Research On Nonlinearity Analysis And Control Method Of RLV Reentry Model

Suborbital Reusable Launch Vehicle(SRLV)is a versatile aircraft. It can perform a flight mission freely in the large airspace between near space and ground. The main goal of SRLV is to go into the space successfully with low cost, and to provide reference for the future development of orbiting vehicle. Thus, many countries have begun to research relevant technologies of SRLV. During the reentrying process, SRLV has the characteristics of wide flight airspace, strong constrained flight, several actuators, complex aerodynamics, flying at high attack-angle, etc. Therefore, in order to finish the arranged task smoothly and safely, the reentry model and the controller design of SRLV should be mainly researched. This paper uses SRLV as the study object, and foces on its reentrying process. It firstly studies analytical model of aerodynamics, then analyzes the nonlinear strength based on analytical model. Combining with the changes of nonlinear strength, the gain scheduling control method based on stability radius is proposed. To reduce the complexity, this paper researchs a high-order sliding mode control method which has great autonomy and robustness. The main contents in this paper are as followed.The shape structure of SRLV is described, and the functions of each operating part are illustrated. The steady and unsteady characteristics of aerodynamic force and moment is analyzed in detail. Then using multivariate orthogonal functions, based on the aerodynamic data base, analytic expressions of the aerodynamic force and moment are given. And on this basis, the reentry model of SRLV is established. Reentry flight process is a time-varying system, which has the close coupling and strong nonlinearity characteristic. By establishing the aerodynamic analytic model of SRLV reentry equations, the nonlinear effect of aerodynamic force and moment on the state variables of SRLV is reflected explicitly, and the nonlinear relationship between the two is descriped accurately. The research of this part lays a good foundation to the following nonlinear analysis of dynamic model and the design of returning controller.For the nonlinear dynamic characteristic of SRLV, based on the nonlinear index method, the nonlinear analysis method of SRLV reentrying model is proposed. To linear the SRLV nonlinear kinetic equation, the state space equation is established, and the state variables are angle of attack, sideslip angle, and Euler angular velocity. The nonlinear strength of SRLV dynamic model is researched by different reference vector, then giving the nonlinear degree of dynamic model on the different regions. The simulation results shows that the changes of state variables can severely affect nonlinear strength of SRLV reentrying model. Therefore, the more suitable gain schedule selection method is considered. It is the function of the reentrying flight controller design.Gain scheduling control is one of the most wide and successful control laws in the aspect of aerospace craft currently. But it is difficult to select a scheduling variable, choose the number of feature points and guarantee the control stability in the arbitrary area between any two feature points. Thus, with nonlinear strength variation of SRLV dynamic model, the theoretical gain-scheduling control method is explored to ensure flight stability in any area between two feature points. And improved gain-scheduling control strategy based on stability radius is given, which is applied to SRLV reentry flight control law design. The simulation results shows that, other than tranditional method, the improved method has more advantages on getting scheduling variable, selecting feature points and control precision.Although gain-scheduling control based on the stability radius improves the traditional method, the process of analysis and design is relatively complex, especially on strongly nonlinear and strong coupling complex systems like SRLV reentry. Because more feature points are needed to design controller, which increases the complexity of design. In order to improve the above control method, based on high order sliding mode control theory, double loop quasi-continuous high-order sliding mode control method is proposed. Sliding mode control method does not need to set the table to adjust the parameters in advance, making the analysis and design process of controller more simple. The dynamic movement of SRLV reentrying process can be divided into slow and fast dynamic stages. Outer loop controller corresponding to the slow motion state, and tracking the angle command offered by guidance systems, then outputting the Euler angular velocities command. Inner loop controller corresponding to the fast motion state, and tracking the Euler angular velocities command offered by outer loop controller, then outputting controlling moment. The simulation results shows that high order sliding mode control method is based on the improvement of the traditional one, which can effectively restrain the chattering phenomenon, and has higher accuracy and better dynamic performance. To the internal uncertainty and external disturbance, this method has certain robustness.