Study on analysis and design of an VIAM- AUV2000 Autonomous Underwater Vehicle (AUV)

This paper presents the design of the VIAM-AUV2000 autonomous underwater vehicle (AUV) with a built-in cylinder for floatation and counterbalance. The modular structure including mechanical design, electronic system, and control algorithm ensures continuous operation for the vehicle at a depth of 50 meters underwater. The main content will consist of two parts: the mechanical implementation and the electrical system. The mechanical implementation part will focus on cal-culating ship hull profile and material selection; computing and simulating stress and distortion on ship hull and waterproof covering using finite element method with NX Nastran; analyzing and planning cylinder and counterbalance arrangements. At the same time, the advantages of hybrid AUV design inspired from the traditional one with thruster and fins as well as the underactuated glider form using counterbalance and cylinder for diving and floating are discussed specifically in the upcoming sections. The electrical system for the robot is also mentioned and clarified through the selection of sensors, actuators and hardware design to ensure stable operation for diving robot at a depth of 50m and operate continuously for long periods under water by using traditional AUV mode and glider mode. Some experimental results of thruster and three-axis tilt estimators with error of less than 1 o are also presented in this paper.


INTRODUCTION
Nowadays, along with the rapid revolution of humankind, science and technology become more modern day by day, we gradually explore and conquer the mysteries of nature. However, the ocean is still a mystery far away from our reach and understanding. The research of ocean, decryption of the mystery in the deep of the sea required modern technology such as unmanned underwater vehicles, which can swim in the deep that no human can reach. In order to investigate the water environment, examine the ecosystem, probe the environmental fluctuation, or use for the military purpose, national defense, and observation… many prototypes of AUVs have been researched and developed. AUV Remus 100 of Woods Hole Oceanographic Institution 1 can dive to 100 meters in more than 10 hours at the velocity of 2.3 m/s. Lightweight AUV 2 developed by Porto University in cooperation with OceanScan work at 20 meters depth in 8 hours at 1.5-2m/s. Autosub6000 of Autonomous Underees Vehicle Applications Center can dive to 6000 meters in 30 hours at 5km/h. Slocum Glider without thruster can work in several months 3 . Vietnam is a coastal country with more than 3.200 kilometers of coastline, and the sea area is about 1.000.000 square kilometers. Economic, scientific, tourism activities and national defense that take place on the sea are playing a very important role. Nowadays, many constructions are built on the sea such as ports, oil platforms, oil and gas pipelines, etc. At the same time, the arising of critical demand for surveying the topography and environment deep down the water surface as well as maintenance and equipment inspection. In the military, the demand for observation and mine removal also experience great increas-ing… That why the research and development of devices working underwater is one of the most important missions in order to take advantage of the sea and marine resources. This paper will focus on the design of AUV hull using Finite element analysis to determine the suitable thickness of hull's part; design the diving and floating mechanism; and design the control system for AUV.

METHODOLOGY OF DESIGN Design Ideas
Design specifications:   Table 1, our team decided to choose plan 4: diving/floating mechanism using one cylinder and counterweight ( Figure 5).

Design Of Shape And AUV Hull
Almost torpedo shaped AUV based on Myring shape (Figure 7) with the cylinder body, the head and stern will be designed according to formula (1) and (2) 6 . Head's shape: (1) Stern's shape: .(x − a − b) 3 (2) Where: r (x) : radius of section at position x. d: the maximum of diameter at the cross-section. a, b, c: length of head, body, and stern of AUV. θ : angle at the end of the stern. n: parameter of the head's shape. Parameters for designed AUV shape included a, b, c, n, θ is shown in Table 2 7 . Base on AUV prototypes that have been built in the world and the other underwater vehicles, especially vehicles work in the sea environment, we decided to use Aluminium Alloy T6 -6061 with mechanical properties shown in Table 3 7 . Using finite element method (FEM) with NX Nastran apply for AUV hull, 4mm thickness, 800mm length, 250mm outside diameter, two end fixed by the flange, the pressure at 50m depth is 0.5MPa. The result in Figure 8 show that the maximum stress on AUV hull SI58 Figure 2: Diving/floating mechanism using a counterweight.         Figure 9, and Table 4. The flange with 6mm thickness is the most suitable for AUV body. However, with the demand for setting up other components, AUV flanges have to bear lots of loads, such as the mass of components inside AUV,… By optimizing the structure of the flange and using FEM, we got the structure of the particular flange shown in Figure 10. The Figure 10 also indi-cates the maximum stress and displacement, which is 48,84Mpa and 0.0577mm respectively.  ing (6).

Piston-cylinder pump
The friction force between the O-ring cylinder wall: • Apiston: area of the piston Pneumatic pressure: F n = P 2 A p (6) Let assume that the process is isothermal: The preliminary diameter of ballscrew is calculated with formula (7) 7 .
Where [σ k ]: tensile yield strength of the material. Torque on ballscrew: The overall parameters of the cylinder's ballscrew are shown in Table 5.

Counterweight
Counterweight includes 8 round battery with is 310g/battery (Figure 13), linear bearing, P counterweight ≃ 3kg After considering (7), (8), (9) and standard specification selection, the parameters of counterweight's ballscrew is shown in Table 6. The axial load F a is calculated while AUV swimming in the glider's journey ( Figure 14). Let assume that F a ≃ P counterweight = 30N. For the counterweight, Faulhaber 2444 motor 51W would be use for counterweitght with some specification such as aximum torque 18mNm, 45000rpm, and

Design and manufacture thruster
The thruster was designed using a magnetic couplingas shown in Figure 16 with specifications Table 7 8 : Figure 17 (above) depicts the relationship between speed and current of the thruster. Considering the motor speed of 1000 rpm, the circuit still withstands 10A currents because of its good heat dissipation. Figure 17 (below) show that the thruster is stable and less noise. At 1000 rpm, the thrust of the engine was able to reach more than 6 kgf corresponding to 55% of the motor's power output.

METHIDOLOGY AND RESULTS OF THE ELECTRICAL SYSTEM
The robot is connected to the control center located on the surface (on the shore, on the mothership,…), data will be transmitted to the central station for management and command control via RF wireless system, GSM/GPRS, and Sonar.
The electrical system structure of the AUV is shown in Figure 18. The high-performance central processing unit allows the AUV to process received data at high speed, creating a premise for the AUV applies the advanced algorithms of guidance and control to serve  Max. thrust (kgf) 8 Number of wings of the propeller 6 Power supply (Vdc) 48 Communication CANBUS each specific operating requirement. Data acquisition systems from sensors and actuator controllers are designed using high-speed ARM core microcontrollers (STM32Fx), which are interconnected via the CAN communication standard with a transfer rate of up to 1Mbit. The robot is equipped with a variety of sensors to collect information of the operating state of the robot and the surrounding environment, thereby assisting the robot to make precise control de-cisions. The sensor system includes: GPS sensor (error < 1m horizontal), DVL velocity sensor (error 1% ± 1 mm/s), altimeter sensor and depth sensor (pressure sensor). In addition, tri-axis rotation angles estimator as shown in Figure 19 with high accuracy (error < 2 degrees) integrated into the AUV.
The algorithm in the tri-axis rotation angles estimator consists of two layers, each with an extended Kalman filter. Table 8 shows the error of the system in the

DISCUSSION
The study has presented many designed options and selected the most optimal designed solution for the AUV-VIAM2000, which is from selecting the Myring shape to using a combination of counterbalance and cylinder structure to support floating/diving. Thus, this help the diving robot can operate flexibly in two modes: AUV and glider that will help save energy. We also used finite element method to compute, simulate stress and distortion on ship hull which is 5mm thickness, and waterproof covering. In addition, tri-axis rotation angles estimator implementation has been tested with error <2 degrees in many cases and integrated into the AUV-VIAM2000. The 600W thruster device that we designed to ensure movableness of AUV-VIAM2000.

CONCLUSIONS
This paper has analyzed and selected the complete design options for the AUV-VIAM2000, capable of diving/floating at a depth of 50m by a combination of cylinder and counterbalance. Through stress simulation, finite element analysis has been used to select materials and suitable shell thickness, ensuring that the robot can operate at a stable design depth. Last but not least, the research has achieved some goals in building electrical system comprising sensors and actuators selection, hardware design, thruster manufacture and control as well as tri-axis rotation angles estimator implementation.