Output tracking with nonhyperbolic and near nonhyperbolic internal dynamics: Helicopter hover control

A technique to achieve output tracking for nonminimum phase linear systems with nonhyperbolic and near nonhyperbolic internal dynamics is presented. This approach integrates stable inversion techniques, that achieve exact-tracking, with approximation techniques, that modify the internal dynamics to achieve desirable performance. Such modification of the internal dynamics is used (a) to remove nonhyperbolicity which is an obstruction to applying stable inversion techniques and (b) to reduce large preactuation times needed to apply stable inversion for near nonhyperbolic cases. The method is applied to an example helicopter hover control problem with near nonhyperbolic internal dynamics for illustrating the tradeoff between exact tracking and reduction of preactuation time.