With the increasing importance of ocean exploitation, providing anti-rolling stability for ships at anchor has become more and more important. The lift-generation theory of traditional fin stabilizers is based on incoming flow velocity, which is not suitable for explaining lift generated at anchor. We analyzed non-steady flows, with forces on fin stabilizers generated by non-incoming flow velocity conditions, and gave a new lift-generation model. The correctness of the model was proven by comparing experimental results of fin stabilizer motion under non-incoming velocity conditions from the fluid computation software with that from the emulator of the lift-generation model. Finally, the model was used in an anti-rolling system on a ship and the reduction of roll was much better than what could be achieved by passive anti-rolling tanks.
This paper studies the linear motion speed control problem of an underactuated spherical robot under unknown disturbances. A novel fractional fixed-time terminal sliding mode control with a nonlinear disturbance observer is proposed for the spherical robot to achieve fast stabilization and robust control performance. First, a novel fixed-time terminal sliding surface is proposed by adding a fractional differential operator in the traditional integer order fixed-time terminal sliding surface. A nonlinear disturbance observer is designed to estimate the unknown disturbances. Then a fractional hierarchical sliding mode speed controller is designed based on the novel fractional fixed-time terminal sliding surface and the nonlinear disturbance observer. Through the Lyapunov stability theorem, the boundedness of each sliding surface is achieved, and the stability of the whole system is guaranteed. The effectiveness of the proposed controller has been verified via simulation work. The simulation results show the fractional sliding mode controller has a shorter settling time and lower overshoot compared to an integer order sliding controller. When subjected to the abrupt changes of rolling friction, the fractional hierarchical sliding mode controller shows stronger robustness than the integer order one.
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