# Performance Evaluation of a Novel Propulsion System for the Spherical Underwater Robot (SURIII)
## 2 Mechanical Design and Analysis
### Mechanical Design and Analysis of Propulsion System  
Taking the requirement of volume into consideration, SUR as a kind of small underwater robot in our lab, its movement is restricted by the structure and space. Thus, its propulsion system needs to be improved in the limited space to enhance the hydrodynamic property. In the propulsion system of the SURII, three water-jet thrusters were homogeneously located on an equilateral triangle support as shown in Figure 3. This kind of layout limited the rotation angle of servo motors, thereby effecting the movement performance, especially the forward movement. Such a structure cannot achieve the maximum propulsive force and the payload capability will be affected. In terms of the current issue, a novel annular symmetrical mechanism is designed for the propulsion system of SURIII as illustrated in Figure 4. This system adopts an annular structure which can provide support for four thrusters. The maximum distance between the two opposite thrusters is 48 cm, the inside diameter of the annular support is 19 cm and its thickness is 0.5 cm. Considering the size of spherical hull, the weight of the robot and advantages of symmetric structure, only one more set thruster is added. Moreover, the stability of robot will be impacted less while improving the dynamic power. In order to enhance the stiffness of the propulsion system, the annular support structure is fixed with the waterproof bin by jackscrews. Figure 2: Propulsion mechanism of the jellyfish. For the novel propulsion system, the static analysis is necessary to evaluate its performance. Above all, unforeseen deformation will adversely affect the robot, particularly the direction of the nozzle. Although the second-generation robot has already enhanced the rotation support between servo motors and the water-jet thruster and reduced the deformation of the system [21], its triangle support is only suitable for three water-jet thrusters and cannot be used for fours. Therefore, the deformation analysis for the annular support is very important. Moreover, the weight of the propulsion system is increased after adding one more set of vectored water-jet thruster, hence the static analysis is quite essential. The basic movements of the robot include three motions: up, down and forward motion. Based on this, the static analysis is carried out by the ANSYS WORKBENCH. Figure 5 illustrates the simulation models of the different motions for SURIII. The up and down motion models are shown in Figure 5a,c, the 2 N force is applied to each nozzle, respectively. The forward motion model is shown in Figure 5b, the 2 N force is exerted on three working nozzles. The position and direction of these forces are the same as the static analysis of the SURIII. Both the triangle support of the SURII and the annular support of the SURIII are fixed with the ground. Figure 6 displays the deformation of the propulsion system. Figure 6a,b describe the deformation of the previous and the novel propulsion system, respectively. According to the rainbow map in the left of the interface, red color indicates the maximum deformation in the result. Observing these two propulsion systems, the maximum deformation occurs at the same location, therefore the maximum deformation of these two propulsion systems is compared to evaluate the stiffness. Figure 7 is the comparison of the maximum deformation between these two propulsion systems. From the comparison, it is evident that no matter what kind of motion state, the deformation of propulsion system in SURIII is smaller than that in SURIII. However, the maximum deformation (0.045 mm) of all  
Figure 4: Propulsion system of SURIII. Figure 3: Propulsion system of SURII. Due to the good characteristics of the vectored water-jet thruster, this part uses the previous design. Each set of vectored water-jet thruster is composed of four parts, which includes a water-jet thruster, a waterproof box for protecting DC motor, two servo motors and a support for them.