# Design and Motion Simulation of an Underwater Glider in the Vertical Plane
## 1 Introduction  
Unmanned underwater vehicles, such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), have been used to investigate the underwater terrain or work under the water. Sometimes, small ROVs are used to identify unidentified targets [1, 2]. However, the working range of ROVs is limited such that AUVs have been used for a wide range of areas. Autonomous underwater gliders, one kind of AUV, have been developed and investigated for decades as devices that enable long-range underwater detection and reconnaissance [3]. An underwater glider (UG) is a new type of underwater monitoring platform [4]; it comprises a pair of fixed wings assembled on both sides of the fuselage, as well as a controllable rudder to realize the change in motion direction. A variable-volume buoyancy engine causes changes in its own buoyancy, when the buoyancy force is less than the gravity force, the engine sinks in the water, and when the gravity force is less than the buoyancy force, it floats in the water. During the sinking and surfacing motions, the fixed wing is subjected to hydrodynamic force, which generates a horizontal force to drive the glider forward. The gliding pitch angle of the glider is changed by adjusting the position of the internal adjustable mass block in the body through the internal structure. A heat engine was investigated for use as an energy source of the buoyancy engine of a SLOCUM glider; additionally, the linearmovement of the battery pack for controlling the glider's pitching motion underwater was analyzed [5]. The mass configuration of a buoyancy-driven underwater glider was decomposed and defined, and the glider motion was established and investigated using MATLAB [6]. System identification was conducted to investigate the ballast system of a USM underwater glider to understand the pumping rate as the input of the system and the corresponding net buoyancy of the ballast system. The estimation was performed using various models available in the MATLAB System Identification Toolbox, in which the ARMAX model was selected as the best model to represent the system and to obtain the transfer function. In addition, the glider depth and pitching angle were monitored [7]. The hydrodynamic performance of the underwater glider was analyzed using FLUENT software to provide a reference basis for the ideal design of each section of the body, and the motion in the vertical plane of the underwater glider was analyzed. The variation curves of the angle of attack (AOA) and attitude angle were obtained for different net buoyancies, different center of gravity displacements, and different speeds [8]. The motion of the glider in the vertical plane was simulated and analyzed to derive a relationship between the pitch angle of the glider and each of the axial displacements of the slider and the net buoyancy [9]. For the underwater glider, under the premise of satisfying the performance, the structure should be designed as compact as possible in order to reduce the manufacturing cost, and the shape should be streamlined to reduce the resistance in an underwater operation so as to reduce the energy loss during the operation and to extend the mission time. In this paper, a new UG resisting 400 m pressure on the buoyancy engine and achieving 2 knots' speed was designed and constructed, and the related piston cylinder-type buoyancy engine and an internal mass block were studied. For the design of the UG, the net buoyancy and the movable range of the movable mass block were studied, and after determining the net buoyancy, the mass of the movable mass block could be indirectly determined. Based on the UG design, the parameters of the buoyancy engine and the movable mass were obtained. In the glider motion simulation, the displacement of the piston and movable mass block were used as input conditions, which allowed a more visual representation of the effect of different displacements on the gliding parameters.