Output-Redefinition-Based Dynamic Inversion Control for a Nonminimum Phase Hypersonic Vehicle

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.