System Design And Dynamical Behavior Of Autonomous Underwater Vehicle With The Capability Of Landing On The Seafloor

As an effective platform for scientific sensors, autonomous underwater vehicle (AUV) has become an intense area of oceanic research because of their emerging applications in oceanographic survey. However, long-term marine environment measuring is impractical for available AUVs because the energy storage is limited. It is useful to develop the variable buoyancy AUV with the capacity of landing on the seafloor and bottom-sitting for an extended measuring period. In this thesis, a low cost modular AUV with the capacity of landing is developed. Because of more attached bodies, the new AUV developed in the thesis is more complicated than conventional AUVs in shape and construction. Accordingly, the dynamic modeling and the analysis of dynamical behavior of the AUV for landing is more complex than conventional AUVs. The AUV with the capacity of landing can be considered as a multibody system consisting of a base body and several attached bodies. In the thesis, the modeling methods based on multibody system dynamics are applied to the modeling process of the AUV with the capacity of landing. Then the dynamical behaviors of the AUV for landing are simulated and analyzed using multibody system dynamic model. Finally, a series of experiments including captive model experiment and at sea trials are conducted to testify the simulating results. Experimental results show that the simulating results are in good agreement with experimental results.The main contributions of the thesis are summarized as follows:1. The mechanical structures and control systems of the AUV with the capacity of landing are designed to achieve the functions of long-range navigation, underwater landing, bottom-sitting and rising. There are two main features in the system design. One is the convertible design for measuring sensor system. During the process of navigation, the measuring sensors onboard can also be used as DVL (Doppler Velocity Log) and the cost is reduced greatly. The other one is the ballast and releasing mechanism. Tow ballast tanks are designed to change the buoyancy of the vehicle.2. The model of the AUV with complex shape and construction is set up based on the theory of multibody system dynamics. In order to reduce the amount of calculation, the vector modeling method which is deduced from multibody system dynamical theory is applied to work out the motion equations of this kind of AUV. The merit of this model lies in its convenience and precision. With this model, the physical coefficients of each body of the AUV can be evaluated and revised easily without any influence on other coefficients in the process of design and optimization. Because the properties of each body are described in detail, this model can achieve eligible precision for engineering simulation. The multibody system dynamic modeling method presents an adaptive and effective tool for the design and optimization of the AUV.3. Based on the above dynamic model, the thesis analyses and optimizes the maneuverability of lateral movement and longitudinal movement of the AUV with the capacity of landing. Consequently, the following conclusions can be made. Firstly, the position of rudder should conform to the position of elevator. Secondly, the position of the ballast tanks fixed on the AUV should be lower than the position of the main cabin to attain the motion stability. Thirdly, the optimized result of the elevator position is 1.3 meters from buoyancy center.4. As for the landing movement of the AUV, four-stage landing trajectory is presented according to the physical requirements such as overload, vertical descent rate,continuity and smoothness. Then, the geometric parameters of the landing trajectory are work out. Compared to the landing method of free-fall in water, the proposed landing method is more reliable according to the simulation results and sea trial results.