| The composite nonlinear feedback(CNF)control law itself is composed of two part:the linear feedback control part and the nonlinear feedback control part.The application of CNF to airship motion control is of great benefit to improve the steadystate and transient characteristics of airship motion.In this dissertation,three issues of airship pitch attitude tracking control,constant wind field disturbance suppression and time-varying wind field disturbance suppression based on CNF are studied.First,based on the structural characteristics,force analysis and assumptions for simplification of the soft airship studied in this dissertation,the six-degree-of-freedom kinematics equations of the airship established by the Newton-Euler model.To facilitate the design of the CNF control law,the original nonlinear kinematics equations are linearized with small disturbance under the working point of the airship mentioned in this dissertation,and the longitudinal(pitch angle control related)motion equations of the airship are obtained.Then use the pole-zero diagram and step response to analyze the characteristics of the airship’s pitching motion model,and point out the necessity of designing the the airship’s pitching motion control law.Then,according to the longitudinal motion equation of the airship,the CNF control law based on the state feedback is designed,and the asymptotic stability of the closedloop system is proved,and the step response characteristic of the system after the pole configuration is given.Compared with the airship pitch angle control research using proportion differentiation and proportion integration differentiation control laws,CNF control law can control the airship pitch angle to track the command value quickly,without overshoot,and without steady-state error.At the same time,the CNF control law also shows better dynamic performance than the existing control laws in the initial pitch angle deviation and angle of attack disturbance.Finally,the pitch angle control based on CNF control law is studied when the airship is disturbed by the longitudinal wind field.In this dissertation,constant and cosine time-varying longitudinal wind field disturbances are considered.For constant wind field disturbance suppression,this dissertation designs enhanced composite nonlinear feedback(ECNF)control law,and proves its stability.The ECNF achieves constant disturbance suppression by introducing additional state variables proportional to the diference product of the input and output of the system.However,the inner membrane principle shows that ECNF cannot suppress time-varying disturbances.Considering the uncertainty of time variables,this dissertation proposes to design a CNF control law for typical cosine time-varying wind field disturbances,which is called TVCNF(Time Varying CNF),and proves its stability.Similar to ECNF,TVCNF introduces two state variables to construct a resonant controller to achieve cosine timevarying disturbance suppression.The simulation results show that under the same constant disturbance,ECNF can suppress the disturbance compared with CNF,and ensure that the airship pitch angle can track the command value without static difference in the steady state.Under the same cosine time-varying disturbance,TVCNF can suppress the disturbance better than ECNF and CNF,ensuring that the pitch angle of the airship will not fluctuate in the steady state. |
