Thrust Vector Nonlinear Control Law Design

Thrust vector control is an important technology for fighter. It was developed earlier in some foreign countries, and several kinds of aircraft with thrust vector control system have been designed successfully. In our country, the investigation is developing from theory to engineering applications gradually. The question for discussion in this paper belongs to items of the pre-investigation of tenth five-year plan.Thrust vector control makes the aircraft controllable in high angle of attack in case of which many nonlinear problems, especially the nonlinearity of six-degree-of-freedom equations, need to be solved. Whether the matter is solved successfully is a key to the quality of the control system. Traditional method, gain schedule, has some intrinsic faults, so it is not the best choice. The author believes that the best method is nonlinear method of modern control theory. In this paper, a nonlinear decoupling approach is used to solve the problem, which is called nonlinear dynamic inversion (NDI) and was developed in early stage of 1990s.NDI is one of the approaches to study control system with linearization using nonlinear negative feedback. The fundamentals of NDI can be described as follow : set an integral inversion system before the plant, which can compensate plant into linear system that can be designed with linear system theory. NDI has some advantages, such as simplicity, having a clear physical image. Because the dynamic of aircraft is a typical singular perturbation system, this makes it more simply to use NDI to solve the problem. So NDI makes a stir in the domain of flight control since it has been presented.The author has sum up the conclusions of other people and offered his own view point in some details. The main contribution includes:1) A batch of equations based on bank angle about the velocity vector, flight path angle and velocity heading angle are deduced with the fundamentals of flightmechanics. The equations and the process of the deduction are scarcely found.2) The decoupling control of aircraft system based on nonlinear dynamic inversion is investigated, and singular perturbation theory is applied to depart the complicated dynamic system into several simple sub-systems for study respectively. Slow-state and fast-state controllers are discussed in detail.3) Control selector schemes in foreign documents are investigated, and a geometry explanation is proposed which make the arithmetic of control selector clearer.4) Maneuver generator has been designed, which make it possible to control the craft to maneuver according to instructions.5) Simulation according to relevant theory is accomplished.We can draw the follow conclusions from simulation and theoretic analysis: combined with singular perturbation theory, NDI solved the problem of nonlinear control of aircraft successfully. A NDI controller can be constructed with high precision, simple arithmetic and can be used for control aircraft in real time.