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ABSTRACT Dynamic positioning ships have different operation modes in the complex ocean environment in accordance with different requirements. Aiming at the problem of thrust allocation under time-varying environmental disturbances and different operation modes, a thrust allocation switching strategy is proposed to improve the stability of dynamic positioning system. The strategy method designs three thrust allocation modes according to the time-varying environmental load and the output of the control system under different operation modes. The switching indicator is set up by considering factors such as environmental disturbance, thrust error and thruster characteristics, and moreover the switching mechanism is established with the hysteretic logic in order to realize the adaptive switching of thrust allocation modes. Three thrust allocation optimization algorithms are adopted to solve the thrust allocation problem under three allocation modes. The simulation results show that the strategy can realize the switching of thrust allocation modes smoothly and effectively, and the thrust allocation task can be completed.
INTRODUCTION Along with the deepening of marine resources development, Dynamic positioning system with multiple thrusters is widely used in various engineering vessels, offshore platforms and new unmanned vessels. Ships are confronted with complex and changeable sea conditions when carrying out operational tasks. Therefore, it is necessary to formulate a variety of allocation modes to meet operational requirements and improve the stability of the system. In the actual thrust allocation process, due to the influence of environmental disturbances and working modes, a single thrust allocation algorithm cannot solve the allocation problem under current sea conditions and working modes. Therefore, it is necessary to establish a reasonable thrust allocation switching mechanism to realize the switching between multiple allocation modes in order to complete the thrust allocation task.
Swanson (1982) divided the ocean environment into calm sea conditions and severe sea conditions. The energy consumption was used as the switching indicator. The allocation modes of fixed angle and variable angle were designed. However, the method has a single switching indicator and does not consider the characteristics of control force. It cannot effectively reflect the actual environmental conditions. Ruth (2008) proposed the characteristics of force and moment to be allocated as reference indicators, and proposed the concept of hysteresis switching, but this method ignores important indicators such as allocation error and energy consumption. Aiming at the characteristics of pseudo-inverse method, Shizhi Yang et al. (2011) proposed three allocation modes. However, this method is limited to pseudo-inverse method, without considering the physical limitation of thrusters, and cannot optimize the angle of thrusters. Fang Wang (2012) established an allocation strategy based on fixed and variable angle modes considering the performance indicators such us energy consumption, control error and response time, but the minimum thrust restriction of the thruster was not considered carefully by this method. Linkai Xu (2015) explored the switching of allocation modes for different typical sea conditions and working conditions, but the selection of various weight factors for a variety of indicators in the paper was difficult. Xing Zhou (2016) used the control force and energy consumption as the switching indicators corresponding to the sea conditions, and established the switching mechanism of the hierarchical processing with the current position and the expected position difference as the switching indicators of the working conditions, but the environmental force characteristics were not considered, it did not reflect the true thruster state. Haijun Fu (2017) designed three kinds of thrust allocation modes and formulated the mode switching standard, but it only set four kinds of environmental force fixed angles, and did not consider the direction of environmental force properly under the fixed angle mode. Moreover, most of them did not match a thrust allocation mode with a matching optimization algorithm, which may affect the accuracy and reliability of the solution of the thrust allocation problem under different conditions.
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