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Mobile robots that can survive in unstructured wildernesses is essential in many applications such as environment detecting and security surveillance. In many of these applications, it is highly desirable that the robot can adapt robustly to both terrestrial environment and aquatic environment, and translocate swiftly between various environments. A novel concept of amphibious spherical robot with fins is proposed in this paper, capable of both terrestrial locomotion and aquatic locomotion. Unlike the traditional amphibious robots, whose motions are commonly induced by propellers, legs or snake-like tandem joints, the proposed amphibious spherical robot utilizes the rolling motion of a spherical shell as the principal locomotion mode in the aquatic environment. Moreover, spinning motion of the spherical shell is used to steer the spherical robot efficiently and agilely; several fins are attached to the outer spherical shell as an assistance to the rolling motion. These two motion modes, rolling and spinning, can be used unchangeably in the terrestrial environment, leading to a compact and highly adaptive design of the robot. The work introduced in this paper brings in an innovative solution for the design of an amphibious robot.

Due to the nonholonomic and underactutated properties of spherical robot, to realize its steady motion control is usually difficult. This paper presents a linear motion control method for an underactuated spherical robot. The linear motion dynamic model of the spherical robot is deduced with Euler-Lagrange method, which is a second order, nonlinear differential system with two coupled variables. Without any linearization a single-input multiple-output PID controller to realize the position control and pendulum angle control simultaneously is designed. Simulation results about the position stabilization and path tracking control are provided separately to show the control effects of the proposed controller.