Parameter Identification Of Time-varying Systems And Its Application To Active Heave Compensation Systems

In practical systems,time-varying behavior of the plant parameters may be unavoidable due to the complex mechanisms,the model-plant mismatch,or unmeasured inputs that affect the system dynamics.The existing identification algorithms are generally based on the assumption of time invariant systems,which can not estimate time-varying parameters.In order to solve this problem,on-line parameter identification algorithms are proposed for time-varying system in this paper.Then,the proposed algorithm is applied to the secondary regulated active heave compensation system.Simulation results verify the effectiveness of the identification algorithm.The main research works are as follows:(1)For linear time-varying systems,a modified Least-squares algorithm which involves the addition of an extra term to the covariance's update law is presented.The boundedness of estimation error,estimation parameters and covariance has been proven.When the input signal of the linear time-varying system satisfies persistent excitation condition,this new algorithm can attenuate the influences of bounded disturbances and the change ratio of time-varying parameters effectively.Contrary to the earlier proposed estimation laws,this algorithm does not require any priori knowledge of upper bounds on the parameters and the disturbances.Simulation verify the effectiveness of the identification algorithm.(2)In the case of unknown excitation condition of the input signal,a modified on-line identification law is proposed for linear time-varying systems,.By adopting the variation trend of the covariance matrix as an on-line explicit indicator of regressor's excitation level,the proposed identification law is switched between a modified least-squares algorithm and a gradient algorithm based on fixed ?-modification.When the input signal satisfies persistent excitation condition,the modified LS algorithm is used to realize parameter estimation accurately.Otherwise,the gradient algorithm based on modification is used to assure the boundedness of the estimates,so this algorithm can be applied in closed-loop systems.Furthermore,it can be found from the simulation that aninput signal which does not satisfy persistent excitation condition in linear time invariant systems can also carry sufficient information to estimate time-varying parameters.(3)For nonlinear time-varying systems,by introducing the X-swapping technology,the nonlinear time-varying system is converted into a linear time-varying system,and the direct calculating the time derivatives of the system states are avoided.The proposed method is used to estimate the system parameters.The simulation verifies the effectiveness and superiority of the proposed algorithm.(4)The proposed identification algorithm is applied to the secondary regulated active heave compensation system.Firstly,based on the secondary regulated hydrostatic drive technology,the crane's winch driving system is constructed,and the complete mathematical model of the heave compensation system is given.A double closed-loop control structure including position loop of variable oil cylinder in the secondary unit and speed loop of secondary unit motor is designed.The proposed identification law is used to estimate the unknown time-varying system parameters.The simulation results show that the proposed algorithm can estimate the unknown time-varying parameters of closed-loop system accurately.