Acceleration Feedback in Dynamic Positioning
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This dissertation contains new results on the design of dynamic positioning (DP) systems for marine surface vessels. A positioned ship is continuously exposed to environmental disturbances, and the objective of the DP system is to maintain the desired position and heading by applying adequate propeller thrust. The disturbances can be categorized into three classes. First, there are stationary forces mainly due to wind, ocean currents, and static wave drift. Secondly, there are slowly-varying forces mainly due to wave drift, a phenomenon experienced in irregular seas. Finally there are rapid, zero mean linear wave loads causing oscillatory motion with the same frequency as the incoming wave train. The main contribution of this dissertation is a method for better compensation of the second type of disturbances, slowly-varying forces, by introducing feedback from measured acceleration. It is shown theoretically and through model experiments that positioning performance can be improved without compromising on thruster usage. The specific contributions are: • Observer design: Two observers with wave filtering capabilities was developed, analyzed, and tested experimentally. Both of them incorporate position and, if available, velocity and acceleration measurements. Filtering out the rapid, zero mean motion induced by linear wave loads is particularly important whenever measured acceleration is to be used by the DP controller, because in an acceleration signal, the high frequency contributions from the linear wave loads dominate. • Controller design: A low speed tracking controller has been developed. The proposed control law can be regarded as an extension of any conventional PID-like design, and stability was guaranteed for bounded yaw rate. A method for numerically calculating this upper bound was proposed, and for most ships the resulting bound will be higher than the physical limitation. For completeness, the missing nonlinear term that, if included in the controller, would ensure global exponential stability was identified. The second contribution of this dissertation is a new method for mapping controller action into thruster forces. A low speed control allocation method for overactuated ships equipped with propellers and rudders was derived. Active use of rudders, together with propeller action, is advantageous in a DP operation, because the overall fuel consumption can be reduced. A new model ship, Cybership II, together with a low-cost position reference system was developed with the aim of testing the proposed concepts. The acceleration experiments were carried out at the recently developed Marine Cybernetics Laboratory, while the control allocation experiment was carried out at the Guidance, Navigation and Control Laboratory. The main results of this dissertation have been published or are still under review for publication in international journals and at international conferences.