# Design and Modelling of an Amphibious Spherical Robot Attached with Assistant Fins
## 2 Mechanical Structure and Electrical Actuation and Control System
### Introduction of the Developed Spherical Robot  
The spherical robot introduced here is a modified type aiming at amphibious applications, which is originated from traditional spherical robots [26; 27]. The spherical robot is mainly composed of two parts: a hollow spherical shell and an IDU, as shown in Figure 1. The spherical shell acts as both a wheel and a shelter; while the IDU, consisting of components inside the spherical shell, acts as the driving unit of the whole robot. The hollow spherical shell consists of two hemispheric shells made of acrylic material, which makes it transparent and with little radio interference. A toroidal collar is attached to the great circle of one hemispherical shell, acting as an installation reference for the other hemispherical shell. Some subtriangular fins are adhered onto the outside surface of the spherical shell, in order to improve the robot's locomotion performance when navigating on the water. The IDU is mounted completely in the inner space of the hollow spherical shell. Its mechanical structure mainly contains a main shaft, a support frame and a rotator. Two  
Figure 1: Mechanical structure of the amphibious spherical robot. basic motion modes of the spherical robot, i.e., rolling motion and spinning motion, can be actuated by the IDU. The main shaft lies along the sphere's main axis which is fixed relative to the spherical shell during motion, making it parallel to the ground in a stationary state. Two DC motors are mounted on the main shaft to actuate the IDU, named rolling motion motor and spinning motion motor, respectively. The rolling motion motor is placed along the main axis. When this motor starts to rotate, the IDU will depart from its balanced position and begin to rotate about the main axis. The torque exerted onto the spherical shell will accumulate as a consequence. The spherical shell starts to roll as soon as this torque exceeds the friction resistance, resulting in the linear motion of the whole spherical robot. The spinning motion motor is placed perpendicular to the sphere's main axis, and is along the diameter that passes through the contact point in a stationary state. A rotator is connected to this motor through a coupler. When the angular velocity of the rotator is changed by activating the spinning motion motor, the angular momentum of the rotator succeeds to change. As a result of the conservation of angular momentum principle, the spherical shell will spin in the direction that opposite to the change of angular momentum. The principle of those two motion modes is illustrated in Figure 2.  
A curved trajectory can be achieved upon combining the rolling motion mode and spinning motion mode, i.e., upon steering the robot when it is moving forward or backward. The rotator is placed in the interspace between the main shaft and the semi-circular support frame, which is fixed beneath the main shaft. The rotator is composed of a few pieces of ballasts, which are made by stainless steel in order to supply enormous mass and moment of inertial. In order to test the relationship between the depth of immersion and locomotion performance of the spherical robot, the number of ballasts is designed to be adjustable by splitting the ballasts into pieces with different mass. So, the total mass of the rotator can be simply changed by adding or removing some pieces of the ballasts. Sixteen subtriangular fins, eight on each side, are fixed on the outside surface of the spherical shell to improve the locomotion performance on the water. By applying prudent control strategies, these fins will not have any collision with the ground when the robot is moving on land. By contrast, they will immerge into the water when the robot is navigating on the water. Two factors--the first is the number of fins attached to the spherical shell, and the second is the shape of the fin--are believed to have significant impact on the locomotion performance of the robot on the water.