In this paper; a nonlinear cascade control strategy is proposed to control the position for underwater platform. Firstly; a double loop feedback controller is constructed to avoid the influence of external disturbance. An enhanced nonlinear PID controller is used to compensate the nonlinear system. Secondary; the Smith prediction method is introduced to overcome the influence of response lag in the hydraulic system. Thirdly; the high frequency noise in feedback signal is eliminated by the tracking-differentiator; which ensures the smoothness and stability of the motion curve. Finally; the effectiveness of the proposed control method is verified by simulation.


In order to develop marine resources and meet the social demand for resources; people gradually began to develop and utilize the ocean. The offshore engineering industry has also achieved considerable development in the past 40 years (Woodacre; Bauer and Irani; 2015). Nevertheless; the problem of work efficiency and safety caused by wind; wave and current interference has not been solved.

To solve this problem; there are many methods proposed; such as motion prediction; passive heave compensation (Ni; Liu; Wang; Hu and Dai; 2009; Huster; Bergstrom; Gosior and White; 2009); active heave compensation (Li; Ma; Li; Meng and Li; 2019; Niu;Gu;Yan and Cheng; 2019; Richter; Schaut; Walser; Schneider and Sawodny; 2017) and so on. However; the influence of wind; wave and current on the platform is still obvious (Woodacre; Bauer and Irani; 2015).

Compared with the surface platform; underwater platform is less affected disturbed by wind; wave and current and have higher efficiency; which has been widely concerned by researchers. According to Wu; Yang; and Wu (2018); primary actuator of most underwater platform is driven by either hydraulic or electric systems. Electric systems have increased in popularity due to their relatively high efficiency. However; high power electric motors tend to be physically large with a correspondingly large moment of inertia; and the safety of electric winch is severely tested in the deep water. Compared with electric actuators; hydraulic motors have the advantage of high energy density and small footprint; which is appealing when volume of equipment is limited. Nevertheless; traditional hydraulic drive systems suffer from various shortcomings; such as throttling losses; long hydraulic time constant and slow dynamic response. To overcome the drawback of the traditional hydraulic systems; researchers put forward a variety of solutions from the structural design; control strategy; motion prediction and other directions. Which includes secondary controlled hydraulic drive (Dabing; Jianzhong and Xin; 2011); motion prediction methods (Yin; Zou and Xu; 2013; Li; Kawan; Wang and Zhang; 2017); adaptive control (Dang; Zhao; Dang; Jiang; Wu and Zha; 2021); fuzzy logic control (Fan and Mu; 2019)); neural network control (Fang; Zhuo; and Lee; 2010); and so on.

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