Low-order unknown input observers

A projection operator approach to the state estimation of dynamical systems is proposed resulting in new types of low-order observers for systems with unknown inputs. The state of the system, whose state is to be estimated, is decomposed into known and unknown components. The unknown component is a projection, not necessarily orthogonal, of the whole state along the subspace in which the available state component resides. Then a dynamical system to estimate the unknown component is constructed. Combining the output of the dynamical system, which estimates the unknown state component, with the available state information results in the observer that estimates the whole state. It is shown that some previously proposed unknown input observer (UIO) architectures can be obtained using the projection operator approach developed in this paper.