Force Field Identification and Positioning Control of an Autonomous Vessel using Inertial Measurement Units

In this thesis, a system of four sensors for measuring tri-axis translational accelerations and rotational velocities is designed. The sensors were mounted with a spatial distribution on board the ship model {C/S Arctic Drillship}. Experiments were carried out and sensor data was collected for waves with various periods and incoming directions. The distributed acceleration measurements were used to estimate translational and rotational accelerations in Center of control. By comparing the estimated accelerations with the high-accuracy measurements from Qualisys, it was found that the system provides good estimates of the accelerations in Center of control. In addition, the local tri-axis translational accelerations in each sensor frame was estimated, to detect spatial variability of accelerations. This method also performs well, and provides a representation of local forces in the hull. Three methods to estimate the direction of incoming waves are proposed, where one is based on correlation between motion in roll, pitch and yaw, and the two others are based on local accelerations inside the hull. Analysis of motion in roll, pitch and yaw gave some results. Analysis of the horizontal accelerations in the hull was not successful as to detect the surrounding force field, while the heave accelerations gave a better image of the surrounding waves and forces. An algorithm for online estimation of direction of incoming waves was proposed, which was able to predict the direction of incoming waves for some periods. For a short video of some of the experiments and findings, the reader is referred to https://vimeo.com/222068786. In this video, the spatially distributed accelerations in each sensor are shown when the waves hit the vessel.