Search

The attitude control of an underactuated reusable launch vehicle (RLV) in the reentry phase involving nonminimum phase problem and control input constraints is investigated in this article. To address the nonminimum phase problem, an approach combining output redefinition and robust backstepping is proposed, where a synthetic output is constructed using the combination of the original output and the internal states to obtain stable zero dynamics, and then robust backstepping is performed on the new output. Besides, the ideal internal dynamics are obtained by using optimal bounded inversion, which are incorporated into the controller as the reference trajectories for the internal states to improve the output tracking accuracy. To cope with the control input constraints, a simple and useful anti-windup strategy is proposed by using feedback error clipping, which is shown to be very effective in mitigating control input saturation. Numerical simulations are given to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The precision output tracking problem of multi-input–multi-output (MIMO) uncertain nonlinear nonminimum phase systems is investigated in this paper. The challenge lies in the difficulty in solving the ideal internal dynamics (IID), which is a bounded solution for the uncertain zero dynamics. First, the experience replay technique is applied to identify the uncertain parameters in the zero dynamics. It uses the recorded past data concurrently with current data which can greatly speed up the identification convergence. With the identified parameters, a novel approach called optimal bounded inversion is proposed to obtain the IID by solving a trajectory optimization problem using GPOPS-II. The boundedness of the IID is guaranteed by setting state constraints and the feasibility is achieved by minimizing the initial condition mismatch. Moreover, a piecewise IID updating scheme is adopted to reduce the computational burden. The benchmark nonminimum phase system, a vertical take-off and landing (VTOL) aircraft is used to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Output-redefinition-based dynamic inversion (ORDI) control is proposed for a nonminimum phase hypersonic vehicle (HSV). When velocity and altitude are selected as control outputs, an HSV exhibits nonminimum phase behavior, preventing the application of standard dynamic inversion control due to the unstable zero dynamics. This problem is solved by the ORDI control architecture, where output redefinition is utilized at first to render the modified zero dynamics stable, and then, dynamic inversion is used to stabilize the new external dynamics. Three kinds of ORDI controllers with different choices of new control output are investigated. The first takes the internal variable as the control output, which exhibits good robustness but with restricted performance. The second utilizes a synthetic output, which is a linear combination of the system output and internal variable, making the zero dynamics adjustable, and thus improves the tracking performance. The third adds an integral item to the synthetic output, and thus ensures zero steady-state error even with model uncertainties. A systematic way is proposed to determine the combination coefficient to achieve zero dynamics assignment by using the root locus method. The efficiency of the method is illustrated by numerical simulations. [ABSTRACT FROM AUTHOR]

In this paper, the nonminimum phase problem of a flexible hypersonic vehicle is investigated. The main challenge of nonminimum phase is the prevention of dynamic inversion methods to nonlinear control design. To solve this problem, we make research on the relationship between nonminimum phase and backstepping control, finding that a stable nonlinear controller can be obtained by changing the control loop on the basis of backstepping control. By extending the control loop to cover the internal dynamics in it, the internal states are directly controlled by the inputs and simultaneously serve as virtual control for the external states, making it possible to guarantee output tracking as well as internal stability. Then, based on the extended control loop, a simplified control-oriented model is developed to enable the applicability of adaptive backstepping method. It simplifies the design process and releases some limitations caused by direct use of the no simplified control-oriented model. Next, under proper assumptions, asymptotic stability is proved for constant commands, while bounded stability is proved for varying commands. The proposed method is compared with approximate backstepping control and dynamic surface control and is shown to have superior tracking accuracy as well as robustness from the simulation results. This paper may also provide a beneficial guidance for control design of other complex systems. [ABSTRACT FROM AUTHOR]