Ultrasonic Positioning System Research Articles

238 自由航走模型船の水中超音波による位置計測システム

Ultrasonic positioning system for free running model ships at the 80m square basin in Ship Research Institute has recently been enhanced with a newly developed signal processing code on personal computer. This positioning system visualizes the trajectory of radio controlled model ship together with time history of her speed, direction, drift angle and turning rate. in time. The accuracy of the positioning is confirmed to be enough for the analyses of model ship motion.

An Experimental Consideration on Control of a ROV for Lifting Objects

ROVs with computer aided trajectory control and acoustic feedback sensors will be needed for lifting and transportation of objects related to the underwater activities such as inspection, installation and repair of offshore structures. Especially in deep seas, environments are hazardous for human and ROVs become very useful tools. These ROVs are also demanded in the activities related to huge offshore platforms for space utilization. In the repair and inspection of such structures, the amount of works are expected to be so large that without these ROVs the works can not be accomplished.These ROVs are thought to have relatively complicated configurations due to large thruster, and the dynamics will be highly nonlinear and have uncertainties in their dynamics compared to the streamlined vehicles. The nonlinearlities depend on velocity, and hydrodynamic coefficients are poorly known. When such system are controlled, control method must be able to handle the nonlinear dynamics and uncertainties in parameters which depend on the motion of ROV. Gain-scheduled technique which uses a set of linear controllers for different combinations of speeds and directions of motions, can handle these problems. Although theoretically possible, the number of linear controllers become so large that this method is not practical. Another technique which is able to handle the problems and adopted in this paper is sliding mode technique. If the bounds of uncertainties in parameters which determine the dynamics of ROV are known, the technique can incorporate them in the formulation and allows uncertainties in the system. Dynamics change in ROV caused by lifting objects can also be treated as a uncertainty in parameter and control is expected to be robust.The trajectory control of ROVs is examined experimentally by model basin test using a small ROV with four thrusters. Surge and Yaw are controlled by two thrusters in horizontal plane. Heave and Sway are controlled by thrusters arranged diagonally in vertical plane. Ultrasonic positioning system was developed for the basin test. The ROV used in the test was relatively small, and high accuracy of one millimeter order was required for the positioning system and realized by using ultrasound of high frequency.

A Study on a Docking System Using Ropes

At the docking of a three-dimensional moving vehicle to a fixed base in deep water, an automatic system is indispensable to assure reliable and safe operation, and authors proposed an automatic docking system using ropes for such situation. In the first report, the investigation was focused on the practical feasibility of real time state estimation which is the most significant at the docking, and it is shown that a near optimum filter can be effectively utilized to estimate the state variables of the moving vehicle, based on which the vehicle can be controlled by the ropes.In the first report, proportional control law was applied to check controllability of the system by the ropes, although it is not sufficient for the docking which requires accurate access to the docking point along the desirable trajectory avoiding collision or unexpected oscillations. Therefore, an optimal control technique is introduced to improve control performance of the system, and practical feasibility, especially that of real time computation is investigated.Considering the system requirements and computational feasibility, the system equations are linearized and linear regulator algorithm is applied for the docking control. The system performance is investigated by simulation study, and the following results are obtained.1) Proposed control algorithm based on the optimal regulator can control the system fast and stably compared with the proportional control law by selecting adequate control parameters through the simulation study.2) It is shown that real time control by conventional mini-computers is possible if only there are enough memory capacity for storing feedback control coefficients computed in advance. The off-line optimizing computation is not so difficult for this problem formulation, and it can be also computed by a small computer.3) The system can control the vehicle stably when it approaches within ±5×5 m area in horizontal plane and 10 m above the base. This is easily realized by conventional control system using ultrasonic positioning systems.4) When the vehicle is controlled based on the filter outputs, i. e., estimated state variables, overall control accuracy of the system is the same order as the filtering accuracy, which is sufficient for the docking requirements.