## 1 Introduction  
With the reduction of land resources, the exploration of marine resources has become particularly important. An underwater vehicle is the main tool to explore the marine resource, and the complex submarine environment requires high flexibility and controllability of the vehicle [1; 2; 3; 4]. At present, most layout schemes of underwater vehicle are as follows:  
(1) Multi-thruster distributed layout  
Multiple thrusters are installed in different positions of the vehicle to realize the multi-degree-of-freedom maneuvering motion, including the rotation and pitch. Because of its favorable operability and flexibility, this layout scheme is widely used in underwater vehicles with high mobility requirements. A typical case is the underwater mine-clearing vehicle named double-head eagle launched by SAAB AB (Saab-ScaniaAB) company, which can achieve omni-directional control at zero speed and backward motion. However, some of these thrusters are invocated only at turning and lie idle most of the time, which causes a waste of the mechanism resource and the increase of volume and underwater sailing resistance [5; 6].  
(2) Vector water-jet propulsion system This method controls the thrust direction by changing propellers' high-speed jet direction, combining the advantages of water-jet propulsion and thrust-vectoring. For example, the water-jet propulsion systems manufactured by the Hamilton corporation in New Zealand and the Kamewa corporation in Sweden, control astern guide units and steering pipes to produce water jets in different directions, so that the turning and pitch and other attitudes could be realized [7,8]. However, the actual propulsive efficiency of the water-jet propulsion system is affected by the thrust loss of the propulsion pump and pipeline. Compared with the propeller propulsion, the energy conversion rate is lower [9].  
(3) Vector propeller propulsion system  
This system uses a single propeller, in which thrust directions are changed by the thrusters' control to achieve vector propulsion, such as the full deflection vector thruster based on the spatial linkage and universal joint proposed by [PERSON] and [PERSON] [10,11], and a similar mechanism, the full deflection vector thruster with single rotator by [PERSON] and [PERSON] [12]. However, these kinds of mechanisms have problems such as redundancy of degree of freedom and relatively complex control [13]. [PERSON], [PERSON], et al. designed a 3-SPS-S (Spherical-Prismatic-Spherical-Spherical) parallel manipulator with passive constraining spherical joints to drive the underwater vehicle [14]. [PERSON] and [PERSON] used a 3-RPS (Revolute-Prismatic-Spherical) parallel mechanism with hydraulic driven to realize the attitude adjustment of the vehicle [15]. The above thrusters all use the external actuator, which puts forward high request to the sealing characteristics of the propulsion device. Meanwhile, the position of the propeller varies greatly during the attitude adjustment, which goes against stability of the underwater vehicle [16].  
On the basis of the above analysis, a RS+2 PRS (Revolute-Spherical+ Prismatic-Revolute-Spherical) vector propulsion device is proposed in this paper, which allows AUV (autonomous underwater vehicle) attitude adjustment to be accomplished with only a single propeller and avoids a waste of the mechanism resource. The device only contains 2-DOF (degree-of-freedom), which avoids the waste of power resources caused by redundant DOF. In addition, this device can realize the in situ attitude adjustment of the propeller, which reduces the volume of the device and makes the sealing easier to realize.