|M.Sc Student||Tal Asaf|
|Subject||Modeling and Evaluating Small Autonomous Underwater|
Vehicle for Improving Navigation Performance
|Department||Department of Mechanical Engineering||Supervisors||Professor Reuven Katz|
|Dr. Itzik Klein|
|Full Thesis text|
The Technion Autonomous Underwater Vehicle (TAUV) is an ongoing project. The TAUV project goal is to develop and to produce a small, modular, autonomous underwater vehicle, which will serve as a technology demonstrator and a platform for various research programs.
The contribution of this thesis is twofold: 1) a six degrees of freedom (6DOF) closed loop simulation to emulate the TAUV performance and 2) The derivation of the extended loosely coupled (ELC) approach directed for situations of partial Doppler velocity log (DVL) measurements.
A six degrees of freedom (6DOF) closed loop simulation of the autonomous underwater vehicle was derived. This simulation includes the following main subsystems: control, guidance, hydrodynamic and navigation. The hydrodynamic subsystem includes: specific TAUV hydrodynamic model, thruster model, tails configuration and hydrostatic forces and moments. The control system goal is to stabilize the AUV orientation and velocity. It contains: servo transfer function, attitude controller and velocity controller. The guidance system purpose is path following, that is maintaining the TAUV in a desired path. The navigation system goal is to estimate the position, attitude and velocity of the TAUV .It includes the modeling of: INS, DVL, magnetometer, pressure sensor a the design of an extended Kalman filter for fusing sensor data and estimating the vehicle state vector.
Utilizing the simulation, the TAUV navigation system performance was evaluated. Sensitivity analysis of the navigation performance to the DVL parameters was made implementing both loosely coupled and tightly coupled techniques for INS/DVL fusion.
In many INSs, such as the one used in TAUV, only the velocity vector (provided by the DVL) can be used for aiding the INS, i.e., enabling only a loosely coupled integration approach. In cases of partial DVL measurements, such as failure to maintain bottom lock, the DVL cannot estimate the vehicle velocity. Thus, in partial DVL situations no velocity data can be integrated into the TAUV INS, and as a result its navigation solution will drift in time. To circumvent that problem, we propose a DVL-based vehicle velocity solution using the measured partial raw data of the DVL and additional information, thereby deriving an extended loosely coupled (ELC) approach. The implementation of the ELC approach requires only software modification. Using the 6DOF simulation, the proposed approach is evaluated and the benefit of using it is shown.