# Design and Modelling of an Amphibious Spherical Robot Attached with Assistant Fins
## 1 Introduction  
Amphibious robots are designed to cope with drudgeries involving miscellaneous terrains, such as lakes, wetlands, shallows and pipelines [1, 2]. Compared with a robot moving only on land or underwater, which is easily stuck in the junction area between the land and the water, amphibious robots are capable of both aquatic locomotion and terrestrial locomotion, which are especially suitable for applications in unknown environments, potentially with complex terrains. Tasks calling for this kind of robots include environment detecting, wild exploring, security surveillance as well as scientific inspecting [3]. A variety of amphibious robots have been developed by researchers in recent years. As one of the typical amphibious robots, snake-like robots are inspired by the locomotion mode of a snake as well as a lamprey. A snake-like robot called HELIX is developed by [PERSON] and [PERSON] to verify the propulsion principle that helical motion can be created by successive distortions of articulated body segments [4]. The design of one unit of HELIX, including a special spherical mechanism and two servo motors, is introduced in detail by the same authors. As a modified prototype of HELIX, ACM-RS bears impressive performance in terms of its dexterous locomotion capacity. Underwater swimming fins are attached around the outer surface of each unit to assist the underwater motion, with passive wheels attached on the tip of each fin in order to improve the on-land locomotion performance [2, 5]. The AmphiBot robots, a series of amphibious snake-like robots capable of swimming in water and crawling on land, are designed by [PERSON] and [PERSON] et al. [6]. A central pattern generator (CPG)-based controller is used to generate lateral undulation for this kind of robot [7, 8]. Some key parameters that produce fast locomotion gaits, such as the amplitude of oscillation, frequency and wavelength, are identified by a series of experiments. An improved version of AmphiBot, named Salamandra robotic, equippedwith a passive tail and four active 3-DOF limbs, is also introduced [9]. To make the snake-like robot to follow a desired path, a straight line path following controller is proposed by [PERSON] et al. [10; 11]. Another type of amphibious robots is equipped with flippers that act as paddles in water and as legs on land. Using six paddles for propulsion, AQUA is suitable for navigating in shallow-water environment [12; 13]. With the help of an acoustic-based localization system and a vision-based localization system, AQUA obtained the ability to revisit a previously visited site autonomously [14; 15]. To enhance its climbing ability, Whegs IV combines two similar body segments with wheel-leg propellers [16; 17]. AmphiHex-I obtains versatile gaits with the aid of transformable flipper legs, which makes it flexible in transforming propulsion mechanisms [18]. Its improved version with variable stiffness legs and CPG-based control strategy is also introduced [19]. A quadrupedal micro-robot weighs only several grams is introduced by [PERSON] et al. [20], which uses passive flaps to swim forward and adjust direction. Some robots with spherical shape have also drawn researchers' attention. The Ground-bot, an amphibious spherical robot that accomplishes its propulsion system by displacing the center of mass, is introduced by [PERSON] et al. [21]. Groundbot shows good robustness in the waypoint following mode. An amphibious spherical robot which combines two actuating systems, including the quadruped walking system for terrestrial locomotion and the water-jet propulsion system for underwater locomotion, is designed by [PERSON] et al. [22; 23]. A micro-robot which is used as a manipulator and a monitor can be carried and deployed by this amphibious spherical robot. An amphibious spherical robot uses pendulum to move on land and propeller to move in the water is developed by [PERSON] et al. [24]. A spherical rotary paddle called Omni-Paddle is proposed to form a robot that mainly moves on the boarder of water and ground [25].