Robust Adaptive Sliding Mode Control For Near Space Vehicle

As a novel vehicle, the near space vehicle (NSV) has attracted considerable attentions indeveloped countries. Compared with traditional vehicles, NSV has significant difference in flightairspace, working states and mission pattern. Hence, there exists great strategic value of NSV in bothmilitary and civilian domains. The NSV control system design, which is considered as the basis offlight security and mission execution, is not only an important branch of NSV researches, but also achallenging and innovating project. Based on sliding mode control, the robust adaptive control ofNSV is investigated in this dissertation. The main results are given as follows:To reduce the chattering phenomenon in the traditional sliding mode control, a dynamic slidingmode control scheme is proposed。The higher-order dynamic sliding mode (HODSM) is defined byLie algebra, whilst the design steps and basic ideas are given for the HODSM. The second-orderdynamic sliding mode controller for the slow control loop and the first-order dynamic sliding modecontrol for the fast control loop are designed. The control scheme could reduce the chatteringphenomenon by embedding discontinuous functions into the differential of controller. Furthermore,2-order dynamic sliding mode controller for the slow control loop of NSV would avoid the singularityin the fast loop controller of NSV. The upper boundary of the compound disturbance is estimatedonline via an adaptive law. Thereafter, the estimated value is utilized to design the compensation item.The whole system has been proved to be a stable system by Lyapunov theorem.Based on disturbance observer, two dynamic sliding mode control schemes are proposed toovercome severely external disturbance. The slackly limiting condition for the nonlinear disturbanceobserver is given to estimate the arbitrary disturbance with any frequency. The formula for theestimation error is given. Then，it is proved that the derivative of the output of fuzzy disturbanceobserver is bounded in all situations. A real-time feedback from the disturbance observer is given tothe dynamic sliding mode controller for the compensation of disturbance. By the adaptive law, arobust term is added into controller based on the upper boundary of the observer error. The strongrobustness, high control accuracy and better performance in steady states have been found in thiscontrol scheme. It is suitable for the engineering application.The discontinuous sign function in higher-order sliding mode differentiator (HOSMD) issubstituted by the terminal attractor function to avoid unsmooth output. The formulas for theestimation error with disturbance and without disturbance are given respectively. Two strategies, the coefficients setting and increasing the order of HOSMD, are utilized to reduce the estimation error.Compared with traditional maximum gain scheme, the new one has obviously advantages in boththeorem and simulation. By inserting a linear item into each sliding mode surfaces of HOSMD, theconvergence speed is increased when the trajectory is far from the equilibrium point. The formula forthe estimation error in the fast HOSMD is also proposed.In coordinated turn of NSV， attack angle is increased to make sure that lift equals to gravity andto avoid height decreasing continuously. Advanced higher-order sliding mode differentiator is used toestimate the derivatives of system states with arbitrary order. Then, the estimation values of themismatched disturbance and its derivatives are obtained by calculating the differences among theestimated differential values with proper order of the system states. To avoid the logic problem in thecontinuous system, three schemes are proposed to approximate the compound disturbance of the lastsubsystem. It is proved that the estimation error could be arbitrarily small by choosing appropriatecoefficients. Ideal simulation results are obtained by applying the previous schemes to the coordinatedturn control of NSV.The fast convergence and sensitive controller is required for the NSV system. To achieve thisperformance, the recursively terminal sliding mode controller for NSV is designed to increase theconvergence speed in the neighbourhood of equilibrium point. The HOSMDs are utilized as theindirect observers to achieve the unmatched compounded disturbances and the estimation of thederivatives. To avoid singularity in the higher-order derivative of nonlinear term, limiting method isused to guarantee bounded output of sliding mode control. It is proved that the states error could bearbitrarily small with this scheme.By substituting the linear term as power term, power fast terminal sliding mode control has fasterconvergence speed than normal fast terminal sliding mode control at any point. Two concrete powerfast terminal sliding mode strategies are designed and the convergence time expression between initialpoint and zero is presented. General principle, method and steps of power fast terminal sliding modemethod are investigated in the paper. A criterion, which is suitable for any sliding mode, is presentedto judge whether the states would be converged to origin in finite time by sliding mode controller.Numerical solutions of convergence time for any sliding mode are given to replace analyticalsolutions which are difficult to be obtained from the complicate sliding mode strategy. Finally, theapplication for NSV shows the ideal performance.