Force Control Of Actuator In Active-truncated Model Test Of Mooring System

The design and construction of offshore platforms and their mooring systems need to be verified by basin tests.However,with the increase of working depth of offshore platforms,the existed wave basin dimension cannot meet the requirements of physical model tests of offshore platforms and their mooring systems in deep water.Therefore,an active-truncated model test is proposed,namely truncating the mooring line at the bottom of the wave basin in the basin test,and tracking the movement of truncated point with an actuator,so as to ensure the equivalence of the truncated mooring system and the full-length mooring system,thus ensuring the reliability of the physical model test.In order to make the truncated point move as the real corresponding motion,it is necessary to connect the truncated point to the actuator to track the input motion signal in real time.The key problem is how to select and control the actuator to realize the real-time high-precision movement of the truncated point.Therefore,the main purpose of this paper is to verify the feasibility of the active-truncated model test of mooring system by studying the actuator and the whole process movement of actuator tracking truncated point.Firstly,the Stewart platform is selected as the actuator of the experiment.Based on the kinematic principle,the inverse equation and Jacobian matrix of Stewart platform are derived.Based on the dynamic principle,Newton Euler method is used to derive the closed-loop dynamic equation of Stewart platform,and the actuator dynamic simulation model based on workspace control is established in Simulink.Single and multi-degree of freedom of Stewart platform motion examples are designed to verify the correctness of the established model.Secondly,the electric cylinder system is studied as the core of Stewart platform.The driving mode of the electric cylinder system is analyzed from the aspects of principle,structure and mathematical model.A three-ring control electric cylinder system model based on current loop,rotational speed loop and output loop is established by Simulink.An electric cylinder driving calculation example is designed to verify the correctness of the established model.Principle of traditional PID algorithm and fuzzy PID algorithm as well as the influence on the output control effect of the electric cylinder system are compared and analyzed.Thirdly,the influence of time delay in the active-truncated model test is analyzed.The influence of time delay on output of system is respectively analyzed when it is in the control channel and the interference channel.In view of the instability and lag characteristics caused by time delay,Smith predictive control algorithm and disturbance compensation control algorithm are used to compensate the time delay,and the application range,advantages and disadvantages of these two compensation methods are analyzed.Finally,the proposed 3-parameters control strategy of the model test is verified.In combination with the established dynamic simulation model,electric cylinder system simulation model and the related control algorithms,a taut mooring system of 1800 m water depth is studied.In the case study,platform motion of mooring system is motivated by the displacement of low frequency and wave frequency.Taking the full-length mooring system as the standard model and the truncated mooring system as the truncated model,compared with the results of standard model,the tracking performance of truncated model at truncated point is analyzed,and the stress conditions of truncated model at the truncated point and the fairlead are discussed to verify the feasibility of the active-truncated model test studied in this paper.