Enforcing State Constraints on a Model of a Hypersonic Vehicle

Inlet unstart is a sudden breakdown of the shock system in an inlet which can lead to departure from controlled flight and sometimes catastrophic loss of supersonic and hypersonic air vehicles. One way to mitigate this problem is to limit the angle-of-attack and sideslip angle that a vehicle can achieve during flight. This paper develops a sliding mode adaptive controller with the ability to enforce state constraints, which makes it particularly useful for hypersonic vehicles. The system tracks a reference trajectory command provided that the command remains within a predetermined, acceptable region of the state-space. When the reference model leaves this region, the adaptive mechanism is “shut off” and replaced by a state limiting mechanism. An advantage of this approach is that no bounds need to be placed on the input command signal. This method is first developed for a general class of nonlinear dynamic systems of the same form as the equations of motion for a hypersonic vehicle. Controller performance is then demonstrated on a high fidelity simulation of a generic hypersonic vehicle. Results presented in the paper show that this controller is capable of constraining the states while maintaining a smooth control signal and can be considered as a potential candidate for prevention of inlet unstart.