation angle of vectored water-jet thrusters is more varied and flexible. Hydrodynamic characteristics are the key point and difficulty for underwater robot research.