Unknown input observers for uncertain systems: a unifying approach and enhancements

A unifying view of existing disturbance decoupling observers is proposed. The different approaches of linear algebraic and geometric design methods and variable structure/sliding modes methods are put together into a single framework. Some new results are obtained: a discrete-time sliding modes observer, a new continuous-time sliding modes observer, and a bound for the degrees of freedom in the error dynamic assignment for some particular cases of the general observer. A systematic view is also posed in already existing disperse results, concerning the class of admissible disturbances, the role of the disturbances input matrix, the mechanism of disturbances decoupling and the obtainable error dynamics in particular cases of the general model. An heuristic procedure is also proposed for dealing with the invariant system zeros.