Jonghui Han

Accurate and Practical Thruster Modeling for Underwater Vehicles

The thruster is the crucial factor of an underwater vehicle system, because it is the lowest layer in the control loop of the system. However, an accurate and practical thruster model has not been utilized yet. In this paper, we propose an accurate and practical thrust modeling for underwater vehicles which considers the effects of ambient flow velocity. In this model, the axial flow velocity of the thruster, which is non-measurable, is represented by ambient flow velocity and propeller shaft velocity. Hence, contrary to previous models, the proposed model is practical since it uses only measurable states. Next, the whole thrust map is divided into three states according to the state of ambient flow and propeller shaft velocity, and one of the borders of the states is defined as Critical Advance Ratio (CAR). This classification explains the physical phenomenon of conventional experimental thrust maps. The proposed model is evaluated by comparing experimental data with numerical model simulation data, and it accurately covers overall flow conditions within ± 2N force error. The comparison results show that the new model’s matching performance is significantly better than conventional models’.

Redundancy resolution for underwater vehicle-manipulator systems with minimizing restoring moments

In this paper, redundancy resolution of underwater vehicle-manipulator system(UVMS) is addressed. In general, UVMS has redundant degrees of freedom(DOFs) as many as DOFs of manipulator and these redundant DOFs can be used to optimize the configuration of UVMS while satisfying given tasks. We propose a performance index for redundancy resolution which minimizes the restoring moments of UVMS. The restoring moment is induced from gravity and buoyancy forces and it can cause unintentional change of poses of UVMS. If the restoring moments remain small, control effort for keeping the poses of UVMS decreases. This means that energy consumption can be reduced by minimizing the restoring moments during conducting tasks. We solve inverse kinematics of UVMS using well-known weighted pseudo-inverse matrixes and we optimize the restoring moments by adopting gradient projection method. Numerical simulations are presented to demonstrate performance of the proposed algorithm.

Active Use of Restoring Moments for Motion Control of an Underwater Vehicle-Manipulator System

This paper proposes a framework for actively using the restoring moments of an underwater vehicle-manipulator system (UVMS) considering both kinematic and control aspects. The kinematic aspect concerns redundancy resolution of the UVMS where the redundant degrees of freedom are used to selectively optimize the restoring moments. For this, a performance index with variable gradient gain is newly proposed, in which the gain is determined by the result in the comparison of the task direction with the direction of the restoring moments. The control aspect concerns compensation of the restoring forces and moments. In this framework, the control input makes up for the difference between the performances due to the desired dynamics and the restoring moments. This is accomplished by compensation of the restoring forces and moments, which are consistently updated under certain constraints. In addition, the compensation and optimal proportional-integral-derivative (PID) control are merged into a robust adaptive control. The proposed framework requires only masses, buoyant forces, and centers of gravity and buoyancy, not any hydrodynamic parameters. Numerical simulations are presented to demonstrate the performance of the proposed framework, in which a UVMS can perform specific tasks with less control input and achieve smaller tracking errors compared to conventional control systems.

Coordinated motion control of Underwater Vehicle-Manipulator System with minimizing restoring moments

In this paper, coordinated motion control of underwater vehicle-manipulator system (UVMS) is addressed. In order for UVMS to carry out manipulation tasks alone, motion planning with consideration of redundancy and tracking control under disturbances are required. We propose redundancy resolution with optimization of restoring moments as motion planning and inverse optimal nonlinear Hinfin control as robust tracking controller. Numerical simulations are presented to demonstrate performance of proposed coordinated motion control of UVMS. As a result, control input for tracking is reduced and UVMS can track generated trajectories under parameter uncertainties.

Accurate thruster modeling with non-parallel ambient flow for underwater vehicles

To improve the maneuverability of underwater vehicles, accurate thrust model is required. However, its modeling is not only difficult but also inaccurate, because it has highly nonlinear dynamics and many parameters which cannot be modeled exactly. Generally, a propellers steady-state axial thrust is proportional to the signed square of propeller shaft velocity under bollard pull condition. And, it is reported that the velocity of the ambient flow of the thruster affects the thrust force largely. From these facts, we can easily make a reasonable inference that the thrust force may also be affected by the thruster spouting angle with respect to the vehicle moving direction. However, the effect of this non-parallel ambient flow is not well understood in the previous works. Hence, in this paper, a new parameter, what we call incoming angle, is introduced and its effect on thrust force is shown. Incoming angle is defined as the angle between the direction of thrust and that of vehicle velocity. The effect of this angle can be dominant during an underwater vehicle changes its direction, or an omni-directional vehicle carries out its task. The effect cannot be compensated by including the only parallel component of vehicle velocity into previous models. In this paper, based on the three axial flow states classification model, the effect of the incoming angle of ambient flow is analyzed, and critical incoming angle (CIA) is also defined to describe the thrust force states according to incoming angle. In order to characterize the effect of non-parallel ambient flow, the proposed model is evaluated by comparing experimental data with numerical model simulation data, and it accurately covers overall flow conditions within /spl plusmn/2N force error.

An Ethology-Based Hybrid Control Architecture for an Autonomous Underwater Vehicle for Performing Multiple Tasks

In this paper, we propose a control architecture for an autonomous underwater vehicle (AUV), implemented in a hybrid architecture with two layers: a hierarchical planning layer and a reactive execution layer. Most of its tasks are interpreted as a set of waypoints and then specified actions at the points. Thus, in the planning layer, the task planner is designed as a waypoint planner using a genetic algorithm. This planner generates an optimized plan, considering given constraints such as positions of obstacles, current velocities, and task priority. In addition, the execution of the task plan is monitored by a mission supervisor, which determines ongoing tasks and can change the original plan if exceptional events occur. In the execution layer, a behavior-based control with an ethology-based action selection mechanism is implemented. As a result, the AUV can always choose the most appropriate behavior, maximizing its motivation, and the robot is controlled by the output of the selected behavior. Consequently, the proposed control architecture has an open and modular structure. Numerical simulations were conducted to verify its performance.

Optimal mission planning for underwater environment

In this paper, we suggest the mission planning method for autonomous underwater vehicle. In order to set the optimal mission scheduling, we solve the traveling salesman problem using genetic algorithm, and we suggest the cost function which considering both Euclidean distance and current information. In additionally, constraints such as priority and obstacles is considered. By checking the simulation, we achieved the valuable simulation results.

Optimal task scheduler design using the underwater risk assessment

In this paper, we propose a multi-task scheduler by considering the underwater risk assessment for autonomous underwater vehicle. We divide the task scheduler into two parts: a path coordinator and a condition interpreter. The path coordinator finds the minimum cost sequence from several combinations of paths by using a genetic algorithm. The condition interpreter limits high risk paths such as obstacle environment and strong tidal current. By considering both the path coordinator and the condition interpreter, we get an optimal path for a given condition, and this algorithm is verified through simulations.

Redundancy Resolution and Robust Control of Underwater Vehicle-Manipulator Systems with Minimizing Restoring Moment

In this paper, redundancy resolution of UVMS (underwater vehicle-manipulate. system) is addressed. In general, UVMS has redundant DOFs (degrees of freedom) as many as DOFs of manipulator and these redundant DOFs can be used to optimize the configuration of UVMS while satisfying given tasks. We propose a performance index for redundancy resolution which minimizes the restoring moments of UVMS. The restoring moment can cause unintentional change of poses of UVMS. If the restoring moments remain small, control effort for keeping the poses of UVMS decreases. This means that energy consumption can be reduced by minimizing the restoring moments during conducting tasks. Proposed performance measure is optimized by gradient projection method. Generated trajectories by this redundancy resolution are tracked by robust PID controller. Numerical simulations are presented to demonstrate performance of the proposed algorithm.