Kohei Ohtsu

Paper: A new ship's auto pilot design through a stochastic model

A new type of ships autopilot system is designed by a statistical approach. A ships motion at sea is described by a multi-variable autoregressive model using minimum AIC (Akaikes Information Criterion) procedure. Through the fitted model, the ships behavior is analyzed and an optimal control law for a ship under a newly introduced criterion function is derived. The feasibility of our control system is checked by both digital and hybrid simulations. The results of the simulation show that with our controller the yaw motion is depressed through smoother and less rudder motion than that of the conventional autopilot systems and the ill effect of rolling is avoided. It is expected that the controller has another merit: it is quite robust for possible changes of external environments. Finally, a successful result of an actual sea test is briefly discussed. Thus, the possibility of realizing an entirely new autopilot system by a stochastic model is demonstrated.

Bayesian estimation of directional wave spectra based on ship motions

Abstract New techniques are introduced into the Bayesian modeling procedure to estimate directional wave spectra based on ship motion data. In the proposed method, the triple-valued function problem in following seas is strictly taken into account and the optimum solution is obtained from the stochastic viewpoint. In order to examine the reliability of the proposed method, towing tank experiments and onboard experiments were carried out. Using the proposed method, it is shown that the directional wave spectra can be estimated from the vector-valued time series recorded on a running ship even in following seas.

Statistical analysis and design of a rudder roll stabilization system

Abstract The multivariate auto-regressive rudder roll control system (MARCS) proposed by the authors has been improved by being designed using a new type of performance index which attempts to keep the movement of the rudder motion as smooth as possible. Furthermore the MARLS is statistically analyzed from the point of view of rudder-roll-yaw coupling motions, using noise contribution functions and impulse response functions.

A FULLY AUTOMATIC BERTHING TEST USING THE TRAINING SHIP SHIOJI MARU

In this paper, the authors discuss a fully automatic berthing system and actual tests using a small training ship, Shioji Maru. The ship is about 440 gross tonnage and furnished with a controllable pitch propeller, a bow thruster and a stern thruster to which a computer can easily be linked. The system developed is composed of an electro-optical position measuring system and the computer software necessary for manoeuvring the ship. The electro-optical position measuring system can measure a distance between the ship and a target on a berth within 4 cm accuracy and cover an area out to about 100 m range. The manoeuvring is divided into two stages. The first one is from a start line to a transition point off the final berth. At this point, the ship must be stopped in an alongside attitude to the berth. In the second stage, the ship is pushed by the thrusters in an athwartships direction towards the berth and the final speed at the berth must be limited to within 0.5 ms. The computer software generates an appropriate strategy following an approach line which has been determined in advance. According to the actual tests under slightly rough sea conditions, the system could manoeuvre the ship along the approach line by a direct path and maintain her final speed within the predetermined limit.

A Study of Minimum Time Berthing Solutions

Abstract The conditions that ship’s masters or pilots face to in ship handling to a berth are various. The minimum time berthing solutions, which the authors have solved, might give good information to them. However, it is impossible to prepare the minimum time berthing methods for whole conditions they will face to. The paper deals with a general guideline for maneuvering of small ship gained from the minimum time solutions. Especially the geometrical points of view of the minimum time berthing solution are focused.

Nonlinear Control of Ships Minimizing the Position Tracking Errors

Abstract In this paper, a nonlinear tracking controller with integral action for ships is presented. The controller is based on state feedback linearization. Exponential convergence of the vessel-fixed position and velocity errors are proven by using Lyapunov stability theory. Since we only have two control devices, a rudder and a propeller, we choose to control the longship and the sideship position errors to zero while the heading is stabilized indirectly. A Virtual Reference Point (VRP) is defined at the bow or ahead of the ship. The VRP is used for tracking control. It is shown that the distance from the center of rotation to the VRP will influence on the stability of the zero dynamics. By selecting the VRP at the bow or even ahead of the bow, the damping in yaw can be increased and the zero dynamics is stabilized. Hence, the heading angle will be less sensitive to wind, currents and waves. The control law is simulated by using a nonlinear model of the Japanese training ship Shiojimaru with excellent results. Wind forces are added to demonstrate the robustness and performance of the integral controller.

Rudder Roll Stabilization Control System Through Multivariate Auto Regressive Model

Abstract In this paper, the authors discuss the rudder roll control system (RRCS) which was newly developed. The model of the RRCS is based on the multivariate auto regressive model which has been fitted by the minimum AIC method to the actual time series records including yaw, roll rate and rudder motions. The feedback gains not only to keep the required course but also to reduce roll motion are given by statistical optimization technique. The salient feature of the RRCS is that a designer does not need any preliminary information about the dynamical properties of the ship except for the yaw, the roll rate and the rudder motions driven by random steering during her navigating. After feasible study and simulations of the rudder roll stabilization using the actual record, the results of full scale experiments at sea are reported.

Optimal Ship Maneuvering Using Bryson and Ho’s Time Varying LQ Controller

Abstract In this paper, the Minimum time, course alteration and the Minimum overshoot, parallel deviation problems in ship maneuvering are treated using Bryson & Ho’s time changing LQ-controller. The basis for the controllers are the simple speedand thrust- dependant linear steering equations in sway and yaw. The algorithms created, recalculate the optimal output at every sampling by integrating the Riccati equations backwards in time, with final time specified. Substantial processing power is needed, but not more than that modern personal computers and software can achieve it. In the case of the course alteration problem, an approach to a minimum time, bang-bang controller is introduced based on the linear model. It is also showed that simultaneous control of speed in parallel with the maneuvering controllers is possible. The algorithms developed are then tested with and without speed change on actual sea tests, showing good results. Even though the controllers was made for a specific vessel, the M.V. Shioji Maru, adapting the algorithms for other ships is straightforward if one possesses a good speed-dependant linear steering model.

The Preliminary Study of Fin and Rudder Multivariate Hybrid Control System- Advanced Rudder Roll Stabilization System-

Abstract The combination of fin and rudder seems to be an attractive alternative for roll damping. However fin motions as well as high frequency rudder motions disturb the heading control system. In order to reduce this interaction, this paper proposes the advanced rudder roll stabilization control system with hybrid fin control system. From results of simulation and model experiments, it can be concluded that the fin and rudder hybrid control system is powerful and useful tool for stabilization of roll and course keeping.

A New Type of Marine Governor Considering Pitch Motion

Abstract In this paper, a new type of marine governor which takes account of pitch rate information is proposed. Using a statistical analysis through a multi-variate auto regressive(MAR) model of the sea trial’s data on an actual small training ship Shioji Maru, it is made clear that the rate of revolution of marine propeller is strongly influenced by especially ship’s pitching motion. And based on this observation, a new type of marine governor considering ship’s pitching motion is designed using the modern multi-variate control theory. Lastly, some full-scale experiments are implemented and the performances of the new system are tested on board.