The trajectory data of vessel AIS (automatic identification system) has important theoretical and application value for information supporting decisions. However, large sizes lead to difficulties in storing, querying, and processing. To solve the problems of high compression ratio and longtime consumption of the existing online trajectory compression algorithm, an SPM (scan-pick-move) trajectory data compression algorithm added sliding window is proposed. In order to better compress vessel trajectory data regarding compression efficiency, the sliding window is added to the classical SPM algorithm. In order to reduce trajectory data storage space, the maximum offset distance reference trajectory point is used as the criterion of whether the current trajectory point can be compressed. In this paper, the multi-dimensional space-time characteristics of trajectory data, such as distance error, compression ratio and compression time, are selected to evaluate the trajectory compression method from three levels: geometric characteristics, motion characteristics and compression efficiency. Compared with the existing SPM trajectory data compression algorithm, parallel experiments are conducted based on AIS data gathered over the duration of a month in the Japan Osaka Bay. The SPM trajectory compression algorithm added sliding window can significantly reduce the compression time and outperforms other existing trajectory compression algorithms in term of average compression error at high compression strengths. Also, the proposed method has high compression efficiency in the range of commonly used compression thresholds.
The dynamic positioning system is a system that ships rely on their own propulsion devices to maintain position and course. Pod propulsion was more and more installed in dynamic positioning unmanned surface vessels because it can rotate 360 degrees freely. The maximum disturbance of unmanned surface vessel dynamic positioning comes from the forces and moments generated by the ocean wind, wave and current. In order to improve its accuracy of disturbance rejection, the mathematical model of the vessel was established according to the special structure of pod propulsion ship, and the calculations of the forces and moments of wind, wave and current are given. Then, the sliding mode controller based on approach law and adaptive backstepping were designed, and optimal controller based on cerebellar model articulation controller (CMAC) was further designed. The tracking differentiator was added to eliminate the large chattering in the initial stage of the system. Finally, the simulation verification was carried out. It can be seen from the results that the control law designed has better control effects than traditional sliding mode control and can realize dynamic positioning of pod driven unmanned surface vessel under certain disturbances. INDEX TERMS Cerebellar model articulation controller, dynamic positioning system, pod propulsion, sliding mode control, unmanned surface vessel.
With the continuous improvement of unmanned ship automation requirements, research into the automatic berthing of underactuated unmanned ships has important theoretical significance and practical value. In order to determine the trajectory of unmanned ships, the line of sight (LOS) algorithm was applied due to the characteristics of underactuated unmanned ships without side thrusters. In order to resist wind disturbance, the active disturbance rejection control (ADRC) method was applied to keep the ship moving on its intended trajectory. Then, to carry out the simulation analysis before the tank experiment, a remote-control simulation system based on a user datagram protocol (UDP) communication was built, and the ability of the ADRC controller to make the ship perform completely automatic berthing in both wind and no wind conditions was verified in simulations. Combined with the simulation results, a tank experiment was accomplished at the Japanese National Research Institute of Fishery Engineering. The experimental results also showed that the ADRC controller has good robustness, that the problems of insufficient autonomous route determination and the disturbance rejection ability in the process of the automatic berthing of underactuated unmanned ships are solved, and the safety of ship navigation is improved, which lays a theoretical and experimental foundation for the further development of unmanned ship control.
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