Cruise And Reentry Tracking Control For Hypersonic Vehicles Based On Terminal Sliding Mode

Hypersonic Vehicle(HSV) has the features of parameter variations, coupling effects, high nonlinearities and multi-uncertainty effects, making it quite challenging to design the flight control system. Due to its great potential value in both military and civil affairs, HSV has attracted much attention around the world. And this paper focus on the control system design for HSV, applying terminal sliding mode control(TSMC), as TSMC has the character of fast convergence even finite-time convergence. To avoid the singular phenomenon existing in conventional TSMC, this research pays attention to TSMC with nonsingular characteristics. Considering that there exists a variety of uncertainties and disturbances in the HSV model which may affect the system stability, adaptive control and disturbance observer(DO) technology are combined with TSMC to alleviate the unexpected effects.Firstly, a control system is designed for Air-breathing Hypersonic Vehicle(AHV), which flies in cruise phase. The elastic states and coupling term are neglected and treated as system uncertainty, simplifying the AHV model of high nonlinearities. The input-output linearization is utilized to get the dynamic inverse model, based on which a kind of polynomial adaptive nonsingular TSMC is designed. The polynomial TSMC will guarantee the nominal system states to converge to equilibrium point in finite time, while the adaptive algorithm is designed to estimate the upper bound of "unknown bounded uncertainty" automatically, the estimation of which will be used as controller gain. Then the controller gain is greater than the system uncertainty, ensuring the stability of the flight control system.Secondly, there exists much interference such as constant wind in reentry phase, and a control system is designed for HSV in this situation. The six-degree-of-freedom model is simplified and the double-loop model is established, thus the disturbance in each loop satisfies the matching conditions. The DO and nonsingular TSMC are designed for each loop, and the two design process can be carried out separately. The DO can be used to estimate unknown external disturbance, and the nonsingular TSMC ensures finite-time convergence of the system. The estimation of DO is introduced in the designed TSMC, so the matched disturbance can be compensated, ensuring safe and stable reentry flight.Thirdly, there is another control strategy for HSV reentry control, that is, design a controller without separating the model into two loops(single-loop control). To avoid the need of the high-order derivative in the traditional single-loop attitude control, a finite-time Disturbance Observer- Controller integrated design strategy is proposed. In this situation, the design of DO and TSMC can't be independent, and the latter has to rely on the output of the former. The DO is given firstly, and then the estimation will be used to design the sliding mode surface and sliding mode control law, completing a nonsingular TSMC design. This special kind of design can suppress matched and mismatched disturbances, ensuring the security and stability of reentry flight.